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In the southern part of the Hruby Jesenik Mts. (Silesicum, Bohemian Massif) granitic orthogneisses and quartz-feldspars mylonites occur that were variously deformed, metamorphosed and imbricated with the overlaying Devonian volcano-sedimentary complex during the Variscan orogeny. Based on combined mapping, petrologic and geochemical studies, three main rock groups are distinguished. The fairly primitive nature of the tonalite suite is shown by low 87Sr/86Sr550 ratios (0.7034-0.7038) and positive [element of]Nd 550 values (+3.8 and +3.1). Isotopic composition of the metagranite suite is similar ([element of] Nd 550 = +1.9 to +2.9). Both suites probably belong to a single Cadomian calc-alkaline tonalite-granite association, which can be correlated with acid-intermediate plutonic rocks of the eastern Brunovistulicum (Slavkov Terrane). All these units are interpreted as representing dismembered fragments of the same continental margin magmatic arc system. The leucogranite suite is most likely early Variscan in age. The initial Nd isotopic composition ([element of] Nd 330 = +0.8 and +2.3) is consistent with its formation by a Variscan remelting of the metaigneous Cadomian crust represented by the rocks of the tonalite-granite association. The within-plate granite affinity of the leucogranite suite is most likely related to the break-up of the Brunovistulicum during the Variscan orogeny. [PUBLICATION ABSTRACT]

A large number of Variscan mesothermal gold deposits are located in the central part of the Bohemian Massif, close to the Central Bohemian Plutonic Complex. The Petráckova hora deposit has many features that distinguish it from other deposits in the region and suggest its mineralization is closely related to the late magmatic processes associated with the Petráckova hora granodiorite. The gold ores occur as sheeted arrays of quartz veins and veinlets hosted by the small Petráckova hora granodiorite stock. Gold is found mainly as free grains of >900 fineness, and is accompanied by abundant pyrrhotite and chalcopyrite, and accessory pyrite, arsenopyrite, loellingite, and molybdenite. Molybdenite from the Petráckova hora deposit has been dated by the Re-Os method at 344.4±2.8 Ma. Hydrothermal alteration in the Petráckova hora deposit exhibits a distinct temporal paragenesis. Selectively pervasive, early K-alteration and silicification are the oldest hydrothermal phases. These were followed by early quartz veins (Q^sub 1^ to Q^sub 4^) that contain most of the gold mineralization. Late quartz veins (Q^sub 5^) and fracture-controlled silicification are gold-poor or barren. Barren calcite veins are the youngest hydrothermal product. Extensive low-temperature, meteoric-water dominated alteration, as is typical of classic porphyry deposits, is absent. However, the lower [delta]^sup 18^O whole rock values for Petráckova hora granodiorite and aplite (+2.4 to +5.1[per thousand] SMOW) compared to other intrusions in the region reflect either interaction with isotopically light external fluids or magma assimilation of small volumes of hydrothermally altered country rock. The [delta]^sup 18^O isotopic compositions for quartz, scheelite and hornblende (7.7 to 13.4[per thousand] SMOW) and the [delta]^sup 34^S compositions for sulfide minerals (-1 to +3.5[per thousand] CDT) from early, gold-rich quartz veins indicate formation at high temperatures (590 to 400 °C) from fluids with a magmatic isotopic signature ([delta]^sup 18^O^sub FLUID^ of 5.7 to 7.2[per thousand]). Fluids related to late quartz veins (Q^sub 5^) suggest the presence of a significant component of non-magmatic water ([delta]^sup 18^O^sub FLUID^: +2.5 to +4.0[per thousand]). The [delta]^sup 34^S values of post-Q^sub 5^ sulfide minerals (-4.5 to -3.5[per thousand]) reflect at least partial derivation of late-stage sulfur from a source external to the intrusions. Aqueous, aqueous-carbonic and nitrogen-bearing fluid inclusions were identified in hydrothermal and igneous quartz, with the aqueous inclusions being the most common. In hydrothermal vein quartz, the salinity of primary aqueous inclusions falls into ranges 6 to 23 and 33 to 41 equiv. wt% NaCl; in igneous quartz, populations in salinity were observed between 5 to 16, 35 to 40 and 62 to 70 equiv. wt% NaCl. The salt component of these fluids is best, and minimally, approximated by the NaCl-KCl-CaCl^sub 2^ system. Low- and high-salinity aqueous-carbonic inclusions are accessory in many of the analyzed samples. Three large successive pulses of fluids are recognized. Each pulse begins with a high-salinity (>30 equiv. wt% NaCl) magmatic fluid and evolves toward a lower salinity (~5 equiv. wt% NaCl) fluid. Data suggest that external (meteoric?) water(s) were significant for only the third fluid pulse, which formed the late Q^sub 5^ quartz veins and the calcite veins. Polyphase fluid inclusions hosted by igneous quartz of the Petráckova hora granodiorite indicate minimum trapping conditions of about 3 kbar and 550 °C. The gold-rich Q^sub 1^ to Q^sub 4^ veins may have formed along a quasi-isobaric cooling path at 2.5 to 1.5 kbar and 590 to 400 °C. This was followed by uplift, and formation of late Q^sub 5^ quartz veins (0.5 to 1.5 kbar; ~300 °C) and post-ore calcite veins (<0.5 kbar; 100 to 140 °C). The characteristics of the Petráckova hora deposit suggest that it may represent a position intermediate between intrusion-related gold systems (e.g., Fort Knox deposit, Alaska) and gold-rich, copper-poor porphyry deposits (e.g., Maricunga Belt in Chile). As such, the Petráckova hora deposit might be an example of the reduced gold sub-type of porphyry deposit.[PUBLICATION ABSTRACT]