The geology and genesis of the Kencana epithermal AU-AG deposit, Gosowong Goldfield, Halmahera Island, Indonesia

The Kencana Au-Ag low-sulfidation epithermal deposit, situated in the Neogene magmatic arc of Halmahera, Eastern Indonesia, has an estimated resource of 4.4 Mt @ 27.9 g/t Au, containing 4 Moz Au. The deposit, forming part of the Gosowong Goldfield, is the third, and most recently discovered (2002) deposit in the goldfield, after the Gosowong and Toguraci deposits. The Gosowong goldfield is situated on the eastern side of the NW arm of Halmahera, which is composed of four superimposed volcanic arcs, produced as a result of subduction of the Molucca sea plate beneath Halmahera since the Palaeogene. Lithologies are dominated by andesite to basaltic andesite volcanic and volcaniclastic rocks and diorite intrusions. Epithermal mineralization is hosted by the upper Miocene Gosowong Formation, a series of interbedded volcaniclastic rocks, ignimbrites and coherent andesitic volcanic flows and diorite intrusions. Andesites and diorites are closely temporally related, with andesite emplacement at 3.73 ± 0.22 Ma followed by diorite intrusion at ~3.50 Ma. Epithermal mineralization post-dates andesite and diorite emplacement with \\(^{40}Ar/^{39}Ar\\) dating of hydrothermal adularia giving a mean age of 2.925 ± 0.026 Ma for the Kencana deposit. The deposit is hosted by two main sub-parallel NW-trending fault structures, (namelyK1 and K2) with a strike length of 400 m, a vertical extent of 200 m and dipping 46°E; joined by link structures, such as K-Link (KL). Bonanza Au-grade zones are located in dilational zones above hematitic volcaniclastic mudstone packages. The deposit does not crop out, but displays a weak surface expression represented by carbonate veining and faults filled with clay and pyrite. Kencana (K1 vein) shows a complex, multiphase history of formation with numerous brecciation and opening events. Eleven infill types are recognized at Kencana, including wallrock (1), quartz stockwork (2), wallrock breccias with crystalline quartz cement (3), red chalcedony infill (4), massive crystalline quartz (5), massive crystalline quartz breccias (6), cockade-banded quartz-chlorite breccias (7), banded quartz-chlorite (8), banded quartz-adularia (9), grey cryptocrystalline quartz stringer veins (10) and black quartz-molybdenite infill (11). Infill types 1, 2 and 3 are distributed throughout the deposit and are particularly prevalent on the margins of the vein. Types 7, 8 and 9 are the main ore-bearing stages and form the bulk of the central section of the vein. Type 11 infill is most prevalent to the north. Types 4, 5, 6 and 10 are variably and sporadically distributed across the vein. Ore assemblages are dominated by high-fineness electrum and sulfides, with selenides and lesser tellurides and sulfosalts. Chalcopyrite is the most common sulfide mineral, with selenian-galena, sphalerite, bornite and pyrite in order of decreasing abundance. Other accessory minerals include aguilarite and molybdenite, with trace tennantite, arsenian-pyrite, silver and lead tellurides, naummanite and rare bismuth minerals. Gangue minerals are crystalline, microcrystalline and cryptocrystalline quartz, adularia, chlorite and calcite. Ore deposition is interpreted to be the result of a combination of processes (mixing, boiling and cooling), with mixing processes inferred to be of particular importance, based on the presence of high-fineness electrum and selenium-bearing minerals. Evidence for boiling processes is present at Kencana after deposition of infill type 5, including bladed carbonate pseudomorphs and abundant adularia. A general transition from early coarse crystalline quartz to micro- and cryptocrystalline quartz is observed in the paragenetic sequence of the K1 vein, reflecting an increase in the rate of silica precipitation in silica-saturated fluids. Fluid inclusion data indicate that early crystalline quartz (type 2c) was precipitated from near-neutral, hot, low salinity, low `CO_2` (203.0 to 248.8°C, 0.1 to 0.5 wt% NaCl (equiv.), <0.015 