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Open File Report 32

Geochemical and hydrogeochemical investigations of alluvium at Mulgarrie, Western Australia

Gray, D.J.

An integrated geochemical, chemical and hydrogeochemical survey of the Mulgarrie gold deposit was conducted by sampling RAB material, pit samples and groundwaters in the south-east margin of a palaeodrainage which overlies Au mineralization. The boundary between basement and alluvium is marked by decreased Mg, Zn, Ni and Au in the alluvium. Both basement and alluvium were identified as ultramafic by examining Ti/Zr and Cr/Fe ratios. Close to the surface, samples appeared to be depleted in Ti, relative to Zr, possibly due to organic acids dissolving Ti.

Elements associated with the surface carbonates are Au, which has been correlated with surface carbonates throughout the southern Yilgarn, and Th. These two metals are similar in that they are ordinarily highly insoluble but have enhanced mobilities in organic-rich horizons. Gold associated with carbonate is highly soluble and would be expected to have a high mobility in soil horizons. The soil enhancement could be caused by the metals been taken up by deeply rooting plants and then deposited on the soil surface.

There is a reduction in the magnitude of the surface Au anomaly from outcropping mineralization towards the middle of the drainage. This could be explained in terms of a lateral transport of Au, with dilution with distance, or by an upward movement of Au from the buried mineralization, with the magnitude of the soil Au anomaly being lesser with greater depth to mineralization. Iodide extraction has shown that surface Au close to the outcropping mineralization has a lower solubility than surface Au near the centre of the palaeodrainage (which has a similar extraction behaviour to Au in carbonate soils at other sites in the Yilgarn). Such distinctions may well be significant in understanding and utilizing exploration data.

Iron oxides control the geochemistry of a number of elements, particularly V, Cr, As and to a lesser extent, Sb, and Mn oxides have a critical role in the accumulation of Co, Ba and the REEs, and possibly Cu, Mg, Ca, Zn, Ni and S. In addition, Au in Mn-rich zones appears to be highly mobile, with only that Au which has been totally occluded by other phases being retained. Gold mobilized from such Mn-rich zones, either at Mulgarrie or at other sites, could be a source for secondary deposits.

Present-day groundwaters at this site are very unreactive, possibly because of low rates of sulphide oxidation. However, there are still several elements which have anomalous concentrations, due either to significant lithological enhancement (e.g., Ni and Cr with ultramafics) or to enhancements related to mineralization (e.g., Mn, Co, Ba, I and Ni), which are similar to groundwater enhancements observed at other sites. In addition, Mulgarrie groundwaters have specific depletions (K) or enrichments (Mg and SO42-) of some of the major ions, which vary at a regional level, and may be useful in distinguishing different lithological regions. These data suggest the potential usefulness of groundwaters at a regional, lithological and exploration scale.

At this site (contrary to observations elsewhere) groundwater geochemistry did not "see through" the alluvium. This may be a consequence of the low activity of the groundwater and/or the presence of adsorptive phases such as Mn and Fe oxides. Additionally, groundwater at Mulgarrie has very low concentrations of Au, probably due to the lack of a means for Au mobilization, which at other sites involves either acid-oxidizing conditions (Au chloride) or neutral sulphide weathering (Au thiosulphate). The Au contents of Mulgarrie groundwater would be classified as background at these other sites.

Last updated: Tuesday, January 04, 2000 04:02 PM


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