Ore-fluid geochemistry and metallogeny of the Dunde iron–zinc deposit in western Tianshan, Xinjiang, China: Evidence from fluid inclusions, REE and C–O–Sr isotopes of calcite

Abstract

The Dunde iron–zinc deposit is a typical submarine volcanogenic iron oxide deposit in the eastern Awulale Metallogenic Belt of the western Tianshan. We report new petrography, rare earth elements (REE), and fluid inclusion microthermometry and C–O–Sr isotopes of the calcite from the skarn and the magnetite ores from the Dunde deposit. Calcite from both the skarn and magnetite ores displays high LREE/HREE (44–172), implying significant REE fractionation. We infer that the complexation of Cl (±F) during the fluid circulation and mineralogical control of calcite at the depositional sites were important for the LREE enrichment in the Dunde calcite. The restricted Y/Ho (26.7–32.1) suggests that calcite in the skarn and magnetite ores was co-genetic and the superchondritic Y/Ho may have resulted from the preferential incorporation of Y over Ho in calcite. Positive Eu anomaly (1.23–1.63) of the skarn calcite may have been inherited from the parental fluids, while the negative or slightly positive anomaly (0.65–1.04) of the magnetite ore calcite may have been resulted from the high Sr2+ content that suppressed Eu2+ in calcite.Fluid inclusion microthermometric results exhibit a wide range of homogenization temperatures (106°C–364°C) and salinities (0.2wt.%–41.5wt.% NaCl equiv.). Three kinds of ore-forming fluids can be distinguished, i.e., high-salinity (>35wt.% NaCl equiv.), medium-salinity (10wt.%–35wt.% NaCl equiv.) and low-salinity (<10wt.% NaCl equiv.) fluids. The high-salinity fluid was likely to be magma-derived and occurred in both the skarn- and magnetite ore calcite. The low-salinity fluid, possibly seawater-derived, was found only in the skarn calcite. The medium-salinity fluid, mainly of CaCl2–H2O aqueous- and brine-featured fluids, may have been the product of the fluid–rock interactions as the magmatic fluids and seawater circulated through the strata. The Dunde calcite has δ13C comparable to marine carbonates, but δ18O substantially lower, suggesting possibly a result of limestone dissolution. Calculated δ18O of the fluids in equilibrium with calcite vary from −0.78‰ to 5.46‰ (−0.78‰–1.85‰ and 2.15‰–5.46‰ for skarn calcite and magnetite ore calcite, respectively), implying magmatic fluids and seawater mixing. The Dunde calcite 87Sr/86Sr (0.7071 to 0.7078) correlate positively with 1/Sr, implying that two sources, i.e., basaltic/andesitic volcanic wall rocks (87Sr/86Sr: 0.7056–0.7066) and limestones/seawater (87Sr/86Sr: 0.7075–0.7089), may have been involved in the mineralization. Mass-balance calculation between fresh basaltic tuff and its neighboring skarn showed that extra Ca, Mg and Fe were added to the basaltic tuff during the hydrothermal alteration. The limestone dissolution during the fluid–rock interactions may have added Ca to the ore-forming fluids, while Fe and Mg may have been introduced from the deep-seated magmatic system via the heated Cl-rich fluid circulation.