Origin of the volcanic-hosted Yamansu Fe deposit, Eastern Tianshan, NW China: constraints from pyrite Re-Os isotopes, stable isotopes, and in situ magnetite trace elements

The Yamansu Fe deposit (32 Mt at 51% Fe) in the Eastern Tianshan Orogenic Belt of NW China is hosted in early Carboniferous volcano-sedimentary rocks and spatially associated with skarn. The paragenetic sequence includes garnet-diopside (I), magnetite (II), hydrous silicate-sulfide (III), and calcite-quartz (IV) stages. Pyrite associated with magnetite has a Re-Os isochron age of 322 ± 7 Ma, which represents the timing of pyrite and, by inference, magnetite mineralization. Pyrite has δ34SVCDT values of − 2.2 to + 2.9‰, yielding δ34SH2S values of − 3.1 to 2‰, indicating the derivation of sulfur from a magmatic source. Calcite from stages II and IV has δ13CVPDB values from − 2.5 to − 1.2‰, and − 1.1 to 1.1‰, and δ18OVSMOW values from 11.8 to 12.0‰ and − 7.7 to − 5.2‰, respectively. Calculated δ13C values of fluid CO2 and water δ18O values indicate that stage II hydrothermal fluids were derived from magmatic rocks and that meteoric water mixed with the hydrothermal fluids in stage IV. Some ores contain magnetite with obvious chemical zoning composed of dark and light domains in BSE images. Dark domains have higher Mg, Al, Ca, Mn, and Ti but lower Fe and Cr contents than light domains. The chemical zoning resulted from a fluctuating fluid composition and/or physicochemical conditions (oscillatory zoning), or dissolution-precipitation (irregular zoning) via infiltration of magmatic-hydrothermal fluids diluted by late meteoric water. Iron was mainly derived from fluids similar to that in skarn deposits.