1. brett gunter.pdf

Majalah Geologi Indonesia, Vol. 26 No. 1 April 2011: 1-17

1Naskah diterima: 05 April 2010, revisi terakhir: 16 Maret 2011

Geology, Alteration, and Mineralization at the Jelai Gold Project, Bulungan Regency, East Kalimantan

Geologi, Alterasi, dan Mineralisasi pada Projek Emas Jelai, Kabupaten Bulungan, Kalimantan Timur

Brett Gunter

PT. GMT Indonesia Jln.TB Simatupang Kav. 1S, Cilandak Timur, Jakarta Selatan 12560, Indonesia

ABSTRACTThe Jelai Gold Project is located approximate ly 1,550 km northeast of Jakarta, the capital city of Indonesia, close to the east coast of East Kalimantan Province. The centre of the Jelai Gold Proj-ect is located at approximately E 117 00 00 and N 03 10 00. The property was first intensely explored in 1996 when anomalous gold contained in stream float samples were traced upstream to the Mewet area. Subsequent exploration programmes, such as stream sediment sampling, soil sampling, mapping, and drilling outlined a series of areas containing anomalous gold and silver. Follow up exploration by PT Jelai Cahaya Minerals has concentrated on shallow drilling to define the extent of the veins, with selected deeper drilling where mineralisation and textures have shown a potential ore shoot that to be developed. In total, there is more than 5 km of aggregate vein within the Mewet area alone and a number of other peripheral prospects have yet to be sampled sufficiently to define drill targets. The mineralization in the area comprises a series of low sulphidation epithermal veins, vein breccias, sheeted vein systems, and silicified zones with anomalous precious metal values hosted within andesitic volcanic, dacite,andesite intrusions, and sediments. The alteration within, and adjacent to, the mineralised zones is typically propylitic on a regional scale, with narrower zones of argillic alteration adjacent to the veins and ubiquitous silicification associated with the precious metal mineralisation. Mineralogically, the gold mineralisation is extremely fine and free milling in the Mewet area. Higher grades of gold are often associated with fine, black sulphide bands in colloform quartz, which may be gold rich electrum or a gold sulphosalt. The silver/gold ratio is generally low, unlike some other Kalimantan epithermal systems such as Mount Muro deposit.Keywords: alteration, mineralization, Jelai, gold, Mewet

SARIProjek emas Jelai berlokasi kurang lebih 1.550 km timur laut Jakarta, ibu kota Indonesia, dekat ke pantai timur Provinsi Kalimantan Timur. Pusat Proyek Emas Jelai berlokasi lebih kurang pada 117o BT dan 03o 10 00 LU. Kekayaan alam ini pertama kali diekplorasi secara intensif pada tahun 1996 ketika anomali yang terkandung di dalam percontoh gelundungan di sungai terdeteksi di hilir daerah Mewet. Program eksplorasi berikut seperti pemercontohan sedimen sungai, pemercontohan tanah, pemetaan, dan pemboran mengungkapkan beberapa daerah yang mengandung emas dan perak anomali. Eksplorasi lebih lanjut oleh PT Jelai Cahaya Minerals berkonsentrasi pada pemboran dangkal untuk menentukan panjangnya urat dengan pemboran dalam yang terpilih bila mineralisasi dan tekstur mengindikasikan bijih potensial untuk dikembangkan. Secara keseluruhan terdapat lebih dari 5 km urat agregat di daerah Mewet saja dan daerah prospek lainnya masih harus diambil percontohnya secara memadai untuk menentukan target pengeboran. Mineralisasi di daerah penelitian terdiri atas rangkaian urat epitermal sulfidasi rendah, breksi urat, sistem urat berlembar, dan zona tersilisifikasi dengan nilai logam mulia anomali yang terdapat di dalam batuan andesit, dasit, intrusi andesit, dan batuan sedimen. Alterasi pada dan dekat zona mineralisasi merupakan propilitik yang


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INTRODUCTION

The Jelai Gold Project is located approxi-mately 1,550 km northeast of Jakarta, the capital city of Indonesia, close to the east coast of East Kalimantan Province (Figure 1). The major towns in the area include Ba-likpapan, approximately 500 km south of the project area and Tarakan 45 km east of the project site. The centre of the Jelai Gold Project is located at approximately E 117 0000 and N 03 10 00. The climate in the area is typical tropical monsoonal.

In the Jelai-Mewet area, the terrain is undulating to rugged, with elevations of

up to 1,000 m. Waterfalls and rapids are developed in the upper reaches of the drainage systems, posing access problems in some areas. The drainage pattern devel-oping in most of the area is dendritic but in the east of the project area, the major Jelai River is linear, controlled by regional fault structures trending north-northeast in the area.

The aim of the study is to show an example of a low sulphidation, quartz-adularia domi-nated, and precious metal epithermal system developed in association with younger vol-canic and intrusive in East Kalimantan as indicated at the Jelai Gold Project.

khas pada skala regional, dengan zona sempit alterasi argilik dekat urat dan silisifikasi menyeluruh yang berasosiasi dengan mineralisasi logam mulia. Secara mineralogis, mineralisasi emas di daerah Mewet sangat baik dan bebas penggilingan. Emas kadar tinggi sering berasosiasi dengan lapisan sulfida halus berwarna hitam di dalam kuarsa, koloform, yang mungkin merupakan elektrum kaya akan emas atau sulfosalt emas. Ratio perak/emas pada umumnya rendah, tidak seperti beberapa sistem epitermal Kalimantan lainnya, seperti deposit Gunung Muro.Kata kunci: alterasi, mineralisasi, Jelai, emas, Mewet

Timor LesteBali

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Figure 1. General location of the Jelai Gold Project, East Kalimantan, Republic of Indonesia.

