Abstract

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 skarn 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 δ34S 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 Gossan 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.

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