Formation of serpentinite-hosted talc in a continental crust setting: Petrographic, mineralogical, geochemical, and O, H and Cl isotope study of the Gilów deposit, Góry Sowie Massif (SW Poland)

Abstract

The Gilów deposit is located in the Góry Sowie Massif of the Central Sudetes (NE Bohemian Massif, SW Poland). Talcose rocks are associated with serpentinite bodies hosted by country migmatitic paragneiss, in an area where paragneiss also hosts amphibolitized eclogite bodies and granitic pegmatite veins. The talcose rocks and wall rocks were subjected to petrographic examination, electron microprobe study of minerals (including monazite Th-U-total Pb chemical dating), bulk rock chemical examination, as well as stable O, H and Cl isotope analysis. The talcose rocks can be used as a carving material and in ceramics, although the presence of tremolite and anthophyllite, and high Cr and Ni contents, preclude application in pharmacy and cosmetics. High Cr (2121.0–2148.4 ppm), Ni (1079–1561 ppm) and Co (57.4–84.8 ppm) contents, low ΣREE (2.13–13.81 ppm), as well as isotopic composition of O and H, classify the talcose rocks as a serpentinite-hosted type of deposit. Location in the strongly folded metamorphic unit, in the intimate vicinity of several faults and mylonitic zones, implies classification along the thrust-bound and fold-related metamorphogenic deposits. The talcose rocks are separated from serpentinites by the metasomatic chlorite schist, which contains two generations of monazite yielding two, different Th-U-total Pb dates. Older (389.8 ± 11.4 Ma) probably dates serpentinites juxtaposition with country paragneiss and chlorite formation. Younger (365.0 ± 18.2 Ma) likely dates amphiboles and talc formation, as overlaps the ages of amphibolite facies, country paragneiss migmatitization and intrusions of granitic pegmatites, and the talcose rocks contain talc-tremolite-anthophyllite assemblage indicative of the low-pressure mid-amphibolite facies. High MgO (24.27–27.21 wt%), Fe2O3 (5.06–5.12 wt%), Cr, Ni and Co contents are inherited from serpentinites. On the other hand, SiO2 (57.30–60.46 wt%), CaO (0.86–4.39 wt%), Al2O3 (1.38–3.04 wt%), TiO2 (0.02–0.04 wt%) and F (197–337 ppm) contents in the talcose rocks are higher than in host-serpentinites, likely due to a metasomatic introduction by fluids derived from migmatitic paragneisses or intruding pegmatites. Introduction of these elements is evidenced by a negative δ37Cl and elevated δ18O of the talcose rocks relative to host-serpentinites that are typical effects of interaction with a felsic crustal material. Interaction with paragneiss- and pegmatite-derived fluids is further evidenced by REE and trace elements patterns of the talcose rocks. The LREE/HREE enrichment, negative Sr and Ti anomalies, and positive Nb-La and Zr-Hf-Sm slopes, of the talcose rocks patterns, are similar as in paragneiss and pegmatite. Serpentinite bodies transformation into the talcose rocks was gradual and associated with increase of δ18O, and decrease of a Cl content, δD and δ37Cl. Pseudomorphic serpentinite, similar to abyssal serpentinites in terms of an isotopic composition (δ18O = +5.7‰, δD = −64‰, δ37Cl = +2.0‰), recrystallized into non-pseudomorphic serpentinite (δ18O = +7.1‰, δD = −48‰, δ37Cl = −0.1‰), which, in turn, was replaced by the talcose rock (δ18O = +8.3‰ to +8.7‰, δD = −52‰ to −48‰, δ37Cl = −2.6‰ to −0.1‰). Moreover, paragneiss- and pegmatite-derived fluids might have interacted with eclogite bodies during amphibolite facies-retrogression. Amphibolitized eclogite has Cl-rich amphibole rims, and negative δ37Cl (−0.5‰) consistent with a Cl-rich crustal fluid.