Linkages between Volcanotectonic Settings, Ore-Fluid Compositions, and Epithermal Precious Metal Deposits
Richard H. Sillitoe, Jeffrey W. Hedenquist, 2005. "Linkages between Volcanotectonic Settings, Ore-Fluid Compositions, and Epithermal Precious Metal Deposits", Volcanic, Geothermal, and Ore-Forming Fluids: Rulers and Witnesses of Processes within the Earth, Stuart F. Simmons, Ian Graham
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Epithermal Au and Ag deposits of both vein and bulk-tonnage styles may be broadly grouped into high-, intermediate-, and low-sulfidation types based on the sulfidation states of their hypogene sulfide assemblages. The high- and low-sulfidation types may be subdivided using additional parameters, particularly related igneous rock types and metal content.
Most high-sulfidation deposits are generated in calc-alkaline andesitic-dacitic arcs characterized by near-neutral stress states or mild extension, although a few major deposits also occur in compressive arcs characterized by the suppression of volcanic activity. Rhyolitic rocks generally lack appreciable high-sulfidation deposits. Highly acidic fluids produced the advanced argillic lithocaps that presage high-sulfidation mineralization, which itself is due to higher pH, moderate- to low-salinity fluids. Similar lithocaps in the Bolivian Sn-Ag belt, some mineralized with Ag and Sn, accompany reduced, ilmenite series magmatism.
Intermediate-sulfidation epithermal deposits occur in a broadly similar spectrum of andesitic-dacitic arcs but commonly do not show such a close connection with porphyry Cu deposits as do many of the high-sulfidation deposits. However, igneous rocks as silicic as rhyolite are related to a few intermediate-sulfidation deposits. These deposits form from fluids spanning broadly the same salinity range as those responsible for the high-sulfidation type, although Au-Ag, Ag-Au, and base metal-rich Ag-(Au) subtypes reveal progressively higher ore-fluid salinities.
Most low-sulfidation deposits, including nearly 60 percent of the world's bonanza veins, are associated with bimodal (basalt-rhyolite) volcanic suites in a broad spectrum of extensional tectonic settings, including intra-, near-, and back-arc, as well as postcollisional rifts. Some low-sulfidation deposits, however, accompany extension-related alkaline magmatism, which, unlike the bimodal suites, is capable of generating porphyry Cu deposits. Extensional arcs characterized by active andesitic-dacitic volcanism do, however, host a few low-sulfidation deposits. Low-sulfidation deposits genetically linked to bimodal volcanism are formed from extremely dilute fluids, whereas modestly saline contributions account for the low-sulfidation deposits in alkaline centers.
Early lithocap-forming and high-sulfidation fluids, as well as low-sulfidation fluids in deposits associated with alkaline igneous rocks, display clear evidence for a close genetic relationship to magmatism and, although the linkage is less intimate, late high-sulfidation fluids responsible for much of the Au introduction along with similar intermediate-sulfidation fluids also seem to owe much to their magmatic parentage. Where ascending intermediate-sulfidation fluids enter lithocaps, they evolve to high-sulfidation fluids. Eventual neutralization and lowering of sulfidation state by wall-rock interaction can convert high- back to inter-mediate-sulfidation fluids, as confirmed by both spatial and paragenetic transitions from high- to interme-diate-sulfidation mineralization. In contrast, low-sulfidation fluids other than those of alkaline affiliation lack such clear-cut connections to magmatism, although Giggenbach's work on the geothermal fluids associated with the Taupo Volcanic Zone in New Zealand suggests that a deep magmatic source different from that of fluids in andesitic arc terranes is probable. In addition, at least in some regions, there appears to be a correlation between the reduced sulfide assemblages of low-sulfidation deposits and the reduced nature of the volcanic rocks with which they are associated. Therefore, it may be argued that the defining characteristics of epithermal deposits are related directly to their magmatic roots, notwithstanding the existence of important unanswered questions, especially regarding the source of low-sulfidation fluids.
This review puts forward several exploration guidelines for epithermal precious metal deposits. Exploration activity in andesitic-dacitic arcs should be restricted to high- and potentially related intermediate-sulfidation deposits containing Au and/or Ag, whereas a variety of rift-related bimodal suites and convergent-margin alkaline rocks offer the prime environments for Ag-deficient, low-sulfidation Au deposits (Ag/Au <∼15). Bonanza Au veins are more likely to be of the low-sulfidation type and to be discovered at relatively shallow paleodepths in bimodal rift settings, where rhyolitic and/or basaltic rocks may be proximal to Au ore. Even tholeiitic basalts in emergent mid-ocean ridge or hot-spot settings might possess underappreciated epithermal Au potential. Subaerial extensions to some volcanic-hosted massive sulfide (VMS) belts may possess low-sulfidation Au potential because of the broadly similar volcanotectonic settings for both deposit types. The reduced, ilmenite series volcanic rocks of the Bolivian Sn-Ag belt are unfavorable for epithermal Au. Deficiency of volcanic rocks in epithermal provinces is typical of highly compressive arcs (high- and intermediate-sulfidation deposits) and some rifts swamped by fluviolacustrine sedimentation with silica sinter occurrences (low-sulfidation deposits). In contrast to high- and intermediate-sulfidation deposits, exploration for low-sulfidation Au deposits, even where exposed, may be hampered by the visually subtle nature of many outcropping examples.
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To be honest, I am surprised to find myself addressing a meeting of the Society of Economic Geologists—being neither a geologist nor economic. And looking at the title of my paper, I wouldn’t be offended if people told me that I may be going to talk about something I know nothing about. After listening to some of this afternoon’s talks, however, it is clear to me that I wouldn’t be the only one. With this I don’t mean that the previous speakers were inept but that there are still quite a few basic problems which have to be solved before we may safely say, we know what’s going on in hydrothermal systems. And by basic, I mean basic.
The title of my talk links two processes: magma degassing, something I have been studying now, from the gases’ point of view, for more than 20 years, and mineral deposition, something I had my nose rubbed into by living in close vicinity to some of the biggest gold freaks like Kevin Brown, Jeff Hedenquist, Dick Henley, and Terry Seward. I myself had, quite early on, declared gold a four letter word and had vowed never to use it in any of my papers, together with other uncouthities, such as zinc or lead. Now that the above have dispersed, each into his corner of the globe, I think myself free to reconsider my earlier pledge.