The Martabe Au-Ag high-sulfidation epithermal deposits, Sumatra, Indonesia: implications for ore genesis and exploration

The Martabe gold district, situated on the north-west coast of Sumatra, Indonesia, consists of four high-sulfidation epithermal gold-silver deposits over an 8 km strike length: Purnama, Baskara, Kejora and Gerhana and one low-sulfidation epithermal gold-silver deposit, Pelangi. Resources have been estimated for the three principal known deposits, Purnama, Baskara and Pelangi. Resources (inferred, indicated and measured) are 91.2 Mt @ 1.5 ppm Au and 19 ppm Ag for Purnama, 36.6 Mt @ 1.0 ppm Au and 4 ppm Ag for Baskara, and 10.4 Mt @ 1.1 ppm Au for Pelangi. Reserves (proved and probable) are based on an optimized open-pit mine design which extends approximately 900 m north-south along strike; total Purnama reserves are identifying 35.7 Mt @ 1.9 ppm gold and 26 ppm Ag. Martabe is located in the western Sunda Banda magmatic arc within and adjacent to a Late Tertiary porphyritic dacite and andesite dome and diatreme complex that was emplaced into a volcano-sedimentary sequence comprising interlayered sandstone, siltstone, carbonaceous mudstone and andesite lava flows. Martabe is located near a series of fault splays of the Sumatra Fault System and this structural framework has played an important role in the formation of the deposits. At Martabe there are a wide variety of distinctive breccias within the dome and diatreme complex. The origins and processes of these breccias are varied and include phreatomagmatic, phreatic, tectonic and hydraulic brecciation. Recognition of stratified and unstratified breccias, base surge deposits and overlying airfall tuffs indicate that the initial setting was a maar volcanic field, containing multiple diatreme vents. Intrusion of felsic magma into a fault-bounded block of brecciated carbonaceous mudstone (part of Sumatran Fault System) within an active hydrothermal system resulted in phreatomagmatic brecciation at Martabe. The presence of juvenile magmatic clasts with delicate wispy texture and cuspate margins, in situ rhyolite clasts associated with dacitic-andesitic dykes and base surge (stratified breccia) deposits are the key pieces of evidence for a phreatomagmatic origin. The flow dome complex formed at ca. 3.8¬± 0.5 Ma (quartz-phyric dacite, U-Pb method) and 3.1¬±0.4 to 2.8¬± 0.3 Ma (hornblende-phyric andesite, U-Pb method), and extensive alteration closely followed emplacement of the domes at 3.30¬± 0.11 to 2.14¬± 0.10 Ma (alunite, K-Ar method). These periods of hydrothermal activity indicate the magmatic/hydrothermal system was active from 3.8 to 2.1 Ma and the main-stage alteration occurred within 0.5‚-0.8 Ma of dome emplacement. All identified economic and sub-economic gold-silver mineralization of the district displays a zonal pattern of alteration typical of high-sulfidation epithermal systems, with the presence a low-sulfidation epithermal system at the periphery (i.e., Pelangi). In general, the alteration at Martabe consists of siliceous (quartz dominant), advanced argillic (alunitic and kaolinitic), argillic and propylitic alteration. The 'alunitic' alteration assemblage consists of quartz+alunite¬±dickite/kaolinite¬±pyrite; the 'kaolinitic' assemblage refers to quartz+kaolinite/dickite¬±alunite¬±pyrite; the argillic assemblage contains illite-smectite-pyrite+quartz; and the propylitic assemblage consists of chlorite+epidote+calcite ¬±illite/sericite¬±pyrite¬±quartz. Alteration is typically zoned from a core of brecciated, massive and vuggy quartz (siliceous alteration) that grades outwards through advanced argillic alteration (alunitic and kaolinitic) to argillic alteration that is surrounded by a peripheral zone of pervasive propylitic alteration. Economic mineralization is hosted within the siliceous and advanced argillic alteration zones. The alteration zones most likely occurred as multiple stages and formed contemporaneously with the Martabe dome magmatism. The majority of epithermal Au-Ag mineralization in the district is characterized as high-sulfidation, based on the sulfide and sulfosalt mineral assemblage and advanced argillic alteration. Ore mineralization consists of