Diatreme Breccias at the Kelian Gold Mine

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Introduction

SOME magmatic-hydrothermal ore deposits in volcano-plu-tonic arcs are associated with large volcanic-hydrothermalbreccia complexes (e.g., Sillitoe and Bonham, 1984; Sillitoe,1997). Such breccia bodies have commonly been inferred to

be diatremes—large-scale breccia pipes that underlie maar volcanoes (e.g., Lorenz, 1986). In the volcanological litera-ture, there are few descriptions of the root zones to this classof volcano, because they can only be exposed by deep-levelerosion or mining activity (Clement, 1982; Cas and Wright,1987; Lorenz and Kurszlaukis, 2007). In contrast, ore depositgeologists have provided numerous examples of what they infer to be breccia-filled diatremes from porphyry and ep-ithermal settings (e.g., Acupan, Philippines: Cooke andBloom, 1990, Cooke et al., 1996; Grasberg, Indonesia: Mac-donald and Arnold, 1994; Martabe, Indonesia: Sutopo et al.,2003, 2007; Wau, Papua New Guinea: Sillitoe et al., 1984; ElTeniente, Chile: Cannell et al., 2005; Yanacocha, Peru:Turner, 1997; Colquijirca, Peru: Bendezú et al., 2003; AguaRica, Argentina: Landtwing et al., 2002; Montana Tunnels,United States: Sillitoe et al., 1985; Cripple Creek, UnitedStates: Thompson et al., 1985, Thompson, 1992; Jensen,2003; Roşia Montanˇ a, Romania: Wallier et al., 2006), al-though the supporting evidence for this genetic interpreta-tion is not always compelling. This is because only a few de-scriptive papers (e.g., Wau: Sillitoe et al., 1984; Montana

Tunnels: Sillitoe et al., 1985; Cripple Creek: Thompson et al.,1985) have adequately described their key textural and mor-phological characteristics.

Kelian is a large, breccia- and vein-hosted epithermal sys-tem of Miocene age (van Leeuwen et al., 1990). Most of the

mineralization at Kelian occurred subsequent to catastrophicbrecciation events that produced a nested complex of car-bonaceous matrix-rich breccia pipes, dikes, and beds (Davies,2002). Open-pit mining and deep drilling have provided ex-cellent exposures of these breccia bodies over a vertical inter- val in excess of 700 m. This environment has provided anideal opportunity for systematic mapping, description, and in-terpretation of the carbonaceous matrix-rich breccia complex.

This manuscript documents the characteristics and faciesarchitecture of premineralization matrix-rich breccia bodiesat the Kelian gold mine and assesses their role as a precursorto the formation of a large-scale auriferous hydrothermal sys-tem. In particular, we document individual breccia lithofa-cies, their facies associations and spatial distributions, and in-

terpret their origins. We comment on the implications of ourresults for understanding fluid flow in and around similarlarge-scale discordant breccia bodies in other epithermal andporphyry ore systems and how this could influence mineralexploration.

Terminology

Breccia nomenclature has been applied inconsistently inthe economic geology literature. Terms that may cause someconfusion are defined below. With regards to grain size andshape terms (breccia, conglomerate, sandstone, mudstone),

Diatreme Breccias at the Kelian Gold Mine, Kalimantan, Indonesia:Precursors to Epithermal Gold Mineralization

ANDREW G. S. DAVIES,* DAVID R. COOKE,† J. BRUCE GEMMELL, AND KIRSTIE A. SIMPSON

CODES ARC Centre of Excellence in Ore Deposits, University of Tasmania, Private Bag 79, Hobart, Tasmania 7001, Australia

Abstract

Early Miocene volcanism associated with a maar-diatreme breccia complex preceded main-stage epithermalgold mineralization at the Kelian gold mine, Kalimantan, Indonesia. Prior to brecciation, andesite intrusions(19.7 ± 0.06 Ma) were emplaced into a package of felsic volcaniclastic rocks and overlying carbonaceous sand-stones and mudstones, and a weakly mineralized geothermal system was established. Intrusion of quartz-phyric(19.8 ± 0.1 Ma) and quartz-feldspar-phyric rhyolite (19.5 ± 0.1 Ma) into the active geothermal system triggered

widespread fragmentation and formation of the maar-diatreme complex.Subsurface phreatomagmatic and phreatic explosions disrupted the preexisting hydrothermal system, pro-

ducing three composite diatreme breccia bodies (the Tepu, Runcing, and Burung Breccias). The diatremesconsist of polymict breccias and sandstones that contain abundant carbonaceous matrix. A distinctive facies as-sociation comprising coherent rhyolite, jigsaw-fit rhyolite breccia, and matrix-rich breccias that contain wispy to blocky juvenile rhyolite clasts define the root zones of the diatremes.

The surficial products of maar-diatreme volcanic activity at Kelian are preserved as large blocks of well-strat-ified breccias. They contain accretionary lapilli and were deposited by a combination of wet, pyroclastic base-surge, fallout, and cosurge fallout processes. Evidence for syneruptive resedimentation of the pyroclastic de-posits is preserved in poorly stratified breccia beds. Megablocks of phreatomagmatic base-surge deposits weredropped down several hundred meters from the maar environment into the underlying diatremes.

Volcanism in the Kelian maar-diatreme complex was dominated by a combination of phreatomagmatic andphreatic processes, with subordinate hydraulic, tectonic, and dry magmatic fragmentation. The carbonaceousmatrix-rich diatreme breccias acted as aquicludes during subsequent hydrothermal activity, focusing fluid flow into the wall rocks adjacent to the diatremes, where epithermal gold mineralization and hydrothermal brec-ciation occurred.

† Corresponding author: e-mail, [email protected]*Present address: Teck Cominco Ltd., Vancouver, British Columbia,

Canada.

©2008 Society of Economic Geologists, Inc.Economic Geology, v. 103, pp. 689–716


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we follow conventional sedimentological and volcanologicaldefinitions (e.g., McPhie et al., 1993).

Maars: Maar volcanoes comprise a central crater surroundedby low rims (tens of meters) of phreatomagmatic base-surgeand fallout deposits and consequently have low aspect ratios(Fisher and Waters, 1970). The craters excavate country rocksas a result of phreatomagmatic eruptions (Fisher and Waters,

1970). The surface morphology and the amount of slumping of blocks derived from the surface back into the crater can be dic-tated by the nature of the wall rocks (e.g., “hard-substrate” vs.“soft-substrate” maars; Auer et al., 2007).

Diatreme: Diatremes are downward-tapering, subsurface volcanic conduits, which may lie beneath maars and areformed at least in part by explosive phreatomagmatic erup-tions (Lorenz, 1986; Cas and Wright, 1987; Martin et al.,2007). Dry magmatic and/or possibly phreatic (steam) explo-sions may also contribute to diatreme formation but in isola-tion do not form diatremes. Diatremes are filled by volcani-clastic deposits and collapsed wall-rock blocks (Lorenz,1973). Their cross-sectional areas are similar to those of maars (


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DIATREME BRECCIAS AT THE KELIAN GOLD MINE, KALIMANTAN, INDONESIA 691

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FIG. 2. Premining surface geology of the Kelian gold deposit (modified after Davies, 2002), showing the major brecciabodies. The Kelian River originally passed through the main ore zone and had to be diverted to the north, providing excel-lent exposures through the Runcing Breccia. Abbreviations: Quat = Quaternary, U. Cret = Upper Cretaceous.


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Brecciation and mineralization occurred synchronous with,and subsequent to, rhyolite emplacement. Previous workersdocumented six types of breccia at Kelian (van Leeuwen etal., 1990): tuff, fault, intrusion, hydrothermal, fluidized, andmuddy breccia. It is the latter group, the muddy breccias, which are the subject of the current study. Muddy breccias were described by van Leeuwen et al. (1990) as dark gray,

massive, polymict and polyphase breccia bodies and dikes.They are aligned roughly north-south, have intrusive contactrelationships, and were found to contain clasts of sediments,tuff, andesite (concentrated at contacts with andesite intru-sions), and rhyolite (van Leeuwen et al., 1990). Coarse- andfine-grained varieties were observed by these previous work-ers, as were late-stage sandy and pebble dikes.

Mineralization at Kelian has been described by vanLeeuwen et al. (1990), Davies (2002), and Davies et al. (2003,2008). Only key features are summarized below. A hydrother-mal system was established prior to the formation of themuddy breccia complex, based on the presence of truncatedillite-pyrite veins and associated illite alteration in brecciaclasts. Main-stage gold and silver deposition commenced dur-

ing the waning stages of muddy breccia formation and contin-ued for some time after their final consolidation. Gold and silver are hosted by hydrothermally cemented breccia bodies,sheeted and conjugate veins, and disseminated sulfides.

Davies (2002) and Davies et al. (2003, 2008) identified fivestages of mineralization that progressed from pyrite- to basemetal-sulfide–dominated (sphalerite + galena ± chalcopyrite)and finally to sulfosalt-dominated mineralization (proustite-pyragyrite, tennantite-tetrahedrite). Deposition of gangueminerals progressed from illite-quartz to adularia and/orquartz and/or illite and finally carbonate-dominated assem-blages. Stage 1 (pre-muddy breccia) mineralization producedproximal illite-pyrite-quartz–cemented veins and brecciasand distal calcite-quartz ± epidote veins. Stage 2 mineraliza-

tion generated pyrite-quartz-illite–cemented breccias and veins with minor base metal sulfides in the northern part of Kelian and adularia-quartz-pyrite in the south. A transition toabundant base metal sulfides (galena, sphalerite, and chal-copyrite) occurred from stage 2 to stage 3A. In addition tobase metal sulfides, stage 3A veins and hydrothermal brecciascontain ubiquitous pyrite, local sulfosalts, and abundant na-tive gold. Stage 3B mineralization was coeval with stage 3Aand was localized at depth and on the flanks of the Kelian sys-tem. It produced base metal sulfides, pyrrhotite, marcasite,and melnikovite. Boiling produced abundant bladed carbon-ate during stage 3C. Stage 4 sulfosalts and sulfides are inter-grown with laminated and bladed rhodochrosite. Stage 5 con-sists of late kaolinite, covellite, digenite, and siderite, possibly

(at least in part) of supergene origin. Gold formed throughoutstages 1 to 4 but most occurred during stages 3 and 4. Goldoccurs principally as inclusions within and intergrown withpyrite, sphalerite, galena, arsenopyrite, quartz, bladed car-bonate, and sulfosalts.

