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Open File Report 70

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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