The carbonate-dominated Mesozoic sequence of the Transdanubian Mountain Range contains Triassic, uranium-enriched phosphorite layers and Cretaceous, REE-enriched nodular phosphorite. Detailed investigation of these deposits may have an economic benefit because of their large U and REE contents. The dominant minerals in the Triassic phosphorite are carbonate-bearing fluorapatite (CFA) and calcite. According to the electron-probe microanalysis (EPMA) the U is mainly associated with the CFA crystals. Laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) measurement shows that CFA contains 137–612 ppm U and 113–261 ppm total REE + Y. The LA-ICP-MS U-Pb age of the uppermost phosphorite horizon is 237 ± 11 Ma, which conforms with the stratigraphic age of the host limestone.The Cretaceous nodular phosphorite occurs on the base of an Aptian crinoid-bearing limestone mostly in the form of encrustations around bio- and silicic-clasts, but the clasts also contain phosphorite. The main minerals in these crusts are CFA, calcite, quartz, glauconite and Fe-oxide-hydroxides. Based on EPMA the REE enrichment is related to CFA and LA-ICP-MS measurements show that it contains 748–2953 ppm total REE + Y.The redox-sensitive proxies and the shape of NASC normalized REE patterns indicate that both phosphorites formed in anoxic environments. There are significant differences between these deposits such as appearance, rock-forming minerals, and U and REE contents which indicate differences in their sedimentary environments. The present results suggest that the Triassic phosphorite was formed by inorganic precipitation in a reducing environment close to sea-mounts. The Cretaceous occurrence resulted from a concentric growth mechanism in cold, ascending seawater at the continental margin environment during the anoxic Selli Event (OAE 1a) and/or Paquier Episode (OAE 1b). The critical raw material contents were derived from other sources.
Comparative studies on hydrothermal alteration of submarine peperitic basalt occurrences related to the Triassic early rifting of the Neotethys were carried out in various parts of the Dinarides and Hellenides. The study areas included the displaced fragments of the Dinarides in the Darnó Unit, NE Hungary, the Kalnik Mts. in Croatia and the Vares-Šmreka area in Bosnia and Herzegovina. In the Hellenides, similar environments were studied in the Stragopetra Mts., Greece. Jurassic pillow basalts formed in a back-arc-basin of the Neotethys were also studied in the Szarvaskő Unit, NE Hungary, which also represents a displaced unit of Dinaridic origin. Within the submarine basaltic lava flows, six volcanic facies were distinguished. The hydrothermal alteration was characterised according to those facies. The first process was the albitisation of the rock-forming plagioclase at ~300°C in all localities. During the higher temperature stage of the subsequent cooling, chloritisation in the ground mass is typical for all types of basalts, however chlorite and rarely quartz formed in the fractures and amygdales of the Triassic basalts, while chlorite, quartz and prehnite precipitated in the fractures of the Jurassic rocks. At lower temperatures of this cooling-related process, calcite is a common mineral filling up the larger amygdales, jigsaw type fractures and other open spaces, but some epidote, pumpellyite, prehnite and laumontite also occur in the Triassic basalts. The late stage alteration (occurring at the lowest temperature) is characterised by argillitisation at every locality. The observed hydrothermal alteration patterns also show slight differences according to the volcanic facies as a function of the distal/proximal setting in relation to the eruptive centres and the presence/absence of water-saturated and unconsolidated carbonate or siliciclastic sediments at the time of the emplacement of lava flows. The study revealed that the most important factors influencing mineralogy and zoning of hydrothermal alteration in these short lived local hydrothermal systems are the rapid cooling of the hydrothermal fluid, the dominance of the poorly evolved seawater as the source of hydrothermal fluid and the local, i.e. effective water/rock ratio, determined by the degree of fracturing in the rock. The mineralogical-textural peculiarities of the highly localised hydrothermal fluid/rock interaction in the studied submarine seamount type volcanoes are clearly different from the products of the large-scale hydrothermal processes occurring at mid-oceanic ridges. Recognition of these differences is important in the evaluation of ore potential in the Neotethyan realm or other areas with occurrences of submarine basaltic units.
The Darnó Unit in NE Hungary contains basalt and associated sediments related to the advanced rifting stage of Triassic age in the Neotethys. A detailed field study of five key outcrops and two deep wells revealed that only distal facies of basaltic lava flows of submarine volcanoes occur as blocks in the Jurassic mélange; the central and the most distal facies are missing. The advanced rifting-related Triassic and oceanic stage-related Jurassic basalt flows of the same mélange can be distinguished on the basis of lithostratigraphic, structural and textural features. The paper contains for the first time the detailed description of the key outcrops of peperitic facies consisting of a mixture of basalt and red micritic limestone. The occurrence of this facies is the principle key feature for discrimination in the field between Triassic and Jurassic basalt. In addition, four types of Triassic basalt were recognized: the Báj-patak-type, the Mély Valley-type, the Nagy-Rézoldal-type and the Reszél Hill-type. The observed peculiarities of advanced rift-related basalt are also compared to characteristics of volcanics encountered in wells drilled in the Darnó Unit. This comparison solves many problems of earlier interpretations of the studied wellbore sections. Central European Geology, Vol. 53/2-3, pp. 181-204 (2010) DOI: 10.1556/CEuGeol.53.2010 Field characteristics and petrography of the advanced rifting-related controversial conclusions concerning the origin of these rocks. However, many of the authors recognized their specific characteristics as differing from the midoceanic ridge basalts, sensu stricto, and from their equivalent, Jurassic basalt, occurring nearby in the Szarvaskõ Unit of the Bükk Mts. A similar origin and possible relationship between these submarine basaltic rocks and occurrences in the Dinarides were recognized on the basis of stratigraphy, petrochemistry and tectonic evolution of the region (Downes et al.
