Theo van Leeuwen

25 years of mineral exploration and discovery in Indonesia

Abstract This paper traces the history of mineral exploration in Indonesia between 1967 and 1992, and discusses various technical aspects, such as area selection, exploration and discovery methods, and significant geologic features of the more important new discoveries. Exploration activity over the past 25 years can be divided into four main phases. Phase 1 (1967–1976) mostly involved investigations of mineral prospects and districts previously identified by the Dutch. These investigations resulted in many discoveries, including: a major copper-gold district (skarn-porphyry copper) in Irian Jaya, where exploration is still in progress (resources identified to date: 28 Mt Cu and 2,700 t Au); large nickel resources in Eastern Indonesia (13 Mt Ni); significant onshore and offshore tin resources in the Sumateran tin belt (0.13 Mt Sn); and large but low grade bauxite deposits in West Kalimantan (300 Mt Al 2 O 3 ). Of the eight Contracts of Work signed between 1967 and 1972, six reached the mining stage. Phase 2 (1970–1975) consisted of an extensive porphyry copper search in the Sunda arc, the western arc of Sulawesi and the central belt of Irian Jaya. Best results were obtained from northern Sulawesi, where follow-up between 1976 and 1982 identified three potentially economic copper-gold deposits (1.7 Mt Cu and 140 t Au) and one subeconomic molybdenum porphyry system (0.8 Mt Mo). During Phase 3 (1981–1988) extensive coal exploration in South and East Kalimantan delineated over 5,000 Mt of coal of varying rank and quality, including 1,500 Mt as measured reserves in 17 deposits, eight of which have been developed to date. Phase 4 (1984–1990) involved a major gold rush, focused primarily on the Cenozoic magmatic belts of Kalimantan, Sulawesi, Moluccas and the Sunda arc. Over 80 primary and alluvial gold prospects were drill tested. Five of these were brought into production (two alluvial deposits, two new hard rock discoveries and one Dutch mine), containing approximately 135 tonnes of mineable gold, and several other projects are under development or undergoing feasibility studies. Total geological resources identified to date are estimated to contain about 700 tonnes of gold. Exploration during phase 4 also resulted in several gold-rich porphyry copper discoveries, including a major deposit in Sumbawa (2.7 Mt Cu and 250 t Au). Intermittent exploration for uranium, diamonds and lead/zinc since 1969 has been largely unsuccessful. Exploration is now passing to the next phase, which is likely to be multi-commodity in nature with a strong focus on gold, copper and coal. A number of deposits outlined during earlier phases will be developed. The unprecedented high level of mineral exploration activity over the past 25 years can be attributed to Indonesias mineral prospectivity and favourable investment climate. Given a continued competitive commercial environment and sustained commodity prices, the next 25 years should see further strong development of the countrys mineral resources.

Spatial and temporal isotopic domains of contrasting igneous suites in Western and Northern Sulawesi, Indonesia

Abstract Palaeocene to Pliocene magmatism in NW Sulawesi shows a progression from an Older Series with calc-alkaline/tholeiitic signatures (51–17 Ma) to a Younger Series of mafic-intermediate high-K magmas (∼14–5 Ma) and felsic K-rich calc-alkaline (CAK) magmas (9–2 Ma). The isotopic and geochemical compositions of the Older Series samples indicate that the more westerly samples have been generated in a continental arc setting and the more easterly samples in an oceanic arc; this distinction defines the boundary between the Western and Northern Sulawesi tectonic terranes. The Younger Series high-K magmas have unusual isotopic compositions, with variable but low 143Nd/144Nd, high 87Sr/86Sr values, and high 208Pb/204Pb for their 206Pb/204Pb ratios compared to subduction-related magmas. The diversity of the isotopic compositions points towards a source with a long and heterogeneous geochemical evolution, most likely located within the Australian subcontinental lithospheric mantle. The Younger Series felsic CAK magmatism has a more homogeneous isotopic and geochemical signature and reflects melting of continental crust of Australian origin. This geochemical progression in time is very similar to that seen in central Sulawesi [Chem. Geol. 156 (1999a) 67], and is best explained by normal subduction of an oceanic plate followed by melting of an underthrust sliver of Australian continent. The size of this microcontinent can be estimated from the areal extent of low-Nd-isotope magmas in Western Sulawesi, ranging from approximately 4°S to 1°N. Underthrusting must have happened prior to 14 Ma, indicating that this event cannot be equated to the collision between Sulawesi and the Sula platform, which occurred at 5 Ma. While subduction beneath western Sulawesi ceased prior to the onset of potassic magmatism in this region, it continued beneath northern Sulawesi producing predominantly calc-alkaline suites.