Geology, Alteration, and Mineralization at the Jelai Gold Project, Bulungan Regency, East Kalimantan (B. Gunter)

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Exploration History

Pre-1994 Exploration

Before 1994, there was little sustained systematic exploration of the region by mining companies. Much of Northeastern Kalimantan was designated a Reserved Area (CTA 39A) and was not available for mineral exploration.

CTA 39A was the reserve for a joint ex-ploration and mapping programme at 1:250.000 scale and a stream sediment geochemical survey conducted by a joint French-Indonesia research team (Bureau de Recherch Geologiques et Mineres and Direktorat Jenderal Pertambangan Umum (BRGM/DSDM) during 1979 - 1986 (re-ports by Lefevre et al., 1982; Le Bel et al. 1985; and Nagel, 1987).

The BRGM/DSDM regional stream sedi-ment geochemical survey resulted in the discovery of an intrusive-skarn metallogenic province in the Long Laai area (100 km south of the Jelai Gold Project). The skarns contain base metal-silver mineralization associated with tin-bearing intrusive. No systematic sampling was undertaken for gold, apart from pan concentrate analysis in some areas.

Follow up stream sediment sampling in 1:50.000 scale geological mapping was undertaken by the BRGM/DSDM over the Long Laai region (Le Bel et al., 1985) and the Long Bia region (Nagel,1987). A drilling was conducted at the Mamak prospect, but the results were disappointing.

The epithermal gold potential of northeast Kalimantan was first discussed by Kirwin and Beaudoin (1985; in Kirwin, 1993). RTZ (Rio Tinto) conducted a three month prospecting survey in 1990. The investi-gation included stream sediment, panned concentrate, stream float, channel sampling, as well as some tape and compass mapping. Drainage investigation included the Turau,

Mewet, and Jelai River. The results were not considered encouraging at that time for RTZ.

In 1993, PT Macan Mas Minerindo (Flem-ing, 1993) conducted a selective reconnais-sance survey over areas considered pro-spective based on the BRGM/DSDM survey results. The reconnaissance survey located epithermal quartz veins in the Jelai River area, silica-pyrite breccias in the Makjun and Pangean River drainages, a presumed base-metal gossans at Gunung Kelapis, and confirmed the presence of base metal-silver mineralized skarns at Long Laai.

Fleming (1993) indicated that some small scale hard rock gold mining activities occur-ring in the Bukit Pondok area are associated with previously worked lead-silver veins. Episodic local alluvial gold panning occurred in the Makjum, Saling, Betal, Geh, Malinau, Pangean, Segah, Yin, and Ptong River.

Indochina Goldfields Exploration

A technical recommendation to system-atically prospect the region for epithermal gold by Kirwin (1993) was the basis for Indochina Goldfields Ltd. committing to exploration in East Kalimantan.

A geological assessment (Sennitt & Kirwin,1994) and an aerial reconnaissance survey (Kirwin & Sennitt, 1994) were undertaken over parts of the region, in order to con-firm the prospective for significant pre-cious metal mineralization. In July, 1994, Indochina entered into a joint venture agreement, covering three CoW (Contract of Work) blocks, totalling 3 million ha, in order to explore the region for precious metal mineralizations. Work commenced in Jelai River as early as 1995 but it was not active until 1995/1996 that the focus shifted to the Mewet River area, where epithermal quartz vein floats were found in many streams in the area and precipitated a full detailed geological survey of the area


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including mapping, rock chip sampling, stream sediment sampling, ridge and spur soil sampling, grid based soil sampling, geophysics (limited), trenching, and drill-ing (3,901.52 m in 26 drill holes).

The project was closed in December 2000, along with all other Ivanhoe Mines Ltd. (formerly Indochina Goldfields Limited) operations in Indonesia.

PT Jelai Cahaya Minerals Exploration

The project remained inactive for a number of years after Ivanhoe closed its projects. It is believed that a local miner attempted to develop a shaft on the Mewet Vein and evidence of this case remains with a number of decaying sacks of quartz vein material dumped at the collar of a shaft a few metres in depth on the vein outcrop. It is thought that the extremely fine nature of the miner-alization did not allow the recovery of any gold from the samples and the development was abandoned.

In 2006, JCM Minerals placed an applica-tion over the known gold occurrences and this was granted on the 1st June 2007. The management of the company, under the ad-vice of a number of consultants, embarked on an aggressive scout drilling programme aimed at outlining the near surface extent of the vein systems. This programme was aimed at persistence, rather than the desire to obtain high-grade results, as it was deter-mined that the soil cover in the project area could hide the mineralized lodes and shal-low drilling was the most effective method for delineating of deeper targets.

METHODS AND EXPLORATION TECHNIQUES

In exploring these styles of deposits and relating to the exploration of the Jelai Gold

Project over time, the following exploration techniques have been relevant or irrelevant in the exploration of the area: Stream sediment geochemistry would

have located the area of the gold miner-alization but the high background level of gold in a large number of drainages would have vectored the area only.

