Genesis of Pyrite-Au-As-Zn-Bi-Te zones associated with Cu-Au Skarns: Evidence from the big Gossan and Wanagon gold deposits, Ertsberg district, Papua Indonesia

The Ertsberg district hosts multiple skarn and porphyry-related deposits, which together comprise one of the largest Cu-Au resources in the world. Earlier skarn Cu-Au deposits at Big Gossan and 2 km along strike to the northwest at Wanagon Gold are overprinted by distinctive late-stage pyrite, sphalerite, arsenopyrite, and native gold with local Bi and Te minerals. The Wanagon Gold deposit contains an estimated 2 million ounces (Moz) of gold; reserves at Big Gossan are 33 million tonnes (Mt) at 2.63 percent Cu, 0.92 g/t Au, and 15.72 g/t Ag. Phlogopite from the Big Gossan occurrence is younger than 2.82 ± 0.04 Ma, based on a new ⁴⁰Ar/ ³⁹Ar age from the Big Gossan skarn, and K-feldspar from the Wanagon Gold deposit has a ⁴⁰Ar/ ³⁹Ar age of 3.62 ± 0.05 Ma. A K-Ar date (3.81 ± 0.06 Ma) from the Wanagon sill constrains formation of the overprinting skarn Cu-Au and late-stage Wanagon Gold deposit to a period of ca. 0.2 m.y. At Big Gossan, earlier skam Cu-Au mineralization displays three-dimensional mineralogical, chemical, and temperature zonation. The high-temperature core (defined by low Zn/Cu) plunges to the northwest and is open at depth. Highest Cu grades and greatest development of the overprinting pyrite-Au-As-Zn-Bi-Te association occur to the northwest coincident with northeast-striking faults. Pyrite-Au-As-Zn-Bi-Te occurrences are also distributed in faults and fractures to the north and south of the Big Gossan skarn Cu-Au deposit. At Wanagon Gold, leaching of skarn and sandstone preceded introduction of the pyrite-Au-As-Zn-Bi-Te occurrences. In the sandstone, the pyrite-Au-As-Zn-Bi-Te mineralization was accompanied by K-feldspar (adularia) and minor quartz gangue. In carbonate rocks, no leaching or secondary K-feldspar is apparent; instead, sulfides are accompanied by quartz and dolomite gangue. The δ ³⁴S of sulfide from skarn Cu-Au and overprinting pyrite-Au-As-Zn-Bi-Te occurrences at both deposits range from -0.7 to +5.1 per mil. Laser ablation-inductively coupled plasma mass spectrometry (LA-ICPMS) analyses show that later pyrite overprinting both occurrences is distinct from that in the earlier skarn Cu-Au deposits and contains up to 60 ppm Au, 2 percent As, 680 ppm Bi, and 40 ppm Te. The mineralogy of the overprinting occurrences includes native gold, argentian tetrahedrite and tennantite, a silver-antimony sulfide, and Bi and Te-(Ag-Au) minerals including cosalite, bismuthinite, petzite, hessite, altaite, and tetradymite. The fineness of native gold varies with sulfide association. The lowest fineness gold (737-863) occurs with Pb minerals (galena and sulfosalts), and the higher fineness gold (904-974) occurs trapped within pyrite or in association with bismuthinite. Fluid inclusions in sphalerite and quartz in the Big Goss an pyrite-Au-As-Zn-Bi-Te occurrence have an average salinity of 8 wt percent NaCl equiv and an average homogenization temperature of 245°C. Stable isotopes indicate that the inclusion fluids were magmatic. However, a direct genetic relationship to earlier skarn Cu-Au mineralization is not obvious. The pyrite-Au-As-Zn-Bi-Te occurrences are considered to have formed from a fluid with a different composition, possibly the magmatic precursor to fluids commonly recognized in low- and high-sulfidation epithermal deposits that develops at shallow levels and contains significant nonmagmatic (i.e., meteoric) water.