Baker Andrews 1991 Eg Kidston

Economic Geology Vol. 86, 1991, pp. 810-830

Geologic, Fluid Inclusion, and Stable Isotope Studies of the Gold-Bearing Breccia Pipe at Kidston, Queensland, Australia

E. MAX BAKER*

Department of Geology, James Cook University of North Queensland, Townsville, Queensland 4811, Australia

AND ANITA S. ANDREW

CSIRO Division of Exploration Geoscience, P.O. Box 136, North Ryde, New South Wales 2113, Australia

Abstract

The Kidston mine, which is situated approximately 280 km west-northwest of Townsville, North Queensland, is currently Australia's second largest gold producer. The mineralization is hosted within a trapezoid-shaped breccia pipe with surface dimensions of 1,100 X 900 m. Brecciation and gold mineralization are spatially and temporally related to a swarm of Permo- Carboniferous rhyolite dikes which intrude middle Proterozoic metamorphic and Siluro-De- vonian granitoid host rocks. The rhyolite dikes are interpreted as being underlain by a Permo- Carboniferous batholith.

Three phases of brecciation have been distinguished within the breccia pipe, all being associated with magmatic and or magmatic hydrothermal processes. Fluid inclusion data indicate that the present level of exposure was approximately 3,500 m below the contemporary land surface at the time of mineralization. The lack of any significant input of meteoric fluid into the hydrothermal system suggests that the breccia pipe, the bulk of which formed by collapse, failed to breach the contemporary land surface. Volatile-rich fluids, envisaged as being an integral part of breccia pipe formation, escaped fractures now occupied by breccia dikes.

Prebreccia mineralization is uneconomic, consisting of stockwork vein mineralization in the carapaces of small prebreccia rhyolite stocks. Stockwork veining and localized brecciation resulted from the multiple buildup and escape of a high-temperature (>500øC), highly saline (>40 wt % NaC1 equiv) magmatic fluid associated with a number of crystallizing rhyolite stocks. This exsolved magmatic fluid comprised a liquid with a b180 value of between 9.4 and 9.8 per mil. The pressure drop associated with fracture propagation and brecciation produced a vapor phase with a salinity of 3 to 19. wt percent NaC1 equiv, which condensed at temperatures between 380 ø and 460øC due to adiabatic expansion to produce a liquid of similar salinity.

Postbreccia mineralization was dominated by magmatic fluids (calculated b180 value of 3- 8%0 and bD value of -50 to -20%0), which in turn were dominated by a liquid with a salinity of 9. to 10 wt percent NaC1 equiv. This liquid resulted from condensation of a vapor produced by the boiling of a highly saline magmatic fluid at a deeper level within the breccia pipe. Fluid inclusion studies indicate trapping temperatures in the ranges of 400 ø to 540øC for early-stage cavity infilling to as low as 170 ø to 300øC during the deposition of the sulfides and carbonate in the late-stage quartz veins and cavities. The economic-grade gold mineralization was deposited during late-stage mineralization. The lateral and vertical decrease in the grade of gold mineralization within the late-stage sheeted veins and cavities is associated with an increase in ratio of pyrrhotite to pyrite, which is interpreted as reflecting increasing temperature.

The structural control on the distribution of postbreccia mineralization was an inverted funnel-shaped zone of enhanced permeability produced by the forceful emplacement of the postbreccia rhyolite into the lower portion of the breccia pipe. The persistence of this zone throughout the postbreccia mineralizing event is further evidence of the close genetic relationship between rhyolite magmatism and gold

Introduction

THE Kidston open-pit gold mine, situated about 280 km west-northwest of Townsville, north Queensland, is currently Australia's second largest gold producer and one of the world's largest producers of gold from

* Present address: E. M. Baker and Associates, 115 Ross River Road, Townsville, Queensland 4812, Australia.

mineralization at Kidston.

a breccia pipe, with production to 31 December 1990 of 23.7 million metric tons at 2.08 g/metric ton gold. Estimated mineral resources are 42.6 million metric

tons averaging 1.43 g/metric ton gold and 1.85 g/ metric ton silver. A further 11.7 million metric tons are inferred.

The Kidston breccia pipe is of particular interest as a gold-rich subvolcanic breccia-hosted deposit be-

810

Au BRECCIA PIPE, KIDSTON, QUEENSLAND 811

cause of the close spatial and temporal relationship between brecciation, mineralization, and rhyolite intrusions. There are significant differences in the style of brecciation and mineralization compared with the diatreme breccia-hosted deposits as exemplified by the Mount Leyshon deposit approximately 200 km to the southeast (Morrison et al., 1988).

Geology

The Kidston breccia pipe is located on the northern edge of the northwest-trending Permo-Carboniferous rhyolite dike swarm which extends between the Lochaber ring complex to the south and the Newcastle Range Volcanic Complex to the northwest (Fig. 1). A regional gravity low which is coincident with this dike swarm suggests that the two volcanic centers and the dike swarm are underlain at depth by a large Permo- Carboniferous batholith. Several generations of rhy-

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FIG. 1. Regional geology of the Kidston area showing the relationship of Kidston to centers of Permo-Carboniferous volcanism, the northwest-trending rhyolite dike swarm, and coincident gravity low. The coincidence of the gravity low and the dike swarm suggests that the area may be underlain by a Permo-Carboniferous batholith.

olite dikes have been recognized, the earliest being truncated by the breccia pipe, one synchronous with the main phase of brecciation, and one later than the breccia pipe. Based on the timing relationships relative to breccia pipe formation, intrusive and mineralization events have been divided into pre-, syn-, or postbreccia (Table 1).

Host rocks to the breccia pipe consist of early to middle Proterozoic Einasleigh metamorphics and Sil- uro-Devonian Oak River granodiorite. The middle Proterozoic Einasleigh metamorphics consist of mul- tiply deformed upper amphibolite-grade biotite gneiss, calc-silicate gneiss, migmatite, and amphibolite with subordinate biotite schist and quartzite (Withnail et al., 1980). The Oak River granodiorite, which consists of the host rocks to the southwestern portion of the breccia pipe, has been assigned a Siluro-De- vonian age by Warnick and Withnail (1985). This granodiorite, in the mine area, has been subdivided into two rock types; a nonfoliated porphyritic unit in the southwest of the pipe and a foliated nonporphyritic unit to the northwest (Fig. 2). In the mine area, the contacts between the host lithologies and the regional metamorphic foliation, which is parallel to the granodiorite-metamorphic contacts, are steeply dipping and strike northwest-southeast (Fig. 2).

Prebreccia rhyolite

The north-northwest-trending swarm of rhyolite dikes and several small stocks of rhyolite in the Wise's Hill area are referred to collectively as prebreccia rhyolite because they have been disrupted during breccia pipe formation. The rhyolite is generally fine grained, a light brown color, and phenocryst poor. Quartz and feldspar phenocrysts make up less than 10 percent of the rock.

The rhyolite stocks which have diameters of approximately 100 m are shaped like inverted tear drops. They were emplaced into the area now occupied by the breccia pipe; although they have been brecciated, the movement of clasts was insufficient to disrupt their internal relationships. The fine-grained, nonporphyritic chilled carapace to these bodies passes downward and inward into a more porphyritic rhyolite. This transition is marked by a zone up to several meters thick of crenulate quartz-layered rock which consists of alternating bands of porphyritic rhyolite and crenulate quartz layers, the quartz layers showing irregular contortions (Fig. 3A). The bands of quartz vary in thickness from 1 mm to as much as 1 m, but most are in the range of 1 to 10 cm thick. Each of the quartz layers grew in the same direction pointing inward away from the walls of the intrusion. Similar textures consisting of bands of orthoclase and peg- matitic rhyolite were observed in one of the rhyolite stocks. Wallace et al. (1978) described these crenulate quartz layer textures in the Henderson porphyry mo-

812 E. M. BAKER AND A. S. ANDREW

TABLE 1. Summary of Geologic Relationships at Kidston

Age Timing Event-Lithology Relationships

Permian Postbreccia Andesite dike

Permo-Carboniferous Postbreccia Postbreccia mineralization

Late Paleozoic

Middle Proterozoic

Brecciation

Prebreccia

Postbreccia rhyolite dikes Phase 3 breccia Phase 2 breccia Phase I breccia

Prebreccia mineralization

Prebreccia rhyolite

Oak River granodiorite

Einasleigh metamorphics

Cuts postbreccia mineralization

Cuts breccia pipe and postbreccia rhyolite dikes

Dikes intrude the breccia pipe Intrudes phase 2 breccia Formation of the breccia pipe Clasts within the breccia, cuts

prebreccia rhyolite dikes and prebreccia mineralization

Cuts prebreccia rhyolite dikes Intrudes host rocks, cut by

breccia pipe

Host rocks to brecciation

Host rocks to brecciation

LEGEND

• [•] Andesire dike o

• Post-breccia • rhyolite dike

Quartz-feldspar porphyry

•) "":"•'[] Pre-breccia rhyolite rhyolite dike

• Oak River

• e> ++• porphyritic I Granodiorite

.:• Phase 3 breccia {with rounded clasts)

[• Phase 2 breccia • Phase I breccia {tourmaline breccia)

[• Crenulate quartz-layered rock [•--• Foliation [-•----] Old workings

Approximate boundary

•'+\+ outer zone breccia of lithological types within

FIG. 2. Surface geology of the Kidston breccia pipe showing the distribution of the three breccia phases, the relationship of pre- and postbreccia rhyolite dikes to brecciation, and the base- ment stratigraphy within the pipe as reflected by dominant clast types.

lybdenum deposit and interpreted them as forming in response to rhythmic fluctuations in the partial pressure of H20 and HF during crystallization of the rhyolite.

