Geological, geochronological and geochemical constraints on the Tianhu iron deposit, Chinese Tianshan Orogen, NW China: A modified Algoma-type BIF deposit

The Tianhu large Fe deposit (104 Mt @ 42%) contains bedded- and lenticular-shaped orebodies hosted by the metamorphosed Neoproterozoic Tianhu Group in the Chinese Tianshan Orogen, NW China. In this study, an integrated geological, age and geochemical investigation was conducted to decipher whether the deposit belongs to the long debated BIF- or skarn-type. A newly-measured cross-section shows that the orebodies are stratiform, in intrusive contact with migmatite, and are divided into the upper, middle and lower units by faults. The well-preserved primary banded ores are hosted in the lower unit, while sulfide veins crosscut the banded formation in the middle and upper units, indicating pre-existing Fe-rich sediments modified by later sulfide-bearing magmatic-metamorphic fluids. Petrographic observation has identified a biotite-sphene-epidote-K-feldspar-plagioclase-ilmenite metamorphic mineral assemblage, whilst phase equilibrium simulation indicates that the ore-hosting Tianhu Group (with volcanic protoliths) were amphibolite-facies metamorphosed at 7.8-8.8 kbar and 605-635 degrees C. The Tianhu migmatite is newly zircon U-Pb dated to be Middle Ordovician (469.6 +/- 2.3 Ma and 469.3 +/- 1.7 Ma). Geochemical features of the Tianhu Fe ores, including low Al2O3, TiO2 and HFSEs and the poor correlations among them, coupled with the presence of positive Y and Eu anomalies, point to a mixed oreforming material source of high-temperature hydrothermal vent fluids and seawater. The negative Eu anomalies and REY patterns, atypical to BIFs, may have been derived from the sedimentary and hydrothermal apatite. The pyrite delta S-34 (-9.23 to +11.07 parts per thousand), Pb-206/Pb-204 (19.513-21.156), Pb-207/Pb-204 (15.691-15.764) and Pb-208/Pb-204 (38.655-39.136) indicate that the pyrite may have been of a mixed primary sedimentary and later hydrothermal origin. Based on the new geological, age and geochemical data, we conclude that the Tianhu Fe deposit is a Neoproterozoic Algoma-type BIF deposit modified by Ordovician magmatic-metamorphic events. (C) 2017 Elsevier B.V. All rights reserved.