Tonalite-trondhjemite fractionation of peraluminous magma and the formation of syntectonic porphyry copper mineralization, Gibraltar mine, central British Columbia

Special Volume, Vol. SV 46, No. 1995, 1995

The Gibraltar porphyry copper deposit is located in central British Columbia 362 km north of Vancouver, British Columbia. Production on the property began in March of 1972. As of December 1992, 241 000 000 tonnes of ore averaging 0.360% Cu have been milled: 147 500 000 tonnes averaging 0.301% Cu (OJXJ8% Mo) have been defined as a mining reserve; and 546 600 000 tonnes averaging 0.287% Cu (0.007% Mo) have been classified as a mineral resource. The Gibraltar deposit consists of six separate ore zones hosted by the Granite Mountain batholith, a zoned peraluminous subalkaline body which intrudes the Permian Cache Creek Group. The batholith and adjacent Cache Creek Group have undergone penetrative foliation and are metamorphosed to the upper greenschist facies. Magmatism, dynamothermal metamorphism and ore deposition occurred as a related chain of events over a period of tectonism extending from Upper Triassic to possibly earliest Jurassic. Fractional crystallization of the batholith involved a trend of increasing Na enrichment without an associated increase in K. The magmatic sequence was: tonalite (Mine Phase), to trondhjemite (Granite Mountain Phase), to leucotrondhjemite (Leucocratic Phase). Ore stage alteration and sulphide mineralization closely followed the emplacement of the leucotrondhjemite as a volatile-rich fraction of the tonalite-trondhjemite differentiation. The alteration process was dominated by K metasomatism, accompanied by the redistribution of Mg and Ca, and the net expulsion of Na. Alteration assemblages consist of quartz, sericite, chlorite, epidote, and carbonate. Major sulphide minerals are pyrite and chalcopyrite. Molybdenite is a minor but economically significant component of the ore. Sulphide zoning is evident, consisting of chalcopyrite + molybdenite, to chalcopyrite, to chalcopyrite + sphalerite, to sphalerite. The orebodies were formed within an evolving environment of low angle, northeasterly directed compressional stress which resulted in the complex deformation of alteration and mineralization.
Keywords: Porphyry, Porphyry copper deposit, Batholith, Magmatism, Dynamothermal metamorphism, Ore deposition