Filipčíková, P.
1*, Dublyansky, Y.
2, Spötl, C.
2& Koltai, G.
21Department of Environmental Geochemistry, Comenius University in Bratislava, Slovakia; 2 Institute of Geology, University of Innsbruck, Austria; *filipcikova2@uniba.sk
Hydrothermal speleogenesis is a specific category of hypogene karst, commonly but not always hosting euhedral calcite crystals lining cave walls and/or isotopic alteration halos in the adjacent wall rock (Dublyansky, 2013; Dublyansky and Spötl, 2009).
Samples of euhedral calcite crystals and bedrock were obtained from three caves in the area of Veľká Fatra (Slovakia). Prior to this study, hydrothermal speleogenesis has not been identified in the area. The presence of euhedral calcite crystals, however, suggest a deep-seated hydrothermal origin of some of these caves. We applied a suite of methods including stable isotope analyses on wall rock and calcite crystals, fluid-inclusion petrography, microthermometry and isotope analysis of fluid inclusion water to identify speleogenetic phases and constrain the nature of the fluids involved.
The initial dissolution phase is represented by isotope alteration halos in the cave wallrock where carbon isotope values significantly decrease towards the cave wall (from 1.5 ‰ to −10.3 ‰) showing a partially sigmoidal shape. The δ18O data show a weak trend towards higher values (about
−2.9 ‰ VPDB) compared to the unaltered Triassic host rock (−4.5 ‰ VPDB). A different (and presumably late-stage) alteration signal was obtained for two locations, where δ18O values range from −8.1 to −7.1 ‰, and δ13C between −4.1 and −1.6 ‰.
Homogenisation temperatures of two-phase fluid inclusions range between 41 and 74 °C in three caves. The temperature variation within a single crystal is 10 to 20 °C and the measured FIA indicate either stable conditions or a cooling trend.
Salinity decreases in the same manner from 3.6 to 0.7 mass% NaCl equiv. from the base to the top of the crystals. The stable isotopic composition of the inclusion water was analysed in five samples. All samples show moderate (1.5-2.0 ‰) positive δ18O displacement relative to the Global Meteoric Water Line, suggesting some degree of water-rock interaction.
The early phase of calcite formation involved fluids of likely sea water origin while the later phase of calcite growth suggests a meteoric water influx.
Changes in crystallisation conditions are also reflected in the changing crystal habit.
The study of scalenohedral calcite and associated bedrock yielded evidence of temperature decrease in a range 74 to 41 °C and the fluid composition evolution from the marine
towards low-salinity meteoric that likely occurred during the uplift and exhumation of the area.
We conclude the cave Tajná Túžba (TT) and Lubená II (LUB-II) formed under the same conditions represented by the early (sea-water) dissolution, however the bedrock signal in TT was likely overprinted by late, meteoric fluid. The overprint signal was detected (along with the marine signal) in samples of loose boulders topped by calcite lying on the cave floor, and corrosive environment is suggested also by the presence of highly corroded crystals lining the cave walls. The precipitation phase was characterised by progressively diluted fluid where the calcite in TT yielded higher temperatures (74 to 52 °C) suggesting earlier precipitation than in LUB-II (66 to 47 °C).
The formation of Lubenna I cave took place subsequent to the marine phase, represented by meteoric signal in the wall rock and in calcite spar, crystallising at temperatures 63 to 41 °C.
Timing of the speleogenetic process, related to tectonic uplift and stage involving marine water is tentatively placed in the Paleogene–Lower Miocene.
References
Dublyansky Y.V. (2013) in: Treatise on Geomorphology 57-71.
Dublyansky Y.V. and Spötl C. (2009) in: Hypogene Speleogenesis and Karst Hydrogeology of Artesian Basins. 45-50.
Fluid inclusion study in the Portas gold deposit (Lugo, NW of Spain): preliminary results Fuertes-Fuente, M.
1*, Cepedal, A.
1, Martin-Izard, A.
1, Arias, D.
1& Aragón, D.
21Department of Geology, University of Oviedo, Spain; 2Sondeos y Perforaciones Industriales del Bierzo S.A., Spain; *mercedf@uniovi.es
Introduction
Gold occurrences are abundant in the NW Iberian Peninsula, and mining companies currently explore several gold prospects. This research focuses on one of them, the Portas deposit (Cepedal et al., 2018). This work presents a preliminary fluid inclusion study to determine the composition of the fluids involved in the mineralising process.
Geological Setting
The Portas deposit is located in the Variscan belt of the NW of Iberian Peninsula. The succession that hosts the deposit is a sequence of alternating slates, metasandstones and quartzites, culminating in the Armorican Quartzite. Slates of Llanvirn age overlie the quartzite. Iron-rich metasediments occur in the transitional stratigraphic levels to the Armorican Quartzite. This Ordovician sequence is affected by three coaxial Variscan deformation phases, and by metamorphism at greenschist facies.
The Portas deposit
The deposit is associated with a quartz-vein system that crosscuts Ordovician metasediments.
The veins show different degrees of deformation, and the less deformed and undeformed quartz-veins are associated with the ore mineralisation.
They are mainly composed of quartz and chlorite, and lesser amounts of, ankerite, apatite and rutile.
In the veins, the ore minerals are arsenopyrite, and pyrite that overgrows or heals fractures in the arsenopyrite. This pyrite is associated with galena, chalcopyrite, sphalerite, and gold.
