Mineralogical, petrological and fluid inclusion study of the Brehov ore deposit (Eastern Slovakia)

Levente Molnár¹, Ferenc Molnár¹, Pavel Bačo²

1Department of Mineralogy, Eötvös Loránd University, Budapest, Hungary

2Štátny geologický ústav d. Štúra, Werferova, Košice

We have analyzed drill core (VSB-2 and VSB-2a) and surface samples from the buried base metal ore deposit near Brehov. The samples were divided into four lithological groups: rhyodacite, hydrothermal breccias, pyroclastics, diorite. The rhyodacitic samples are heavily altered to illite-sericite, while kaolinite is the product of steam heated acid-sulfate alteration in pyroclasics. The ore forming minerals occur in stockwork and veins (sphalerite, galena, pyrite, hematite). The gangue minerals are quartz together with barite in pyroclastics and rhyodacite and with calcite in diorite. Results of fluid inclusion study which was carried out on quartz samples show that boiling occurred at different depths of the same paleohydrological system, and thus the 10 wt% NaCl boiling curve can be fitted to the location of samples from various depths. This data shows that we can count about 200 meters of erosion in relation to the paleogroundwater table. Compared to the Tokaj Mts., the age of the volcanic rocks and mineralization fits into the trend observed in the Tokaj Mts., where hydrothermal centres exposed on the surface are progressively younger towards south. Fluid inclusion data for the Brehov deposit show similar salinity as fluids in the transitional zone between the porphyry and epithermal mineralization in the Zlatá Baňa stratovolcano. According to these results, a base metal sulfide rich low sulphidation type epithermal mineralisation is present near Brehov and the hydrothermal system can be connected to the subvolcanic diorite intrusions of the area.

Oxide and sulfide ore formation sequence modeling with reference to ore deposits of the Udokan-Chiney ore district

Murashov K.Yu., Gongalskiy B.I.

The Institute of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry RAS, Moscow

gon@igem.ru

In spite of the long-standing history of the studies of large layered complexes containing the bulk of world Cr, Ti, V, PGE, and Ni reserves, the selectivity of metallic mineralization in ultramafic–mafic massifs still remains an open problem. Most of plutons containing unique magmatic ore deposits were formed during the Proterozoic within ancient cratons. A classic example is the Bushveld pluton. Numerous gabbroid intrusions with unique titanomagnetite ores rich in vanadium and containing platinum-group metals, lithologically and structurally similar to the Upper zone of that pluton, are clustered along the periphery of the Siberian craton in Russia. As a rule, magmatic ore deposits are associated with ore deposits of different formation types. A case in point is the Udokan–Chiney ore district with Paleoproterozoic magmatic, hydrothermal, and hydrothermal-sedimentary copper deposits. Hyperbasite–basite intrusions are represented by the Chiney, Maylav, Luktur, and other massifs. More than 45 Mt copper have been estimated in the deposits of this district including 25 Mt in the Udokan deposit; 12 Mt, in other deposits in sedimentary rocks (Pravoingamakitskoye, Unkur, Krasnoye, Burpala, and others) and 8 Mt, in the deposits of the Chiney massif (Rudnoye, Verkhnechineyskoye, Skvoznoye, and Kontaktovoye). Ore bodies of these deposits exhibit zonal distribution: magmatic disseminated ores in the central zones of massifs; veiny and disseminated, in endomorphic and exomorphic zones, and veiny, brecciated, and disseminated ores, at large distances from massif contact zones.

The best known in the 2.5 km thick Chiney anorthosite–gabbronorite massif with unique vanadium and ordinary-sized copper deposits. It was formed as a result of consecutive invasions of genetically related basite magmas that produced four rock groups with fine and coarse layering and irregular rhythmicity. The first group consists of large pyroxenite, anorthosite, and transitional gabbroid xenoblocks and xenoliths. The larger part of the massif consists of the second and third rock groups: titanomagnetite–gabbro and leucogabbro series and the intruding norite series (third group). The fourth group consists of hybrid rocks (fluid-magmatic breccias) that cross-cut the massif as steep dykes along with basal sills. The differently composed melts are probably the underlying magma chamber fractionation products. A specific feature of the Chiney gabbroids is the absence of olivine, which remained in the lower chamber. The latter fact is illustrated by computer simulation using COMAGMAT-3.5 software [1]. The sequence of high-Ti and low-Ti melts crystallization was studied in more detail with reference to samples corresponding to the melts which the gabbroids of titanomagnetite-gabbro and norite series crystallized from. Titanomagnetite concentration peaks were recorded at the beginning (pyroxene-titanomagnetites) or by the end (plagioclase-titanomagnetites) of melt crystallization (early and late magmatic ores).

Unlike oxide ores, copper and other metals accumulated at final crystallization stages in close association with minerals containing volatile components. Consecutive olivine, pyroxenes, and plagioclase crystallization demonstrated by simulation suggests of residual liquor enrichment in SiO₂, K₂O, P₂O₅, most rare and rare-earth elements, and copper. Abundances of the elements listed above, except SiO₂, in the 10% residual liquor grow up more than tenfold, and this has led to the liquation of such residue producing silicate and sulfide melts. The latter formed veiny and stockwork-type ore bodies in the contact zones of the massif (Rudnoye, Verkhnechineyskoye, Skvoznoye, and Kontaktovoye). Decrease in temperature and progressive invasion of such residue into clastic host rocks led to hydrothermal system formation and hydrothermal ore bodies emplacement at various distances from the massif (Pravoingamakitskoye, Unkur, Krasnoye, Burpala, and others), to the extent of forming ore deposits in water basins (Udokan). Multiple melt invasions extended hydrothermal system lifetime and changed fluid migration pathways.

Thus the results of crystallization modeling using the COMAGMAT-3.5 software suggest of matching between simulated fractionation sequence and real rock compositions as well as rapid volatile, rare, and ore elements accumulation in residual liquors and their subsequent passing into hydrothermal systems. Copper deposits in different (magmatic and sedimentary) host rocks may belong to a single ore-magmatic system [2,3]. Ores of these deposits make up a natural series: magmatic ores of the central parts and endomorphic zones of the layered massifs; exomorphic ores of the layered massifs; veiny and disseminated ores of the immediate surroundings of the layered massifs; and cupriferous sandstones.

References:

1. Gongalskiy, B.I., , Krivolutskaya, N.A., Ariskin, A.A., Nikolayev, G.S. Structure, composition and formation history of the Chiney anorthosite-gabbronorite massif in Northern Transbaikalia. Geokhimiya, no. 7, 2008, pp. 691–720.

2. Gongalskiy, B.I., Safonov, Yu.G., Krivolutskaya, N.A., Prokofyev, V.Yu., Yushin, A.A. A new type of copper–noble metal mineralization in Northern Transbaikalia. Doklady Ross. Akad. Nauk, 2007, vol. 414, no. 5, pp. 645–648.

3. Large and superlarge ore deposits. Volume 2. Strategic mineral deposits. Moscow: IGEM RAS, 2006 (in Russian).