Geochemistry of multi-stage scheelite in the skarn: Constraints on ore-forming processes of the Machangqing Cu-Mo polymetallic deposit in Yunnan Province, southwest China

Abstract

The Machangqing Cu-Mo (-Au) deposit lies in the middle of the Jinshajiang-Ailaoshan alkaline porphyry metallogenic belt, and skarn is one of the main mineralization styles in the deposit. Scheelite from the Machangqing skarn occurs as discrete grains present in the early to late skarnization stages of the deposit, which can be divided into three types based on their occurrence: scheelite I in the prograde skarn, scheelite II (including the early scheelite II-a and the late scheelite II-b) in the retrograde alteration and scheelite III in the quartz-sulfide stages. In the CL images, scheelite grains of all three types have core- (mantle-) rim textures displaying CL-dark cores and CL-bright rims. The REE content could act as quenchers reducing CL intensity of the Machangqing scheelite. The Mo content of these scheelite types ranges from 0.5% to 13.6%, and Scheelite II-b is significantly enriched in MoO3 compared to other scheelite. Scheelite I and scheelite II-b exhibit right-dipped REE patterns, whereas scheelite II-a and scheelite III exhibit slightly middle rare MREE enriched patterns and flat REE patterns, respectively. The Eu anomaly differs between different types of scheelite, most of which exhibit variable negative Eu anomalies. Our study reveals that the substitution mechanism of REE for Ca in the Machangqing scheelite was probably 3Ca2+ =2REE3+ + □Ca and 2Ca2+ = REE3+ + Na+. The ore-forming fluids may be magma-derived. Mo contents of the Machangqing scheelite are generally controlled by the temperature and fO2 of the ore-forming fluids. The Mo variations in scheelite from different stages indicate a temperature decrease from the prograde to the early retrograde skarn stages, followed by fO2 increasing from the early retrograde to the late retrograde skarn stages, then decreasing to the quartz-sulfide stage. The Eu anomaly of scheelite was probably inherited from the ore-forming fluids and not dependant on the fO2 of hydrothermal fluids in the Machangqing deposit.