Scheelite deposition induced by long cooling time in the Longjiaoshan skarn tungsten deposit in South China: Evidence for petrography and fluid inclusions

Abstract

Tungsten (W), one of the most important critical strategic metals, is mainly formed in skarn deposits from magmatic hydrothermal systems. The precipitation mechanism of W in hydrothermal environments remain debated, primarily due to scarcity of direct in-situ fluid evidence from scheelites that could constrain W mineralization process. This study systematically reconstructs the fluid evolution history of the Longjiaoshan deposit, a representative large skarn W deposit in South China, based on integrated field investigations, petrographic observations, infrared microthermometry, cathodoluminescence (CL) imaging, Raman microspectroscopy and LA-ICP-MS analysis of intergrown scheelite and gangue mineral assemblages. The scheelite mineralization ore at Longjiaoshan deposit developed through two principal stages (i.e., retrograde skarn formation and subsequent sulfide stages). Scheelite (Sch1: euhedral, crystals with homogeneous gray CL, response and well-defined oscillatory zoning) precipitated during a discrete interval between garnet crystallization and subsequent quartz (Q1) formation. Then Sch2 (anhedral morphology with gray CL signature, displaying microcracks and fissures) co-precipitated with Q1 quartz). Sch3 (exhibiting dark CL luminosity and irregular oscillatory zoning) was overprinted by later Q2 quartz veins. Then the sulfide mineralization (chalcopyrite and pyrite) cross-cut pre-existing mineral phases. Microthermometric analysis of 71 fluid inclusion assemblages (FIAs), comprising 90 individual inclusions in scheelite and quartz from the Longjiaoshan deposit, yielded the following homogenization temperatures and salinities (468 to 575.1 ℃ and 10.6 to 17.4 wt% NaCl eq. for Sch1, 371.0–482.1 ℃ and 10.6 to 17.7 wt% NaCl eq. for Sch2, 397.4–458.1 and 10.6–13.7 wt% NaCl eq. for Sch3, 265.0–395.1 ℃ and 5.3–12.5 wt% NaCl eq. for Q1, and 209.3–300.8 ℃ and 8.3–12.4 wt% NaCl eq. for Q2, respectively). The Rb/Na versus K/Na ratio plot of fluid inclusion compositions demonstrates magmatic fluid signatures, consistent with exsolution from a homogeneous magma reservoir. The progressive temperature decline through successive mineralization stages therefore corresponds to downward gradual migration of the magma interface. The B and As diagram from fluid inclusion data reveal limited fluid phase segregation and vapor phase during boiling. In-situ microanalysis of primary / pseudosecondary fluid inclusions from the different crystallization stages indicates that inconspicuous fluid phase segregation and vapor phase during boiling. Integration of SEM-CL imaging, infrared microthermometry, Raman microspectroscopy, and fluid inclusion LA-ICP-MS analyses demonstrates that scheelite precipitation at the Longjiaoshan deposit was controlled by prolonged cooling.