CRC LEME
Open File Report 70
ABSTRACT
Geochemistry of weathered rocks at the Telfer Gold Deposit, Paterson
Province, WA
Wilmshurst, J.R.
A study has been made of a suite of surface rocks and sub-surface
profile samples from the Telfer Gold Deposit, Paterson Province,
W.A.
The surface rocks are weathered material, generally gossanous,
from the East and West limbs of the Middle Vale Reef (MVR) and from
the West Dome, together with three from the Fallows Field prospect;
color illustrations are available for definition and reference.
Profile samples are from the East and West limbs of the MVR, and
from the overlying E-1 Reef.
Chemical analysis was performed by ICP-OES, ICP-MS, INAA and XRF
methods, with data being obtained for the following elements: Ag
Al, As, Au, Ba, Bi, Br, Ca, Cd, Ce, Co, Cr, Cs, Cu, Eu, Fe, Hf,
Ir, K, La, Lu, Mg, Mn, Mo, Na, Ni, P, Pb, Rb, S, Sb, Sc, Se, Si,
Sm, Sr, Ta, Ti, Th, TI, U, V, W, Y, Yb, Zn, Zr.
In general, the subordinate elements can be divided into those
with an affinity for the iron-oxides, residual phases (primary and
secondary), secondary oxidate minerals and secondary silicates.
While the affinity of the several elements for the iron-oxides
is a function of pH during the formation of the oxides, and hence
the absolute concentrations may vary in a carbonate bearing system,
cobalt, copper, nickel, zinc, arsenic and molybdenum vary with iron
as expected. Tungsten, phosphorus and sulphur also associate with
iron-oxides but each is bimodal in its associations as are bismuth
and lead. Uranium, also, has an apparent affinity for the iron-rich
rocks. It is notable that zinc is present in only very low concentration.
Residual phosphate minerals, monazite and xenotime, probably account
for a high proportion of the lanthanide and phosphorus content;
the titanium-thorium association, as found in this study, might
be accounted for by 'residual' anatase, and zirconium by zircon.
The high degree of correlation between thorium and titanium was
unexpected, but is not unique. In the surface rocks, and in the
upper part of the profile, gold is resistate.
Secondary oxidate minerals are rare in the surface rocks and hence
are of minor importance except in that silver, antimony, bismuth
and lead are, in varying degree, hosted by such minerals which are
subject to leaching, thus accounting for variation in concentration
of these elements. In spite of the moderately high content of copper,
and of arsenic, secondary minerals are little in evidence in hand
specimen or at the macro-scale.
Although thorium is not so evident in the high-iron gossans, the
concentrations of both thorium and uranium are exceptionally high
in the surface and profile rocks and, while this may be a function
of the weathering history of the total sedimentary sequence, it
warrants further consideration.
The clay mineral, illite, might be expected to host boron but there
is no evidence for associations with other clays, although there
is evident distinction between potassic and magnesian zones. The
latter implies a higher local pH and buffer capacity.
The gossans are very largely directly derived from the sulphidic
precursor with general indication of a sulphide content in the form
of boxworks or pseudomorphs; some, however, are evidently deposited
from solution more or less remotely from the site of oxidation,
and the Fallows Field material shows indication of ferruginisation
well in excess of the initial sulphide content. Pyrite of larger
scale was common in the Telfer rocks, as evidenced by pseudomorphs
of both cubic and pyritohedral forms, and a number of the West Limb
gossans are derived from rock carrying well crystallised carbonate,
probably at the centimetre scale. Many of the West Limb gossans
carry visible gold, but it is not so visually evident in the East
Limb material, despite a high analytical content.
The profile samples, in their intersections of the MVR, show many
of the associations noted for the surface rocks, but since there
is a lower proportion of iron, and a higher clay content, the absolute
concentrations of the several iron-associated elements is usually
much lower; however, the association between titanium and thorium
is more clearly evident.
The peak concentration of the lanthanides is found at the upper
contact of the MVR, in association with phosphorus; this is, therefore,
possibly a primary feature. The data also suggest that there has
been mobilisation of the lanthanides since there is some separation
of the light and heavy members, although this could well be explained
by the presence of a second lanthanide phase with a much higher
proportion of the heavy elements.
If the epithermal model of Goellnicht et al., (1989) is correct,
the data, as given here, might be used with little modification;
however, it is likely that there would be lateral and vertical haloes
of the more mobile elements around the mineralisation channelways,
and, hence, variation in relative abundances. The present study
suggests that the elements can be prioritised for likely effectiveness
in further exploration in the Province, thus:
- High - Au, As, Cu, Co, W, Bi, Sb, ?Mo, ?Ag, Zn
- Probable - Lanthanides
- Possible - Th, Ti, U, Zr
- Informative - Fe, Si, Al, Ca, Mg, S, Mn, P
- [both Sn and B have a high potential priority, but further data
are needed].
However, if gold itself was not introduced largely by epithermal
activity in association with the above elements, and the mineralisation
has a more complex and perhaps multi-episodic origin, the relationship
of this suite of elements to gold, as seen in the Main Dome data,
might not hold on a more regional scale. (The relative intensity
of the several phases of mineralisation might vary from one locality
to another.) It is important to note, however, that the above suite
of elements would not necessarily be 'absent' in the case of a non-epithermal
genesis since their occurrence is not restricted to epithermal style
mineralisation. Nonetheless, it would seem prudent to regard each
member of the above suite as very significant in reinforcement or
confirmation, but not as an essential factor in a multi-element
strategy.
Last updated: Thursday, January 06, 2000 11:45 AM
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