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Controls and distribution of Cu-Au mineralization that developed the Island Mountain deposit, Whistler property, southcentral Alaska
Gross, Timothy G.
Gross, Timothy G.
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2014
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2014
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Abstract
The Island Mountain Au-Cu deposit, located in southwestern Alaska, USA, has been classified as a reduced Au-Cu porphyry deposit on the basis of alteration mineralogy. Due to the reduced oxidation state of the ore fluids, alteration mineralogy, metallogeny, and zoning of ore-mineral associations can be greatly affected in comparison to classically oxidized porphyry deposits. The broad aim of this study was to provide insight to the genesis of reduced porphyry deposits using the Island Mountain system as an example. Focus was given to the spatial, temporal, and chemical relationships of rock types and alteration mineralogy hosting Au-Cu and Au-only mineralized zones. An additional goal of the thesis was to determine the controls and timing of emplacement of the Breccia Zone (host to the majority of Au and Cu) and its regional context within the Whistler area. The Island Mountain deposit is composed of a cluster of multiple ore-bearing intrusive- and hydrothermal-matrix breccia pipes that are hosted in diorite, monzodiorite, and carbonaceous flysch and metasedimentary rocks. U-Pb isotopic ages indicate the diorite and monzodiorite intruded the carbonaceous sedimentary rocks during the Late Cretaceous (~77 Ma). These host rocks were intruded by magmatic and hydrothermal breccias. Alteration associated with the emplacement of hydrothermal breccias developed the economic grades of Au and/or Cu in two primary zones at Island Mountain: the Main Breccia Zone and the Lower Sulfide Zone. The morphology of the Main Breccia Zone is characterized by a northeast-trending (structural domain 1) cluster of ovoid breccia pipes that plunge steeply ([greater than]80[degrees]) towards the north-northeast. Three major stages of alteration and mineralization resulted in development of six distinct alteration types at Island Mountain. The stages are 1) Early Stage potassic, interpreted to be proto-ore, 2) Main Stage sodic-calcic and propylitic, ore-related, and 3) Late Stage.Located within the Lower Sulfide Zone and crosscutting the Main Breccia Zone are hornblende phyric dikes interpreted to be syn or slightly post-hydrothermal alteration based on crosscutting relationships. An Ar-Ar isotopic age obtained from hornblende of the dikes indicate they were emplaced at ~65 Ma. Hhydrothermal biotite (63 Ma) indicate the dikes were emplaced 1-2 Myrs before the cooling of hydrothermal fluids below ~200[degrees]C. Au-Cu mineralization (chalcopyrite and electrum) occurred towards the end of Main Stage alteration primarily with epidote, which is spatially related to Early Stage potassic and Main Stage sodic-calcic altered zones. Abundant Au (as electrum) is associated with pyrite veins and pyrite overgrowths on pyrrhotite, indicated Au mineralization occurred subsequent to deposition of pyrrhotite. Comprehensive documentation of the host rock and alteration mineralogy of the Island Mountain "reduced" porphyry indicates that mineralogy typical of oxidized deposits (anhydrite, magnetite, abundant quartz) is observed in the Main Breccia Zone where intrusive-matrix breccias are observed. Mineralogy indicative of reduced ore fluids (pyrrhotite, arsenopyrite, scapolite) is associated with the increased interaction with biotite hornfels and graphitic-rich sedimentary rocks (clasts in breccias, Lower Sulfide Zone relationships) of the Kahiltna terrane. Reactivated, or subsequent deformation parallel to northwest trending faults (Domain 2) offset the breccia zone.
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