IntroductionFOR MORE than 25 years, the association between specific rhyolite suites and volcanogenic massive sulfide (VMS) deposits was promoted as an effective tool for exploration. Thurston (1981) and Campbell et al. (1982) first identified this relationship using Archean VMS deposits in the Canadian Shield. Lesher et al. (1986) and Barrie et al. (1993) subsequently developed formal classifications for felsic volcanic rocks based on trace element concentrations and suggested that certain types of rhyolites were more prospective for VMS deposits than others. Recently, Hart et al. (2004) revitalized the concept by proposing a conceptual petrogenetic model to account for the association of specific rhyolite geochemical signatures with VMS deposits. The classification geochemically discriminates rhyolites as FI, FII, FIIIa, or FIIIb types. The general conclusions drawn from Lesher et al. (1986), Lentz (1998), and Hart et al. (2004) are that Archean VMS deposits are hosted mainly by FIII rhyolites, whereas most post-Archean VMS deposits are hosted predominantly by FII rhyolites, and FI rhyolites are unfavorable for VMS mineralization. The link between VMS formation (hydrothermal activity) and rhyolite petrogenesis invokes a particular geodynamic setting and, more specifically, the depth of rhyolite generation (Lentz 1998; Hart et al., 2004).Although exceptions and limitations were recognized, the concept was rapidly adopted by exploration companies, and rhyolites with the most potential were commonly selected without any consideration for the mineral potential of other
AbstractThe relationship between rhyolite geochemistry and volcanogenic massive sulfide (VMS) mineralization has been proposed as an exploration tool to discriminate prospective felsic volcanic centers. The most widely used classification discriminates between four types of rhyolite: FI, FII, FIIIa, and FIIIb. The FI rhyolites are calcalkaline, with strongly fractionated REE patterns and strongly negative Ta and Nb anomalies. They are usually considered barren, unless associated with FII or FIII felsic volcanic rocks. The FII rhyolites are calc-alkaline to transitional with moderately fractionated REE patterns and moderate Ta and Nb anomalies. They range from barren to having a high potential to host VMS mineralization. The FIIIa and FIIIb rhyolites are tholeiitic and show weakly fractionated REE patterns and weak to absent Nb and Ta anomalies. They have the highest potential to host VMS mineralization. The FIIIb rhyolites are high-temperature rhyolites with flat REE patterns and no Ta or Nb anomalies.The Abitibi greenstone belt, especially in Quebec, is well known for its abundant and diverse VMS deposits. Representative samples of VMS-associated rhyolites within and outside of mining districts, including the classic Noranda VMS district, were analyzed for major and trace elements to validate their proposed favorability for hosting VMS deposits. Results indicate that all of the rhyolite types are prospective, but mineralization may differ from...
Orogenic gold deposits are the main source of gold for humanity. These deposits formed from the Archaean to Phanerozoic at crustal depths of more than 4 km. The key parameters accounting for their formation remain under discussion. I review these parameters in light of recent scientific advances using a systemic approach, where the relative pertinence of each component of a complex system in time and space is addressed. From the base to the top of the mineralising system, the following components and processes are reviewed: (1) the fluid sources; (2) the appropriate ligands and their sources; (3) the solubility of gold and its potential sources; (4) the function of faults as conduits; and (5) the mechanisms for precipitating gold. This review provides further support for the role of carbon-rich and pyritic sedimentary rocks in the formation of orogenic gold deposits throughout geological time.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.