In the northeast part of the pipe, the contact between one of these prebreccia dikes and the breccia pipe is flow banded against the breccia contact, indicating that the rhyolite was fluid and still being intruded when it was truncated by the breccia pipe.

Syn- and postbreccia rhyolite

The breccia pipe itself was intruded by a coarser grained and more porphyritic (Fig. 3B) postbreccia rhyolite dike. Phenocrysts are larger (9.-5 mm) and more abundant (greater than 25%) than in the prebreccia rhyolite. The postbreccia dikes have a radial distribution pattern and do not extend outward beyond the breccia pipe margin (Fig. 9.). A small irregular and discontinuous synbreccia porphyritic rhyolite dike has intruded the breccia in the Wise's Hill area.

Clasts of this synbreccia rhyolite dike, some of them showing a slightly flattened pumaceous texture, are present in the adjacent breccia.

Postbreccia andesite dikes

Two andesite dikes, up to several meters in width, trend northwest-southeast and cut through the breccia pipe and surrounding host rocks. The dikes also cut the sheeted veins indicating that they are postminer- alization. The andesite dikes are medium grained and nonporphyritic with up to 15 percent acicular ilmen- ite. The dikes are strongly altered to muscovite, carbonates, and chlorites with amygdales filled with calcite and quartz.

AU BRECCIA PIPE, KIDSTON, QUEENSLAND 813

"' lcm, '

FIG. 3. Rock types within the Kidston breccia pipe. All scales in cm except A which is close to actual size. A. Crenulate quartz-layered rock consisting of quartz (white) and porphyry (dark). The quartz layers have all grown in one direction, resulting in one regular and one irregular surface quartz layers. B. Postbreccia rhyolite porphyry showing the coarse grain size and abundance of phenoerysts. C. Phase 1 quartz-tourmaline cemented breccia, with tabular clasts of prebreccia rhyolite. D. Phase 2 breccia consisting of angular dasts of prebreccia rhyolite and granodiorite supported by smaller dasts and a matrix of comminuted elast material.

Oxidation

Sulfides have been weathered to an average depth of 30 m throughout the orebody. The upper half of the oxidization profile is stained a red-brown color due to goethite and hematite after pyrite and pyrrhotite. In the lower half of the weathering profile, which is bleached, pyrite and pyrrhotite are partly replaced by jarosite, and cavity space originally filled with carbonate is filled with fine-grained porcelaneous supergene alunite. Sulfur and hydrogen isotope studies (Bird et al., 1989) suggest all the alunite minerals found at Kidston are supergene.

Brecciation

The Kidston breccia pipe is trapezoid in shape with surface dimensions of 1,100 X 900 m (Fig. 2). The

breccia pipe extends downward beyond the level of the deepest drilling, approximately 300 m below surface. The pipe margins dip steeply, generally inward at greater than 80 ø , but locally outward. Where the pipe margins are exposed, the contact with the host rocks is abrupt. Three phases of breccia have been distinguished within the Kidston breccia pipe on the basis of timing relationships (Table I and Fig. 2).

Phase I

The broad outline of this breceia as shown in Figure 2 reflects the broad distribution of phase I breccia clasts within the phase 2 breccia. Prior to the disrup- tion by the phase 2 brecciation event, the phase 1 breccia consisted of several steeply dipping dikelike zones. The breccia consists mainly of angular, tom-


monly thin tabular clasts ofprebreccia rhyolite, many containing stockwork quartz-pyrite-molybdenite veins, cemented in a quartz tourmaline matrix (Fig. 3C). Other clasts are from metamorphic or granodiorite rocks. The parallel alignment of thin tabular clasts of rhyolite, called shingle breccia, has been described in other tourmaline-bearing breccias where they are considered to have formed by regular break- age and detachment of zones of sheeting like those around pipe walls and large fragments (Sillitoe, 1985).

Phase 2

The phase 9. breccia comprises the bulk of the breccia pipe (Fig. 2). The clasts are angular to sub- angular, mostly ranging in size from 1 to 20 cm. The distribution of clast types within the phase 2 breccia reflects the host-rock-type stratigraphy prior to brecciation. Schist and gneiss clasts are dominant in the northeast of the pipe where the surrounding host rocks are metamorphics and granodiorite in the southwest of the breccia pipe (Fig. 2). Contacts between lithologic breccia types are gradational over several meters. These contacts can be mapped through the breccia pipe out into the host rocks on either side of the pipe (Fig. 2). The internal geometry of the small rhyolite stocks, in the Wise's Hill area, can still be recognized on the basis of clast distribution. A number of large blocks of unbrecciated host rock, up to several hundred meters in diameter, are present within the breccia (Fig. 2). In contrast with the smaller clasts, these large blocks show no evidence of significant rotation during brecciation as the metamorphic foliation roughly parallels that in the adjacent host rocks. Commonly, clasts of two or more differing lithologies will be present, even within a hand-sized specimen, with less common lithotypes such as prebreccia rhyolite occurring as isolated clasts within a mixture of the dominant clast types (Fig. 3D). The metamorphic foliation within these relatively small clasts shows no preferred orientation or alignment.

The matrix to the breccia, which comprises less than 30 percent of the rock, consists of smaller chip- sized clasts and finer material derived from commi-

nuted clast material. The clasts are partly supported by this matrix material and partly in contact with each other. Cavities up to several centimeters in diameter and infilled with hydrothermal minerals comprise several percent of the breccia by volume. Two generations of cavities are present; cavities representing open spaces between clasts which were not filled by rock flour at the time of breccia pipe formation and later cavities and veins produced by fracturing of the already cemented breccia. Phase 3

This breccia is distinguished from the phase 2 breccia by the presence of spheroidal clasts of rhyolite

up to 40 cm in diameter. However, angular to sub- angular clasts of metamorphics, granodiorite, and rhyolite make up the bulk of the breccia. Some of the angular clasts contain stockwork vein mineralization which is not found in the adjacent phase 9. breccia. The vein assemblage differs markedly from the only other area of exposed stockwork vein mineralization at Wise's Hill. The breccia is matrix supported, although clasts comprise at least 50 percent of the total volume. The matrix, like that of the phase 2 breccia, consists of comminuted clast material. In the northern

part of the pipe, a 1-m-wide dike of this breccia cuts the phase 2 breccia. No fragments of the phase 2 breccia have been recognized within the phase 3 breccia, although they would be difficult to recognize in a breccia which is distinguishable only by the presence of clasts of spheroidal rhyolite and clasts containing stockwork vein mineralization.

In addition to these three main breccia types, several 1- to œ-m-wide dikes of breccia were found cut-

ting the host rocks adjacent to the breccia pipe margin. These breccias consist of semirounded clasts of host-rock material supported in matrix of comminuted clast material.

Mineralization and Alteration

The mineralization at Kidston has been divided into

pre- and postbreccia on the basis of timing relative to breccia pipe formation. Prebreccia mineralization is present within breccia clasts as stockwork quartz veins. Postbreccia mineralization occurs as veins cut-

ting the breccia and infilling of breccia cavities. The economic-grade gold mineralization at Kidston is of postbreccia age and is confined to an inverted funnel- shaped zone of quartz-carbonate-sulfide veins and cavities, referred to as the sheeted vein zone.

Prebreccia assemblages

The quartz stockwork veins within breccia clasts consist of 2- to 20-mm-thick veins of gray, fine-grained glassy quartz with thin, dark-colored bands of sulfides and oxides parallel to the vein walls. In thin section, the quartz is equigranular and shows no preferred growth orientation. These stockwork veins are present within the clasts of the phase 2 and 3 breccia. The sulfide and oxide assemblages of stockwork veins within the two areas differ significantly.

The quartz-magnetite-pyrite stockwork veins occur only within the phase 3 breccia. Veins in which magnetite is more abundant than pyrite are associated with selvages of epidote-chlorite-muscovite-carbonate alteration overprinting an earlier, more pervasive phase of orthoclase-albite alteration. Veins in which the pyrite is more abundant than magnetite are sociated with either silicification or muscovite-carbonate alteration. Centimeter-wide microdikes of

rhyolite cut early generations of stockwork veins in


some of these clasts. These microdikes are themselves

cut by later generations of similar stockwork veins (Fig. 4A), indicating that the stockwork veining and emplacement of rhyolite are spatially and temporally related. The magnetite-pyrite stockwork veins associated with silicification are highly anomalous in gold.

Quartz-molybdenite_-pyrite _ arsenopyrite _ chalcopyrite-bearing stockwork veins are concentrated within the chilled carapaces of the prebreccia rhyolite bodies at Wise's Hill. These stockwork veins do not extend downward into the zone of crenulate quartz layers. Associated alteration consists of weak silicification and muscovite-carbonate replacement of primary plagioclase within the rhyolite. Clasts of this mineralization occur within the phase 1 breccia. Clasts

of the phase 1 breccia containing this stockwork vein mineralization are present within the phase 2 breccia.

