Petrology and geochemistry of Variscan dykes from the Jchymov (Joachimsthal) ore district, Czech Republic

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Petrology and geochemistry of Variscan dykes from the Jchymov (Joachimsthal) ore district, Czech Republic Powered By Docstoc
					Petrology and geochemistry of Variscan dykes
Description: Mafic and silicic dyke rocks, which occur in the Ag-U ore district of Jchymov (Joachimsthal) in the Krusn hory (Erzgebirge) belong to kersantites, diorite porphyries, and granite porphyries, subordinately to spessartites. Dykes of minettes are located outside the district and penetrate the Nejdek-Eibenstock granitic massif. Mafic and silicic dykes exhibit diverse cross-cutting relationships and form in places composite dykes. Lamprophyres correspond to shoshonitic magmas with high abundances of compatible (Cr, Ni) and incompatible elements (Rb, Ba, P). All mafic dykes were affected by a postmagmatic alteration manifested by replacement of primary olivine and pyroxenes by amphibole, biotite and chlorite. Granite porphyries crystallized from leucocratic, weakly peraluminous, low-P magmas with moderate enrichments in Rb and depletions in Ca, Sr and Zr. By contrast, diorite porphyries are intermediate rocks containing 62-65 wt. % SiO^sub 2^ whose geological occurrence and major-element composition suggest origin by magma mixing involving lamprophyric and silicic melts, with simultaneous fractionation of biotite. The Th/U ratios in all dyke rocks correspond to chondritic and crustal values and demonstrate that whole-rock uranium contents were neither significantly increased nor affected by contemporaneous mineralization events. The stable isotope composition has been determined for whole rocks (O, C, S) and carbonate minerals (C, O). Wholerock δ^sup 18^O values range between 1.6 to 7.8 [per thousand] SMOW and do not show any systematic variations with the rock types. The isotopic composition of carbonates is represented by the following values: δ^sup 18^O = 19.6 to 25.0 [per thousand] SMOW and δ^sup 13^C = -5.7 to -9.0 [per thousand] PDB. The high positive values of δ^sup 18^O of carbonates are in a remarkable contrast to compositions of primary magmatic carbonates and indicate the carbonate formation during low-temperature hydrothermal event(s). Sulphides in kersantites
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