m `CO_2`) fluids. Temperatures and salinities increase during formation of the main ore-bearing stages of the deposit. Fluids associated with precious metal deposition in type 7 and 8 infill are 202.7 to 306.9°C, 0.0 to 1.0 wt% NaCl (equiv.), <0.015 m `CO_2`, with fluids in quartz-adularia (type 9) mineralization forming at (95.8 to 258.7°C, 0.0 to 0.8 wt% NaCl (equiv.), and <0.015 m `CO_2`). Pressure estimates used to calculate minimum depth of entrapment infer that most fluid inclusions were trapped 50 to 200 m deeper than their current location. Mineralizing fluids are strongly dominated by meteoric water, with a marginally increased magmatic input during formation of type 7 and 8 infill. Gold hydrosulfides (in particular, Au(HS)2 are most significant in terms of gold transport at Kencana. Metal-bearing fluids were sourced from the down-dip extension of the K1 vein, flowing upwards through the dilating structure. Metal distribution is vertically and laterally zoned at the Kencana deposit, with precious metals enriched at shallow levels of the system, and base metal values increasing systematically with depth. Lateral zonation implies a hydrothermal fluid temperature gradient with a metal source to the north of the Kencana deposit. Ten alteration facies are recognized: SCG (Argillic 1) facies, ISP (Argillic 2) facies, IC (Argillic 3) facies, QAS (Phyllic) facies, EC (Propylitic 1) facies, CEP (Propylitic 2) facies, P (Sub-propylitic) facies, BM (Calc-potassic) facies, IDP (Intermediate-advanced argillic) facies, and KH (Advanced argillic) facies. Zonation is also observed in the distribution of alteration facies: (1) phyllic alteration (QAS facies) in the form of pervasive silicification and quartz-adularia-sericite alteration in the immediate vein zone, (2) argillic alteration (IC facies) enveloping the vein zone, (3) high-temperature propylitic alteration (CEP facies) filling fractures, extending up to 50 m from the vein, and (4) low-temperature argillic alteration and regional propylitic alteration (SCG, ISP and IC facies) distal to the vein. Supergene advanced argillic alteration (KH facies) blankets the top 5 m of stratigraphy at Kencana and represents intense tropical surface weathering. The geochemical signature of altered rocks hosting the Kencana deposit is variable depending on position relative to the mineralized structure, either within the vein, in the upflow zone, in the outflow zone or in the alteration halo. It is proposed that a sub-class of low sulfidation epithermal deposits (Se-rich low sulfidation epithermals), characterized by bonanza-grade Au, bimodal volcanism and a Se-rich ore mineral assemblage, such as Midas (USA), Hishikari (Japan), and Broken Hills (New Zealand), be considered, and that Kencana is a classic example of such a deposit. It is suggested that as well as similarities in their general geological setting, these deposits may form under similar physiochemical conditions, including the relatively more oxidized conditions than typical for low sulfidation systems. The volcanic-hydrothermal evolution of the Kencana deposit is relevant to understanding the genesis of, and exploring, for other low sulfidation epithermal gold deposits in volcanic settings. Several geological and geochemical features observed in the Kencana deposit may be useful for helping to vector towards mineralized zones in the Gosowong Goldfield, and other low sulfidation epithermal districts, including zonation of chlorite chemistry from Fe-rich to Mg-rich with increasing proximity to mineralization, lateral metal zonation from high to low temperature assemblages, elevated Pb and Sb values in altered host rocks with strong zonation around the ore zone, and the quantitative increase of potassium metasomatism (represented by mK/(2Ca+Na+K) values) towards the Kencana deposit. High As and Sb values relative to mK/(2Ca+Na+K) values can be used as distal pathfinders to ore mineralization, as these elements are enriched at low mK/(2Ca+Na+K) values, where precious metals (e.g. Au) are enriched at higher mK/(2Ca+Na+K) values.