JOURNAL O! GEOCHIMICAL EXPLORATION

ELSEVIER Journal of Geochemical Exploration 50 (1994) 409~,28

Structural controls and genesis of epithermal gold- bearing breccias at the Lebong Tandai mine,

Western Sumatra, Indonesia

David H. Jobson, Clive A. Boulter, Robert P. Foster Department of Geology, University of Southampton, Southampton S09 5NH, UK

(Received 31 August 1993; accepted after revision 28 October 1993)

Abstract

Lebong Tandai is a low-sulphidation, volcanic-hosted epithermal gold deposit of Neogene age, located within the foothills of the Barisan Mountains, Sumatra. To date, the mine has produced approximately 40 tonnes of gold and 400 tonnes of silver. The mineralisation is exclusively in the form of tabular quartz-cemented breccias bodies which are localised along faults. The breccias comprise angular to sub-rounded clasts of the wallrocks and earlier barren breccias cemented by banded or massive quartz, and in many instances, the clasts are supported within the quartz cement.

The sulphide minerals occur as either a single cockade band around the clasts in the breccia, or as polymineralic aggregates disseminated throughout the breccia cement. The main precious-metal- bearing phase is electrum, with silver-sulphosalts and silver-tellurides also present. Highly variable concentrations of pyrite, sphalerite, galena and chalcopyrite are associated with the precious-metal phases.

With the exception of two minor lodes, the mineralised breccias are localised along strike-slip faults which display changes in orientation indicative of D-, R- and P-shears and T-fractures, with individual segments ranging from a few metres to a few hundred metres in length. Two strike-slip fault systems are recognised, one sinistral, trending east-west and the other dextral, trending northwest, the latter of which is parallel to the Sumatran Fault System. The majority of gold and silver production is from breccias localised along faults formed during the sinistral tectonism. The breccias are believed to have been generated during compressional reactivation of the east-west sinistral strike-slip faults in response to the subduction of the Indian-Australian plate beneath Sumatra. Supralithostatic fluid pressures are a necessary pre-requisite for such reactivation, and the sudden drop in fluid pressure during reactivation is thought to have resulted in both the formation of the breccias by hydraulic fracturing, and the deposition of amorphous silica, precious metals and base metal sulphides. High rates of fluid flow subsequent to fracturing are thought to have led to fluidisation of the breccia clasts and abrasion to their current morphologies.

Microthermometry of fluid inclusions in sphalerite indicates that the mineralising fluids were of low salinity, less than 3 wt% NaClequ~va~e,t, and that mineralisation took place at temperatures of 260-

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410 D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409-428

280°C. Variations of salinity and homogenisation temperature due to boiling are poorly developed, although if boiling occurred, the metalliferous minerals would have been deposited early in the boiling process before the fluid had cooled appreciably.

1. Introduction

The Lebong Tandai gold deposit is located in the Bengkulu province of Sumatra, Indo- nesia, at latitude 03°01 '48.20" south, longitude 101°51 '39.19" east (Fig. 1 ). Gold and silver ores in the area have been expoited since the fourteenth century by both local and Hindu miners, although modem mining methods were not initiated until the Dutch company M.M. Simau commenced mining in 1910 (Van Bemmelen, 1949). During the period 1900 until 1940 the Bengkulu province was the most important producer of gold and silver in what was then the Dutch East Indies, with most of the gold production coming from the Lebong Tandai (then called Simau) and Lebong Donok mines. During this period, Lebong Tandai produced approximately 37 tonnes of gold and 422 tonnes of silver, with Lebong Donok producing slightly more gold (41 tonnes) and approximately half as much silver (228 tonnes). Gold production ceased at the start of World War Two, although copper was mined

2S0km

PLATE CONVERGENCE

N

!

Fig. 1. Location map of the Lebong Tandai mine, showing the locations of the Sumatran Fault System, the Sumatra Trench and the Lebong Donok deposit. The large arrow represents the direction of convergence of the Indian- Australian and Eurasian plates.

D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409~428 41 I

at Lebong Tandai by the Japanese u.ntil the end of the war. No large-scale mining was then undertaken until the mine was re-opened in 1983 by CSR Minerals Ltd. The mine is currently operated by P.T. Lusang Mining, and produces approximately 110,000 tonnes of ore per year at an average grade of 8.8 g/t gold and 42 g/t silver (Iskander, 1990).

Much of the previously published work was undertaken when the Dutch operated the mine, and is written in Dutch. A general account of the deposit was given by Van Bemmelen (1949) and more recently, the results of a preliminary structural appraisal of the mineral- isation were presented by Harris (1989).

2. Regional geology

Much of the geology of Sumatra is influenced by the past and present subduction of the Indian-Australian Plate along the Sunda subduction system (Hamilton, 1979). Relative to the Eurasian plate, the Indian-Australian plate is moving northwards, resulting in oblique subduction along the northwest-trending Sumatran sector of the arc. The most obvious products of subduction are the Barisan Mountains, a continental-based volcanic arc which extends the entire length of Sumatra and rises to altitudes in excess of 4000 m. A discontin- uous line of northwest-trending straight river valleys and intermontane depressions is trace- able for virtually the entire length of the Barisan Mountains, effectively defining the Sumatran Fault System, a series of dextral strike-slip faults which parallel the Sumatra trench, and lie some 300 km inland of it (Fig. 1 ). The Sumatran Fault System forms the northeast boundary of a prism of lithosphere, the other boundaries of which are the trench to the southwest, and the subducting oceanic crust at the base. The oblique subduction of the Indian-Australian plate beneath the Eurasian plate results in the whole prism moving northwestwards relative to the craton of northeast Sumatra and Malaysia (Hamilton, 1979), the movement being taken up along the Sumatran Fault System. Total offset along the Sumatran Fault System has been estimated at between 130 km and 400 krn. The Lebong Tandai mine is located approximately 20 km west of the Sumatran Fault System, within the allochthonous crustal block.

