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ABSTRACT: Mineralization of Porphyry Copper-Gold Deposits: Paragenesis and Fluid Inclusions

Akira Imai

The paragenetic sequence and fluid inclusions of several porphyry copper-gold deposits in the Western Pacific Region were studied. The studied deposits include Mamut (Sabah, Malaysia), Santo Tomas II, Dizon, and Lepanto-FSE (Philippines). The typical paragenetic sequence is from early biotitization with dissemination, followed by chloritization with accompanying quartz stockworks, and then fracture-controlled base metal mineralization including molybdenum and/or antimony mineralization at later stages.

Secondary biotite flakes that replaced igneous hornblende phenocrysts were hydrothermally altered to chlorite along the stockwork quartz veinlets. The metal grade data accumulated by the mining geologists suggest that gold may be associated with chloritization and not with biotitization. Fluid inclusions found in the stockwork quartz are highly saline and are boiling. Some of them are homogenized at temperatures as high as 500°C upon heating, while most of them are not homogenized up to 520°C. Biotitization and dissemination may take place just after the solidification of the ore bringing porphyry intrusives.

High halogen contents in the hydrous minerals, both of igneous and hydrothermal origins, and high oxygen fugacity estimated in the hypersolidus and hydrothermal conditions may be due to the selective degasing of the immiscible light vapor from the system. The other dense highly saline fluid remaining in the system may be responsible for the porphyry type mineralization as ore fluids as noted in the fluid inclusions of the quartz vein stockworks.

AAPG Search and Discovery Article #90097©1990 Fifth Circum-Pacific Energy and Mineral Resources Conference, Honolulu, Hawaii, July 29-August 3, 1990