Postbreccia assemblages Postbreccia mineralization consists of the infilling

of open spaces within the breccia such as cavities and parallel fractures (sheeted veins). In contrast to the prebreccia stockwork quartz veins, the postbreccia sheeted veins cut through the breccia clasts and matrix. Also, the quartz in the postbreccia sheeted veins and cavities is combed, indicating open-space filling. The centers of most of these veins and cavities are filled with carbonate and sulfides. The alteration associated with vein and cavity infilling is most strongly developed within the breccia matrix and rims of clasts.

FIG. 4. Examples of mineralization types within the Kidston breccia pipe. A. Prebreccia quartz- pyrite-magnetite stockwork vein. Note the rhyolite microdike cutting early generations and being cut by later generations ofstockwork veins. B. Postbreccia early-stage quartz-epidote _ pyrite _ pyrrhotite cavity infi!ling surrounded by a narrow rim of white orthoclase alteration of the breccia matrix and clasts. Note the radiating epidote needles (dark) in the center of the cavity adjacent to the calcite (massive white). Some of the epidote needles (white) are replaced by a mixture of muscovite and calcite. C. Early-stage calcite _ pyrite ___ pyrrhotite cavity-infilling mineralization surrounded by weak muscovite- calcite-chlorite alteration. D. Late-stage quartz-ankerite-sulfide cavity-infi!ling mineralization surrounded by muscovite-ankerite alteration.


The centers of some of the larger clasts are unaltered. Within the breccia matrix, the intensity of alteration decreases away from the cavities and veins, making it possible to relate the various cavity and vein assemblages to specific alteration types (Table 2). Based on overprinting relationships, the postbreccia cavity infilling and vein assemblages can be divided into early, transitional and late stage (Table 2). A three-dimen- sional zoning pattern for the cavity-infilling assemblages has been constructed on the basis of drill core information (Figs. 5 and 6).

Early stage: The quartz-epidote _ pyrite _ pyrrhotite and quartz-orthoclase _ pyrite ___ pyrrhotite cavity-infilling assemblages are confined to an inverted funnel-shaped zone around the periphery of the breccia pipe (Fig. 5). Cavity infilling consists of quartz with either radiating needles of green epidote (Fig. 4B) or squat crystals of white orthoclase. Sub- hedral to euhedral pyrite and/or pyrrhotite grains are present in some of these cavities and are intergrown with the quartz, epidote, and orthoclase. Sphalerite where present is intergrown with pyrite and pyrrhotite. Hydrothermal allanite is present as an accessory mineral within these cavities. Where calcite is present within these cavities, the epidote needles adjacent to calcite are altered to a mixture of muscovite, calcite, and quartz. Cavities are surrounded by a distinctive

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TABLE 2. Relationship between Brecciation, Mineralization, and Alteration in the Kidston Breccia Pipe

Stage Mineralization Alteration

Prebreccia

Postbreccia

Early

Transitional

Late

qtz-py-mt stockwork veins (mt > py)

qtz-py-mt stockwork veins (py > mt)

qtz-mo-py stockwork veins

qtz-epi q- py q- po q- cal cavity infilling qtz-or q- py q- po

cavity infilling cal q- py q- po cavity

infilling bio-mt-po cavity

infilling bio-sid-py cavity

infilling Transitional between early-stage and late-stage

cavity-infilling assemblages qtz-ank-cal-sulfide mus-qtz, mus-ank

sheeted vein and

cavity infilling

ep-chl-mus-carb

qtz

qtz-mus-carb

or, or-alb-mus-cal-chl

or, or-alb-mus-cal-chl

mus-cal-chl

bio-and-qtz-mus-mt-chl

bio-sid

Abbreviations: alb -- albite, and = andalusite, ank = ankerite, bio -- biotite, carb -- carbonate (ank and/or cal), cal = calcite, chl = chlorite, cpy = chalcopyrite, epi = epidote, gal = galena, or -- orthoclase, mt = magnetite, mo = molybdenite, mus = muscovite, po -- pyrrhotite, py = pyrite, qtz -- quartz, sid -- siderite, sph = sphalerite

MINERALIZATION ALTERATION

:: i• Ou(3rtz-epidotezpyrite_* pyrrhotite

':"::"• B iot ite- s iderite- pyr ite Biotite-siderite

--• C(3lcite *_ pyrite_* pyrrhotite Mu scovite-c(31cite- chlorite o26 88 I Drillhole(prefix PAK)

FIG. 5. Distribution of early-stage mineralization and alteration assemblages for the 520-m level. The potassium silicate assemblages are restricted to a circular band around the periphery of the pipe.

Orthoclose, orthoclose-olbite- muscovite- colcite

centimeter-wide white alteration rim (Fig. 4B) in which the primary plagioclase and biotite have been completely replaced by hydrothermal orthoclase. Throughout the surrounding breccia which is altered to a pale green color, primary orthoclase is unaltered and plagioclase and biotite are replaced to varying degrees by a fine-grained mixture of hydrothermal orthoclase, albite, muscovite, calcite, and chlorite. These mineral assemblages are associated with subeconomic gold mineralization.

Isolated patches of biotite-siderite-pyrite and biotite-magnetite-pyrrhotite cavity infilling are restricted to the Wise's Hill area (Fig. 5). Rare quartz-magnetite sheeted veins are also restricted to the Wise's Hill


.-7911000N N _ • .:iii :'h-

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A

'•:.- ;? -'.'X•

LEGEND

'• Quortz-pyrite-lx•se roelois ':.':'• Quartz-pyrite-arsenopyrite -base metols

!!• Ouortz-pyrrhotite > pyrite - bose metols - mo[ybdenite

B

..,

,,."//..'/ / I

• Zone of ß lppm Au 026 Drillhole (prefix PAK)

88 I Drillhole( prefix PAK)

FIG. 6. Distribution of late-stage quartz-sulfide mineralization within sheeted veins for the 520-m level. The zone of +1 ppm Au is shown. The associated sericite-carbonate alteration zone (not shown) is most strongly developed within the sheeted vein zone and weakly developed 50 to 100 m into the hanging-wall zone.

area, representing the first evidence of postbreccia fracturing within the pipe. The alteration surrounding biotite-siderite-pyrite cavities consists of the intense replacement of the breccia matrix by secondary biotite and siderite (Table 2). The biotite-magnetite- pyrrhotite infilling assemblage was not strongly developed and the associated alteration consists of secondary biotite with or without andalusite, muscovite, magnetite, and chlorite replacing primary plagioclase and biotite. Magnetite and pyrrhotite partly replace epidote needles in some of the quartz-epidote _ pyrite ___ pyrrhotite cavity-infilling assemblage, indicating that the magnetite-pyrrhotite-bearing assemblages were introduced later than the quartz-epidote- bearing assemblages. The biotite-pyrite-siderite cavity infilling is later than the magnetite-pyrrhotite assemblage. The secondary biotite-bearing cavity mineralization is associated with ore-grade gold miner-

However, the mineralization is too weakly developed to contribute any substantial tonnage.

Calcite _ pyrite _ pyrrhotite cavity infilling is present throughout the remainder of the pipe (Fig. 5). The cavities are devoid of any matrix material and consist of massive white calcite (Fig. 4C). The amount of pyrite and pyrrhotite within these cavities decreases outward away from the contact with the zone of quartz-epidote _ pyrite _ pyrrhotite mineralization (Fig. 5). Ankerite is also present in many of these cavities, either as intergrowths with the calcite or filling irregular fractures within calcite and the surrounding breccia. Within the breccia matrix and clasts, primary plagioclase and biotite grains are replaced by a fine-grained mixture of muscovite, calcite, and chlorite (Table 2). This alteration has a characteristic pink color making it easy to distinguish from the other alteration assemblages. Early-stage assemblages are associated with only weakly anomalous grades of gold mineralization.

The outward progression from the quartz-epidote ___ sulfide- to calcite ___ sulfide-filled cavity assemblages lacks any recognizable intermediate phase. However, some of the quartz-epidote-sulfide-filled cavities on the periphery of this zone are surrounded by a rim of pink orthoclase which grades outward into the typical pink-colored muscovite, albite, quartz, calcite, and chlorite alteration associated with calcite- filled cavities.

Transitional stage: The distribution of transitional- stage cavity assemblages is similar to that of the early- stage quartz-epidote-sulfide assemblage. Transitional cavities show an internal zoning from an outer quartz- epidote _ pyrite _ pyrrhotite or quartz-orthoclase _ pyrite _ pyrrhotite assemblage to an inner assemblage of quartz-ankerite-pyrite _ pyrrhotite ___ base metals. In contrast to early-stage cavities, transitional- stage cavities contain only minor amounts of epidote or orthoclase lining the walls of cavities and significant amounts of ankerite and/or calcite in the center of cavities. Alteration consists of an early-stage orthoclase-albite-muscovite-calcite assemblage overprinted by a muscovite-ankerite assemblage identical to the late~stage alteration assemblage (Table 2).

