The Big Gossan Cu-Au skarn is hosted in the Big Gossan Fault at the contact between the Tertiary New Guinea Limestone Group and the Jurassic-Cretaceous Kembelangan Group. Meinert (1997) recognised vertical and lateral metal zonation in the Big Gossan skarn whereby Mo increases with depth and Cu, Au, Ag, Pb, Zn, As, Co increase towards the top of the system and towards the western and eastern margins. The current data indicate that most of this zonation is a product of overprinting sulphide stages and the availability of structural channelways.

Several generations of brecciation in the Big Gossan skarn are associated with infill and alteration by pyroxene, garnet, magnetite, amphibole, and chalcopyrite-pyrite with a Cu-Au-Ag element association, whereas sphalerite, galena and minor pyrite postdate the Cu-Au mineralisation. Breccia zones occur in the Big Gossan Fault above the skarn and in the hanging wall adjacent to the skarn. The major hanging wall breccia contains infill and alteration composed of amphibole, calcite, magnetite and talc and is crosscut by fractures and faults containing gold associated with massive pyrite, sphalerite, galena and localised Bi-Pb sulphides (Rae et al., 1994). Gold in the minor fault breccias above the skarn is associated with massive banded sphalerite, pyrite, galena and minor arsenopyrite.

One or more late stage galena-sphalerite-pyrite-gold events overprints the main Cu-Au-Ag orebody, and the apparent vertical and lateral zonation is not the result of a single-stage evolving fluid.

References: Meinert, L.D. 1997. Econ. Geol. v92, pp509-533. Rae, A.J., Allen, J.M. and Powell, N.G., 1994. Unpublished internal report for PT Freeport Indonesia.