AIG Article

Groundwater is a chemically reactive medium. With its dissolved gases (O₂ and CO₂) it contributes to the break down of rocks and minerals and production of the regolith. At depth, groundwater can transport ore and host rock elements both laterally and vertically. Groundwater therefore constitutes an important geochemical exploration sample medium in regions where thick cover caps targets such that they are invisible to surface geochemical or geophysical exploration techniques.

The landscape of many older continental cratonic regions of Australia, South America or Africa, and tectonically active continental margins such as the Andes, are typically deeply weathered. Many of these regions have undergone extensive erosion with thick valley fill deposition or are capped by younger sediments or outcrops of ferricrete, silcrete or calcrete duricrust. Geochemical sampling of surface sediments, rocks and soils in such regions is likely to result in collection of mixed-age, mixed-source, lithologically diver materials. The geochemical signature from deeply buried mineralisation interpreted from, for example, geophysical or other remote exploration, or even from the geology or alteration about such a body, would be subtle or non-existent.

Ore body or country rock signatures can however be interpreted from comprehensive field and laboratory chemical analyses of groundwater samples collected using standardised procedures from widely spaced drill holes or existing water bores. Field measurements include pH, Eh, salinity, temperature and reduced Fe; and trace element analyses are undertaken in the laboratory. Changes in groundwater geochemistry across a region can reflect changes in country rock type, alteration, or interaction of the groundwater with a body of mineralisation.

Effective interpretation normally requires an understanding of relatively simple solution geochemistry. One of the reasons groundwater geochemistry has been underutilised as an exploration tool in covered terrains is the lack of understanding of the basic methods of data interpretation. This is due in part to the general paucity of teaching of field applications of solution geochemistry as part of university course work in geology. To overcome some of this deficiency a handbook has been prepared by Angela Giblin and published by CSIRO Exploration and Mining. The handbook illustrates principles and methods of application, modelling and interpretation of groundwater analyses in exploration programs, using real field examples.

An ongoing CSIRO project, focused on this work, has during the last three years compiled analytical data on some 5500 Australian groundwaters into a Microsoft ACCESS database. These samples represent groundwaters from most of the established mineral provinces and active exploration areas across Australia.

DETERMINATION OF PRESENCE OF MINERALISATION

Interpretation of exploration geochemical anomalism from stream, rock or soil analyses is normally based on the relative abundances of target commodity elements. With groundwaters however, this approach is only appropriate if the commodity element is rare, or poorly reactive with low temperature groundwaters, for example gold.

For elements that are less rare and exhibit higher degrees of reactivity with groundwaters and whose solubility varies with the groundwater composition (including changes in pH, redox and salinity conditions), anomaly identification is not simply based on absolute abundance. It is necessary to take into account whether prevailing conditions favour or reduce the element’s solubility. Because these conditions might vary across an exploration project area, the relative solubility of an element in any individual groundwater sample has to be considered separately from other samples to take into account any local differences in the groundwater geochemistry.

The general procedure used is to calculate the concentration (or solubility) of a target element in the actual groundwater assuming a minute but fixed mass of an appropriate target mineral is present. For example, in exploration for Zn mineralisation a mineral indicator might be a zinc carbonate, ZnCO₃. For available groundwaters across an area, the calculated Zn values for a given water composition where Zn (as this carbonate) is present in the rock are then plotted against the actual analysis values for Zn. By this means it is possible to determine which groundwaters (if any) correspond to groundwaters that are in equilibrium with zinc carbonate mineralisation.

Figure 1: Comparison of measured concentrations of Zn solutes with those calculated for the same groundwaters if zinc carbonate were present. This allowed assessment of proximity of groundwater sample locations to a VMS Pb-Zn-Ag deposit.

DETERMINATION OF ROCK TYPE

Exploration indicators in groundwaters are not restricted to the ore elements. Strategic elements include both major constituents (Ca, Mg, Na, K, Cl, S and carbonate species) and trace elements that occur at low and sub-ppb ppb concentrations such as U, F, Cr, Ni, Rb and Mo that may be useful lithological indicators to targeted host rocks.

The principal sources of major elements, Ca, Mg, Na and K in groundwaters are rock- forming silicate minerals. Dominant aquifer lithologies can be recognised from the ratios of these major element concentrations in a groundwater. Consistent relative abundances include: Na and K > Ca and Mg in waters from aquifers that geochemically match felsic igneous rocks such as granites or rhyolites; Ca and Mg > Na and K in those that match mafic igneous rocks such as basalts. Waters reflect intermediate compositional relationships where aquifer geochemistry has compositions between these extremes. Waters from ultramafic aquifers show Mg > Ca , Na and K. A simple and effective procedure for identifying the variations across an exploration area for concealed mafic rocks is, for example, to plot the relative concentration of Mg in groundwaters. This relative or normalised value, referred to as NMg, is the absolute Mg content of the groundwater, normalized (divided) by the total concentration of all major cations using equivalents/L as the unit of concentration.

Figure 2 illustrates a clear change in rock type across a base metal prospect centred on a gravity anomaly. The distribution of NMg indicates a zone in the middle of the gravity anomaly of low NMg, possibly felsic units, surrounded by a region of high NMg, indicative of rocks of more mafic character.

Figure 2: Variation of NMg (normalised Mg) across an exploration target centred on a gravity anomaly.

