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Abstract

The Black Mountain Southeast Cu-Au-(Mo) porphyry system of the Baguio district, Northern Luzon, consists of two orebodies with a total resource of 65 Mt @ 0.40% Cu and 0.38 g/t Au. Detailed mapping, petrography, and geochemistry have identified six intrusive phases within the Black Mountain area. From oldest to youngest these are as follows: the Liw-Liw Creek hornblende megacrystic mafic dikes (Liw-Liw Creek; 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 hornblende-phyric basalt, and the aphanitic to plagioclase microphenocrystic fine-grained mafic dikes. The rocks of the Black Mountain area are low to medium K calc-alkaline intrusions; however, the intrusive history of the Black Mountain Southeast intrusive suite demonstrates an abrupt shift from megacrystic mafic dikes to voluminous stocks and plugs of relatively felsic equigranular and porphyritic intrusions, followed by a gradual transition to mafic fine-grained dikes. Hornblendes from the intrusive rocks fall into two groups: one formed at depth in a mafic magma and the other at shallower levels in a felsic magma. The presence of both groups within a single sample suggests mixing of a mafic and felsic magma. 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.

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