The oldest rocks in the mine are the Old Andesites of Van Bemmelen (1949), which are now termed the Hulusimpang and Painan Formations, and dated as Oligocene to Lower Miocene in age (Gafoer and Purbo-Hadiwidjoyo, 1986). They are overlain by a series of volcaniclastic conglomerates, agglomerates and tufts of Miocene age. The host rocks have undergone very little deformation, folding being rarely observed, and overall, the strata dip gently towards the northwest. Polymict volcaniclastic conglomerates are the most com- monly observed host rock. They comprise angular to rounded volcanic rock clasts contained in a generally fine-grained matrix. The clasts are usually poorly sorted, matrix supported, and range in size up to 15 cm. The youngest rocks observed are Pliocene to Pleistocene volcaniclastics which unconformably overlie the earlier strata, and apparently post-date the mineralised breccias.

Rhyolite occurs as vertically-oriented, dyke-like bodies up to I m in thickness, particularly at the west end of the mine. They are usually spatially associated with the mineralisation, often emplaced along the structures which have localised the orebodies. Their emplacement

412 D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409-428

clearly pre-dates the mineralisation, as blocks of rhyolite are observed within the mineralised

breccias.

3. Wallrock alteration

The wallrocks are typically silicified within 1-2 m of the mineralised breccias. Sericitic alteration, comprising pyrite, illite and mixed-layer illite-smectite, is dominant in zones 15-20 m wide around the orebodies. The mixed-layer illite-smectite comprises 75-80% illitic layers, and systematic variations in composition are not observed with increasing distance from the orebody. The composition of mixed-layer illite-smectite is temperature dependent, although illite-smectite comprising 80% illite suggests temperatures of approx- imately 200-220°C (J.W. Hedenquist, pers. commun., 1993) which is at least 50°C less than temperatures indicated by fluid inclusions studies of the mineralised breccias. This suggests either that thermal equilibration of the hydrothermal fluids and the wallrocks did not occur, possibly due to the short-lived nature of the hydrothermal events, or that the composition of the illite-smectite is controlled by a factor other than temperature, probably the chemistry of the fluid and the wallrocks. Chlorite and pyrite occur in virtually all of the wailrock lithologies, even 150-200 m from the mineralised breccias, possibly indicative of wide-scale propylitic alteration.

4. Description of orebodies

Gold-silver mineralisation at Lebong Tandai occurs entirely as quartz-cemented breccias which contain variable concentrations of base metal sulphides, chlorite, adularia and calcite. The breccias form a broadly continuous series of mainly steeply-dipping tabular bodies (Fig. 2). Individual breccia bodies may be up to 700 m in length and up to 6m wide, although 1-2 m is more common. Known vertical extents are up to 500 m (Fig. 2). The breccia bodies are highly discordant to the bedding in the host volcaniclastics, and commonly display well defined, planar hangingwall and footwall contacts with the host rocks. Where such contacts are observed, either dip-slip or strike-slip striations are common. In places, however, both dip-slip and strike-slip striations may be observed on a single surface, or the footwall and hangingwall contacts may display striations of different orientations. Planar contacts are not always observed, and breccias may display a planar footwall contact, with the hangingwall contact being an irregular zone of quartz veining up to 1 m wide where the intensity of veining decreases away from the orebody.

4.1. Brecc ia types

The breccias at Lebong Tandai are extremely heterogeneous, both along the strike of the mineralised structures and within individual lodes, although there does not appear to be any systematic spatial distribution of the breccia types. The breccias comprise angular to sub- rounded clasts of the wallrocks cemented by quartz. The clasts range widely in size, and may be up to I m in diameter, although most are between 5 cm and 20 cm. Silicified chloritic

D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409-428 413

Magazine lONG SECTION Lu~River Lode Slmsn A

Aer Noar Oslam ::i:i:i:i:i:i:i: : i : i : i : i : i : i : iJ '~ =4,v...,

T ~ l a l

PLAN ~ _ 7 ~ . . ~ - ~ ~* Manazine "',,-.--t-. 41rob ~X ~ Aer Noar Aer Noar

Eut • Derek ~" L Kip $atu

Siman 7s" ~" I DeJern t ~

Tandal Lode Lusang , ,500m , River

Lode

EUitu

Fig. 2, Long section and plan view of the Lebong Tandai mine showing the locations of the different orebodies. The stippled areas on the long section are areas of past and present production; the 6 and 8 level drives are shown for reference. The plan view shows the positions of the mineralised breccia bodies on 6 level, and the dips of the orebodies are indicated.

volcaniclastic clasts are the most common, and rhyolite clasts are locally common within Aer Noar East. Most of the clasts display variable amounts of pre-mineralisation quartz veining and brecciation, and clasts of quartz-cemented breccias are ubiquitous.

4.2. Cockade breccias

These breccias are the most abundant, and are typified by cockade bands of quartz with chlorite and/or base metal suiphides around the clasts (Fig. 3), and crustiform bands, particularly of quartz and sulphide minerals, along one or both of the margins of the orebody.

