Ore genesis of the Baimashan Fe-S crypto-explosive breccia deposit in Fanchang volcanic basin, eastern China: insights into the ultra-shallow intrusive metallogenesis

Abstract

The Baimashan iron-sulfur polymetallic deposit exemplifies an ultra-shallow cryptoexplosive breccia-type deposit situated in the Fanchang region of the Middle-Lower Yangtze Metallogenic Belt (MLYMB) in eastern China, with its formation occurring at a depth of approximately 400 m. Analyzing its metallogenic characteristics and processes offers significant insights into the late Mesozoic ultra-shallow mineralization within the MLYMB. A thorough petrographic analysis reveals that magnetite (Mag, 0.5–1 mm) is associated with a specific metallogenic stage, while pyrite can be categorized into Py Ⅰ (early stage, 0.5–1 mm) and Py II (late stage, <0.1 mm). Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) techniques, including element point and mapping analyses, in addition to in-situ sulfur isotope analysis of pyrite, are employed to determine the genesis of the deposit and the evolution of the ore-forming hydrothermal fluid. The temporal signal diagram derived from LA-ICP-MS point analysis, element binary diagrams, and LA-ICP-MS mapping outcomes collectively demonstrate that the majority of trace elements present in both magnetite and pyrite exist as solid solution within the mineral structure. However, magnetite contains partial incorporation of Co, Ni, As, Sb, Ti, Sn, and W, while pyrite incorporates Ag, Bi, Tl, and Sb into its sulfide inclusions. Additionally, LA-ICP-MS analysis reveals that Py Ⅰ exhibits relatively higher concentrations of Co, Ni, and As, whereas Py II shows enrichment of Cu, Pb, Zn, Ag, Sb, and Tl. Notably, the Co/Ni ratios in both Py Ⅰ and Py II exhibit similar trends and temperature ranges, suggesting a shared variation in temperature during their formation. Moreover, the Co/Ni mapping image’s cross section provides a detailed depiction of the temperature fluctuations during the process of pyrite formation. The discriminant diagram provides evidence supporting the hydrothermal origin of magnetite, and all suggests that pyrite was formed in a hydrothermal environment with lower temperatures with two hydrothermal pulses. Analysis of in-situ sulfur isotopes reveals that the δ³⁴S values of Py Ⅰ range from +3.41 ‰ to +5.54 ‰, with an average of 4.63 ‰, while Py II has δ³⁴S values ranging from +2.47 ‰ to +4.50 ‰, with an average of 3.62 ‰. Through a comparative analysis and discussion of regional skarn-type and porphyrite-type iron ores, it is inferred that the ore-forming fluid exhibits characteristics indicative of a mixture between magmatic hydrothermal fluid and meteroic water, with minimal involvement of brine from gypsum layers. Building upon this research, this paper develops a metallogenic model for the Baimashan ultra-shallow intrusive deposit. By examining the mineralization processes of porphyrite-type iron deposits, it is suggested that too early occurrence of cryptoexplosion, resulting from ultra-shallow emplacement, is detrimental to the enrichment and mineralization of magnetite. Consequently, the appropriate emplacement depth is deemed crucial for the successful formation of such deposits.