Skarn formation and trace elements in garnet and associated minerals from Zhibula copper deposit, Gangdese Belt, southern Tibet

Abstract

Trace element concentrations in garnet and associated minerals from the mid-Miocene Zhibula Cu skarn, Gangdese Belt, Tibet reflect a diversity of local environments, evolving fluid parameters and partitioning with coexisting minerals. Exoskarn occurs as massive but narrow intervals within a Lower Jurassic volcano-sedimentary sequence containing limestone, the main skarn protolith. Endoskarn is present at the contact with mid-Miocene granodiorite dikes. Prograde skarn associations are garnet-dominant but also include diopside-dominant pyroxene in variable amounts. Garnet compositions in exoskarn change from andradite (And)- to grossular (Gr)-dominant from the massive intervals to bands/lenses within marble/tuff, but not in endoskarn. In both cases however, associations at the protolith contact include anorthite and wollastonite, both indicative of skarnoid or distal (relative to fluid source) skarn formation. Exoskarns also contain vesuvianite. Retrograde clinozoisite, actinolite and chlorite replace pre-existing skarn minerals. Garnet displays brecciation and replacement by Al-richer garnet. Depending on partitioning among coexisting minerals, chondrite-normalised REY (REE+Y) fractionation trends for garnet depict endo- to exoskarn diversity, the dominance of And- vs. Gr-rich garnet (in turn related to proximal-to-distal relationship to fluid source), as well as prograde-to-retrograde evolution in the same sample. A strong variation in Eu-anomaly, from positive to negative, in And-dominant garnet can be correlated with variation in salinity of ore-forming fluids, concordant with published fluid inclusion data. Trends depicted by And- and Gr-dominant garnets are consistent with published data from skarns elsewhere, in which the dominant substitution mechanism for REY is YAG-type. Zhibula garnets are enriched in a range of trace elements less commonly reported, including W, Sn, and As, but also Mo (as high as 730ppm), an element seldom analysed for in silicates. Molybdenum, W, and Sn display excellent co-correlation and shared zonation patterns on LA–ICP–MS maps of garnet, indicating substitution in the crystal lattice. As well as assisting in interpreting skarn evolution in time and space, and providing constraints on ore genesis, the trace element data for garnet explain the range of colours observed. The discovery of garnets carrying significant concentrations of W, Sn and Mo is a valuable finding that deserves evaluation in post-collisional skarns elsewhere, and is potentially of critical significance in prospecting. Together with a conspicuous trace ore mineral signature, garnet compositions at Zhibula support a genetic connection and sharing of ore-forming fluids between the skarn and the Qulong porphyry Cu-Mo deposit, 2km to the north. Within the Gangdese belt, or in analogous settings elsewhere, the presence of deep-seated porphyry mineralization beneath exposed skarns could be tested for by studying garnet chemistry. As more data become available, such trace element signatures could be viable tools for distinguishing barren from mineralized skarn systems.