Report on the composition and assemblage of minerals associated with the porphyry Cu-Mo mineralization at the Gibraltar deposit, south central British Columbia, Canada

Abstract

Glacial sediments commonly contain resistate heavy minerals, such as zircon, rutile and epidote. To identify the sources of these minerals and use them as a vectoring tool for mineral deposits, it is critical to understand the compositional variations and assemblages of these minerals in mineral deposits and whether they are distinct from those in barren rocks. This paper reports the occurrence and assemblages of igneous and alteration minerals in the Gibraltar porphyry Cu deposit, southcentral British Columbia. The Gibraltar deposit is the largest of three porphyry-Cu deposits in the region, the others being the Mount Polley mine and the Woodjam prospect. The Gibraltar deposit is hosted by the Granite Mountain batholith and has reserves of over 749 million tons at 0.249% Cu and 0.008% Mo. The batholith is mostly tonalite, with minor variation in modal abundances of felsic and mafic minerals. Igneous minerals are plagioclase, quartz, biotite, hornblende, titanite, zircon and apatite. The mineralization is accompanied by extensive phyllic alteration in the tonalite, which produced illite, quartz, rutile, titanite, magnetite, apatite, chlorite and epidote. Hydrothermal titanite is distinctly different from igneous titanite in crystal habits, optical properties and chemical compositions. Epidote shows a large compositional variation even within one sample from Al-rich (high clinozoisite component) to Fe-rich epidote. Some epidote grains contain significant La and Ce (allanite component), which could be diagnostic of the porphyry mineralization. Preliminary observations suggest the presence of a potassic alteration zone forming hydrothermal biotite, but is now replaced by chlorite.