The Kelian disseminated gold deposit, East Kalimantan, Indonesia

Kelian is one of a number of recently discovered Tertiary volcanic-hosted gold deposits which occur within a 400-km-long, northeast-trending belt in the interior of Kalimantan, the Indonesian part of Borneo. It consists of two main orebodies, namely West and East Prampus, and four smaller mineralized ore zones. These have a combined resource potential of +75 Mt at 1.8 g/t Au, making Kelian the largest known gold deposit in Indonesia. The deposit was discovered in 1976 during follow up work of alluvial gold occurrences within the Kelian River, using traditional exploration techniques of stream sediment, pan concentrate, rock float and outcrop sampling. Subsequent detailed investigations included soil sampling, trenching, deep augering, ground magnetics, induced polarization and 60,000 m of diamond drilling. The geology of the deposit consists of a pile of silicic pyroclastics grading upwards into a series of sediments of Late Eocene age. The sequence was folded and faulted along northerly and northeasterly trends and intruded by a number of subvolcanic andesitic to trachyandesitic bodies in the Early Miocene. Some time after the emplacement of the andesites a hydrothermal system was established. It brought about extensive alteration, mineralization and hydrothermal brecciation. The preserved part of this system is 1 km2 in area and has a vertical extent of at least 600 m. Limited radiometric age data suggest an age of around 20 Ma for the alteration. Four partly overlapping stages of alteration/mineralization are recognized: 1. Stage I: chlorite+carbonate+sericite+pyrite±epidote alteration in the cores of the larger andesite bodies. 2. Stage IIA: localized fracturing and sericite±quartz deposition. Stage IIB: a major period of polyphasal fracturing and hydrothermal brecciation with associated boiling of hot (290–330°C), dilute (0.5–4.2 eq. wt.% NaCl) and CO2-rich fluids, and development of replacement/vein alteration of adularia+sericite+quartz+pyrite accompanied by subordinate As-Ag mineralization and possibly some gold mineralization. Progressive loss of CO2 upon boiling caused a progressive increase in adularia and a corresponding decrease in sericite deposition. Deeper in the system a quartz-sericite-carbonate assemblage was formed without adularia. 3. Stage III: renewed magmatic activity, focused on East Prampus, released volatiles to the hydrothermal system, producing a hot (300–330°C), relatively saline (> 10 wt.% NaCl), CO2-rich brine, and extensive fracturing and hydrothermal brecciation. This was accompanied by boiling, deposition of various carbonates±quartz±sericite and gold-base metal mineralization; preliminary stable isotope data on the carbonates support the presence of a magmatic component. Mixing of the upwelling hot, saline fluids with descending cooler, relatively low pH, steam-heated fluids has resulted in a complex zonation in carbonate mineralogy and a wide range of fluid salinities and homogenization temperatures. Away from the centre of brecciation and boiling, gold and base metals were deposited in a more passive environment together with carbonate and/or quartz. 4. Stage IV: late-stage widespread influx of relatively cool (<200°C), low pH (3–4), CO2-rich, steam-heated fluids from the top and margins of the system forming a kaolinite+Fe/Mn carbonate+pyrite ±hematite±cinnabar assemblage. The magmatic-hydrothermal event was followed by uplift and erosion of more than 900 m during the Midddle Late Miocene and basaltic volcanism in the Plio-Pleistocene. A second cycle of erosion, which began in the Pleistocene, has removed most of the young volcanic cover exposing the deposit as known today.

