An Evaluation of Partial
Extraction Techniques
at the Wyoming Cu-Au Prospect, Parkes Region

B.I. Cruikshank¹,  K.M. Scott² 
and I. Baillie³

 

1.     
CRC LEME c/-AGSO, GPO 378, Canberra, ACT 2601

2.     
CRC LEME  c/-CSIRO
Exploration and Mining, PO Box 136, North Ryde, NSW 1670

3.     
Formerly  SIPA Exploration NL now Omya Southern Pty. Ltd., PO Box 52, Bathurst, NSW 2795

 

Key  Words: gold exploration, regolith, transported
cover, partial leaching, Parkes

Abstract

By focussing on geochemical species likely to be derived
from mineralisation, and minimising the contributions of lithochemical species,
partial extraction techniques have the potential to dramatically improve signal
to noise ratios during geochemical surveys. 
However the proliferation of different techniques using proprietary
methods makes the use of the information difficult.  This study evaluates several of the partial extraction techniques
commonly used in the temperate to semi-arid areas of the central western part
of New South Wales.

 

At the Wyoming Prospect, near Tomingley, 60 km north of
Parkes, vein-related mineralisation occurs below 50 m of transported cover.
Partial leach methods (Regoleach, MMI and Enzyme Leach) and aqua regia
extractions all show an Au anomaly on a soil traverse at the Wyoming
Prospect.  Regoleach appears to be the
most aggressive of the leaching procedures, at least for Mn and  base metals.  MMI (Digest B) is also quite aggressive for Au and shows results
similar to those from the aqua regia digest. Consideration of these factors,
and the cost of analysis, suggests that aqua regia may be the most
cost-effective digest for Au and Cu under these conditions.

Introduction

The Wyoming Cu-Au Prospect, 60 km north of Parkes and 1.5 km
south of Tomingley (Figure 1), has been explored by SIPA Resources and
Michelago Resources to test strike extensions to the Myall United (McPhail)
Gold Mine.The area is covered by up to 50 m of 
alluvium (red silt with some pebble bands and quartz grit) and is
underlain by Goonumbla Volcanics (Sherwin, 1996).  The volcanic unit comprises andesitic lavas and volcanogenic
sandstone and conglomerate, and is the host to mesothermal vein Au
mineralisation at the Myall United Mine (Bowman et al., 1982; Sherwin, 1996) and to epithermal Au-Cu mineralisation
at Peak Hill (Sherwin, 1996).  Copper-Au
porphyry-related mineralisation also occurs at Goonumbla (Northparkes).  Mineralisation at the Peak Hill Mine contains
pyrite, tennantite, gold, chalcopyrite, sphalerite, galena, chalcocite and
covellite in quartz pyrophyllite breccia (Bowman et al., 1982), which suggests that pathfinder elements would be Au,
Cu, As, Pb and Zn, plus Ag and Sb. 
Mesothermal vein deposits in the Tomingley area, including the Myall
United Mine, are less well characterised, but Ag and Cu are associated with the
Au (Bowman et al., 1982) and
gossanous veins from the  McGregors
Prospect (south of Peak Hill) contain 3.3 ppm Au, 1.9% As, 24 ppm Sb, 110 ppm
Cu, 110 ppm Zn and 16 ppm Pb.

 

Forty-five aircore holes were drilled at the prospect,  with 5 containing greater than 1 g/t Au
(Michelago Resources, 1997).  The best
intersection was 16 m at 1.7 g/t  Au in
hole WY29.  Drill hole samples were analysed
for As and Cu by SIPA.  An idealised
profile of the 6393440N section shows 
mineralisation  in residual
saprolite covered by up to 50 m of alluvium (Figure 2).

 

Aliquots of the samples from along that traverse were
submitted for analysis by MMI (Mobile Metal Ion), Regoleach, Enzyme Leach and
after aqua regia and total digestions to evaluate the potential of partial
leaches in the temperate to semi-arid areas of central western New South Wales.

The Partial Extraction Method

The three partial extraction methods, used on samples from
the 6393440N section, aim to minimise lithochemical masking effects from
surficial material by extracting only metal ions which have a high probability
of having migrated into the surficial environment from mineralisation at depth.
Each  of the methods claims to target
different species in the samples, as shown in Table 1.

 

Table 1.  Nominated targets for each of the partial
extraction methods used.

 

Method	Target
Regoleach	Hydrated Fe-Mn oxides, clays, and carbonate and sulfate precipitates
MMI	Exterior of soil particles without affecting the substrate
Enzyme Leach	Highly-reactive, amorphous Mn oxide coating mineral grains

 

Comparison of results from the three methods

Amounts of chalcophile elements extracted

Table 2 summarises results obtained for the three partial
extraction methods, and the aqua regia digest, on the twenty-eight soil samples
from the traverse. The most obvious difference in the partial leaching results
is in the Cu, Pb and Zn values for Regoleach which are about 40 to 120 times
those for MMI, and between 130 and 1300 times those of Enzyme Leach.  In fact, Cu, Pb and Zn values for Regoleach
vary between 18% (Zn) and 65% (Pb) of the total values (Table 3). Average Au
values are of similar magnitude for the aqua regia digest (2.63 ppb), Regoleach
(0.79 ppb) and MMI (1.27 ppb), but Enzyme Leach again is very much less (0.12
ppb).  The relative results are shown in
Table 3, as a percentage extraction of the total value.  In all cases Enzyme Leach is the lowest,
with less than 1% extracted for most elements.