Late stage: The late-stage quartz-ankerite ___ pyrite ___ pyrrhotite assemblage occurs within the sheeted veins and, to a lesser degree, the late-stage cavities. These late-stage sheeted veins and cavities, which are host to the economic-grade gold mineralization, are restricted to an inverted funnel-shaped zone around the margins of the breccia pipe (Fig. 6). This distribution roughly coincides with the distribution of early-stage quartz-epidote and quartz-orthoclase cavity-infilling assemblages (Fig. 5). The veins and cavities consist of inward-facing comb quartz on the margins, followed by quartz and sulfides, then ankerite and sulfides but without quartz, and finally in


the centers of veins and cavities ankerite without sulfides (Fig. 4D). Adjacent to veins and cavities the breccia matrix is completely replaced by muscovite and quartz, giving the breccia a gray to white color. Away from cavities and veins the intensity of alteration decreases from complete to partial replacement of primary plagioclase and biotite by a fine-grained mixture of muscovite and ankerite. With decreasing intensity of alteration the breccia matrix and clast color changes from gray to white to creamy yellow to the color of unaltered rock. Sulfides present within sheeted veins and cavities include pyrite, pyrrhotite, sphalerite, chalcopyrite, galena, molybdenite, bis- muthinite, and bismuth tellurides.

The sulfide assemblage within sheeted veins shows a zoning pattern consisting of pyrite-galena-sphalerite-chalcopyrite in the highest part of the system cen- tered on Wise's Hill, outward and downward through pyrite-pyrrhotite-galena~sphalerite-chalcopyrite plus arsenopyrite to pyrite-pyrrhotite-galena-sphalerite- chalcopyrite and molybdenite (Fig. 6). The economic- grade gold mineralization is associated with the upper- level pyrite-(minor pyrrhotite)-galena-sphalerite- chalcopyrite zone. More than 90 percent of the gold occurs as discrete grains, generally from 20 to 100 ttm in diameter. The remainder of the gold is believed to be present as small inclusions or in solid solution with pyrite, arsenopyrite, and bismuth tellurides. Sil- ver abundance is similar to that of gold, but no silver mineral has been identified.

Fluid Inclusion Studies

Techniques

The majority of inclusions studied were contained in quartz. No suitable primary inclusions were observed in the carbonate phases. Although only a few of the inclusions selected for study were unequivo- cally of primary origin, all inclusions selected were relatively large and irregularly distributed. In contrast, inclusions of secondary origin were smaller and confined to planes. The inclusions studied have been trapped during the hydrothermal event responsible for the deposition of the host mineral. Homogeniza- tion and freezing was undertaken on a Chaixmeca dual-purpose heating-freezing stage, with some parallel checks being made with a stage designed by the U.S. Geological Survey. Identification of daughter phases was based on microscopic identification and by opening inclusions and analyzing them using scan- ning electron microscopy (SEM) and qualitative energy dispersive analysis (EDA), as described by Le Bel (1976) and Anthony et al. (1983).

Fluid inclusions have been classified into types S, L, V, and C, based on their relative volumetric properties at room temperature, as proposed by Nash and Theodore (1971) and Weisbrod (1981). Inclusions

containing only liquid and vapor phases have been divided into type L and V, depending on which of the phases is dominant at room temperature. Inclusions containing liquid CO•. at room temperature have been classified as type C. Type S inclusions contain one or more daughter mineral phases in addition to liquid and vapor. Salinities were calculated using the method described by Roedder (1971) for type S and Potter and Clynne (1978) for Wm,ce. Observations and results

Prebreccia mineralization: Prebreccia stockwork

veins and the quartz in crenulate quartz layers are characterized by the abundance of type S and V inclusions and relatively rare type L inclusions. The timing relationships appear to be complex with all inclusion types occurring as early (primary) inclusions as well as within planes of secondary inclusions. In many cases type S inclusions show local variations in the number and relative proportions of daughter phases present as well as in homogenization temperature. However, small clusters of type S inclusions, all with the same number and relative proportions of daughter phases and homogenization patterns, were also present in many of the samples examined. Daughter phases identified using SEM-EDA techniques include halite, sylvite, iron chloride, potassium-iron chloride, potassium-iron-aluminum chloride, calcium carbonate, calcium-iron carbonate, and aluminosilicates. Halite homogenizes over a temperature range of 200 ø to 500øC, whereas iron chloride invariably homogenizes below 400øC and sylvite below 150øC. Undifferentiated phases homogenize between 200 ø and 600øC (Table 3). In all but a few cases, the vapor phase homogenizes at a lower temperature than the final dissolution of daughter minerals as shown in the example of crenulate quartz layers and quartz-molybdenite-pyrite stockwork veins in Figure 7A. Quartz ___ magnetite ___ pyrite stockwork veins showed similar homogenization behavior. Type V inclusions homogenize over the temperature range of 380 ø to 460øC, whereas the bulk of type L inclusions homogenize below 400øC (Fig. 7C).

Postbreccia mineralization: Postbreccia mineral-

ization is characterized by abundant type L and less common type V inclusions. Rare type S inclusions are present within early- and transitional-stage mineralization. All the inclusions studied were from quartz- bearing assemblages taken from the semicircular zone around the periphery of the pipe. No reliable data were obtained from the carbonate cavity-infilling assemblages throughout the remainder of the pipe. The results are summarized in Table 3 and Figure 8.

Early- and transitional-stage mineralization show similar homogenization patterns with type S, C, and V inclusions and a small but significant portion of the type L inclusions clustering in the range 360 ø to

TABLE 3.

Au BRECCIA PIPE, KIDSTON, QUEENSLAND

Summary of Homogenization and Salinity Data for Fluid Inclusion

819

Inclusion

Stage type

Homogenization temperature (øC)

NaCI Undifferentiated Th (L-V) Tm (ice)

Salinity wt % NaCI

equiv

Prebreccia

qtz-mo-py, S crenulate L

quartz V layers Critical qtz-py-mt S

L

V

Critical Postbreccia S

Early L V

Critical Transitional S

L

V

C (L and V) Critical

Late L

V

C (L and V) Critical

275-500 300-600

200-500 200-600

•480 340-500

320-600

130-350

200-540 -3 to 20 260-520 -8to -2 380-440 -3.9

•250

260-520 -14 to -1

320-600 -8to -4 420-460 -7.1 to -3

•250

160-500 -7to -1

340-500 -4 380-500

•250

180-480 -3to -2 300-420 -1 320-520 360-400

120-420 -6to -1 340-420 -4to -2 320-480

380-420

40-50

5-20 3-12

6

40-50 2-20

6-13

5-10

•50

2-10 6.5

<50

3-5

2

7-11

2-10 3-7 0-10

See Table 2 for abbreviations

500øC (Fig. 8A and B). The cluster of type L inclusions in the 260 ø to 360øC range corresponds with the range ofhomogenization temperatures of the bulk of type L inclusions in the late-stage mineralization. These lower temperature inclusions are probably of secondary origin, trapped during the late-stage mineralization event.

Late-stage mineralization contains lower temperature inclusions with the bulk of type C and V inclusions homogenizing between 300 ø and 400øC and the bulk of type L inclusions in the range 260 ø to 380øC (Fig. 8C). Although many of the lower temperature type L inclusions in the range 120 ø to 260øC appear to be of primary origin, inclusions of secondary origin with a similar homogenization temperature range are also present in the quartz. These lower temperature inclusions were most probably trapped during the deposition of carbonate and sulfide in the center of quartz veins and cavities.

The salinities of type L, V, and C inclusions from all three phases of postbreccia mineralization are similar and range from 2 to 10 wt percent NaC1 equiv (Table 3).

Interpreted conditions of trapping

Prebreccia mineralization: Homogenization of type S inclusions is by the dissolution of daughter phases with the vapor phase disappearing prior to complete dissolution of halite. Therefore, the fluid trapped in these inclusions could not have been in equilibrium

with a vapor phase at the time of trapping. Based on the experimental data of Sourirajan and Kennedy (1962) pressure >800 bars would be required to in- hibit boiling in a fluid with a temperature •500øC and a salinity •40 to 50 wt percent NaC1 equiv.

The vapor trapped as type V inclusions is interpreted to have formed by boiling of the highly saline fluid trapped in type S inclusions. At a temperature >500øC and pressure of approximately 800 bars, a vapor with a salinity range of 5 to 10 wt percent NaC1 equiv would be produced by boiling a fluid with a salinity similar to that present in type S inclusions. These type V inclusions could not result from trapping of vapor produced by boiling of the liquid in type L inclusion because the salinity ranges are similar. Since both type L and V inclusions exhibit critical homogenization behavior over a temperature range of 380 ø and 460øC, and the overall range of homogenization temperatures is higher for type V inclusions, it is possible that the type L inclusions represent the condensate of the vapor phase trapped under near critical conditions at a temperature above 380øC. The experimental data of Sourirajan and Kennedy (1962) indicate that at these temperatures, a fluid with the salinity of type L and V inclusions would exhibit near critical behavior over the pressure range of 300 to 500 bars. Assuming that confining pressure was close to hydrostatic during times ofbrecciation and fracture propagation, the pressure range of 300 to 500 bars is indicative of a depth of approximately 3,500 m. A


600

5OO

400

Th(S) 3OO

200

100

(a)

ß

mO •o ø

•0

0 0 0 •' KCI o NaCI

ß e ß Unidentified 1

0 d I i I I • 0 1_ 0 200 300 400 500 600 Th(L-V!

