Abstract Ophiolite complexes are tectonically transported slices of ancient oceanic lithosphere, ranging in age from Early Proterozoic to Paleocene, that occur in orogenic belts formed by convergent plate motions (Fig. 1). They are important markers of fossil plate boundaries. Complete ophiolite sections have a pseudostratigraphy that includes, from the bottom upward, tectonized ultramafic mantle rocks, layered ultramafic to mafic cumulate rocks, isotropic gabbros, a sheeted dike complex, and a mafic volcanic section dominated by pillow lavas (Penrose Conference participants, 1972; Fig. 2). Associated rocks (of minor volume) include amphibolite at the base of the complex, chromitites in the ultramafic tectonite cumulates, stocks and veins of trondhjemitic composition (“plagiogranites”), and Fe- and Mn-rich sedimentary rocks in the volcanic section. Of the more than 200 ophiolite suites recognized throughout the world, at least 25 contain significant volcanogenic massive sulfide (VMS) mineralization (Fig. 1; Table 1). Ophiolite-hosted VMS deposits represent some of the oldest recorded sources of copper and gold, with these metals being recovered from gossans in the Troodos ophiolite, Cyprus, since 2500 B.C. (Bear, 1963). Ophiolite-hosted VMS deposits in Turkey supplied copper to the Assyrian Empire before 2000 A.D. and similar deposits in Oman exported copper over a span of ancient time to a area that included the Sumerian and Roman empires (Griffitts et al., 1972). Along with the advent of the volcanogenic massive sulfide model, exploration and development of this deposit type was renewed in the 1960s, and continues in many parts of the world (Table 1). Ophiolite-hosted VMS deposits form a significant base metal resource. Individual deposits are up to 30 million tonnes (Mt), averaging approximately 5 Mt (Table 1; Fig. 3). They can contain between 10 5 and 10 6 tonnes of copper (Fig. 4), plus appreciable quantities of sulfur, iron, zinc, cadmium, and gold. They are commonly thought to have high Cu/Cu + Zn ratios (Fig. 5); this may be an artifact of pre-1930s metallurgical methods, where zinc could not be effectively extracted from these ores. They in fact vary in composition through Cu- to Zn-Cu rich. Appreciable amounts of gold have been recovered from the oxidized sulfide caps of some deposits, particularly from those of Tethyan (Cretaceous) age.

Abstract Of the 37 major massive sulfide deposits in the Bathurst Mining Camp, 14 have well-developed gossan and/or supergene zones that formed prior to Pleistocene glaciation. Of these, the Caribou, Heath Steele, and Murray Brook gossans were mined for Au and Ag; the Caribou supergene zone was mined for Cu. Other deposits have had their gossan and/or supergene zones removed during glaciation, a fact that makes their detection and characterization in tills important in mineral exploration. Gossans associated with massive sulfide deposits in the Bathurst Mining Camp formed under a temperate climate from late Pliocene until the onset of glaciation in the Pleistocene. Pliocene weathering of these deposits and sulfide-rich host rocks has resulted in the formation of two distinct supergene zones and gossans. The gossans have been classified into the following types on the basis of primary textures and mineralogy: (1) massive sulfide gossans, (2) stockwork and/or disseminated zone gossans, and (3) ferruginized wall-rock gossans. At some deposits, gossans have been eroded by streams and glaciers producing chemically anomalous stream sediments and gossan boulder trains in tills. Mass balancing of massive sulfide gossan data using the conservative element Sn has shown that Au, As, and Sb are consistently enriched and Fe, Mn, Mg, Ca, Sr, S, Cu, Pb, Zn, Cd, In, Ag, and Tl are consistently depleted in all deposits; Ba, Hg, Bi, Se, Co, V, and Mo are variably enriched or depleted. Two massive sulfide gossans have been chemically reworked with significant loss of most elements, including Au. In terms of the relative mobility of elements out of the gossans into surrounding aqueous regimes, the following trend has been observed: Sn << As < (Sb, Hg) < (Au, Bi) (Mo, Ba) < (Se, V) << Ag < (Fe) < (Pb, In) < (Tl) < Cu < (Co, Ca, Mg) << Cd < (Zn, S). Well-developed Cu-rich supergene zones occur in at least two Bathurst deposits. Profiles of Zn/Pb ratios and Cu contents indicate sequential replacement of galena and sphalerite by secondary Cu minerals. The extent of sphalerite and galena replacement varies with depth. The supergene sulfide zones show major enrichment of Cu (up to 7 fold), Sb, and In and significant loss of Ca, Mg, Sr, Mn, and CO 2 (carbonate dissolution), Zn, Cd, Pb, Ag, Ni, Co, and V. Average Au contents of massive sulfide gossans vary from 125 to 5,350 ppb and the element shows mass-balanced enrichments from 7 to 290 percent. Gold has only been identified as an Au silica gel. Gold correlates strongly with Sn, Ag, and SiO 2 in these gossans and this four-element grouping is a strong remnant marker in gossans of primary Zn-Pb sulfide zones. Differences in percentage of Au enriched in massive sulfide gossans are not due to overall Au contents of precursor pyrite and arsenopyrite that hosts most of the Au in massive sulfide deposits. The Au is released to solution as Au 0 during the breakdown of pyrite and arsenopyrite accompanying the late stages of sulfide oxidation, percolates down through the gossan cap, and precipitates in void linings as Au silica gels. Later maturation of some of these gels may form submicron (<1- μ m) native Au. “These Gossans or Ochres, are commonly called the Feeders of their respective Metals” Pryce (1778,p.44)