Hydrothermal alteration is zoned about contacts, faults,breccias, and veins (Davies, 2002; Davies et al., 2003, 2008).The alteration assemblages in andesite intrusions grade fromproximal quartz-illite-pyrite through illite-carbonate-pyrite,and illite-chlorite-carbonate to distal chlorite-calcite-illite. Al-teration zonation in the volcaniclastic host rocks grades from

proximal quartz-illite-pyrite to distal smectite-illite assem-blages. Local, intense adularia- quartz-illite and/or carbonatealteration assemblages are spatially associated with adulariaand carbonate cement, respectively. Alteration patterns atKelian were controlled by lithologic units, structure, andhost-rock permeability (Davies, 2002).

Carbonaceous Breccia and Sandstone Lithofacies

From detailed mapping and drill core logging, we have rec-ognized two distinctive groups of matrix-rich breccias at Ke-lian. Both are characterized by abundant carbonaceous com-ponents (clasts and/or matrix) derived from the surroundingsedimentary units. Collectively, these are the breccias that were described by van Leeuwen et al. (1990) as muddy brec-cias and are here referred to as carbonaceous matrix-richbreccias, in order to emphasize their common, distinguishingcomponent: carbonaceous matrix.

We have divided the carbonaceous matrix-rich breccias intotwo groups: unstratified (A facies) and stratified (B facies).Each group consists of several facies and subfacies (Tables 1,2). Breccia facies A include monomict carbonaceous mud-

stone and sandstone-clast breccias, polymict carbonaceousmatrix-rich breccias, and monomict rhyolite breccias withcarbonaceous matrix. Facies have been defined by variationsin (1) breccia composition (monomict vs. polymict), (2) diag-nostic clast types, and (3) grain size. Subfacies are distin-guished by subtle variations in the dominant clast type, otherthan carbonaceous mudstone and sandstone (i.e., andesite, volcaniclastic rocks, rhyolite), or modal mineralogy of theclasts (QP vs. QFP). The breccias vary in appearance due toclast abundance, size, lithology, and degree of clast roundingbut are generally black, dark gray or, where intensely illite ±quartz-pyrite altered, light gray in color. Although there areclast-supported breccias in this group, most are matrix sup-ported and massive. The polymict facies all have similar ma-

trix components (sand-sized fragments of quartz, polymictlithic clasts, and abundant carbonaceous mudstone and sand-stone clasts) but have variable clast populations (various com-binations of andesite, rhyolite, volcaniclastic rocks, earlierformed breccias, accretionary lapilli, charcoal or coal, andrare vein fragments).

The B facies consist of both breccias and sandstones andhave similar clast and matrix components to the A facies butdiffer in that they are stratified. B facies vary in the styles of stratification and the principal clast components (rhyolite- or wall rock-dominated). Subfacies are distinguished by grain-size variations (sandstone to breccia; Tables 1, 2).

Unstratified breccias and sandstones

Facies A1: These are dark gray to black, monomict, matrix-rich (locally up to 60%) carbonaceous mudstone-clast brec-cias that contain abundant clay gouge (Fig. 3A–D; Tables 1,2). Some A1 breccias have formed at the contact between theRuncing Rhyolite and carbonaceous mudstones (Fig. 3A), butmost are spatially associated with faults (e.g., Fig. 3B). Many A1 breccias are internally banded (e.g., aligned clasts, gouge,foliation). Sulfide vein clasts are present locally (Fig. 3C), asare late hydrothermal cements (Fig. 3D).

Facies A2: These breccias occur in contact with coherentflow-banded rhyolite dikes (Fig. 4A). Facies A2 are clast-rich,

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TABLE 1. Summary of Breccia Facies A and B at Kelian: Composition, Internal Organization, and Hydrothermal Features

Facies and Alteration andfacies name Subfacies Internal organization Clast shape and type Matrix mineralization

A1: Monomict None Monomict, jigsaw-fit, Shape: angular to Mud- to sand- Pervasive ill-sme-pyrcarbonaceous clast rotated or massive subrounded sized fragments alt , local clay gougebreccia Clast to matrix supported Type: Cms, local Local clay gouge

Some imbricated clasts sulfide vein

Local banding or foliation fragmentsA2: Monomict, QP: QP clast Jigsaw-fit, clast to matrix Shape: angular or Black to dark Weak to moderate kao ±

jigsaw-fit rhyolite bearing supported irregular with gray matrix of ill alt of rhyolite clastsbreccia with QFP: QFP clast cuspate and/or milled A1 and ill-pyr alt ofcarbonaceous bearing wispy margins facies and/or matrixmatrix Type: QFP, QP clay gouge

A3: Polymict, QP: QP clast Polymict, massive and Shape: irregular to 30–50% mud- to Weak to intensecarbonaceous, bearing matrix supported angular with wispy sand-sized qtz-ill-pyr altmatrix-supported QFP: QFP clast or cuspate margins fragments And and V clasts locallybreccia with wispy bearing Type: Cms, And, V, QP, contain stage 1A veinsand/or blocky QFP, charcoal, coal, (i.e., some mineral-rhyolite clasts vein, breccia ization predated

brecciation)

A4: Polymict, QP: QP clast Polymict, massive and Shape: angular to Mostly sand-sized Tepu Breccia: qtz-ill-carbonaceous, bearing matrix supported subangular, some Up to 90% matrix pyr±crb alt , local ill-

matrix-supported QFP: QFP clast subrounded in fine-grained qtz -pyr-crb altbreccia with bearing Type: Cms ± And, V, QP, breccia Qtz-ill-pyr±crb alt inrhyolite clasts QP/QFP: QP and QFP, coal, charcoal, Burung Breccia near

QFP clast bearing AL, vein and breccia 393 Breccia

A5: Polymict, V: volcaniclastic Polymict, jigsaw-fit, clast Shape: angular to Sand-sized Weak to moderatecarbonaceous, clast dominated rotated and massive subangular fragments pervasive ill-qtz-crb-matrix-supported And: andesite Type: V, And, Cms, rare pyr altbreccia clast dominated C1, C7 breccia clasts Local ill-qtz-pyr±crb alt

A6: Polymict, QP: QP clast Polymict Shape: abundant Up to 20% A6-QFP: weak to intensecarbonaceous, bearing Generally clast supported subrounded to minor sand-sized ill-pyr altclast-supported QFP: QFP clast Minor matrix supported subangular clasts fragments A6-V: weak sme-ill altbreccia and bearing Type: QFP, Cms,conglomerate V: volcaniclastic And, V

clast bearing

A7: Discordant, AL: accretionary Polymict Shape: angular to Up to 100% Weak to moderatecarbonaceous, lapilli bearing Generally massive subangular mud- and sand- pervasive ill-pyr alt incrystal and lithic Locally stratified Type: Cms, And, V, sized fragments Tepu Brecciasandstone with QFP, QP, broken qtz Intense qtz-ill-pyr±crbrhyolite fragments and fsp, local AL alt in Burung Breccia

B1: Well-stratified, A: medium-grained Polymict, poorly to Shape: angular to Sand-sized Weak sme-ill ± kao andrhyolite-clast-rich, volcanic breccia moderately sorted subangular fragments trace disseminated pyrcarbonaceous B: fine-grained Matrix to clast supported Type: V, QP, Cms,breccia and volcanic breccia Planar, wavy, dune, and minor Alsandstone C: volcanic low-angle cross stratified

sandstone Normal, reverse, double-graded beds

B2: Well-stratified A: medium-grained Polymict, poorly to Shape: angular to Mud- to sand- Weak sme-ill ± kao altcarbonaceous volcanic breccia moderately sorted subrounded sized fragments and rare disseminatedbreccia and B: fine-grained Matrix to clast supported Type: Cms, QP, V, pyrsandstone volcanic breccia Planar, wavy, dune, and abundant Al

C: volcanic low-angle cross stratifiedsandstone Diffuse normal, reverse

and double-graded beds

B3: Weakly stratified A: medium-grained Polymict, poorly to Shape: angular to 40% mud- to Weak sme-ill altcarbonaceous volcanic breccia moderately sorted subrounded sand-sizedbreccia and B: fine-grained Matrix supported Type: V, Cms, QP matrixsandstone volcanic breccia Diffuse tabular and trough-

C: volcanic sandstone shaped beds, graded

Abbreviations: And = andesite, AL = accretionary lapilli, alt = alteration, Cms = carbonaceous mudstone and/or sandstone, crb = carbonate, fsp = feldspar,ill = illite, kao = kaolinite, pyr = pyrite, QFP = quartz-feldspar-phyric rhyolite, QP = quartz-phyric rhyolite, qtz = quartz, sme = smectite, V = volcanic sand-stone and/or mudstone


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TABLE 2. Summary of Breccia Facies A and B at Kelian: Contact Relationships, Morphology, and Distribution

Facies Contacts Morphology Distribution

A1 Sharp or gradational into unbrecciated, Irregular to tabular inclined to subvertical Common in Runcing Breccialaminated and thinly bedded carbona- zones m to 10s of m across Also occurs in hanging wall to Westceous mudstone and/or sandstone Forms a shell up to 200 m across around the Prampus, Burung, and North Burung

Crosscuts B1, B2, B3 breccias sharply Runcing Breccia and Runcing Rhyolite faults, shallow levels of Burung Breccia,Irregular dikes (0.25–5 m thick) cut B facies and on margins of Tepu Breccia in south

on Gunung Runcing wall of the East Prampus open pitA2 Highly irregular at centimetrer scale, with Tabular to irregular sheets along margins of Runcing and Tepu Breccias, Burung fault

transition from coherent rhyolite into fault-zone–hosted QP rhyolite dikes hanging wall, NW-trending faults, cross- jigsaw-fit and then clast-rotated rhyolite- Tabular dikes (10 cm to 1 m width) in fault cutting the Burung faultclast breccia over 0.5 to 10 m zones, f ingers and irregular pods in A1, Locally, in A1 and A4 breccias adjacent to

Complicated zones of fault gouge mingled A3 and A4 breccias the Runcing Rhyolite with rhyolite occur at margins of rhyolitedikes in fault zones

A3 Convoluted, irregular and penetrating Tepu Breccia: subvertical A3 pipes are Common in Tepu Breccia, typically internalcontacts with earlier breccia phases; local commonly >30 m, and up to 80 m, across to the breccia body, rarely at marginsgradation into A2 breccias and crosscut A4, A3, and A2 breccia pipes Minor occurrences peripheral to Runcing

