Reiner Klemd

High-density CO2−N2 inclusions in eclogite-facies metasediments of the Münchberg gneiss complex, SE Germany

The eclogite-facies metasedimentary rocks in the Münchberg gneiss complex (T=630±30° C/P≥17–24 kbar) locally contain CO2−N2-rich fluid inclusions of extremely low molar volumes (32 cm3/mol) in quartz. These fluid compositions are mainly found in rocks intercalated with calcsilicate bands. Densities were determined from low-temperature phase transitions like stable or metastable homogenization (L+V→L), partial homogenization (S+L+V→S+L) and the transition S+L→L (L = liquid, V = vapour, S = solid). The high fluid densities are in agreement with eclogite-facies pressure and temperature and subsequent amphibolite facies. CO2−N2 inclusions were not observed in adjacent eclogites nor in non-calcareous metasediments. These rock types contain predominantly H2O-rich inclusions correlating with amphibolite-facies conditions. The variation of fluid composition with lithological differences indicates local fluid gradients and speaks against a pervasive fluid flow during eclogite-facies metamorphism.

Characterisation of amphibolite-facies fluids of Variscan eclogites from the Orlica-Snieznik dome (Sudetes, SW Poland)

Abstract Eclogites from the Orlica-Snieznik dome (Sudetes, SW Poland) exhibit evidence for Variscan high-pressure metamorphism and a subsequent amphibolite-facies overprint. Three main types of fluid inclusions were observed in quartz of partially retrogressed eclogites. Type 1: aqueous inclusions, mostly two-phase inclusions (L + V). Type 2: (a) CO2 ± N2 ± CH4 inclusions, normally one-phase inclusions, some contain graphite as daughter mineral; (b) N2 ± CH4 inclusions, one-phase inclusions. Type 3: mixed H2OCO2 ± N2 inclusions, normally two-phase inclusions (LH2O + LCO2 ± VCO2) consisting of 70–80 vol% CO2. The texturally earliest inclusions are the CO2 ± N2 ± CH4 (type 2a) inclusions, which were followed by H2OCO2N2 (type 3) and H2O (type 1) inclusions in chronological order. Late aqueous (type 1) and N2CH4 (type 2b) inclusions were the last to be trapped. Isochores for fluid inclusions are not conformable with peak-metamorphic conditions which are estimated at temperatures between 640° and 790°C and minimum pressures of >14 kbar. At least three possible explanations must be considered: (1) no fluid phase was present during the eclogite-facies metamorphism; (2) all fluid inclusions which were trapped during the high-pressure event decrepitated during retrogression; (3) only under medium-pressure conditions fluid inclusions were trapped. We did not succeed to identify fluid inclusions which clearly are related to the eclogite-facies metamorphism. However, the isochores derived from microthermometric measurements can be used to constrain the retrograde P-T path. The highest-density isochore of the early CO2 ± N2 ± CH4 inclusions indicates a pressure of 8 kbar at 600°C. The isochores of the H2OCO2 ± N2, the H2O inclusions and the re-equilibrated CO2 ± N2 ± CH4 inclusions suggest an almost isothermal decompressional uplift to 2 kbar at 600°C, which was followed by isobaric cooling to 200°C. Therefore, the rate of uplift was low relative to the rate of cooling, indicating a relatively long period of tectonic inactivity.

Fluid inclusions in quartz-pebbles of the gold-bearing Tarkwaian conglomerates of Ghana as guides to their provenance area

Quartz-pebbles of the early Proterozoic Au-bearing Tarkwaian conglomerates in Ghana reveal several original (inherited) pre-sedimentary fluid inclusions. These inclusions are CO2-N2 rich and display a distinct high density (up to 1.15 g/cm3). The unusual high density and composition compare well with CO2-N2-rich inclusions in quartz-vein type gold deposits of the Birimian Supergroup in Ghana and Burkina Faso. This type of fluid inclusions has not been reported from any other lode-gold deposit of greenstone affiliation and is thus a specific characteristic for Birimian-hosted gold deposits. Therefore, it can be used as an unequivocal pathfinder for epigenetic as well as for syn-sedimentary gold mineralization of the early Proterozoic of West Africa. The inherited fluid inclusions with the unique physicochemical characteristics suggest that the Tarkwaian quartz-pebbles and possibly some gold were derived from Au-quartz vein deposits comparable in mineralogy, petrography and genesis to those along the NW-margin of the Ashanti belt (e.g. Ashanti Mine, Prestea Mine).

