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Orogenic Gold Deposits : Genesis and Exploration
| Content Provider | Semantic Scholar |
|---|---|
| Author | Goldfarb, Richard J. |
| Copyright Year | 2013 |
| Abstract | gold deposit for which many economically important examples are preserved throughout the geological record. These gold deposits are hosted by a variety of mafic igneous, felsic igneous, and sedimentary rocks in Precambrian cratons and late Neoproterozoic-Phanerozoic fold belts. A diversity of structures, from shear zones to fold hinges, are recognized controls for the ores, which possess highly variable gold grades. Pre-, syn-, or postgold intrusions generally occur at, or within a few kilometers of the deposits. Greenschist to lower amphibolite metamorphic conditions, having been reached within a few tens of million years prior to ore formation, represent peak P-T conditions in the wallrocks for most deposits. However, orogenic gold deposits in eastern North China craton, in extremely high-grade metamorphic terranes, provide globally unique examples where important deposits post-date metamorphism of their host rocks by billions of years. Ore fluids are predominantly reduced, low salinity, CO2-rich systems.Ages of large orogenic gold deposits cluster between ca. 2750-2520, 2100-1730,, and 650-50 Ma, which are within the welldefined times of lithospheric growth at continental margins. It is the orogen-scale processes during these times that control release of gold and related elements during prograde metamorphic events, but lithosphericscale relationships in eastern China suggest that some gold provinces definitely require deeper and larger fluid and/or metal source regions, such as subducted, oceanic slabs. Orogenic gold deposits are most readily defined as gold deposits in deformed metamorphic rocks. As their name implies, this type of hydrothermal gold deposit represents regional fluid flow that is inherent to orogeny. The goldbearing orebodies typically form in host rocks between 20 and 200 million years after deposition of the rocks and relatively late in their deformation history. Historically, these were thought of as a relatively high-grade type of gold deposits, with gold-bearing quartz veins mined underground at grades of at least 5 g/t, and commonly at >10 g/t. With rising metal prices, improved mining methodologies, and more efficient gold extraction techniques, the character of the ores has dramatically changed during the past few decades. Many of these deposits are now mined by large open pit operations, recovering both high-grade veins and surrounding lowergrade hydrothermally altered wallrock that had been considered waste, or even just very distal geochemical anomalies, in the past. Thus, present-day open pit mining of orogenic gold deposits allows for much lower ore grades, including some examples of large orebodies grading < 1.0 g/t Au, such as throughout northern Sonora, Mexico and at Paracatu, Brazil. The orogenic gold deposits are found in three different broad temporal/geotectonic settings (Goldfarb et al., 2001). First, ores hosted in Late Archean greenstone were mainly formed between 2750 and 2520 Ma in granitoidgreenstone terranes. This group of deposits includes those of the Yilgarn craton (Western Australia), Superior province (Canada), Slave province (Canada), Tanzania craton (central Africa), Dharwar craton (India), Zimbabwe-Kaapvaal craton (southern Africa), and Sao Francisco craton (Brazil). Second, ores dated between 2100 and 1730 Ma are hosted by Paleoproterozoic sedimentary and volcanic rocks deposited on the margins to the granitoid-greenstone terranes. Banded iron formation and ferruginous chert are commonly favorable host lithologies in the Paleoproterozoic terranes. Important examples include the ores of western Africa, northern South America, and Homestake (USA). A final group of ores formed between ca. 650 Ma and 50 Ma in marine sedimentary rocks adjacent to active continental margins. These include the Mesozoic to Tertiary gold deposits of the North American Cordilleran orogen, Colombian Andes, eastern China, eastern Russia, and the Otago area of New Zealand, and the latest Neoproterozoic-Paleozoic gold deposits of the Tasman orogen, Central Asia orogenic belt, Pan-African orogen, and Eastern Cordillera of the main Andean range. Most Phanerozoic orogenic gold deposits occur in the fore-arc Richard J. GOLDFARB 2013. Orogenic Gold Deposits: Genesis and Exploration Acta Geologica Sinica (English Edition), 87(supp.): |
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| Alternate Webpage(s) | http://www.geojournals.cn/dzxben/ch/reader/create_pdf.aspx?falg=1&file_no=dzxben2003z10400&quarter_id=Z1&year_id=2013 |
| Language | English |
| Access Restriction | Open |
| Content Type | Text |
| Resource Type | Article |
