In situ compositions and sulfur isotopes of the copper sulfides from the Jiama giant copper polymetallic deposit: implications for ore-forming material sources, gold enrichment mechanism and mineral exploration

• Bornite is the most efficient gold carrier in Jiama deposit, and the correlation of Au, Ag, Bi, Te and Se in bornite and chalcopyrite may indicate the spatial location of mineralization. • The metallogenic materials are mainly derived from the nascent lower crust with the addition of mantle-derived materials and possibly some organic S from the wall rocks. • Bi melts scavenged Au from hydrothermal fluids and fluctuation of oxygen fugacity are the key factors for gold enrichment in Jiama deposit. The Jiama giant copper polymetallic deposit is one of the most important deposits in the Gangdise metallogenic belt, which consists of hornfels-type Cu-Mo, skarn-type Cu-polymetallic, porphyry-type Mo-Cu, and distal vein-type Au orebodies with associated Ag, W, and Bi mineralizations. This deposit develops a large number of copper sulfides, such as bornite, juxingite and chalcopyrite. This study carried out LA-ICP-MS and EPMA analyses on the aforementioned copper sulfides to (1) assess the elements’ partitioning and distribution pattern in the mineral assemblage, (2) trace the ore-forming material source, (3) decipher the metallogenetic physicochemical environment, and (4) ascertain the gold enrichment mechanism. The study results show that both bornite and juxingite are enriched in Bi, in particular with the juxingite, whose Bi peak content reaches 80425 ppm. Besides, as the most important gold carrier in the Jiama deposit, bornite contains significantly more Au and Ag contents than those in other sulfides, and the content of Bi, Au and Ag in bornite shows a decreasing trend from the distal skarn to the proximal skarn. In contrast to bornite and juxingite, chalcopyrite possesses much lower Bi, Au and Ag contents. Elemental correlation analysis shows that the contents of Au and Ag in bornite and chalcopyrite are positively correlated with those of the elements, such as Bi, Te and Se, which could be indicative of the spatial location of mineralization. The δ 34 S values of the most sulfide samples in Jiama deposit range from −3.42 to −0.9, which are similar to those in the porphyry of the Gangdise metallogenic belt, indicating an ore-forming material source of the neogenic lower crust with addition of the mantle-derived materials. Locally, some bornite samples possess much lower δ 34 S values (−7.52 to −6.81), which implies that there maybe a mixing of the organic sulfur from the wall rocks of the Linbuzong Formation or sulfur isotope fractionation caused by cooling. The formation temperature of bornite-chalcopyrite assemblages ranges from 236℃ to 345℃, and the mineral assemblages reveal that the log f S 2 was greater than −12 during the main stage of skarn mineralization, while the f S 2 was less than −11 during the late mineralization of skarn. The gold enrichment in bornite and juxingite was closely relevant to existence of the Bi-Au melt. During the mineralization process, Bi melts scavenged Au from hydrothermal fluids, and the fluctuation of oxygen fugacity led to precipitation of the Bi-Au melts and enrichment of gold in bornite. With the temperature cooling, the juxingite exsolved from the bornite and captured the residual gold in the fluid.