The morphology of the quartz varies from chalcedonic to comb-textured, and alternating bands of different grain size are typical (Fig. 4). Quartz bands range in thickness from < 1 mm to 20 mm, although chalcedonic quartz bands are usually thinner than those composed of comb-textured quartz. Chalcedonic quartz commonly displays colloform banding, result- ing in fine-scale botryoidal or reniform textures. Bands of comb-textured quartz are usually a single layer of quartz crystals oriented orthogonal to their substrate (Fig. 4). Adularia forms up to 10% of the breccias and occurs as rhomb-shaped crystals within the bands of comb-textured quartz.

Quartz is commonly accompanied by a single polymineralic band of sulphide minerals usually less than 4 mm in thickness (Fig. 4). In breccias which display cockade sulphide bands, not all of the clasts have sulphide rims, although there is no correlation between the presence of sulphide minerals and the composition of the clast. The sulphide minerals are usually associated with fine-grained ( = 50/xm) quartz, and typically comprise 1-2% of the breccias.

Cockade bands of chlorite cemented by fine-grained quartz may be up to 4 mm in

414 D.tl. Jobson et al. /Journal of Geochemical Exploration 50 (1994) 409-428

Fig. 3. Cockade breccia from Lusang River Lode. The sulphide minerals form one of the dark bands around the clasts, the remainder of the dark bands comprising chlorite. Cockade quartz bands are visible around all of the clasts. Note the earlier brecciation and quartz veining visible in some of the clasts.

thickness. The margins of the chlorite bands are commonly irregular in morphology, usually as a result of the irregular nature of the quartz substrate on which they occur. Chlorite is normally associated with finer-grained quartz bands, and has not been observed in contact with sulphide bands. In breccias where cockade sulphide bands do not occur, the chlorite bands may contain sulphide minerals.

4.3. Disseminated-sulphide breccias

Some breccias do not display banding and the sulphide minerals are disseminated through- out the cement of the breccias (Fig. 5). The clasts are typically of similar shape, size and composition to those in the cockade breccias, although they may have a much higher cement to clast ratio and are commonly cement supported. The cement is composed of quartz of variable morphology, ranging from very fine-grained, virtually cryptocrystalline silica to relatively coarse, equant or euhedral grains. The quartz forming the cement of disseminated- sulphide breccias is usually massive, with quartz bands rarely present. Where coarser quartz occurs, vugs up to I0 mm in diameter may be observed.

The mineralogy and paragenesis of the disseminated-sulphide breccias is the same as for the cockade breccias, with the sulphide minerals occurring either as polymineralic aggre- gates up to 10 mm in diameter, or as isolated grains up to 2 mm in diameter. The sulphide mineral content of these breccias is very variable, with 1-2% sulphides typical, although

D.H. .lobson et al. / Journal of Geochemical Exploration 50 (1994) 409-428 415

Fig. 4. Photomicrograph of cockade quartz bands around a clast of earlier vein quartz (bottom). Note the variable morphology of the banded quartz, and the band of opaque sulphide minerals at the top of the photograph. Cross- polarised light. Field of view 5.44 ram.

they may form up to 75% of the cement of some breccias. Sulphide-rich breccias occur locally in all parts of the mine, although they are usually very limited in extent. In Lebong Baru, however, breccias comprising a minimum of 30% sulphide minerals extend for at least 40 m both vertically and horizontally, and in some cases virtually the entire cement of the breccia may comprise sulphide minerals. These sulphide-rich breccias at Lebong Baru are of lower gold grade than the breccias elsewhere in the mine, although Lebong Baru is the only area where there is a definite correlation between low gold grade and high sulphide mineral content.

Chlorite is commonly observed disseminated throughout the cement of disseminated- sulphide breccias. Where the quartz cement of the breccias is very fine-grained, the chlorite occurs as rounded inclusions up to 200/xm in diameter within the quartz, resulting in a prominent green colouration of the breccia cement. In breccias where the quartz cement is more coarsely grained, discrete grains of chlorite up to 5 mm in diameter are observed. Chlorite usually forms 5-10% of the cement of the breccias, although chlorite-rich breccias occur where up to 50% of the cement may be chlorite intergrown with fine- to medium- grained quartz. In these breccias, sulphide minerals are associated with both chlorite and quartz.

Both varieties of breccia contain amethystine quartz and carbonate which post-date the brecciation process. Amethystine quartz is usually observed within vugs in the breccias, where the terminations of comb-textured quartz crystals are typically purple in colour.

416 D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409--428

Fig. 5. Disseminated-sulphide breccia from Lebong Baru. The sulphide minerals form the dark areas within the quartz cement of the breccia. Post-mineralisation quartz veining has fractured the elongate clast at the bottom of the sample.

Calcite also occurs within vugs in the breccias where it forms crystals up to 20 cm in diameter.

