Magmatic evolution and alteration geochemistry of the Black Mountain Southeast porphyry copper-gold deposit, Baguio mineral district Luzon, Philippines

Abstract

The Black Mountain porphyry Cu-Au deposit is located in the central-western portion of the Baguio mineral district, Philippines, approximately six kilometers southwest of Baguio City in the Upper Bued River. It consists of two orebodies hosted within the Black Mountain Intrusive Complex (BMIC). The 'Main' or 'Kennon' orebody occurs at the northwest end of the elongate intrusive complex. Main was block caved from 1969 1983 and had a preproduction reserve of 47 Mt at 0.38% Cu and 0.35 g/t Au + 0.01% Mo. The Southeast orebody was block caved from 1977 - 1983 from surface at 790 meters to a subsurface elevation of 560 meters and had a preproduction reserve of 15 Mt ∼0.37% Cu + 0.26 g/t Au. The Southeast orebody has a surface expression that is approximately 150 meters wide and 600 meters long, trending northwest. Trace element analysis of alteration epidote demonstrated a spatial chemical zonation within the propylitic alteration halo of the Black Mountain Southeast system for a suite of pathfinder elements, originally defined by the AMIRA P765 project to be used for regional porphyry exploration (Zn, Sn, Mo, Cu, La, Y, Zr, Sr, As, Pb, Sb, U, Bi). Comparison of the alteration epidote trace element compositions with the trace element compositions of the host rocks and host phenocrysts suggested that for the majority of the suite of elements examined, host rock chemistry did not have an impact on epidote trace element composition. However, a weak correlation between epidote, whole rock and plagioclase phenocryst Sr composition was detected. It is therefore recommended that Sr be discarded from the possible suite of pathfinder elements.