In-situ trace element and Fe-isotope studies on magnetite of the volcanic-hosted Zhibo and Chagangnuoer iron ore deposits in the Western Tianshan, NW China

Abstract

The Carboniferous Zhibo and Chagangnuoer iron deposits are situated within a caldera centre and along the flank of the same volcanic edifice, respectively, in the Awulale Iron Metallogenic Belt of the Western Tianshan orogen. Several stratiform 10 to 100m large, tabular to lenticular shaped magnetite-dominated ore-bodies occur in (trachy-) andesitic to rhyolitic host rocks. The magnetite mineralization mainly occurs as massive iron ores, partly with columnar-network or flow textures, and as disseminated magnetite ores. Trace element and isotope investigations of the different ore types reveal two major groups of magnetite: Group I, represented by the massive, partly brecciated ores from both deposits, is enriched in Ti, V, Ni, and HFSE such as Y, with concentrations similar to Iron Oxide-Copper-Gold (IOCG) ores. The δ56Fe values (up to 0.4‰) support an ortho-magmatic origin corresponding with an isotopic source calculation at ~800°C. Positive correlations between Fetotal and δ56Fe (from +0.4‰ to −0.1‰) and incompatible trace element contents (e.g. Si, Al, Nb, Ti and Y) in Group I magnetite are interpreted to be the consequence of a Raleigh-type fractionation process. Decreasing V, Ni and Mn values indicate changing fO2 conditions at the time of ore genesis. Group II, which is represented by the disseminated ores from Chagangnuoer, is - compared to Group I - relatively depleted in elements like Ti, V, Ni and Y and further spans a dominant δ56Fe range from about 0‰ to −0.5‰. These textural and chemical characteristics and the garnet-actinolite-diopside-epidote-carbonate-K-feldspar paragenesis are in accordance with hydrothermal Fe-skarn ores.The similar multi-element patterns of magnetite from all investigated samples, the overlapping δ56Fe ratios of the same massive ore-type from Zhibo and Chagangnuoer and the close proximity of both deposits indicate a common source of Fe-enrichment for the different iron ore types. In contrast to the ortho-magmatic Group I magnetite, reverse trace element trends with decreasing δ56Fe ratios (from 0‰ to −0.5‰) among the disseminated ores cannot simply be explained by a straightforward Raleigh fractionation or alteration processes. Therefore, a bimodal formation model is suggested for the Group II magnetite formation, including a partial remobilization of iron from the proximal, ortho-magmatic ore bodies and a subsequent distal re-precipitation. These processes were driven by late-stage hydrothermal fluids, which originated from deeper- seated granitic/granodioritic intrusions in the immediate vicinity.