LA-ICP-MS trace element geochemistry of garnets: Constraints on hydrothermal fluid evolution and genesis of the Xinqiao Cu-S-Fe-Au deposit, eastern China

The Xinqiao Cu-S-Fe-Au deposit in the Tongling ore district, an important part of the Middle-Lower Yangtze River Valley metallogenic belt in eastern China, is located along the northern margin of the Yangtze Block. The deposit consists of two major mineralization types: stratiform and skarn (the genesis of the former is disputed). The more important stratiform orebody (90% of the total Cu, S and Fe reserves) has abundant garnet as the main gangue mineral, similar to the skarn orebody hosted in the intrusive contact. Garnets from the Xinqiao stratiform orebody range from pure andradite (And(100)) to And(50)Gro(46), and can be divided into the early (Grt1) and late (Grt2) generation. Both generations contain low MnO (0.19-0.89%), wide range of Y/Ho (39.0-40.7 for Grtl, 29.8-67.9 for Grt2), and do not contain melt or fluid-melt inclusions, indicative of a magmatic-hydrothermal replacement origin. Grtl and the Grt2 cores are grandite, whilst the Grt2 rims are mainly andradite with some grandite zones. Grandite is enriched in heavy rare earth elements (HREEs), and displays low Sigma LREE/Sigma HREE, La-N/Yb-N and U concentrations with negative Eu anomalies, whereas andradite is HREE-depleted, and displays high Sigma LREE/Sigma HREE, LaN/YbN ratios and U concentrations with positive Eu. anomalies. The distinct trace element features suggest that the hydrothermal fluids for Grt1 and the Grt2 cores were of nearly neutral (pH), relatively oxidized and HREE-enriched, whereas those for the Grt2 rims bad experienced episodic inflections between a mildly acidic, relatively reduced and HREE-depleted fluid and a nearly neutral, relatively oxidized and HREE-enriched fluid. Meanwhile, they also reveal that Grtl and the Grt2 cores may have formed by diffusive metasomatism in a closed system, whereas the Grt2 rims may have formed by infiltration metasomatism in an open fracture system, possibly related to the unconformity that hosts the Xinqiao stratiform ore body. We propose that the Xinqiao stratiform orebody may have formed from Early Cretaceous magmatic-hydrothermal fluids associated with the jitou stock, similar to the skarn-type orebodies in the district. (C) 2017 Elsevier B.V. All rights reserved.