High-pressure relics in meta-sediments intercalated with the Weissenstein eclogite, Münchberg gneiss complex, Bavaria

AbstractThe mineral assemblages in the eclogites and meta-sediments of the Münchberg gneiss complex suggest minimum pressures of about 15 kbar, and temperatures of 600±30° C for the eclogite-facies metamorphism. Both rock types underwent a subsequent amphibolite-facies metamorphism at almost the same temperature range. In the Weissenstein borehole the eclogites and meta-sediments show an intimate interlayering on a centimetre scale. Contacts between the two rock types are often sharp and untectonized suggesting in-situ metamorphism of the eclogites. The following features indicate that the gneisses were subjected to eclogite-facies metamorphism:1.Phengite relics and phengite inclusions in garnet contain up to 3.45 Si per formula unit while amphibolite facies phengites have considerably lower Si-contents (3.0–3.25 p.f.u.).2.Omphacite relics occur in the form of Na-augite (6–9 mol% jadeite)-oligoclase symplectites.3.Garnets with up to 40 mol% of the pyrope component occur. The geological and mineral-chemical data indicate that large volumes of crustal material have been buried to depths possibly exceeding 70 km. The preservation of primary eclogitic textures as well as symplectitic textures in the presence of a fluid phase, are indicative of a rapid decompression. This would suggest a tectonic uplift (e.g. underthrusting) as is also indicated by the virtually constant temperature range during the uplift.

Metamorphism of the country rocks hosting gold–sulfide-bearing quartz veins in the Paleoproterozoic southern Kibi-Winneba belt (SE-Ghana)

Abstract Many sections of Paleoproterozoic greenstone belts in the Birimian of Ghana are found to have experienced subgreenschist- to greenschist-facies metamorphism. However, mineral–chemical and textural examinations in combination with conventional geothermobarometry of metapelites and amphibolites from the southern Kibi-Winneba belt suggest peak amphibolite-facies conditions of 500–610 °C at 4.5–6 kbar. This is in accordance with peak-metamorphic P–T estimates of 490–580 °C at 4–6 kbar for Birimian rocks of the southern Ashanti belt [Precambrian Res. 98 (1999) 11] and the northwestern Sefwi belt [Geol. Jahrbuch, in press]. Hydrothermal alteration and gold mineralization in this region apparently has occurred during retrograde greenschist-facies conditions at temperatures of 400–540 °C as indicated by the sulfide paragenesis arsenopyrite–pyrite (with lollingite inclusions) and arsenopyrite–pyrrhotite. These data suggest that alteration and lode-gold mineralization occurred under post-peak-metamorphic conditions.

Fluid Composition and Source of Early Proterozoic Lode Gold Deposits of the Birimian Volcanic Belt, West Africa

Gold-bearing veins of Early Proterozoic lode gold deposits of Ghana and Burkina Faso contain predominantly CO2-N2-rich fluid inclusions, often displaying a very high density (up to 1.15 g/cm3), in contrast to most other lode gold deposits of greenstone affiliation. Mineral modeling shows that retrograde and upper-level fluids will have higher XH2 O because of wall-rock interaction. The high-density CO2-N2-rich fluid is considered to represent the internal unbuffered ore-fluid composition, which may have originated in the deep crust or mantle.

Gold-telluride mineralisation in the Birimian at Diabatou, Burkina Faso: the role of CO2N2 fluids

Abstract Gold-telluride bearing quartz veins at Diabatou occur in rocks of the Palaeoproterozoic Birimian Supergroup of Burkina Faso. The main Au-telluride mineralisation and wallrock alteration occurred at temperatures between 300 and 380°C and pressures of about 3–5 kbar. The Au-telluride- bearing quartz veins predominantly contain (>90%) CO 2 N 2 fluid inclusions, some of which display densities of up to 0.99 g cm −3 , much in contrast to other lode-Au deposits of greenstone affiliation. The highest density isochores match the P-T conditions of the mineralisation. CO 2 N 2 inclusions and Au-telluride mineralisation occur in all investigated samples, while aqueous inclusions are almost absent. Therefore, the CO 2 N 2 inclusions at least represent residues of mineralising fluid, although the fluid composition calculated from thermodynamic modelling of wallrock mineral assemblages disagrees with the fluid inclusion composition. Post-entrapment modifications due to grain-boundary migration recrystallisation during deformation are suggested to be responsible for the formation of most CO 2 N 2 inclusions.

New carbonatite occurrences along the Hatta transform fault zone (northern Oman Mountains, United Arab Emirates)

Abstract New large occurrences of extrusive carbonatite have been found within metasedimentary rocks below the Semail Ophiolite Complex of the Hatta transform fault zone, United Arab Emirates. Geochemically they can be classified as ferrocarbonatites. Their chemistry is different from the carbonatites occurring in the Dibba transtensional fault zone about 60 km to the north, however, they have a similar stratigraphical position. The textures of the carbonatites indicate that they are of extrusive origin. They are either related to Triassic rifting during the early phases of the formation of the Neo-Tethyan Sea or to Late Cretaceous, intra-plate alkaline magmatism. These carbonatite occurrences highlight the possibility of further carbonatite discoveries in tectonic environments, such as transform and transtensional fault systems.