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The North China Craton (NCC), one of the oldest continental blocks in Asia, has a complicated evolutionary history with the age of the old crust up to 3.8 Ga and records the important geological events of the Earth. The Sijiaying BIF, the largest banded iron formation located in eastern Hebei province, in the east part of NCC, records an important thermo-tectonic event; Previous studies focused on the geological description, but the timing and tectonic setting have been unclear so far. The Sijiaying BIF is hosted in Neoarchean metamorphic rocks, which includes biotite-leptynite, hornblende plagioclase-gneiss and biotite plagioclase-gneiss. Using major element contents and ratios of the host rocks, the protoliths of the hornblende plagioclase-gneiss and biotite plagioclase-gneiss are shown to be dacite, whereas those of the biotite-leptynites are sedimentary rocks. Based on geology and geochemistry of the host rocks, we infer that the Sijiaying BIF is an Algoma type deposit. SIMS zircon U–Pb dating shows: igneous zircons from the biotite plagioclase-gneiss and hornblende plagioclase-gneiss have U–Pb ages of 2535 ± 8 Ma and 2543 ± 14 Ma, respectively; zircons from the biotite-leptynite have an age of 2537 ± 13 Ma. We infer that the ages of 2543–2535 Ma represent the time of the Sijiaying BIF. PAAS-normalized REY profiles of the ore samples are characterized by LREE depletion and HREE enrichment, positive La and Eu anomalies and an average Y/Ho weight ratio of 32, indicating a mixture of submarine hydrothermal fluids and deep seawater. REE data was combined with the δ18O analyzed by previous researcher of the ore from individual magnetite bands to infer that the Sijiaying BIF precipitated from hydrothermal fluids discharging on the sea floor. The hornblende plagioclase-gneiss and biotite plagioclase-gneiss that are enriched in LILE and LREE and depleted in high field strength elements (HFSE) Nb, Ta and Ti, have geochemical signatures that are similar to those of island-arc volcanic rocks. The biotite-leptynite shows the characteristics of continental island-arc. Based on the geology and geochemistry of associated wall-rocks and ore samples, we suggest that the Sijiaying BIF formed in a back-arc basin in Neoarchean.

Scheelite (CaWO4), a hydrothermal mineral commonly displaying enrichment in Mo (up to 16%) and/or rare earth elements (REE), is the main economic mineral in the Gaojiabang porphyry-skarn type W-Mo deposit, East China. Based on microscopic observations and in-situ LA-ICP-MS and LA-MC-ICP-MS analysis, three groups of scheelite, each with different geochemical characteristics, can be recognized. This evidence provides a good basis for considering the behavior of some trace elements in scheelite and how they may constrain ore genesis in porphyry-skarn systems. P-group scheelite occurs inside the porphyry rocks in the form of vein-hosted or disseminated scheelite. These have the lowest (87Sr/86Sr)i values (0.7089–0.7108), lowest Mo concentration (mean 213 ppm; n = 51), LREE/HREE ratios of 16.33–84.91, and highest ΣREE concentration (191–405 ppm) with downwards-sloping REE fractionation trends. S-group scheelite occurs in skarns, coexists with skarn minerals (garnet, diopside, epidote, etc.) and has the highest (87Sr/86Sr)i values (0.7103–0.7113), highest Mo concentration (mean 1,323 ppm; n = 29), highest LREE/HREE ratio (62.6–164), and lowest ΣREE concentrations (68.5–112 ppm), also with strongly downwards-sloping REE fractionation trends. H-group scheelite occurs within veins hosted by hornfels and displays moderate (87Sr/86Sr)i (0.70104–0.70122), Mo (mean 806 ppm; n = 17), LREE/HREE ratio (16.32–42.55), and ΣREE concentration (98.3–167.6 ppm). During hydrothermal precipitation of scheelite, changing redox state plays a major role in controlling Mo behavior. Both the precipitation of early skarn minerals and changing redox states of ore-forming fluids likely result in an increase in Mo, and corresponding decrease of HREE, in scheelite (particularly in S-group scheelite). The Sr-isotope study indicates that crustal materials provided the main source for W-Mo-bearing ore-forming fluids. Furthermore, both fluid mixing and fluid-rock interaction played an important role in the evolution of ore-forming fluids: Formation water or groundwater was likely involved in ore formation. A geochemical model is described combining data for the different types of scheelite, and the behavior of Mo, REEs and Sr-isotopes, to constrain the evolution of ore-forming fluids and constrain ore genesis in the porphyry-skarn system at Gaojiabang. This study contributes to the fingerprinting of ore deposits using scheelite geochemistry.