Geology and Hydrothermal Vein System of the Jáchymov (Joachimsthal) Ore District

Journal of the Czech Geological Society 48/34(2003) 3 Geology and hydrothermal vein system of the Jáchymov (Joachimsthal) ore district Geologie a hydrotermální ilný systém jáchymovského rudního okrsku (26 figs, 1 tab) PETR ONDRU1 FRANTIEK VESELOVSKÝ1 ANANDA GABAOVÁ1 JAN HLOUEK2 VLADIMÍR REIN3 1 Czech Geological Survey, Klárov 3, CZ-118 21 Prague 1 2 U Roháèových kasáren 24, CZ-100 00 Prague 10 3 Institute of Rock Structure and Mechanics, V Holeovièkách 41, CZ-182 09, Prague 8 This contribution provides a brief review of geology of the Kruné hory Mts. (Erzgebirge) and specifically of the Jáchymov (Joachimsthal) deposit. Main types of rocks in the Jáchymov ore district and the role of tectonic processes are described. The position of the ore district is defined much like its tectonic borders. Geological factors controlling the mineralization (especially the so-called five-element mineralization) are summarized. Hydrothermal veins are divided into Morning and Midnight veins according to the classical historical speak of miners, and all vein clusters are listed. Vein hydrothermal five-element mineralization is chronologically divided into separate mineralization stages and their age is estimated. Key words: Geology, Kruné hory Mts., Jáchymov deposit, rocks, tectonics, mineralization. Review of geology of the Kruné hory Mts. region of kinetic models, including important thrusting (nappe) tectonics [409], [411], [496], [552]. The Kruné hory Mts. region belongs to the Saxothurin- In the central part of the Kruné hory (Erzgebirge) gian Zone of the Variscan orogenic belt. In the Czech Mountains, the parautochthonous metasedimentary base- Republic, the zone is exposed in the Kruné hory Mts., ment has been overthrust by a crustal nappe of orthog- Smrèiny Mts., and in the basement of the Ohøe Rift (Eger neisses [496]. Eclogites tied to structural base of the thick Graben). The zone represents a fairly autonomous unit orthogneiss thrust unit show Variscan equilibration at with a specific history of sediment accumulation, tecton- 650700 °C and 25 kbar. Phengitegarnetkyanite mica ics, granitic plutonism and metamorphism. The Kruné schists from near the base of the orthogneiss thrust unit hory Crystalline Complex is composed of two main parts: at Mìdìnec yielded estimates of 640 °C and 22 kbar 1. the Kruné hory Crystalline Complex s.s., comprising (probably P ) [496]. min Precambrian metamorphic rocks, 2. metamorphosed Low- The crystalline complex shows a complicated Variscan er Palaeozoic (VogtlandSaxony Palaeozoic) volcanosed- structural record, with D structures preserved in large 1 imentary unit. The main features of palaeotectonic evo- eclogite boudins, D structures with EW-trending lin- 2 lution of the Kruné hory Complex belong to the ear elements and flat-lying foliations corresponding to Cadomian geotectonic cycle, including the deposition of westward transport of the orthogneiss thrust unit, with marine sediments, intensive deformation and metamor- late D NS-trending folds with flat-lying axes, and D 2 3 phism which transformed the volcanosedimentary com- structures corresponding to a NS compression resulting plex to phyllites, mica schists and gneisses. Large bod- in refolds such as major EW-trending Klínovec antiform ies of granite, emplaced in the Cambrian and dated to 524 and Mìdìnec antiform. Late brittle-ductile D kink-band 4 ± 10 Ma [579] and ca. 550 Ma [580], were transformed folds correspond to nearly vertical compression. into red orthogneisses during the Variscan orogeny. With regard to structural analysis, the traditional de- Relict contact metamorphism associated with the Cam- scription of the regional structure in terms of NESW- brian granite intrusion is documented in some subareas. trending Kruné hory Mts. (Erzgebirge) antiform and The Arzberg Group of supposed Neoproterozoic to Smrèiny (Fichtelgebirge) antiform (kvor [551]) may be Cambrian age is usually considered to belong to the Ca- more suited in morphological context or in connection with domian cycle. The Lower Palaeozoic (VogtlandSaxony the Upper Carboniferous Kruné hory granite pluton. Palaeozoic) volcanosedimentary unit was involved, to- An extensive contact metamorphism accompanying the gether with intra-crustal thrust slices of the Cadomian emplacement of the Upper Carboniferous Kruné hory basement, in the Variscan orogeny. The accompanying re- granite pluton is important as the granite pluton under- gional metamorphism features a wide range of conditions lies much of the Kruné hory Crystalline Complex at sub- from the greenschist to eclogite facies. surface level. The mobilistic concepts of nappe tectonics were pro- The Kruné hory Mts. region is characterized by prom- posed by Suess ([548], [549]), Kossmat ([417], [439]) inent