Sediment-hosted Cu–Co deposits in the Zhongtiao Mountains are hosted by the metasedimentary rocks of the Paleoproterozoic Zhongtiao Group in the southern part of the trans-North China orogen, North China Craton. The ore genesis is still disputed with proposed genetic models including metamorphosed sedimentary exhalative (M−SEDEX), metamorphosed sediment-hosted stratiform copper (M−SSC) and metamorphogenic mineralization type. Pyrite occurs as a ubiquitous mineral throughout all ore-forming stages and is ideal for clarifying this issue via the integration of in situ (LA–ICP–MS and EPMA) elemental and LA–MC-ICP–MS sulfur isotope composition analyses. Two main types of pyrite are identified based on petrographic and SEM observations: fine- to medium-grained, variably deformed and inclusion-rich Py1 (including Py1a and Py1b subtypes) in the disseminated–veinlet mineralization stage (S1), and coarse-grained, fractured Py2 (including Py2a and Py2b subtypes) in the fracture-controlled vein-type mineralization stage (S2, main ore stage). Deformed and inclusion-rich Py1a is overgrown by inclusion-poor Py1b, and nonporous Py2a is rimed by porous Py2b. Geochemical results show that the studied pyrite samples have considerable contents of Co (mean > 1000 ppm) and Ni (mean > 100 ppm). These features could be related to both the Co and Ni-rich sedimentary environment (e.g., a volcaniclastic-rich sulfidic marine) and metamorphic enrichment. The content of Se (mean > 200 ppm) in Py2 is significantly higher than that in Py1, indicating the extensive interaction of mineralizing fluids with Se-bearing graphite schist. LA–ICP–MS mapping and textural studies indicate the presence of coupled dissolution-reprecipitation (CDR) reactions that could lead to trace metal compositions (e.g., Co, Ni, Se, Pb, Bi) in the product phases (Py1b, Py2b) that differ from those of their parent phases (Py1a, Py2a). These variations were most likely controlled by fluid temperature and precipitation of specific minerals. The grain-scale chemical zoning of Py2a revealed by elemental mapping indicates fluctuating fluid parameters (e.g., temperature, fS2, and/or parent fluid compositions). The bimodal distributions in Py1 (10–14 ‰ and 18–24 ‰) are consistent with the inheritance of sulfur from sedimentary pyrite and the input of thermochemical sulfate reduction (TSR) -related sulfur from reworked evaporite sulfates in an increasingly open system. The relatively lighter and wider variation in δ34S values (3.2–22.4 ‰, mean = 15.1 ‰) of Py2 implies that the evolution of the ore fluid in S2 was far more complicated, probably as a result of fluid–rock interactions and fluid cooling. During this process, hydrothermal rims with lighter δ34S values (8.4 ‰) grew on older metamorphic pyrite cores with relatively heavier δ34S values (17.4–18.2 ‰). Chalcopyrite displays overlapping S isotopic compositions with pyrite from different mineralization stages, suggesting that copper sulfides inherited reduced sulfur from earlier formed pyrite. Cooling and fluid–rock interactions serve as the critical controls triggering Cu precipitation from the fluid. The combined textural and compositional data of pyrite and chalcopyrite are suggestive of a syn-orogenic copper deposit model, in which all the ore-grade copper was introduced during the Zhongtiao orogeny.
Abstract A new mineral species, yuchuanite-(Y), ideally Y2(CO3)3·H2O, has been discovered and characterized in the Yushui Cu deposit in South China. The mineral occurs in bedded/massive ore and is associated with bornite, chalcopyrite, galena, sphalerite, bastnäsite-(Y), xenotime-(Y), anhydrite, and quartz. Individual crystals range in size from 30 to 300 μm. No twinning is observed. The mineral is colorless and transparent with a vitreous luster. The calculated density is 3.62 g/cm3. An electron microprobe analysis yields the empirical formula (based on 10 O apfu), (Y1.61Yb0.11Er0.11Dy0.08Ho0.03 Gd0.02Tm0.02)Σ1.99(CO3)3·H2O. Yuchuanite-(Y) is triclinic, with space group P1 (#2), Z = 6, and unit-cell parameters a = 6.2134(3) Å, b = 8.9697(3) Å, c = 19.9045(7) Å, α = 91.062(3)°, β = 90.398(3)°, γ = 91.832(3)°, and V = 1108.54(8) Å3. The structure is constructed from (110) sheets of eight-coordinated Y polyhedra and C trigonal planar groups. All Y polyhedra are linked by shared edges. The Y atoms occupy six independent crystallographic sites of two different coordination types: [YO7(H2O)] and [YO8]. The chemical composition of yuchuanite-(Y) is similar to tengerite-(Y), Y2(CO3)3·2–3H2O, but is distinct in the crystal structure, such as crystal system, space group, and unit cell, from that of tengerite-(Y). The Y polyhedra of tengerite-(Y) are nine-coordinated, while those of yuchuanite-(Y) are eight-coordinated. Moreover, their structures could be both described as sheet structures built up from Y polyhedra and CO3 trigonal planar groups but link together in significantly different ways. Thus, yuchuanite-(Y) is not a polytype of tengerite-(Y) but is an independent mineral species.
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