(b)

e -• T•(L-V)

ß Th(S] Freq

100 200 300 400 500 600

Temperature (øC)

(c)

1!f Type L&V • Th (critical)

ijjj• ..,. :.:•• [•] Type V Freq / [] Type L

•oo 260 3•0 4(•o 5oo 600

Temperature (øC)

FIG. 7. Homogenization data for fluid inclusions in prebreccia stockwork quartz veins and crenulate quartz layers. A. Plot of Th(S) versus Th(L-V) for type S inclusions. B. Histogram of Th(S) and Th(L-V) for type S inclusions. C. Histogram for Th(critical), Th(L), and Th(V) for type L and V inclusions.

similar depth is obtained for type S inclusions (800 bars) if a lithostatic pressure is assumed.

Postbreccia mineralization: In the early-stage postbreccia mineralization, type L inclusions are much more abundant than type S and V inclusions. Using the data of Potter (1977) to apply a pressure correction to type L inclusions, which homogenize in the temperature range 360 ø to 500øC, and assuming the pressure obtained for prebreccia mineralization (350-500 bars), the trapping temperature for this fluid would have been in the range 400 ø to 540øC. The type L inclusions which homogenize over a lower temperature range are probably of secondary origin, introduced during transitional- to late-stage mineralization. The rare type S inclusions, which homogenize by halite dissolution, would have been trapped during periods of fluid overpressuring. A high-temperature, high-salinity, overpressured fluid could migrate upward into the breccia column before boiling if the breccia were sufficiently impermeable due to

sealing of cavities by hydrothermal minerals. Type V inclusions are interpreted as having been produced by intermittent boiling of the highly saline fluid trapped in type S inclusions. No type C inclusions were observed in early-stage postbreccia mineralization.

Transitional-stage mineralization shows an overlap in fluid inclusion types and homogenization temperatures with early- and late-stage mineralization (Table 3). The introduction of CO2 into the system during transitional-stage mineralization was accompanied by a drop in temperature of the fluid. However, since there is no marked difference in salinity of type C and type L and V inclusions, there is no fluid inclusion evidence to suggest that a lower temperature, lower salinity fluid of possible meteoric origin was introduced into the system during transitional- to late-stage mineralization. The absence of any evidence for mixing of fluids at the time CO2 was introduced suggests that the COa was probably derived from the same source as the fluid trapped in type L inclusions.

Late-stage mineralization differs from early-stage mineralization in containing no type S inclusions and abundant type C inclusions. Type C and V inclusions homogenize under near-critical conditions over a

(a)

Freq11I O/ IO0 200 300 4do 560

Temperature (o C)

[] Th (critical)

E• Type V

[] Type L

ß Type S (Th(S))

(b)

20 L• Type C, Th(V)

• •JTypeC, Th(L! 16 ['-J Th (critical) 12

Freq L•l Type V

i [] Type L ß Type S. Th(S)

100 200 300 400 500 600

Temperature (øC)

(c)

• • . Type C, Th(V) 30 • •J Type C. Th(L) 20 • ['-J Th (cr,tica,)

me. ..... , = Type v •'•'• x [] Type L

100 200 300 400 500

Temperature (øC)

FIG. 8. Histograms of homogenization temperatures for type S, L, V, and C inclusions in early- (A), transitional- (B), and late- stage (C) postbreccia mineralization.


temperature range of 300 ø to 420øC. Applying the same assumptions in pressure correction as above, the high-temperature cluster of inclusions would have been trapped in the range 300 ø to 400øC. Type L inclusions in the lower temperature cluster would have been trapped at temperatures between 170 ø and 300øC. These lower temperature inclusions are interpreted as having been trapped at the time of carbonate and sulfide deposition in the centers of late- stage veins and cavities. Since gold occurred within the sulfide grains, the economic-grade gold mineralization would therefore have been deposited over this temperature range.

Stable Isotope Studies

Minerals and whole rocks were analyzed using conventional preparation techniques (McCrae, 1950; Bigeleisen et al., 1952; Clayton and Mayeda, 1963; Robinson and Kusakabe, 1975) on a Micromass 602D mass spectrometer for O, C, and H and on a modified Micromass 602 mass spectrometer for S. All analyses are reported as • values in per mil relative to V-SMOW for O and H, V-PDB for C, and CDT for S. Routine analytical precision for standard material is _0.1 for O and C, +_2 for H, and ___0.2 for S.

Analytical results

Prebrecciation stage: Quartz from stockwork veins in rhyolite has a well-defined, narrow range of •180 values between 9.4 and 9.8 per mil (Table 4). This narrow range of values contrasts with the spread in •180 values measured in quartz from the granodiorite and metamorphic hosts (•80 = 8.9-10.8%0). Epidote and chlorite from the selvages to quartz-pyrite-magnetite stockwork veins have •D values of -57 and

-81 per mil, respectively. A single analysis of quartz from crenulate quartz

layers is enriched in 180 relative to the stockwork veins. If these veins represent a zone of boiling and volatile loss, this enrichment probably reflects pref- erential partitioning of 160 into the volatile phase.

Postbrecciation stage: Quartz from early-stage cavities has a narrow range of •180 values which are enriched in •80 compared with stockwork veins. The •D values of chlorite and muscovite from alteration

halos around the early-stage cavities are the same as those from minerals from the alteration selvages to the stockwork veins.

Quartz from late-stage cavities and quartz-ankerite- pyrite cavities and veins are enriched in •80 relative to early- and transitional-stage quartz. The similarity in •180 values of quartz in sheeted veins and cavities supports the geologic and fluid inclusion evidence that suggests filling of late-stage cavities and formation of sheeted veins were contemporaneous. Textural re- lations, as well as large quartz-calcite fractionations (•2%0), suggest calcite and ankerite filling these cav-

ities and veins (•180 = 6.7-14.8%0) postdate quartz precipitation. Carbon isotope values of calcite and ankerite from late-stage cavity infilling and sheeted veins have values between -7.8 and -5.8 per mil. Hydrogen isotope analyses from fluid inclusion ex- tracted at •300øC (Table 5) from sheeted veins and cavity infilling have •D values between -90 and -71 per mil.

Oxygen isotope values of whole-rock samples of fresh and altered granodiorite (Table 6) were analyzed to measure the effects of the postbrecciation alteration on the isotopic composition of the host rock and to estimate the amount of water that passed through the system. Petrographically unaltered granodiorite re- mote from the pipe (from drill core at the plant site) and adjacent to and within the pipe has values between 8.3 and 10.0 per rail. Pervasive chloritic, sericitic, and carbonate alteration around sheeted veins has resulted in an increase in the •180 value of the whole rock to a maximum value of 10.9 per mil in chloritized selvages.

Sulfur isotope values in pyrite, pyrrhotite, and sphalerite from early- and late-stage cavities and veins and their alteration halos have a narrow range of values between 2.2 and 4.3 per mil (35 analyses; Table 4). Where sulfide minerals are in textural equilibrium, they are also in isotopic equilibrium with fractionations suggesting temperatures around 300 ø +_ 80øC (Kajiwara and Krouse, 1971). There is no apparent isotopic difference between the late-stage sulfides which are associated with most of the mineralization

and those in prebrecciation or early postbrecciation assemblages.

Evolution and origin of the hydrothermal fluid Fluid evolution at Kidston is traced for each rec-

ognized phase of alteration (Table 7; Fig. 9) by using the isotopic composition of alteration fluids, temperature estimates of fluids derived from fluid inclusion

studies, and fluid inclusion salinity data. The calculated •80 composition and high salinities

of fluid inclusions from prebreccia stockwork veins and crenulate quartz layers are typical of waters in equilibrium with igneous rocks at high temperatures (Fig. 9). Calculated •D values of -50 to -20 per rail are in the range for a vapor separating from a crystallizing magma which has not undergone extensive degassing (Taylor, 1988). The isotopic composition of the computed early-stage fluid is similar to the prebreccia fluid except for a wider range in •80 values, due mainly to a larger temperature uncertainty. The associated fluid inclusions also have somewhat lower

salinity. Uncertainties in temperature estimates for the fluid

responsible for late-stage cavity infilling and sheeted vein formation are a problem in determining the fluid composition for this phase of alteration. For a lower


TABLE 4. Oxygen, Carbon, Hydrogen, and Sulfur Isotope Values in the Kidston Breccia Pipe

CSIBO DDH/m blsO bD blaC ba4S no. Description Mineral (%o V-SMOW) (%o V-PDB) (%o CDT)

Prebrecciation

Oak River granodiorite 70075 86/91.3 72257 111/260.8 102647

Einasleigh metamorphics 70085 87/129.4

72206 101/64.6 72230 103/144.5 72268 113/207.4

Prebrecciarhyolite 70094 93/238.2 70096 93/240.7 72210 101/112.7 72226 103/63.2

70098 93/242.9 WH23-1

WH23-2

Postbrecciation

Early stage 70069 53/107.8 72203 101/53.4 72207 101/80.9 72208 101/106.75 72260 111/299.5

