Wau Sillitoe Et Al 1985 p0638-p0655

Economic Geology Vol 79, 1984, pp. 638-655

Gold Deposits and Hydrothermal Eruption Breccias Associated with a Maar Volcano at Wau, Papua New Guinea

RICHARD H. SILLITOE,

8 West Hill Park, Highgate Village, London N6 6ND, England

E. MAX BAKER,

E. M. Baker and Associates Pty. Ltd., 115 Ross River Road, Mundingburra, Townsville 4812, Australia

AND WILLIAM A. BROOK

Geopacific Services Pty. Ltd., P.O. Box 619, North Sydney, New South Wales 2060, Australia

Abstract

The Wau district has produced much of the 18 metric tons (0.58 million oz) of lode gold attributed to the Morobe goldfield in Papua New Guinea. Geologic mapping and reappraisal of the district have shown that Pliocene epithermal gold mineralization was associated with generation of a maar volcano and associated endogenous domes.

Emplacement of endogenous domes and eruption ofpyroclastic flows in the Morobe goldfield took place in the interval from 4 to 2.4 m.y. ago. An undated dome at Edie Creek is spatially, and perhaps temporally, associated with lode gold mineralization.

During the later part of this time interval, following tilting of the ignimbrite pile, a maar was generated at Wau. The subjacent diatreme is believed to extend downward for about 1 km to a regional dip-slip fault zone which hosts deeper level gold mineralization at the nearby Ribroaster mine. Interaction of dacitic magma and ground water in depth on the fault zone caused a series of hydrovolcanic explosions which resulted in pyroclastic deposits of ballistic fall and base surge origin, the latter carrying abundant accretionary lapilli. The pyroclastics accumulated within the maar and constructed an encircling tuff ring, only a small part of which is preserved overlying Late Cretaceous-Paleogene phyllites to the south and west of the maar.

Following initial maar formation, hydrothermal brecciation and gold deposition were controlled by low-angle extensional faults which developed during incipient subsidence of a wedge of poorly supported rock between the regional fault zone and the ring fault delimiting the maar. Fluid overpressures, which developed at depths of >100 m beneath self-sealed cap rocks, triggered hydrothermal eruptions which gave rise to a ramifying system of irregular veins and bodies of hydrothermal breccia surmounted by subaerially deposited breccia aprons. Self-sealing was caused by early gold-bearing calcite-manganocalcite-quartz vein mineralization, now found as fragments in breccia conduits and aprons. Later stages of gold deposition, accompanied by the same gangue assemblage, occurred in gently dipping, lenticular lodes and overlying stockworks, many of them hosted by hydrothermal breccias.

During the late stages of, and after, accumulation of pyroclastic fall and surge deposits, epiclastic sedimentation was widespread in the maar. It gave rise to reworked breccias, grits, and prominent organic-rich mudstones deposited in ephemeral lakes. Hydrothermal activity at this time produced subaqueous chert-pyrite beds and possibly subaerial hot spring sinter and travertine and, in depth, a quartz-calcite-kaolinite-smectite-(illite) alteration assemblage. No significant gold is present in any of these hydrothermal products.

Volatile-depleted magma reached the surface around the perimeter of the maar during active sedimentation and constructed two principal endogenous domes, one dacitic and the other andesitic. The dacitic dome, dated previously at 2.4 m.y., gave rise to an apron of crumble breccia along the northwestern edge of the maar.

Accumulation of the intramaar sequence was accompanied and followed by widespread subsidence and slumping which resulted in steep dips and locally overturned beds. On com- pletion of the filling of the maar, coherent blocks of basement phyllite slid for distances of as much as 1 km into the maar from its steep western wall. Some of these slide blocks are overlain by remnants of the tuff ring and carry hydrothermal breccias and economically important gold mineralization, all mechanically transported to their present positions.

At this time, an opal-pyrite-marcasite horizon was forming at a boiling water table, with an overlying cristobalite-kaolinite-alunite zone believed to be a product of acid leaching

0361-0128/84/300/638-1852.50 638

Au DEPOSITS AND HYDROTHERMAL ERUPTION BRECCIAS, PAPUA NEW GUINEA 659

caused by dissolution of boiled-off H2S in cool ground waters. This alteration lacks gold mineralization, except where it overprints a gold-bearing allochthonous slide block.

The regional drainage system eventually breached the crater rim and resulted in deposition of channel conglomerates and fiuviolacustrine beds, all carrying alluvial gold. Radiocarbon dating of wood from this unit showed that it accumulated >42,000 years ago.

The most recent events at Wau are believed to be a series of hydrothermal eruptions which constructed a composite breccia apron at Koranga crater on the southeastern edge of the maar. The most recent eruption is tentatively inferred to have taken place in 1967.

Introduction

MARKEDLY increased gold prices since 1979 have focused renewed attention on high-level, epithermal precious metal deposits. It has recently been em- phasized that a significant number of such deposits are intimately related to a variety of little-eroded volcanic landforms (R. H. Sillitoe and H. F. Bonham, Jr., unpub. data). One of these is a maar volcano, the surface manifestation ofa diatreme. The writers' geologic mapping and reappraisal of the Wau sector of the Morobe goldfield in Papua New Guinea (Fig. 1) have demonstrated that epithermal gold mineralization accompanied hydrothermal brecciation as part of the late-stage development of a typical maar volcano.

Gold was discovered at Wau in 1922 by W. "Sharkeye" Park, an Australian prospector, and fol-

lowing the location of extraordinarily rich alluvial deposits at nearby Upper Edie Creek (Fig. 3) in 1926, the district became the focus of one of the world's

last great gold rushes (Nelson, 1976). Except for the war years, gold has been produced from the Morobe goldfield ever since, although since the late 1950s {n reduced amounts. To date the production has amounted to about 120 metric tons (3.8 million oz), some 15 percent of it from lode deposits (Fisher, 1975; Lowenstein, 1982).

Small-scale open pit mining at Wau is currently conducted by New Guinea Goldfields Limited, since late 1981 a subsidiary of Renison Goldfields Con- solidated Limited of Australia, and a small amount bf alluvial gold is recovered as a result of ground sluicing both by the company and by private individuals, including tributors. Annual production by the company

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FIC. 1. Location of Wau with respect to Pliocene-Pleistocene volcanic rocks (after Dow, 1977), major faults (after Dow, 1977), and late Cenozoic plate tectonic elements (after Hamilton, 1979) of Papua New Guinea. Volcanic outcrop A comprises Yelia and Marble Peak centers (see text).

640 SILLITOE, BAKER, AND BROOK

currently runs at about 0.35 metric tons (11,000 oz), approximately 90 percent of it from the Upper Ridges open cut (Fig. 2). Although this is only some 2 percent of the total produced each year as a by-product from the Panguna porphyry copper deposit on Bougainville Island (Fig. 1), it makes a small contribution to Papua New Guinea's position as the world's ninth largest gold producer (Du Boulay, 1983). Although now overshadowed by Panguna, the Morobe goldfield ranks as one of the premier goldfields of the western Pacific region.

