Search CRC LEME :

powered by FreeFind

Publication Policy

Open File Report Series

OFRS Index


Regolith Maps

Annual Reports

Articles & Papers


Minerals Briefs

"Focus on Salt"

Other LEME Reports

Order Form

Open File Report 39

The petrography, mineralogy and geochemistry of a felsic, mafic, ultramafic and metasedimentary weathered profile at Rand Pit, Reedy Mine - Cue, WA

Robertson, I.D.M., Chaffee, M.A. and Taylor, G.F.

A near vertical sequence of variably-deformed, alternating, mafic and ultramafic metavolcanic rocks, interleaved with mica schists and black shales and intruded by porphyry pods, is exposed on the south face of the Rand Pit at Reedy to a depth of 75 m. The face has been mapped and each lithology sampled at approximately 20 m intervals from the base to the top. The samples have been studied mineralogically and petrographically to illustrate the mineral and fabric changes caused by weathering. Each sample has been analysed for 55 (major and trace) elements and the drying losses, ignition losses and densities were measured in order to understand further the geochemical changes due to weathering and to find ways of distinguishing the rock types on a geochemical basis, despite their weathering state.

Where fresh, the ultramafic rocks are talc-chlorite+tremolite schists and the mafic rocks are schists of granular quartz and albite with muscovite, chlorite and talc. Initially weathering solutions have penetrated the margins of quartz veins and some cleavage planes. Sulphides are among the first minerals to weather and, in the mafic rocks, these are closely followed by plagioclase which is almost completely altered at 70 m depth. At a depth of 50 m, needles of tremolite in the ultramafic rocks have largely dissolved, leaving voids, some of which have been filled by goethite. Chlorite in all rocks becomes progressively more turbid, Fe-stained and altered to smectite, and both sphene and ilmenite alter to anatase. Above 30 m depth, and particularly in the top 10 m, kaolinite becomes very abundant where it is an alteration product of talc, muscovite, feldspar, chlorite and smectite. It forms secondary fine-grained mats, coarser-grained stumpy stacks and accordion structures which progressively destroy the schistose saprolite fabric. The rocks become pockmarked with vesicles and solution channels which are, in part, filled with secondary clays. Muscovite and talc, in the mica schists and ultramafic rocks respectively, are relatively stable minerals which persist to close to the surface where both are partly altered to kaolinite.

The regolith exposed at Rand may be divided into a shallow zone (0-15 m), marked by rocks of low density (<2.0) in which there has been extensive leaching and element dispersion, and a more dense deep zone, extending to saprock and relatively fresh rock. In the deep zone, some elements (As, Au, Cu, Pb, Sb, Sn, W, Zn and, to a lesser extent, Ag, Mo, Se, Te and Tl) appear to be ore-related and many show an exponential relationship of their maximum concentrations to their distance from the main ore shoots. Of the remaining lithologically related elements, Cr differentiates the mafic from the ultramafic suite, Al and Ga separate the mafic rocks from the mica schists, low Fe concentrations mark the mica schists, black shales and porphyries, and abundant Ba characterises the mica schists and porphyries.

The number of useful pathfinder elements (As, Au, Cu, Se and W) is less in the low density shallow zone of the regolith where Al, B, Ba, Fe, Ga, Nb, Si, Sr, Ta, Th and V have been enriched but Ag, Al, Ca, Cd, Ce, Co, Eu, Fe, Ge, La, Li, Lu, Mg, Mn, Na, Ni, Sm, Y and Zn have been leached. Chromium, Ti, K, Cs, Rb, Zr and Hf are relatively unaffected by weathering; K, Rb and Cs are probably sited in residual micas and Zr and Hf in stable zircon.

Discriminant analysis has been used to separate the five lithologies at Rand. Effective discrimination (better than 94%) may be achieved by using power transformed Cr, Nb, Co, Sc, Lu, Hf, Ba, Eu and Zr data and using curved boundaries on canonical plots. Log transformed data is almost as successful. Discrimination may well be improved if samples below 10-15 m depth are used as, above this level, the geochemical and fabric changes are most severe.

Last updated: Thursday, January 06, 2000 08:30 AM


Cooperative Research Centres Australia

About Us | News & Events | Research
Publications | Education | Staff Only | Links

Contact Us | Disclaimer | Sitemap
© CRC LEME 2004

CRC LEME is established and supported under the Australian Government's Cooperative Research Centres Program. The CRC Program is an Australian Government initiative which brings together research groups with common interests.

CRC LEME Core Parties