Search CRC LEME :

powered by FreeFind

Publication Policy

Open File Report Series

OFRS Index


Regolith Maps

Annual Reports

Articles & Papers


Minerals Briefs

"Focus on Salt"

Other LEME Reports

Order Form

Open File Report 33

Investigation of hydrogeochemical dispersion of gold and other elements in the Wollubar Palaeodrainage, Western Australia

Gray, D.J.

The hydrogeochemistry, and the usefulness of groundwater as an exploration medium, was investigated for a 30 km length of the Wollubar palaeodrainage, an acid groundwater system that passes over mineralized Archaean rocks. Limited sampling was also conducted at the Golden Hope pit, about 1.5 km north of the palaeodrainage.

The Golden Hope groundwaters are very similar to deep waters from other mineralized sites, having neutral pH, low to moderate Eh and anomalous concentrations of Fe, SO4 (from sulphides), Mg, Ca, Sr and HCO3 (from carbonates). These data, and speciation results indicating groundwater equilibration with calcite, dolomite and magnesite, suggest that sulphides are dissolving at the weathering front, with the resultant acidity being neutralized by carbonate dissolution. Other minerals that appear to be in equilibrium with some or all of the Golden Hope groundwaters are gypsum (CaSO4.2H2O), barite (BaSO4), amorphous alumina [Al(OH)3] and ferrihydrite [Fe(OH)3.nH2O]. In addition, the groundwaters at Golden Hope are enriched in Ga, Mo, W, Ag, Hg, Tl, I, PO4 and Cs. These elements (possibly with As, which was not determined) may also have value for exploration. Most of these elements appear to be sulphide associated, so they may not be directly related to Au. However, a system for easily pinpointing sulphide enrichments may still have exploration value.

The groundwaters at Golden Hope are different from other mineralized sites in that they are not Au rich. The reasons for this difference are not clear, because in all known respects these groundwaters should be just as effective in dissolving Au, as a thiosulphate complex, as other sites previously investigated. If dissolved Au is to be used as an exploration tool, it is critical to understand why strong groundwater enrichments are only occurring at some sites.

Total salinity, K and Br data indicate that groundwaters in the palaeochannel to the west and to the east of the main Boulder-Lefroy shear are hydrochemically distinct. This is consistent with the proposal that the two systems join near the shear and then flow south into Lake Lefroy. The palaeodrainage system is acid, with pH varying from near 6 at the northern part of the study area down to 3 in the western arm. In general, results for Wollubar closely matched observations at other sites with acid groundwaters, with the major difference that the Wollubar groundwaters were Fe-rich, and therefore tended to have lower Eh values. The mineral phases that appear to be equilibrating with some or all of the groundwaters, and the elements being controlled are:

  1. fluorite (F);
  2. gypsum (Ca);
  3. barite (Ba);
  4. amorphous silica, for pH <4 (Si);
  5. jurbanite, for pH <5 (Al);
  6. amorphous alumina, for pH >5 (Al);
  7. ferrihydrite, for pH >4.5 (Fe);

With the exception of Au, for which speciation analysis works poorly, the minor elements were undersaturated with respect to their least soluble mineral phase, indicating that dissolution has occurred slowly and/or that concentration is being limited by other mechanisms such as sorption on, or co-precipitation with, iron oxides. Most metals, and particularly the base metals (other than the higher charge ions Al, Sc, Cr and U), showed no clear relationship with pH, possibly because their abundance was also affected by other hydrogeochemical or lithological factors. The concentration of REE is very high at Wollubar, both in the palaeodrainage and where acid waters are directly contacting Archaean rocks, being at least 5 times greater than for any other known surface water or groundwater in the world.

The palaeodrainage samples adjacent to the main Boulder-Lefroy shear showed particularly anomalous characteristics, being enriched in a similar "sulphide suite" as for the Golden Hope samples (Ga, Fe, Mo, W, Ag, Hg and Tl), in acid soluble elements (Sc, Y, REE and, relative to the observed pH, Al, Si and U) and also Au and Pb. This may represent acid weathering of a similar mineralized material to that at Golden Hope, and indicates that even high flow palaeochannel groundwaters can have solution characteristics relating to underlying mineralization.

Last updated: Tuesday, January 04, 2000 04:11 PM


Cooperative Research Centres Australia

About Us | News & Events | Research
Publications | Education | Staff Only | Links

Contact Us | Disclaimer | Sitemap
© CRC LEME 2004

CRC LEME is established and supported under the Australian Government's Cooperative Research Centres Program. The CRC Program is an Australian Government initiative which brings together research groups with common interests.

CRC LEME Core Parties