The geology of the Wau sector of the Morobe goldfield is dealt with in papers by Fisher (1944, 1945), Fisher and Branch (1981), and Rebek (1975), and aspects of it are also the subject of several unpublished reports and maps, notably those prepared in the early 1930s by H. M. Kingsbury. Existing geologic information is ably summarized in the mono- graph by Lowenstein (1982), which also provides much new geochemical data. Although we rely

heavilyon published and unpublished data for details of lode gold deposits previously worked underground and either no longer accessible or removed by open pit mining, our work at Wau, which included logging of 2,700 m of new diamond drill core, has resulted in redefinition of many of the geologic units and reinterpretation of the geologic setting, timing, and genesis of the gold deposits. Although previous workers clearly recognized the volcanic setting of gold mineralization at Wau (e.g., Fisher, 1944, 1945; Rebek, 1975), and some have suggested an association with diatremes (e.g., Dow et al., 1974), the complete volcanic context of ore deposition and its analogues elsewhere were not previously appreciated.

Regional Setting

Geologic setting

The southwestern part of Papua New Guinea comprises a Paleozoic crystalline basement terrain over-

.

FIG. 2. Oblique aerial photograph looking south over the Golden Peaks (GP), Golden Ridges (GR), and Upper Ridges (UR) sectors of the Wau gold deposit, Papua New Guinea. Namie Creek and the mill are in the foreground; the regional fault zone is in the left background. Dark gray rocks behind and to the left of the mill are parts of Kaindi Metamorphic slide blocks. The degraded maar wall is visible between Golden Ridges and Upper Ridges. Crumble breccia can be seen to the right of the mill.


lain by Mesozoic-Cenozoic shelf sediments which, along its northern side, collided with a Late Creta- ceous-Paleogene intraoceanic island-arc system in middle Tertiary times. Collision induced tectonism, metamorphism, ophiolite obduction, and uplift (Dow, 1977).

The Wau district is located in the Owen Stanley Ranges, approximately 30 km west of the Owen Stan- ley fault system (Fig. 1), which separates the obducted Papuan ophiolite from the Mobile Belt. The Mobile Belt consists of a highly deformed and metamorphosed Late Cretaceous to Eocene sedimentary pile which is associated locally with basaltic volcanic rocks and overlies oceanic crust (Dow, 1977). Hamilton (1979) considered the Mobile Belt rocks to represent a m•- lange terrain accreted during northward subduction beneath the intraoceanic island arc and subsequently deformed by the continent-island arc collision. In the Wau district, the volcano-sedimentary prism is represented by the Kaindi Metamorphics (Figs. 3 and 4), which are dominated by partly graphitic blue-

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FIG. 3. Selected geologic features of the Morobe goldfield, Papua New Guinea, modified after Dow et al. (1974).

gray phyllites but also include chlorite, chloritoid and sericite schists, and subsidiary quartzites and marbles. Clasts of garnet-bearing schist were also encountered in breccias and conglomerates (see below), but the rock was not observed in situ. The low-grade metamorphic rocks are characterized by foliation parallel to bedding and by numerous pods and lenses of metamorphogenic quartz. An Rb-Sr isochron age of 21.0 _ 4.0 m.y. was determined for Kaindi samples collected 4 km west-northwest of Wau and is interpreted as the age of regional metamorphism (Dow et al., 1974; Page, 1976), probably a late-stage facet of middle Tertiary collisional orogeny.

A partly subaerial volcanoplutonic arc--the Mar- amuni arc (Dow, 1977)--was constructed along the Mobile Belt in middle to late Miocene times. In the

Papuan peninsula, the arc was probably a consequence of northward subduction from the now-buried Port Moresby trench (Hamilton, 1979; Fig. 1). In the area of the Morobe goldfield, magmatism is represented by the dominantly granodioritic Morobe batholith, which at its nearest point to Wau was ra- diometrically dated by the K-Ar and Rb-Sr methods at 12 to 13 m.y. (Page, 1976). In the Wau district, three small bodies of weakly foliated hornblende-biotite diorite (Fig. 4) are considered as apophyses of the Morobe batholith.

During the Pliocene to Holocene interval, major uplift and subaerial volcanism (Fig. 1) were widespread in Papua New Guinea during southward subduction and included the activity, detailed below, which gave rise to the gold mineralization at Wau. The Owen Stanley fault system underwent strike-slip displacement and, in common with the major northwest-trending fault mapped at Wau (Fig. 3), also ap- preciable normal movement.

Metallogenic setting

Except for the Morobe goldfield, little economic mineralization is known from the Papuan peninsula of eastern Papua New Guinea. Minor alluvial gold concentrations, derived mainly from middle Miocene to Pliocene magmatic rocks, are worked sporadically. Several porphyry copper prospects of possible Plio- cene age, one some 75 km northwest of Wau (Fig. 1), have been investigated, and a laterite nickel pros- pect is known in the Papuan ophiolite belt some 30 km southeast of Wau at Lake Trist (Fig. 1).

Late Cenozoic volcanotectonic setting The Wau district is not mentioned in recent

syntheses of Pliocene to Recent volcanism in Papua New Guinea (e.g., Johnson, 1979, 1982), probably because of the absence of either Recent activity or any large volcanic edifice. The Wau district is part of a tectonically complex region situated between the southeastern extremity of the Highlands volcanic


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•O. 4. Geologic map of the Wau gold deposit and vicinity, Papua New Guinea.


province, which overlies the continental basement, and the northwestern termination of the Eastern

Papua volcanic province (Fig. 1). The Eastern Papua volcanic province may be paired with southwestward subduction of Solomon Sea lithosphere at the probably now-inactive Trobriand trench, which developed in the Pliocene (Hamilton, 1979; Fig. 1). The High- lands volcanic province is believed by Hamilton (1979) to be a product of southwestward subduction from the eastward extension of the New Guinea

trench along the north coast of Papua New Guinea (Fig. 1) which became inactivated by collision with the westward extension of the New Britain arc-trench

system (Hamilton, 1979). Since the dormant Yelia andesitic volcano and the extinct Marble Peak volcanic

center, some 95 km west-northwest of Wau (Fig. 1), are currently underlain by a remnant of a southwest- dipping Benioff zone (Dent, 1976), it seems likely that Pliocene volcanism at Wau may be attributed to subduction at either the New Guinea or Trobriand trenches.

Volcanic Geology Introduction

Our reappraisal of the Wau district and its environs has resulted in recognition for the first time of a sequence ofunwelded ignimbrites, several endogenous domes, and a maar volcano (Figs. 3 and 4). Hydro- thermal activity was associated with dome emplace- merit and maar formation. These Pliocene volcanic

units and related sedimentary rocks, of which only the maar volcano is restricted exclusively to the Wau district (Fig. 4), overlie, and partly cut, the Kaindi Metamorphics described above.

