Traces of ore-forming heat-and-flow paleo-systems
|Location||International Geological Congress,oslo 2008|
|Holding Date||15 September 2008|
Genesis of mineral deposits is being carefully studied in terms of geochemistry since the beginning of last century. But, to form the deposit or any anomalous accumulation of chemical element in the Earth crust, we need not just chemical elementsas such, but, also, source of energy that facilitates relocation of the elements and their chemical interaction with other elements, solutions, minerals, and rocks.
At the same time, each geologist knows that any report on geological mapping, exploration or prospecting contains information on spatial relation of ore objects with geophysical anomalies. The goal of this study is to define, if geophysics can trace energy paleo-sources, and if so, how position of these sources correlates with location of ore objects.
In the ongoing project on evaluation of mineral potential for frames of crystalline shields in Russia, it was established that deep seismic sounding (DSS) and gravity data contain information correlating with two kinds of endogenic ore objects, specifically - the ones related with 1)mafic-ultramafic and 2)acid magmatism. In both cases we observe zones of seismic transparency in the DSS cross-sections, but gravity pattern is different. In the first case, layered density model of the Earth crust reveals correlation of positive density anomalies from Moho through upper crust, which can be interpreted as a conductor of mafic and ultramafic intrusions containing PGE, Cr, and Ni-bearing deposits. In the second one, zones of density deficit in the middle crust correlate with zones of seismic transparency, and the ore objects locate in the marginal parts of the projection of the said zones to the surface. Correlation of ore objects with the margins can be illustrated by a simple example: up to 95 percents of known gold deposits in Aldan shield and Yenisei Ridge are located inside 12-km lent drawn along the axes of middle-crust maximum of density gradient encircling the zones of density deficit. Similar results have been obtained for Sn, W, Mo, Ag, sometimes - base and rare metals.
Interpretation of the said results can vary, but, from the author’s point of view, we have learned how to reveal elements of two kinds of heat-and-flow paleo-systems - purely conductive system, which brings to the surface basic and ultrabasic intrusions, and contaminative system, in which deep energy source initiates remelting of crust, and zone of this remelting acts as an engine for sub-surface processes.
From the point of technology:
- all the work have been done using 1:1,000,000 scale data;
- for gravity data processing, frequency separation have been used to build the layered density model;
- for DSS data processing, original method have been worked out in order to formalize zones’ of seismic transparency revelation.
The technology shows itself to be really good for orogenic environment, but in the shields and folded areas it also works well.
The approach can be of interest for metallogenic analysis, and tectonic interpretations.