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

The Gilow deposit is located in the Gory 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 TREE (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 lowpressure 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 delta Cl-37 and elevated delta O-18 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 ZrHf-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 818O, and decrease of a Cl content, delta D and delta Cl-37. Pseudomorphic serpentinite, similar to abyssal serpentinites in terms of an isotopic composition ( delta O-18 = +5.7 parts per thousand, delta D =-64 parts per thousand, delta Cl-37 = +2.0 parts per thousand ), recrystallized into non-pseudomorphic serpentinite ( delta O-18 = +7.1%o, 8D =-48%o, delta Cl-37 =-0.1%o), which, in turn, was replaced by the talcose rock ( delta O-18 = +8.3 parts per thousand to +8.7 parts per thousand, delta D =-52 parts per thousand to-48 parts per thousand, delta Cl-37 =-2.6 parts per thousand to-0.1 parts per thousand). 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 delta Cl-37 (-0.5%o) consistent with a Cl-rich crustal fluid.

Automated summary

The Gilow deposit in southwestern Poland is associated with talcose rocks and serpentinite bodies. The talcose rocks, formed from the transformation of serpentinites, have mineral compositions suitable for carving and ceramics. Other metamorphic rocks and intrusions are also present in the area near the deposit.