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Geology of the Ajax East and Ajax West, silica-saturated alkalic copper-gold porphyry deposits, Kamloops, South-Central British Columbia Ross, Katherina V.

Abstract

Ajax East and Ajax West deposits are two of a number of porphyry copper-gold deposits that are hosted by the silica-saturated alkaline Iron Mask batholith, in the southern part of Quesnellia. The northwesterly trending batholith is an Early Jurassic composite intrusion emplaced in the Nicola Group, a well defined volcanic island arc package. Pit mapping delineated eleven significantly different rock units: Nicola Group volcanic rocks, picrite and nine intrusive units ranging in composition from diorite or gabbro to quartz monzonite. The Ajax deposits occur at the intersection of two major dioritic phases of the Iron Mask pluton, the hybrid diorite and the younger Sugarloaf diorite, which is recognized as the probable source of mineralization. Porphyry-style mineralization consists of pyrite and chalcopyrite. Alteration has been divided into four categories: (i) pre-main stage alteration, (ii) main stage porphyry alteration and mineralization, (iii) late main stage alteration, and (iv) post porphyry alteration. Four main stage porphyry alteration assemblages have been defined: propylitic, albitic, potassic and scapolitic. Propylitic alteration, which occurs peripheral to albitic alteration, appears to be a weaker manifestation of the albitic assemblage. Albitic alteration, which is spectacularly developed along the contact of the Sugarloaf diorite and the hybrid diorite is associated with high grade copper-gold mineralization. Potassic and scapolitic alteration occurs as veins that cross-cut propylitic and albitic alteration. Pyrite and chalcopyrite are present in all main stage alteration assemblages but are most closely associated with albitic alteration. Albitic alteration liberates Fe⁺ and may decrease pH, assisting in the precipitation of chalcopyrite. Mineralization also appears to be controlled to some extent by host lithology with the Sugarloaf diorite as the most favourable host. Main stage alteration assemblage minerals overprint several deuteric alteration events and are overprinted in turn by a low grade metamorphic assemblage. Screens of hornfelsed Nicola Group volcanics and serpentized picrite indicate the presence of major fault systems that have also controlled the intrusion of the younger, mineralizing phases of the batholith, and thus, are good indicators of potential mineralization.

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