5. Structural interpretation

The orebodies at Lebong Tandai form three essentially east-west-trending segments: Lebong Baru, Tandai Lode, and Aer Noar/Aer Noar East ( Fig. 2). Between these segments are the northeast-trending Lusang River Lode and Derek Satu orebodies, and a relatively unmineralised area containing only the minor Siman Lodes (Fig. 2). This pattern closely resembles that of a low-displacement strike-slip system, where straights, which are parallel to the shear zone boundaries, step both to the left and the right. The style of segmentation and the nature of orientation changes within the straights (e.g. Tandai Lode; Fig. 6) strengthens the comparison with strike-slip tectonics, which is also compatible with the widespread strike-parallel striations. Unfortunately, the available kinematic indicators do not provide independent evidence for the sense of movement on the east-west shear zone. Despite this, discrimination is straightforward because the central and eastern straights are linked by the steeply dipping northeast-trending Lusang River and Derek Satu lodes. Only sinistral shear could result in this geometry, as dextral shear would only generate shallowly dipping contractional faults with a northeast strike. Thus, the Lusang River Lode-Derek

D.H. Jobson et al. / Journal o f Geochemical Exploration 50 (1994) 409-428 417

TANDAI LODE R

R . . . . . R R . . . . . R . P . . . . . . . . . . . . . . - . . . . . . . . . . . / ~

D ' k:~m '

Fig. 6. Composite plan of the mineralised breccias on 6 level (continuous line) and 8 level (dashed line) of Tandai Lode (see Fig. 2) which are localised along the east-west sinistral fault system. The different shear

orientations are indicated. Shafts are shown as black squares.

AER NOAR A BLOCK

7 - ; ..............

T X •

MINERAUSED EAST-WEST SINISTRAL SYSTEM

MINERALISED NORTHWEST . . . . DEXTRAL SYSTEM

AMBIGUOUS ORIENTATIONS

N

Fig. 7. Plan view of the mineralised breccia bodies in Aer Noar East and Aer Noar on 8 level. Breccias are localised along both the east-west-trending sinistral and northwest-trending dextral fault systems. The orientations of the different shears are indicated, based on comparisons with Tchalenko diagrams for the two strike-slip shear-zone

orientations.

Satu link is a dilational jog where the separation of the straights is approximately 200 m, and the lodes are parallel to the tensional fracture direction.

Interpretation of the step from Tandai Lode to Aer Noar East is complicated by the lack of continuity of the mineralisation. This step, which involves a separation of 650 m, could either be entirely an anti-dilational jog where the east-west system stepped to the right, or solely the result of cross-faulting generated in sympathy with the Sumatran fault. Although some combination of the two is likely, the lack of significant northwest-trending faults in the drives between Siman Dalam and Magazine Lode suggests that the jog is the major contributor to the step. Extensive zones of 030 °- and 060°-trending quartz stockwork and silicification in the unmineralised section have geometries indicative of subsidiary shears of the Sumatran Fault, but these would not result in direct northwest dextral offsets between Siman Dalam and Magazine Lode. The intermediate dip of the Siman Dalam oreboby is probably the result of its formation within an anti-dilational jog with a significant contrac- tional component.

Variations in the orientation of the mineralised breccia bodies occur on all scales, with individual segments varying in length from a few metres (Fig. 7), to tens or even hundreds of metres (Fig. 6). When the orientations of the lodes are compared to the predicted

418 D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409~128

orientations of fractures within shear zones, it is clear that approximately 75% of the total strike length of mineralised breccias corresponds to D-, R- and P-shears and T-fractures ( as defined by Tchalenko and Ambraseys, 1970) of the sinistral strike-slip fault system, although the total production from lodes localised along these faults is much higher than this because of the greater depth of mining in Tandai Lode. The east-west-trending lodes are localised along D-shears, which are parallel to the strike of the shear zone. Lodes that trend approximately 075 ° are along R-shears, whilst those which trend 105 ° are localised along P-shears (Fig. 8). Poles to the northeast-trending T-fractures cluster around a bearing of 135 °. The angles between the groupings representing the D-, R- and P-shears are approx- imately 15 ° (Fig. 8). This is in close agreement with the orientations predicted by the models of Tchalenko and Ambraseys (1970), if the angle of internal friction (~b) is 30 °, a good average for most rocks (Bartlett et al., 1981 ). Thus, it appears that no transpression or transtension occurred.

The orientations of Siman A, Lebong Kip and parts of Aer Noar East C block ( Fig. 7 ) do not correspond well with those predicted for a sinistral system. They are, however, consistent with the predicted orientations of a northwest-trending strike-slip fault system. The sense of movement of this fault system cannot be constrained on the basis of its geometry. However, its average orientation is 316 °, virtually parallel to the 320 ° trend of the adjacent segment of the dextral Sumatran Fault System, and anything other than dextral movement along it would require a highly improbable re-alignment of regional stresses within Sumatra. Within Aer Noar East C block, D-, R- and P-shears of this system are mineralised (Fig. 7; Fig, 8), and the shallowly-dipping Siman A orebody is a reverse fault. Lebong Kip is localised along R- and X-shears. No T-fractures are observed within the dextral system.

East-west-trending sinistral system

N

n

Northwest-trending dextral system

N

Fig. 8. Lower-hemisphere equal-area stereographic projections of downward-pointing poles to mineralised breccias localised along both fault systems. The clusters relating to different shear orientations are labelled.

D,H. Jobson et al. / Journal c~' Geochemical Exploration 50 (1994) 409~128

6. Mineralogy of the mineralised breccias

419

The mineralised breccias comprise quartz and sulphide minerals, together with chlorite and adularia in some instances. Whilst quartz and chlorite are spatially and temporally associated with the base metal sulphides and precious-metal-bearing phases, adularia is associated with quartz which is paragenetically later than the mineralisation. Truscottite (Ca~4Si2aOss(OH)s.2H20), the type locality of which is Lebong Donok, has not been observed in the mineralised breccias at Lebong Tandai.

