Genesis of the Gold Deposit in the Indus-Yarlung Tsangpo Suture Zone, Southern Tibet: Evidence from Geological and Geochemical Data

The Nianzha gold deposit, located in the central section of the Indus-Yarlung Tsangpo suture (IYS) zone in southern Tibet, is a large gold deposit (Au reserves of 25 tons with average grade of 3.08 g/t) controlled by a E-W striking fault that developed during the main stage of Indo-Asian collision (approximate to 65-41 Ma). The main orebody is 1760 m long and 5.15 m thick, and occurs in a fracture zone bordered by Cretaceous diorite in the hanging wall to the north and the Renbu tectonic melange in the footwall to the south. High-grade mineralization occurs in a fracture zone between diorite and ultramafic rock in the Renbu tectonic melange. The wall-rock alteration is characterized by silicification in the fracture zone, serpentinization and the formation of talc and magnesite in the ultramafic unit, and chloritization and the formation of epidote and calcite in diorite. Quartz veins associated with Au mineralization can be divided into three stages. Fluid inclusion data indicate that the deposit formed from H2O-NaCl-organic gas fluids that homogenize at temperatures of 203 degrees C-347 degrees C and have salinities of 0.35wt%-17.17wt% NaCl equivalent. The quartz veins yield O-18(fluid) values of 0.15 parts per thousand-10.45 parts per thousand, low DV-SMOW values (-173 parts per thousand to -96 parts per thousand), and the C-13 values of -17.6 parts per thousand to -4.7 parts per thousand, indicating the ore-forming fluids were a mix of metamorphic and sedimentary orogenic fluids with the addition of some meteoric and mantle-derived fluids. The pyrite within the diorite has S-34(V-CDT) values of -2.9 parts per thousand-1.9 parts per thousand (average -1.1 parts per thousand), Pb-206/Pb-204 values of 18.47-18.64, Pb-207/Pb-204 values of 15.64-15.74, and Pb-208/Pb-204 values of 38.71-39.27, all of which are indicative of the derivation of S and other ore-forming elements from deep in the mantle. The presence of the Nianzha, Bangbu, and Mayum gold deposits within the IYS zone indicates that this area is highly prospective for large orogenic gold deposits. We identified three types of mineralization within the IYS, namely Bangbu-type accretionary, Mayum-type microcontinent, and Nianzha-type ophiolite-associated orogenic Au deposits. The three types formed at different depths in an accretionary orogenic tectonic setting. The Bangbu type was formed at the deepest level and the Nianzha type at the shallowest.