Hydrothermal Alteration and Exploration Vectors at the Island Gold Deposit, Michipicoten greenstone belt, Wawa, Ontario
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The Island Gold deposit is a high-grade orogenic gold deposit located in the Goudreau Lake Deformation Zone (GLDZ), within the Michipicoten greenstone belt. This belt is situated within the larger Wawa-Abitibi terrane which is an area of significant gold endowment in the Superior Province. This underground mine is operated by Alamos Gold Inc. (formerly operated by Richmont Mines Inc.). At the end of 2016, the Island Gold Mine had estimated indicated and measured resources of 478,800 tonnes at 5.94 g/t Au (91,400 oz) as well as inferred resources estimated at 3,041,800 tonnes at 10.18 g/t Au (995,700 oz). Simultaneously, proven and probable mineral reserves were estimated at 2,551,000 tonnes at 9.17 g/t Au (752,200 oz). This thesis integrates petrology, geochemistry, electron probe microanalysis, U-Pb zircon geochronology, and multiple sulphur isotopic study to characterize the nature and timing of alteration associated with the auriferous zones at this deposit. The protoliths of pre-gold mineralization lithologies identified at the Island Gold deposit, listed from oldest to youngest, include dacitic volcanic rocks, iron formation, gabbro, and tonalite–trondhjemite. VGD extensional and V1-V2 shear-related quartz (± carbonate) veins, that often contain visible gold, post-date and alter these lithologies. Protoliths of post-mineralization gold lithologies include quartz diorite, gabbro/low-K lamprophyre (spessartite), and silica-poor diorite–monzodiorite. Late V3 extensional quartz-carbonate veins result in local, inconsistent alteration and remobilization of certain elements. Later tourmaline ribbons invade pre-existing structures. Lastly, diabase–quartz diabase dykes sharply cut all lithologies and vein types. Greenschist facies metamorphism has variably affected all lithologies, resulting in metamorphic minerals that mainly include actinolite, carbonates (mainly calcite), chlorite (ripidolite), chloritoid, epidote, hematite, plagioclase, quartz, and white mica. Archean U-Pb LA-ICP-MS ages of 2735 ± 8 Ma and 2738 ± 9 Ma obtained from zircons in altered and least-altered dacitic samples are consistent with previous geochronological studies. Analysis of uniform cathodoluminescent zircons and rims in the altered sample yielded anomalous Proterozoic ages ranging from c. 975 to 1499 Ma, which are attributed to unidentified alteration event(s) that post-date auriferous quartz veining. Alteration related to auriferous quartz veining at this deposit results in the significant and consistent enrichment of Au, K2O, Rb, S, and Te as well as typically the depletion of Na2O in alteration envelopes hosted by dacite, gabbro, and the tonalite–trondhjemite. Alteration minerals associated with V1-V2 auriferous quartz veining include biotite, Ca-Mg-Fe carbonates, chlorite (ripidolite), plagioclase, quartz, sulphides (pyrite ± pyrrhotite ± chalcopyrite), and white mica (muscovite ± phengite). The alteration associated with V1-V2 veining remains relatively consistent with varying distance parallel to the GLDZ and elevation. Aside from auriferous alteration resulting in Na2O depletion, alteration-derived biotite, and insignificant proportions of alteration-derived potassium feldspar at the Island Gold deposit, the gold-related alteration at orogenic gold deposits east of the Kapuskasing Structural Zone is similar to the alteration identified at the Island Gold deposit. This overall consistency of the alteration between these deposits and the Island Gold deposit supports a common genetic process for these orogenic gold deposits. Multiple sulphur isotope study of sulphide minerals from auriferous quartz veins and associated ore zones dominantly suggests a mantle/igneous parentage. The auriferous fluid that produced these veins was likely derived from the mantle, exsolved from a cooling crustal intrusive body, or the dehydration of igneous rocks due to metamorphism.
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Tyler Justin Ciufo (2019). Hydrothermal Alteration and Exploration Vectors at the Island Gold Deposit, Michipicoten greenstone belt, Wawa, Ontario. UWSpace. http://hdl.handle.net/10012/14609