Early volcanism

A widespread sequence of volcanic rocks, up to several hundred meters thick, was mapped in parts of the Morobe goldfield and beyond by Dow et al. (1974) and previous workers (Fig. 3), and was des- ignated as the Bulolo Agglomerate. North of Wau, the sequence dips eastward at 40 ø to 65 ø . The sequence in the Wau district and its environs is, however, dominated by massive, unbedded, and unwelded ignimbrite, • which carries uncollapsed pumice fragments and accessory lithic clasts, including Kaindi Metamorphics and Morobe Granodiorite, in an ash matrix. Interbedded sediments were also recognized, including a distinctive boulder conglomerate in Namie Creek, northeast of the mine area (Fig. 4). Page and McDougall (1972) presented apparent K-Ar ages of 3.7 to 3.2 m.y. for the ignimbrite eruption with 3.5 m.y. selected as a preferred maximum age, although

• Terminology used in this report for volcanic rocks is process oriented and wherever possible follows Sparks and Walker (1973) and Wright et al. (1980).

it should be stressed that their sampling sites were 20 km northwest of Wau. The source of the pyroclastic flows that generated the unit, which we pro- pose to rename the Bulolo Ignimbrite, remains un- proven, although eruption from, or collapse of, early endogenous domes (see Smith and Roobol, 1982) is a distinct possibility and would fit well with the timing of dome emplacement summarized below.

In the Wau district, the Bulolo Ignimbrite is overlain, with little sign of discordance, by another re- gionally extensive unit, the Otibanda Formation (Fig. 3), which comprises poorly sorted fiuviatile conglomerates, sandstones, and reworked tuffaceous material, the last particularly abundant immediately above the Bulolo Ignimbrite. Thin ignimbrite flows carrying fragments of carbonized wood were also observed as interbeds in places. In the Koranga crater 2 area (Fig. 4), Kaindi Metamorphics, including garnetiferous schist, dominate clast lithologies and are accompanied by isolated fragments of wood and, according to Plane (1967), vertebrate remains. The Formation is worked for alluvial gold in the Koranga crater area and elsewhere. Plane (1967) determined that this part of the Otibanda Formation was deposited as coalescing fans along the margin of an intermontane lake. Page and McDougall (1972) concluded that the Otibanda For- mation was deposited not more than 3.5 to 3.1 m.y. ago.

Two and possibly three varieties of biotite-hornblende andesite to dacite porphyry intrusive stocks-- the Upper, Unclassified, and Lower Edie Porphy- ries-were distinguished in the Morobe goldfield by Fisher (1945) and most subsequent workers. All major bodies of Edie Porphyry are now recognized as endogenous domes (Fig. 3), some of which (see below) have undergone only limited erosion. Parts of domes exhibit well-developed flow foliation, and a more deeply eroded one located in Upper Edie Creek (Fisher's Unclassified Porphyry) is characterized along parts of its steep contacts by spectacular hydrothermal breccias which carry fragments of both dacite porphyry and Kaindi Metamorphics in a rock flour matrix. Lithologically similar hydrothermal breccias locally border restricted dikelike bodies of biotite dacite

porphyry, which intruded the Kaindi Metamorphics along and immediately west of the major fault zone depicted in Figures 3 and 4. These truly intrusive dacite porphyries commonly carry disseminated and veinlet pyrite, and in places in the fault zone they are sericitized and silicified.

We are unable to confirm the sequence of dacite

ß 2 A name introduced at least 50 years ago (N.H. Fisher, written commun., 1983) because of the craterlike form of the area. Most workers (Dow et al., 1974; Fisher and Branch, 1981; Lowenstein, 1982) considered it as a site of Recent volcanic activity, whereas we favor activity of hydrothermal origin (see below).


porphyry emplacement proposed by Fisher (1945) which, on the basis of our mapping, also appears to derive little support from the detailed K-Ar dating study conducted by R. W. Page (Page and McDougall, 1972; Dow et al., 1974). His biotite ages for porphyry samples from the Morobe goldfield range from 4.2 to 2.4 m.y. Several corresponding plagioclase separates yielded older ages, apparently a result of in- corporation of various amounts of excess radiogenic argon (Page and McDougall, 1972). In the absence of more precise data, we accept his range of biotite ages as broadly representative of the time interval during which dome emplacement took place, given that the flow-banded dacite porphyry which yielded the youngest age of 2.4 m.y. is not a late feature, as suggested by N. H. Fisher, but is an integral part of an endogenous dome (Fig. 4)--Fisher's (1945) Lower Edie Porphyry. The age of dike intrusion along the major fault zone is unknown but tentatively correlated with early stages of dome emplacement.

Maar-diatreme formation Maar formation at Wau was localized 600 to 1,000

m east of the trace of a major dip-slip fault zone, marked at surface by both tight gouge and open-fault breccia. The northwest-striking fault plane is inclined eastward at about 40 ø (Fig. 5) and is partly exposed as faceted spurs (Fig. 4). Fault movement is believed to have both pre- and postdated maar formation, which therefore probably took place on a steep, east- erly inclined palcoslope comparable to that in exis- tence today (Fig. 3). Continuing displacement may be evidenced by the major landslide along the southern part of the fault in Figure 4. The progressive decrease in dip of the Bulolo Ignimbrite-Otibanda Formation succession northeast of the Wau area is

interpreted as a result of fault-induced tilting which was largely completed prior to maar formation.

The oldest recognizable rock unit directly associated with generation of the Wau maar is a distinctive lithic breccia long known as the Namie Breccia. It is preserved 200 to 300 m west of the maar at Upper Ridges and, partly as landslide material, some 600 m south of the maar (Figs. 4 and 5). At both localities it unconformably overlies a highly irregular surface cut across Kaindi Metamorphics. Thicknesses in excess of 200 m have been intersected during drilling at Upper Ridges but are suspected to be partly a consequence of repetition of the succession by fiat faults (Fig. 5). The Namie Breccia is composed of angular to rounded fragments of Kaindi Metamorphics and dacite porphyry set in a gray-colored matrix of finely comminuted rock material (Fig. 6A). Dacite porphyry clasts include recognizable pumiceous material of either juvenile origin or derived from the Bulolo Ig- nimbrite, and fiow-foliated dome material, which might suggest the presence of an endogenous dome in the Wau area prior to maar formation. Fragments are mainly <10 cm in size but locally attain >30 cm. Disseminated pyrite, at least partly of clastic origin, is ubiquitous in the rock flour matrix.

The Namie Breccia is dominated by a coarse, poorly stratified and poorly sorted facies (Fig. 6A) but contains interbeds of a fine, silty, and well-stratified facies (Fig. 6C). The poorly stratified parts correspond to the explosion breccia of Wohletz and Sheridan (1983) and accumulated mainly by ballistic fall of ejecta. The well-stratified parts are thinly bedded and characterized by well-developed low-angle cross stratification (Fig. 6C), as well as horizons carrying abundant accretionary lapilli (Fig. 6B), which exhibit the characteristic concentric internal structure. The ac-

cretionary lapilli unambiguously testify to a subaerial origin for the well-stratified Namie Breccia and were probably formed by progressive accretion of numerous layers of wet volcanic particles around water

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FIG. 6. Selected geologic features of the Wau gold deposit, Papua New Guinea. A. Varieties of coarse-grained Namie Breccia from drill hole WD-14 (150-200 m), Upper Ridges. Pale fragments are dacite porphyry and sericite schist, and dark fragments are phyllite, the last dominating in the lowest of the three co•e samples. B. Accretionary lapJill in Namie Breccia, Upper Ridges. C. -I•ow-angle cross stratification of base surge origin in fine-grained Namie Breccia, Upper Ridges. D. Mesoscopic recumbent fold formed by slumping of pyritic mudstones in the intramaar sequence. Dark-colored beds are rich in syn- or diagenetic pyrite. Sample from vertical drill hole WD-2 (72 m), Golden Peaks. E. Flow foliation in the dacitic dome from the northwestern side of the maar. F. Crumble breccia from the apron flanking the dacitic dome along the northwestern side of the maar.

