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MINERALS and MINERAL VARIETIES from METAMORPHOSED Mn DEPOSITS of BISTRITA MOUNTAINS, ROMANIA

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MINERALS and MINERAL VARIETIES from METAMORPHOSED Mn DEPOSITS of BISTRITA MOUNTAINS, ROMANIA Acta Mineralogica-Petrographica, Abstract Series 1, Szeged, 2003 MINERALS AND MINERAL VARIETIES FROM METAMORPHOSED Mn DEPOSITS OF BISTRITA MOUNTAINS, ROMANIA HÎRTOPANU, P.1 & SCOTT, P.2 1 Geological Institute of Romania, Caransebeş 1, RO-78344 Bucharest, Romania. E-mail: [email protected] 2 Camborne School of Mines, Redruth, Cornwall, United Kingdom. The Bistrita Mountains belong to the Crystalline Meso- mation of some amphiboles and some pyroxenes into other zoic Zone of the East Carpathians, which consists of super- phases, there are drastical transformations of pyroxenes into posed Variscan and Alpine Nappes, overthrusted eastwards pyroxenoids (johannsenite into rhodonite), pyroxenoids into over the Flysch Zone. The manganese ore is contained by pyroxenoids (pyroxmangite into rhodonite), pyroxmangite Tulghes Group (Tg2 level) of the Variscan Putna Nappe, into manganogrunerite, garnets into garnets (spessartine- situated over the Pietrosu Bistritei Nappe and supporting the calderite into spessartine, spessartine into anisotropic spes- thrusting of the Rebra Nappe. All these Variscan nappes sartine-andradite-grossular), calderite into pyroxmangite- constitute the Alpine Sub-Bucovinian Nappe localised be- magnetite, etc. are the best evidences of continuous variation tween Alpine Infrabucovinian Nappe in the East and the of formation conditions. Alpine Bucovinian Nappe in the West. The Mn ore have a predominant carbonate rather than The mineralogy of Mn metamorphosed deposits from silicate mineralogical composition, which means a great CO2 Bistrita Mts. includes 328 minerals and mineral varieties. fluid control in the carbonation and dehydration processes They may count among the mineralogically the most com- along the many stages of the whole history of the ore and the plex deposits of the world. Prior to 1970 there were known Tg2 level. The mineral reactions for the tephroite assem- ca. 50 minerals. The mineral number grew in 1970-1976 blages were of decarbonation type, their temperatures were period to 70 minerals and in very recent period (1994-2002) strongly influenced by composition of metamorphic fluid, the mineral number reached 328. Minerals and mineral va- that is, the decarbonation reactions took place at high tem- rieties from almost all mineral classes have been identified: perature and high XCO2 (corresponding to amphibolite fa- carbonates 17, silicates 157, oxides 47, sulphides 48, sul- cies). phates 5, phosphates 11, wolframates 2, borates 1, arsenates The olivines, carbonates, Mn-humites, garnets, pyrox- 5, vanadates 1, native elements 1 and 33 minerals from oxi- enes, pyroxenoids, amphiboles, some oxides, some phyllo- dised zone. silicates and some sulphides offer useful petrological infor- The minerals and mineral varieties were determined by mation. Metamorphic reactions and P-T path of the Bistrita combined methods: X-ray, IR, AAS, SEM analysis and opti- ores suggest that they have undergone at least five stages of cal microscopy. Several of them are very rare species (nam- recrystallisation in a subduction zone. The clockwise trend of bulite, natronambulite, norrishite, bannisterite, parsettensite, metamorphism is in agreement to the structure of the compli- manganpyrosmalite, friedelite, schallerite, nelenite, minne- cated tectonic setting of the Crystalline Mesozoic Zone. sotaite, kellyite, etc.). A lot of determined minerals have Mn Many minerals are accessory phases having a scientific im- as major constituent: tephroite, manganese humite group portance, enriching the national mineralogical patrimony. (manganhumite, sonolite, alleghanyite), leucophoenicite In the Bistrita Mn-deposits three types of assemblages group (ribbeite, leucophoenicite, jerrygibbsite), some of the were determined on the basis of the bulk oxidation ratios: oxide group, manganiferous phyllosilicates group, etc. Many oxidised (i.e. containing Fe3+ and Mn3+), reduced (with Fe2+ are secondary, as they occur in veins or are product of retro- and Mn2+) and neutral, with Mn2+ and Fe3+. gressive transformation from the granulite to amphibolite Closely associated assemblages of diverse mineralogy facies, from the amphibolite to blueschist or greenschist from Bistrita Mn ore suggest that XMn and Xfluid rather than facies. Each metamorphic event was a source for new miner- physical conditions of metamorphism are the decisive factors als. Frequently, each mineral grain presents chemical varia- in forming the observed mineral diversity. The Bistrita tional function of P, T, fO2, fCO2, fCl2 etc. on the route of pro- metamorphosed Mn-rich mineral assemblages evolved under grade and retrograde polyphasic metamorphism. The zona- a variety of constraints, including the diversity in the char- tion of the pyroxenes and amphiboles – marginal and sectoral acter of the protolith and the nature of buffering of the fluid – is a good evidence of changes of metamorphic conditions. phase during metamorphism. Such a rich mineralogy when In the case of zoned amphiboles, the core is man- compared with the limited number of minerals occurring in ganogrunerite (amphibolite facies) and the rim is constituted the country rocks, make manganese ore from BM very of alkali blue amphiboles (blueschist facies). The sector- promising potential markers for P,T, fO2, fH2O, fCl, fB, fAs, etc. zoned arrangement of pyroxenes developed during rapid reconstructions, to supplement the country rocks mineral crystal growth and involved differences of both composition records (Tg2 level), strongly transformed or even erased by and cation order. It consists of bands or hour-glass texture of metamorphism. sodic augite and omphacite pyroxenes. Beside the transfor- 43.
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