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Late Eocene tectono-magmatic evolution and genesis of reduced porphyry copper-gold mineralization at the North Fork deposit, west central Cascade Range, Washington, U.S.A. Smithson, David Mark

Abstract

The North Fork porphyry Cu-Au deposit is located in the west central Cascades Mountain Range, Washington, U.S.A., and belongs to a belt of Eocene to Miocene porphyry Cu (Au) deposits that extend northward into the Coast Mountains of southern British Columbia. The deposit has a geological reserve of 80.4 million tonnes @ 0.44% Cu and 0.003 ounces (oz) Au (a 218,000 oz Au reserve) and is hosted in three main rock units. The oldest and most spatially extensive unit is the Mount Persis andesite (38.9 ±0.3 Ma). It is intruded by quartz monzodiorite (2 samples dated at 37.2 ±0.1 Ma and 37.0 ±0.2 Ma) and mafic latite porphyry (2 samples dated at 37.1 ±0.2 Ma and 36.8 ±0.2 Ma). Plutonic rocks are weakly to moderately peraluminous, calc-alkaline, I-type granitoids that have crystallized at uncommonly low oxygen fugacities (ƒO2's) ranging from the quartz-fayalite-magnetite (QFM) oxygen buffer to one log unit above (QFM+1). The older andesites are even more reduced and have crystallized at ƒO2's approximating QFM-1. Field relationships, age constraints, mineralogy, oxidation state, and whole-rock trace-element data indicate that plutonic and volcanic rocks are consanguineous. It appears that the reduced I-type granitoids have intruded into their own volcanic pile during construction of a late Eocene volcanic arc. Hypogene Cu-Au mineralization is associated with three stages of vein formation, but primarily occurs with banded and crustiform Main-stage quartz-actinolite-albite-chlorite-sulfide veins and accompanying sodic-calcic (albite-actinolite) alteration. Main-stage veins contain abundant hypogene pyrrhotite and lack primary hematite and sulphate minerals indicating formation from relatively reduced hydrothermal fluids. Studies of quartz-hosted fluid inclusions in Main- and Early-stage veins reveal that the North Fork deposit has formed from a thermally prograding system with Cu-Au sulfide deposition occurring at pressures of ~ 400 to 690 bars and temperatures between 348° to 576°C. These pressures are hydrostatic and correspond to depths of ~ 4 to 7 km because fluids were undergoing immiscible phase separation (boiling) into a dense aqueous brine (up to 51 weight % NaCl equivalent) and coexisting low-density vapor (1.4 to 3.4 weight % NaCl equivalent) at the time of trapping and Main-stage vein formation. These physicochemical conditions of ore formation are typical of porphyry Cu-Au deposits worldwide, and together with a direct genetic association with reduced I-type magmas, classify the North Fork deposit as a "reduced porphyry copper-gold" deposit. Measurement of 671 brittle structures (fractures and faults) define three main structural trends that are consistent with the various stress fields operative during the Eocene. The most important of these structures are the NNW-striking (320-340°) fractures and faults that have focused the intrusion of ~ 37 Ma mafic latite porphyry, hypogene Cu-Au mineralization, and related hydrothermal alteration. Repeated use of these structural conduits has resulted in overprinting episodes of magmatism, hydrothermal alteration, and Cu-Au mineralization. Argonargon dating of hydrothermal sericite from halos surrounding Late-stage quartz-sulfide veins yields an age of 35.5 ± 0.2 Ma, which is at least 900,000 m.y. younger than the emplacement age of the youngest mafic latite porphyry, but well within the time-frame that many porphyry Cu- Mo±Au deposits form. The δ34S values of hypogene sulfide minerals from all stages of mineralization lie between 2.3 to 3.0 %o, a range typical of magmatic sulfur and further support of a magmatic origin for the North Fork fluids. The recognition of reduced porphyry Cu-Au mineralization and related arc magmatism at -37 Ma, highlights the prospectivity of the Mount Persis andesites and raises the possibility that Late Eocene porphyry Cu-Au mineralization may be more common in the west central Cascades than has been predicted previously from the localized exposures of quartz monzodiorite and mafic latite porphyry at North Fork.

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