The concentration of base metal sulphides within the breccias is highly variable, ranging from < 1% to virtually 100% of the cement. Electrum is the only gold-bearing phase present in the mineralisation, with additional silver sulphosalts and silver tellurides. Precious-metal- bearing minerals usually form less than 1% of the metalliferous minerals, with the remainder being base metal sulphides, principally pyrite, chalcopyrite, sphalerite and galena. Within individual breccia samples, all of the base metal sulphides are usually present, although the relative amounts of each of mineral vary greatly.

Grains of sulphide minerals are up to 5 mm in diameter, although the larger grains are typically aggregates of smaller grains. Where intergrown sulphide minerals occur, the textures indicate replacement has taken place along rounded fronts, resulting in lobate embayments of the later mineral in the earlier one (Fig. 9). Between these lobate embay-

Fig. 9. Reflected light photomicrograph showing typical ore mineral textures from Lebong Tandai. Pyrite (Py) was deposited first, followed by galena (GI), chalcopyrite (Cp), acanthite (Ac) and electrum (El). Later sphalerite (Sp) partially replaces all of the earlier base-metal sulphides and electrum. Field of view I. 15 mm

420 D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409~128

ments, the early mineral remains as cuspate relics which are often completely isolated as irregularly shaped inclusions with cuspate morphologies.

Pyrite was the first of the base metal sulphides to be deposited, and where crustiform bands of base metal sulphides occur, pyrite forms the earliest of the bands. Pyrite may occur as euhedral grains, although more commonly it is subhedral in morphology. Pyrite grains are of variable size, although most are between 250/xm and 2 mm in diameter. They are usually surrounded by later base metal sulphides, with some replacement, especially by galena and sphalerite. Replacement of pyrite by chalcopyrite is less pronounced, and pyrite which is completely enclosed within chalcopyrite often shows no evidence for replacement. Where the sulphides are enclosed by chlorite, pyrite is the most abundant sulphide mineral observed and is typically euhedral or subhedral.

Galena and chalcopyrite usually occur together and are intergrown. They form irregularly shaped grains, and the boundaries between them are commonly very irregular. The textures do not indicate the consistent replacement of one mineral by the other and are thought to represent contemporaneous deposition of the galena and chalcopyrite.

Sphalerite was the last of the base metal sulphides to be deposited, and the partial replacement of the earlier base metal sulphides by sphalerite is typically observed ( Fig. 9). Sphalerite forms irregularly shaped aggregates of grains up to 6 mm in diameter, or indi- vidual grains which are up to 2 ram. The sphalerite is translucent, with a pale-brown to honey-yellow colour, and electron microprobe analysis indicates the iron content to be less than 1.5 wt%. Coiour zoning of the sphalerite is usually very poorly developed. Chalcopyrite inclusions in the sphalerite mostly occur as trails, although some sphalerite grains display marginal zones rich in chalcopyrite inclusions.

Precious-metal-bearing phases are closely associated with the base metal sulphides, and all of the base- and precious-metal-bearing phases were deposited during a single mineral- ising event. Electrum is associated with all of the base metal sulphides, either as inclusions within them (Fig. 9), along the boundary between adjacent grains, or on the surface of pyrite. It also occurs as isolated grains within the quartz matrix of the breccias, and associated with chlorite. Electrum grains are typically rounded or elongate in morphology, and up to 200 /xm in diameter. Where electrum is observed in the quartz matrix of the breccias, individual grains or groups of grains are often completely isolated within a single quartz grain, indicating that quartz deposition paragenetically overlapped the deposition of elec- trum. Electrum within chlorite may either be associated with sulphide minerals in the chlorite or occur as isolated inclusions. Inclusions of electrum in chlorite tend to be small (less than 50/xm), and rounded or irregular in shape.

As electrum occurs in spatial association with all of the base metal sulphides, it is difficult to place it within the paragenetic scheme. However, it appears that the deposition of electrum was synchronous with that of the pyrite, chalcopyrite and galena. The composition of the electrum grains ranges from 49 to 72 wt% gold, although the gold content is virtually constant throughout individual grains.

Acanthite is the main silver-sulphosalt present, with pearceite and members of the pear- ceite-polybasite solid-solution series also evident. They are found in association with the base metal sulphides and precious-metal phases (Fig. 9) or within the quartz matrix of the breccias, and do not replace any of the base metal sulphides, although they may be partially replaced by chalcopyrite and sphalerite. The silver-sulphosalt grains are usually irregular

D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 4 0 9 4 2 8 421

in shape, especially when intergrown with other minerals. The acanthite contains up to 7 wt% selenium, indicating that it may contain fine-scale lamellae of aguilarite (Petruk et al., 1974).

Hessite (Ag2Te) and cervelleite (AgaTeS; Criddle et al., 1989) are the only telluride phases which have been observed. Hessite typically occurs as rounded inclusions, up to 50 /~m in diameter, in galena or sphalerite, although it can form larger grains intergrown with the base metal sulphides. Grains of hessite which are intergrown with the base metal sulphides are usually irregular in shape, and up to 200/zm in diameter. Cervelleite has only been observed as inclusions in galena, or within hessite inclusions in galena. Cervelleite inclusions in hessite are up to 10/xm in length, and typically irregularly shaped, whilst inclusions of cervelleite in galena are rounded in morphology, and up to 20/xm in diameter.

