Magnetite and garnet trace element characteristics from the Chagangnuoer iron deposit in the western Tianshan Mountains, Xinjiang, NW China: Constrain for ore genesis

Located in the eastern Awulale metallogenic belt of western Tianshan Mountains, the large-scale Chagangnuoer iron deposit is hosted in the andesite and andesitic volcaniclastics of the Lower Carboniferous Dahalajunshan Formation, with one lentoid marble as footwall rock beneath the main ore bodies which exhibit as lamellar, stratoid and lenticular. The alteration zonation is similar with typical hydrothermal deposits. According to ore fabric and mineral paragenesis, this deposit can be divided into two ore-forming stages, which are magmatic stage and hydrothermal stage (included prograde sub-stage and quarts-sulfide sub-stage). In the magmatic stage, REE in magnetite is very low, rich in LREE and HREE but depleted in MREE with a "U" type pattern. In addition, this kind of magnetites has a higher Ti, V, Cr, indicating that Fe might come from the crystallization differentiation of andesitic magma. On the other hand, in the prograde sub-stage, magnetites have a lower REE content, a bit rich in LREE but other REE strongly depleted. Compared with the magnetites in magmatic stage, these magnetites are poor in Ti, V but a bit abundant in Ni, Co and Cu content. Garnets in barren and ore-bearing skarn distribute the same REE patterns, having a relatively high REE content, enriched in HRRE but depleted in LREE, and with a not pronounced positive Eu anomaly, which displays the feature of garnet with metasomatic origin in the calcic skarn. And this hints that the magnetites, which have a paragenesis relationship with ore-bearing garnets, should be also a product of hydrothermal fluid replacement with wall rocks, and most of the mineralizing materials (Fe) probably are derivate from andesitic strata. In combination geological characteristics with trace element geochemistry, we hold that the Chagangnuoer iron ore is probably one polygenetic deposit with the skarn type (predominated) superposition upon the magmatic type.