Sulfide mineral oxidation is a basic process that is central to a number of economically and environmentally important topics, including the formation of acid mine drainage, migration of heavy metals and ecogeochemical effects of tailings influence. Reflected light microscopy is routinely applied to the study of ore minerals and textures in polished sections prepared from a wide variety of ore specimens. With the increasing interest in environmental problems, the important role of the reflected light microscope in the precise phase identification and the determination of the textural relationships between those phases needs to be recognized. In this report we present the results of mineralogical and geochemical investigations of sulfide minerals to demonstrate the peculiarities of sulfide tailings weathering, release of heavy metals and environmental pollution.
Dzhida tungsten-molybdenum Plant (Transbaikal, Russia) is processing rare-metal ores with high contents of sulfide minerals within the 60 years. About 4 mln.t. of tailings were accumulated during this period close to the town Zakamensk. Authors have studied the accumulation of sulfide intermediate product which are separately stockpiled. Its resources are 50 th.t. and sulfide contents (mainly pyrite) add up to 90% (an average of 50%). This industry dump is enormously favourable and suitable model for investigation in detail because of sulfide oxidizing dissolving, their transformation to other minerals as well as associated processes of heavy metal carrying out are focused with space and time.
The sharp zonality is indicated in degree and nature of substance weathering according to the location of conforming layer up to the surface. The zonality is registered by mineralogical and geochemical methods. In particular, the uppermost zone of coarse-grained pyrite sand (its thickness is 20 cm) is characterized by intensive dissolving of pyrite and some other sulfides. Dissolving is clear fixed through a redeposition of Fe, Cu, Pb and Ag in sulfate forms. Greater part of them is confined to narrow horizon (near 5 cm) adjacent from above to clay layer with a weak penetrability for solutions and acting as geochemical barrier. In the pyrite zone with similar composition and visual appearance located in the bottom of column (1.8 - 2.0 m from the surface) it was found that substance weathering does not range up equally significant degree. Only anglesite was determined as a sulfate form; bornite rims are developed over chalcopyrite; goethite and lepidocrocite are presented here, also. The two last penetrate also in underlying zone and impart an intensive bright-reddish color.
The zonality described above can be explained by the decrease of oxidizing conditions down cross section, which is generally fixed by the increase of the ratios Fe2+/Fe3+, Mn2+/Mn4+, S2-/S6+ for samples from various depths in the same direction. Similar processes of sulfide dissolving and heavy metal carrying out are taking place across the whole volume of tailings judging by mineralogical investigation of sulfide grains. These observations allow to recognize the following order of tendency to be leached, based on partial Cu fixation in covellite rims over primary sulfides and Pb fixation as anglesite rims over galena as well as Fe fixation in the form of hydroxides:
Zn (Cd) > Cu > Fe > Pb
The factors favourable to migration of these metals from tailings are: 1) significant excess of pyrite in relation to other sulfides as a source of free H2SO4; 2) predominance of quartz and acid plagioclase in hosted rocks; 3) the absence of carbonate and dark-colored minerals acting as neutralizers of acid solutions.
It was calculated, that about 15% of whole sulfides were oxidated during the last 50 years and hence a significant carrying out of H2SO4 on environment resulting in favorable conditions for heavy metal migration by acid water streams has been taking place.