645


globules. The presence of accretionary lapilli and low- angle cross stratification, in combination with features such as penecontemporaneous slumping and chan- neling of beds and lack of sag structures beneath large fragments, accords well with an origin for the well- stratified Namie Breccia as pyroclastic surge deposi.ts of the low-temperature base surge type. Base surge deposits are ubiquitous products of maar-forming phreatomagmatic or phreatic eruptions (Moore, 1967; Fisher and Waters, 1970; Lorenz, 1973). The accretionary lapilli may have formed in eruption columns and then fallen as a component of ash or, more probably, grew in the base surges themselves (Lorenz, 1974; Self and Sparks, 1978). The presence of Namie Breccia up to at least 1.6 km from the Wau maar (Fig. 4) agrees well with Wohletz and Sheridan's (1979) observation that the maximum radial distance attained by pyroclastic surge deposits approximates the vent diameter.

Since the Namie Breccia is present largely beyond the maar, it is interpreted as an erosional remnant of a maar-encircling tuff ring which was originally of substantially greater extent. The observed remnants of the tuff ring were preserved by being let down on the regional fault zone, a mechanism which would also have contributed to the steep south-westward dip of the Namie Breccia (Fig. 4). Smaller bodies of Namie Breccia at Golden Peaks and Golden Ridges (Figs. 4 and 5) are interpreted as blocks that slid into the maar at a later date (see below). We therefore refine previous interpretations of the Namie Breccia as a volcanic breccia (Fisher, 1944, 1945; Fisher and Branch, 1981) or as a diatreme breccia (Dow et al., 1974), and reject Rebek's (1975) concept of it as a talus accumulation partly reworked by water.

The volcano-sedimentary sequence within the maar is incompletely known because of its structural com- plexity and the effects of widespread hydrothermal alteration. The sequence occupies a surface area of approximately 1.4 X 1.4 km (Fig. 4) and defines the extent of the maar. Available evidence, both at surface and from drill holes, shows that the intramaar sequence is delimited by a ring fault which dips at 50 ø to 70 ø (Figs. 4 and 5) and truncates the regional strike and dip of the Bulolo Ignimbrite and Otibanda Formation. The intramaar sequence dips centripetally at angles greater than 30 ø , with vertical and overturned (Fig. 6D) beds present locally. Inward dips and associated structural complexities resulted not from regional tectonism but from subsidence and slumping of unconsolidated, partly water-saturated beds during accumulation of material within the maar.

The intramaar sequence is both pyroclastic and epiclastic in origin and is estimated to exceed 200 m in thickness. The pyroclastic part is dominated by a second unit of lithic breccia of ballistic fall and base

surge origin. It is easily distinguished from the Namie

Breccia by the dominance of dacite porphyry clasts, the relative paucity of Kaindi Metamorphic clasts, and the much paler rock flour matrix. It is characterized by the same clast sizes and degrees of clast rounding as the Namie Breccia and also possesses prominent fine-grained, well-bedded horizons with accretionary lapilli. Since these dacite porphyry-rich breccia deposits are absent from the preserved parts of the tuff ring and were observed to overlie a small allochthonous block of Namie Breccia within the maar

(Fig. 5), they are attributed to later phreatomagmatic or phreatic explosive activity, with its products ac- cumulating mainly within the confines of the maar.

The epiclastic part of the intramaar sequence is interbedded with and overlies the pyroclastic component, and ranges from reworked breccia through grits and sandstones to finely laminated mudstones, the last particularly prominent and widespread. The mudstones include horizons rich in plant matter as well as cherty horizons with bedded pyrite of syn- or diagenetic origin (Fig. 6D). Travertine and sinter deposited as hot spring aprons are also poorly exposed at two localities (Fig. 4) within the maar, although it is not certain whether they are part of the intramaar sequence or unconformably overlie it.

Two of the principal endogenous domes in the Mo- robe goldfield abut the Wau maar (Figs. 3 and 4) and were emplaced during its development. The larger one, to the northwest of the maar, is composed of unaltered biotite-hornblende dacite pori•hyry and exhibits well-developed flow foliation which dips in various directions, but commonly inward, at 50 ø to 60 ø (Fig. 6E). The smaller dome, on the maar's southern contact, is a biotite-hornblende andesitc porphyry and is characterized by vertical flow foliation close to its contacts, which are marked by gouge development in places. Two much smaller outcrops of altered and fiow-foliated andesitc porphyry on the western side of the maar (Fig. 4) appear to be parts of buried domes which were emplaced somewhat earlier than the two principal masses. Sills and dikes of altered dacite to andesitc porphyry cutting both the intramaar sequence and the Namie Breccia are believed to be offshoots of the domes. The only con- cordant K-Ar ages on coexisting plagioclase and biotite obtained by Page and McDougall (1972) in the Mo- robe goldfield were from a sample taken where lower Edie Creek is incised into the dacitic dome (Fig. 4). Their two ages of 2.4 _+ 0.1 m.y. effectively date emplacement of the larger of the two domes, which followed the explosive stage of maar formation and probably much of the intramaar sedimentation (see below).

The southeastern side of the dacitic dome, which originally constituted a steep rim to the maar, is flanked by a thick and extensive apron of dacitic breccia (Fig. 4), which is believed to have accumulated


by spalling off of the cool outer crust of the dome during its continued growth--talus or crumble breccia. If such breccia originally flanked the northwestern side of the dacitic dome, it has since been removed by the deeper erosion along lower Edie Creek, The breccia near the dome margin is monomict and clast supported, and comprises nonvesiculated blocks of dacite porphyry up to at least 2 m across (Fig. 6F). Clast size decreases, and the amount of ashy matrix increases, away from the dome, in which direction it is intercalated with the epiclastic component of the intramaar sequence. Pyroclastic rocks erupted from the domes flanking the maar were not recognized, and the ignimbrite mapped east of Golden Peaks is part of an exotic slide block (Fig. 4; see below).

Our work at Wau has documented the importance of exotic slide blocks within, and especially overlying, the intramaar sequence. Most of the identified slide blocks are composed of Kaindi phyllite (Fig. 2), partly overlain by Namie Breccia (Fig. 4). The largest block measures 500 X 450 m at surface (Fig. 4) and was shown in one place to be 120 m thick (Fig. 5). They are important as the source of all gold ore mined at Golden Ridges and Golden Peaks. These Kaindi Metamorphic-Namie Breccia blocks clearly originated from Upper Ridges and, on the basis of the position of the Kaindi-Namie contact and fault pat- terns, may be assigned to their approximate original positions (see below; Fig. 9). Since the maar was constructed on a steep mountain side, the western wall of the maar crater, still partly preserved below Upper Ridges (Figs. 2, 4, and 5), was probably the most unstable and therefore provided the majority of the landslide blocks. However, some landsliding also took place from other parts of the crater walls, as supported by recognition of a block of Bulolo Ignimbrite east of Golden Peaks and a block of Otibanda Formation

north of Koranga Crater (Figs. 4 and 5). The slide blocks became detached from the unstable crater

walls and subsided into the maar on low-angle faults, marked by substantial zones of crushing and shearing beneath the blocks (Fig. 7A).