Ridge and spur soil geochemistry is use-ful in areas of cover and did define the major vein structures but drilling of soil geochemistry anomalous high areas is not the ultimate answer for targeting drilling, the highest geochemistry over the Nyabi vein, which is the most spectacular in outcrop, was drilled with no economic grades encountered at depth in essentially a carbonate breccias lode.

Geophysics has a limited use in the area but magnetic surveys (very closed spaced) may define magnetite destruction in the argillic altered zones adjacent to the main vein structures, resistivity may define resistant zones of silica in the general wall rocks.

Mapping in areas of poor outcrop is dif-ficult and vital features in the oxidised zone may be missed by the novice explor-er but the recognition of quartz textures in relation to the general vein textures adjacent is an extremely important clue to the presence of bonanza grades at depth.

Drilling is an extremely important as a tool, shallow drilling can adequately collect good vein samples for visual analysis, geochemical analysis, and vein orientation. It is not necessary to get too ambitious from the beginning of the drill programme. It is far cheaper to miss with a 50 m drill hole and give yourself the chance to accurately drill the target at 200 m depth by knowing the exact vein orientation.

Epithermal vein targets are extremely rewarding to successfully explore. They


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require persistence and an understanding of the relationship between structures, miner-alization, fluid flows, and epithermal geo-chemistry to successfully explore for such deposits. Throughout recent exploration stories, this aspect is proven time and again.

REGIONAL GEOLOGY

Tectonic History

Geological mapping by AGSO and GRDC(Pieters et al., 1993) indicates that a suture formed in Borneo, was due to the collision of two continental fragments. The collision occurred during the Cretaceous to the Late Tertiary, with possible after effects continu-ing until the Quaternary

Gold deposits are widespread in Borneo. The deposits are generally located within areas of uplifted, faulted basement highs that arose during post-collisional uplift and extensional tectonics. Recently, recognized epithermal gold-silver mineralization were most likely deposited as a result of hydrothermal activity associated with a volcanism coeval with the uplift of the basement highs (e.g. Kutai Basin-Kelian, Barito Basin-Mount Muro, and Mount Wullersdorf in Sabah). However, there remain a number of points that are yet to be clearly answered regarding the tectonic evolution of the area and the volcanic episodes responsible for epithermal, and other mineralization.

The development of the present tectonic framework of the northeast Kalimantan area probably commenced during the Late Cretaceous, when suite of alkali hot gran-ites containing high concentration of U and Th, were emplaced, associated with a transitional tectonic environment from sub-duction to extension (Pieters et al., 1993). These granites can be genetically linked to the base metal skarns in the Long Laai area

and may represent the oldest hydrothermal mineralisation in the region, with the intru-sions dated by the BRGM at 222Ma (Le Bel et al., 1985).

The BRGM (Le Bel et al., 1985) postulated that the hot granites resulted from arc ac-cretion phenomena. The double convection model proposed by Katili (1973) explains the formation of volcanic arcs and related basins up to the Early Eocene, the northwest subduction zone being located at Sarawak, the southeast zone at the Mangkalihat Pen-insula (Mangkalihat Ridge). The transition from this regime to a collisional regime took place when the southeast subduction gently stopped, while the northwest one was still active. Partial melting of arc-derived mate-rial at depth, generated thehot granites. The next series of dated volcanics in the Kelay area, south of the Jelai Gold Project derived, again from the BRGM work (Lefevre et al, 1982), show an age of 16 Ma. These are likely to be related to a suite of dacite porphyry intrusions in the Mantam Complex, south of the Segah River near Tepianbuah.

Younger volcanic and intrusive rocks in the area of Mount Rian/Rukak, to the northwest from the Jelai Gold Project, have been age dated as 7 Ma from a porphyritic basalt and 9.4 Ma from an andesite (Lefevre et al, 1982).

From this characteristic, it can be seen that the Jelai Gold Project host rocks are likely to be Late Miocene to Pliocene in age but no age dating has been conducted in the specific area of the project and this remains postulate. Evidence of extremely young volcanic and intrusive rocks can be made based on geomorphologic observations, such as the Keluh Caldera, the intrusive emplaced within young Tarakan basin sedi-ments to the east of Jelai and the presence of andesitic volcanic complexes within the


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centre of the Tarakan Basin such as Dulun, Seruyung, and Sinelak.

It is possible that the basic components of the Late Cretaceous tectonic mechanism produced several pulses of magmatic events over time and there are still several theories on the origin of the volcanism continuing in phases until the Late Miocene-Pliocene. It is possible that the Late Miocene-Pliocene volcanism relates to a crustal thickening in the area beneath northeast Kalimantan as a result of the collision of a continental frag-ments but other theories include westward subduction beneath northeastern Kaliman-tan, volcanism as a result of rotation of Borneo during the Tertiary or postsubduc-tion I-type volcanism.

It is beyond the scope of this paper to discuss this in more detail, mainly due to the lack of definitive evidence for any of the theories of the origin of the intrusive and volcanic responsible for the Jelai mineralisation.

Regional Stratigraphy

The geology of northeastern Kalimantan is still poorly understood due to the lack of detailed mapping programmes and pub-lished data. Several petroleum companies have compiled their own stratigraphic sub-divisions, including Pertamina-Beicip and Total Indonesia. However, the most useful regional work remains that of the various BRGM/DSDM surveys undertaken from 1979 to 1986 (Figure 2).

