Mineralogy, fluid inclusion and isotope signatures: Implications for the genesis of the Early Paleozoic Yangjiashan scheelite-quartz vein deposit, South China

Abstract

• Magmatic fluid mixing with mantle-derived components and meteoric water were involved in the formation of scheelite-quartz vein deposit. • Fluid pH changes is responsible for scheelite precipitation. • No significant homogenization temperature gap between scheelite and quartz. Decoding the formation mechanism of scheelite-quartz vein deposits is critical for advancing the understanding of quartz vein tungsten mineralization systems. In this study, we take the Early Paleozoic Yangjiashan scheelite-quartz vein deposit as an example, applying electron probe, He–Ar–H–O isotopes, fluid inclusions, and laser Raman spectroscopy analyses, to investigate the nature and origin of mineralizing fluids and the conditions of scheelite precipitation. Fluid inclusions in quartz and scheelite from both greisen and quartz vein stages include vapor-rich and dominant liquid-rich types. The measured homogenization temperatures and salinities of the two types of fluid inclusions within scheelite from both stages range from 215° to 336 °C (mean 250 °C) and 2.4 to 13.2 wt% NaCl equiv (mean 9.3 wt% NaCl equiv), respectively, for Stage 1, and from 195° to 290 °C (mean 231 °C) and 1.2 to 12.8 wt% NaCl equiv (mean 6.6 wt% NaCl equiv), respectively, for Stage 2. All these values are comparable to those of quartz from both stages, indicating an insignificant temperature gap between the stages. Raman spectroscopy confirms the presence of H 2 O with detectable CH 4 , CO 2 and N 2 within the fluid inclusions. The mineral assemblage of native bismuth, pyrrhotite, and marcasite and the presence of CH 4 in the fluid inclusions suggest reduced ore-forming fluids, and the calculated log f O 2 and log f S 2 values vary from approximately -45 to -40 and -15.0 to -7.0, respectively. The δD and δ 18 O H2O values in equilibrium with scheelite range from -84 to -56‰ and 3.2 to 5.1‰, respectively, for the two stages. The 3 He/ 4 He ratios in arsenopyrite for both stages are between 0.13 and 0.69 Ra (Ra, 3 He/ 4 He ratio of air = 1.39 × 10 -6 ); 40 Ar/ 36 Ar and the percentages of 40 Ar* (non-atmospheric) range from 556 to 1,327 and from 47 to 78%, respectively. All these isotope signatures support the involvement of magmatic fluid mixing with mantle-derived components and meteoric water in the formation of the Yangjiashan deposit. Fluid pH change associated with boiling and mixing processes is responsible for scheelite precipitation.