81/91.3 70075 86/91.3

72234 104/108.1 72257 111/260.8

116/50.6

Late stage 70062 32/246.05

70064 45/74.9 70067 53/98.65 70070 53/109.3 70073 60/50.8 70074 60/51.3 70080 86/101.6 70082 86/117.2 70083 87/19.8

70087 88/100.85 70088 88/101.15

70092 93/207.1 70093 93/220.3

70094 93/238.2 70095 93/239.4

70097 93/241.2

70098 93/242.9

Granodiorite qtz 10.8 Granodiorite qtz 11.6 Granodiorite po

Gneiss

Gneiss

Gneiss

Pegmatite

Stockwork vein Stockwork vein Stockwork vein Stockwork vein

Crenulate-rock Alteration Alteration

qtz 10.2 bio 6.9

qtz 8.9 qtz 10.3 qtz 9.3

qtz 9.4 qtz 9.6 qtz 9.6 qtz 9.8 PY

qtz 10.6 epi chl

Cavity infilling qtz 10.3 Cavity infilling qtz 10.5 Cavity infilling py Cavity infilling qtz 10.3 Cavity infilling py Alteration chl Alteration chl 7.7

mus 9.3

Alteration py Alteration or 10.8

mus 8.8

Alteration epi PY

Sheeted vein qtz 13.3 Cavity infilling cal 9.4

ank 11.0

Sheeted vein qtz 11.8 Sheeted vein qtz 11.9 Sheeted vein qtz 13.5 Sheeted vein qtz 13.0 Sheeted vein qtz 12.8 Sheeted vein py Sheeted vein po Cavity infilling cal 8.4

ank 9.8

Cavity infilling cal 8.6 Cavity infilling cal 7.4

ank 7.3

PY

gal Sheeted vein qtz 12.6 Sheeted vein qtz 13.7

cal 7.1 Sheeted vein cal 9.1

Sheeted vein qtz 12.6 cal 8.6

Cavity infilling qtz 11.7 cal 8.0

Sheeted vein qtz 12.5

-81

-57

-81

-87

-80 -71

-68

-51

-6.6

-6.7

-6.5

-6.6

-5.8

-5.9

-6.0

-6.5

-7.8

-7.4

-7.1

0.2

3.8

3.4

3.7

3.0

3.0

3.0

2.2

3.6

1.1

Au BRECCIA PIPE, KIDSTON, QUEENSLAND

T^BLE 4. (Cont.)

823

CSIRO DDH/m b•80 fid b•aC ba4S no. Description Mineral (%o V-SMOW) (%o V-PDB) (%o CDT)

70099 93/264.9 Cavity infilling cal 6.7 -6.9 po 2.3

70100 93/271.7 Sheeted vein qtz 12.7 cal 7.6 -5.9

py 3.2 72211 101/131.6 Sheeted vein qtz 13.2 72212 101/132.3 Sheeted vein py 3.2

sph 2.4 72213 101/135.85 Sheeted vein ank 11.4 -6.3

py 3.6 sph 2.5

72214 101/136.5 Cavity infllling ank 14.1 -6.1 py 2.8

72219 101/182.8 Sheeted vein qtz 12.5 qtz 13.0

72222 101/190.75 Sheeted vein qtz 12.9 py 3.3 sph 2.6

72225 103/57.3 Cavity infllling py 4.0 sph 2.4

72227 103/69.5 Cavity infllling py 3.8 sph 2.5

72228 103/119.5 Sheeted vein py 3.8 sph 2.9

72229 103/127.6 Sheeted vein py 3.5 sph 2.4 cpy 2.8

72231 104/42.2 Cavity infllling cal 11.1 -6.5 ank 11.6 -7.1

72232 104/49.7 Cavity infilling cal 11.6 -6.7 72233 104/96.5 Cavity infllling cal 11.3 -7.1 72236 108/5.8 Cavity infilling cal 11.5 -6.1 72237 108/15.8 Cavity infllling ank 14.8 -6.2 72239 108/19.8 Cavity infilling ank 14.4 -6.3 72241 108/43.0 Cavity infllling ank 13.5 -6.0 72242 108/45.6 Cavity infllling py 3.2

sph 2.6 72243 108/52.25 Cavity infllling qtz 10.5

ank 12.3 -6.2

py 3.7 sph 2.7

72244 108/57.5 Sheeted vein ank 14.0 -6.1 py 3.4 sph 2.4

72250 111/121.2 Cavity infllling ank 14.6 -6.6 72253 111/227.4 Sheeted vein qtz 13.3 72259 111/280.5 Sheeted vein cal 10.3 -7.5

py 3.3 72260 111/299.5 Sheeted vein ank 13.1 -6.4 72261 111/288.9 Sheeted vein qtz 12.6 72264 111/314.7 Cavity infilling qtz 13.3

cal 11.2 -6.0

72270 113/255.15 Sheeted vein py 4.6 70089 88/146.4 Alteration py 4.3 72257 111/260.8 Alteration py 4.2

See Table 2 for abbreviations

temperature limit of 300øC, the calculated b•sO composition of the water in equilibrium with quartz and the bD value of inclusion fluids suggest that an evolved meteoric water may be a component in the latest, and gold-depositing, phase of the mineralizing

fluid. Interaction of a strongly •sO depleted fluid, such as an unmodified Permian meteoric water, with the rocks would result in •SO-depleted values at any geo- logically reasonable temperatures.

These values can also be obtained from a magmatic


TABLE 5. Hydrogen Isotope Data from Decrepitated Inclu- sions within Surface Samples of Late-Stage Sheeted Veins and

Cavity Infilling in the Kidston Breccia Pipe (•D reported in %0 V-SMOW)

CSIRO Temperature range (øC) no. Description 200-300 300-400 400-500

70052 Cavity infilling -76 -71 qtz

70053 Sheeted vein -88

qtz 70054 Sheeted vein -111 -90

qtz

-78

action of a fluid with •sO value of 8 per mil at 300øC and water-rock ratios (at. %) of about 0.4 to 1.2. The resultant hydrothermal fluid has •sO values of 5.4 to 6.4 per mil, within the range calculated to be in equilibrium with quartz from late-stage veins and cavities. An initial fluid with lower •sO values of between 5 and 8 per mil would result in a similar shift in •sO at higher water-rock ratios up to two for closed system circulation and one for an open system. Fluids with •sO values of less than 5 per mil at 300øC cannot cause 1sO enrichment of the altered rocks using this model under the assumed conditions.

fluid which has undergone extensive crystallization and degassing to lower •D values (Taylor, 1988) and exchange during alteration with rocks to lower •sO values (Pollard et al., 1991). A modified magmatic fluid also explains the similarity in salinities and composition of type L and V fluid inclusion salts between the late- and early-stage fluids. Using the rock-water interaction model of Taylor (1977), the enrichment •sO values of sericitic and chloritic altered rocks (•sO = 10-11%0; Table 6) relative to unaltered granodiorite (•sO: 9%0) can be obtained by inter-

Origin of sulfur and carbon

Sulfur isotope values for coexisting minerals from late-stage sheeted veins and cavity infilling are in equilibrium and suggest temperatures of between 250 ø and 430øC (Kajiwara and Krouse, 1971), con- sistent with fluid inclusion estimates. The narrow

range of•34S values from the prebreccia Mo and postbreccia Au mineralization sequence at Kidston (Table 6), together with the absence of primary sulfates and presence of pyrrhotite, indicates relatively reducing conditions prevailed, so the values measured for sul-

TABLE 6. Oxygen Isotope Analyses of Granodiorite from within and around the Kidston Breccia Pipe

•8 0

CSIRO no. DDH/m Description (%o V-SMOW)

Plant site

102702 98/33.6 Biotite granodiorite 9.4 102705 98/40.4 Biotite granodiorite 8.7 102706 98/44.8 Biotite granodiorite 8.3 102717 100/52.0 Biotite granodiorite 9.2 102722 100/80.3 Biotite granodiorit e 8.7

Outside breccia pipe, beyond sheeted vein zone 102643 45/111.7 Biotite granodiorite 9.1 102646 45/121.3 Biotite granodiorite 9.7 102648 45/130.1 Biotite granodiorite 10.0

Outside breccia pipe, within sheeted vein zone 102639 45/100.6 Biotite granodiorite 9.4 102640 45/104.8 Biotite granodiorit e 9.6 102642 45/107.2 Biotite granodiorite 9.1

Within breccia pipe, adjacent to sheeted veins 102619F 16/67.0 102619G 16/67.0 102619E 16/67.0 102619D 16/67.0

102619C 16/67.0 102619B 16/67.0 102619A 16/67.0

Within breccia pipe, clasts 102575C 10/87.2 102579A 10/91.5 102579B 10/91.5 102579C 10/91.5 102607B 10/146.0

Biotite granodiorite 10.1 Biotite granodiorite 9.8 Chloritized granodiorite 9.9 Outer green altered granodiorite 9.2 Inner brown altered granodiorite 10.3

Sericitized granodiorite 10.6 Qtz vein 12.4

Biotite granodiorite Pale green chloritized selvage Deep green chloritized selvage Sericitized granodiorite Sericitized granodiorite breccia

9.8

10.9

10.2

10.6

10.6

Au BRECCIA PIPE, KIDSTON, QUEENSLAND 8 25

-10 / ß SMOW / post -• -30 / breccia •.•.• pre-breccia / 'early'•

-50 • •///J--'magmatic .•.•...// V/• water'

-70 •"•/ •ate •llJl i i i { i• i {i •--post-breccia iiiiii iii i

-110

-130 / / meteoric water t / (low latitude)

-150 / I I I I I I I -20 -15 -10 -5 0 5 10 15

(•180 (% V-SMOW)

FIG. 9. Calculated fields for oxygen and hydrogen isotope composition of pre- and postbreccia alteration fluids at Kidston. Fields for low-latitude Permian meteoric waters from eastern

Australia (Sun and Eadington, 1987) and magmatic water (Shep- pard et al., 1969) are also shown. MWL -- meteoric water line.

fides approximate the sulfur isotope composition of the mineralizing fluid (Ohmoto, 1986). Like porphyry Cu-Au and Mo deposits elsewhere, the sulfur was probably derived from an A-type magma, specifically the parent for the rhyolite clasts and dikes.