Late Cretaceous - Early Eocene

Mapping by the BRGM/DSDM (reports by Lefevre et al., 1982; Le Bel et al., 1985 & 1986, and Nagel et al., 1987) has indicated the widespread distribution of a thick sequence of laminated siltstone, car-bonaceous siltstone, shale, and micaceous feldspathic sandstone. These rocks are well laminated, generally steeply dipping and

tightly folded and display varying degrees of metamorphism, ranging from unaltered through contact hornfels to lower green schist facies.

The BRGM/DSDM team has named the Mentarang, Paking, Lurah, Malinau, Long Bawan, and Long Geh Formations as what is an essentially synchronous sedimentary cycle but in different palaeogeographic do-mains. The lithologies observed are flysch type sequences, with a marine environment found to the north and east. Lefevre et al. (1982) estimate a combined thickness of 7,000 m for the various units. For simplic-ity, the Mentarang Formation is preferred.

Late Oligocene - Early Miocene

Several corridors of north-northeast trending granitoid intrusive complexes occur within the region. One belt occurs at Long Laai, the other southwest of Long Bia. The Long Laai complex has been studied in some detail by the BRGM/DSDM (Le Bel et al., 1985) and found to consist of leuco adamellite, with the development of exo and endo skarn facies, at the contact of the intrusive with the older sediments.

The complex situated southwest of Long Bia has been termed the Yamuk Intrusive Complex by Sennitt et al. (1994). It con-sists of biotite and hornblende granodio-rite, granodiorite porphyry, quartz diorite porphyry, andesite porphyry, and dry quartz-feldspar porphyry. Age dating by the BRGM (Le Bel et al., 1985) obtained a 22.6 Ma age for the Long Laai leucoadamellite, indicating a Late Oligocene - Early Miocene age. A similar age had been interpreted for the Yamuk Intrusive Complex but it is also possible these intrusives are the same age as the Kelay intrusive, being Middle Miocene in age, and also represented by a granodio-rite intrusive mapped east of the Jelai Gold Project in Bengara River.


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REGIONALGEOLOGICAL MAP

50 10 km

Longbawan Formation

Quaternary IntrusivesSembakung FormationKaramuan FormationIntrusive RockJelai Volcanic RockTendehhantu FormationPlug, Dyke

Alluvial DepositsSinjin Formation

Mentarang Formation

Jelai Cahaya Minerals (PT)

Malagensing

Ambalat

Figure 2. Regional geology of northeast Kalimantan (Pieters et al., 1990b).


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It seems likely that the intrusive complexeswere emplaced along existing north-north-east trending structures and that deformation and metamorphic effects observed in the older sediments, resulted from this phase of intrusive activity. Pieters et al. (1993a) believes similar rocks situated in west Ka-limantan were associated with a transitional tectonic environment, from subduction to extension.

Middle - Late Miocene

Pieters et al. (1993b) suggested extension was active during the Miocene and Pliocene, producing numerous fault-bounded graben and small subbasins, over the deformed, older sedimentary sequence, with contacts between these rocks and the older sedi-mentary sequence, being either faulted or unconformable.

The observed sequence consists of basalt conglomerate and debris flow conglomer-ate (?), overlain by sandstones, siltstones, and bio- calcarenite. The bio-calcarenite grades into reef limestone, with abundant foraminifera. A maximum thickness of 300 m has been estimated by BRGM/DSDM (Lefevre et al., 1982). Interbedded within the limestone are tuffs and tuffaceous sedi-ments, indicating a coeval volcanism.

Numerous formation names have been ad-vanced for the sequence including Sebuku, Bayangkara, Tempilan, Tenampak, Teku, and Tahling, depending upon the location of each subbasin. For simplicity, the Sebuku Formation is preferred. The interbedded dacitic tuffaceous unit is known as the Kelay Volcanics.

Pliocene

Apparently conformable with the Middle-Late Miocene sequence is a volcano-sedimentary sequence of Pliocene? Age-

restricted to small subbasins developed over deformed older sediments. Examples of this type of sedimentation include the Langap Basin, where coal seams are currently ex-ploited, and south of the town of Malinau. The sequence is confined to a small, fault-bounded, basin adjacent to a dacite porphyry complex, possibly of a similar age to the dacite porphyry complex in the Kelay.

The sequence includes a basalt pebble-cobble conglomerate unit, being overlain by a clayey and tuffaceous sandstone one. In-terbedded within these sediments are several thin, sub-bituminous coal seams. A number of these basins can be observed in the region and each group is normally referred to from its location (Tepianbuah Formation and La-ngap Formation). In the Langap area, these basins definitely postdate the adjacent dacite complexes, as thin coal seams within the Sebuku Formation have been devolatilised by intrusive, whereas the coal seams in the Langap Formation themselves retain their volatile content. However, the tuffaceous and volcanic material observed in the se-quence indicates coeval volcanism in the region during the Pliocene.

Recent

Numerous alluvial plains and river drainage system is developed in the project areas, with lithologies including semi - to uncon-solidated clays, silts, sands, and gravel to cobble conglomerates. Also observed are deposits of organic debris, including leaf litter and logs within gravel bars, as well as layers of reduced and oxidized sediments.

Regional Structures

The regional structure of the project area is dominated by the north-northeast trending Jelai Fault System, which can be traced for at least 200 km south-southwest from the project area and defines the main trend of


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volcanics and intrusive in the area. This fault system is interpreted to represent an arc- parallel structure, reactivated on numerous occasions and initially formed during the Cretaceous subduction event below Borneo from the northwest. The northern extension of the fault system is hidden beneath younger sediments of the Tarakan Basin.

