A Review of Silver-Rich Mineral Deposits and Their Metallogeny
Frederick T. Graybeal, Peter G. Vikre, 2010. "A Review of Silver-Rich Mineral Deposits and Their Metallogeny", The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries, Richard J. Goldfarb, Erin E. Marsh, Thomas Monecke
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Mineral deposits with large inventories or high grades of silver are found in four genetic groups: (1) volcanogenic massive sulfide (VMS), (2) sedimentary exhalative (SEDEX), (3) lithogene, and, (4) magmatichydrothermal. Principal differences between the four groups relate to source rocks and regions, metal associations, process and timing of mineralization, and tectonic setting. These four groups may be subdivided into specific metal associations on ternary diagrams based on relative metal contents.
The VMS deposits rarely contain more than 15,600 t Ag (500 Moz). Grades average 33 g/t Ag. Variable Ag-Pb-Zn-Cu-Au ± Sn concentrations are interpreted as having been derived both from shallow plutons and by leaching of the volcanic rock pile in regions of thin or no continental crust and the mineralization is syngenetic. Higher silver grades are associated with areas of abundant felsic volcanic rocks.
The SEDEX deposits rarely contain more than 15,600 t Ag (500 Moz). Grades average 46 g/t Ag. Silver, lead, and zinc in relatively consistent proportions are leached from sedimentary rocks filling rift-related basins, where the continental crust is thin, and deposited as syngenetic to diagenetic massive sulfides. Pre-mineral volcanic rocks and their detritus may occur deep within the basin and gold is typically absent.
Lithogene silver-rich deposits are epigenetic products of varying combinations of compaction, dewatering, meteoric water recharge, and metamorphism of rift basin-related clastic sedimentary and interbedded volcanic rocks. Individual deposits may contain more than 15,600 t Ag (500 Moz) at high grades. Ores are characterized by four well-defined metal associations, including Ag, Ag-Pb-Zn, Ag-Cu, and Ag-Co-Ni-U. Leaching, transport, and deposition of metals may occur both in specific sedimentary strata and other rock types adjacent to the rift. Multiple mineralizing events lasting 10 to 15 m.y., separated by as much as 1 b.y., may occur in a single basin. Gold is absent at economic levels.
The magmatic-hydrothermal silver-rich deposits are epigenetic and related to cordilleran igneous and volcanic suites. Six magmatic-hydrothermal districts each contain more than 31,000 t Ag (1,000 Moz) with grades of veins >600 g/t Ag. Mineralization occurs as veins, massive sulfides in carbonate rocks, and disseminated deposits including porphyry silver deposits, a proposed exploration model. Most deposits are epithermal with low-sulfidation alteration assemblages. Deposits are often telescoped and well-zoned. All large and high-grade magmatic-hydrothermal deposits appear confined to regions of relatively thick continental crust above Cenozoic consuming plate margins on the eastern side of the Pacific Rim. Silver in these deposits may be partly derived by hydrothermal leaching of rocks under or adjacent to the deposits.
Specific metal associations in SEDEX and lithogene deposits may reflect confinement of fluid flow to and derivation of metals from specific source rock types. Variable metal associations in VMS and magmatichydrothermal deposits may reflect derivation of metals from a more diverse suite of rocks by convecting hydrothermal systems and processes related to the generation of magma. The discovery rate for silver-rich deposits has accelerated during the past decade, with new deposit types, metal associations, and exploration models being identified that provide numerous exploration and research opportunities.
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The Challenge of Finding New Mineral Resources: Global Metallogeny, Innovative Exploration, and New Discoveries
There seems to be general consensus throughout much of the global mining industry that the supply of base and precious metals and some other commodities (e.g., ferrous metals, uranium) is reasonably well assured into the oreseeable future because increases in total resources continue to keep pace with or outstrip global consumption. The basic assumption is that market forces and technological advances will combine to promote and perpetuate this trend (e.g., Tilton, 2003; Crowson, 2008). Others disagree, however, andpredict that shortages are inevitable if metal consumption continues to escalate (Beaty, 2010).
It is already becoming clear that many known resources seem unlikely to be mined, irrespective of commodity prices, because of their low grade and/or quality. Hence, many mineral resources that were uneconomic in the early 2000s are likely to remain so, both today and into the foreseeable future because of increases in both the direct (e.g., energy, labor) and indirect (e.g., environmental, social) production costs. This situation is being further exacerbated by the perceived decrease, over at least the past decade, in the discovery rate of base and precious metal resources measured in terms of both the number of major discoveries made and the exploration dollars spent per discovery (e.g., Dummett, 2000; Horn, 2002; Schodde, 2004). There is also a suggestion that the discoveries made are, on average, becoming both smaller and lower grade. Therefore, it seems reasonable to ask whether current exploration practices and success rates are going to be adequate to provide for the massive increases in metal consumption that world population growth, rising living standards, and rapid industrialization and urbanization in China, India, and other emerging markets appear to portend. For example, Rio Tinto's projections suggest that "by 2030 the additional supplyrequired will be equivalent to replicating the iron ore output of the Pilbara region of Australia every five years, adding another aluminium production complex the size of Canada's Saguenay every nine months, and developing another copper mine the size of Escondida in Chile each year. Future energrequirements are such that an entire Hunter Valley coal supply chain needs to be created each year plus a uranium mine the size of Ranger every four years" (Albanese, 2010, p. 7). Clearly, the exploration business has to become increasingly effective if it is to rise to the challenge of finding mineral resources of the right caliber to assure that this burgeoning demand can be adequately satisfied.