Trace element geochemistry, oxygen isotope and U–Pb geochronology of multistage scheelite: Implications for W-mineralization and fluid evolution of Shizhuyuan W–Sn deposit, South China

Abstract

The Shizhuyuan oxidized skarn-greisen W–Sn deposit (South China) has widely developed scheelite in the prograde skarn stage, retrograde skarn stage, oxide stage, sulfide-quartz vein stage, greisen stage, and quartz-calcite-fluorite vein stage that are related to the reduced A-Type Qianlishan granite. In this contribution, we used trace elements, oxygen isotopic compositions, and U–Pb isotopes for tracing the W metallogenic processes in skarn and greisen, and the genesis of the Shizhuyuan W–Sn deposit. The total rare-earth element (∑REE) contents of scheelite grains from prograde and retrograde skarn stages to oxide stage show a decreasing fractionation trend (means from 1647 to 181 ppm) and the increasing trend in sulfide-quartz vein stage and greisen (mean = 428 ppm and 2074 ppm) indicate multi-stage fluid activities. Scheelites formed in skarn show HREE depletion REE patterns (LaN/YbN values means from 159 to 1.40) which results from paragenetic mineral precipitation (e.g., garnet and diopside), and negative or small positive Eu anomaly. In contrast, scheelite grains from the greisen stage have a flat REE pattern and a pronounced negative Eu anomaly identical to those of the Qianlishan granite that mirror the magmatic-hydrothermal fluid. The MoO3 contents in scheelite decrease from the prograde and retrograde skarn stages through the oxide stage to the sulfide-quartz vein stage (mean = 27.4 wt%, 1.97 wt%, 0.51 wt%, and 0.12 wt%). However a slight rise in MoO3 contents was observed in scheelite grains from the greisen to the latest-stage quartz-calcite-fluorite vein stage (mean = 0.28 wt% and 0.52 wt%, respectively). It is suggested to be the result of a change in fluid fO2 conditions and periodic transition from oxidizing to reducing metallogenic environment.The mean δ18OH2O values of the ore-forming fluid in the different mineralization stages as calculated by δ18Oscheelite are 7.98 ‰ (prograde skarn), 6.20 ‰ (retrograde skarn), 4.96 ‰ (proximal greisen), 2.83 ‰ (distal greisen), and 3.29 ‰ (quartz-calcite-fluorite vein). Scheelite grains formed in the retrograde skarn stage yielded a U–Pb isochron age of 164.4 ± 7.6 Ma, which is within the uncertainty of the emplacement age of the Qianlishan granite. Although scheelite grains formed in different stages have different trace elements and oxygen isotope compositions, they are all associated with the Qianlishan granite. We assume that the giant skarn-greisen W–Sn deposit has mainly resulted from multistage fluid activities derived from reduced intrusions and long-term fluid-rock interactions with thick marbleized limestone in an oxidizing setting.