Magmatic Evolution and Alteration Geochemistry of the Black Mountain Southeast Porphyry Copper-Gold Deposit, Baguio Mineral District, Luzon, Philippines

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.;The magmas of Black Mountain Southeast porphyry system are a product of complex sub-arc generative mechanisms and intra-crustal modification: Liw-Liw Creek magmas were generated and emplaced prior to ∼3.2 Ma, possibly as a product of resumed subduction of an aseismic seamount chain. Locally compressive tectonics facilitated intra-crustal stagnation and the subsequent fractional crystallization of a Liw-Liw Creek magma chamber. A second mafic melt (a hybrid mixture of evolved slab-derived basalt and primitive depleted mantle wedge material) was generated and, upon ascension into the crust, underplated the crystallizing Liw-Liw Creek magma chamber. The additional heat and volatiles provided by the hybrid melt resulted in partial melting at the base of the magma chamber and possibly energized the remaining melt within the intrusion, giving rise to felsic and intermediate partial melt products (as a mixture of the hybrid magma and partial melt product) as well as mineralizing fluids. A shift to local extension allowed for the emplacement of the partial melt products, and a gradual increase in the representation of the hybrid mafic melt in the upper porphyry system.;Alteration associated with the Black Mountain Southeast orebody constitutes an early sub-circular zone of potassic alteration (quartz+biotite) imposed over the central plug of EMD that is spatially and temporally linked to mineralization, followed outward by a contemporaneous propylitic alteration halo (epidote+chlorite+carbonates). A later northwest-elongate zone of pyritic and sericitic alteration overprinted both of the earlier alteration assemblages. Weak phyllic alteration is present locally in quartz-pyrite veins crosscutting the Black Mountain Southeast area.;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.;Sampling and detailed mapping at a 1: 1000 scale of the Black Mountain Southeast area has identified six intrusive phases, termed the Black Mountain Southeast Intrusive Suite (BMSIS). From oldest to youngest, these are: the hornblende megacrystic Liw-Liw Creek basaltic dikes (LLC), the quartz vein stockworked and quartz flooded early mineralization diorite plug that is the main host of mineralization at Black Mountain Southeast (EMD), the plagioclase and variably hornblende-phyric diorite stock and dikes (PHD), the hornblende megacrystic and clotted gabbro dikes (HMG), the hornblende clotted basalt dikes (HCB), and a series of young aphanitic to plagioclase microphenocrystic mafic dikes (AM). Over the lifetime of the porphyry system, they mark a temporal shift in composition from mafic to felsic to mafic. These rocks are derived from low- to medium-K calc-alkaline island arc melts. U-Pb and 40Ar- 39Ar dating has confirmed the relative chronology of intrusive phases, and major and trace element whole rock geochemistry confirms the temporal shift from mafic to felsic to mafic magmas. Isotope systematics (Rb-Sr and Sm-Nd) show that both radiogenic Sr and Nd increase during the lifetime of the Black Mountain Southeast system. Major element geochemistry of hornblende phenocrysts indicate two spatially and/or temporally distinct crystallization events; one occurred at shallower levels as a product of quenching intermediate magma, whereas the other occurred at deeper crustal levels in the presence of mafic magmas.