enargite-luzonite+tetrahedrite-tennantite-pyrite in veins, vugs and as breccia matrix. Jarosite, hematite and goethite are the most common products of oxidation. Gold is present as micron-sized native gold grains associated with quartz, Fe-oxides, enargite-luzonite, tetrahedrite-tennantite and covellite-digenite. Purnama consists of disseminated Au-Ag mineralization distributed sub-horizontally within and adjacent to the western perimeter of the diatreme with both strong lithological and structural control. A quartz matrix-supported breccia contains the majority of high-grade ore, although lower grade mineralized zones occur within pervasive advanced argillic-alunitic altered breccias and advanced argillic-kaolinitic altered andesite. At Baskara and Kejora, mineralization is sub-vertical and spatially associated with phreatic and phreatomagmatic breccia bodies emplaced along NE-striking faults. The best gold mineralization occurs at the contact between breccia bodies and the dacite-andesite dome. At Pelangi, gold- and silver-bearing, low-sulfidation, quartz veins and stockworks cross-cut the advanced argillic alteration. At Gerhana, the mineralization are with both lithology and structural controls and are likely similar to Purnama. Results of a LA-ICP-MS study of trace-element mineral chemistry in pyrite and enargite indicate significant variability in trace-element compositions, particularly in Fe, Te, Bi, Sn, Se, Au, Pb, Mo, W and Ba. Three generations of pyrite are recognized. An early stage of well-crystallized pyrite (stage-1) is enclosed by fine (submicron) overgrowths of second stage of poorly crystalline pyrite (stage-2). The stage-1 cores have relatively low trace element concentrations. Conversely, the stage-2 pyrites contain an abundant variety of trace elements with high concentrations. Stage-3 pyrite occurs as filling vugs or fractures and encompasses stage-1 and stage-2 pyrite. Stage-3 pyrite has a similar range of trace elements as stage 2 but at lower concentrations. In general, the most abundant trace elements in enargite are those that also form discrete sulfosalt, selenide and telluride accessory phases. Enargite in the Purnama and Gerhana deposits is enriched in Au, Se and Te. The sequence of events that formed the Martabe diatreme/dome complex and associated alteration and mineralisation is interpreted to be: (i) Down faulting of pre-Miocene sedimentary units. (ii) Intrusion of felsic magma along existing structures. Interaction of meteoric water with magma initiated early hydrothermal convection (low-sulfidation fluid). Formation of argillic and propylitic alteration. (iii) Felsic intrusions continued to move up along the existing faults and triggered phreatomagmatic and phreatic eruptions (diatreme formation). (iv) Phreatomagmatic eruptions continued to excavate the conduit and widen the diatreme producing multiple crosscutting breccias. (v) High temperature, extremely acid (high-sulfidation) fluids from condensation of magmatic volatiles caused advanced argillic alteration. No precious-metal mineralization accompanied this alteration. (vi) The hydrothermal system returned to convection of meteoric water (low-sulfidation fluid). Formation of Au-Ag-bearing, quartz-chalcedony veins. (vii) Intrusion of dacite and hornblende andesite into the diatreme creating the dome complex. Triggering of phreatomagmatic and phreatic eruptions. (viii) Domes interacted with meteoric water causing hydrothermal convection of a low-sulfidation fluid with overpressure leading to hydrothermal brecciation. (ix) Introduction of magmatic, metal-rich, high-sulfidation fluid depositing Cu-Au mineralization (disseminated and fracture-controlled pyrite-enargite-luzonite-tennantite-tetrahedrite). (x) Late-stage phreatic brecciation. (xi) Oxidation, erosion and weathering. The Martabe Au-Ag deposits show a complex interplay of intrusive events, phreatomagmatic, phreatic, and hydrothermal brecciation and differing stages of hydrothermal (low- and high-sulfidation) fluid introduction. The superposition of both high- and low-sulfidation mineralizing events increases the precious metal content of the mineralization, and adds to the overall exploration potential of the district.