GEOLOGY, GEOCHEMISTRY, AND GEOCHRONOLOGY OF THE CONTACT COPPER PROSPECT: AN INVESTIGATION OF A POTENTIAL PORPHYRY ROOT, NEVADA, USA

The Contact Copper prospect is a Jurassic copper oxide resource hosted in an equigranular granodiorite and likely represents an eroded copper porphyry root associated with the Elko Orogeny. Mineralization primarily consists of chrysocolla and malachite with minor sulfides within 1-3 meter wide quartz veins which dip 45-60° to the southwest. Hydrothermal alteration styles consist of sodic-calcic, potassic, phyllic, intermediate argillic, and distal endoskarn alteration.

Hydrothermal fluids at Contact Copper evolved from early, oxidizing, and circumneutral pH conditions associated with sodic-calcic and potassic alteration to later, relatively reducing, and slightly more acidic conditions associated with phyllic and intermediate argillic alteration. Early hydrothermal alteration is texturally destructive and quartz-leaching, producing a vuggy texture, but late hydrothermal alteration is associated with quartz veins.

A-type veins at Contact Copper are represented by 1) barren, whispy, quartz-dominant veins with local alteration envelopes; or 2) quartz veins with trace chalcopyrite±bornite and weak alteration halos of albite, epidote, actinolite±potassium feldspar±white mica. B-type veins are 1) quartz+chlorite±molybdenite±sphalerite veins with alteration envelopes of albite, epidote, actinolite±white mica; or 2) quartz+molybdenite±chalcopyrite veins with alteration halos of potassium feldspar and/or albite±epidote±white mica. The youngest vein types observed are chlorite±chalcopyrite veinlets with thin halos of white mica. The Al-OH absorption feature of white micas occurs between 2198 and 2202nm near vein centers and between 2204 and 2213nm at distance, implying a transition from muscovitic illite to aluminoceladonite.

Crosscutting relationships and geochronology suggest that early hydrothermal alteration occurred before the complete solidification of the Contact pluton at 160.6±1.5Ma (U-Pb age, 2σ errors) and continued until at least 157.9±0.6Ma (Re-Os age, 2σ errors). Mineralized andesite porphyry dikes (160.0±1.4Ma; U-Pb age, 2σ errors) mingled and mixed with granodiorite mush and likely contributed volatiles and metals which migrated upwards and reacted with solidified wallrock, forming mineralized quartz veins at a structurally higher level of the pluton.