Fluid inclusion study
Quartz samples from the ore-bearing veins were collected for fluid inclusion petrography and microthermometry. Two fluid inclusion types were recognised in all the sampled veins:
Type 1: Carbonic fluid inclusions. They are abundant in all the studied samples. At room temperature, they are monophasic and have a dark appearance (Fig. 1). They form three-dimensional arrays in the quartz crystals. Microthermometry data show a Tm(CO2) range of between 64 and -61.8 ºC. The volumetric fraction of the carbonic liquid phase varies between 0.25 and 0.5. The Th(CO2) is to the vapour state with two temperature intervals from 26 to 21.5 ºC, and from 10.5 to -7.2 oC. The homogenisation temperatures are consistent within the fluid inclusion assemblages.
Type 2: Aqueous fluid inclusions. At room temperature, they are biphasic inclusions with φliq = 0.7-0.8. They occur as intragranular and transgranular fluid inclusion planes. Te is
around -20.8 ºC and Tm(ice) is from -5.3 ºC to -2.8 ºC. Th(total) is between 230 ºC and 316 ºC to the liquid state.
Conclusions
From this preliminary study, a mixture of volatiles, CO2-CH4-N2,(Van den Kerkhof and Thiéry, 2001) seems to have had a role in the ore mineral deposition. Further studies are necessary to establish the actual composition of this fluid, and its implication for the genesis of this gold deposit.
Acknowledgement
This work has been financed by the CGL2016-76532-R project of the Spanish National Plan I+D+i.
References
Cepedal A. et al. (2018) in: 15th IAGOD Symposium Proceedings Anales 56-SEGEMAR:148-149.
Van den Kerkhof A. and Thiéry R. (2001) Lithos 55:49-68.
Fig. 1. Type 1 fluid inclusions containing a single fluid phase in a quartz crystal. The crystal also shows abundant inclusions of vermicular chlorite (on the right).
Fluid inclusion study and burial depth of deep-seated Evate carbonatite deposit Gajdošová, M.
1*, Huraiová, M.
1& Hurai, V.
21Department of mineralogy and petrology, Comenius University, Slovakia; 2Earth Science Institute, Slovak Academy of Science, Slovakia; *misa.gajdos@gmail.com
The Evate carbonatite deposit is located in the Monapo structure in Mozambique. As calcite and dolomite are the major mineral phases, the rock can be classified as calciocarbonatite and ferric calciocarbonatite. Apatite, olivine, amphibole, phlogopite, magnetite, ilmenite, anhydrite and spinel are other abundant minerals of the carbonatite and associated nelsonite (Hurai et al., 2017; Barbosa et al., 2016). No comprehensive research has been carried out from the Evate deposit yet and the recent views on its genesis remain contentious. The Evate deposit may represent an example of deep-seated post-orogenic carbonatites occurring rarely in the Earth.
We studied fluid inclusions in the metasomatic fenite, subjacent to the Evate deposit, consisting of quartz, albite, K-feldspar, amphibole, magnetite, pyrrhotite and pyrite. Accessory minerals are titanite, monazite and rutile with small apatite and calcite inclusions. The observed mineral assemblage allowed us to combine data from various thermometres with the isochores determined from the fluid inclusions.
The presence of rutile needles in quartz (Fig. 1) indicates the system saturated in Ti. Therefore, the crystallisation temperature of quartz could be calculated by using Ti-in-quartz thermometre (Thomas et al., 2010). The Ti-content determined by EPMA (13-38 ppm) returned crystallisation temperature of 42118 °C (at 1 bar). Temperature calculated by using the Zr content in rutile determined by EPMA (40-78 ppm) corresponded to 49311 °C (at 1 bar), according to the calibration of Tomkins et al. (2007).
Numerous monophase fluid inclusions in quartz had rounded or elongated shapes and diametres ranging from 1 to 16 μm. Raman spectroscopy revealed low-density secondary gaseous inclusions of N2-CO2 composition and higher density primary fluid inclusions of CO2-rich fluid with minor admixture of CH4 and N2. In the primary inclusions, the homogenisation of V (vapour phase) and L (liquid phase) took place into the L between -31.5 and 21.8 °C with a top frequency at -3.2 °C. Temperatures of melting ranged from -57.7 to -56.6 °C. A super-dense inclusion was also observed, showing a metastable total homogenisation of V into the L at -57.6 °C, stable partial homogenisation of V into the L at -57.4 °C and total homogenisation by the dissolution of solid in the L at -56.9 °C.
Nahcolite was a ubiquitous daughter phase identified by Raman spectroscopy within CO2-rich primary inclusions (Fig. 1). Dawsonite frequently occurred only within CO2-rich inclusions adhered to
albite. Thenardite was also rarely present. Solitary monophase nahcolite inclusions were also observed in quartz.
Isochores of CO2 inclusions combined with the Ti-in-quartz and Zr-in-rutile thermometres returned temperatures and pressures of 420-530 °C and 3-7.2 kbar, respectively. Burial depths are estimated to be 26-29 km, assuming the changing lithostatic and hydrostatic fluid regimes, thus corroborating our assumption about the deep-seated carbonatite formed during the post-collisional break-up of Gondwana. This interpretation is supported by the EPMA U-Pb-Th monazite age of 57313 Ma in the studied rock, corresponding to the thrust-faulting exhumation phase of east-African orogen (Grantham et al., 2013).
Acknowledgement
This work was supported by the Comenius University grant for young scientists (UK/205/2019).
References
Barbosa R.O. et al. (2016) 35th IGC, Cape Town, Abstracts volume.
Grantham G.H. et al. (2013) Precambrian Res. 234:85-135.
Hurai V. et al. (2017) Ore Geol. Rev. 80:1072-1091.
Thomas J.B. et al. (2010) Contrib. Mineral. Petr.
160:743-759.
Tomkins H.S. et al. (2007) J. Metamorph. Geol. 25:703-713.
Fig. 1. Quartz-hosted fluid inclusion containing nahcolite-nah, CO2 phase-V and rutile-rtl (size 11x5 μm).