CRC LEME
Open File Report 85
ABSTRACT
Geochemical exploration for platinum group elements in weathered
terrain: P252 Final Report
Butt, C.R.M., Williams, P.A., Gray, D.J., Robertson, I.D.M.,
Schorin, K.H., Churchward, H.M., McAndrew, J., Barnes, S.J. and
Tenhaeff, M.F.J.
This report summarizes the results of over three years of research
that had the objectives of (i) supplementing existing knowledge
of the aqueous geochemistry of platinum group elements (PGE) in
the weathering environment, (ii) obtaining information concerning
the distribution of these elements in the lateritic regoliths and
the potential for supergene enrichment, and (iii) establishing criteria
for exploration in lateritic terrain. These objectives were met
by conducting detailed laboratory experiments and field-based geochemical
investigations.
Laboratory studies demonstrated that, in addition to chloride ion
and organic compounds, thiosulphate ion and arsenious acid could
mobilize the PGE under weathering conditions. Mobilization as thiosulphate
complexes might occur in the vicinity of sulphides oxidizing under
neutral to alkaline conditions, whereas arsenious acid could only
mobilize Pd. and then only in acid environments. The potential for
mobilization by these ligands has not previously been considered,
but may have significance in appropriate environments. Studies were
also made of the processes that cause immobilisation of the PGE
in the regolith. The PGE tend to be sorbed by most regolith materials,
thus restricting their potential mobility, although in more organic
samples, some redissolution occurred after a few months, possibly
due to soluble organic species produced by biological activity.
The results demonstrate that despite the theoretical models postulating
mobility under specific physico-chemical conditions, the substrate
will strongly influence the actual behaviour.
Field investigations on the distribution of PGE were carried out
on the Ora Banda Sill, Western Australia, and the Tout Intrusion
at Fifield, New South Wales. Lateritic regoliths are well preserved
on the pyroxenites of the Ora Banda Sill and the duricrusts locally
contain 1-2 ppm Pt + Pd. There has been some relative loss of Pd
in the duricrust and, particularly, the ferruginous lag derived
from it, but otherwise there seems to have been little mobilization
of the PGE. The PGE enrichment appears to be residual and occurs
in similar horizons, and by a similar factor, as other relatively
immobile elements such as Cr, Cu, V, Ti and Zr. No PGE minerals
or alloys were found in the regolith and selective leaching experiments
suggest that both Pt and Pd occur predominantly in the minus 2 µm
fraction. Platinum is mainly associated with hematite throughout
the profile, implying early release from its primary host; in comparison,
Pd is present in goethite, but only high in the profile, and is
presumed to be released from a primary phase (e.g., chromite) late
during weathering. Chromite compositions can discriminate between
peridotite and pyroxenite in the regolith and could possibly be
useful for indicating sulphide-rich zones within the bedrock. There
has been rather greater secondary mobility of PGE, especially Pt.
in the regolith over the serpentinized dunites of the Tout Complex,
but again no secondary PGE phase was located and the mechanism of
remobilisation could not be established, although organic or chloride
complexes are most probable. PGE enrichment in alluvium is probably
mechanical.
The surface exploration procedures of soil and lag sampling are
effective in exploration in lateritic environments. Routine sampling
of the laterite itself by shallow drilling may be the best general
procedure, especially in areas where much of the laterite is buried.
High Pt and Pd concentrations are themselves insufficient indicators,
so that it is probably necessary to analyse selected samples for
the other PGE. Copper, Cr and Ni are not effective pathfinder elements
for none is necessarily associated with primary or secondary mineralisation
at either Ora Banda or Fifield. The restricted mobility of the PGE
and lack of suitable pathfinder elements may pose problems to effective
exploration of areas eroded to the saprolite or shallow fresh subcrop,
since the target will be small and any enlargement is thus dependent
on limited physical dispersion at the surface.
Last updated: Friday, July 21, 2000 04:24 PM
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