Mineralogy and mineral chemistry of Bi-Te minerals: Constraints on mineralization process of the Dulanggou gold deposit, Dadu River Metallogenic Belt, China

There is a widely recognized association of bismuth (Bi)-tellurium (Te) minerals with gold in many hydrothermal gold deposits. However, the specific process for gold enrichment related to Bi-Te minerals remains enigmatic. The Dulanggou gold deposit, located on the southeastern margin of the Songpan-Garzê orogenic belt, China, provides an opportunity to research this process. Gold in the deposit is mainly associated with coarse-grained Bi-Te minerals and bismuth-rich polyphase metal droplets. Three mineralization stages are identified in the deposit. In Stage 1, a small amount of gold was deposited within dominate pyrrhotite. In Stage 2, a large amount of visible gold grains were precipitated with coarse-grained Bi-Te minerals. The various Bi-Te phases are mainly tsumoite, pilsenite, unnamed minerals (Bi3Te2, Bi2Te), joséite-B and trace hedleyite, which have a common characteristics of 2 ≥ Bi/Te > 1. In stage 3, around the Stage 2 large Bi-Te phase patches, bismuth-rich polyphase metal droplets pervasively occurred with Bi/Te ≥ 2. Other Bi-Te minerals in Stage 3 include hedleyite, unnamed minerals (Bi3Te, Bi8Te3, Bi5Te), joséite-B, and joséite-A, all intergrowth with gold and maldonite in the metal droplets. Mineral chemistry and phase diagrams of the Bi-Te mineral assemblages indicate that the mineralization temperatures at Stages 2 and 3 are > 325 ℃ and around 235 ℃ respectively, and the fTe2 and fS2 gradually decrease from Stage 2 to Stage 3. Importantly, a large amount of native gold was deposited with native Bi during Stage 3 and Bi contents in Bi-Te minerals increase with gold fineness from 799 ∼ 876 at Stage 2 to 833 ∼ 939 at Stage 3. These indicate that the metal droplets could be Bi-rich melts which were separated from Bi-Te phase patches and scavenged gold from earlier mineral assemblages to enhance gold enrichment.