Download Surface Geochemical Sampling:

This presentation was prepared for delivery at the MCA (Victoria) Technical Day in Melbourne, in
August 2013. The Technical Day was cancelled, but as I had put some effort in preparation, I felt that
publishing it was the next best option.
I also took the opportunity to present this at the GPIC-Bendigo meeting on the 12th November, 2013
For Further Information:
Geoff Turner
BSc(Hons), MSc, FAIG, RPGeo, GSA, SEG
Mobile:
0412 114322
Email:
[email protected]
Web:
www.explorems.com.au
Disclaimer
I am an economic geologist with more than 30 years field exploration experience. While my credo is to
never stop learning, I am not a geochemist. Some geochemists may have issues with certain elements
in this dissertation, but this is based on my findings and is supported by research by experienced and
practical geochemists.
This method works.
31st October, 2013
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Lockington – 80 km north east of Bendigo
In 2004 an AirCore drill hole sited on a coincident soil anomaly and structure
interpreted from regional gravity imagery intersected 15 metres with an average gold
grade of 0.53 ppm, under 54 metres of unconsolidated sediments of the Murray
Basin. Best intersection of the program was 15 metres averaging 1.27 ppm gold under
78 metres of cover.
This discovery AirCore program was based on near-surface soil sample lines that
produced near coincident anomalous silver, gold and arsenic geochemistry. Northsouth trend lines were interpreted by lining up peaks in the stacked profiles of the
pathfinder metals – shown here as magenta stipple.
Subsequent AirCore drilling on 40 or 80 metre x 400 metre spacing (coloured
triangles) show how close these soil geochemistry trends follow the basement
mineralisation.
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Four Eagles – 75 km north of Bendigo
Near-surface soil geochemistry was also applied to the Four Eagles gold discovery
near Pyramid Hill to test its effectiveness in a similar environment. Gold, arsenic and
antimony data reveal coincident peaks along roadside reserves, which can line up in
north-south trends. These surface geochemical trends roughly approximate the
known basement gold-arsenic mineralisation that had already been, and is still being
defined by AirCore drilling (Catalyst Metals Ltd, ASX Code: CYL).
While not exactly mirroring the basement geochemistry as at Lockington, the soil
geochemistry is active enough to generate drilling interest when it is seen to support
other tools such as regional geophysics interpretation.
But still there is scepticism –
• How can the metals migrate from basement to surface?
• What is the mechanism?
• Surely the presence of aquitards (dense clays such as Torrumbarry Formation) and
saturated aquifers (Calivil Sand and stringer sands within the Shepparton
Formation) will prevent migration of metals as ions.
• And many of the aquifers within the Murray Basin cover sequence have northflowing water – surely this should smear out any geochemical response?
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Shallow auger sampling during the mid-2000s along roadside reserves surrounding
the old Nagambie Mine produced variable results that were difficult to interpret.
Samples were taken at depths up to 3 metres within Shepparton Formation clays, and
were not of weathering basement – even though the samples were recorded as ‘C’horizon!
The high values do not relate to the geology, which is controlled by dominant EastWest striking faults and folds.
And samples were subject to a range of assay techniques –
• Fire assay for gold and silver,
• Bottle Roll cyanide leach for gold,
• Four acid digest and AAS for gold and silver,
• Aqua regia digest and AAS for arsenic and antimony,
• Sometimes no pathfinder metal support.
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The area was resampled during 2011 to 2013, taking samples from near surface
(immediately below the root zone of the grasses). Gold determination by 4-acid
digest and AAS, and arsenic/antimony by partial leach extraction. Anomalous gold
values are supported by anomalous arsenic and antimony (not shown) and
anomalous trends relate to the regional geology.
So near surface sampling can give a clearer picture of the regional geochemistry than
deeper auger sampling.
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Two common methods of determining gold in soils have been compared.
MMI (gold) incorporating a weak acid leach routinely gives lower results and poorer
anomaly definition than Cyanide Leach methods. This variation cannot be explained
by sample size – there must be gold that is amenable to leaching by the stronger
cyanide solution than by the weaker MMI acid solutions.
