Open File Report 21
Hydrogeochemistry in the Mt Gibson Gold District
Research was conducted into the hydrogeochemistry of groundwaters
within the Mt Gibson mine area and in the surrounding district.
This work involved determination of field parameters such as pH
and Eh, laboratory analysis of water samples for major and trace
elements, isotope determinations (D and O18), computer speciation
of analytical data, and statistical analysis of the water data.
The groundwater system is dominated by a northward saline drainage
system. Groundwater flow along this drainage appears to be restrained
by an underground sill about 7 km north of the mine area, resulting
in highly saline groundwaters within the mine region. This saline
groundwater appears to flow back, south into the mine area, at depth.
Thus, the north section of the mine area has fresher waters (about
3% TDS) overlying hypersaline water (>13% TDS).
Based on the major element and isotope analyses, the mine groundwaters
were resolved into a number of hydrogeochemically distinct water
masses. In particular, the waters from drill hole sample sites 600
m west of the major area of supergene Au mineralization at Midway
were identified as probably originating from contact with granitic
rocks. The other mine groundwaters appear to be associated with
mafic or ultramafic systems.
Waters within the Midway area showed highly anomalous characteristics,
being high in dissolved Au, Fe, Mn, Co, Cd, Ba and I, and having
low HCO3 concentrations. These observations are explained as being
due to weathering of sulphide minerals. Down gradient of the Midway
area, the groundwater becomes acidic, due to oxidation-hydrolysis
of the dissolved Fe. This has led to major dissolution of many metals,
particularly (in order from least to most enriched) Cd, Co, Ni,
Zn, Cu, Cr, Al and Ag. This enrichment is related to the base affinity
of the metals.
Soluble Au was only observed above the detection limit (0.05 µg/L)
within the mineralized area. Two major anomalies were recognized:
the first, within the Midway area, may represent dissolution of
Au by thiosulphate; while a second, more localized anomaly, within
the N2 pit area may represent dissolution by chloride.
On the basis of this work, soluble Au analyses could be used at
this site to indicate areas of Au mineralization at both a district
and a mine scale. Thus, measurements of dissolved Au may represent
a useful adjunct to drilling during Au exploration, particularly
with respect to buried mineralization.
Last updated: Tuesday, January 04, 2000 01:42 PM