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Further Re-Os Arsenopyrite Geochronology from Selected Meguma Au Deposits, Meguma Terrane, Nova Scotia: Possible Evidence for a Protracted Gold-forming System
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- Author / Creator
- Chen, Lin
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The Meguma terrane, Nova Scotia, is dominated by two rock types; regionally deformed and metamorphosed Cambro-Ordovician metasedimentary rocks and ca. 380-370 Ma meta- to peraluminous granites. The metasedimentary rocks host numerous orogenic-type, vein-hosted gold deposits which occur throughout the metasandstone-dominated Goldenville Group rather than the overlying metasiltstone- and metamudstone-dominated Halifax Group. These mineralized veins are dominated by quartz-carbonate-sulfide assemblages and occupy structures consistent with emplacement during late-stage fold tightening of the regional, northeast-trending, upright folds that formed during the Acadian orogenic event at ca. 410-400 Ma. From previous work, vein formation, hence gold emplacement, spanned 30-40 Ma, as constrained from field observations and radiometric dating. The former indicates veins post-date cleavage formation given that cleaved wall-rock fragments occur in some veins, and that rarely; veins post-date hornfels related to 380 Ma granites. Existing absolute age dating indicates two events at 408 Ma (Re-Os Arsenopyrite; 40Ar/39Ar whole rock) and 380-362 Ma (Re-Os Arsenopyrite; 40Ar/39Ar Muscovite, Biotite, whole rock). Here we report new Re-Os geochronological data generated from arsenopyrite in gold-bearing veins for two deposits sampled, all of which lie in the same stratigraphic-structural position in the lower part of the Goldenville Group. The Re-Os analysis of arsenopyrite from three veins from the Beaver Dam deposit, which is dominated by bedding-concordant type veins, yielded the ages of ca. 461 Ma, 464 Ma and complex ages of 456 Ma and 446 Ma. In contrast, Re-Os analysis of one vein-hosted arsenopyrite sample from the Touquoy deposit of the Moose River gold district, where mineralization occurs in a vein-poor sequence of carbonate-rich metasiltstone rocks, yielded two isochron ages of ca. 380 Ma and 438 Ma; the other one sample from the same deposit but from a different vein provided Re-Os model ages from ca. 400 Ma to 440 Ma. Obvious zoning patterns are observed from thin sections, SEM images and EDS element mapping of the arsenopyrite, which indicates multiple generations of mineral growth. From the host rock Re-Os analyses, the extremely low Re and Os concentrations are recorded, indicating that the host rocks are unlikely to significantly affect the sulfide Re-Os ages. The new data suggests that in the Meguma terrane there exists more than two periods of gold mineralization, which started before the Acadian deformation and metamorphism of the host rocks, implying the gold deposit type in the Meguma terrane is not only orogenic related. Similar 440-460 Ma Ordovician and Silurian ages are reported in the adjacent Avalon terrane and Yarmouth area of the Meguma terrane respectively, representing tectonic related magmatic events. Therefore the 460 Ma and 440 Ma vein ages in this study may provide new evidence for the pre-Acadian history of the Meguma terrane. The Meguma terrane might be close to the Avalon terrane during Middle Ordovician age and there may be a wide-spread magmatic event in the Meguma terrane around 440 Ma.
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- Subjects / Keywords
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- Graduation date
- Fall 2015
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- Type of Item
- Thesis
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- Degree
- Master of Science
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- License
- This thesis is made available by the University of Alberta Libraries with permission of the copyright owner solely for non-commercial purposes. This thesis, or any portion thereof, may not otherwise be copied or reproduced without the written consent of the copyright owner, except to the extent permitted by Canadian copyright law.