Petrology and U-Pb Geochronology of Footwall Porphyritic Rhyolites from the Wolverine Volcanogenic Massive Sulfide

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• journal_title：Economic Geology
• Contributor：Stephen J. Piercey ; Jan M. Peter ; James K. Mortensen ; Suzanne Paradis ; Donald C. Murphy ; Terry L. Tucker
• Publisher：Society of Economic Geologists
• Date：2008-
• Format：text/html
• Language：en
• Identifier：10.2113/gsecongeo.103.1.5
• journal_abbrev：Economic Geology
• issn：0361-0128
• volume：103
• issue：1
• firstpage：5
• section：Articles

摘要

<p id="p-1">Porphyritic rhyolite sills form an important component of the footwall of the Wolverine volcanogenic massive sulfide (VMS) deposit, Yukon, Canada, and occur proximal to mineralization in the immediate deposit area (Wolverine/Lynx zone) and at similar stratigraphic levels along strike (Fisher, Puck, and Sable zones). Porphyritic rhyolites are of two types: an older quartz-feldspar porphyritic (QFP) rhyolite suite; and a younger feldspar porphyritic (FP) suite. Both the QFP and FP suites of intrusions are semiconcordant, suggesting a sill-like morphology, and are altered and crosscut by veinlet mineralization, suggesting that they are pre- to synmineralization. The margins of QFP suite of intrusions contain minor xenoliths of surrounding shales and poorly developed chilled margins suggesting emplacement into partially consolidated sedimentary rocks, whereas the FP suite of intrusions shows well-developed chilled margins indicative of emplacement into fully solidified sedimentary rock. These features suggest that the QFP suite of intrusions represents an older phase of rhyolitic magmatism, whereas the FP suite represents a younger event. This is supported by U-Pb zircon ages, which indicate a 352.4 ± 1.5 Ma emplacement age for the FP suite and a ~347 to 346 Ma emplacement for the FP suite (two ages at 347.8 ± 1.3 and 346.0 ± 2.2 Ma). Both suites of porphyries have inherited Proterozoic zircon and have ratios of La/Sm_UCN ~1 and Nb/Th_UCN ~1 (UCN – upper continental crust normalized), indicating derivation from and/or extensive interaction with ancient upper continental crustal materials. The FP suite, however, has elevated high field strength element (HFSE) and rare earth element (REE) contents, high zircon saturation temperatures, and higher Nb/Ta ratios and lower Ti/Sc ratios than the QFP suite. These features are interpreted to reflect that the FP suite of magmas was hotter (>900°C) melts with a greater mantle component in their genesis. Both suites, however, are interpreted to have formed due to basaltic upwelling, crustal melting, and crust-mantle mixing during ensialic back-arc basin activity. The presence of mantle heat within the Wolverine basin from ~352 to ~347 to 346 Ma, a minimum of 5 m.y., suggests that sustained mantle heat flow was critical to the genesis of the Wolverine porphyries. It is also suggested that this sustained mantle heat was responsible for the Wolverine hydrothermal system and that upwelling mantle may be essential in providing the heat to drive hydrothermal systems even in continental margin-type VMS environments (e.g., Bathurst, Iberian pyrite belt). p><p id="p-2">The Wolverine porphyries are among the most HFSE- and REE-enriched felsic rocks associated with VMS mineralization globally. The high HFSE and REE concentrations in these rocks are interpreted to be due to high-temperature melting of continental crust and the efficient dissolution of HFSE- and REE-bearing accessory phases (e.g., zircon, monazite) during crustal melting. The presence of HFSE- and REE-enriched felsic rocks indicates thermally anomalous geodynamic settings, which may have had sufficient heat flow to form high-temperature crustal melts and, by association, the potential to generate robust and long-lived hydrothermal systems. Thus, in ancient continental margin settings identification of HFSE-REE–enriched rhyolites may be useful in outlining potentially prospective areas for VMS mineralization. p><p id="p-3">Formation of the Wolverine deposit, and other Devonian-Mississippian VMS deposits in the Finlayson Lake district, coincided with widespread extension in the Finlayson Lake region and elsewhere in the northern Cordillera. This extensional geodynamic activity was related to arc-rifting and ensialic back-arc basin activity in the Yukon-Tanana terrane and the opening of the Slide Mountain back-arc basin (ocean). The age of the Wolverine deposit is also broadly coincident with a global pulse of syngenetic VMS and SEDEX mineralization (e.g., Iberian pyrite belt, Selwyn basin) and an episode of regional, and possibly global, ocean anoxia. The combination of (1) high-temperature magmatism capable of driving vigorous hydrothermal circulation, (2) widespread extension to provide structural conduits to focus hydrothermal discharge, and (3) anoxic bottom waters and abundant associated black shales to act as a trap to prevent hydrothermal fluid dissipation into the water column, were important factors in the formation of the Wolverine VMS deposit. These conditions existed elsewhere in the world during the Devonian-Mississippian and may explain the abundance of syngenetic sulfide deposits that formed at this time. p>