Fine-scale intergrowths of late-stage covellite and digenite are common within some samples, although their occurrence is sporadic. They commonly form rims up to 40/xm thick around the base metal sulphides, or veinlets within them. They are rarely seen in association with galena, and covellite or digenite rims around sphalerite or chalcopyrite often terminate against galena grains.

7. Fluid inclusion studies

When thin sections of the quartz matrix of the breccias are viewed under cross-polarised light, many of the quartz crystals display anomalous, plumose extinction (Fig. 10). This results from the presence of small quartz crystallites arranged in a radial pattern around the c-axis of the quartz crystal, so that each of the quartz crystallites has a different maximum extinction position. In some samples from Lebong Tandai, the quartz crystallites are restricted to a zone around the margins of quartz crystals (Fig. 10), whilst elsewhere the entire crystal may be composed of quartz crystallites. Where the plumose quartz forms only the margins to quartz crystals, the core of the crystal is usually optically homogenous, displaying neither anomalous extinction nor refractive index variations. The cores of the crystals may be euhedral, although they are mostly irregular in morphology.

Plumose textures are thought to result from the recrystallisation of originally amorphous or cryptocrystalline silica to form crystalline quartz (Sander and Black, 1988). This would result in fluid inclusions within the quartz which were not representative of the conditions of deposition of the original silica minerals (Sander and Black, 1988). Microthermometric analysis of fluid inclusions in quartz did not yield any meaningful data and so fluid inclusions in sphalerite were analysed, as these should provide evidence of the conditions of base metal, and hence gold, mineralisation.

7.1. Petrography of fluid inclusions in sphalerite

Fluid inclusions within sphalerite are of variable size and morphology. Individual inclu- sions are up to 50/xm in diameter, although the majority are between 5/xm and 15 /xm. Fluid inclusions of two different morphologies are recognised. The most abundant are rounded, either roughly spherical, ellipsoidal or elongate in shape. Less frequent are inclu- sions with angular morphologies which are usually multi-faceted, the different facets appar-

422 D.H. Jobson et a l . / Journal of Geochemical Exploration 50 (1994) 409~128

Fig. 10. Transmitted light photomicrograph of the quartz cement of the breccias. The cores of the crystals are optically homogenous, whilst the margins display plumose extinction. The dark spots seen within both the cores and the margins of the crystals are fluid inclusions, although recrystallisation of the quartz means that these inclusions are unlikely to be indicative of the conditions of mineralisation. Cross-polarised light. Field of view 1.36 ram.

Table I Characteristics of fluid inclusions in sphalerite from Lebong Tandai

Type Characteristics

A Isolated inclusions conforming to primary criteria of Roedder (1984). Trails of inclusions parallel to edges of crystals or colour banding.

Inclusions in trails at high angles to the edges of crystals or colour banding. Conform to criteria for secondary inclusions (Roedder, 1984).

Inclusions in trails which are parallel to trails of chalcopyrite inclusions. Individual inclusions occurring in same trail as chalcopyrite inclusions. May be attached to chalcopyrite inclusions.

ent ly co r respond ing to c leavage faces of the host sphaleri te. They may adopt very irregular

and complex shapes, but do not appear to represent a d i f ferent popula t ion o f inclusions, as

trails conta in ing both rounded and angular inclusions are observed.

Three d i f ferent modes of occurrence of inclusions have been recognised , and are sum-

mar ised in Table 1. The mos t abundant fluid inclusions are aqueous, usually conta in ing

D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409-428 423

both liquid and a vapour phase. Their degrees of fill are variable, ranging from 0.50 to 0.95, with most between 0.70 and 0.85. Monophase inclusions are relatively rare. Where seen, they are usually dark in appearance, and occur in association with the primary inclusions. They may contain predominantly vapour, with refractive index variations between the vapour and the sphalerite resulting in diffraction of light along the vapour-sphalerite inter- face, and thus accounting for their dark appearance.

Microtbermometric analysis of fluid inclusions was undertaken on samples from through- out the mine, although Tandai Lode, which is now largely worked out and inaccessible, could not be sampled. The ice which forms in the inclusions upon freezing has a refractive index very close to that of the fluid, and recognition of the temperature of first melting of the ice proved impossible. Laser Raman analysis of the inclusions did not reveal the presence of any volatile phases above the minimum detection limits of the microprobe. Although the detection limits are dependent on the size and depth within the sample of individual inclu- sions, Raman analysis indicates that the volatile content of the inclusions is less than 1 mole% (e.g. about 2.5 wt% CO2, the dominant gas in the epithermal environment; Hed- enquist and Henley, 1985).

