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91.
The Bereznjakovskoje gold trend, southern Urals, Russia 总被引:2,自引:0,他引:2
B. Lehmann J. Heinhorst U. Hein M. Neumann J. D. Weisser V. Fedesejev 《Mineralium Deposita》1999,34(3):241-249
The East Uralian Zone south of Cheljabinsk consists of a Silurian to Early Carboniferous volcanosedimentary sequence on Proterozoic
basement, intruded by postorogenic Permian granitic rocks. The 7 by 1–2 km Bereznjakovskoje gold trend is hosted by dacitic
to andesitic volcanic-subvolcanic units of Late Devonian age. Epithermal gold mineralization is of the sulfide-rich low-sulfidation
(adularia-sericite) type with the metal spectrum of Au-Ag-As-Sb-Cu-Zn-Pb and a characteristic Te component. Silicic and sericitic
alteration overprints barren propylitic pyrite-rich assemblages in stockworks and shear zones. Precious-metal mineralization
is related to a fahlore-telluride stage overprinting earlier pyrite. Late stage redistribution and weathering liberates gold
from telluride mineral phases with fixation of native gold (high fineness) on vugs and grain boundaries, and in intergrowth
aggregates with supergene jarosite/limonite. Epithermal mineralization formed below the boiling level and is possibly related
to a porphyry system at depth.
Received: 23 February 1998 / Accepted: 12 July 1998 相似文献
92.
Diatomite associated with the Kolubara Coal Basin was studied to better understand early stage silica diagenesis of shallow water deposits. The Kolubara Basin consists of Neogene siliciclastic rocks, diatomite, marlstone and rare carbonates. Palaeozoic metamorphic and Mesozoic sedimentary and igneous basement rocks are transgressively overlain by Upper Miocene sandstone, siltstone, shale and mudstone. This Upper Miocene section is transgressively overlain by the Pontian section, which contains diatomite and coal beds. White and grey diatomite forms beds 0.7-2.2 m thick that are continuous over an area of about 2 km2. Siliceous rocks vary in composition from diatomite (81-89 per cent SiO2) to diatom-bearing shale (58-60 per cent SiO2). Siliceous deposits are laminated in places, with the laminae defined by variations in clay minerals, organic matter and diatoms. Diatomite shows only incipient diagenesis characterized by the fragmentation of diatom frustules, the minor to moderate corrosion of frustules and the formation of minor amounts of opal-A' (X-ray amorphous inorganic opal) cement. The low degree of diagenesis results from the young age of the deposits, low burial temperatures and possibly also from the presence of abundant organic matter and the dissolution of kaolinite. The presence of only weak diagenesis is also reflected by the characteristically poor consolidation of the rocks and low rank of the associated coal. 相似文献