EQUILIBRATION OF GROUNDWATER WITH VARIATION IN AQUIFER HOST MINERALOGY

A useful groundwater exploration application is the “Stable Mineral Assemblage” (SMA) indicator that can be calculated for a groundwater from its major and trace element composition. This can give a direct indication of the likely mineralogy of the rocks though which the groundwater has passed and equilibrated with, remembering that water is a reactive chemical medium. It can provide some indication of rock alteration and host rock mineralogy.

The SMA is defined as the mineral assemblage that is associated with a fluid in which no mineral is either undersaturated or supersaturated. The calculation of SMAs involves running a “reaction model” for each groundwater to determine a stable, albeit theoretical, equilibrium state. The model starts by allowing all supersaturated minerals to precipitate, and then continues with interactions among the system constituents with consequent fluid composition changes, and mineral precipitation or dissolution, until no further changes can occur. The suite of minerals (the SMA) in equilibrium with the groundwater at this point provides a portrayal of the groundwater that characterises its chemistry, and may thus constitute an exploration indicator, for example, indicate alteration assemblages or favourable host rock types.

An application of this procedure is in determining which ground waters are in equilibrium with serpentinite units in Cambrian greenstones sequences in Victoria. The greenstones are considered to be the likely source of gold that was later incorporated into the structurally hosted Ordovician gold deposits. In this case a water in equilibrium with serpentinite minerals (e.g. talc and antigorite) in the SMA would be considered to be a positive indicator of a favourable host environment in exploration for new Victorian gold deposits. This might help an explorer to undertake regional sampling to detect geochemically favourable zones in areas hidden beneath the the extensive younger basalt cover or sediments of the Murray Basin. Figure 2 below illustrates this.

Figure 3: Locations of groundwaters in which the Stable Mineral Assemblage includes talc and antigorite.

The technique might also have wide application in South America, for example in Argentinian Patagonia, where large areas prospective for low sulphidation epithermal gold are concealed below basalt cover, or in the Peruvian Andes, where thick glacial material covers extensive areas with high sulphidation gold potential.

Another related exploration application from reaction modelling of groundwater compositions is one that determines mineral ratios that appear to be responsible for the changing compositions of a series of groundwaters across a region. Iterations of this approach with various mineral ratios (related to possible local rock units) can determine which waters in an exploration area are potentially contacting (altered) rocks that are expected to host target mineralisation. The procedure involves the use of groundwaters known to be associated with a specific ore deposit. These include both low salinity, recently formed groundwaters and end member, more saline groundwaters. Starting with the low salinity water, and progressively modelling reaction with the relevant minerals (kaolinite, chlorite, muscovite etc) in chosen ratios, produces a reaction path that links groundwaters known to be in contact with the mineralisation. Drawn across an appropriate silicate mineral phase diagram, this path can identify other groundwaters in the exploration area that are likely to be in contact with similar (altered) host rocks.

Figure 4: Kaolinite - Muscovite - Chlorite phase diagram illustrating evolving paths of groundwater composition for a low salinity groundwater from the Stawell gold deposit in Victoria as it interacts with various mass ratios of chlorite and muscovite. The solid line depicts reaction with fixed masses of 1 millimole of muscovite and 0.002 millimoles of chlorite. This reaction progressively evolves into groundwater compositions similar to those of higher salinity Stawell groundwaters. The ellipse illustrates the fields covered by approximately 900 groundwaters studied in the region that hosts the northern Victorian goldfields. Crosses are individual analyses.

OTHER METHODS FOR PROCESSING GROUNDWATER DATA

In addition to standard statistical inferences from groundwater constituents considered separately, there may be additional exploration information that can be extracted by considering part or whole of a groundwater chemical data matrix as an entity using methods such as multivariate descriptive data analysis.

An example of this might be the application of K-means clustering of Principal Component scores 1, 2, 3 and 4 of a wide range of variables, for example, Cu, Pb, Zn, U, Al, Fe, Mn, Ba, Co, Sr, Ni, As, Li, Si, V, Rb, Cs, NCa, NMg, NK, NSO4, Sc, F, Au, Mo and Ge from say, a base metal exploration target centred on a deep geophysical anomaly. A cluster identified by this process may delineate discrete zones within the overall geophysical anomaly that might reflect critical host lithologies or an alteration envelope.

CONCLUDING REMARKS

Groundwater geochemistry has been under utilised historically in routine exploration in Australia and elsewhere. This is due to the ease by which simple surface (geochemical and geophysical) sampling methods have been able to delineate mineralization in exposed or shallowly disposed bodies of mineralization. With readily accessible targets becoming harder to find and exploration now extending into areas with deep, often intensely weathered, cover these surface methods have become much less effective or entirely inappropriate. Groundwater geochemistry however enables exploration to be undertaken in areas where cover is extensive, where deep weathering has obliterated any surface expression of mineralisation, or where mineralisation is at such a depth that there is at best only a remote geophysical signature.

Groundwater geochemistry offers scope to assist in determination of the character of deep geological or geophysical targets from a limited number of boreholes, to refine location, size and orientation of host target zones, or directly demonstrate presence of alteration or mineralisation. For deep mineralisation the cost savings may be significant.

The methods outlined highlight the potential for application of groundwater geochemistry to explore in areas hitherto out of the range of traditional exploration techniques.

REFERENCES

The handbook, “Groundwaters – Geochemical Pathfinders to Concealed Ore deposits” by Angela Giblin is published by CSIRO, is available for download from the web at:

http://www.syd.dem.csiro.au/unrestricted/dempublication/publicat.htm