Origin of serpentinite-related nephrite from Jordanóv and adjacent areas (SW Poland) and its comparison with selected nephrite occurrences

The Gogolow-Jordanow Massif (GJM) in the Fore-Sudetic Block, SW Poland, hosts nephrites traditionally interpreted as serpentinite-related (ortho-nephrite). This contribution confirms the serpentinite-related origin of the nephrites on the basis of mineralogy, bulk-rock chemistry, and 0 and H isotopes. Rock-forming amphiboles from nephrites of the GJM have 7.73-7.99 Si apfu, comparable to 7.76-8.03 Si apfu of serpentinite-related Crooks Mountain nephrite amphibole (Granite Mountains, Wyoming, USA). The GJM amphiboles also have Mg/(Mg + Fe2+) values ranging from 0.82 to 0.94, similar to serpentinite-related Crooks Mountain and New Zealand nephrites amphiboles with Mg/(Mg + Fe2+) values of 0.86-0.90 and 0.91 to 0.92, respectively. The GJM nephrite amphiboles differ from the Val Malenco dolomite-related nephrite (Italy) amphibole, e.g., Val Malenco has a higher Si content (similar to 8.0 Si apfu), although it overlaps with some of the GJM nephrite samples, and similar to 1.0 Mg/(Mg + Fe2+), also higher than the GJM samples. Also, apatite in the nephrite studied from the GJM has a slightly higher Ca content than apatite from dolomite-related nephrite. Chlorites found in the Jordanow nephrite have similar compositions to that of chlorites in the serpentinite-related nephrites and also to chlorites associated with serpentinisation/rodingitisation. The bulk-rock FeO vs. Fe/(Fe + Mg), Cr, Ni, and Co are also typical of the serpentinite-related nephrites. The delta O-18 values range from +6.1 to +6.7/0. (+/- 0.1 parts per thousand), and the average delta D values = -61 parts per thousand, corresponding with the serpentinite-related nephrites range. Based on petrographic observations, we suggest four crystallisation stages (including rodingitisation prior to nephrite formation): 1-leucogranite rodingitisation and black-wall formation; 2-tremolite formation at the expense of rodingite diopside and black-wall chlorite (nephritisation) and garnet break-down, with spinel and chlorite formation (chlorite can be a product of garnet break-down or spinel with serpentine reaction); 3 - prehnite vein formation; 4 - tremolite formation at the expense of prehnite veins and actinolite veins formation. Spinels composed of 0.29-1.96 wt.% MgO, 24.87-29.67 wt.% FeO, 8.72-22.82 wt.% Fe2O3, 3.11-4.36 wt.% Al2O3, and 39.07-54.46 wt.% Cr2O3 suggest nephritisation in the greenschist to lower-amphibolite-facies conditions.