(Samples taken on Shepparton Formation clays north of Bendigo)
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Orientation Study at Lockington supervised by Keith Scott (CSIRO Mining & Exploration)
showed that:
• The near surface material is up to 80% clay (<63 micron), with less than 10%
greater than 1 mm
• The coarser fraction is dominated by ferruginous concretions
• Gold and silver were found in the finer fraction
• Arsenic, antimony and base metals did not report to the fine fraction, but were
present in the coarse fraction
Recommendations
• Assay finer fraction (-25 micron) for gold and silver
• Assay coarse fraction (+1 mm) for arsenic, antimony and other pathfinders
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Geological Survey of Victoria Study
In 2007 Gold Fields & EMS collaborated with the GSV to investigate the possibility of
using soil geochemistry in areas of thick cover sequences.
This study applied near surface sampling techniques, various assay techniques, and
downhole geochemistry through the cover sequence.
Results and all associated data are available from the DPI Online Store as Gold
Undercover Report 10.
Why do Partial Leach methods work for As, Sb and other base metals, but do not
work for gold and silver?
The main conclusion was that soil geochemistry can be used in thick sequences of
Murray Basin cover.
But while the report canvassed various methods of ion transport, it could not reach a
conclusion on the transport mechanism.
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The second phase of the GSV study involved comparing the geochemistry down hole in an
attempt to track the migration of the metals. Samples were taken at 3 metre intervals during
the course of routine AirCore drilling over the suspected Main-Lees Trend at Lockington.
The image shows graphically the variation of arsenic with depth (ppm along top axis), with
an image of the interpreted cover sequence stretched to fit the graph. Top clays in beige or
tan colour, overlying orange and yellow stringer sands. Below an iron rich horizon (Karoonda
Surface shown in red) are the plastic clays of the Torrumbarry Formation (grey) and brownish
sands of the Calivil Formation.
These overly the clean and organic clays of the Olney Formation (brown) and underlying
Warina Sands of the Renmark Group (grey). The Calivil and Warina Sands are normally water
saturated, and often over-pressured.
Arsenic is slightly elevated near surface, and elevated in the basement saprolite zone (pink
hatching). Arsenic is also elevated in the organic clays (brown, just above saprolite).
Metal values are very low between basement source, and surface. If the ions migrated
through the cover sequence, then there should be evidence of this migration, such as steadily
decreasing metal values.
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It has long been recognised that trees and shrubs can absorb precious and base
metals and store them in various organs such as leaves, woody material and bark.
This fact has driven many companies to employ vegetation sampling as a primary
geochemical exploration tool.
Example in this slide was taken from a study of metal concentration in tree organs
over gold deposits in PNG.
Workers at Massey University in NZ have studied plants for their ability to
decontaminate gold tailings dumps, by absorbing arsenic and storing it in leaves.
They have also investigated the potential of plants for phyto-mining, whereby gold
and silver is extracted from the dumps by shrubs and the vegetation harvested for its
gold content.
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NANO -Particles
Other workers in this field have recognised the potential of plants to produce nanoparticles of gold and silver on a commercial scale, for later uses in medicine,
electronics and others.
These images show how silver is stored as very small nano-particles in leaves of
alfalfa plants. Some nano-particles are as small as a dozen or so atoms!
The coloured image is of gold nano-particles in a leaf.
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(a) Synchrotron image showing distribution of Cu (blue), Sr (oxalate crystals, green)
and Au (red) in natural leaf specimen. Strontium is a proxy for Ca; boxed area in a is
shown in b.
(b) Detailed view showing isolated particulate Au grains (red, arrowed) and two of
which are attached to the surface of Ca oxalate crystals (green).
(c) SEM–EDS map of twig section showing examples of Au (purple) in laboratorygrown plants.
(d) SEM BSE image of laboratory-grown plants in c showing plant structure with
brighter areas showing Au; boxed area in (d) is shown in e.
(e) Detailed BSE view showing distribution of Au (bright areas). Arrows point at some
examples of Au (including those particles attached to Ca oxalate crystals); boxed area
in (e) is shown in f.
(f) Detailed BSE view showing Au particle 8 μm in length.
(g) Spectra showing identification of Au by SEM–EDS. Scale bars in a, c, d and b, e and
f indicate scale as 500, 50 and 5 μm, respectively.
