Genesis of the gold deposit in the Indus-Yarlung Tsangpo suture zone, southern Tibet

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 (∼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 mélange in the footwall to the south. High-grade mineralization occurs in a fracture zone between diorite and ultramafic rock in the Renbu tectonic mélange. 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°C–347°C and have salinities of 0.35wt%–17.17wt% NaCl equivalent. The quartz veins yield δ¹⁸O_fluid values of 0.15‰–10.45‰, low δD_V-SMOW values (−173‰ to −96‰), and the δ¹³C values of −17.6‰ to −4.7‰, 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 δ³⁴S_V-CDT values of −2.9‰–1.9‰ (average −1.1‰), ²⁰⁶Pb/²⁰⁴Pb values of 18.47–18.64, ²⁰⁷Pb/²⁰⁴Pb values of 15.64–15.74, and ²⁰⁸Pb/²⁰⁴Pb 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.