URANIUM-SULPHIDE MINERALISATION IN THE BISTRITA MTS, EAST CARPATHIANS, ROMANIA

Transcription

1 URANIUM-SULPHIDE MINERALISATION IN THE BISTRITA MTS, EAST CARPATHIANS, ROMANIA HIRTOPANU 1 Paulina, UDUBASA 1 Gheorghe, SCOTT 2 HARTOPANU 1 Ion, BARBIR 3 Gabriel Peter, 1 Geological Institute of Romania, 1 Caransebes street, Bucharest, Romania, Paulina. Hirtopanu@igr.ro 2 Exeter University, Cornwall, TR11 5RL, UK, peter.w.scott@lineone.net 3National Company of Uranium, Suceava Branch, Romania The uranium mineralisations at Crucea are situated in Bretila Group at the proximity contact with the Tg3 level. Geollogically and structurally it belongs to a complex area, the so called Eastern Central Carpathians Nappes (ECCN). The ECCN are constituted from top to bottom by a system of Alpine Nappes which in their turn consist of several Variscan tectonic units named also pre-alpine Nappes. The Crucea Uranium deposit is situated in the Variscan Rarau Nappe (represented by the oldest Bretila Group) which thrusts over the Putna Nappe (represented by the youngest Tulghes Group). This is the only major thrusting in the area. Because its tectonic setting, the host rocks of uranium minerals are strongly retromorphosed, highly carbonatated, forming a tectonic zone, characterised by crushing, cataclasis and microbreccia formation. The samples for analyses were collected from Bretila Group rocks hosting the Crucea uranium mineralisation. The microscopical study and SEM analayses allowed us to determine uranium oxides, uranium silicates, uranyl silicates, uranium (uranyl?) silicate sulphates, uranyl sulphates, uranyl carbonates and uranyl phosphates. The primary uranium ore from Crucea deposit is mainly constituted by uraninite (Plate I, fig A, point A). The uraninite is white and has a irregular shapes, enclosed in black carbonaceous matter (point B on the fig A/plate I). Its chemical compostion is shown in the SEM profile A (Plate I). Beside uranium, it contains very little Ca, Al, Si and P. In the SEM profile B of the same plate on can see the chemical composition of the carbonaceous matter, indicating only C with very little S. In polished thin section the carbonaceous matter is intensely anisotropic and has relatively high reflectance, which means a high degree of crystallisation. The pitchblende is the metamictic, amorphous substance which results from the transformation of uraninite, containing up to 5wt % water. Coffinite, the uranium tetragonal silicate differs from uraninite, being anisotropic. Other uranium silicates determined by SEM analyses in the Crucea deposits are: ursilite, uranophane, uranosilite, weeksite, soddyite and clarkeite. The last one is a result of hydrothermal transformation of the uraninite. The high Ti and Ca content of some U- silicates indicate the probable presence of a mineral from the sklodowskite group. The chemical analyses on intensely anisotropic and high reflectance carbonaceous matter, which has a high degree of crystallisation, indicate only C with very little S (SEM analyses)(plate I, profile B). The amount of S from carbonaceous matter could have been derived from biogenetic material and/or reduction of sulphate. The Ba and Si together with uranium in the chemical profile of some grains are a proof of the kasolite presence in this mineralisation. The SEM analyses indicate a U- 108

2 sulphate-silicate, which probably is a new mineral (mineral X) (Hirtopanu & Barbir, 2004). Its chemical profile is shown in fig.4. The uranyl-sulphates are: uranopilite, zippeite. The uranylcarbonates are: zellerite, uranocalcarite. The uranyl phosphates contains generally Ca and Mg: upalite, uranospatite, autunite, mundite, saleeite, novacekite. In the Plate II on can see the image of some Uranium silicate and their chemical composition variation (3 chemical profiles from point 1, 2 and 3 on the picture). The sulphides comprise three mineral associations: (1) dominated by pyrite, pyrrhotite (2) polymetallic dominated by Pb-Zn-Cu, represented by chalcopyrite, sphalerite, galena, bornite, bournonite; (3) Ni-Co-As sulphides dominated by pentlandite, polydymite, gersdorffite, rammelsbergite, bravoite, associated with calcite, hematite, siderite. The U mineralisation is situated between (2) and (3) association, because frequently it substitutes pyrite and pyrrhotite There are relics of ursilite in pyrite (Plate I, fig B). Because its association, the uranium mineralisation seems to be hydrothermal in origin, but it was tectonically strongly transformed and remobilized after leaving the original source. There is some concentric zonations with gersdorffite in the centre and with pentlandite at the rims of the grains. Also, a very nice zonation is shown in Plate I fig C, with pentlandite at the margin (a), Ni-pyrrhotite in the middle (b) and mineral X in centre of the grain (c). The chemical profile of mineral X is shown in the Plate I, profile G1c. In the plate I fig B on can see the relics of pyrite (grey) in uranium silicate, ursilite (white), with concentric zonation. In the other grain the zonation shows: As-Ni sulphide (gersdorffite) in the margin, pentlandite (in the middle) and ursilite in the centre of the grain. Other sulphides, like tetrahedrite, tennantite, marcasite, bravoite, etc, were also determined. The protore of U mineralisations of Bretila Group rocks was probably a carbonaceous matter with a very high carbon concentration deposited on the Precambrian see floor. Subsequently the U mineralisations were accumulated by complex metasomatic oxidationreduction processes, which promoted destruction (degradation) of the organic matter and the repetead precipitation of the ore components, follwed/acompanyied by a strong remobilization/migration. Recently the uranium mineralisations, through it geologic setting and mineralogical composition looks like a hydrothermal vein one, but it was tectonically strongly transformed and remobilized from the original source. The U mineralisation and the sulphides, especially Ni-Co-As seems to be contemporary. To solve this problem we need some isotopic analyses to establish the nature (organic or mineral) of carbonaceous matter. If the carbonaceous matter will prove to be of mineral origin, then it can be formed by reducing of carbonates. The decarbonation reactions usually produce CO 2 and CO, which decompose into graphite and oxygen, below the graphite buffer curve (at very low oxygen fugacities). The special tectonical conditions to form the carbonaceous matter in this way were available in this area. If the isotopic analyses will give a mineral nature of the carbonaceous matter, its age could be younger, at last contemporanous with the thrusting of Rebra Group over Tulghes Group. References Hirtopanu P. and Barbir G., (2002), Rom.J.Petrology, v.79, Suppl. Nr1. 109

3 110

4 111

5 112