CRC LEME
Open File Report 57
ABSTRACT
Gold and associated elements in the regolith - dispersion processes
and implications for exploration. P241A Final Report
Butt, C.R.M., Gray, D.J., Lintern, M.J. and Robertson, I.D.M.
This report summarises the results of the two-year continuation
of the CSIRO-AMIRA Project P241 "Gold and associated elements
in the regolith - dispersion processes and implications for exploration."
The continuation had the principal objective of extending the research
into some important topics identified during the original project.
The data complement and expand those obtained previously and the
discussion of each topic draws on the results and conclusions of
each phase of the Project. The principal fields of research and
the outcomes are as follows:
- Elemental dispersion in deep regoliths was investigated at Lights
of Israel, Davyhurst and a background site at Mt Percy, Kalgoorlie,
with a minor study at Mulgarrie. The data confirm that, at local
to sub-regional scales, Au is the best indicator of Au mineralization.
However, sampling and interpretation must account for the distribution
of Au in the regolith, in particular, its accumulation in ferruginous
and calcareous surface horizons, its depletion for 5 to 20 m below
them, and further sub-horizontal enrichment at depth. The most
suitable pathfinders suite is As, Sb, W, +/- Bi, Mo, Pb; Ba, K
and Rb give expression to some alteration zones. These elements
provide confirmatory evidence for mineralization where Au has
been depleted or where the distribution is patchy or has little
focus.
- The determination of bedrock lithology from its weathered counterpart
is a major problem in regolith dominated terrain. The processes
of rock weathering and procedures for geochemical, mineralogical
and petrographic identification of different lithologies have
been summarized in an "Atlas of Weathered Rocks" produced
specifically for this Project. The Atlas illustrates the changes
in fabric, mineralogy and composition that take place during weathering.
It is far from an exhaustive catalogue of all lithologies, but
demonstrates the amount of lithological information that can be
obtained from chemical analysis and by careful observation, even
at the hand lens scale.
- The close relationship between the distributions of Au and pedogenic
carbonate in areas south of the Menzies Line was further confirmed
by detailed orientation studies at Lights of Israel and Zuleika
(Ore Banda), with a minor study at Mulgarrie. Zuleika and Mulgarrie
represent sites where soil Au anomalies directly overlie mineralization
concealed beneath barren palaeochannel sediments. Unfortunately,
due to natural contamination from nearby outcropping mineralization,
no unequivocal evidence could be obtained to relate the anomalies
and the concealed deposits, although such a relationship has been
demonstrated during routine exploration programs elsewhere. Further
soil investigations were conducted north of the Menzies Line,
where Au tends to be associated dominantly with ferruginous materials
in residual soils. However, in hardpan at Granny Smith, Au is
associated with lithorelics and the siliceous cement. There is
no obvious association with Mn oxides. Gold enrichment occurs
towards the base of the hardpan, overlying leached and possibly
depleted saprolite. Secondary carbonate is present in the lower
hardpan and upper saprolite; this has much less Au than pedogenic
carbonates, although much of it is highly soluble.
- Laboratory investigations of the chemistry of Au in soil have
led to an improved understanding of its associations with other
constituents and the mechanisms of the formation of Au anomalies.
Gold in carbonates is highly soluble and can be distinguished
from that associated with Fe oxides by partial extraction. Gold
and Ca are probably brought to the surface biochemically, being
cycled via vegetation, but the Au carbonate association in soil
is probably largely physical in origin, arising from precipitation
via evaporative processes. Gold in soil is highly labile and,
at least in part, as water-soluble complexes, so that under wet
conditions can readily be redistributed.
- Hydrogeochemical data show that the clearest indication of Au
mineralization is given by Au itself. Data interpretation depends
upon knowledge of the dominant dispersion mechanism. Where Au
is dissolved as a thiosulphate complex (Boags, Bottle Creek; Hornet,
Mt Gibson), Au distribution matches that of mineralization. Where
dispersion is by halide complexation (Panglo, Wollubar and parts
of Mt Gibson), the distribution of dissolved Au is controlled
by physico-chemical parameters unrelated to the presence of mineralization,
in particular where a high Eh is maintained by Mn oxidation in
acid groundwaters. Additionally, there is a strong antipathetic
relationship between dissolved Au and Fe, due to Fe2+ acting as
a reductant. The presence of other elements, such as As and Sb,
may indicate sulphides associated with Au mineralization. However,
groundwaters in some drainages have very high concentrations of
base metals and REE unrelated to the presence of such mineralization,
probably having been leached from country rock and concentrated
by evaporation.
- The spectral characteristics of Fe oxides and clays in the 0.4
to 2.5 µm wavelengths may have use in distinguishing different
minerals not always apparent by routine XRD analysis and, in field
logging or remote sensing, for mapping regolith materials such
as lateritic horizons, lag and exposed saprolite. Spectral identification
and semi quantitative estimation of micas has potential for identifying
alteration zones and micaceous lithologies, but practical application
is limited to materials having over 20% mica. Similarly, poor
discrimination of soil carbonates means that the presence of this
important sample medium cannot be recognized.
Last updated: Thursday, January 06, 2000 09:07 AM
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