Discrimination of Ore-Bearing and Barren Porphyries in the Yulong Porphyry Copper Ore Belt, Eastern Tibet

摘要

The Yulong porphyry copper ore belt is the largest belt of porphyry copper deposits in China. Detailed geological and geochemical comparison indicates that ore-bearing and barren porphyries in this belt were derived by progressive partial melting of veins of phlogopite-gamet clinopyroxenite in Iherzolitic lithosphere. Ore-bearing porphyry represents the earliest melt derived by partial melting of accessory phases such as apatite, carbonate, and phlogopite, and thus contains high contents of volatiles F, Cl, and H_2O. These volatiles promoted copper extraction and enrichment. Barren porphyry is the product of relatively high degrees of partial melting involving clinopyroxene and garnet, and thus is depleted in volatiles, hence barren of copper. This is the essential petrogenetic discrimination between ore-bearing and barren porphyries. Geochemical discrimination includes the following: Cu > 100 ppm, W > 5 ppm, F + Cl > 900 ppm, K_2O/Na_2O > 1.2, and Sm/Yb > 6.5 for bulk rock of the ore-bearing porphyries, and Mg/(Mg + Fe_T) > 0.57 and Fe~(3+)/Fe~(2+) > 0.7 for biotite in the ore-bearing porphyries. In contrast, such values are all less for barren porphyries. These geochemical parameters have important significance for copper exploration in the Yulong porphyry copper belt.