Although generally considered a poor cousin of Au-rich deposits such as orogenic or epithermal deposits, a significant number of volcanic-hosted massive sulfide (VHMS) deposits are significant repositories of Au. Several of these deposits had original Au resources exceeding 8 Moz and in some recently discovered deposits Au, not base metals, is the primary economic metal.
Although most Au in volcanic-hosted massive sulfide districts is hosted by massive sulfide lenses, recent discoveries, both on land and on the ocean floor, indicate that significant Au occurs outside of these lenses. In most deposits, Au has a metallogenic association with either Cu or Zn. When associated with Cu, Au is concentrated toward the base of the massive sulfide lens. Gold-rich deposits of this metallogenic assemblage commonly are associated with (metamorphosed) advanced argillic assemblages and are inferred to have formed from acidic, high-temperature (>300°C), oxidized fluids. These deposits have been equated to high-sulfidation epithermal deposits and may be detected using recently developed spectral techniques such as PIMA (Portable Infrared Mineral Analyzer) and airborne hyperspectral scanners.
When associated with Zn, Au is concentrated near the top of massive sulfide lenses, in some cases in baritic zones. Gold-rich deposits of this metallogenic assemblage tend to be formed from low-temperature (200° ± 50°C) and/or near-neutral fluids as indicated by fluid inclusion studies or by alteration assemblages (e.g., K feldspar or carbonate). A small number of deposits cannot be classified into the Au-Zn or Au-Cu association. In these deposits, Au is concentrated in pyritic zones that contain relatively low amounts of base metals. Moreover, a consistent relationship with Zn or Cu is not present. Although this group is small, it includes deposits such as Horne.
Mineralogically Au can occur in electrum or native gold, Au tellurides, or auriferous pyrite or arsenopyrite. In deposits of the Au-Cu association, Au tends to occur as native gold or tellurides, whereas electrum and auriferous pyrite and/or arsenopyrite is more common in the Au-Zn association. Metamorphic recrystallization tends to liberate Au held in auriferous pyrite or arsenopyrite, potentially enhancing metallurgical recoveries.
Figures & Tables
Gold in 2000
THIS Gold in 2000volume is organized around a classification of hypogene gold deposits that emphasizes their tectonic setting and relative time of formation compared to their host rocks and other gold deposit types (e.g., Sawkins, 1972, 1990; Groves et al., 1998; Kerrich et al., 2000). The temporal division of orogenic gold deposits into Archean, Proterozoic, and Phanerozoic follows closely the recently published classification of orogenic gold deposits (Groves et al., 1998) which incorporates the previously identified “mesothermal” gold deposits. The newly recognized intrusion-related and sedex gold deposits represent new gold deposit classes even though their exact genetic classification remains open, with more research considered a priority. Proterozoic Au-only and Cu-Au-(Fe) deposits are also a relatively recently recognized class of structurally controlled epigenetic gold deposits. Particularly, the origin and classification of Cu-Au-(Fe) deposits (e.g., Olympic Dam) remains equivocal, as pointed out by Partington and Williams (2000). In fact, Kerrich et al. (2000) discuss the anorogenic iron oxide copper-gold deposits as one of six world-class gold deposit classes. Low- and high-sulfidation and hot spring epithermal gold deposits are dealt with as one genetic gold class. Alkalic epithermal and porphyry gold deposits are dealt with as a separate gold deposit class owing to their specific host-rock association and element enrichment (e.g., Mo, F, Be, Hg, W, and Sn).
The gold deposit classes are described from both industry and academic points of view, with emphasis on a balanced account of the descriptive geology, genetic interpretations, exploration significance, as well as open questions and future research avenues. The volume contains 13 papers covering 10 major classes of gold deposits and three summary papers, and was presented as a Society of Economic Geologists-sponsored short course held November 10 and 11, 2000, at Lake Tahoe, Nevada.
Orogenic gold ores are associated with regionally metamorphosed terranes of all ages (Kerrich and Cassidy, 1994) and are spatially linked to subduction-related thermal processes (Kerrich and Wyman, 1990)(Fig. 1). These metal concentrations formed during compressional to transpressional deformation processes at convergent plate margins in accretionary (oceanic-continental plate interaction) and collisional (continental-continental collision) orogens (i.e., Bohlke, 1982; Groves et al., 1998). In both cases hydrated marine sedimentary and volcanic rocks have been added to continental margins over a long period of collision (10 to >100 Ma). Accretionary or peripheral orogens contain gold deposits in the Archean of Australia, Canada, Africa, India, and Brazil and the Mesozoic and Cenozoic gold fields of western North America, i.e., the famous Mother Lode belt. Collisional or internal orogens contain gold deposits in the Proterozoic of Australia, North America, West Africa, and Brazil, and the famous Phanerozoic gold fields in the Variscan, Appalachian, and Alpine regions of North America and Europe. In Phanerozoic orogenic gold deposits, subduction- related thermal events, episodically raising geothermal gradients within the hydrated accretionary sequences, initiate and drive long-distance hydrothermal fluid migration.