Paleomagnetic studies combined with fission-track datings on the western arc of Sulawesi, east Indonesia

Abstract Paleomagnetic studies in conjunction with fission-track dating on the western arc of Sulawesi yield important evidence bearing on the tectonic history of the area. During the Paleogene to Early Miocene time interval the paleomagnetic pole for southwestern Sulawesi was situated at 36.5°E 44.8°N. This pole position is significantly different from that in the time interval Middle Miocene to Recent, which is consistent with the north pole of the axial geocentric dipole. This fact suggests that subsequent to the Paleogene to Early Miocene period, possibly 19–13 m.y. B.P., a major tectonic event occurred which caused about 40 degrees of anticlockwise rotation of the area. It is suggested by the present work that the postulated collision followed by welding of eastern Sulawesi with western Sulawesi during the Pliocene (Katili, 1978) may be the tectonic event mentioned above. In addition, our data does not support the hypothesis that western Sulawesi has been derived from the dispersal of Gondwanaland.

Porphyry molybdenum mineralization in a continental collision setting at Malala, northwest Sulawesi, Indonesia

Abstract The Malala deposit in northwest Sulawesi is the only known porphyry molybdenum occurrence in Indonesia. It is typical of the fluorine-poor (quartz monzonite or differentiated monzogranite) class of molybdenum deposits and belongs to the plutonic sub-type. The mineralized system is associated with porphyritic intrusive rocks of predominantly granitic composition (Malala porphyries) which occur as late differentiates in the roof zone of a composite pluton (Dondo batholith). Major oxide, trace element, isotope, and mineralogical data indicate that the various intrusive phases of the Malala-Dondo suite had a common magma source, are magnetite-series, LILE-enriched, (Caledonian) I-type granitoids, and belong to the high-K calc-alkaline series. The intrusive suite forms part of a 600 km long belt of granites and granodiorites, which were emplaced in a continental margin (“Western Sulawesi”) in Late Mio-Pliocene time, during and following the collisions between several microcontinents and the Mesozoic-Tertiary western magmatic arc/eastern subduction complex which forms the island of Sulawesi. The granitoids have initial Sr isotope ratios of 0.71–0.72, and are interpreted to be the results of partial melting of lower crust (possibly underthrusted continental crust of Precambrian to Paleozoic age) due to lithospheric thickening in a continental collision regime. Alteration and mineralization at Malala are erratically, and in most places weakly, developed over an area of 4 km 2 , predominantly as a “shell” up to 50 m thick at the intrusive contact. Highest grades are found in the East Zone, an elongate, steeply dipping, NW trending, fault controlled mineralized zone, that has an estimated resource of 100 Mt at 0.14% MoS 2 . No well-defined zoning of discrete alteration and sulphide mineral assemblages has been recognized. Petrographic, fluid inclusion and oxygen isotope data suggest that fluids of two origins were involved in the evolution of the mineralized system at Malala. Early hypersaline (40–65 eq. wt.% NaCl), hot (400–700°C) fluids were of magmatic derivation and circulated through fractures while the host intrusion was initially still in a semi-consolidated state, producing potassic alteration, barren quartz veins (stage 1) and quartz-K-feldspar-apatite-molybdenite veins (stage II). This early event was terminated upon introduction of predominantly meteoric, less saline fluids, which flowed through newly formed fractures and older reopened veins at temperatures in the range of 200 to 400°C, resulting in wall-rock alteration to, and vein deposition of, sericite, chlorite, carbonate and base metal sulphides (stage III). Finally, carbonate and kaolinite/dickite were deposited in late fractures (stage IV). Malala shows both significant similarities with and differences from other F-poor porphyry molybdenum deposits. The similarities appear to be in the nature of the cogenetic igneous rocks (granite, quartz monzonite and granodiorite) having relatively high Sr and Ba, and low Rb and Nb contents compared to F-rich Climax-type deposits, the geochemistry of the hydrothermal system (high Cu, low F and Sn), general vein paragenesis and lack of multiple ore shells. Malala differs from most other deposits with respect to its continent-continent collision setting, the late magmatic (“deuteric”) nature of the molybdenum mineralization and the dominance of carbonate alteration.