Geology and genesis of variscan porphyry-style gold mineralization, Petrackova hora deposit, Bohemian Massif, Czech Republic

A large number of Variscan mesothermal gold deposits are located in the cen tral part of the Bohemian Massif, close to the Central Bohemian Plutonic Co mplex. The Petrakova hora deposit has many features that distinguish it fro m other deposits in the region and suggest its mineralization is closely re lated to the late magmatic processes associated with the Petrackova hora gr anodiorite. The gold ores occur as sheeted arrays of quartz veins and veinl ets hosted by the small Petrakova hora granodiorite stock. Gold is found ma inly as free grains of >900 fineness, and is accompanied by abundant pyrrho tite and chalcopyrite, and accessory pyrite, arsenopyrite, loellingite, and molybdenite. Molybdenite from the Petrackova hora deposit has been dated b y the Re-Os method at 344.4+/-2.8 Ma. Hydrothermal alteration in the Petrac kova hora deposit exhibits a distinct temporal paragenesis. Selectively per vasive, early K-alteration and silicification are the oldest hydrothermal p hases. These were followed by early quartz veins (Q(1) to Q(4)) that contai n most of the gold mineralization. Late quartz veins (Q(5)) and fracture-co ntrolled silicification are gold-poor or barren. Barren calcite veins are t he youngest hydrothermal product. Extensive low-temperature, meteoric-water dominated alteration, as is typical of classic porphyry deposits, is absen t. However, the lower delta O-18 whole rock values for Petrackova hora gran odiorite and aplite (+2.4 to +5.1 parts per thousand SMOW) compared to othe r intrusions in the region reflect either interaction with isotopically lig ht external fluids or magma assimilation of small volumes of hydrothermical ly altered country rock. The delta O-18 isotopic compositions for quartz, s cheelite and hornblende (7.7 to 13.4 parts per thousand SMOW) and the delta S-34 compositions for sulfide minerals (-1 to +3.5 parts per thousand CDT) from early, gold-rich quartz veins indicate formation at high temperatures (590 to 400 degreesC) from fluids with a magmatic isotopic signature (delt a O-18(FLUID) of 5.7 to 7.2 delta parts per thousand). Fluids related to la te quartz veins (Q5) suggest the presence of a significant component of non -magmatic water (delta O-18(FLUID): +2.5 to +4.0 parts per thousand). The d elta 34S values of post-Q(5) sulfide minerals (-4.5 to -3.5 parts per thous and) 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 obs erved between 5 to 16, 35 to 40 and 62 to 70 equiv. wt% NaCl. The salt comp onent of these fluids is best, and minimally, approximated by the NaCl-KCl- CaCl2 system. Low- and high-salinity aqueous-carbonic inclusions are access ory in many of the analyzed samples. Three large successive pulses of fluid s are recognized. Each pulse begins with a high-salinity (>30 equiv. wt% Na Cl) magmatic fluid and evolves toward a lower salinity (similar to5 equiv. wt% NaCl) fluid. Data suggest that external (meteoric?) water(s) were signi ficant for only the third fluid pulse, which formed the late Q(5) quartz ve ins and the calcite veins. Polyphase fluid inclusions hosted by igneous qua rtz of the Petrackova hora granodiorite indicate minimum trapping condition s of about 3 kbar and 550 degreesC. The gold-rich Q(1) to Q(4) veins may have formed along a quasi-isobaric coo ling path at 2.5 to 1.5 kbar and 590 to 400 degreesC. This was followed by uplift, and formation of late Q(5) quartz veins (0.5 to 1.5 kbar; similar t o 300 degreesC) and ($) double under bar post-ore calcite veins (< 0.5 kbar ; 100 to 140 <degrees>C). The characteristics of the Petrackova hora deposi t suggest that it may represent a position intermediate between intrusion-r elated gold systems (e.g., Fort Knox deposit, Alaska) and gold-rich, copper -poor porphyry deposits (e.g., Maricunga Belt in Chile). As such, the Petra ckova hora deposit might be an example of the reduced gold sub-type of porp hyry deposit.