The Jelai Fault System contains a number of jogs to the south of the project area and the major rivers in the area, such as the Kayan River, follow the main fault line as a series of kinked drainages extending to the south from the project area.

Subsidiary faults lie parallel to the main fault system and a number of splays can be seen extending from the fault line on the north - western side, one of which hosts the epithermal vein system at the Jelai Gold Project. These splays may actually be con-jugate fault sets developed from the main Jelai Fault System.

Other structures noted on a regional basis around the project include west-northwest trending arc-normal structures, again as-sumed to have developed during the Cre-taceous subduction event beneath Borneo and remained active until the Quaternary, where basin development continues in the northeastern Kalimantan are.

RESULTS AND DISCUSSION

Rock Types

Mentarang Formation Sediments

The Mentarang Formation in the project area is composed of shale and argillite, be-ing light to dark grey in colour when fresh, in 2 - 15 cm thick beds. Sometimes grey sandstones to 75 cm thick underlie shale bands. The thin shale sequence with occa-

sional sandstone couplets is typical of distal turbidites. In some areas, thick sedimentary breccias, composed of randomly distributed angular fragments of shale approximately 10 cm in size in a light grey clay matrix possibly represent slump zones in the distal turbidite sequence. This argillite sequence is characterised by strong presence of the S1 cleavage which, in places, turns the rock into a slate.

Volcanics

The predominant host rocks in the projectarea is fine to coarse andesitic lava. The rocks have a distinctive lava texture and comprise clasts containing crowded fine-to coarse-grained plagioclase laths, rare fine-grained subhedral granular quartz and scattered fine- to medium-grained opaque. These are hosted within a groundmass of broken feldspar, mafic minerals, minor quartz phenocrysts, and glass. Irregular compressed cavities are common.

In addition, andesitic basalt occurs and is characterised by fine- to medium-grained mafic and felsic phenocrysts set in a fine grained, glassy groundmass. It is essentially unaltered.

Lithic tuffs occur as rare thin units within theandesitic volcanic sequence. These units are typically poorly sorted and fine to coarse grained with subangular to well rounded clasts. Clasts are predominately silty mud-stone of the Mentarang Formation and me-sothermal quartz. The lithic tuff is normally sericite altered.

Intrusive

Andesite is characterised by a porphy-ritic texture of euhedral or partly resorbed phenocrysts set in a fine grained, glassy groundmass. The andesite generally dis-plays pervasive alteration with a strongly bleached, pale green colouration. It com-


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monly contains vesicles infilled with quartz and in zones of strong propylitic alteration, chlorite, haematite, and calcite. Irregular patchy alteration gives rise to a pseudobrec-cia texture.

The dacite porphyry occurs as irregular stocks, dykes, and plugs in the area and com-prises a grey to greenish-grey fine-grained rock with prominent quartz eyes to 10 mm in diameter. In some areas, the epithermal vein structures are associated with the contact of the dacite porphyry, such as the Sembawang Vein but in other areas the drilling has inter-sected significant mineralization without the presence of the dacite porphyry.

The rhyolite is restricted to the east of the concession area, where the rock has intruded into the main fault line of the Jelai Fault System as a single stock. Little is known about the genetic relationship between the rhyolite and the other intrusive and it is not known if there is a genetic relationship to any mineralization style in the area.

Structures

The local structural setting of the Jelai Gold Project is relatively simple in broad terms. The most prominent structure is the north-northeast trending Jelai Fault System, defining the eastern margin of the mineral-ization and generally perceived to be the driving structure in the project area. From the main structure, a number of splays can be seen, particularly in the Mewet area. These splays trend north-south and, in the case of the Mewet area, dip to the west at approximately 60 - 70. These splays may represent conjugate fault systems developed from either the main Jelai Fault System or form as a conjugate set of faults from the arc-normal structures.

Another interpretation is that the faults have developed as horse-tail fault sets from jogs in the main Jelai Fault, where the nearest

jog is in the Jelai River, immediately east of Mewet and defined by a significant change of the river course with a rhyolite intrusive emplaced into the area of the jog. A number of epithermal veins occur in this area also, including the Mangkuling and Abang areas and the Nyilut River.

It is likely that the most important structures for hosting mineralization include the north-northeast and north-south structures.

Alteration

Pervasive propylitic alteration represents the predominant alteration throughout the an-desitic volcanic sequence. The fine-grained mafics always show a certain degree of chlo-ritic alteration. Haematite is often present in the fine-grained matrix of volcanic flow breccias, fracture coatings, and in vesicles as coatings to chlorite and calcite. Up to 1% fine disseminated pyrite occurs with the chloritic alteration. Sporadic epidote occurs in zones of intense propylitic alteration and preferentially replaces feldspar phenocrysts and also occurs rarely as fracture coatings. In zones of intense propylitic alteration, epidote occurs in vesicles with calcite and chlorite. Epidote alteration was not observed in the vicinity of the Mewet vein.

Argillic alteration (comprising illite-smec-tite in the andesite sequence and sericite dominated in the dacitic rocks) occurs as a halo to the hydraulic brecciation and to the epithermal mineralization. The intensity of the alteration is a function of the proximity to the mineralisation. Argillic alteration is in all cases observed overprinting the pro-pylitic alteration. The argillic alteration is not specifically restricted to any one single phase of quartz veining/silicification. The intensity of the argillic-silica alteration is also a function of the intensity of breccia-tion with the strongly brecciated hanging wall acting as a more permeable medium for


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the alteration fluids than did the relatively unfractured footwall. Therefore, the argillic alteration is moderate to strong in the hang-ing wall while it quickly dissipates, over a few metres, to strong propylitic alteration in the footwall.