It is difficult to equate the intramaar sequence with the stratigraphic schemes of previous workers, although it appears to be equivalent to Rebek's (1975) Early Volcanics, and part of it was probably assigned to the Koranga Volcanics of Dow et al. (1974) and to the volcanic products of the Koranga crater by Fisher and Branch (1981).

Late sedimentation

Two stages of fiuviatile sedimentation took place in the Wau district after the formation of the maar

was essentially complete. The earlier, and topo- graphically higher, is represented by a series of erosional outliers up to at least 50 m thick which are

the remnants of one or more stream channels and a

restricted lake (Fig. 4). Stream channels were incised into the intramaar sequence and are occupied by conglomerates carrying a significant proportion of Kaindi Metamorphic (including garnet-bearing schist) and Morobe Granodiorite clasts. They are therefore interpreted as products of an externally derived drainage system. In the Golden Peaks area (Fig. 4), the unit comprises well-stratified conglomerates, sandstones, and mudstones which were rapidly deposited where one or more streams entered a small lake in

the northern part of the maar. The principal remaining outcrop of the fiuviolacustrine beds is recumbently folded and crosscut by a northeast-trending fault (Figs. 4 and 5). Although the origin of these two structures remains uncertain, their position along the leading edge of an exotic block of Bulolo Ignimbrite suggests that they are due to very late sliding of the block. Prominent clasts of acid-leached rock, including opal, confirm that the fiuviolacustrine beds postdated alteration and mineralization. A remarkable feature of the fiuviolacustrine beds is the abundance of transported woody material, some of it partly pyritized (Fig. 7B). Logs up to several meters long, twigs, and leaves are all present. An earthy, but microcrystalline, dark blue mineral is sparsely scattered through the unit and was identified using X-ray diffraction analysis as the iron phosphate vivianitc. The channel fill and fiuviolacustrine beds are all sluiced for their alluvial

gold content. Radiocarbon dating by the U. K. Atomic Energy'Authority, Harwell, of a sample of little- weathered wood which we collected from the flu-

violacustrine beds at Golden Peaks yielded an age of •42,000 yr, the limit of the analytic procedure employed. In view of the special care taken because of the suspected antiquity of the sample, this minimum age is preferred to that of 34,000 yr, which was determined previously at the Australian National Uni- versity, Canberra, for wood collected from the same locality (H. A. Polach, in Fisher and Branch, 1981).

A similar fiuviolacustrine sequence is present in Upper Edie Creek (Fig. 3), where it is incised by the present drainage. It also contains abundant woody material and scattered vivianitc, and continues to provide rich alluvial gold. Wood collected from this unit also proved too old to be dated by the radiocarbon method and, like that in the fiuviolacustrine beds at Golden Peaks, has a minimum age of •42,000 yr.

The second and younger fiuviatile sequence, also worked for its alluvial gold content, occupies an extensive area to the southeast and east of the maar

(Fig. 4), where it constitutes a broad piedmont fan. It was constructed at the mountain front as a result

of renewed uplift (Fisher, 1944) on the northwest- trending fault zone and is incised by the present drainage.

ß

c

12t I I t I t I

FIG. 7. Additional selected geologic features of the Wau gold deposit, Papua New Guinea. A. Zone of intense low-angle shearing developed in the intramaar sequence (light) beneath hydrothermal eruption breccia (dark) along the eastern side of the principal slide block, Golden Ridges. B. Fragment of partly pyritized wood in fluviolacustrine beds, Golden Peaks. C. Fragment of calcite-manganocalcite vein material in hydrothermal breccia cutting Namie Breccia in drill hole WD-14 (102 m), Upper Ridges. Each small division of the scale equals I cm. D. Bedded hydrothermal eruption breccia, Golden Ridges. E. Crustiform banding in a gold-bearing calcite-manganocalcite-quartz vein, Upper Ridges. F. Opaline silica horizon developed in the intramaar sequence exposed in collapsed benches of Golden Peaks open cut.

648


Hydrothermal Brecciation, Gold Mineralization, and Alteration

Hydrothermal brecciation

Several bodies of breccia, clearly distinct from the Namie Breccia, were mapped at Wau in the three centers of gold mineralization at Upper Ridges, Golden Ridges, and Golden Peaks, and also in the Koranga crater area (Fig. 4). The breccias in the first three areas were not distinguished from Namie Brec- cia by previous workers, and those at Koranga crater were either not recognized at all or considered as volcanic breccia.

Although the original extents of breccias at Upper Ridges, Golden Ridges, and Golden Peaks have been severely reduced by open pit mining, their overall geometries may still be deduced from existing remnants (Fig. 4), old records, and the recent diamond drilling. Overall geometric relations permit subdi- vision into massive crosscutting breccia bodies and weakly stratified accumulations of subaerial breccia (Fig. 4). Both types are present at Golden Ridges and Golden Peaks, but only crosscutting breccias have been recognized at Upper Ridges and only subaerial breccias at Koranga crater.

Both breccia types at Upper Ridges, Golden Ridges, and Golden Peaks are similar and characterized by their poorly lithified nature. They comprise a chaotic assemblage of angular to rounded fragments of Namie Breccia and Kaindi Metamorphics set in a muddy and often plastic matrix. Fragments of gold-bearing calcite, manganocalcite (largely oxidized to manganese oxides), and quartz comprise up to 5 volume percent of both breccia types (Fig. 7C). The Koranga crater breccias are lithologically distinct and carry prominent fragments of intramaar pyroclastics, Morobe diorite, silicified and pyritized material (including wood), and balled-up plastic mud. All breccias contain fragments up to 1 m or so in size, but lack any essential (juvenile) material.

The crosscutting breccias occur as a ramifying system of irregular pods and veins which, especially at Upper Ridges, preferentially follow a series of gently dipping planes, some of which are clearly faults (Figs. 4 and 5). These bodies transect mainly the Namie Breccia but are locally hosted by Kaindi Metamor- phics. At Upper Ridges, breccia bodies extend to depths of • 100 m, and reinterpretation of old records suggests that at greater depths they steepen and are transitional to veins (Fig. 5). The subaerial breccias unconformably overlie Namie Breccia or are sepa- rated by low-angle zones of shearing from either Kaindi Metamorphics or the intramaar sequence. At least at Golden Ridges, the subaerial breccias attain a thickness of 60 m and are made up of thick, nearly fiat beds defined on the basis of different clast lith-

ologies (Fig. 7D).