Microthermometric data from Aer Noar East A block are presented in the form of histograms and bivariate plots in Fig. 11. The fluid inclusions which were analysed are from 10 and 11 level in Aer Noar East A block, corresponding to depths of 300-350 m below the current surface. Primary (Type A) inclusions have degrees of fill of 0.60 to 0.95. Histograms of homogenisation temperature indicate the presence of two mean homogeni- sation temperatures, one at 220-230°C and one at 270-280°C. The majority of the high- temperature primary inclusions have salinities of approximately 2 wt% NaClequiwlem. The lower temperature inclusions have salinities ranging from 2 to 5.5 wt% NaClequiv,te m. Sec- ondary, Type B, inclusions form a well defined cluster on bivariate plots of salinity versus homogenisation temperature. Homogenisation temperatures are between 220 and 250°C, with a mean at 230-240°C. Salinities range from 2.5-4 wt% NaClequivale m. Chalcopyrite- associated, Type C, inclusions also display two means on histograms of homogenisation temperature, one at 230-240°C and a higher temperature mean at 280-290°C. The low temperature inclusions have salinities of 2-4 wt% NaClequivalent whilst the higher temperature inclusions have salinities of approximately 2 wt% NaCleqmva~cm. The low temperature appar- ent primary inclusions are interpreted as being secondary inclusions which have been mis- identified, and mineralisation occurred at temperatures of 270-280°C. The chalcopyrite- associated fluid inclusions comprise both primary and secondary inclusions, indicating that the formation of chalcopyrite inclusions in sphalerite occurred both during the deposition of the sphalerite and during post-sphalerite activity.

Trends of salinity and homogenisation temperature which may be indicative of boiling are not evident from the microthermometric data. The high-temperature chalcopyrite-asso- ciated inclusions record a slight increase in salinity with decreasing homogenisation tem- peratures, possibly indicative of the concentration of solutes due to steam loss. The microthermometric data from other areas of the mine are very similar to that from Aer Noar East A block, although primary inclusions from Aer Noar East C block display homogen- isation temperatures of 30(O320°C, with salinities between 1 and 1.5 wt% NaCleqmval~,,. Elsewhere in the mine, homogenisation temperatures are in the range 260-280°C, with salinities between 1 and 3wt% NaCl~q,+v~m. Siman A records homogenisation temperatures

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D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409428 425

of approximately 250°C, although this is believed to be a function of the more shallow palaeodepths of the mineralisation in this area ( 200 m below the present surface). Possible boiling trends are rarely observed in the microthermometric data from the other mine areas studied, although in Lebong Baru salinity decreases by approximately 1.5 wt% over a very narrow range of homogenisation temperatures (less than 20°C). This may result from the loss of dissolved CO2, which acts as a solute, to the vapour phase during boiling (Hedenquist and Henley, 1985). The dark, apparently monophase fluid inclusions may contain the vapour exsolved from such a boiling fluid, although analysis of these inclusions has proved impos- sible, and such a conclusion is tentative at best.

Boiling point versus depth relationships, as tabulated by Haas ( 1971 ), indicate that a 2 wt% NaC1 solution at 270°C would boil approximately 600 m below the watertable, assum- ing that the confining pressure was hydrostatic. The presence of even small amounts of CO2 in the hydrothermal fluid would increase this depth. Thus there has been at least 250 to 300 m of erosion in the area since mineralisation.

8. Genesis of the mineralised breccias

The east-west trending sinistral fault cannot be a Riedel-shear of the Sumatran Fault System, as this is not consistent with both the observed orientation and the inferred sense of movement. Instead it may be the result of doming of the Barisan Mountains (Harris, 1989). The northwest-trending dextral system is parallel to the Sumatran Fault System, and is a response to north-south relative motion of the Indian-Australian plate with respect to Sumatra and Asia.

Of major importance in developing a structural model of a deposit is the relative timing of mineralisation and tectonism. If mineralisation occurs synchronously with strike-slip faulting, high rates of fluid-flow would occur in dilational areas, especially tension fractures, and along the intersection of different shear segments of the fault. Such enhanced fluid-flow may lead to high-grade mineralisation on dilational segments, fault intersections, and at 90 ° to the slip direction. Conversely, if the mineralising fluids merely utilised a pre-existing fault as a high permeability pathway, pronounced high-grade oreshoots in these orientations would not be expected, although some grade-enhancement may occur at the junction of two segments of different orientations. In both cases there may be considerable variations in the grade of the mineralisation from one planar segment to another.

Within the sinistral system, many potentially dilational zones occur, the most obvious of which are the tensional fractures at Lusang River Lode and Derek Satu. Dilation would also occur where active faulting stepped from D- to R-shear or T-fracture orientations, such as can be seen in Tandai Lode (Fig. 6). Hams (1989) stated that the mineralisation within the dilational jog at Lusang River Lode and Derek Satu comprised syntaxial quartz veins, citing this as evidence that mineralisation was syntectonic with the east-west sinistral tectonism. In the current study, syntaxial quartz veins were not observed, and no consistent differences in the style, mineralogy, or gold-grade of the breccias were found between lodes of different orientations; zones which must have been dilational during east-west sinistral faulting do not display enhanced gold-grades. Contouring of gold-grades recorded within stopes localised along the east-west fault systems does not indicate the presence of steeply-

426 D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409~128

plunging higher-grade oreshoots where the orientation of the mineralisation changes, sug- gesting that mineralisation was not synchronous with east-west-oriented sinistral strike- slip tectonism, contrary to the interpretation of Harris (1989).