Although kaolinite is occasionally observedin the alteration assemblage, it is suspected to be a supergene alteration product, rather than a primary component of the alteration. Adularia is commonly found as fine dis-seminations within the colloform-crustiform banded quartz veins in this zone and is thought to be intimately associated with the gold mineralisation. The general mineral assemblage found in the primary alteration indicates most assemblages formed at near- neutral pH conditions.

An advanced argillic zone (lithocap) is not present at Mewet. This may be due to the system being eroded down to the top of the precious metal zone, well below the original paleo-surface.

Fluid inclusion studies from vein material in the project area have returned homog-enization temperatures ranging between 210 - 240C with salinities calculated be-tween 1.4 - 1.8 wt%NaCl. These data are consistent with the interpretation that the mineralization is of low sulphidation epith-ermal origin. More work is required in this area, particularly in establishing lateral and vertical patterns of the vein systems.

Mineralization

The Jelai Gold Project contains a number of mineralized areas, of which the best known is the Mewet prospect. There has been in-sufficient work in all areas to degrade or promote them in terms of prospectively, although broad geological domains and mineralization styles have guided the pri-oritisation of the areas previously, with the development of epithermal veins being

the highest priority (Mewet, Mangkuling, Nyilut, and Mipi). Other mineralization styles include silicified zones containing anomalous precious metal values with high arsenic, hosted in either tuffaceous sand-stones or Mentarang Formation sediments (Balangan, Lian, and Bakayan), breccias vein zones adjacent to dacite intrusive and sheeted epithermal vein systems (Inyang).

In general, high grade gold values are commonly confined to finely laminated chalcedonic quartz-adularia-sulphide veins (Figure 3). Variable high gold values are en-countered in brecciated chalcedonic quartz veins with the grade being dependent on the frequency of chalcedonic-colloform banded quartz vein clasts within the breccias.

cm

Figure 3. Typical high grade colloform-crustiform banded quartz vein, Mewet Vein.


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Brecciation of the colloform banded quartz and subsequent additional deposition of chalcedonic quartz + adularia lead an up-grade of the gold grade. The mineralisation appears tightly structurally controlled with little gold mineralisation observed in the footwall of the veins. Low grade gold values are occasionally obtained within the breccias column in the hanging wall of the quartz veins. Silver grades are sporadic with the most elevated value occurring with the high grade gold in the finely laminated, sulphidic quartz veins.

In the Mewet prospect area, a number of veins are known to be gold-bearing, includ-ing the Sembawang Vein, the Mewet Vein, the Lipan Vein, and the Salam Vein. Drill-ing has tested only the shallow portions of the veins over a limited strike length. The Mewet Vein itself is the best known regard-ing the potential for defining geological resources.

The mineralization in the Mewet Vein com-prises a discrete lode structure containing mixed hydrothermal vein breccias, mas-sive quartz veins, and colloform-crustiform quartz veins in a zone trending generally north-south and dipping at approximately 60 to the west.

Over the strike length of the vein, a pinch and swell effect is observed with the width being variable at a similar RL along the strike of the vein. The lode contains a late cross-cutting quartz-carbonate vein and fracture fill phase at 65 - 70. Late phase veins are typified by glassy to milky, fine-grained drusy quartz + carbonate which are generally barren with assays rarely exceed-ing 0.02 ppm Au. The late phase quartz is rarely weakly amethystine in nature and crosscuts all phases of brecciation and veining. Postmineralization andesitic basalt dykes were intersected in some drill holes. Weak to moderate propylitic alteration oc-

curs within the andesitic dykes, predomi-nately as fracture coatings.

Mineralogically, the gold mineralization is extremely fine and free-milling in the Mewet prospect area. Higher grades of gold are often associated with fine, black sulphide bands in colloform quartz, which may be gold-rich electrum or a gold sulphosalt. Free gold has never been noted in any of the prospects in the project area. Attempts by local miners to amalgamate the gold have failed but recent metallurgical test work has shown almost complete extraction of the gold in less than 24 hours by cyanide. The silver/gold ratio is generally low, unlike some other Kalimantan epithermal systems such as Mount Muro.

Recent geological modelling of the MewetVein has established an exploration target of100,000 ounces of gold over a 250 m strikelength of the vein in the best drilled area. The exploration target is between 850,000 - 950,000 tonnes at between 2 - 3 g/t Au and a similar grade of silver. The mineralization extends from the surface to 150 m below the surface, or over a vertical interval of 245 m. Higher grade shoots within the main vein structure plunge to the north at approxi-mately typical drill hole intercepts from the drilling in the Jelai Gold Project area are outlined in Table 1 and shown on Figure 4.

Interpreted Genesis of the Epithermal Veins

Multiple phases of brecciation and vein formation are evident in the project area. Similar systems in other parts of Indonesia, and indeed the world, follow similar pat-terns of development. Although no specific studies have been undertaken to collaborate the observations , the following section describes the development of a typical low sulphidation quartz vein, within the context of the Mewet Vein.


