Abstract

The Black Mountain porphyry Cu-Au deposit in the Baguio District of the Philippines, consists of two orebodies with a total resource of 65 Mt @ 0.40% Cu and 0.38 g/t Au hosted by the Black Mountain intrusive complex. Detailed mapping, petrography, and geochemistry have identified six intrusive phases within the Black Mountain area. From oldest to youngest these are: the Liw-Liw Creek hornblende megacrystic mafic dikes (3.20 ± 0.02 and 4.73 ± 0.17 Ma), the early mineralization quartz diorite, the plagioclase- and variably hornblende-phyric diorite (2.87 ± 0.08, 2.98 ± 0.02 and 2.83 ± 0.23 Ma), the hornblende megacrystic gabbro (2.81 ± 0.15 Ma). The geochemical and isotopic characteristics of the mafic dikes in and around the Black Mountain system are consistent with primitive mantle-derived melts that underwent minimal crustal contamination. In contrast, the intermediate to felsic rocks show geochemical signature indicative with contamination by young arc crust, consistent with ponding and fractionation within shallow-crustal magma chambers.

The presence of amphibole in felsic rocks aged 6.39 to 2.83 Ma with similar crystallization temperatures and pressures, suggests a long-lived and hot felsic magma chamber under the Black Mountain deposit. However, mineral textures and chemistry recorded by phenocrysts of plagioclase and amphibole indicate repeated mafic magma recharge. The common occurrence of rounded or elongated sulfide inclusions in the megacrystic amphibole from the mafic rocks (2.81 ± 0.15 Ma) suggests enrichment of Cu in mafic rocks relative to the ore-forming porphyries. Large-scale mafic magma recharge (particularly at ~2.8 Ma), likely introduced ore-forming metals to the long-lived felsic magma chamber. Porphyry mineralization in the Black Mountain area is interpreted to have formed as a result of underplating of a felsic magma chamber by a mafic magma that formed as a result of mantle recharge related to the subduction of the aseismic Scarborough Ridge.