Post-collisional porphyry copper deposits in Tibet: An overview

Abstract

This paper presents a review of the geology and geochemistry of post-collisional PCDs in Tibet, including their spatial–temporal distribution, features of the ore-forming porphyries, magmatic origin and evolution, water–metal–S sources, alteration and mineralization features, fluid sources and evolution, conditions of Cu–Mo mineralization, and geodynamic models of their formation.

The post-collisional PCDs in Tibet contain total resources of ∼46 million tonnes (Mt) Cu at an average grade of 0.3–0.6 % Cu. They are mainly concentrated in the Gangdese, Yulong, and Ailaoshan–Red River belts, with ages of 30–13, 43–37, and 36–34 Ma, respectively. Their ore-forming porphyries have compositions that vary from granodiorite to monzogranite, syenogranite, and granite, and are high-K calc-alkaline to shoshonitic, with adakite-like signatures and highly variable Sr–Nd–Pb–Hf isotopic compositions. The ore-forming porphyries were mainly generated by partial melting of subduction-modified, thickened mafic lower crust with contributions from metasomatized lithospheric mantle. The causes of lower-crustal melting include asthenospheric upwelling associated with delamination of lithospheric mantle or slab tearing/break-off, and/or underplating of mafic magmas derived from metasomatized subcontinental lithospheric mantle. Ore-forming metals and S were mainly sourced by remelting of sulfide phases introduced into the lower crust during pre-collisional arc magmatism. Water necessary for mineralization was concentrated by dehydration reactions in the upper part of the subducting continental plate and/or degassing of water-rich ultrapotassic and/or alkaline mafic magmas derived from the mantle.

Similar to subduction-related PCDs, post-collisional PCDs in Tibet exhibit typical alteration zoning from inner potassic to outer propylitic zones, but with more intense overprinting of phyllic alteration on the former two alteration zones, likely due to higher rates of syn-mineralization uplift. Copper mineralization in post-collisional PCDs is mainly associated with phyllic alteration (particularly chlorite–sericite alteration) and, to a lesser extent, with potassic alteration, which is different from the typical association with potassic alteration in subduction-related PCDs. The initial ore-forming fluids in the post-collisional PCDs are single-phase, intermediate-density, and low-salinity fluids derived from evolved magma reservoirs. With ascent and decompression, the single-phase fluids separate into immiscible metal-rich hypersaline liquids responsible for potassic alteration and a low-salinity vapor. The evolved single-phase fluids are possibly diluted by meteoric waters, which leads to phyllic alteration. Cooling of magmatic–hydrothermal fluids may control metal precipitation in some post-collisional PCDs.

The development of post-collisional PCDs in Tibet indicates that other collision zones worldwide also have the potential to host economic PCDs. On a regional scale, high whole-rock Sr/Y, V/Sc, Eu/Eu*, and 10,000*(Eu/Eu*)/Y ratios; high zircon Eu/Eu* and 10,000*(Eu/Eu*)/Y ratios; high ΔFMQ values; high apatite SO₃ contents; and zircon Lu–Hf isotopic mapping can be used as fertility indicators during regional exploration for post-collisional PCDs. On a district scale, field geological and geochemical methods are effective in searching for outcropping or near-surface post-collisional porphyry mineralization.

There are many issues to be clarified by future research, such as the mechanisms of Cu enrichment and migration in the lower crust; sources of Cl; mechanisms of Cu precipitation; occurrence, resource potential, and enrichment processes of critical elements (Re, Se, and Te); and use of mineral chemistry to aid PCD exploration.