 

Table 2.
Means and standard deviations for different dissolutions (ppb, unless otherwise
indicated)

 

 

 * estimate only,
most values at, or below, the detection limit

 

Table 3.  Relative results for the three extraction
methods, Regoleach, MMI and Enzyme Leach, and 
for the Aqua Regia digest, as a percentage of the total value.

 

 

              Note:        *   MMI Digest A   ** 
MMI Digest B

 

 

The low values reported for Enzyme Leach are consistent with
the method’s modus operandi, that is
to dissolve amorphous MnO₂ while leaving crystalline MnO₂
and other mineral phases unaffected (e.g. Clark et al., 1990).  At less than
2% of the total Mn, this would represent a very much less abundant source than
the hydrated iron-manganese oxides, adsorbing clays, carbonate and sulfate
precipitates, etc which are postulated as sources for Regoleach and,
presumably, the metals adsorbed on soil particles for MMI.  The complexing agent, gluconic acid (CH₂OH
(CHOH)₄COOH), formed during the Enzyme Leach dissolution process
(Clark et al., 1990), could have
difficulty in preventing Au from readsorbing (Hall et al., 1995; Gray et al.,
1998) after dissolution with the reactive MnO₂.  In contrast, MMI uses two digestion
solutions: Digest A for Cu, Pb, Zn and Cd, and Digest B for Au, Ag, Ni, Pd and
Co (Mann et al., 1993). Digest A is
acidic whereas Digest B is reputedly alkaline and may be cyanide-based since
all five elements determined on that digest form soluble, stable cyanide
complexes.  This should have little
trouble keeping Au in solution and MMI Au values are closest in magnitude, and
distribution, to the aqua regia and ‘total’ values. The Regoleach extractant
must be considered as the most aggressive since it dissolves significantly more
of the major base metals with extractions ranging from about 10% for Ag to over
90% for Mn, far more than the other methods. 
However, it appears to be a less effective at extracting, or retaining,
Au than does MMI Digest B.

Distribution of the Elements

Gold: The
distribution of Au values along the 6393440N traverse for aqua regia digest,
Regoleach, MMI and Enzyme Leach are shown in Figure 3. MMI and aqua regia
digestions both show twin peaks whereas Regoleach and Enzymeleach show only
single peaks.  Because the  MMI digests remove only the loosely-bound
metal ions from the exterior of soil particles (Birrell, 1996), it appears that
all the Au in the soil at Wyoming is ‘loosely’ bound.  All four methods would have drawn attention to the area near or
above the Au mineralisation  but the
shape and position of their peaks varies in detail.

 

Copper: Copper has
also been reported for all three partial extraction methods and for the aqua
regia digest (Figure 4). All methods show increased Cu to the east of the area
drilled and may indicate high sub-surface Cu values. However, only Enzyme Leach
would have inspired any interest in Cu in the area above the mineralised zone
and this may have been confused somewhat by the higher and unconstrained values
in the east.

Evaluation of the methods

A Scorecard (Table 4) indicates how the individual methods
performed overall.  Included is the cost
of analysis per sample, the number of elements reported (Enzyme Leach winning
easily), the success or otherwise of Au values in defining the mineralised
zone, and the success of other elements in defining the zone.  Other elements which are considered as
potential indicators of the mineralisation, either through precious metal or
chalcophile associations (in chalcopyrite, chalcocite, covellite, galena,
sphalerite and tennantite), are shown in bold.

 

Table 4.  Scorecard for partial extraction methods
including assessment of success, and costs.

 

 

As indicated above (Section 3.2) and in a more extensive
study (Cruikshank et al., 1999),  ‘Yes’ indicates the element/method showed
clear peak(s) over the mineralised zone, with little or no confusion due to
peaks/noise outside the zone, and 
‘Maybe’ indicates positive responses over the zone which were confused
or compromised by significant responses and/or noise outside the zone.

 

While providing a minimum of information with only two
elements (Au and Cu), in this exercise the aqua regia digest defines the mineralised
zone by its Au response as effectively as any of the more expensive partial
extraction methods.  The fact that the
magnitudes and patterns of the aqua regia, Regoleach and MMI Au values are
similar (no more than a factor of 3 times different) suggests that there is
little or no particulate gold from the transported soil to confuse the issue,
and, therefore, the partial leach methods show little real advantage.  The limited depth to mineralisation (about 50
m) may not allow these methods to show their full advantage. Of the partial
leach methods, MMI probably had the best success rate with a smaller number of
relevant elements and was easy to interpret, Regoleach defined the mineralised
zone through its Au values alone, albeit with a different pattern to the other
methods, and Enzyme Leach showed some good successes but interpretation is
complicated by a plethora of information (60 elements reported).

Acknowledgements

The authors thank Michelago Resources NL and SIPA
Exploration NL for their cooperation in this study,  and Drs Ken McQeen and David Gray for their  constructive criticisms of  the manuscript.

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