faulting. A majority of faults strike NWSE and NE and Scheumann ([541], [542], [543]) and later rejected. SW, and also NS- and EW-striking faults occur in the Modern studies, using quantitative estimates of metamor- Jáchymov area (Fig. 1). Among transversal (NWSE) phic pressures and temperatures, result in the renaissance faults, the Jáchymov Fault is the most prominent one, with 4 Journal of the Czech Geological Society 48/34(2003) brian age. Two-mica and biotite mica schists, sometimes with graphite, garnet and pyrite, are the most widespread rocks. Minor intercalations of calc-silicate gneiss, amphibolite, quartzite and orthogneiss are also present. The youngest metasedimentary rocks are phyllites of inferred Cambrian and Ordovician age. Petrological studies indicate a complicated history of metamorphic evolution. Relics of kyanite in muscovite garnet mica schists indicate elevated pressures. Mica schists of the Pøíseènice Group contain chloritoid show- ing variable relations to garnet, including both chloritoid inclusions and overgrowths. The documented chloritoid lo- calities constitute a belt between Mìdìnec, Antonín skarn deposit near Jáchymov and Gross Hammerberg near Ober- wiesenthal. Some mica schists free of garnet carry chloritoid porphyroblasts up to 2 cm in size. Samples of schists Fig. 1. Sketch map displaying the geographic localization of the Já- from Jáchymov carry garnet with inclusions (including chymov ore district (black square). rutile), indicating syn-kinematic growth of garnet. Ther- mobarometry estimates on a muscovitegarnetchloritoid interpreted continuation in deep crustal levels (so-called schist from Srní near Jáchymov point to equilibration un- Boí Dar axial ramp of Máka and Zoubek [521], [537]). der amphibolite-facies conditions [496] (see Fig. 4). Numerous subparallel faults with spacings of several km show the same trend. The most important NESW-stri- Granite plutons king fault is the Kruné hory Fault. During the Late Ter- tiary and Early Quaternary, the Kruné hory Block was up- Postmetamorphic Variscan granitoids of the central part lifted by 700 to 1000 metres along this fault (kvor [551]). of the Kruné hory Mts. are represented mainly by the Karlovy Vary pluton. It is a compositionally inhomoge- Geology of the Jáchymov ore district neous body of a lenticular or tongue-like shape up to 25 km wide, with a probable vertical extent of 10 to Large-scale mining produced extensive and detailed in- 12 km. Several successive intrusions are usually classi- formation on geology of the district [365], [390]. The fied in terms of an older igneous complex (332323 Ma), main tectonic structure of the district is the EW-trend- including a minor dioritic members and a dominant gran- ing Klínovec antiform. Steep flanks of the antiform are ite. The granite prevailing by volume is a medium- complicated by folds of a lower order. Metamorphic rocks grained porphyritic biotite granite B with elevated thori- are intruded by Upper Carboniferous granites of the ex- um and uranium contents (Th > U). A younger igneous tensive Kruné hory pluton. Granites are exposed on the complex comprises dominantly leucocratic granite with surface in an elevated area southwest of the Bludná and uranium prevailing above thorium, and increased Nb, Sn Pernink fault zones and in greisenized small isolated mas- and Be contents. Apical parts of these granite units are sifs. The suite of dyke rocks associated with the granite often greisenized and carry Sn mineralization, accessory pluton including granite porphyry, porphyrites, aplite, topaz, fluorite and lithian micas. A biotite-rich zone up pegmatite and lamprophyres is also exposed on the sur- to 10 cm wide occurs at the contact of granite with meta- face. Tertiary magmatic rocks including alkali basalts, morphic country rocks (schists). Some components en- tuffs, volcanic breccias and minor trachyte belong to the riched in the leucocratic younger granite (F, B, Li) func- OligoceneMiocene neovolcanic phase of the Bohemian tioned as solidus depressants, and the late melt fractions Massif (see Figs 2, 3). were strongly enriched in fluids. Metasomatic alterations superimposed on solid granite portions were dominantly Metamorphic rocks of the acid leaching type resulting in greisenization, i.e., removal of primary feldspars, crystallization of second- Metamorphic rocks of the district represent an envelope ary albite, topaz, fluorite, tourmaline, Li-micas, wol- of the Upper Carboniferous granite pluton, which reach- framite, cassiterite, minor molybdenite, chalcopyrite and es to the surface only on the southern margin of the dis- arsenopyrite. A portion of the metals was probably de- trict, near Mariánská. Nevertheless, subsurface extension rived from a similar source as the

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