Recent developments in geochemical and electrical prospecting for Au and PGE; methods and case studies


Marchenko, A.; Blinov, K.; Ilchenko, V.

Vuorimiesyhdistyksen Tutkimusseloste. Sarja B 92-1: 118

2011


Prospecting for Au and PGE with the use of conventional techniques is often difficult, especially if noble metal deposits are deep seated, of small size, or covered by drift. Joint application of geochemical and geoelectrical methods makes it possible to get hidden information about gold-bearing and PGE-bearing objects, that helps to encounter prospective local areas and discover ore bodies. However, low-sulfide Au and PGE ores are weakly differentiated in geochemical and geophysical fields; geochemical and geophysical anomalies are usually of low contrast. These may require application of specific and/or advanced methods of geochemical and geophysical prospecting. First of all, the on-ground geochemical and geophysical prospecting should include: (a) stream sediment geochemical prospecting at reconnaissance stages and soil geochemical prospecting at follow-up stages; (b) Induced Polarization (IP) electrical prospecting, including surveys and soundings, with measurements of apparent or differential polarizability and apparent resistivity. Other methods are added if necessary. In areas of residual overburden, soil samples are analyzed for Au, PGE and other pathfinder elements by FA/AAA, ICP/AES, ICP/MS and other quantitative techniques. For ICP analyzes, near total multi-acid digestion is preferable, but aqua regia digestion is appropriate as well. It is essential to determine not only direct indicators such as elements forming enrichment zones, but also indirect indicators such as elements forming depletion zones and negative geochemical anomalies. Compilation of results of our case studies and published data allows choosing of most probable direct and indirect geochemical pathfinders of Au and PGE for main types of ores. In areas covered by drifts, it is necessary to develop and apply new geochemical exploration techniques which are deep-penetrating ones and enable revelation of concealed ore deposits. Modified Method of Diffusive Extraction of elements (MDE-M) is aimed at leaching from soils water-extractable and weakly adsorbed forms of elements that allows revelation of superimposed dispersion halos. Another promising technique is Modified Gaseous Geochemical Prospecting method with catching ground gases by means of the Dew-Type Traps (MGGP-DTT). These methods, in complex IP prospecting, were successfully applied to geochemical exploration for PGE in Kola Peninsula. IP electrical prospecting makes it possible to find zones with both massive and disseminated sulfides, graphite, quartz, other altered zones and ore-bearing veins. Special algorithms of IP data processing and interpretation enable construction of 2D and 3D geoelectrical models and estimation of sulfide content in depth. In turn, geochemical investigations of geophysical anomalies give information about element composition of their sources.