Geology, zonation, and fluid evolution of the Big Gossan Cu-Au skarn deposit, Ertsberg District, Irian Jaya

Abstract

The Big Gossan Cu-Au skarn deposit is the highest grade copper deposit in the world-class Ertsberg district, Irian Jaya. Current reserves are 37.4 million metric tons (Mt), grading 2.69 percent Cu, 1.02 g/t Au, and 16 g/t Ag. Mineralization is associated with a series of 3 to 4 Ma granodioritic dikes which have intruded close to the near-vertical faulted contact between the Shale Member of the Cretaceous Ekmai Formation and the stratigraphically overlying Paleocene Waripi and Eocene Faumai Formations. Most mineralization and alteration occurs in the purer carbonate rocks of the Waripi Formation, although biotite and calc-silicate hornfels alteration also occurs in the footwall rocks adjacent to mineralization.Prograde skarn alteration consists dominantly of pyroxene and garnet with an overall ratio of approximately 2:1. The average pyroxene and garnet compositions are Di 73 Hd 24 Jo 3 and Ad (sub 84 7) Gr 135 Sp (sub 1.5) Py (sub 0.3) , respectively. The skarn is zoned in three dimensions relative to the main fluid conduit along the Ekmai-Waripi contact. Proximal zones are garnet rich and the garnet has a dark red-brown color, whereas the pyroxene is pale in color and is iron poor. Intermediate zones contain subequal amounts of relatively coarse-grained, green pyroxene and brown garnet. Distal skarn zones are dominated by dark green, iron-rich (up to Hd 75 ) pyroxene. Garnet in distal zones typically is pale green to greenish brown. At the skarn-marble contact, retrograde alteration (mostly amphibole and epidote) and sulfide minerals (mostly pyrite and pyrrhotite) are abundant. Chalcopyrite and anhydrite are present in all skarn zones. Although the skarn-marble contact is sharp, tiny dark veinlets (containing chlorite, serpentine, clay, sulfides, and/or carbon) which locally resemble stylolites record the passage of hydrothermal fluids for tens to hundreds of meters beyond the skarn. Skarn zonation in terms of mineral ratios, colors, and compositions can be used for exploration on both a local and district scale.In addition to mineralogy, Big Gossan is zoned with respect to metals. Cu, Au, Ag, Pb, Zn, As, and Co in the skarn all increase toward the top of the system, whereas Mo increases with depth. Similarly, Cu, Au, Ag, Pb, Zn, As, and Co increase (for a given elevation) toward the western and in most cases, toward the eastern margin of the system. Only Mo is inverse to this trend, defining a central core zone which is interpreted to represent the main locus of fluid flow overlying the source pluton. Relative to the host rocks (Kembelangan and Waripi Formations), mineralized skarn is enriched in Si, Fe, S, Cu, Ag, Au, As, Co, Se, and W. Both the host rocks and mineralized skarn are significantly depleted in rare earth elements relative to typical Phanerozoic sedimentary rocks. Particularly striking is the deep negative europium anomaly, mostly < 1 ppm Eu.Fluids associated with prograde skarn are high-temperature, low CO 2 (<0.05 mole %), NaCl-KCl brines. Pressure-corrected temperatures for fluid inclusions in prograde skarn range from 360 degrees to 535 degrees C and average 460 degrees C. Most pyroxene fluid inclusions contain multiple daughter minerals including halite, sylvite, chalcopyrite, hematite, and anhydrite. Total salinity ranges from 38 to 65 wt percent NaCl + KCl and mean salinities are 22 wt percent KCl, 35 wt percent NaCl, and 57 wt percent NaCl + KCl. Fluid inclusions in quartz and anhydrite associated with retrograde alteration homogenize from 250 degrees to 410 degrees C and average 369 degrees C. These fluid inclusions do not contain daughter minerals, average 7.1 wt percent NaCl equiv, and are vapor rich with evidence for boiling at temperatures of 370 degrees to 380 degrees C. This corresponds to a pressure of 20 MPa and a depth of 2 km under hydrostatic conditions. Pressure-depth reconstructions from fluid inclusion data indicate that 1.4 km of erosion has occurred in the Ertsberg district in the past 4 million years. Both the high- and low-temperature fluids can be modeled as originating from 6 to 8 wt percent solutions, most likely exsolved from an underlying pluton, which followed different cooling paths on ascent such that the early fluid reached its solvus at high temperature and the later fluid did not. This resulted in the early fluid separating into a hypersaline brine, whereas the later fluid maintained its original salinity. Overall, the Big Gossan salinities are similar to, but the homogenization temperatures slightly lower than, those determined for other skarn deposits in the Ertsberg district. This is consistent with the geologic setting of the Big Gossan deposit in which skarn mineralization is associated with dikes and fluid flow zones rather than with the main pluton and intrusive contact as at the original Ertsberg discovery, the Gunung Bijih Timur-IOZ (Intermediate Ore Zone)-DOZ (Deep Ore Zone), Dom, and Grasberg deposits.