Carlin-Type Gold Deposits In The Dian-Qian-Gui "Golden Triangle" Of Southwest China

The Dian-Qian-Gui "Golden Triangle" area of southwest China has the second-largest concentration of Carlin-type gold deposits in the world, containing more than 800 tonnes of Au (25.7 Moz). All of the deposits are located along long-lived, deep-penetrating crustal structures inherited from Devonian rifting of the Precambrian Yangtze craton. They are hosted in Cambrian to Middle Triassic platform carbonate, transitional, and siliciclastic rocks of the Youjiang basin, and locally in Late Permian diabase intrusions or volcaniclastic rocks. These deposits have many characteristics in common with Carlin-type gold deposits in Nevada, USA, including lithology of host rocks, alteration types, elemental associations, and occurrence of gold.Our recent work has identified two episodes of gold mineralization in the Dian-Qian-Gui area that have distinct geologic settings, radiogenic and stable isotopes, and fluid inclusions. Gold deposits hosted in diabase intrusions along the southern margin of the Youjiang basin formed in the Middle-Late Triassic (232-212 Ma) and have low-salinity (similar to 2 wt % NaCl equiv), high-temperature (similar to 245 degrees C) fluid inclusions with high-density CO2 that are similar to those in orogenic gold deposits. Sediment-hosted gold deposits along the northern margin of the Youjiang basin formed in the Late Jurassic-Early Cretaceous (148-134 Ma) and have moderate salinity (similar to 5 wt % NaCl equiv) and temperature (similar to 210 degrees C) fluid inclusions, with variable CO2, low Fe, and high As, Sb, and Au contents, based on microanalysis of fluid inclusions. Deposits on each margin contain gold-bearing arsenian pyrite and arsenopyrite that precipitated from H2S-rich fluids by sulfidation of Fe minerals in the host rocks. Oxygen and hydrogen isotopes indicate metamorphic fluid sources for deposits on both margins, but sulfur isotopes indicate different sources of reduced sulfur. The narrow range of high delta S-34 values for arsenian pyrite and arsenopyrite from districts along the southern margin of the Youjiang basin suggests derivation from a sedimentary source. Some of the deposits along the northern margin of the Youjiang basin have delta S-34 values near zero that permit a magmatic or sedimentary sulfur source, while others have high values indicative of a sedimentary source.We propose a model in which metamorphic ore fluids were generated by regional metamorphism of sedimentary rocks during the Indosinian orogeny along the southern margin and the Yanshanian orogeny along the northern margin of the Youjiang basin. Metamorphic ore fluids were focused into reactivated basement-penetrating rift faults and flowed upward into structural highs in response to stress relaxation during each orogeny. Gold-bearing sulfides precipitated where the ore fluids reacted with carbonaceous and Fe-rich host rocks and mixed with variably exchanged meteoric ground water.The pressure-temperature conditions and compositions of ore fluids are intermediate between those of the mesozonal orogenic and the shallow Carlin-type gold systems. The Chinese Carlin-type gold deposits may, therefore, represent a link between orogenic and Carlin-type gold deposits that formed during transitions between compressional and extensional environments.