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Hole From To Metre Au (g/t) Ag (g/t) Vein

JCM-60 60.60 62.10 1.50 1.24 0.90 Lipan

JCM-53 28.75 31.50 2.75 1.09 1.05 Lipan

JCM-50includes

26.4526.45

34.5027.80

8.051.35

4.5215.40

2.746.00 Lipan

JCM-38includes

21.9523.45

27.3524.95

5.401.50

11.7430.68

5.0513.80 Lipan

JCM-01 21.55 24.70 3.15 5.76 2.60 Lipan

JM024and

60.7358.07

60.8559.35

0.121.28

53.88.40

18.003.00 Lipan

JCM-66 includes

43.8046.70

48.0548.05

4.251.35

1.642.99

1.181.90 Sembawang Cent.

JCM-51 128.85 130.10 1.25 1.02 BD Sembawang Cent.

JCM-49 7.10 10.40 2.20 1.07 1.03 Sembawang Cent.

JCM-25and

14.6032.00

15.5035.50

0.903.50

2.961.01


JCM-26 includes

42.0043.40

47.2044.60

5.201.20

5.6017.33


JCM-27includes

12.5516.70

18.5017.35

5.950.65

2.1510.30


JCM-04 22.35 23.10 0.75 5.75 4.20 South Mewet

JCM-16 34.50 38.50 4.00 2.27 1.10 Sembawang South

JCM-09 37.73 38.30 0.57 1.16 BD Sembawang South


JCM-13and

19.4532.00

20.4536.75

1.004.75

2.2310.43

2.4014.60 Sembawang South


JCM-19 43.30 44.30 1.00 5.48 2.00 Balangan

JCM-69and

22.7552.00

28.7554.85

6.002.85

15.842.09

81.196.91 Mewet

JCM-68and

36.6559.70

39.1060.15

2.450.45

4.452.83

11.0333.00 Mewet

JM011 26.31 28.43 2.12 6.90 16.00 Mewet

JM012 57.00 58.70 1.70 11.10 10.00 Mewet

JM013 38.33 40.20 1.87 7.50 13.00 Mewet

JM015and

67.5069.27

69.0070.77

1.501.50

7.307.40

14.0020.00 Mewet

JM018 115.00 119.51 4.51 6.40 8.00 Mewet

JM019 185.10 191.20 6.10 5.70 15.00 Mewet

Table 1. Significant Gold Intercepts from the Drilling at the Jelai Gold Project


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Figure 4. Typical cross section of drilling on the Mewet Vein.

-50 m

0 m50 m

-100

m

-150

m

9 1 0 m4 8 5

9 5 m4 78 0

497900 m

497950 m

4 80 0 9 0 m

49805 m0

1498 00 m

9 0 4 78 0 m

4 77 0 m9 5

JM020

J019 M

5050

100

150

200

J018 M

200

50 2 255m2

00

50

010

JM012

209.9

5m

224m

50

150

1JM

01

CM7

J8

100

150

010

115.7

m75.7m

0 50

010

150

186m

Histo

gran

Au (

g/t)

Lith

olog

y

0 - 05

05 -

1

1 - 15

15 -

26

26 -

6

5 - 10

10 -

15

15 -

20

VADOL

HOX

QV VAL

VEL

VOP

VEX


15

North-south structures, originating from the major Jelai Fault System to the south, pro-vided a conduit for ascending near-neutral chloride fluids which led to rapid silicifica-tion. The silica solution, with the silica origi-nating from the leaching of the wall rocks at high temperatures, remained saturated and was deposited slowly, resulting in the formation of mesothermal/crystalline quartz observed in the early phase of silicification. Sealing of the system with silica resulted in an over pressuring effect, which was relieved through explosive brecciation of the system. Brecciation of the earlier phase of quartz veining resulted with subrounding of some clasts indicating that there has been some transportation from depth. No clasts of the underlying sedimentary basement were encountered in the drill core within the vein structures.

Mixing of CO2 with meteoric water led

to the deposition of rhombic and bladed carbonate as breccias matrix flood and cavity infill. Subsequent mixing of the supersaturated acidic fluids with meteoric water resulted in the rapid deposition of fine grained microcrystalline to chalcedonic quartz. Overprinting acidic/silica enriched fluids partially replaced the carbonate with amorphous silica, leading to massive chal-cedonic silica with patches of bladed calcite pseudomorphs, lattice infilled textures and colloform-crustiform banded quartz veining.

Massive carbonate cavity fill in some veins has partially been replaced by chal-cedonic silica with the zone proximal to the colloform- crustiform banded quartz + adularia vein being completely replaced to massive chalcedonic silica. Continued boiling of the epithermal system led to several telescoping phases of overprint-ing events, representing varying degrees of pH and concentrations of silica + CO2. The system represents several phases of sealing and rebrecciation with a number

of phases of colloform-crustiform quartz clasts observed in the chalcedonic quartz.

Colloform banded quartz clasts formed as a product of an acidic fluid are observed within a matrix of carbonate. Drusy quartz is in places overprinted with rhombic calcite with the calcite rarely displaying a late drusy quartz coating. Clasts of earlier chalcedonic quartz are often observed within the mas-sive appearing carbonate veining and the quartz veining is also often represented by several phases of sealing and cracking. The collapse of the hydrothermal system led to the draw- down of meteoric waters leading to the deposition of glassy crystalline quartz + calcite. Rarely amethystine quartz veining is noted as a late overprinting event.

CONCLUSIONS

The Jelai Gold Project contains some classic examples of high-level epithermal precious metal mineralization. The deposit charac-teristics indicate the veins formed at low temperatures from low salinity fluids at near neutral pH conditions. All of these features are typical of low sulphidation epithermal systems. It is also interpreted that a number of mineralized areas within the concession area require further exploration work to truly prioritise the prospective ranking of each target.