The carbon isotope compositions of carbonates from early- and late-stage breccia infilling and sheeted veins also have a narrow range in values, which are depleted in 13C relative to marine carbonates but within the range of values for CO2 degassed from a magmatic source (Taylor, 1986).

Model for Breccia Pipe Formation

The spatial and temporal relationships of both brecciation and mineralization to rhyolite emplacement suggest that magmatic hydrothermal processes as well as the mechanical process directly associated with rhyolite emplacement were involved in breccia formation. Figure 10 shows the various stages of breccia pipe formation and the interpreted relation-

ships of the breccia and intrusive phases in the third dimension.

The quartz-tourmaline cemented breccia (phase 1) is spatially associated with the small rhyolite stocks in the southwest portion of the breccia pipe. The shingle breccia texture which is a characteristic feature of this breccia has been recognized in other tourmaline-bearing breccia pipes by Sillitoe (1985) and Allman-Ward et al. (1982), and it is interpreted as forming due to decompression associated with a drop in pressure as volatile-rich magmatic hydrothermal fluids escape from the upper portions of a crystallizing melt (Fig. 10A). The close spatial and temporal relationship between this breccia and the quartz stockwork veins, which formed in the carapace of the rhyolite stocks during crystallization, is consis- tent with a magmatic hydrothermal origin for the phase 1 breccia. Fluid inclusion data from the stockwork veins indicate that these processes occurred at a depth of approximately 3,500 m.

The phase 2 breccia comprises the bulk of the breccia pipe. This breccia contains partly disrupted dikes of synbreccia rhyolite and flattened pumaceous fragments of dike material indicating a direct magmatic involvement in breccia pipe formation. The presence of large unrotated blocks of host rock within the breccia pipe and the preservation of host-rock stratigraphy within the breccia itself indicate that brecciation occurred without large-scale movement of the brecciated material. However, on a scale of several meters, the smaller clasts are well mixed and rotated, indicating that brecciation involved more than in situ fracturing. Therefore, the most likely mechanism for brecciation appears to have been collapse (Fig. 10B). Since the host-rock stratigraphy is steeply dipping, there are unfortunately no fiat marker horizons which can be used to determine what

degree of downward movement was associated with collapse. Two possible processes which could have caused collapse at this scale are magma withdrawal and the escape of a large volume of hydrothermal fluid from the apical portion of a crystallizing intrusion as envisaged by Norton and Cathies (1973). At a

TAI3LE 7. Summary of Fluid Compositions in the Kidston Breccia Pipe

•80 •D Salinity T (øC) (%0 V-SMOW) (wt % NaCI equiv) Composition

Prebrecciation 500-600 8 -50 to -20 40-55 NaCI rich Postbrecciation

Early 340-500 4-8 -50 to -20 20-30 NaC1 rich Late

Quartz 300-400 2-8 -90 to -71 2-10 NaCI rich Carbonate 275-350 0-8 ? 0.5-0.7 CO• rich

Fractionation factors used for calculations of fluid compositions are from O'Neil and Taylor (1967, 1969), O'Neil et al. (1969), Suzuoki and Epstein (1976), Graham et al. (1980, 1987), and Clayton et al. (1989)


LEGEND :

•r• stockwork veins

• Crenulote quartz-layers

'*;'•*'•..- • Aphonffic rh¾ohte

I Phase •. breccia ( tOurmoline breccla)

;• Pre- post-brecclO porphyrytic rhyolite

J• Phase 2 breccio (gronodiorlte/metomorphic clasts

:•=] Phase 3 brecclo Sheeted vein zones


deeper level of the pipe where the breccia and underlying intrusions are juxtaposed, it may be possible to determine which of these processes was responsible for collapse.

The presence of phase 3 breccia dikes cutting the phase 2 breccia indicates that this breccia is intrusive in origin (Fig. 10C). A deep-level origin for this breccia is supported by the presence of quartz-pyrite- magnetite stockwork veins bearing clasts which are not characteristic of the style of mineralization exposed at the present level of erosion and therefore are interpreted as having become entrained in the breccia at a deeper level in the system and transported upward to their present position. The spheroidal clasts which are a characteristic feature of this breccia have

been observed in other breccia pipes by Sillitoe (1985) and are interpreted as forming by exfoliation associated with decompression during rapid ascent of material to shallower levels within the system. Con- sidering the intrusive nature of the breccia, the deeper source for some of the clasts, and the evidence for relatively rapid ascent provided by the spheroidal clasts, brecciation is more likely to have been related to the rapid upward escape of hydrothermal fluids than to mechanical processes associated with the emplacement of the postbreccia radial dikes which intrude it. Therefore, third-stage brecciation is interpreted as resulting from either the explosive interaction of hydrothermal fluid already resident within the breccia pipe with the postbreccia rhyolite or the explosive escape of magmatic volatiles from a phase of postbreccia rhyolite which crystallized in the deeper levels of the breccia pipe.

Subsequent to breccia pipe formation, a series of postbreccia dikes of porphyritic rhyolite were emplaced into the breccia pipe (Fig. 10D). These dikes do not transgress the breccia pipe margin, suggesting that the stress fields which were responsible for cre- ating the radial pattern were restricted to the breccia pipe itself. The inward-dipping sheeted veins cut these radial dikes (Fig. 10E), indicating that the fracturing which localized the sheeted veins was produced subsequent to or during the emplacement of these dikes. Koide and Bhattacharji (1975) recognized that radial and concentrically inward dipping fractures

could be produced in the host rocks by forceful emplacement of intrusive bodies. The forceful emplacement of the postbreccia rhyolite porphyry into the lower portion of the breccia pipe resulted in the formation of the radial fractures, occupied by the dikes, and the inverted funnel-shaped zone of enhanced permeability which played a major role in localizing early-, transitional-, and late-stage mineralization.

The upper portion of the pipe did not breach the contemporary land surface; stable isotope evidence indicates that no significant amounts of meteoric water were introduced into the system during postbreccia mineralization. Therefore, the breccia pipe is likely to have graded upward through a zone of fracturing and faulting, without significant brecciation, into an overlying column of undisturbed host rock. The fractures which acted as conduits for the escaping hydrothermal fluid would presumably contain some evidence of brecciation. The tourmaline breccia in the

Wise's Hill area (phase i breccia) and a narrow breccia dike peripheral to the breccia pipe may represent such structures (Fig. 10A and B). Figure 10F shows the position of the present erosion surface.

The postbreccia and postmineral andesitc dike which cuts through the breccia pipe is part of a northwest-trending dike swarm. The localization of this dike is presumably controlled by a more regionally induced stress pattern.

Model for Hydrothermal Fluid Evolution Prebreccia

Petrographic, fluid inclusion, and stable isotope data indicate that the original prebreccia mineralizing fluid was a high-temperature (+500øC), highly saline 40-50 wt % NaC1 equiv), magmatic fluid (•sO -- 8%0, •D = -50 to -20%0) exsolved from a number of small rhyolite bodies. The growth of crenulate quartz layers within the rhyolite bodies and the formation of stockwork vein mineralization and breccia-

tion in the overlying rocks occurred in response to the cyclic buildup and release of fluid pressures within the crystallizing rhyolite. Repeated episodes of brecciation and stockwork vein formation appears to have resulted in a drop in fluid pressure from approximately 800 bars (lithostatic) to 300 to 500 bars (hydrostatic),

FIG. 10. Model for formation of the Kidston breccia pipe. A. Development of prebreccia stockwork quartz veins in the rhyolite carapace and phase 1 quartz-tourmaline cemented breccia associated with the buildup of hydrothermal fluid during crystallization of the underlying batholith. B. Formation of phase 2 breccia, resulting from the collapse of the overlying rocks into the space produced by magma withdrawal and/or the escape of magmatic hydrothermal fluid from the crystallizing rhyolite body. C. Intrusion of phase 3 breccia produced by the rapid escape of hydrothermal fluids either directly from a crystallizing rhyolite melt or due to the explosive interaction of a rhyolite magma with hydrothermal fluids already resident within the breccia pipe. D. Intrusion of the postbreccia rhyolite. E. The forceful emplacement of the postbreccia rhyolite into the lower portion of the breccia pipe produced radial fractures, which the rhyolite has intruded, and inward-dipping concentric fractures which host the economic-grade gold mineralization. F. Present level of erosion through the breccia pipe.


causing periodic boiling of the highly saline fluid to produce a vapor with a salinity range of 5 to 10 wt percent NaC1 equiv. The vapor condensed over a temperature range of 380 ø to 460øC under near critical conditions due to adiabatic expansion and/or ad- sorption of heat by wall rocks to form a liquid of similar salinity.

