Origin of siderite mineralization in western Guizhou, SW China: Constrains from REEs, C, O, Sr and S isotopes

Numerous siderite deposits in western Guizhou of SW China are mainly hosted in the Middle Devonian dolostones. The larger Caiyuanzi (CYZ) and Xiongxiongjia (XXJ) deposits have an ore reserve in excess of 300 Mt with an average ore grade of similar to 35% total iron. There are banded and massive siderite ores and sulfide-siderite ores. The major ore mineral is magnesian siderite and gangue minerals are ankerite, dolomite, kaolinite, quartz and carbon. There are also metallic minerals including pyrite, chalcopyrite, and minor galena, tetrahedrite and enargite. Rare earth elements (REEs) and carbon, oxygen, and strontium isotope data indicate a marine origin of the host dolostones and a hydrothermal metasomatic origin of the siderite mineralization. The variations of REEs and carbon and oxygen isotopic compositions of siderite from the CYZ and XXJ deposits reflect the variable mineralization processes and different fluid/rock ratios. The LREE depletion of siderite from the CYZ deposit could be due to crystallographic control, whereas the LREE depletion along with MREE enrichment and positive Eu anomalies of siderite from the XXJ deposit was possibly controlled by both crystallography and fluid composition. Siderite and calcite have delta C-13(PDB) and delta O-18(SMOW) values ranging from -8 to -4.01 parts per thousand and from 13.42 to 20.07 parts per thousand, respectively. The variations in C and O isotopic compositions of siderite and calcite may reflect metasomatic replacement of marine carbonates by hydrothermal fluids at different fluid/rock ratios. Dolostones have (Sr-87/Sr-86)(initial) values of 0.70844-0.70920, whereas siderite and calcite have values ranging from 0.71396 to 0.71708. Negative correlations of initial Sr isotope ratios versus carbon and oxygen isotopes of the host dolostones, siderite and calcite indicated that carbonate minerals were metasomatic products of marine carbonates by hydrothermal fluids leaching from the Proterozoic basement rocks. Iron was possibly derived from the basement leaching rather than from the continental weathering. The delta S-34(CDT) values (-19.9 to 13.6 parts per thousand) of sulfides associated with siderite indicated that the sulfur was probably derived from the marine sulfates through either bacterial sulfate reduction or organic mediated thermochemical reduction. (C) 2014 Elsevier B.V. All rights reserved.