Origin and evolution of W mineralization in the Tongshanling Cu–polymetallic ore field, South China: Constraints from scheelite microstructure, geochemistry, and Nd–O isotope evidence

Abstract

Multistage, skarn-type W mineralization has recently been discovered in the Tongshanling Cu–polymetallic ore field, South China. This contribution reports new results from cathodoluminescence, trace element, and Nd–O isotopic analyses of scheelite samples from Tongshanling, designed to better constrain the W mineralization process and origin of associated metals and metallogenic fluids. Three generations of scheelite were identified that can be attributed to three paragenetic phases: (i) an early prograde skarn stage dominated by massive garnet and diopside intergrown with fine-grained scheelite, (ii) an intermediate retrograde skarn stage characterized by an assemblage of abundant scheelite, medium-grained chlorite and epidote, and (iii) a late sulfide-quartz vein stage typified by coarse-grained, subhedral scheelite grains that are typically intergrown with chalcopyrite, sphalerite, and galena. Our data show that although WO3 and CaO concentrations are similar in scheelite of all three stages and rare earth element (REE) patterns vary, the ∑REE + Y values of scheelite markedly and progressively decrease from the prograde skarn (mean = 901 ppm) to retrograde skarn (70.1 ppm) to the sulfide-quartz vein (34.1 ppm) stages. This signature is consistent with either of the following scenarios: The coprecipitation of REE-enriched minerals (e.g., garnet and scheelite), a change in fluid oxygen fugacity change and/or minor distortions in crystal texture. The δ18OH2O values of fluids linked to the prograde skarn and sulfide-quartz vein stages (5.02‰–6.19‰) are consistent with a magmatic fluid input. The δ18OH2O values of fluids associated with the retrograde skarn stage (2.85‰–3.45‰), on the other hand, are indicative of fluid mixing, possibly reflecting dilution of magmatic fluids by meteoric waters. Interestingly, scheelite samples of the sulfide-quartz vein stage returned low εNd(t) values (−7.46 to − 7.07) and moderate two-stage Nd model ages (T2DM = 1526 –1557 Ma) that are identical to the whole rock Nd isotope composition of an adjacent Jurassic I-type granodiorite porphyry pluton. Overall, the results of our study indicate a close genetic relationship between this pluton and the W mineralization, a scenario that is at odds with the common association between I-type granodiorites and Cu–Pb–Zn mineralization in South China.