Vein stratigraphy, mineralogy, and metal zonation of the Kencana low-sulfidation 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 million tonnes (Mt) @ 27.9 g/t Au, containing 4 Moz Au. The deposit, which forms part of the Gosowong Goldfield, is the third and most recently discovered deposit in the Gosowong Goldfield, after the Gosowong and Toguraci deposits. The Gosowong Goldfield is on the eastern side of the northwest arm of Halmahera, which is composed of four superimposed volcanic arcs. Epithermal mineralization occurs in the Pliocene Gosowong Formation, a series of interbedded volcaniclastic rocks, ignimbrites, and coherent andesitic volcanic flows and diorite intrusions. Andesite emplacement at 3.73 Ma was followed by diorite intrusion at approximately 3.50 Ma. Epithermal mineralization postdates andesite and diorite emplacement with 40Ar/39Ar dating of hydrothermal adularia giving a mean age of 2.93 Ma for the Kencana deposit. The deposit is in two main subparallel NW-trending fault structures (K1 and K2) that have a strike length of 400 m and a vertical extent of 200 m, dip 46°E, and are joined by link structures (K-Link). 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: (1) wall rock, (2) quartz stockwork, (3) wall-rock breccias with crystalline quartz cement, (4) red chalcedony, (5) massive crystalline quartz, (6) massive crystalline quartz breccias, (7) cockade-banded quartz-chlorite breccias, (8) banded quartz-chlorite, (9) banded quartz-adularia, (10) gray cryptocrystalline quartz stringer veins, and (11) black quartz-molybdenite. Infill types 1, 2, and 3 occur throughout but are particularly prevalent on the margins of the deposit. Types 7, 8, and 9 are the main ore-bearing types and form the bulk of the central section of the vein. Type 11 is most prevalent to the north. Types 4, 5, 6, and 10 infill are variably distributed throughout the vein. Ore assemblages are high-fineness electrum and sulfides, with selenides and lesser tellurides and sulfosalts. Chalcopyrite is the most common, with lesser selenian galena, sphalerite, bornite, and pyrite. Accessory minerals include aguilarite and molybdenite, with trace tennantite, arsenopyrite, Ag and lead tellurides, naummanite, and rare bismuth minerals. Gangue minerals are quartz, adularia, chlorite, and calcite. Metal distribution is vertically and laterally zoned, with precious metals enriched at shallow levels of the system and base metal values increasing systematically with depth. Lateral zonation implies a fluid source from the north. 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. Mineralizing fluids are primarily meteoric water, with a magmatic content added during formation of ore stages. Characteristics of the geology, vein stratigraphy, mineralogy, and metal zoning at Kencana were combined to develop an ore genesis model, which can be used to generate new targets in the Gosowong Goldfield and other low-sulfidation epithermal districts.