PLATINUM GROUP ELEMENTS GEOCHEMISTRY AND Re-Os ISOTOPIC COMPOSITIONS OF DAPING GOLD DEPOSIT IN AILAOSHAN GOLD BELT, YUNNAN PROVINCE, CHINA AND THEIR METALLOGENIC IMPLICATIONS

Abstract

Concentration of platinum group elements(PGE) of various gold ores and around rocks and Re-Os isotopic compositions of major ore minerals of Daping gold deposit were analyzed by using an inductively coupled plasma mass spectrometry (ICP-MS) with nickel sulfide fire assay pre-concentration and carius tube decomposing technique with acetone extracted Re method. The results show that the total PGE (∑PGE) in the gold ores are greatly varied, varying between 3.91 × 10-9 to 61.64 × 10-9 and averaging 25.93 × 10-9, which are similar to that of the Himalayan lamprophyre dykes(22.19 × 10-9) and much higher than those of the Caledonian diorite around rocks ((2.79-4.75) × 10-9). Parameter Pt/Pt* of the gold ores are 0.01-0.9, with an average of 0.249, closing to that of the lamprophyre dykes (0.45), and much lower than those of the diorite (1.38-1.57). Chondrite normalized patterns of the gold ores show marked negative Pt and Ir and positive Ru and Rh anomalies, which are also similar to that of the lamprophyre dykes but obviously different from those of the diorite, suggesting that gold mineralization was closey related to the lamprophyre intrusion, and the oreforming materials didn't derive mainly from the diorite. Element ratios of the PGE show that the lamprophyre dykes was formed by partial melting of a depleted mantle which had undergone extraction of mafic magma and metasomatism, and the primary magma was sulfur saturated. Re/Os ratios of the ore minerals are quite high, varying between 6.36 to 121.67, and γOs(t) are 290.21-810.24, averaging 492.92. Besides 187Os/188OS(t) of the ore minerals are 0.495-1.154, which are much higher than the initial 187Os/188Os ratio of chondrite (0. 12679) at the gold metallogenic age (33.55Ma), implying that crust-derived radiogenic 187Os might have been added during gold mineralization. PGE geochemistry and Re-Os isotopic compositions indicate that genesis of the Daping gold deposit was closely related to strong crust-mantle interaction in the Ailaoshan area since the Cenozoic era, and the ore-forming fluids was mixture of the mantle-derived fluids formed by the upper mantle degassing and the CO2-riching fluids from the lower crust released by dehydration and roasting triggered by the rising hot mantle magma's heat. The shear zone provided channel for fluid migration and mixing, and the ore-forming fluids migrated up along micro-fractures in the shear zone, reacted with the diorite, and finally deposited the ore minerals in the brittle structures of the shear zone because of water-rock interaction and boiling. Thus, the Daping is a typical shear zone controlled mesothermal gold deposit.