The geology of the Tangse porphyry copper-molybdenum prospect, Aceh, Indonesia

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The porphyry mineral deposit at Tangse is hosted by a multiphase intrusion of predominantly quartz diorite composition termed the Tangse stock. Emplacement along a segment of the transcurrent Sumatera fault system was related to the northward subduction of the Indian plate. The intrusive rocks belong to a normal K calc-alkaline suite and are characterized by low abundances of K (1.09-1.35 wt %), Rb (26-40 ppm), Nb (<5 ppm), Th (<4 ppm), and U (<1.5 ppm), high Sr (314-488 ppm) abundance, moderately high K/Rb and K/Ba ratios, and low Rb/Sr and Th/U ratios. Initial strontium isotope ratios (0.70423-0.70453) are uniformly low and prohibit any significant involvement of a sialic crustal component in magma genesis. K-Ar analysis yields middle to late Miocene ages for the intrusion and cooling of the Tangse stock (13.1 m.y.) and the development of the associated hydrothermal alteration-mineralization (9.0 m.y.). The porphyritic intrusive phases are divisible into three groups (older, intermediate, and younger porphyries). The older porphyries form the bulk of the Tangse stock; the other phases are volumetrically unimportant, occurring typically as dikes. The distribution of the hypogene sulfide minerals, chalcopyrite, molybdenite, and pyrite, within the older porphyry mass is related to the development of a complex series of aluminosilicate alteration assemblages. A core of early chalcopyrite-bearing biotite alteration (Cu = 0.1-0.2%, Mo: 60-90 ppm, total sulfide = 1 vol %) is enclosed within a halo of barren chlorite-epidote alteration. Both have been overprinted by feldspar-destructive, sericitic alteration with quite variable metal contents. Copper and molybdenum grades in the chalcopyrite-molybdenite-bearing sericite-chlorite-quartz alteration (total sulfide = 2-4 vol %) typically exceed 0.2 percent and 100 ppm, respectively, and locally may reach values of 0.8 percent and 300 ppm, reflecting an enrichment relative to the early biotite alteration. By contrast the quartz-sericite and quartz-sericite-andalusite alteration assembldges (total sulfide content = 2-10 vol %) with grades of less than 0.1 percent Cu and 20 ppm Mo are depleted in both metals.