Contacts with pipes of same breccia facies are Runcing Breccia: irregular pipes and pods QFP and QP intrusions in NW-trendinggradational over 2 to 5 m; crude increase vary from 1 to 20 m across faults, and NW-trending faults in thein matrix abundance toward contact Burung fault hanging wall

A4 Sharp or diffuse, and/or erosional Subvertical irregularly shaped pipes, dikes Principally in the Tepu, Burung, andIn the Tepu and Burung Breccias, several Thin breccia dikes range from centimeter to Runcing Breccias

of the bounding surfaces are post- meter scale and extend tens of meters Smaller bodies along NW-trending faults inbrecciation faults vertically East and West Prampus zones

Larger irregular pipes range from 10 to over Narrow dikes (75 m north side of the Kelian River diversionand penetrative Subfacies A6-V: Small (


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monomict, jigsaw-fit rhyolite breccias that have a carbona-ceous mudstone matrix (Tables 1, 2; Fig. 4B-C). Rhyoliteclasts (QP or QFP) are angular and blocky with curviplanarmargins (Fig. 4B) or wispy with ragged margins (Fig. 4C).Some rhyolite clasts have partially wispy and partially blocky margins. A2 breccias occur in narrow dikes (10 cm to 1 m) inpreexisting faults or as irregular fingers and pods in monom-ict A1 or polymict A4 breccias. In several locations, a com-

plete gradation from coherent rhyolite (Fig. 4A) to monomict jigsaw-fit rhyolite breccia with carbonaceous mudstone ma-trix (Fig. 4B) to polymict carbonaceous matrix-supportedbreccia that contains blocky and wispy rhyolite clasts (A3 fa-cies; Fig. 5) occurs over strike lengths of 0.5 to 10 m.

Facies A3: These unsorted, matrix-supported, polymict car-bonaceous breccias are characterized by texturally distinctive wispy and blocky QP or QFP rhyolite clasts (Fig. 5A-F; Tables1, 2). Wispy rhyolite clasts have delicate, irregular margins(Fig. 5B-D), whereas blocky rhyolite clasts have curviplanarmargins (Fig. 5E-F). Some rhyolite clasts have partial wispy

and partial blocky, curviplanar margins. All other clast typesidentified in the A3 facies are more rounded than the rhyoliteclasts (Fig. 5). Apart from the common clast types (carbona-ceous mudstone and sandstone, volcaniclastic sandstone, an-desite, and rhyolite), unusual clast types in A3 breccias in-clude carbonized wood (Fig. 5C), A4 breccia, A7 sandstone,and finely bedded B2 sandstone. Accretionary lapilli (2–8 mmdiam) comprise up to 5 percent of some A3 breccias.

Facies A4: These are the most abundant of the carbona-ceous matrix-rich breccia facies at Kelian. They are poorly sorted, carbonaceous matrix-supported polymict breccias(Fig. 6; Tables 1, 2). Discrete A3 and A4 breccia pipes canoccur adjacent to each other (e.g., Fig. 6A), and the A4 brec-cias are similar in composition to A3 breccias (e.g., Fig. 6B-C). There are, however, no wispy rhyolite clasts in the A4breccias, and rounded rhyolite clast shapes are more common(e.g., Fig. 6D-E). Unusual, distinctive and rare clasts thathave been observed in A4 breccias include accretionary lapilli, hydrothermally cemented breccia, A3 and A1 breccia,


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FIG. 3. Breccia facies A1. A. North wall of East Prampus pit (March 1999), showing domains of A1 breccia that have cutand/or surrounded laminated carbonaceous mudstone and sandstone (CMS) in the hanging wall to the Runcing fault and 394ore zone. The Runcing Rhyolite intrusion is surrounded by A1 breccia and carbonaceous sedimentary rocks. Light gray, bed-ded B facies are crosscut by dikes of A1 breccia. B. Jigsaw-fit to clast-rotated A1 breccia. Larger clasts are outlined in white,as are some internal fractures. A zone of matrix- to clast-supported A1 breccia with aligned clasts between the yellow lines

is interpreted to be a fault zone (Runcing Breccia, 1090 mRL). C. Matrix-supported A1 breccia with clasts of carbonaceousmudstone, stage 2A or 3A pyrite vein fragments (PYR) and clay gouge in the matrix (Runcing Breccia, 1060 mRL). D. Clast-supported A1 breccia with minor matrix and minor carbonate (CRB) cement. There are ball and pillow structures in the bed-ded carbonaceous sandstone and mudstone clast (Burung Breccia, K317-188.0 m).


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blocks of stratified facies B breccias (Fig. 6C), and unstrati-fied A4 breccia blocks that are up to 50 m in diameter. An-desite and volcaniclastic rock clasts that contain truncated veins and prebreccia alteration assemblages have also beenobserved (Fig. 6B). There is a correlation between increasedmatrix abundance and clast roundness and decreased clastsizes in A4 breccias. Small dikes of A4 breccia have been ob-

served locally (Fig. 6F).Facies A5: These occur both as discrete bodies (e.g., Fig.7A) and with other A facies. They contain angular to sub-rounded clasts of carbonaceous mudstone, sandstone, volcani-clastic rocks, and andesite (Fig. 7B-C; Tables 1, 2) and are dis-tinguished from A4 breccias by a lack of rhyolite clasts. Clastsof A1, A4, and hydrothermally cemented breccia occur locally.

Facies A6: A6 breccias and conglomerates occur as irregu-lar subvertical pipes and dikes that have crosscut and erodedA4 and A3 breccias (e.g., Fig. 8A). The A6 facies is distin-guished by abundant (60–90%), subrounded to rounded QFPclasts (Fig. 8B), and ranges from clast- to locally matrix-sup-ported, poorly to moderately sorted breccia and conglomer-ate (Tables 1, 2). A3-QFP dikes have cut A6 breccias locally

(Fig. 8C).Facies A7: Massive to locally stratified, poorly sorted, dis-cordant, carbonaceous sandstone characterizes facies A7 (Fig.9; Tables 1, 2). The most abundant components are fragmentsof carbonaceous mudstone and sandstone. However, finefragments of QP and/or QFP rhyolites are ubiquitous and lo-cally have wispy and/or curviplanar margins. Accretionary lapilli are present locally. A7 discordant sandstone occurs asthe only facies in breccia pipes that are up to 20 m in diame-ter, as fine-grained facies that grades into A3 or A4 facies within larger breccia bodies, and as dikes less than 3 m wide(e.g., Fig. 9). Some A7 sandstones are laminated subparallelto the walls of the larger breccia bodies.

Stratified breccias and sandstones

Facies B1: These are well-stratified breccias that have an-gular to subangular clasts of aphanitic to finely (


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FIG. 5. Breccia facies A3. A. Coarse-grained marginal A3-QFP facies of Tepu breccia with large blocks of andesite (And)and wispy quartz-feldspar-phyric rhyolite clasts (QFP). Contact with coherent andesite is 1 m to right of area shown in frame(Tepu Breccia, 1000 mRL). B. Medium-grained A3-QP breccia with abundant wispy and ragged quartz-phyric rhyolite clasts

(QP) in a carbonaceous, sand-sized, lithic, and crystal matrix (Tepu Breccia, 1070 mRL). C. Fine-grained A3-QP breccia with20% wispy and blocky rhyolite clasts and fragments of carbonized wood (Tepu Breccia, 1070 mRL). D. Detail of ragged QPrhyolite clasts in fine- to medium-grained polymict A3-QP breccia with mudstone clasts (CMS). The two large QP clasts nearthe top of the photograph display small-scale jigsaw-fit texture (Tepu Breccia). E. Detail of blocky finely porphyritic rhyoliteclasts in fine- to medium-grained A3-QFP breccia. There is a range of clast shapes in the rhyolite clast population—somehave delicate curviplanar margins, whereas others are subrounded, as are the sedimentary clasts. The blocky clasts are in-terpreted to be juvenile clasts that have not been recycled, and the subrounded clasts are interpreted to be juvenile claststhat have undergone transport and abrasion possibly during multiple brecciation events. The breccia also contains clasts of carbonaceous mudstone and sandstone (CMS) and volcanic mudstone and sandstone (V; Tepu Breccia). F. Medium- tocoarse-grained A3-QFP breccia with blocky rhyolite clasts (Tepu Breccia, 1070 mRL).


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FIG. 6. Breccia facies A4. A. Fault contact between A4 and A3 breccias of the Tepu Breccia (right) and volcaniclastic rocks(V) and hydrothermal breccias (left). Bottom and top of photograph are 1060 mRl and 1110 mRL, respectively (50-m verti-cal exposure). The massive A4 and A3 breccias in this exposure crop out at the northern margin of the Tepu Breccia. Rhyo-lite intrusions (R) occur in a curved, possibly faulted, contact between the A3 and A4 breccia pipes. Internal contacts be-tween A4 and A3 facies are diffuse, irregular, and gradational in this area. B. Coarse-grained A4 breccia with large blocks of

pervasively illite-pyrite-carbonate–altered andesite that contain stage 1A pyrite-carbonate-sericite veins. The veins are trun-cated at clast margins and provide evidence for an early hydrothermal system (Tepu Breccia, 1060 mRL; scale bar = 10 cm).C. Medium-grained A4-QP and/or QFP breccia with rare blocks up to 5 × 3 × 4 m of stratified B1 and B2 facies (RuncingBreccia, 1090 mRL). D. Medium-grained polymict A4-QP and/or QFP breccia cut by irregular stage 3A pyrite-sphalerite

veins. The breccia contains subangular to subround clasts of carbonaceous mudstone and sandstone (CMS), volcanic mud-stone and sandstone (V), andesite, and quartz-feldspar-phyric rhyolite (QFP; Tepu Breccia, drill hole K777-213.7 m). E.Fine- to medium-grained A4-QP and/or QFP breccia with subround to subangular clasts of quartz-phyric rhyolite (QP).There is a single clast of A3-QP breccia at top of frame. Light color (cf. Fig. 6D) is due to pervasive, intense quartz-illite-pyrite alteration of the Burung Breccia in the vicinity of the 393 ore zone. Irregular stage 2A and/or 3A pyrite veins havecrosscut the breccia (Burung Breccia, drill hole K450-291.2 m). F. A4-QFP dikes in the Runcing Rhyolite (QFP). Clasts andmatrix in the dikes consist of carbonaceous mudstone and QFP fragments. QFP clasts in this example are angular but do nothave wispy or curviplanar margins. The QFP clasts were derived from the local wall rocks and are not a juvenile magmaticcomponent (Runcing Rhyolite, Kelian River diversion east end, south side, 1100 mRL).


