CRC LEME
Open File Report 39
ABSTRACT
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
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