Geochronology and geochemistry of intrusion and multi-generation garnet at the Jia'erbasidao Fe skarn deposit: Implications for two-stage Fe mineralization in the Chinese Altay Orogen (NW China)

Over a hundred iron ore occurrences have been discovered in the Chinese Altay Orogenic Belt (AOB), which is a key Fe-ore belt in NW China. In the Jia'erbasidao ore district, Fe skarn mineralization was mainly developed along the contact zone between biotite granite and the marble of Altay Formation, which is different from most Fe mineralization (non-skarn contact zone) in the Chinese AOB. Two generations of garnet (Grt1and Grt2) have been identified to be related to the two stages of Fe-ore formation at Jia'erbasidao, and Grt1 and Grt2 can be further divided into Grt1a, 1b and Grt2a, 2b, respectively. Our U-Pb dating indicates that the biotite granite and garnet from prograde skarn stage 1 (Grt1a) were formed at 275.6 +/- 3.1 Ma and 270.6 +/- 4.4 Ma, respectively. The mineralized biotite granite is metaluminous to peraluminous, LREE-enriched and HREE-depleted. Meanwhile, two Grt2a samples from the prograde skarn stage 2 yielded lower intercept 206Pb/238U ages of 248.3 +/- 2.9 Ma and 246.0 +/- 6.0 Ma, respectively. Grt1 and Grt2 may have formed from different ore-forming fluids: Grt1a was precipitated from a LREE-depleted, near neutral and low fO2 fluid (8Eu = 1.92-2.84); Grt1b and magnetite (Mag1) were precipitated from the same fluid with higher fO2 (8Eu = mostly 1.22-2.63); Grt2a was precipitated from a fluid with relative LREE enrichment, relatively lower pH and higher fO2 (8Eu = 0.98-1.69); Grt2b and magnetite (Mag2) was formed from a fluid that was HREE-enriched, near neutral, and higher fO2 (8Eu = 0.76-1.09). Our study shows a two-stage Fe mineralization at Ja'erbasidao, including the Early Permian Fe skarn mineralization and the Early Triassic skarn mineralization.