On the basis of their lithologic and geometric characteristics, the breccias are assigned a hydrothermal origin. The subaerial veneers of breccia are interpreted as remnants of composite aprons of ejecta accumulated around shallow vents which were fed by the crosscutting bodies. The subaerial veneers are therefore classifiable as hydrothermal eruption breccias (Lloyd, 1959) or hydrothermal explosion breccias (Muffler et al., 1971); the former name is employed here in conformity with current usage by most workers in New Zealand (e.g., Hedenquist, 1983). The examples described by Muffler et al. (1971) in the Yellowstone geothermal field, Wyoming, and comparable occurrences in the geothermal systems of the Taupo volcanic zone of North Island, New Zealand (Lloyd, 1959; Cross, 1963; Nairn and Wiradiradja, 1980; Hedenquist, 1983), were generated during the last 15,000 yr, and the formation of some has been observed. They were generated by shallow hydrothermal eruptions which caused aprons of ejecta to accumulate around well-defined hydrothermal eruption craters; these craters were from several tens of meters to •1 km in diameter and up to 100 m deep.

In the Golden Ridges and Golden Peaks areas, the hydrothermal breccias are restricted to Namie Breccia and Kaindi Metamorphics in the landslide blocks and, moreover, do not contain any clasts of the subjacent pyroclastic and epiclastic material from the intramaar sequence. The breccias in these two areas are therefore believed to have been generated in spatial con- tinuity with the Upper Ridges breccias and to have subsided as parts of the landslide blocks. The precise age of brecciation is unknown but clearly postdated initial maar formation and predated the late landsliding event.

Development of a fumarolic vent at Koranga crater in May 1967 followed a major landslide which occurred during the night. Although activity is widely believed to have been restricted to landsliding (Pi- gram et al., 1977), we tentatively suggest that the main event could have been a hydrothermal eruption, with the landslide merely being a consequence of oversteepening of the valley side during accumulation of water-saturated breccia. This reinterpretation of the event is based entirely on the presence of poorly stratified muddy breccias in the vicinity of the vent, since there were no reliable witnesses and hydrothermal eruption is not accompanied by loud noise. Examination of the area several days after the inferred eruption revealed the presence of SO2-rich fumaroles at temperatures of 600 ø to 700øC (Pigram et al., 1977), the site of which is now marked by encrus- tations of native sulfur. We suggest that these gaseous emissions could have been an aftermath of the inferred

hydrothermal eruption, although Pigram et al. (1977) invoked spontaneous combustion of carbonaceous matter and pyrite as an explanation.


The Koranga crater area, which is believed to have been the site of hydrothermal eruptions prior to 1967, is presently delimited on three sides by stepped walls, produced by recent subsidence, and is breached on the fourth as a result of the 1967 landslides. It contains

a small cold water pond and has now cooled com- pletely following the 1967 activity. One of the earlier eruption breccias, in the southern part of the Koranga area, carries abundant fragments of Morobe diorite, a rock which does not crop out around the crater and is only known from a small inlier 400 m southwest and 40 m lower in elevation. It is concluded that the

diorite fragments were carried upward from beneath the crater area by hydrothermal eruptions.

Gold mineralization

The principal hydrothermal gold mineralization known in the Wau district is closely associated with the hydrothermal breccias at Upper Ridges, Golden Ridges, and Golden Peaks (Fig. 4). The high-grade mineralization (10-30 g/metric ton) is present mainly in gently dipping, grossly lenticular lodes, which are up to 10 m thick and 300 m long but are generally rather discontinuous. These appear to have been emplaced within or at the base of hydrothermal breccias, apparently of both subsurface and subaerial origin. The major veins appear to be overlain by one or more less extensive veins and are partly paralleled by preexisting, sill-like bodies of dacite porphyry.

In an unoxidized state, the veins carry calcite, manganocalcite, and quartz, along with generally mi- nor pyrite, rhodochrosite, sphalerite, galena, chal- copyrite, tetrahedrite, and silver sulfosalts (see Rebek, 1975; Lowenstein, 1982). The carbonates commonly exhibit crustiform banding (Fig. 7E) and cockade structure, and the quartz is white or transparent and normally vuggy. Hypogene gold in the veins is present in the native state and is probably 500 to 600 fine; the overall Ag/Au ratio of the veins is 3:1 (Lowenstein, 1982). Much of the ore is oxidized and is believed by Lowenstein (1982) to have undergone supergene gold enrichment as an accompaniment to the trans- formation of manganocalcite and rhodochrosite to wad and crystalline manganese oxides.

Gold mineralization of lower grade but commonly minable in bulk, with grades of up to 5 g/metric ton of gold, occupies substantial volumes of hydrothermally brecciated rock. The gold is present, with the same gangue assemblage as in the main lodes, in either short multidirectional veinlets up to several centimeters wide above the main lodes, or in vein fragments within the breccias.

At Upper Ridges, the gently dipping, breccia- hosted lodes are transitional downward to steep, albeit mineralogically similar, vein structures. These are similar to the main veins exploited previously at Upper

Edie Creek (Fig. 3). There the veins, including many less persistent veins and veinlets, mainly transect the Kaindi Metamorphics (Fisher, 1939; Rebek, 1975) in an area adjacent to an endogenous dome, although locally they cut the dome itself. The veins constitute a northwest-trending belt parallel to the regional fault zone at Wau (Fig. 3). The Upper Edie Creek veins carry quartz, calcite, and manganocalcite, as at Wau, but contain a greater abundance of silver sulfosalts (Lowenstein, 1982).

The intrusive dacite porphyries, and their imme- diate metamorphic wall rocks, along the regional fault zone west of Upper Ridges, are locally pervasively sericitized and, at the Ribroaster mine (Fig. 4), have been worked for gold. There, up to 10 volume percent sulfides accompany intense sericitic alteration localized by a cross structure. The sulfide assemblage in- cludes important pyrite, pyrrhotite, and arsenopyrite, and the gold is 750 fine (Lowenstein, 1982); both features are in marked contrast to the Wau and Upper Edie Creek lodes.

Gold mineralization at both Wau and Upper Edie Creek is believed to have been generated during dome emplacement and maar formation (Fig. 8). If Page and McDougall's (1972) biotite ages are accepted (see above), then mineralization probably falls within the 4- to 2.4-m.y. interval. At Wau, however, gold introduction preceded sliding of Kaindi Meta- morphic-Namie Breccia blocks, as shown by the abrupt truncation of gold mineralization at their sheared bases.

As noted above, alluvial gold in the Wau district is present locally as economic concentrations in all the clastic sedimentary units distinguished during the mapping, namely the Otibanda Formation deposited prior to maar formation, the fiuviolacustrine beds and channel conglomerates, and the piedmont fan accumulated after maar formation. The sources of gold in these alluvial deposits have yet to be precisely determined, but in the postmaar units it is probably largely derived from hydrothermal gold associated with dome emplacement and maar formation, as supported by the relatively low fineness (558-611) documented in the Koranga crater area (Fisher, 1975). Gold introduced during Morobe Granodiorite intrusion (Fisher, 1945) in the mid-Miocene, or even during metamorphogenic quartz segregation in the Oli- gocene-early Miocene, is probably the main source of Otibanda gold, which is reported to average 876 fine elsewhere in the Morobe goldfield (Lowenstein, 1982).

The major proportion of the gold produced from the Morobe goldfield was dredged from the Bulolo alluvials (Fig. 3) and was derived by erosion of the Wau and Upper Edie Creek lode gold deposits (Fisher, 1945, 1975).


