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Abstract Apatite as an excellent tracer of the magmatic and metallogenic processes, can incorporate large amounts of trace elements sensitive to melting and/or ore-fluid evolution. This paper reports a combined study of in situ apatite geochemistry and whole-rock geochemistry for Early Cretaceous ore-bearing (Group I) and ore-barren porphyries (Group II) in the Liguo Fe-Cu-Au deposit for the deciphering of its petrogenesis and the control factors of mineralization. The apatite from both Group I and Group II is magmatic fluorapatite, characterized by Eu negative anomalies, an enrichment of LREEs, and a depletion of HREEs. Meanwhile, both groups have high Sr/Y and δEu, indicating porphyry adakitic characteristics in origin. Compared to isotopes of whole rocks, the variable 87Sr/86Sr (0.70250–0.71262) and δ18O (6.22–9.00) values of both groups of apatite imply contributions of mantle-, crust- and/or sediment-derived materials. Although Group I apatite and II apatite show some similar geochemical characteristics, Group I apatite precedes the crystallization of plagioclase with no Sr-(La/Yb) N /(La/Sm) N /(Sm/Yb) N correlations, whereas Group II apatite coincides with plagioclase crystallization, displaying positive correlations. The geochemistry of the elements (δEu, δCe, MnO and V) of these apatite groups, which are sensitive to a redox state, shows high oxygen fugacity (between HM and NNO), and Group I apatite has systematically higher oxygen fugacity than Group II apatite. More importantly, these different trace elements and oxygen fugacity characteristics between Group I apatite and Group II apatite can be used as an indicator of mineralization, and allow the skarn Fe-Cu-Au mineralization range to be drawn for the first time. Furthermore, the estimated F and Cl contents of the host parent magma (F = 1300–2446 ppm, Cl = 140–4780 ppm) are higher than those in a primitive mantle and average continental crust, suggesting an enrichment process of F and Cl. According to the above adakitic characteristics, high oxygen fugacity, and high F and Cl contents, the Pacific Plate subduction may be the main dynamic mechanism of the diagenesis and mineralization found in Liguo. [ABSTRACT FROM AUTHOR]

The upper Ediacaran-lower Cambrian (551–526 Ma) sedimentary succession of the Yangtze Block, South China, hosts abundant chert that formed contemporaneously with the Neoproterozoic Oxidation Event and the Cambrian Explosion. The effect of biological evolution, especially the rise of silica-secreting organisms, on marine Si cycling during the Ediacaran-Cambrian (E-C) transition remains controversial. This paper describes results of a petrological and geochemical investigation aimed at elucidating the origin of the chert and silica biogeochemical cycling during the E-C transition. Our results reveal that the studied chert is dominated by quartz and carbonate. The widespread presence of relict carbonate in inferred peritidal chert deposits suggests that carbonate replacement was an especially common silicification pattern of this facies, whereas the lack of carbonate precursors in subtidal strata argues in favor of primary and biogenic silica precipitation in deposits of this facies. Silicon isotope compositions, relative abundances of Al, Fe, and Mn and rare earth element (REE) + Y concentrations suggest that seawater rather than high-temperature hydrothermal fluid was the primary silica source of the studied cherts. Secular variation of chert Si isotopes reveals that marine silica cycling was closely linked with biological activity. Although silicification was abiological during the Precambrian, it may have been facilitated by microbial activities, especially the widespread formation of cyanobacterial mats. The flourishing of siliceous sponges and radiolarians in the early Cambrian ocean may have served as a global silica sink thus enhancing biological control of the silica cycle after Cambrian time. The studied chert deposits not only demonstrate the silica-rich nature of the global ocean during the E-C transition, but also record the transition of silica cycling from abiological to biological processes and the effect of the evolutionary radiation of silica-secreting organisms during the Cambrian Explosion on silica cycling. Unlabelled Image • Carbonate replacement is the principal silicification pattern of peritidal facies. • Primary and biogenic silica precipitation dominates silicification of subtidal facies. • Seawater was the primary silica source of widespread E-C chert deposits. • Post-Cambrian marine silica cycling is closely linked to biological activity. [ABSTRACT FROM AUTHOR]