Re-Os Dating of Pyrrhotite from the Chaoshan Gold Skarn, Eastern Yangtze Craton, Eastern China

摘要

The Chaoshan gold skarn, located in the central part of the Tongling Cu-Au-Mo district in eastern China, consists of 10 ore bodies occurring along an intrusive contact between the Early Cretaceous Baimangshan pyroxene diorite (BPD) and limestones of the Middle Triassic Nanlinghu Formation. Less commonly, ores occur in interlayered fractures within the calcareous wall rocks. Endoskarn alteration is well developed and manifested by the presence of massive skarn bodies consisting of garnet, scapolite, vesuvianite, epidote, actinolite, plagioclase, and lesser amounts of diopside. Toward the intrusion, the endoskarn alteration grades into potassic alteration composed of K-feldspar and biotite, suggesting that skarn formation, and thus gold mineralization, is directly related to the BPD. Metallic minerals are mainly pyrite, pyrrhotite, arsenopyrite, chalcopyrite, galena, and sphalerite, of which pyrrhotite, pyrite, and arsenopyrite are the dominant Au-bearing phases. Re-Os dating of auriferous pyrrhotite was performed to determine the age of mineralization of this deposit. Fourteen duplicate analyses from seven samples define a rough isochron on the ~(187)Re/ ~(188)Os versus ~(187)Os/~(188)Os diagram, yielding an age of 146 +- 47 Ma (2 sigma) and an initial ~(187)Os/~(188)Os of 0.97 +- 0.25 (MSWD = 17). A greatly improved Re-Os isochron was obtained by plotting seven analyses from four samples that have similar initial ~(187)Os/~(188)Os and thus are assumed to have been derived from the same source, yielding an isochron age of 141.7 +- 9.9 Ma (2 sigma) and an initial ~(187)Os/ ~(188)Os of 1.027 +- 0.058 (MSWD = 1.15). This age is consistent with the SHRIMP zircon U-Pb age (142.9 +- 1.1 Ma) of the BPD and molybdenite Re-Os ages (141.7 +- 2.5 to 136.9 +- 2.2 Ma) of Cu-Mo deposits throughout the Tongling district, and therefore is interpreted as the timing of gold mineralization. The extremely radiogenic initial ~(187)Os/~(188)Os implies a significant crustal component in the Os budget of the pyrrhotite samples, likely derived from an enriched mantle source and crustal contamination of the BPD magma during its ascent through the crust.