The Crater Mountain Deposit, Papua New Guinea: Porphyry-related Au–Te System

摘要

Ore mineralization and wall rock alteration of Crater Mountain gold deposit, Papua New Guinea, were investigated using ore and host rock samples from drill holes for ore and alteration mineralogical study. The host rocks of the deposit are quartz-feldspar porphyry, feldspar-hornblende porphyry, andesitic volcanics and pyroclastics, and basaltic-andesitic tuff. The main ore minerals are pyrite, sphalerite, galena, chalcopyrite and moderate amounts of tetrahedrite, tennantite, pyrrhotite, bornite and enargite. Small amounts of enargite, tetradymite, altaite, heyrovskyite, bismuthinite, bornite, idaite, cubanite, native gold, CuPbS_2, an unidentified Bi-Te-S mineral and argentopyrite occur as inclusions mainly in pyrite veins and grains. Native gold occurs significantly in the As-rich pyrite veins in volcanic units, and coexists with Bi-Te-S mineral species and rarely with chalcopyrite and cubanite relics. Four mineralization stages were recognized based on the observations of ore textures. Stage I is characterized by quartz-sericite-calcite alteration with trace pyrite and chalcopyrite in the monomict diatreme breccias; Stage II is defined by the crystallization of pyrite and by weak quartz-chlorite-sericite-calcite alteration; Stage III is a major ore formation episode where sulfides deposited as disseminated grains and veins that host native gold, and is divided into three sub-stages; Stage IV is characterized by predominant carbonitization. Gold mineralization occurred in the sub-stages 2 and 3 in Stage III. The fS_2 is considered to have decreased from ~10~(-2) to 10~(-14) atm with decreasing temperature of fluid.