Stratabound Cu-Au sulfide deposits are widely distributed in the Middle -Lower Yangtze River Mineralization Belt, eastern China. These deposits are strictly distributed along the Carboniferous stratum, but occur in close proximity to the Cretaceous diorite rocks. The simultaneous presence of sedimentary and magmatic evidence leads the genesis of these stratabound massive sulfide deposits to be an open question. Colloform pyrite (Py-c) is widely distributed in stratabound sulfide deposits and its genesis remains controversial which contrarily limits the establishment of a metallogenic model for the stratabound sulfide deposits. In this study, the mineral composition, morphology, microstructure, and organic composition of Py-c were systematically studied using optical microscopy, powder X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), micro-area laser Raman spectroscopy, and gas chromatography-mass spectrometry (GC–MS). Results reveal three types of textures of Py-c: oolitic, concentric-banded, and micritic (no other characteristic texture). The concentric-banded texture shows alternating light and dark bands which are composed of fine and coarse Py-c grains, respectively. All the Py-c ores are similar in phase composition, where the main phase is pyrite, the minor phases are siderite, dolomite, and quartz, and the trace phases are clay minerals (e.g., illite) and organic matter. The size of pyrite crystals varies from 50 nm to 1.4 μm. Most of the pyrite crystals are euhedral and subhedral, where the euhedral crystals show cubic and spheroidic shapes. The surfaces of cubic crystals are smooth and stripless. Detrital and authigenic quartz are found in Py-c ores. Detrital quartz grains are highly round. Py-c mold impressions are widespread on the grain surface. Py-c around the detrital quartz did not recrystallize. Authigenic quartz grains are euhedral and subhedral, cover a size range of hundreds of nanometers to several micrometers, and aggregate to form crystal clusters. In addition, small amounts of organic matters are found co-existing with Py-c. Results of n-alkanes analysis show that organic matters in Py-c consist mainly of microorganisms and terrestrial higher plants, indicating a possibility of forming Py-c by microorganisms which used terrestrial organic matters as electron donors. These systematic analyses indicate that Py-c in Tongling stratabound sulfide deposits was formed through biogeochemical-mediated sedimentation processes in a semi-enclosed sea basin. In Yanshanian period (Jurassic-Cretaceous), Py-c was transformed to macrocrystalline pyrite (Py-m) through recrystallization or to pyrrhotite and chalcopyrite through thermal metamorphism and mineral-fluid reaction. Py-c ores, which have high porosity, high chemical activity, and organic matter, could have played key roles in precipitating Cu and Au from Yanshanian magma-derived hydrothermal fluids and constrained the formation of stratabound sulfide orebodies in the Tongling mineralization cluster.
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