Lamprophyre magmatism triggering the formation of the Zhuxi granites related to the world-largest scheelite skarn deposit in South China

Abstract

The Zhuxi deposit (3.44 Mt. WO3 @0.54%) is the largest tungsten deposit in the world. Although ore-related granitic magmas are correlated with the partial melting of fertile supracrustal material, the heat source triggering the partial melting of fertile supracrustal material has been poorly constrained. To unravel the petrogenesis of the granites, we study the widespread lamprophyre dike swarms coeval with the ore-related granites. Phenocrysts within the lamprophyres are dominated by amphibole, plus a small amount of eastonite. The amphibole phenocrysts exhibit complex internal structures, but the similar geochemical characteristics reveal that they are “antecrysts” recycled from earlier stages of the same magma system that underwent the repeated injection of mafic magmas. Pressure estimates using the Al-in-amphibole geobarometer reveal that most amphibole crystals crystallized at depths of ∼22 km and/or ∼ 10 km, suggesting that lamprophyric magmas most likely ponded at depths of ∼22 km (i.e., the Conrad discontinuity) to form magmatic reservoirs, which in turn fed higher-level magmatic reservoirs that formed at depths of ∼10 km. Both calculations based on energy conservation and simulation results obtained from the quantitative energy-constrained assimilation fractional crystallization (EC-AFC) model consistently reveal that intense partial melting of metagraywackes above the Conrad discontinuity could occur in the temperature interval between 800 °C and 820 °C due to the underplating of lamprophyric magmas. In combination with previous studies showing that the ore-related granitic magmas of the Zhuxi deposit formed at depths of ∼22 km (∼6 kbar) through the biotite dehydration melting of fertile metagraywackes, it can be inferred that the underplating of lamprophyric magmas could have induced the effective partial melting of the overlying fertile supracrustal material, thus forming granitic magmas with significant W endowment. Therefore, the initial appearance of lamprophyres in a district with W-enriched metasedimentary basement rocks should serve as an important prospecting criterion for large-scale tungsten deposits.