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B2 facies from overlying weakly stratified B3 breccia at onelocation (Fig. 11A). B3 facies also occur as large blocks (me-ters to tens of meters) in polymict carbonaceous matrix-richbreccia pipes (Fig. 11B). These blocks contain thick (0.75–5m), weakly graded beds and are crosscut by A4 breccia pipes.The B3 blocks have disaggregated transitional contacts withthe surrounding breccias (Fig. 11B).

Breccia Geometries, Facies Distributions,and Spatial Associations

Three large, discordant bodies of carbonaceous matrix-richbreccia are exposed in the Kelian mine area: the Tepu, Bu-rung (van Leeuwen et al., 1990), and Runcing Breccias (S.Hunt, pers. commun., 1997; Fig. 2). Other less voluminous,unnamed carbonaceous matrix-rich breccia bodies also occurat and around Kelian. Each of these breccia bodies has a com-plex internal arrangement of breccia facies.

The composite Runcing, Tepu, and Burung Breccia bodiesconsist of multiple breccia pipes and dikes that each containsone or more of the A facies breccias. The bodies formed by multiple brecciation episodes and have complex internal con-

tact relationships. Both internal and country-rock contacts aretypically gradational over meters to tens of meters, especially where earlier breccias have been crosscut and stoped or re-moved by emplacement of later breccias. Some internal faciesoccur as subvertical pipes that are smaller in volume than buthave similar gross geometry to, the enclosing Runcing, Tepu,and Burung Breccia bodies.

The Tepu and Burung Breccia geometries were con-strained partly by andesite intrusions (Fig. 2). Because theRuncing Breccia was not constrained by competent andesiticbodies, it developed a more equant geometry partly con-trolled by and subsequently disrupted by faults.

In the area of both the Tepu and Runcing Breccias, car-bonaceous mudstone to fine-grained sandstone is juxtaposed

against Upper Cretaceous volcaniclastic rocks (Davies et al.,2008) by a series of high-angle normal and wrench faults. Well-developed fault breccia zones are present in the car-bonaceous strata, especially in the hanging wall to the Burungfault (Fig. 2). Fault breccias crosscut and are crosscut by mo-nomict and polymict carbonaceous matrix-rich breccias, indi-cating that fault movement continued during formation of thebreccias.

Tepu Breccia

The northeast-trending Tepu Breccia (Table 3) is locatedbetween the Crusher Andesite and Eastern Andesite intru-sions (Figs. 2, 12A) and formed in a fault-bounded slice of carbonaceous mudstone located along the East Prampus fault

(Fig. 12B; Davies, 2002). The breccia body is downward ta-pering in cross-sectional view. Premining maps (PT KelianEquatorial Mining, unpub. data) suggest that the Tepu Brec-cia flared markedly above the roof of the Crusher Andesite.An east-trending arm of the Tepu Breccia extends along theTepu fault across the Eastern Andesite and into a mineralizedhydrothermal breccia body known as the 255 Breccia (Figs. 2,12A; Davies, 2002; Davies et al., 2008).

The Tepu Breccia consists of multiple crosscutting pipes(e.g., Fig. 12C). Each pipe contains either a single breccia or,less commonly, multiple pipes and dikes of A and minor B


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FIG. 7. Breccia facies A5. A. Dark gray-colored A5 breccia body in northwest wall of the East Prampus pit (base of photograph at 1030 mRL, top at1110 mRL). A prebreccia andesite intrusion (And) crops out to the left of thebreccia and bedded volcaniclastic rocks (V) to the right. Left side of A5 brec-

cia is andesite-clast dominated (A5-And subfacies), right side is volcaniclas-tic-clast dominated (A5-V subfacies), and center is undifferentiated A5breccia with subequal andesite and volcaniclastic clasts. B. Coarse-grained,matrix-supported A5-A facies, with andesite (And), carbonaceous sedimentary (Cms), and minor volcaniclastic (V) clasts. Andesite clasts are pervasively il-lite-pyrite-carbonate altered (East Prampus, 1060 mRL). C. Coarse-grained,clast-supported A5-V breccia. Image shows transition from carbonaceous-mudstone-clast–dominated polymict breccia (left-hand side) to polymict vol-canic-sandstone-clast–dominated breccia (right-hand side; Tepu Breccia,AD97127, 1090 mRL).


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facies (Table 3; Fig. 13). Pipes are typically tens to less than100 m across and are vertically extensive (> several hundredmeters; Figs. 6A, 8A). Several megablocks (10 × 50 m) of diffusely bedded B3 breccia occur in pipes of polymict A4

breccia. Bedding in these blocks is truncated at the blockmargins (Fig. 11B).

Formation of the Tepu Breccia postdated andesite intru-sion, based on the presence of andesite clasts containingearly-formed pyrite-illite veins (Fig. 6B; Table 3; Davies 2002;Davies et al., 2008). Brecciation was contemporaneous withrhyolite intrusion and early stage 2 mineralization and alter-ation. The juvenile magmatic component changed from QPto QFP with time (Table 3). Mineralized stage 3 carbonate veins crosscut the Tepu Breccia (Table 3), indicating that

brecciation ceased prior to stage 3 vein formation.Runcing Breccia

The Runcing Breccia is located north of the Burung-Runc-ing-Discovery fault system (Fig. 2). At the preserved levels, itis enclosed by carbonaceous sedimentary host rocks andunconformably overlain by Pliocene-Pleistocene mafic volcanicrocks. Of the three carbonaceous matrix-rich breccia bodies atKelian, the Runcing Breccia is the largest in map view (750×750 m; Fig. 2; Table 3). Its geometry is poorly constrainedbelow approximately 200 m due to the lack of deep drilling, but

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FIG. 9. Breccia facies A7. Contact between in situ carbonaceous mud-stone (CM) and a polymict A4-QP and/or QFP breccia facies of the TepuBreccia. Both units are cut by subvertical dikes of A7 facies carbonaceoussandstone that have undergone moderate illi te-quartz-pyrite alteration (TepuBreccia, 1070 mRL).

FIG. 8. Breccia facies A6. A. View of Tepu Breccia from 1060 RL. Sub- vertical pipes of A6-QFP breccia have intruded massive A3-QFP breccia.The A6-QFP contacts are sharp, discordant, and locally contorted. B. Detailof clast-supported domain in A6-QFP breccia pipe shown in (A), with sub-

rounded and faceted clasts of quartz-feldspar-phyric rhyolite (QFP) andminor volcanic sandstone (V) and carbonaceous sandstone (CMS) clasts setin a polymict sand-sized matrix (Tepu Breccia, 1070 mRL). C. A6-V con-glomerate crosscut by a pod of A3-QFP breccia. The A3-QFP breccia has en-trained rounded clasts from the A6-V conglomerate (Runcing Breccia, 1100mRL).


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it appears to taper with increasing depth. The Runcing Brecciais surrounded by a series of arcuate faults which form a crudecircular boundary around the breccia body. These normalfaults are interpreted here as ring faults along which gravita-tional collapse of the breccia pipe has occurred (Fig. 14).

The Runcing Breccia consists of a polyphase polymict brec-cia pipe surrounded by a shell of brecciated and in situ car-bonaceous mudstone and sandstone. The breccia pipe alsocontains a megablock (300 × 250 × >75 m) of bedded B fa-cies. A late-stage QFP intrusion has been emplaced mostly


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FIG. 10. Breccia facies B1 and B2. A. Low-angle cross- and planar-stratified fine- (B1-B) to medium-grained (B1-A) brec-cia. B. Normally graded B1 breccia interbedded with B2 breccia. Broad dune bed forms and low-angle cross stratificationare present, which is consistent with transport in high velocity currents. C. Interbedded B1 and B2 breccias. Dark B2-C beds

contain abundant accretionary lapilli. D. Detail of accretionary lapilli (AL) in thin B2-C and B1-C beds. Upper B2-C bedconsists of closely packed 1- to 3-mm accretionary lapilli. E. Flames and synsedimentary faults in interbedded B1 and B2breccia. Dark B2 beds are in general finer grained than B1 beds and commonly contain abundant accretionary lapilli, clastsof B2-B breccia and B2-C sandstone.


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into a monomict A1 breccia domain. The QFP intrusion isoriented northeast and has been emplaced along the inferredprojection of the prebrecciation Burung fault. A smaller QFPplug (450 m)of the three carbonaceous matrix-rich breccia bodies at Ke-lian (Table 4). This downward-tapering pipe is located at the

intersection of the northwest-striking fault corridor with thenortheast-striking fault and breccia corridor that forms thecore of the Kelian system (Fig. 2).

The Burung Breccia consists of a polymict breccia pipe sur-rounded at shallow levels on the west by monomict carbona-ceous breccias and in situ mudstone. The breccia tapers toward its base into several subvertical fins that are alignedparallel to northwest-trending faults.