The dip-slip striations on the surfaces of many of the original strike-slip faults indicate that they were reactivated, and Turvey (1984) recorded reverse offsets. North-south com- pression would not lead to the reactivation of the east-west-trending faults under conditions of normal crustal fluid pressure, although greatly enhanced fluid pressure would allow the reactivation of such severely mis-oriented faults. In order to reactivate faults at high angles ( > 55 °) to the maximum compressive stress, the fluid pressure must exceed the vertical stress (Sibson et al., 1988), which is equal to the lithostatic load. To accumulate suprali- thostatic fluid pressures, the original strike-slip faults must have been sealed to the migration of hydrothermal fluids. Silica deposited along the faults would act as a seal and prevent the preferential mineralisation of the northwest-trending dextral faults. As the fluid pressure below the silica seal increased, pressurised fluid would be forced into fractures in both the silica seal and the wallrocks. Dip-slip reactivation of the fault, probably in response to increasing shear stress, would result in a drop in the pressure of the fluid along the fault plane, and the fluid contained in the fractures in both the silica seal and the wallrocks being overpressured relative to the fluid along the fault plane. Subsequent explosive decompres- sion of this fluid into voids created along the fault plane formed the breccia clasts by hydraulic fracturing of the wallrocks and the silica seal. The occurrence of cement-supported breccias suggest that, subsequent to the hydraulic fracturing, the ascent of the hydrothermal fluids was rapid enough to fluidise the clasts, such that the frictional drag exerted by the fluid on the clasts was sufficient to support them within the ascending hydrothermal fluid ( McCallum, 1985). Variable degrees of clast abrasion during fluidisation are likely to have led to the sub-angular to sub-rounded morphologies which are now preserved (Phillips, 1972).

Rapid decompression of the hydrothermal fluid is likely to have resulted in boiling if the confining pressure of the fluid dropped below its vapour pressure. Boiling is likely to be isoenthalpic (Drummond and Ohmoto, 1985), and deposition of the base- and precious- metal phases from the hydrothermai fluids probably occurred in response to the loss of volatiles, specifically H2S and CO2, during boiling. The loss of these volatiles occurs early in the boiling process, before the hydrothermal fluid has cooled appreciably, so the narrow band of sulphide minerals around many of the clasts represents a restricted temperature interval. Primary fluid inclusions contained in sphalerite are thus indicative of only the early stages of boiling of the fluid, and would not be expected to record large variations in homogenisation temperature, resulting in possible boiling trends being very poorly devel- oped. The presence of adularia within the comb-textured quartz bands is consistent with boiling (Browne, 1978). The deposition of adularia paragenetically after the sulphide minerals may reflect an increase in the pH of the mineralising fluid to values above that where adularia is stable. If due to boiling, this pH increase is a response to the loss of CO2 during boiling, although the late occurrence of the adula:ria does not prove that boiling occurred during the earlier deposition of the precious metals and base metal sulphides.

Rapid decompression of a hydrothermal fluid will lead to supersaturation of the fluid with silica and the rapid deposition of amorphous silica from solution (Fournier, 1985). As fluid velocities waned, fluidisation of the clasts was no longer possible, leading to the

D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 409~428 427

formation of cement-supported breccias. The re-crystallisation of amorphous silica to the currently observed varieties of quartz may have occurred in response to elevated geothermal gradients due to extensive magmatic activity during the Neogene, although elevated tem- peratures are not a prerequisite for such recrystallisation.

The reasons for the absence of orebodies in the region between Siman Dalam and Magazine Lode are not clear. If no steeply dipping faults existed in the area, as would occur if the area represents an anti-dilational step-over within the sinistral system, there would be few structures which could be reactivated under conditions of elevated fluid pressure. Additionally, high fracture permeability due to the presence of subsidiary Sumatran fault- related fractures allowed a more dispersed fluid flow, resulting in the formation of the observed zones of quartz stockwork and silicification. If east-west trending faults are present in this area, they may have prevented significant fluid pressure increases, and hence fault reactivation and brecciation, from occurring.

9. Conclusions

The mineralised breccias at the Lebong Tandai mine are hosted by a series of volcanic and volcaniclastic rocks of Neogene age. The ore-bearing breccias comprise clasts of the wallrocks and earlier barren breccias cemented by quartz of variable morphology, together with precious-metal bearing phases, base metal sulphides, adularia and chlorite. The pres- ence of adularia and chlorite within the breccias, together with a halo of sericitic alteration around the mineralisation indicates that the deposit is low-sulphidation in character, as defined by White and Hedenquist (1990). Regional tectonics played a vital role in both the localisation and formation of the deposit, and the geological structures did not simply act as high permeability pathways for the mineralising hydrothermal fluid. Instead, compression at high angles to pre-existing structures, combined with supralithostatic fluid pressures, resulted in reverse reactivation of original strike-slip faults, with consequent hydraulic brecciation and fluidisation of the clasts. The deposition of silica, base metal sulphides and precious-metal minerals was in response to boiling associated with fluid decompression, and was rapid enough to form cement-supported breccias before fluid velocities waned to the extent that fluidisation of the clasts could not be maintained. Mineralisation took place at temperatures of 260-280°C from fluids that were of low apparent salinity, and at least 250-300 m deeper than the current levels of mineralisation, corresponding to depths of at least 500-600 m below the palaeosurface.

Acknowledgements

The authors thank P.T. Lusang Mining, Billiton Companies in Indonesia, and the Uni- versity of Southampton, for their financial support in the course of this study. Additionally, thanks are due to all the employees of P.T. Lusang Mining and Billiton Companies in Indonesia lbr their assistance and friendship during fieldwork at Lebong Tandai. We also acknowledge Journal of Geochemical Exploration reviewers Stuart Simmons, Jeff Heden-

428 D.H. Jobson et al. / Journal of Geochemical Exploration 50 (1994) 4 0 9 4 2 8

qu i s t , T h e o v a n L e e u w e n a n d Y a s u s h i W a t a n a b e w h o s e c o m m e n t s i m p r o v e d t h i s m a n u -

sc r ip t .

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