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Hydrothermal Alteration Associated with the Gosowong Epithermal Au-Ag Deposit, Halmahera, Indonesia: Mineralogy, Geochemistry, and Exploration Implications

J. Bruce Gemmell

ARC Centre of Excellence in Ore Deposits (CODES), University of Tasmania, Private Bag 79, Hobart, Tasmania, Australia 7001

E-mail, Bruce.Gemmell{at}utas.edu.au

The Gosowong epithermal Au-Ag deposit, located on Halmahera Island, eastern Indonesia, has a resource of 0.99 million metric tons (Mt) at 27 g/t Au and 38 g/t Ag. Host rocks consist of Miocene shallow marine, intermediate-basic volcanic and volcaniclastic rocks. The structural setting consists of an elongate dome with associated tensional fracturing trending parallel to the long axis of the dome. Gold mineralization is hosted within multiphase, epithermal quartz-adularia and quartz-chlorite fissure veins, breccias, and stockwork veining within two gently south plunging ore shoots along a 400-m section of the north-striking, east-dipping Gosowong fault.

Alteration is zoned around the vein from proximal silicic to argillic to vein-related propylitic to distal regional propylitic alteration. Prior to the onset of the Gosowong deposit hydrothermal system the host rocks in the district were altered to a regional propylitic (epidote, Fe chlorite, albite, pyrite) assemblage. As hydrothermal fluids progressed up the Gosowong fault, vein-related propylitic (epidote, Fe chlorite, smectite, calcite, pyrite) alteration formed within approximately 50 m of the deposit. As the hydrothermal system intensified argillic (illite, quartz, chalcedony, adularia, Mg-Fe chlorite, smectite, pyrite) alteration formed within a few 10s of meters of the Gosowong fault and overprinted the vein-related and regional propylitic alteration. Silicic (quartz, chalcedony, adularia, illite, Mg chlorite, pyrite) alteration overprints all other types of alteration and is confined to within a few meters of the vein.

Analysis of the whole-rock geochemistry of altered volcanic rocks indicates there are varying element concentrations depending on location: (1) at surface, Hg, Au, Ag, Pb, Mo, Tl, As, K₂O, and Li are enriched and the Alteration Index (AI = 100(MgO + K₂O)/(MgO + K₂O + CaO + Na₂O) has high values; CaO, MgO, Fe₂O₃, Sr, and Na₂O are depleted, and the chlorite-carbonate-pyrite index (CCPI = 100(MgO + FeO)/(MgO + FeO + K₂O + Na₂O) has low values; (2) proximal to the deposit, Au, Ag, As, Cu, and Pb are enriched, whereas Na₂O is depleted; (3) in the alteration halo, K₂O, Mo, Tl, As, and S are enriched, and the AI has high values; Na₂O and Sr are depleted and the CCPI has low values; and (4) along the downward extension of the Gosowong fault zone, K₂O, Tl, Ba, and Li are enriched and the S/Na₂O ratio has high values. These mineralogical and geochemical features provide a set of exploration vectors for use in the Gosowong goldfield.

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