Melvyn Lintern, Ravi Anand, Chris Ryan & David Paterson, 2013. Natural gold
particles in Eucalyptus leaves and their relevance to exploration for buried gold
deposits. Nature Communications 4, Article number: 2274
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Vegetation Sampling by Proxy
Vegetation sampling in Victoria is unsuitable as:
• Do not present the same species at a consistent density of cover over the project
area,
• Sap movement (hence metal movement and storage) varies according to season,
• Field assistants require extra training in species recognition,
• Some sampling media (such as leaves) are difficult to get to,
• There is evidence that the gold concentration within leaves can be quite variable
within a single tree.
But we can assume that that just about every square metre has had a tree or shrub
on it over the last 10,000 years - the time for the soil profile to develop.
So there is an even vegetation distribution over time.
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Vegetation Sampling by Proxy
All vegetation material eventually falls to the ground and is incorporated into the soil
near surface. Polarised gold and silver nano-particles attach strongly to clay particles.
Clay particles are layered, consisting of layers of molecular groups. Each layer is
polarised, which is why wet clay is slippery – water molecules (also polarised) can
attach to both sides of the clay molecular layers, allowing the layers to slide past each
other by reducing the interlayer electrostatic attraction.
Similarly, gold nano-particles can attach to the inter-layers of clay particles.
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Observation
Explanation
Cover sequence tends to be very low in
metals, with enrichment at surface
Metals transported through the cover
sequence by deep-rooted trees
Surface gold & pathfinder metal
anomalies are vertically above basement
mineralisation
Metals transported vertically by trees, so
are not dispersed by groundwaters.
Near surface sampling is more effective
than deeper sampling
Gold, silver and other metals are
incorporated into topsoil through
decaying vegetation
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Observation
Explanation
Gold and silver report to clays
Polarised nano-particles of gold and silver attach to
polarised clay layers
Arsenic, antimony and base
metals report to coarse fraction
Pathfinder metal salts are in solution in vegetation
cellular fluids. These salts are later absorbed onto
ferruginous concretions as hydrated oxides or salts
through normal geochemical processes during
vegetation decay
Partial Leach methods are not
very effective for gold and
silver
Au and Ag are bonded too tightly to the clays, so only
stronger solutions like cyanide or total digest methods
work
Partial Leach methods work on
As, Sb and pathfinders
Metals loosely bonded to matrix as soluble salts or
oxides
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Recommended sampling protocols for Murray Basin exploration
The horizon to be sampled must be carefully selected at each site. Consistency of
sampling is the key.
Ensure that plastic sieves and receiving pans are used.
Initial sieving to remove gravels (transported?) and coarse organic matter (including
spiders).
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Sieving out the ferruginous concretions (pisolites) can be time consuming, and
sometimes can result in a very small sample for assay.
If Total Iron is determined from the whole sample, Metal:Iron ratios can substitute for
the coarse fraction determination.
There are limitations. This does not apply for soil that is sand dominant, although
you could try extracting the clay component from the soil and treating this for gold
and silver.
And the methodology does not apply to some parts of the Murray Basin.
EMS has extensive knowledge of the Murray Basin sequence in Victoria, and can
assist in planning your geochemical survey.
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Classic treatments of geochemical data can lead to confusion. On rare occasions,
contouring or gridding can work, but stacked profiles give a better picture of the
geochemical variation along a soil line.
Increased depth of cover can (and will) lead to more subdued surface geochemical
response. Prior knowledge of cover thickness helps.
Different soil types will inevitably have different geochemical responses. Alluvial
influxes through regional flood events will dilute (or at least contaminate) the surface
geochemistry. Other influences to consider include aeolian input, soil erosion,
downslope movement of soil, etc.
By “domaining” the soil types, you can determine threshold values for each metal
within each domain, and produce “Anomaly Factors” according to the specific
geochemical response of each soil type.
This helps in regional interpretation.
Soil geochemistry should not be used as a “stand-alone” method of drill target
selection. Soil geochemistry is just another tool in the explorer’s toolbox.
Contact:
Geoff Turner – mob: 0412 114322 or [email protected]
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