Mobile forms of elements: their use in geochemical mapping and exploration

Exploration Geochemistry
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Mobile Forms of Elements: Their Use in Geochemical Mapping and Exploration
Goldberg, I.S.[1], Abramson, G.J., Haslam, C.O.[2], and Los, V.L.

1. Senior Consultant, Ionex Pty Ltd., Sydney, New South Wales, Australia 2. Principal Consultant,Mineral Strategis, Melbourne, Victoria, Australia

ABSTRACT This short paper summarises some empirical aspects of mobile ion geochemical surveys; in particular, it highlights the significance of enrichment and depletion anomalies, which occur at all scales.

A new group of methods have been developed in various countries for exploring covered areas: MPF, CHIM, MDE, MMI, enzyme leaches, Fe and Mnoxide phase leaches (extraction) and others. All are based on the principle of selective extraction of the mobile and weakly bonded elements (MWBE). MWBE anomalies have their own specific qualities, the main one being a vertically-flowing migration of material from ore bodies. As a result, and MWBE anomaly is formed over an area broadly coinciding with the ore body’s projected surface area(Antropova, 1992). Information for the exploration of the geological situation at depth is, as a rule, limited to the registration of the MWBE anomaly. On a larger scale, complex structures of geochemical fields of various metals can often be observed in covered areas. Regional surveys of large areas frequently reveal major linear structures, with significant sections containing increased or decreasedconcentrations of metals. Figure 1 (a, b, c, d) contains part of a map (scale 1:500 000) showing geochemical fields of Ni, Mn, Zn and Ga. The area surveyed is in the northern part of the Bendigo-Ballarat Zone of Victoria, within the Lachlan Fold Belt of Eastern Australia. Overlying Murray Basin sediments are 20–50 m thick, and cover the gold-prospective Ordovician sedimentary rocks. The maps of thegeochemical fields (one sample point per 5–6 km2) show the distribution of metals (fulvate and humate) as found by the MPF method (Antropova et al., 1992). Apparent is a complex linear structure of increased concentrations of Ni/C % up to 0.55 and Mn/C% up to 4.2 A strong direct correlation between these elements can be observed. The correlation coefficient is +0.74. In this zone Zn/C% and Ga/C% showa linear structure with a dramatically decreased concentration. The correlation Zn/C% with Mn/C% and Ni/C% is characterised by an inverse relationship. The correlation coefficient is –0.5 for Ni/C% and -0.46 for Mn/C%. The width of the linear submeridional structure of these three elements is approximately 7.5– 10 km. It was monitored for 45 km in a meridional direction. Spatially, this structureis a continuation of the Ballarat-Wedderburn line (O’Shea et al., 1992).

Another example shows the distribution of elements in the Sunshine area; the diagram covers some 1000 m2 (one sample point per 5–6 km2) within the Melbourne Zone, immediately to the east of the BendigoBallarat Zone. Here the turbidite host rocks of Silurian-Ordovician age are covered by a Tertiary sand and clay overburdenapproximately 50– 100 m in thickness, which in turn is covered by basalt flows 20–60 m in thickness. Here is a series of Ni/C% and V/C% local anomalies, grouped in linear mutually perpendicular zones, 5–7 km wide, northwest and northeast in direction, the former being more clearly defined. A strong, direct relationship can be noted between these elements. Their correlation coefficient is +0.71.Another structure detected in the geochemical field is that of gold. The distribution of Au/C% has a cell structure, and is manifested as a combination of areas (cells) that contain positive concentrations, with other areas that contain decreased concentrations of Au/C%. These combinations create areas that are polar in form, and can be connected into systems according to this principle. The size...