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Sorin Silviu Udubasa, University of Bucharest (Romania)
Lucian Petrescu, University of Bucharest (Romania)
Gheorghe Udubasa, University of Bucharest (Romania)
Paulina Hirtopanu, University of Bucharest (Romania)
Serban Constantinescu, National Institute of Materials Physics (Romania)
Nicoleta Popescu-Pogrion, National Institute of Materials Physics (Romania)
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By routine investigation of some shear-zone related ores and metamorphic Mn-Fe ores it had been seen that in the mineral associations nanominerals do exist as revealed by structural methods such as NGR (Mössbauer) and TEM/SAED.
In the medium grade metamorphics of the South Carpathians, Romania, there are several shear-zone related Au ores forming quartz-rich lenses with sulfides (arsenopyrite, pyrite, chalcopyrite, pyrrhotite etc.) accompanied by Au. The Au is present as macrograins (seen with naked eye), micrograins (visible under the optical microscope) and infragrains (detected by EPMA). In some cases, the Au infragrains in arsenopyrite are surrounded by areas with gradual decreasing Au contents, down to less than 0,2 ppm Au (beyond the detection limit of EPMA), that is the so called invisible gold. Further investigation by TEM/SAED showed however that Au as physical entity is existing at the nanometric scale either as isolated spheres some nm in size or coral-like or clouds of spherical grains forming aggregates up to 100 nm in size. The invisible gold considered to be chemically bond in sulfides (substituting Fe and As) may be thus accompanied by nanogold, detectable only by using TEM/SAED.
The nanogold is seemingly selectively distributed over grain surfaces of the sulfides present, all of them showing locally crystalochemical peculiarities such as: variable Fe/As ratio in arsenopyrite, zoned pyrite with As contents, pyrrhotite as well as chalcopyrite with both bivalent and trivalent Fe (NGR spectra). All these sulfide features represent favorable loci for nanogold to precipitate, which seemingly appeared during repeated shearing of protore-containing metamorphics.
The same is probable true for some inframinerals (identified only by investigations using structural techniques such as NGR, XRD, TEM/SAED) associated with pyrrhotite, i.e. uytenbogaardtite, with arsenopyrite, i.e. auricupride, cobaltite and moganite and with sphalerite, i.e. löllingite.
The metamorphosed Mn-Fe ores at Rãzoare, North Romania, contain about 70 mineral species. However, the main components are manganoan fayalite, Mn-humites, mangangrunerite, jacobsite, magnetite, rhodochrosite etc. The optical study revealed the presence of Mn-orthoferrosilite, pyrite and pyrrhotite (Udubasa et al., 1996). A deeper investigation by structural methods (NGR, TEM/SAED) revealed further inclusions, this time of nanometric size, i.e. wstite and pyroxferroite.
The combination of mineralogical and physical methods has been proved to be a valuable key in identifying infra- and nanominerals. In the case of Au ores the nanogold represents a new form of Au occurrence, probably of high interest in recovery processes. The mineral association in the Mn-Fe ores, nanominerals included, witness the high PT and reducing conditions of early phases of the metamorphic evolution.
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