The Mesozoic complex of Darnó Hill area in NE Hungary, according to well core documentation, is made up of two units. The upper unit, the Darnó Unit s.s., consists predominantly of blocks of ophiolitic rocks (pillow and massive basalt, gabbro) and subordinate abyssal sediments (red radiolarite and red pelagic mudstone of either Ladinian-Carnian or Bathonian-Callovian age, as well as bluish-grey, sometimes blackish siliceous shale of the latter age). The basalt is geochemically of MOR type, based on earlier evaluations. However, it comes in two types: reddish or greenish amygdaloidal pillow basalts with peperitic facies containing reddish micritic limestone inclusions, and green basalts without any sedimentary carbonate inclusion. The former type is probably Middle-Triassic, advanced rifting stage-related basalt, whereas the latter is probably of Jurassic age, corresponding to the Szarvaskõ-type basalt of the western Bükk Mountains. Pre-Miocene presence of an ultramafic sheet above the complex is indicated by serpentinite pebbles in the Lower Miocene Darnó Conglomerate. The lower unit, corresponding to the Mónosbél Unit of the western Bükk Mountains, consists of lower slope and toe-of-slope type sediments: dark grey shale and bluish-grey siliceous shale of Jurassic age, both showing distal turbiditic character, with frequently interbedded carbonate turbidites and debris flow deposits containing cm-to dm-sized limestone and micaceous sandstone clasts. One to ten m-sized slide blocks of reddish, siliceous Triassic Bódvalenke-type limestone associated with the above-mentioned reddish, amygdaloidal basalt also occur. In one of the studied cores a block comprising evaporitic siliciclastics akin to those of the Middle Permian Szentlélek Formation and black, fossiliferous limestone similar to the Upper Permian Nagyvisnyó Limestone Formation of the Bükk Mountains, was also encountered.
The NE Hungarian Darnó Hill is geologically complex; it is composed of an accretionary mélange complex, which contains Permian, Triassic and Jurassic sedimentary and magmatic blocks. The succession can easily be correlated with the NW Dinarides, using evidences of the different evolutionary stages of the Neotethyan Ocean (rifting, marginal basin opening, closure). Several ore indications are known from this area, but their genesis is poorly understood, mostly because of the complexity of the geological structures. The present study deals with one of these indications from deep drilling. A framboidal pyrite bearing Permian-Triassic marly limestone series was investigated, which was previously described as a possible analogy of the Polish-German copper shales (Kupferschiefer). Framboidal pyrite, euhedral pyrite overgrowths (both with high Au content), disseminated euhedral and anhedral pyrite (without overgrowth), anhedral chalcopyrite, galena and sphalerite were recorded by detailed microscopy, EPMA and whole rock geochemical analyses. There is a slightly enriched total metal content of 100-200 ppm in the mineralized sediments compared to the unmineralized ones. The minerals formed under reducing, anoxic, marine sedimentary conditions, in several steps; during synsedimentary, early diagenetic and epigenetic processes. The characteristics confirmed by the research are typical of the weakly mineralized Kupferschiefer type.
Several Cyprus-type volcanogenic massive sulphide (VMS) deposits occur in the Jurassic ophiolitic series of the Northern Apennines. Stratabound, stratiform and stockwork deposits were formed in the western limb of the Neotethys (Ligurian Ocean) and are observed today in basalt, gabbro and serpentinised peridotite host rocks. The studied stockwork deposit at Boccassuolo occurs in basalt and basalt breccia. Detailed petrography, fluid inclusion study, Raman spectroscopy analyses and chlorite thermometry calculations were used to determine the P, T, X conditions of the fluid circulation system. The veins contain three quartz generations, calcite, chlorite, epidote and sericite as gangue minerals and pyrite, chalcopyrite, sphalerite, pyrrhotite and galena as ore minerals. Based on the fluid inclusion study, the earlier defined three vein types (1, 2 and 3) precipitated from the same type of evolving fluid, though at slightly different stratigraphic positions. The determined ranges of temperature (370-60°C), salinity (6.2-11.4 NaCl equiv. wt%), pressure (30-44 MPa) and methane content (average 0.28 mol/kg) suggest an evolved seawater origin for the hydrothermal fluid, modified by fluid-rock interactions and possibly by mixing of magmatic volatiles. The fluid characteristics and the mineralogical observations have proven a slightly distal position in relation to the centre of the fluid flow for all the studied locations, but more central and more distant blocks were also recognised. The temporal evolution of the system developed into a low temperature event, occurring after the main mineral stage of formation, but still within the same overall process.
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