The Burung Breccia is nested within the 393 Breccia, a hy-drothermal breccia body that consists of polymict and mo-nomict open space-filling hydrothermal breccias cementedby sulfide and carbonate minerals (paragenetic stages 2 and 3;Davies, 2002; Davies et al., 2008). The hydrothermal brecciasconsist of wall-rock clasts and only minor carbonaceous mud-

stone clasts (Fig. 15). Near surface, the 393 Breccia forms athin (


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TABLE 3. Characteristics and Genesis of the Tepu and Runcing Breccias

Dimensions,geometry, contacts Facies Facies distribution Timing relationships Interpretation

Tepu Breccia

750 × 250 × > 500 m Carbonaceous Multiple crosscutting pipes, each Formation of the Tepu Breccia Phreatomagmatic breccia body,Downward tapering mudstone and containing single or multiple postdated andesite intrusion with subordinate phreatic and

Crude elliptical sandstone breccia facies and was contemporaneous with tectonic brecciasshape aligned A1 A1 breccias confined to northern rhyolite intrusion and stage 1 to A1: tectonic breccias associatedalong a NE ori- A3-QP end, at the transition into the early stage 2 mineral ization with the East Prampus faultented long axis A3-QFP East Prampus fault and alteration zone; precursor to the polymict

East-trending arm A4-QP A3 and A4 breccias occur as pipes Juvenile magmatic component phreatomagmatic brecciasextends across A4-QFP up to 100 m diameter with sharp changed from QP to QFP QP and QFP clasts: juvenilethe Eastern A4-QP/QFP to diffuse contacts inside the rhyolite with time magmatic component—wispyAndesite into A5-V main breccia body A1: oldest facies clasts and dense clasts withthe 255 Breccia A5-And A3 breccias contain distinctive, A3-QFP: crosscuts A3-QP and cuspate margins are analogous

The Tepu Breccia A6-QFP wispy and dense clasts of either A4-QP, and has mutual cross- to magmatic clasts in dis-is aligned along A7 QP or QFP rhyolite cutting relationships with persed peperitethe East Prampus A7-AL A5: concentrated along the mar- A4-QP/QFP A5: abrasion and collapse offault B3 gins of the Tepu Breccia, locally B3: crosscut by A3-QP/QFP wall rocks into the pipe,

Confined by Eastern grades into in-situ brecciated wall A6-QFP: crosscuts all facies possibly also implosion of walland Crusher rocks, occurs marginal to, and except A7 rocksandesite intrusions grades into A3 and A4 pipes A7 and A7-AL dikes and narrow A6-QFP conglomerate: phreatic

A6: discrete, single-facies pipes pipes crosscut all other facies breccias, gas streaming elutri-that crosscut all facies except A7 Local clasts of A7-AL occur in ated fines and milled clasts

A7: thin (0.1 to 10 cm) sandstone A3-QP facies A7 sandstone dikes: gas stream-dikes and irregular pods that All polymict facies contain clasts ing, may be phreatic orcommonly contain AL of illite-carbonate-pyrite- phreatomagmatic

altered andesite B3 stratified breccias: turbulentAll facies crosscut by stage 3 veins flow inside the breccia body,

or collapse of surficial materialinto evacuated portions of thebreccia pipes

Runcing Breccia

750 × 750 × > 500 m Carbonaceous Polyphase polymict breccia pipe Onset of Runcing Breccia forma- Phreatomagmatic, phreatic,Roughly circular and mudstone and surrounded by a shell of brec- tion is inferred to have been tectonic breccias

downward tapering sandstone ciated and in-situ carbonaceous during stage 1 mineralization A1: tectonic brecciation whichMargins defined by A1 mudstone and sandstone Intrusion of the QFP plug occurred occurred prior to phreato-ring faults A3-QFP The pipe contains a mega-block prior to stage 3 mineralization magmatic brecciation, and

Polymict A facies A4-QP/QFP (300 × 250 × > 75 m) of Collapse along ring faults occurred also during collapse of thebreccias are en- A6-V bedded breccia (B facies) before and probably during Runcing Breccia pipe on ringclosed by a shell B1 Polyphase A4 breccia occupies stage 3 mineral ization, but faultsof A1 breccia B2 the NE two-thirds of the predated stage 4 A3-QFP: phreatomagmatic

Southern margin B3 Runcing Breccia and contain A4 breccias contain clasts of older breccias formed by intrusiondefined by Runcing numerous clasts (1 cm to 5 m) A4 breccia and B1 and B2 breccia of QFP magma into wet A1and Discovery faults of bedded B1 and B2 facies Blocks of B facies breccia have breccia

Northern margin breccia and sandstone, clasts been down-dropped along ring A4-QP/QFP: multipledefined by NW- of AL-bearing sandstone faults during A4 breccia formation phreatomagmatic explosionsstriking faults (either A7 or B2) A6 conglomerate dikes crosscut B1 and B2: phreatomagmatic

QFP plug (550 × A1 breccia occupies the SE third A1 breccias eruptions—these are wet300 m) intruded of the Runcing Breccia and A1 breccias are crosscut by pyroclastic base surgeparallel to the has been crosscut by dikes of A3-QFP breccias deposits; cycling between lowBurung fault polymict A3-QFP, A4-QP/QFP A1 and A4 breccias are baked at and high water supply

breccia and A6-V conglomerate their contacts with the QFP produced the B1 and B2A3-QFP: pods and dikes around intrusion facies respectively

the QFP intrusion, locally sur- B1, B2 and B3 breccias in the B3: collapse and resedimenta-rounded by A1 breccia mega block have been tilted by tion of unconsolidated B1 and

Mega-block of B1, B2 and B3 intrusion of the QFP plug B2 brecciasbreccia located on the southern A1 dikes crosscut B1 and B2side of the polymict pipe breccias

A6 conglomerate dikes up to1 m wide

Abbreviations: And: andesite; AL: accretionary lapilli; QFP: quartz – feldspar – phyric rhyolite; QP: quartz – phyric rhyolite; V: volcanic sandstone / mudstone


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FIG. 12. Tepu Breccia geometry and contact relationships. A. Aerial view looking to the east of the Tepu Breccia (field of view is ~500 m horizontal; pit wall in center of view is ~ 200 m high). This view illustrates the facies distribution in the TepuBreccia and its position between the Crusher and Eastern Andesite intrusions. Note the color difference between the two an-desite intrusions. The Eastern Andesite has been intensely quartz-illite-pyrite and illite-carbonate-pyrite altered, but theCrusher Andesite is only weakly illite-chlorite-carbonate altered. It is interpreted that the relatively impermeable Tepu Brecciainhibited fluid flow to the east. Also in this view is the arm of the Tepu Breccia that extends along the Tepu fault into the 255Breccia. Blue lines highlight the margins of several discrete breccia pipes in the Tepu Breccia. White lines are nonstructuralcontacts and red lines are faults and fault contacts. Yellow dashed line (X-X’) indicates location of graphic section illustrated inFigure 13). B. Intrusive contact between carbonaceous mudstone and the Crusher Andesite (left) and breccia contact betweenmudstone and A4-QP and/or QFP breccia (right). The mudstone has a 1-m-wide baked zone adjacent to the intrusion. Themudstone is inferred to be the protolith to the Tepu Breccia and is only preserved in this location (1070 mRL). C. A5-V brec-cia developed on contact between A4-QP and/or QFP breccia and the Upper Cretaceous Kelian Volcanics (KFV). The contactbetween the two breccia facies is in part gradational but also shows pods of A4 breccia in the A5 breccia (1070 mRL).


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A3-QFP40-50% matrix of lithic sandstone20% QFP rhyolite clastsQFP rhyolite clasts are sub-

rounded to wispy, with smallerclasts more wispy-shaped

Carbonaceous sedimentary clastsare sub-rounded to rounded

Up to 20% QFP wispy rhyoliteclasts as


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FIG. 14. Schematic illustration of Runcing Breccia formation (plan view). A. Intrusion of QP rhyolite dikes along northwest-striking faults and the Burung fault triggered phreatomagmatic explosions at the fault intersection. Polymict facies A

breccias were emplaced into the carbonaceous mudstone wall rocks. B. After the main brecciation event(s) collapse of thebreccia pipe and inferred overlying maar breccias occurred along ring faults. The megablock of B facies may have collapsedseveral hundred meters into the breccia pipe, based on depth estimates from fluid inclusion studies (Davies 2002, Davies etal., 2008). The ring faults nucleated on preexisting northeast- and northwest-striking faults. This resulted in downdroppingof a segment of the Burung fault along the Runcing and Discovery faults. This was a period of major subhorizontal extensionat Kelian (Davies, 2002) so faults may have been under extension during collapse. C. The Runcing QFP intrusion was em-placed along the projection of the Burung fault. Minor phreatic and phreatomagmatic explosions took place during this pe-riod. Doming of the bedded breccias occurred during QFP emplacement.


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Facies Interpretations

Several origins for the carbonaceous matrix-rich brecciasat Kelian have been proposed by previous workers. They have been interpreted as lahars, volcanic breccias, and intru-sive breccias (Ferguson, 1986), and as hydrothermal brecciasproduced by late-stage intrusion of magma into the hy-drothermal system (Lawless, 1988). Van Leeuwen et al.(1990) proposed that the muddy breccia was emplaced dur-ing the main hydrothermal event by fluidization and diapiric

emplacement of the mud component of the mudstone and/orsandstone sequence. They considered fluidization to havebeen the result of either tectonic forces, increased heat fromthe hydrothermal system, or depressurization resulting fromhydrothermal eruptions. R.H. Sillitoe (1993, unpub. reportfor PT Kelian Equatorial Mining) reinterpreted the muddy breccias and associated felsic intrusions to be components of a maar-diatreme complex. Large blocks of bedded carbona-ceous matrix-rich breccias with low-angle cross stratificationand accretionary lapilli in the Gunung Runcing area wereinterpreted to be pyroclastic surge deposits and to indicate

a prior connection to the paleosurface. He also suggestedthat clasts of aphanitic felsic rock in the Runcing diatreme were fragments of the igneous intrusion that generated thediatreme.

The carbonaceous matrix-rich breccia bodies at Kelianshare many features with breccia pipes that have been inter-preted as diatremes (e.g., Montana Tunnels: Sillitoe et al.,1985; Cripple Creek: Thompson et al., 1985; Thompson,1992; Wau: Sillitoe et al., 1984; Balatoc: Cooke and Bloom,1990). Diatremes are inferred to form by phreatomagmatic

plus magmatic and/or phreatic brecciation processes (Lorenz,1973, 1986; Lorenz and Kurszlaukis, 2007); identification of the juvenile magmatic component in the phreatomagmaticdeposits is essential for this interpretation.