I . - -- Dome emploccment

...... Buloio Ignimbrite

- Otibanda Formotion

--- ß Regionc•l tilting

I I ? - Gold mincroiizotion

-- •./? MAAR-FOI•MING EVENTS I Hydrothcrmol br½cciotion c•t Korcmgo ß

4 J 2 ! O

- • - Declte porphyry intrusion, Ribroaster gold rn•neralizet•on - Dletrerne/rneer/tuff-ring forrnetmn

Hear sedirnentotlon

MAAR - FORMING - - Hydrotherrnel brecc•et•on + gold rn•nerohzotion -•-Deep hydrothermel olterobon m moor

[VœNTS .•.. • Dome ernp•scernent b. - - Descent of shde blocks

- Generobon of oc•d-leached zone Fluvmble sed•rnentnbon with (111uv•el •old

FIG. 8. (a) Generalized time sequence of volcanic and hydrothermal events at Wau and vicinity. K-Ar age control from Page and McDougall (1972), (b) Amplification of events associated with the formation of the Wau maar.

Hydrothermal alteration

In contrast to the gold mineralization at the Ri- broaster mine, which is associated with intense sericitic alteration, the main gold lodes at Wau lack any noticeable associated alteration effects, although mi- nor kaolinite and sericite were recorded locally. Nev- ertheless, two broad varieties of hydrothermal alteration essentially devoid of significant gold values are recognized in association with the maar.

A zone of feldspar-destructive alteration, some 1.5 by up to 0.9 km at the surface (Figs. 4 and 7F), occupies internal parts of the maar and was developed at the expense of the intramaar sequence and, locally, of the main andesitic dome. It occupies only high ground and has clearly been largely removed by erosion and open pit mining from the lower Golden Peaks and Golden Ridges areas. The alteration zone does not appear to exceed 70 m in thickness and is characterized at its base by a prominent flatly dipping zone of silicification composed of opal, giving a dis- ordered cristobalite (opal CT) X-ray pattern. The opal is locally inverted to chalcedony. Silicification is accompanied by up to 20 volume percent of partly col- loform, fine-grained pyrite and marcasite that are largely oxidized to supergene jarosite and hematite. Feldspar-destructive alteration above the opalized

horizon is characterized by cristobalite, kaolinite, alunite, and much smaller amounts of iron sulfides, all confirmed by X-ray diffraction analysis of representative samples. Cristobalite-rich rock is porous and low in density.

In the central part of the altered area, restricted to the intramaar sequence around Golden Peaks and Golden Ridges, a second variety of feldspar-destructive alteration is found patchily to depths exceeding 300 m. Samples examined by X-ray diffraction are composed of quartz, calcite, kaolinite, and smectite, and obviously lack alunite. A pale green micaceous mineral that is present locally was determined to be a mixed layer illite. The alteration is associated with veinlets of chalcedony or calcite carrying pyrite.

Field evidence supports hydrothermal alteration at several times during maar formation (Fig. 8). The deep feldspar-destroying alteration appears to be partly early, because drilling has shown that it does not extend upward into either the dacitic crumble breccia or the Kaindi Metamorphic-Namie Breccia slide blocks. In contrast, the near-surface opalized horizon and overlying cristobalite-kaolinite-alunite alteration resulted from a late-stage event which clearly postdated descent of gold-bearing slide blocks but predated deposition of the fiuviolacustrine beds. The cristobalite-bearing rocks have been shown to


contain <0.005 ppm Au, except where they were developed at the expense of auriferous hydrothermal breeeia.

Volcanic and Hydrothermal Development

In the Morobe goldfield, hydrothermal gold deposition accompanied emplaeement of endogenous domes, the earlier of which may have given rise to pyroelastic pumice- and ash-flows preserved as the Bulolo Ignimbrite. On the basis of Page and Me- Dougall's (1972) radiometric ages, the overall maximum time span for gold mineralization was 4 to 2.4 m.y. (Fig. 8). At Wau itself, the initial generation of the maar and underlying diatreme preceded gold mineralization, which is thought to have taken place prior to emplaeement of the youngest dated daeitie dome 2.4 m.y. ago. Filling of the maar, including eraplacement of landslide blocks, was completed before deposition of the fluviolaeustrine beds at an unknown time >42,000 yr ago.

Following accepted models of maar-diatreme formation (e.g., Lorenz, 1973), the Wau system is believed to have been generated by a series of hydrovolcanic (phreatie or phreatomagmatie; Sheridan and Wohletz, 1981) explosions triggered by interaction of hot andesitie-daeitie magma and cool meteoric water. The choice between a phreatie and a phreato-

magmatic (carrying juvenile material) origin for the Wau maar is difficult to make because it is unclear

whether dacite porphyry clasts are juvenile (cognate lithics) or not (accessory lithics). In common with many diatremes (e.g., Self et al., 1980), the Wau diatreme is believed to have been localized by the nearby regional fault zone, which dips eastward at about 40 ø beneath the maar and should therefore be present at a depth of about 1,000 m beneath it (Fig. 8). A per- meable fault zone would provide ideal conditions for descending ground waters to encounter ascending bodies of magma. Early batches of this magma, which rose along the fault zone, are believed to be represented by the dikelike bodies ofdacite porphyry west of Upper Ridges (Fig. 9). Following the main period of hydrovolcanic activity, residual magma depleted in volatiles by explosive eruption was emplaced more than once as viscous domes, with intrusion apparently partly controlled by the diatreme walls. The tuff ring and parts of the intramaar sequence were constructed of ballistic fall and base surge deposits, the latter resulting from laterally directed blasts of turbulent, expanded gas-solid dispersions (Moore, 1967; Sher- idan and Wohletz, 1981). The tuff ring probably ex- tended some 2 km from the perimeter of the maar, but because of its poor preservation potential, it is now largely destroyed.

RIBROASTER-'FYI• GOLD

DEGRADED TUFF-RING

HYDROTHERMALLY BRECCIATED CONDUITS WITH AUTOCHTHONOUS RECONSTRUCTED GOLD DEPOSIT POSITION OF

/ PRINCIPAL SLIDE-BLOCI•

.... ;'/; OVERTUItNED / • ENDOGENOUS ALLOCHTHONOUS FLUVIO•STRINE BEOS / DOME •OLI) DEPOSIT

HYOROTHERMAL BUU:ILO I•NiMBRITE ERUPTION SLIDE BLOCK

PRINCIPAL SLIOE BLOCK OPALINE SILICA

DIATREHE'.

FIG. 9. Partly reconstructed schematic section of the Wau maar-diatreme system to illustrate aspects of the genetic model discussed in the text. Most of the legend as in Figure 4.


During or immediately following initial explosive activity, the ring fault would have formed and subsidence of the enclosed rocks, including parts of the tuff ring, probably took place (e.g., Hearn, 1968; Lorenz, 1973). Material which subsided at this time may have been disaggregated by subsequent explosions, although the Namie Breccia block--intersected by drilling at a depth of 300 m within the maar-- survived. Once subsidence had commenced, the crater would have undergone progressive expansion by slumping and sliding from its unstable walls.