Regionally, the deposits are hosted within younger (Late Miocene to Pliocene) volca-nics comprising andesite and dacite lava, dacite porphyry intrusive, minor tuff, and andesite intrusive. There is evidence to sug-gest a final basaltic andesite phase occurred after the mineralizing system ceased.

The geology of the prospect area shows a dynamic volcanic and sedimentation regime since the Cretaceous until the Pliocene or perhaps younger. Structurally, faults asso-


16

ciated with the oldest tectonic event have formed host structures and fluid conduits that contain the mineralization or controlled the emplacement of intrusive bodies that may be genetically linked to the epithermal mineralization in the area.

The mineralized zones are typically siliceous veins and vein breccias zones containing multiple episodes of chalcedonic to crystal-line quartz phases, with or without precious metal mineralisation, surrounded by immedi-ate argillic alteration grading out to regional propylitic alteration. The alteration is reason-ably typical of these types of deposits.

The main area of exploration focus, includ-ing drilling, in the Mewet prospect has defined areas which can be geologically modelled to produce exploration targets of sufficient scope to encourage further work. One such example is the Mewet Vein, where250 m of the vein has produced a pre- re-source target of 850,000 - 950,000 tonnes grading 2 - 3 g/t Au for approximately 100,000 ounces of gold.

The exploration of the Jelai Gold Project is an example of persistence and detail, often required to adequately explore epithermal systems, where the bonanza shoots and economic portions of the veins may not be apparent at surface. The drilling of these targets is also difficult, requiring targeting a shoot sometimes less than 50 m in width along the vein.

It is important when exploring these types of deposits that surface observations of vein texture, alteration, and structural setting are interpreted correctly to target an essentially hidden bonanza zone at depth. Surface geo-chemistry and geophysics are useful tools for exploration but care is required to ensure what may be geochemically barren quartz at surface does not contain textural or struc-tural signals of a bonanza ore zone at depth.

ACKNOWLEDGEMENTS

Thanks go to Kalimantan Gold Corporation for allowing a presentation of the data on the Jelai Go ld Project and to Mansur Geiger for numerous dis-cussions on the ins and outs of the project, from an operational to geological context. Dr. Peter Pollard has also contributed his viewpoint on aspects of the Jelai mineralizing system. There are also a number of KGC geologists who have contributed knowledge to the deposit in recent years. My thanks to Abdi Dame and Adi for putting the maps together. In addition, my acknowledgement is made for the thousands of man-days spent by Indochina Gold-fields/Ivanhoe Mines geologists, many who still re-main close friends and associates and some who lost their lives, on traversing the 3 million ha of Contract of Work between 1996 and the end of 2000. Without their efforts and the efforts of those that follow, there would be little chance that a deposit such as the Jelai Gold Project can come to the point that we believe it should be.

REFERENCES

Fleming, G., 1993. Borneo Project, selective recon-naissance survey, Co. Rept. (PT Macan Mas Miner-indo). Unpublished

Katili, J.A., 1973. On Fitting Certain Geological and Geophysical Features of the Indonesian Island Arc to the New Global Tectonics. In: Coleman, P.J. (ed.), The Western Pacific Island Arcs, Marginal Seas, Geochemistry, University of Western Australia Press, p.287-305.

Kirwin, D.J., 1993. Technical notes concerning the epithermal gold-silver potential of Northeast Kalimantan, Indonesia. Internal Company Report (Unpublished).

Kirwin, D.J. and Sennitt, C.M., 1994. Aerial re-connaissance conducted in northeast Kalimantan, Indonesia. Internal Company Report. (Unpublished).

Le Bel, L, Nagel, J.L., Lecomte, P., and Machali Muchsin, A., 1985. CTA39A, Follow-up work in the Longlaai area, NE Kalimantan (The Longlaai Project), Phase 1 (1984-1985), BRGM Report 86 IDN 001, 107pp. (Unpublished).

Lefevre, J. C., Collart, J., Joubert, M., Nagel, J. L., and Paupy, 1982. BRGM-Geological mapping and mineral exploration in Northeast Kalimantan 1979-


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1982. Final Report. Report Bureau de Recherches Geologiques et Minieres, Orleans, France No. 82 RDM 007 A. (Unpublished).

Nagel, J. L., 1987. Drilling in the northern part of the Long Laai skarns. BRGM - Anekatambang, J.V. - 1987, BRGM Report 87 IDN 224, 25p. (Un-published).

Pieters, P.E. and Supriatna, S., 1990b. Geological map of the West, Central, and East Kalimantan Area, Scale 1 : 1.000.000. Geological Research and Development Centre, Bandung, Indonesia.

Pieters, P. E., Abidin, H. Z., and Sudana, D., 1993. Geology of the Long Pahangai sheet area, Kaliman-

tan, 1 : 250.000, Geological Research and Develop-ment Centre, Bandung, Indonesia.

Pieters, P. E., Surono, and Noya, Y., 1993. Geology of the Putusibau sheet area, Kalimantan, 1 : 250.000. Geological Research and Development Centre, Bandung, Indonesia.

Sennitt, C.M. and Kirwin D.J., 1994. Geological As-sessment and Evaluation of The Jelai River Kelapis and Long Laai Areas, Northeast Kalimantan, Indo-nesia. Internal Report (Unpublished).


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