International Geologiical Congress - Oslo 2008

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MRD-13 Ore deposits associated with black shales: from their origin to their environmental impacts

 

Mesothermal gold deposits as an ore component of the granite - diorite - dolerite fluid - magmatic complexes

 

Igor Kucherenko, Tomsk Polytechnic University, Geology and Petroleum Engineering Institute (Russian Federation)
Roman Gavrilov, Tomsk Polytechnic University, Geology and Petroleum Engineering Institute (Russian Federation)
 

 

Late Proterozoic, Early to Late Paleozoic mesothermal gold deposits within the crystalline substratum and thick black shale series of the South Siberian mountain folded structures formed as an ore component of the granite-diorite-dolerite magmatic complexes as result of mantle-crust fluid-magmatic systems performance.
Ore-free high temperature mantle fluid injection caused the crust substratum palingenesis and formation of the early granitoid masses, stocks, dikes, mature focus-dome structures. At the late basaltoid stage of the complexes evolution, intermediate post-granite diorites (products of the basalt fusions differentiation) are replaced by dikes, small lenses, stocks of moderate alkali dolerites of different generations, where pre-, inter-, post-ores are identified. Hot inter-ore dikes are heat conductors for metalliferous solutions. They contain within the epigenetic mineral complexes an indicator amphibole-biotitic association ? that has higher temperature than ones in the pyrite-quartz-sericite-ankerite wallrock metasomatites (beresites). Metalliferous solutions fractional injections alternate with basaltic fusion intrusions.
Precious and accompanying metals go with the solutions into the substratum with their clarke content. In addition to the deposition in ore bodies, they form geochemical fields in the wallrock (near-ore) area. Geochemical haloes structure corresponds with the wallrock metasomatic zoning.
Geochemical haloes take up less volume than metasomatic ones. Content, dispersion of distribution, Au, Ag, Hg, As and other metals correlations increase from the original rock values at the further margin and out of the metasomatic haloes towards the internal zone, and reach its maximum in the latter. Au/Ag-ratio increases to the values comparable with the values in ore bodies. All this indicates that both types of haloes and ores form within one and the same ore forming process.
In combination with the given facts mantle isotope relations of sulfur in sulfides and carbon in carbonates, femic elements contrast anomaly (phosphorus, titanium, magnesium, ferrum, manganese), that are present in ore and metasomatites, the phenomenon of mass dissolution of quartz replaced by carbonates are considered to be signs of the fluids alkaline deoxidizing regimes and these generation in the anomalous mantle. Fluid regimes inversion from alkali deoxidizing to acidiferous oxidizing at the mixing border of hot depth metalliferous solutions with ground waters causes organometallic and non organic complexes destruction and ore formation.

 

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