Postbreccia

Early-stage, postbreccia quartz-epidote-sulfide cavity-infilling mineralization was deposited from a magmatic fluid (•lSO = 4-8%0, •D = -50 to -20%0) with a salinity ranging from 5 to 10 equiv wt percent NaC1, at temperatures clustering in the ranges 300 ø to 400 ø and 400 ø to 540øC (homogenization temperatures corrected for pressure). The early-stage postbreccia fluid was a liquid with similar salinity, range of homogenization temperatures, and isotopic composition to condensate trapped as type L inclusions in the prebreccia mineralization. This liquid is interpreted as having condensed from a vapor produced by boiling of a highly saline magmatic fluid at a deeper level in the system, under similar conditions to those described for the formation of the liquid trapped as type L inclusions in the prebreccia mineralization.

During the latter part of the early-stage mineralizing event, the deposition of the quartz-epidote _ pyrite _ pyrrhotite cavity-infilling mineralization appears to have made the breccia su•ciently impermeable that a highly saline magmatic fluid was able to migrate upward into the system with only a limited amount of boiling. The introduction of this highly saline, higher temperature fluid resulted in the trapping of type S and V inclusions and the localized overprinting of quartz-epidote-sulfide mineralization by a hydrothermal biotite-magnetite-pyrrhotite assemblage.

The lower temperatures associated with transitional- to late-stage mineralization resulted in a change from orthoclase to muscovite stable alteration. The extensive replacement of biotite by muscovite led to the fluid becoming su•ciently enriched in Fe that the composition of the transitional- to late-stage carbonate was ankerite. During transitional- to late- stage mineralization, the bulk of the quartz infilling was deposited over a temperature range of 300 ø to 380øC. The lower temperature population of fluid inclusions ranges from 170 ø to 300øC and is interpreted as having been trapped during the deposition of the sulfides and carbonate.

The econominc-grade gold mineralization which is hosted by sulfides within the late-stage sheeted veins and cavities would have been deposited over a temperature range of 350 ø to possibly as low as 170øC. The data of Seward (1973) suggest that a drop in the temperature from 300 ø to 170øC would have caused a drop in solubility of gold bisulfide complexes by one

or two orders of magnitude. Therefore, cooling of the fluid as it ascended through the breccia column must have played an important role in localizing the late- stage gold mineralization over a restricted vertical and horizontal interval of the breccia pipe. This is demonstrated in the Wise's Hill area, where the decrease from economic to subeconomic gold grades over a vertical and lateral distance of 200 m is accompanied by an increase in the ratio of pyrrhotite to pyrite, reflecting increasing temperature. Fluid inclusion and stable isotope data suggest no significant quantities of a lower temperature, lower salinity fluid were introduced into the system at this stage.

Fluid inclusion data indicate that the transitional-

to late-stage fluid was similar to that associated with early-stage quartz-epidote-sulfide mineralization except of the presence of CO2. Stable isotope data indicate that the CO2 was introduced from a cooling rhyolite magma.

Summary and Conclusions

The Kidston breccia pipe is spatially and temporally associated with the emplacement of a series of Permo- Carboniferous rhyolite dikes and stocks into middle Proterozoic metamorphics and granitoids. The coincidence of a regional gravity low with the dike swarm and the association of these dikes with areas of caul-

dron subsidence and ring dikes is interpreted as indicating that the breccia pipe is underlain by a large Permo-Carboniferous batholith. The earliest phase of brecciation (phase 1) is interpreted to have formed as a result of the escape of a boron-rich magmatic hydrothermal fluid from the apical portions of one or more bodies of crystallizing rhyolites. The bulk of the breccia pipe, phase 2 breccia, formed by collapse in response to either magma withdrawal or the escape of a large volume of magmatic hydrothermal fluid, presumably from the apical portion of the postulated underlying batholith. The final phase of brecciation, the phase 3 breccia, which was intruded into the center of the breccia pipe, is presumed to have formed either in response to the explosive interaction of the postbreccia rhyolite and hydrothermal fluids already present within the breccia pipe or by the escape of magmatic hydrothermal fluids from the postbreccia rhyolite body at a deeper level in the pipe. Fluid inclusion data and the absence of any significant amount of meteoric water in the hydrothermal system suggest that the breccia-forming processes occurred at a depth of approximately 3,500 m and that the breccia pipe did not breach the contemporary land surface. Pre- sumably any hydrothermal fluids and volatiles which escaped from the system during brecciation and mineralization did so via fractures, some of which have been mapped as breccia dikes.

Prebreccia mineralization consists of stockwork

quartz veining in the carapace of several rhyolite stocks. The similarities in fluid inclusion and stable


isotope data from the stockwork veins and crenulate quartz layers within the rhyolite bodies indicate that the fluid responsible for stockwork vein mineralization was a high-salinity, high-temperature magmatic fluid exsolved directly from the crystallizing rhyolite bodies. Hydraulic fracturing associated with localized brecciation and stockwork vein formation resulted in intermittent fluctuations between lithostatic and hydrostatic pressure conditions allowing the highly saline fluid to boil. The vapor produced by the boiling of this fluid was cooled by adiabatic expansion and condensed under near-critical conditions as a mod-

erately saline lower temperature liquid. The postbreccia mineralizing fluid was a moder-

ately saline (5 to 10 wt % NaC1 equiv) magmatic liquid, interpreted as having formed by condensation of a vapor produced by the boiling of a highly saline magmatic fluid at a deeper level within the breccia pipe.

During the early-stage prebreccia mineralization, the fluid was channeled up the inverted funnel-shaped zone of enhanced permeability. Within this zone of enhanced permeability, cavities were infilled with quartz-epidote _ sulfide mineralization and associated orthoclase-albite-muscovite-calcite alteration.

Throughout the remainder of the breccia pipe, cavity- infilling mineralization consisted of calcite _ sulfide with muscovite-calcite-chlorite alteration. The change in cavity assemblages presumably reflects the decreasing temperature and fluid-rock ratio as the fluid percolated out of the more permeable zone into the surrounding breccia. The infilling of cavities resulted in decreased permeability within the breccia pipe allowing the highly saline magmatic fluid to penetrate to higher levels in the pipe before boiling. The biotite- magnetite-pyrrhotite and biotite-siderite-pyrite assemblages which overprint the earlier quartz-epidote _ sulfide mineralization are probably associated with this highly saline fluid.

The economic-grade gold mineralization is hosted by the transitional- to late-stage quartz-ankerite _ sulfide cavity infilling and sheeted vein mineralization localized within a zone of sheeted fractures more

or less coincident with the inverted funnel-shaped zone of enhanced permeability present during early- stage postbreccia mineralization. A transitional-stage cavity-infilling and vein assemblage represents the progression from early-stage to late-stage mineralization. A decrease in temperature during transitional- to late-stage mineralization resulted in the partial overprinting of early-stage cavity-infilling assemblages by muscovite-ankerite alteration. Economic- grade gold mineralization was deposited over a temperature range of 300 ø to 170øC. At these temperatures gold would have been transported predom- inantly as a bisulfide complex (Seward, 1973). The lateral and vertical decrease in the grade of gold mineralization within the late-stage sheeted veins and

cavities is associated with an increase in ratio of pyrrhotite to pyrite which is interpreted as reflecting increasing temperature. Therefore, the primary controls on deposition of gold within the sheeted veins appears to have been temperature.

The inverted funnel-shaped zone of enhanced permeability which was produced by the forceful emplacement of the postbreccia rhyolite into the lower portion of the breccia pipe has been the main control on the distribution of postbreccia mineralization assemblages. The persistence of this zone through time as reflected by the late-stage sheeted veins cutting through the breccia already cemented by early-stage assemblages indicates that throughout the entire postbreccia hydrothermal episode rhyolite magma was being forcefully intruded into the lower portions of the breccia pipe. This relationship adds further support to the hypothesis that both brecciation and gold mineralization at Kidston are genetically related to rhyolite magmatism.

Acknowledgments

Placer Pacific and Kidston gold mines kindly made available data and samples and assisted with logistical support. The assistance of Garth Wilson, John Gallo, and Frank Tullemans with these matters is greatly appreciated. Andrew Bryce assisted with stable isotope analyses. R. A. Binns provided samples of granodiorite for isotopic analysis. Research undertaken by EMB as part of this project was sponsored by Austra- lian Mineral Industry Research Association under the supervision of Gregg Morrison at James Cook Uni- versity. EMB wishes to acknowledge the considerable effort G. W. Morrison, P. J. Pollard, and R. W. T. Wilkins put into discussing various aspects of this research. The manuscript has benefited from thoughtful reviews by P. E. Brown, R. H. Sillitoe, G. F. Taylor, and an anonymous Economic Geology reviewer.

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