Abstract

Porphyry copper mineralization at Yandera is located within the Bismarck intrusive complex, a synorogenic batholith emplaced during folding and faulting accompanying collision of the Australian plate and an island-arc zone in mid-Miocene time. Regional stresses during solidification of the pluton produced fracture systems along which younger quartz diorite and dacite stocks, plugs, and dikes were emplaced, an event which produced further complex fracturing and intrusion breccias about 12 to 14 m.y. ago. Still younger, relatively small plugs and dikes of fine-grained 6.5 m.y. aplitic quartz monzonite porphyry and quartz latite porphyry are centrally located within a broad, diffuse zone of disseminated copper mineralization and associated biotite-orthoclase alteration. K-Ar age dates suggest that copper mineralization is genetically linked to emplacement of these intrusions, with better overall hypogene copper grades clearly controlled by spacing and orientation of pre-existing fractures. The potential ore zone is a broad, diffuse area with relatively high fracture density peripheral to the earlier quartz diorite intrusions.Telescoped and overlapping apparent sequences of alteration and sulfide assemblages are evident: an early quartz-orthoclase-biotite alteration with weak, low-sulfide, disseminated and veinlet molybdenite-chalcopyrite-pyrite-bornite mineralization is pervasively developed. Overprinted is strong vein and veinlet chalcopyrite-pyrite-bornite mineralization with associated sericite-chlorite-magnetite-epidote alteration. These are, in turn, overprinted with late pyritic veins with associated strong quartz-clay-sericite alteration.Yandera does not exhibit marked concentric zoning of alteration and mineralization about a centrally located hypabyssal stock as in most porphyry copper deposits of the world, but diffuse, poorly developed zoning is suggested by:a. restriction of bornite to the center of the deposit;b. potassium-silicate alteration and chalcopyrite mineralization in the central area;c. development of pyrite-clay-sericite fracture-controlled mineralization and alteration peripheral to the central area; andd. lead-zinc soil geochemical anomalies peripheral to copper mineralization.The Yandera deposit is similar to some porphyry copper deposits in British Columbia and the Philippines with its location near an internal contact within an area of complex, multiple intrusion in a major batholith, dominance of fracture control of sulfide mineralization, and lack of significant lead-zinc-silver deposits fringing the copper mineralization.

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