The Kelian carbonaceous matrix-rich breccia bodies havelarge vertical extents, taper downward, contain abundantmatrix clasts derived from the adjacent wall rocks, distinc-tive juvenile magmatic clast components (QP, QFP), andlarge blocks of surface-derived stratified breccia that con-tain low-angle dune forms and accretionary lapilli. In isola-tion, many of the breccia facies at Kelian are not diagnostic


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TABLE 4. Characteristics and Genesis of the Burung Breccia

Dimensions,geometry, contacts Facies Facies distribution Timing relationships Interpretation

Burung Breccia

275 × 175 × >450 m Carbonaceous The polymict breccia pipe is sur- Formation of the Burung Breccia A1 breccias formed by tectonicN- to NE-trending sandstone and rounded at shallow levels on the commenced during stage 1 and brecciation along the North

long axis, with a mudstone west by monomict carbonaceous continued into early stage 2 Burung fault and by implosioncrude rectangular A1 breccias and in situ mudstone A1: partly crosscut by A4 or grade around the margins of theshape in plan view A4-QP The Burung Breccia is dominated laterally into A4 breccias phreatomagmatic breccia pipe

Tapers downward A4-QP/QFP by an A4-QP/QFP breccia pipe Rare A3-QP breccia clasts occur A4 QP and A4 QP-QFP brecciasBecomes elongate A5-V A5 breccias occur parallel to the in the A3-QP-QFP breccia are phreatomagmatic, formed

with depth along A5-And western Burung Breccia margin, A6-QFP crosscuts all other facies by intrusion of QP and QFPNE axis A6-QFP in the footwall to the North QFP and QP rhyolite plugs and magmas into wet, A1 facies

Western margin Burung fault dikes have intruded the Burung carbonaceous breccia and wetdefined by North In situ and clast-rotated A1 Breccia faults in mudstoneBurung fault breccias occur on the west side Minor clasts of hydrothermal QP and QFP clasts are the

Southeast margin of the Burung Breccia between breccias, which have been juvenile magmatic componentconfined by the the A4-QP/QFP polymict breccia cemented by stage 2 cements, in A4 breccias, but cuspateEastern Andesite pipe and the North Burung fault occur in the Burung Breccia, and wispy clast margins havebelow ~1000 mRL A1 breccias grade laterally into indicating partial overlap with been milled

Above 1000 mRL, unbrecciated carbonaceous hydrothermal breccia formation A5 breccias formed by tectonicthe Burung Breccia mudstone Contacts with the enclosing brecciation on the Northflares to the north A6-QFP conglomerate containing hydrothermal breccias of the Burung fault, progressive

The Burung Breccia faceted clasts of pyritized QFP 393 Breccia are diffuse and brecciation of wall rocks adja-is enclosed by a occurs in a narrow pipe have been overprinted by cent to A4 facies breccias, in-conical shell of (


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FIG. 15. Diamond drill hole K422 graphic log illustrating relationships between the Burung Breccia and enclosinghydrothermal breccias of the 393 Breccia. The 393 Breccia consists of multiple hydrothermal breccia phases. Its earliestphase, which is cemented by stage 2A quartz and pyrite, formed during the late stages of phreatomagmatic brecciation in theBurung Breccia.


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of any single brecciation process. However, in combination with facies distribution, facies associations, breccia body geometry, and wall-rock relationships, the origins of the Aand B facies can be determined. Table 5 summarizes key textures and spatial associations used to interpret the originsof individual carbonaceous matrix-rich breccia facies at Ke-lian, based on the classification scheme of Sillitoe (1985).

A1 breccias

A1 breccias are interpreted to have formed by a combina-tion of tectonic and phreatic and/or hydraulic processes(Table 5). The tectonic interpretation is based on their discor-dant nature, spatial association with faults, local occurrence of clast imbrication, and presence of shear fabrics and gouge, andtectonic fragmentation was most likely triggered by seismic


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TABLE 5. Summary of Breccia Facies Interpretations

Facies Interpretation(s) Key features

A1 Tectonic breccia Discordant; spatial association with faults; planar A1 breccia zones surrounded by in-situcarbonaceous mudstone; shear fabrics and fault gouge in the A1 breccias

Phreatic and/or hydraulic breccia Dikes of A1 breccia have crosscut other breccias; broad areas of fragmentation not aligned alongfault trends or tabular in shape; spatial association with margins of QFP intrusion in RuncingBreccia; occur at the margins of A4 breccia pipes against in-situ brecciated wall rocks—hydraulicimplosion suspected; occur at contacts between hydrothermal breccias and mudstone units

A2 Phreatomagmatic breccia and/or Discordant; jigsaw-fit textures and sedimentary matrix; facies association of QP or QFP rhyolitepeperite dikes, A2-QP, or -QFP breccia and A3-QP or -QFP breccia

A3 Phreatomagmatic breccia and/or Discordant; distinctive QP and QFP clast morphology (wispy or dense and cuspate) is inconsistentpeperite with rounding of other clasts in A3 breccia; abundance of QP and QFP clasts not consistent with

QP and QFP abundance in wall rocks; consistent phenocryst abundances in felsic clasts and clasttextures across individual A3 breccia pipes suggests common and contemporaneous origin for thedistinct clasts; interpretation of wispy and cuspate clasts as juvenile magmatic component; highdegree of fragmentation and abundance of wall-rock fragments consistent with phreatomagmaticbreccias; facies associations

A4 Phreatomagmatic breccia Presence of andesite and exotic rhyolite clasts similar to the intrusions at Kelian; alteration± phreatic reworking assemblages and veins in the andesite clasts eliminated these as juvenile components; facies

association with A3 breccias and rhyolite dike-A2-A3 facies association lead to interpretation ofQP and QFP clasts as juvenile component; high degree of fragmentation and abundance of

wall-rock fragments consistent with phreatomagmatic breccias

A5 Phreatic breccias, indirectly Discordant; location at margins of phreatomagmatic breccia pipes and parallel to wall-rock contactsphreatomagmatic, and/or Gradational contacts with both the phreatomagmatic breccias and wall rocks—brecciation may havehydraulic (implosion) breccias been by collapse or implosion

Facies association with A3 and A4 breccia—phreatomagmatic, phreatic, and hydraulic implosionFacies association with hydrothermal breccias adjacent to the Burung Breccia—phreatic and

hydraulic implosionTectonic breccia As above, but with addition of fault gouge or sheared fabrics

Location along projection of faults at margins of breccia bodies

A6 Phreatic or intrusion-related Abundance of one clast type—QFP or QP rhyolite; spatial association with late-stage QP and QFPphreatic intrusions; faceted clast shapes and lack of matrix; discordant (pipe and dike) geometries

A7 Phreatomagmatic or phreatic High degree of fragmentation; accretionary lapilli indicate wet gas-rich transport; plane and ripplebreccia and sandstone laminations parallel to dike walls; same composition as A4 breccias, but finer grain size; discordant

geometry Not possible to distinguish definitively between phreatic and phreatomagmatic processes

B1 Phreatomagmatic base surge Low-angle dune bed forms, diffuse gradingdeposit (low water/magma ratio) Cuspate clast margins, relict perlite, and interpretation of QP juvenile clast component

Abundance of juvenile clasts and relatively coarse grain size consistent with low water/magma ratioFacies association with B2 facies, A4 breccia pipes, and QFP intrusions

B2 Phreatomagmatic base surge and Low-angle dune bed forms—consistent with base surgecosurge fallout deposits (high Graded accretionary lapilli beds with constant bed thickness—consistent with cosurge fallout

water/magma ratio) Abundance of wall-rock clasts, accretionary lapilli, soft-sediment deformation structures, andrelatively fine grain size—consistent with low water/magma ratio

Cuspate clast margins, relict perlite, and interpretation of QP juvenile clast componentFacies association with B1 facies, A4 breccia pipes, and QFP intrusions

B3 Phreatomagmatic Diffuse bed forms, compositional and textural similarities to A4 brecciasInternal position of blocks within Tepu Breccia; facies associations with deep A4 and A3 breccias

Resedimented base surge Massive to diffuse bedding; channel bed forms that crosscut B1 and B2 breccias; lack of high-velocitydeposits (phreatomagmatic) bed forms

Facies association with B1 and B2 facies; equivalent components to B1 and B2 breccias but morerounding

Abbreviations: QFP = quartz-feldspar-phyric rhyolite, QP = quartz-phyric rhyolite


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ruptures. In the case of the Runcing Breccia, A1 breccias arespatially associated with the margin of a QFP intrusion, wherephreatic brecciation is inferred to have occurred due to boil-ing of pore waters (Davies, 2002). Adjacent to the QFP intru-sion, anastamosing A1 dikes have crosscut the bedded B faciesmegablock and can be traced back to the brecciated mudstonesurrounding the intrusion. The irregular and penetrative con-

tacts of the A1 breccia dikes suggest that the components of the A1 breccias were forcefully injected into the polymict car-bonaceous matrix-rich breccia. The observed textures are at-tributed to fluidization of sediment due to the intrusion of fel-sic magma and boiling of pore water, in a manner akin to thatdescribed by Hanson and Wilson (1993).

A2, A3, and A4 breccias

Breccia facies A2, A3, and A4 are interpreted to haveformed primarily by subsurface explosive phreatomagmaticbrecciation (Table 5). This process of direct magma-water in-teraction incorporates a juvenile magmatic component intothe resulting deposits (Sheridan and Wohletz, 1981). Rhyoliteclasts (QP or QFP) in the carbonaceous matrix-rich breccias

at Kelian (e.g., Figs. 4B-C; 5B, D) were first interpreted to be juvenile magmatic clasts by R.H. Sillitoe (1993, unpub. reportfor PT Kelian Equatorial Mining). Our observations supporthis findings.

The QP and QFP rhyolite clasts are texturally and compo-sitionally (based on modal mineralogy) equivalent to the QPand QFP rhyolite intrusions. The spatial association and, insome locations, gradational transitions from in situ rhyolite toA2 and then A3 breccias indicate that there is a direct link be-tween magma intrusion and generation of QP and QFP rhyo-lite clasts. This type of evidence is seldom preserved in brec-cia pipes interpreted to have phreatomagmatic origins (e.g.,Montana Tunnels, Montana: Sillitoe et al., 1985; stage 3 frag-mental porphyry, Black Cloud mine, Colorado: Hazlitt and

Thompson, 1990). More commonly, an igneous-clast compo-nent is inferred to be juvenile (i.e., fragments of the parentalmagma, such as dacite porphyry clasts at Wau: Sillitoe et al.,1984).

There is a problem with the occurrence of subsurfacephreatomagmatic explosions (e.g., Wohletz, 1986): how doesmagma mix with water efficiently enough to drive explosive“fuel-coolant” interactions? At Kelian, the A2 breccias occur where rhyolite dikes have intruded (1) preexisting fault zonesconsisting of monomict and polymict fault breccia and faultgouge, (2) A1 breccia, and (3) A4 breccias. In all of these sites,the host rock is interpreted to have been unconsolidated orpoorly consolidated, because sharp intrusive contacts arerare, whereas irregular interfingering and contorted contacts

are common. We propose that rhyolitic magmas were em-placed into poorly consolidated or unconsolidated, early-formed breccias that had undergone preintrusion disaggrega-tion either by tectonic, phreatic, or earlier phreatomagmaticbrecciation events, which facilitated efficient mixing of thefuel and coolant (Sheridan and Wohletz, 1981) to result insubsurface phreatomagmatic fragmentation.

