The Taysan porphyry copper deposit, southern Luzon Island, Philippines

Abstract

The Taysan batholith consists of hornblende diorite to trondhjemite on a calc-alkaline trend intruded into pre-Jurassic schist on the northeast and probably Cretaceous meta-volcanics on the south and west. The youngest dikes, postmineral, were dated at 14.8 + or - 0.9 m.y., putting the intrusive sequence in the middle Miocene, the same time frame as nearly all of the Philippine disseminated deposits. The area was nearly peneplaned and in the Pleistocene the whole of southern Luzon was blanketed with volcanic ash, covering at least 95 percent of the mineralized zone.A disseminated copper deposit formed at the western edge of the batholith seemingly on the margin of a volcanic breccia pipe. It plunges about 33 degrees to the east northeast. The average copper grade is 0.43 percent with significant gold, a trace of molybdenum, and 2 to 3 percent each of magnetite and pyrite. A late pulse of mineralization superposed 0.13 percent zinc in the upper portion of the orebody.Alteration is poorly exposed but an outer propylitic zone with a strong development of epidote and some calcite, chlorite, and pyrite is present. Representing a phyllic zone, there is very strong pyritization in some portions of the fringe of the mineralized zone, much weaker in others. Soda sericite is strongly developed outside the orebody and less so within the ore zone, where silicification is prominent. Potassic alteration is represented by strong hydrothermal development of biotite with weaker and spotty orthoclase, the potassic zone coinciding with strongest mineralization.Strontium 87 Sr/ 86 Sr ratios in the Philippines are low in porphyry copper plutons ranging from 0.7035 to 0.7037, but at Taysan it is 0.7040.