Heterogeneous lead isotopic compositions of sulfide minerals from a hydrothermal replacement deposit (Janggun mine, South Korea)

Kang, Jisuk; Wille, Martin; Hofmann, Beda A.; Strauss, H.; Villa, Igor M. (2020). Heterogeneous lead isotopic compositions of sulfide minerals from a hydrothermal replacement deposit (Janggun mine, South Korea). Ore geology reviews, 122, p. 103527. Elsevier 10.1016/j.oregeorev.2020.103527

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The Janggun lead-zinc-(silver) deposit in South Korea was formed by hydrothermal-metasomatic processes. We collected 12 samples over a 100 m transect in one of the five orebodies recognized in the deposit and analyzed a total of 68 sulfide minerals. Detailed mineralogical observations show successive modifications and overprint of pristine sulfide by replacement reactions, suggesting several distinct petrogenetic events. We distinguished four different phase assemblages, which occur in contiguous zones of the transect. This suggests multiple alteration fronts. Major and minor element concentrations of sulfide phases were measured for the different sulfide generations. Milligram-sized sulfide mineral samples were micro-drilled for Pb and S isotope analyses with a resolution of 200 µm to 1 mm. Pb and S isotopic compositions (206Pb/204Pb, 207Pb/204Pb, 208Pb/204Pb and δ34S) are variable, showing 206Pb/204Pb ranges from 18.334 to 18.879, 207Pb/204Pb; 15.667 to 15.692, and 208Pb/204Pb; 38.682 to 38.899, and δ34S ranges from + 4.4 to + 6.2‰. Although Pb isotopic compositions and δ34S data are similar to literature data from different areas of the Janggun deposit, the Pb isotopic variations between the different samples (at the 10 m scale), and within individual polished sections (at the cm scale) are well over 100 times larger than the analytical uncertainty. The scatter in sulfur isotopic compositions confirms the Pb isotopic heterogeneity at both the cm and 10 m scales. At least seven isotopically well distinct fluid circulation episodes are required in the study area to explain different projections of common-denominator Pb isotope diagrams. This suggests that fluid pulses of distinct chemical and isotopic compositions circulated in distinct spatial and temporal episodes. To investigate the role of country rocks as sources of the metals transported by the ore-forming fluids, we leached 13 samples of country rocks (Janggun limestone, Chunyang granite, granophyre, and dyke) with dilute acid and analyzed both leachates and residues. Correlations are observed between Pb isotopic compositions and trace ratios (e.g. Co/Ni, Cu/Ag, and Zn/Cd). The Pb isotopic compositions of country rocks and Precambrian basement suggest that leaching of country rocks by circulating fluids is able to produce the observed Pb isotope and trace elements variations in the sulfide minerals, whereby different end-members in the isotope correlation diagrams can be linked to different country rocks.

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