Comparative Fe and Sr isotope study of nephrite deposits hosted in dolomitic marbles and serpentinites from the Sudetes, SW Poland: Implications for Fe-As-Au-bearing skarn formation and post-obduction evolution of the oceanic lithosphere

Nephrites belonging to both the dolomite-related and serpentinite-related genetic types, from the Złoty Stok deposit, and Jordanów Śląski and Nasławice prospects in the Ślęża Ophiolite, respectively, were analyzed for Fe and Sr isotope compositions. These deposits in the Central Sudetes (SW Poland), are unusually situated relatively close to each other. This study is a first attempt to perform combined Fe and Sr isotope study of nephrites. Our results show that Fe in the Fe-As-Au-bearing skarn samples from Złoty Stok (2.77–7.00 wt% Fe, δ56Fe +0.07 to +0.14‰, and δ57Fe +0.12 to +0.21‰), to which the dolomite-related nephrite belongs, is not inherited from the host dolomitic marbles (ca. 0.4 wt% Fe; δ56Fe −0.08 to −0.07‰, and δ57Fe −0.12 to −0.09‰), but derived from the nearby, ca. 340 Ma granite intrusion (2.64–5.64 wt% Fe). In contrast, iron in the serpentinite-related nephrites (2.06–3.88 wt% Fe, δ56Fe +0.41 to +0.45‰, and δ57Fe +0.59 to +0.66‰) was likely inherited from host serpentinites (5.27–4.81 wt% Fe), rather than metasomatically derived from adjacent granite intrusions (0.13–0.45 wt% Fe). Moreover, both the marble transformation into a dolomite-related nephrite (and Fe-As-Au-bearing skarn), and the abyssal serpentinite – through ophiolite – towards serpentinite-related nephrite transformation, are accompanied by a progressive increase of δ56Fe and δ57Fe. This increase is regardless of changes in the bulk Fe content, increasing during the dolomite-related nephrite formation, and decreasing during serpentinite-related nephrite formation. This phenomena may be caused by one of the below listed factors, or combinations: a) preferential incorporation of heavy iron by tremolite and diopside; b) preferential incorporation of the heavy iron in the Fe3+-bearing phases, crystallizing during nephrite formation; c) preferential incorporation of the heavy Fe from migrating granite-derived fluids (positive fractionation between aqueous Fe-bearing fluid, and tremolite/diopside); d) positive isotope fractionation between dolomite/antigorite precursor, and newly formed amphibole/clinopyroxene; e) change of the Fe oxidation state. Furthermore, Fe in the rock-forming tremolite from the serpentinite-related nephrites is isotopically heavier than Fe in tremolite from the dolomite-related ones. In contrast to the iron, the isotopic composition of strontium, combined with its content and structural evidence, suggest multiple sources for this element. In the case of the dolomite-related nephrite (ca. 3–4 ppm Sr; initial 87Sr/86Sr = 0.7085), Sr inheritance from the host-dolomitic marble (ca. 90 ppm Sr; initial 87Sr/86Sr = 0.7081), as well as its derivation from intrusions of both the syn- to late-tectonic, ca. 340 Ma granites (ca. 400–440 ppm Sr), and post-tectonic ~305 Ma granites (86.4 ppm Sr), is suggested. In the case of the serpentinite-related nephrite (6.3–61.7 ppm Sr; initial 87Sr/86Sr 0.7037 to 0.7081), the situation is similar, i.e., Sr in the most primitive samples tends to be sourced from ophiolitic rocks (up to 2.4 ppm Sr in serpentinite), whereas in the more evolved types, it was likely delivered from the adjacent, ca. 340 Ma partially rodingitized granites (8.8–102.6 ppm Sr), as well as ~305 Ma granites (ca. 80 ppm Sr). Moreover, these granites caused the fluid-flow, responsible for Sr-bearing clinozoisite vein formation; these veins are likely related with nephrite formation, and with a metallic mineralization.