Abstract

Widespread, currently subeconomic molybdenite mineralization of possible porphyry type occurs in an oval, epizonal, late tectonic Archcan (2,643 m.y.) porphyritic granodiorite-quartz monzonite stock intruded into a sequence of mafic flows and sedimentary rocks. The stock has an area of 5.7 km 2 and is largely a single intrusive phase with an average composition of 32 percent quartz, 43 percent oligoclase, 20 percent potassium feldspar, 2.5 percent biotite and chlorite, and 2.5 percent accessory minerals. Most of the stock is medium grained (2-5 mm) with sparse, 1- to 2-cm, flow-oriented potassium feldspar phenocrysts, but a fine-grained chilled zone up to 500 m wide is preferentially developed on the north and west sides of the stock. Eccentric development of the chilled zone suggests that the stock was emplaced over an extended period of time with the main magma feeding system in the south part of the stock.The east-trending mineralized zone is confined to the more rapidly cooled, early north end of the stock and is up to 450 m wide and 2,500 m long. It consists of closely spaced, molybdenite-sheathed quartz veinlets that form a single set trending 068 degrees and dipping 75 degrees S. The veinlets have an average width of 1 to 2 mm. The average grade in the east part of the zone is 0.06 percent MoS 2 with traces of Cu. The mineralized zone is characterized by weak phyllic alteration in which oligoclase has been largely replaced by albite-sericite pseudomorphs and by minor carbonate and epidote; biotite has been replaced by chlorite and muscovite; pyrite has been introduced; and, in the west part of the zone, there has been some potassium metasomatism. Narrow alteration halos envelop some of the veinlets. Surrounding the phyllic alteration is a 1-km-wide zone of decreasing alteration intensity in which oligoclase is replaced by partial albite-sericite pseudomorphs and biotite partly chloritized.The occurrence has been classified as an Archcan porphyry deposit because of the wide-spread but low-grade mineralization, deposition of the mineralization in joints, zoned alteration of phyllic and propylitic character, and association with an epizonal porphyritic pluton.

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