Newly recognized gold-rich sedimentary-exhalative (sedex) mineralization in Nevada, with an average gold grade of 14 g/tonne (t), and the occurrence of significant amounts of gold in classic sedex deposits like Rammelsberg, Germany (30 Mt at 1 g/t), Anvil, Canada (120 Mt at 0.7 g/t), and Triumph, Idaho (? at 2.2 g/t) demonstrate that basin brines can form gold ore. The sedex Au mineralization in Nevada represents a previously unrecognized end member in a spectrum of sedex deposits that also includes large Zn-Pb, intermediate Zn-Pb-Ba ± Au, and barite deposits. Study of ore deposits, modern brines, and chemical modeling indicates that variation in metal ratios and their abundance in sedex deposits are dominantly controlled by the concentration and redox state of sulfur in brines. For example, Au and Ba solubilities are highest in H2S-rich, SO4-poor fluids, whereas base metal solubilities are highest when H2S is not present. Chemical modeling indicates a typical reduced brine (15 wt % NaCl equiv, pH = 5.5, H2S = 0.01 m) at 200°C is capable of transporting as much as 1 ppm Au in solution.
The H2S content in brines is controlled by the rate of its production through thermochemical reduction of sulfate by organic matter and the rate of its removal from the fluid through the sulfidation of reactive Fe in the sediments. Thus, sedimentary basins with high organic carbon and sulfate in rocks low in reactive Fe, such as carbonates and shales, are most likely to produce H2S-rich brines that may form gold-rich sedex deposits. Because of the tremendous scale of sedex hydrothermal systems, evidence that basin fluids can transport gold identifies a new mechanism for concentrating gold in sedimentary basins and opens extensive areas to further gold exploration.
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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.