Tectonic evolution of the porphyry copper system at Yandera, Papua New Guinea

Abstract

The porphyry copper system at Yandera, Papua New Guinea, occurs within a north-west-trending, faulted body of porphyritic rocks intrusive into the Miocene (13-15 m.y.) Bismarck batholith. Three separate intrusive events have emplaced porphyries assigned to older, intermediate, and younger ages. Copper-molybdenum-gold mineralization has been localized in strongly fractured rocks spatially related to the belt of axially localized younger porphyries.Resuts of drilling have indicated 338,000,000 metric tons 0.42 percent Cu, 0.0183 percent Mo, and 0.1 g/T of Au which occur in a low (1.5 percent) total sulfide mineral assemblage. Both preore and interore veins and veinlets have been recognized. Veinlet alteration which attends sulfide mineralization is of several types, all of which occur in the same rock volumes in and near the mineralized centers and which give rise to a diffused zoning pattern. Field relationships indicate the following paragenesis of vein alteration types: quartz, biotite, feldspar, variations of quartz-sericite-clay-chlorite, and a late stage of clay-chlorite-zeolite. The vein alteration types are asymmetrically distributed with respect to an unusual center of closely spaced, apparently barren, quartz veins. From this center outward, superimposed quartz and K-silicate vein combinations consist of quartz, quartz and biotite veins, biotite and feldspar veins, and feldspar veins. Quartz-sericite vein alteration types overprint the older K-silicate-altered veins. No pyrite aureole has been recognized. Superposition of these vein types indicates both continued fracturing during the life of hydrothermal processes and changing fluid and wall-rock conditions.Results of the analysis of orientations of contacts and of both preore and interore veins reveal a consistent, orthogonal geometry that is consistently different for each of the three interpreted ages of intrusion and fracturing activity. Contacts of the older porphyries and the preore veins are consistent with a south-directed principal stress direction which activated transcurrent faulting. The resulting northwest-oriented couple produced a zone of low compression into which the older porphyries were passively emplaced. A change of orientation of maximum principal stress to south-west-directed is revealed by northeast-striking dikes of the intermediate group. Orthogonal fracturing which has affected all rocks of the porphyry system and the contact geometry of the younger porphyries suggests a shift from the tectonics of the older transcurrent faulting to tectonics of vertical uplift. In addition to strictly geometrical considerations, the presence of breccias and the interpretations of ore fluid chemistry as suggested by vein alteration types further enforce the interpretations of uplift and continuous systematic fracturing during the hydrothermal event. The tectonic style and its suggested origin are consistent with interpreted aspects of plate history in this part of the southwestern Pacific.