Gold Deposits Related to Reduced Granitic Intrusions
Published:January 01, 2000
The role of intrusions in the formation of many types of gold deposits has been widely debated. Magmatic- hydrothermal processes are proposed by some authors for gold-rich porphyry systems, while many other authors claim that intrusions only serve as convective heat engines or structural hosts for mesothermal gold deposits. Based largely on well-documented examples from Alaska and the Yukon Territory, we recognize a class of deposits that display a spatial and temporal relation to reduced (low fO2) granitoids. Most intrusion-hosted and intrusion-proximal deposits and prospects of this class display a consistent and striking Au-Bi-Te-As (W,Mo,Sb) metal association; evidence for a series of alteration and mineralization events spanning a significant range of temperatures (>500°–<300°C); a consistent pattern of early feldspathic (albite and/or K feldspar) alteration and younger sericite-carbonate alteration; evidence for change in sulfidation state from well below pyrite-pyrrhotite (early) to pyrite-arsenopyrite (late); and the presence of high-CO2 and/or high-salinity fluid inclusions. Fluid inclusion and other geobarometric data indicate formation over a wide range of pressures. Deposits that formed at pressures >1 kbar generally lack evidence for rapid magmatic water exsolution (e.g., porphyritic textures, random stockworks, magmatic breccias). Deposits horizontally and vertically distal from intrusions within a given belt or district typically lack the strong Au-Bi association of proximal deposits and contain higher As, Sb, and base metals and only yield evidence for relatively low-temperature and low-salinity fluids. These latter types possess some characteristics of orogenic (mesothermal), epithermal, or Carlin-type deposits; however, their spatial and temporal association with the higher-temperature deposits suggests a common origin.
Although the clearest examples of these reduced intrusion-related deposits (e.g., Fort Knox, Alaska; Dublin Gulch, Yukon Territory) are only of moderate size (<150 tonnes Au), worldwide deposits of apparently similar character and origin host major amounts of gold. Significant global examples include Mokrsko in the Czech Republic, Kidston in Australia, and possibly the world-class Murantau deposit in Kazakstan. Belts hosting deposits of this type occur in a variety of continental arc, back-arc, and collisional settings, most of which contain intrusion-related Sn-W deposits.
A key characteristic of this class of deposit is the occurrence of a wide variety of mineralization styles, depending on formation depth, distance from the parent intrusion, and structural control. It is likely that new variants of this type of deposit are yet to be found in many areas of the world, an assertion supported by the relatively recent discovery of the unusually high-grade Pogo deposit in interior Alaska.
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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.