Interaction between the intruding rhyolite and wet, uncon-solidated, or poorly consolidated sediment could have re-sulted in purely explosive phreatomagmatic fragmentation,nonexplosive quench fragmentation, or a combination of both

explosive and nonexplosive fragmentation. Although grada-tions between A3 breccia, A2 breccia, and coherent rhyolitehave been observed locally, typically the A3 breccias have noidentifiable magmatic roots. It is interpreted that rhyolite dis-aggregation into wispy and blocky clasts generally resulted infragmentation of the upper parts of the intruding magma batchinto the unconsolidated, wet, subsurface breccia. Explosive

magma-wet sediment interaction was sufficiently energetic todisperse the clasts away from the coherent rhyolite feeder intothe overlying breccia. The A2 jigsaw-fit breccias (e.g., Fig. 4B)are interpreted to be the preserved roots of the A3 breccias andtheir rare occurrence most likely relates to destruction of themagmatic root zone by explosive fragmentation at the currentlevel of exposure. The predominance of subrounded clasts inthe A3 breccias (Fig. 5D-E) suggests that they resulted fromgreater clast transport and abrasion than the juvenile rhyoliteclasts (cf. Roache et al., 2000), perhaps due to clast recyclingduring multiple brecciation events (e.g., Houghton andSmith, 1993) with introduction of new juvenile clasts in eachevent. Preservation of the wispy and blocky juvenile mag-matic clasts in A3 breccias (Fig. 5B, D, F) is interpreted to in-

dicate only minor clast transport for these clasts.In the A4 breccias, compositionally and texturally equiva-lent juvenile magmatic components occur; however, they lackthe wispy or cuspate morphology of those in the A3 facies. A4breccias are interpreted to be the product of progressivetransport and abrasion of the A3 breccia components, eitherin single brecciation events or during repeated brecciation cy-cles (either phreatic or phreatomagmatic; Houghton andSmith, 1993). Lithification is not likely to have occurred be-tween these cycles, as clasts of A3 or A4 breccia are rarely ob-served in A3 or A4 breccia. Instead, the unconsolidated brec-cia deposits are inferred to have been disaggregated andreworked during subsequent events.

Some A2 and A3 breccias are interpreted here to be vari-

eties of peperite (e.g., Busby-Spera and White, 1987; Hansonand Wilson, 1993), produced by intrusion of rhyolite magmainto (1) wet, unconsolidated fault breccia and gouge, (2) wetA1 breccia, (3) wet carbonaceous mudstone, or (4) wet A4breccia. Blocky and angular clast domains in A2 breccia (Fig.4B) are similar to textures described by Hanson and Wilson(1993) as closely packed peperite and by Busby-Spera and White (1987) and Hanson and Hargrove (1999) as blocky peperite. The A3 breccias are matrix supported and lack the jigsaw-fit textures of the A2 breccias, indicating that disaggre-gation of the rhyolite magma was more advanced in the A3breccias. Some A3 breccias have similar internal organizationto the dispersed peperite of Hanson and Wilson (1993). A2and A3 breccias containing wispy rhyolite clasts (e.g., Figs.

4C, 5B, D) are interpreted to be varieties of fluidal peperite(cf. Busby-Spera and White, 1987; Hanson and Hargrove,1999). Roache et al. (2000) described similar wispy andblocky rhyolite clasts set in a matrix of polymict breccia andinferred that the wispy clast morphology was produced by squashing of plastic rhyolite clasts by the relatively coarsegrains of the host breccia.

A5 breccias

Where A5 breccias occur as marginal phases to A3 and A4rhyolite-bearing breccias, they are interpreted to have formed

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during the phreatomagmatic explosions that generated the A3and A4 breccias by a combination of (1) wall-rock collapseinto open space at the margins of A3 and A4 breccia pipes, (2)fragmentation of wall rocks by mechanical abrasion, and/or(3) hydraulic implosion or phreatic brecciation in response topressure gradients between the wall rocks and evacuatedpipes (Table 5). In the Burung Breccia, the A5 breccias are

not spatially associated with rhyolite intrusions and are, there-fore, interpreted to have formed by explosive phreatic frag-mentation and tectonic fragmentation on late-stage normalfaults. The occurrence of clasts of hydrothermally cementedbreccias in the A5 breccias, and vice versa, indicates that for-mation of some A5 breccias occurred during the post-phreatomagmatic phase of breccia development.

A6 conglomerates and breccias

The A6 facies is distinct from all other A facies as it is dom-inated by subrounded to rounded clasts and has only minormatrix. The high degree of clast rounding is inconsistent within situ fragmentation and indicates significant clast abrasion.The paucity of matrix suggests that the sand- to mud-sized

component was removed (winnowed?) during breccia forma-tion. Localization of A6 breccias above a QFP intrusion in theBurung Breccia and the dominance of rhyolite clasts imply that the rhyolite intrusions triggered localized phreatic explo-sions, focused in and above the intrusions (Table 5). Gasstreaming resulted in milling of the clasts and removal of thefine matrix component. These breccias may be equivalent tothe intrusion-related phreatic breccias described by Sillitoe(1985), which have been described elsewhere as pebble dikes(e.g., Gustafson and Hunt, 1975).

A7 sandstones

Based on their contact relationships and geometry (Table2), the A7 facies discordant sandstone and accretionary lapilli-

bearing sandstone formed in the subsurface. Since the com-ponents and textures of the A7 facies are identical to those of the A3 and A4 breccias, their origin is inferred to be equiva-lent (Table 5). A7 sandstone is considered to have formedduring high-water/magma ratio explosions and/or as a resultof highly efficient fuel-coolant mixing. This resulted in theanomalously intense fragmentation of wall rock and magma. Water supply and magma-sediment mixing may have becomegreater with time due to the increased abundance of thesand- to mud-sized component in the pipes after successiveexplosion events. The occurrence of abundant accretionary lapilli is consistent with deposition from wet, gas-rich particlesystems (Walker, 1984). Accretionary lapilli are sometimesused to indicate formation in a subaerial environment; how-

ever, they have been identified in subvolcanic breccias atMount Leyshon (Wormald, 1991), in gas segregation pipes inthe Oruanui Ignimbrite in New Zealand (Self, 1983), and indikes cutting breccias at the Rain mine (Williams et al., 2000),Cripple Creek (T. Thompson, pers. commun., 2003), andLihir (Carman, 1994, 2003). It is possible that the accre-tionary lapilli at Kelian formed in a surficial setting and werethen reworked into the subsurface environment, but we pre-fer the interpretation of a subsurface environment of forma-tion, based on the lack of broken accretionary lapilli in the A7dikes and pipes.

B facies

B1 and B2 facies contain the same juvenile magmatic com-ponent as the A3-QP subfacies (QP rhyolite). Planar, dune,and minor low-angle cross beds are consistent with depositionfrom turbulent, gas-rich flows at the transition from low- tohigh-flow regimes (Walker, 1984; Valentine and Fisher, 2000;Table 5). Accretionary lapilli (Fig. 10D) and soft-sediment or

plastic deformation structures (Fig. 10E) are consistent with wet surge deposits. B1 and B2 are interpreted to have beendeposited at the surface by a combination of wet, pyroclasticbase-surge fallout and cosurge fallout, generated by explosivephreatomagmatic eruptions (Fisher and Waters, 1970; Walker, 1984). Cosurge fallout is an important process for de-positing fine (sand- to silt and/or clay-sized) grains and abun-dant accretionary lapilli in uniformly thick beds up to a few centimeters thick (Walker, 1984). Accretionary lapilli-rich B1and B2 beds probably formed as fallout of fine ash after pas-sage of the turbulent wet surge. Deposition is inferred to haveoccurred in a wet, terrestrial environment based on the pres-ence of interbedded carbonaceous mudstone (with woodfragments) in the sandstones and breccias. B facies blocks

then slumped into the diatremes, including the megablock inthe Runcing Breccia.

B3 breccias are interpreted to have formed by syneruptiveresedimentation of the B1 and B2 facies or collapse of the in-ferred maar deposits into the maar crater (Table 5). Blockscontaining B3 beds in the Tepu Breccia are interpreted tohave been deposited in the breccia pipe by slumping of ma-terial back down the evacuated conduit.

Genetic Model for Diatreme Formation at Kelian

The facies of the Tepu, Burung, and Runcing Breccias andthe surrounding sedimentary rocks and cogenetic intrusiverocks record a complex sequence of tectonic brecciation and

phreatic and phreatomagmatic brecciation and eruption.Collectively, these breccias are interpreted to be the prod-ucts of maar-diatreme volcanism and related tectonism andhydrothermal activity. The following section outlines a se-quence of events that could have produced these brecciasand highlights the key evidence for each phase of volcanismand brecciation.

Ground preparation: Eocene to Oligocene carbonaceoussedimentary strata were juxtaposed against Upper Cretaceous volcaniclastic rocks by normal and strike-slip faults prior to orduring the Miocene (Fig. 16A).

Andesite intrusions: Intrusion of late Miocene andesiticstocks was controlled by the same faults that were active dur-ing ground preparation (Fig. 16B). Early phreatic breccias

probably formed at the andesite margins at this time, andsome A1 breccias formed in the carbonaceous sedimentary rocks by phreatic, tectonic, or hydraulic processes. Evidencefor a prediatreme hydrothermal system is preserved in laterformed carbonaceous matrix-rich breccias as altered andesiticand volcaniclastic rock clasts that contain truncated stage 1 veins (e.g. Fig. 6B).

