Tectonic control of porphyry copper genesis in the Southwestern Pacific island arc region

Abstract

Seiya UYEDA and the author recently expressed a hypothesis that the extensional regional stress environment is favorable to the kuroko or similar volcanogenic massive sulphide mineralization and the compressional stress environment to the porphyry type copper concentration (UYEDA and NISHIWAKI, 1980). The investigation for the tectonic control of the 23 porphyry copper mines and important prospects, with more than 3×10 ⁵ tons of copper metal contents (Fig. 1) in the Western Pacific island arc region (Fig. 2) was attempted and reported here. They are all young in age, 20 to 1.5 m.y. BP except 3 probable Eocene deposits. Possible application for prognosis and exploration was also considered.
The distribution of these porphyry coppers in the region is confined in the island arcs under the collisional tectonics in a broad sense, which includes;
(1) The mobile zone between two subduction zones, facing each other as a probable consequence of the polarity reversal, one subducts from the trench in the Pacific side and the other from the trench in the peripheral sea or back-arc side. Some amount of crustal shortning between two arcs is also observed (Examples: Philippines and Solomons, Figs. 2, 3, 4).
(2) Arc-arc collision (Example: Sabah, East Malaysia, Fig. 3).
(3) Continent-arc collision (Example: Papua New Guinea, Fig. 13).
Many other large island arcs like Kuril, Japan, Izu-Bonin, Mariana, Ryukyu, Sunda, Sumatra, et al. have no large concentration of copper of this type, so far inspite of the extensive exploration. Observing above-mentioned distributional pattern the author speculates that the compressional stress regime, common in collisional tectonic zone, might be the favorable tectonic environment. Consequently stress fields at the time and place of ore emplacement were investigated through studies of the direction of (1) elongation, alignment or protrusion of the mineralized intrusive stocks, (2) swarms of dykes including breccia or pebble dykes, (3) mineralized fissure systems, (4) elongated alteration zones, (5) faults, (6) certain geophysical anomalies and foldings of the host formations (Figs. 5, 6, 7, 9, 10, 11, 12, 14, 15).
As a result, 19 among 23 of them are found to be under compressional horizontal stress with its maximum direction being roughly orthogonal to the arc direction. Other 4 deposits are developed along the conspicuous transcurrent Philippine fault or its branch zones under the stress and shear caused by the coupling force of their eminent left lateral movement (Fig. 8). This coupling ultimately results in the compressional stress more or less parallel to the direction of the plate movement which is also roughly orthogonal or with an obtuse angle to the general arc direction.