Hydrothermal brecciation and gold introduction were essentially coeval and took place within the tuff ring and underlying Kaindi Metamorphics of the Up- per Ridges area. In this wedge-shaped body of rock, between the regional fault zone and the maar ring fault, gently dipping extensional faults were generated as this poorly restrained wedge of rock tended to slip downward toward the maar (Fig. 9). Furthermore, the regional fault zone and associated subsidiary structures were ideally located to act as a plumbing system for the ascent of hydrothermal fluids from depth beneath the maar.

In conformity with the mechanism accepted for the genesis of comparable breccias elsewhere (Muffler et al., 1971; Henley and Thornley, 1979; Nairn and Wiradiradja, 1980; Hedenquist, 1983), hydrothermal brecciation at Upper Ridges is believed to have been a consequence of fluid overpressures generated beneath relatively impermeable cap rocks. Permeability was reduced as a result of the self-sealing of fissures by early deposition of gold-bearing calcite, manganocalcite, and quartz, which are now present as ubiquitous fragments in the hydrothermal breccias. Hy- draulic fracturing is thought likely to have been triggered by rapid reductions of confining pressure induced by faulting and sliding of material into the maar. Decompression in one or more fissures would have caused water to flash to steam, disrupt the fissure walls and confining rocks, and carry the resulting fragmental material to the surface. Continued violent discharge of fluids and ejecta caused entrainment of additional material from the walls of the ramifying system of conduits and their progressive widening during the course of the days, months, or years of intermittent eruptive activity. In contrast to the early hydrovolcanic stage, the loci of eruption were very much shallower (•100 m), more restricted, and lack- ing in any direct magmatic involvement.

As suggested for hydrothermal eruption vents in general (Henley and Thornley, 1979), those at Wau appear to have acted as effective channelways for the focused ascent of hydrothermal fluids during and immediately following brecciation. Although no stable isotope studies and only preliminary fluid inclusion studies of samples from Wau have been carried out to date, the fluids are believed to have been dilute

and dominated by meteoric water, in common with those inferred to have been responsible for similar epithermal precious metal mineralization elsewhere (e.g., Berger and Eimon, 1982), although some workers suggest that a crucial metal-bearing magmatic component may have been added (e.g., White, 1981). At Wau, any magmatically derived fluid must have been released during final crystallization of andesitic- dacitic magma bodies beneath the diatreme (Fig. 9). Gold mineralization on the upthrown side of the regional fault zone, at the Ribroaster mine, appears to have been emplaced at greater depth by fluids chan- neled up the fault. The fluids are thought to possess a overall parentage similar to those responsible for gold deposition at Upper Ridges, but they were more saline and in the boiling condition (M. Jones, written commun., 1983).

Since most of the major veins have now either been removed by mining or are inaccessible, it is difficult to ascertain the precise nature of their structural controls. However, all veins, including early ones fragmented by hydrothermal brecciation, were evi- dently produced by repeated opening and filling of laterally restricted, gently dipping fractures. As proposed above, fractures are thought to have been generated by extension of the rock wedge between the regional fault zone and the ring fault owing to incipient downward movement toward the maar (Fig. 9).

Explosive volcanic activity must have continued locally during early stages of sedimentation within the maar, because of the alternation of pyroclastic and epiclastic units, although higher horizons of the intramaar sequence tend to be dominated by epiclastic material. Maar sediments are mainly epiclastic, derived locally from within the confines of the crater, and accumulated partly in shallow ephemeral lakes around which vegetation grew profusely. On the northwestern side of the maar they intertongue with crumble breccia accumulated on the flank of the dac-

itic dome during its active growth. Hydrothermal fluids which debouched at this time gave rise to subaqueous chert-pyrite beds and possibly also to subaerial travertine and sinter. At least part of the deep quartz-calcite-kaolinite-smectite-(illite) alteration probably also dates from this period. The lack of gold in all these hydrothermal products suggests their formation from fluids different from those responsible for mineralization at Upper Ridges and Ribroaster.

The intramaar succession was deformed by continued subsidence and slumping, mainly after explosive activity had ceased, as documented for maars elsewhere (Lorenz, 1979). Detachment of rigid slide blocks from the unstable walls of the maar crater took

place after filling of the maar was essentially complete and may have caused deformation of parts of the intramaar sequence. Most of the recognized blocks originated on the oversteepened western wall, from


which the largest traveled for 600 m (Figs. 4, 5, and 9). Movement on the regional fault zone may have triggered the sliding. Hydrothermal breccias and gold mineralization at Golden Ridges and Golden Peaks underwent mechanical transport for distances of up to i km to their present positions as integral parts of slide blocks. The gently dipping veins at Golden Ridges and Golden Peaks became detached at this time from their steeper feeder veins, which are only present in the area of autochthonous mineralization at Upper Ridges.

In view of its planar, gently dipping form, the opalized horizon is interpreted to have been a boiling water table, with the suprajacent cristobalite-kaolinite-alunite assemblage representing acid leaching by dissolution of boiled-off H2S in cool ground waters (cf. Buchanan, 1981). The zone of acid leaching would have been represented at surface by steaming ground, and cristobalite-rich rock is interpreted as a product of steam leaching. Shallow-level alteration within the maar appears, at least in part, to have postdated arrival of the major mineralized slide blocks (Fig. 8).

After alteration had ceased, the maar crater was breached before 42,000 yr ago by regional drainage, and fluviolacustrine beds were deposited where an externally derived stream exited onto fiats in the middle of the maar. The lithologic and possible geo- chronologic similarities between alluvial gold-bearing fiuviolacustrine beds within the maar and at Upper Edie Creek suggest that the external drainage referred to above was an ancestral Edie Creek. Vivianite pre- cipitation and pyritization of wood in fiuviolacustrine beds at both localities is attributed entirely to diagenetic processes.

Epithermal gold deposition at Wau was clearly a shallow event, but it is not possible to determine the amount of cover lost to erosion since mineralization

took place. However, the preservation of subaerial hydrothermal eruption breccia at Golden Ridges and Golden Peaks strongly suggests erosion of only a few tens of meters of breccia at these localities. Although erosion rates are high under the tropical climatic re- gime prevailing at Wau, denudation has been mini- mized by both downfaulting on the regional fault zone and subsidence of the maar.

If our interpretation of the Koranga crater area is correct, hydrothermal brecciation was renewed at Wau, this time on the southeastern edge of the maar, following accumulation of the piedmont fan and perhaps even as recently as 1967. The fragments of massive chalcedonic silica in the Koranga breccia apron support the occurrence of self-sealing in depth but contrast with the dominance of carbonate fragments in the older breccias.

Acknowledgments Our work at Wau and elsewhere in the Morobe

goldfield was undertaken on behalf of Renison Gold-

fields Consolidated Limited. We thank R. A. Shak- esby, Exploration Manager, and J.P. McKibben, As- sistant Exploration Manager (South West Pacific), for permission to present this summary of the geologic aspects of the work. Useful discussions were held at various times with J.P. McKibben, R. M. D. Meares, D. Pascoe, and J. V. Wright. Reviews of the manu- script were generously provided by N. H. Fisher, P. Lowenstein, and R. W. Page.

August 16, 1983; January 16, 1984

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Wau Sillitoe Et Al 1985 p0638-p0655