Rhyolite intrusions and commencement of volcanism: Fine-grained QP intrusions were emplaced along faults into thefault-bounded carbonaceous sedimentary blocks and domainsof A1 breccia at 19.8 ± 0.1 Ma (Davies, 2002; Fig. 16C). The


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main phreatomagmatic phase was triggered by intrusion of QP into the already-active hydrothermal system. It is notknown if the initial explosions were phreatic, in advance of the ascending magma, or phreatomagmatic. The subsurfaceproducts of phreatomagmatic explosions at Kelian were thediscordant, monomict to polymict facies A breccias and sand-stones. The A4 breccias probably underwent greater degrees

of transport and abrasion or recycling (e.g., Houghton andSmith, 1993) than the A3 breccias because the latter havepreserved the delicate-textured juvenile magmatic clasts. Atthe surface, eruptions contemporaneous with the subsurfaceexplosions produced base- and cosurge fallout deposits. B1and B2 facies in the Runcing Breccia are interpreted to be wet base- and cosurge fallout deposits on the basis of their

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FIG. 16. Schematic model for the formation of the Kelian diatremes, showing their relationships to enclosing units, thestructural framework, and present-day erosion level. A. Ground preparation: downfaulting juxtaposed Eocene-Oligocenecarbonaceous sedimentary rocks against Upper Cretaceous volcaniclastic rocks (Kelian Volcanics). Preserved remnants of fault-bounded blocks have been sheared and tectonically brecciated. B. Intrusion of Miocene andesitic stocks. Andesitic in-trusions were localized by same fault system as in (A). The early hydrothermal system at Kelian is interpreted to have com-menced during or after emplacement of these intrusions and before formation of the carbonaceous matrix-rich breccias.Ingress of ground water deep into the carbonaceous sedimentary rocks may have occurred due to andesite-related breccia-tion, along the bounding faults for the sedimentary units. Arrow and bubbles indicate possible locations of phreatic explo-sions at this time. C. Quartz-phyric rhyolite dikes intruded along the same fault system as previous events, encounteringground water in the fault zone and brecciated sedimentary rocks. Phreatomagmatic explosions produced massive, unstrati-fied polymict carbonaceous matrix-rich facies A breccias in the subsurface (thick arrows) and wet pyroclastic base-surges and

fallout eruptions (facies B1 and B2) at the surface (thin arrows). Synvolcanic resedimentation of volcaniclastic deposits intomaar (dashed arrows) also occurred (facies B3). D. Continued phreatomagmatic explosions and eruptions excavated the con-duit, widening the diatreme, increasing its depth, and producing multiple crosscutting breccias. Excavation of the diatremeaccompanied by collapse of the maar and upper conduit along ring faults and preexisting structures. Blocks of wall rocks, ear-lier diatreme phases and bedded pyroclastic deposits slumped several hundred meters into the conduit, based on fluid in-clusion estimates of the depth of hydrothermal activity (van Leeuwen et al., 1990; Davies, 2002; Davies et al., 2003). Thesemegablocks of facies B breccias have been preserved locally (e.g., Runcing bedded facies) but most were probably disaggre-gated and incorporated into later diatreme breccias. E) Late-stage intrusion of QFP plugs into the Burung and RuncingBreccias accompanied by minor phreatomagmatic and widespread phreatic eruptions. Maar sediments were fluidized andpumped through older breccia phases along contemporaneous faults. Collapse of the maar and diatreme along ring faultsand other structures continued. F. Miocene mineralization (hydrothermal brecciation and veining) occurred in the wall rocksaround the Kelian diatreme breccias. The ore deposit was then uplifted and exhumed. Mafic lava and volcaniclastic depositsoverlie a Plio-Pleistocene paleosurface.


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bed forms and occurrence of cuspate QP fragments and thepresence of accretionary lapilli.

Diatreme excavation: Ground-water infiltration to greaterdepths would have been facilitated by excavation of the con-duit and relatively permeable breccia deposit infilling (Fig.16D). Phreatomagmatic eruptions thus initiated at progres-sively deeper levels under hydrostatic conditions. Gravita-tional collapse of the walls is an integral process in the pro-gressive widening of maars and diatremes (Self et al., 1980;Sillitoe et al., 1984; Lorenz, 1986). At Kelian, collapse oc-

curred by spalling of wall rocks into the evacuated pipe andby subsidence of the crater walls or crater-fill deposits via nor-mal movements on ring faults (cf. Sillitoe et al., 1984; Lorenz,1986) during and after eruptions. Syn- and posteruptive col-lapse of the eruptive breccia facies and collapse and resedi-mentation of volcaniclastic deposits into the maar probably filled the diatreme between eruptive events and contributedto recycling of clasts in subsequent eruptions. Blocks of B fa-cies slumped back into the pipes at this time, including amegablock of bedded breccia in the Runcing Breccia.

Subsidence and phreatic explosions: Continued subsidenceoccurred during intrusion of late-stage QFP domes into theRuncing and Burung Breccias at 19.5 ± 0.1 Ma (Davies, 2002;Fig. 16E). Ragged QFP clasts surrounded by brecciated

black, carbonaceous mudstone have been observed within100 m of the QFP contacts and indicate that minorphreatomagmatic eruptions may have occurred during intru-sion. Phreatic eruptions during this phase of magmatism re-sulted in formation of the A6 conglomerate pipes and flu-idization of brecciated carbonaceous sediments. Thesefluidized monomict breccias (A1 facies) crosscut all polymictfacies in the Runcing Breccia, including the bedded polymictfacies.

Postdiatreme evolution: Main-stage gold mineralization wasassociated with widespread hydrothermal brecciation and

veining. Mineralization at Kelian mostly postdated diatremeformation because the auriferous pyrite-base metal ± carbon-ate veins crosscut diatreme breccias, and no clasts of basemetal ± carbonate mineralization have been identified in thediatreme breccias (van Leeuwen et al., 1990; Davies, 2002).Uplift and erosion produced a Pliocene-Pleistocene surfacerecorded by a paleosol over the Runcing Breccia. This ero-sional surface is overlain by mafic lavas and associated resed-imented volcaniclastic and mafic pyroclastic rocks. Erosion by the Kelian River and its tributaries exposed the upper levels

of the diatreme complex in the Runcing area and deeper lev-els in the Tepu and Burung areas (Fig. 16F).

Implications for models of diatreme volcanism

In the traditional models for diatreme formation (e.g.,Lorenz, 1973, 1986, Lorenz and Kurszlaukis, 2007), phreato-magmatic and dry magmatic eruptions are triggered whenmagma ascends to shallow depths (constrained by the avail-able heat energy and hydrostatic head) and explosively inter-acts with shallow ground waters (e.g., Fig. 17A). Interactionbetween magma and superheated liquid water is the mostproductive mechanism for generating phreatomagmatic ex-plosions due to the large volume changes that occur on va-porization (White and Houghton, 2000). As the explosions

continue, a conduit is excavated progressively and the watertable is depressed around the evacuated cone. Drying out of the aquifer can cause cycling between phreatomagmatic anddry magmatic explosions and can result in surge deposits in-tercalated with ballistic fallout layers (Cas and Wright, 1987).

At Kelian, felsic magmas are inferred to have intruded anactive hydrothermal system, rather than a cold ground-wateraquifer. The Kelian geothermal system was probably at ornear its boiling point, or may even have been overpressured,during initial rhyolite intrusion. We infer that the addition of magmatic heat into the geothermal system triggered hybrid


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FIG. 16. (Cont.)


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phreatomagmatic and phreatic explosions at depth (Fig. 17B-D). Magma intrusion catastrophically disrupted the hydrol-ogy, P-T conditions, and thermal and chemical gradients of the geothermal system. We believe that the initial sites of phreatomagmatic explosions were deep in the hydrothermalsystem, rather than shallow as suggested for diatremes relatedto cold ground-water aquifers (e.g., Lorenz, 1973). This in-

terpretation is based on the observations of root zone feederdikes, truncated veins within altered clasts in the breccias(e.g., Fig. 6B), and paleodepth estimates from our fluid in-clusion studies (Davies et al., 2008). Drying out of the hy-drothermal system may have been difficult to achieve at thesedepths (estimated to be several hundred meters on fluid in-clusion evidence: Davies et al., 2008), resulting in a predom-inance of wet eruptive products derived from phreatomag-matic and phreatic, rather than magmatic explosions. Weinfer that the onset of widespread phreatic brecciation at Ke-lian was contemporaneous with the waning phases of phreatomagmatic brecciation, as shown by observed parage-netic relationships between veins, facies A, and early hy-drothermal breccias (e.g., Tables 3, 4). This situation is com-

parable to the effects documented when a basaltic dikeintruded the active Rotomahana-Waimangu geothermal sys-tem in New Zealand during the early 1900s. An initialphreatomagmatic eruption was followed by several years of phreatic explosive activity and then a return to boiling hotspring conditions (Nairn, 1979; Simmons et al., 1993). Kelianmay contain the subsurface products of this type of transitionfrom phreatomagmatic to phreatic conditions.

Implications for fluid flow and mineralization

Although the diatreme breccias formed prior to the mainstages of hydrothermal mineralization and alteration at Ke-lian, they are only weakly mineralized and altered. Instead,strongly mineralized hydrothermal breccias and veins formed

peripheral to the diatreme breccias in the surrounding wallrocks (Davies, 2002; Davies et al., 2008). It appears that thecarbonaceous matrix-rich breccias at Kelian acted asaquicludes during main-stage hydrothermal activity, causingfluids to be focused into the wall rocks outside of the main di-atreme breccia pipes. The relative impermeability of the dia-treme breccias with respect to the adjacent Cretaceous vol-caniclastic rocks and Miocene andesite intrusions appears tohave been caused by the widespread distribution of muddy carbonaceous matrix through the breccia bodies. This situa-tion contrasts markedly with other diatreme-related hy-drothermal systems, such as the Cripple Creek deposit inColorado, where mineralization and alteration was restrictedalmost exclusively to the diatreme breccia pipes, rather than

the adjacent Precambrian granites that form the wall rocks tothat deposit (Thompson et al., 1985; Thompson, 1992;Jensen, 2003). There are also cases where mineralization andalteration occur both in diatreme breccias and adjacent wall

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(i) Magma intrusion into earlyhydrothermal system at low-

water rock ratios, possibility offluid contributions from hydrous

magma and magmatic-hydrothermal brecciation

Hydrothermal system collapse

(ii) Late magma intrusion into active high-Thydrothermal system causes

sub-surface phreatomagmatic explosions,generates huge volumes of steam andresults in maar - diatreme volcanism

(300-350°C)

Catastrophic volatileloss/pressure

reduction

Breccia pipe inhibits fluid flow- hydrothermal systemenhanced in wall rocks

Mineralization inwall rocks

Phreatomagmatic explosionsthrough active system triggersyn- and post-diatreme hybrid

phreatic explosionsLarge scale hydrothermalexplosions and brecciation

Structurally controlledmineralization at

margins of diatreme

g n i x i m

d i u l

F

m 0 0 5 2

Abundant hot fluids in activehydrothermal system, at or near

critical point

Magma intrusion triggershybrid phreatomagmatic and

phreatic explosions

Catastrophic disruption ofchemical and physical conditions

in h

Documents

Diatreme Breccias at the Kelian Gold Mine