CRC LEME
Open File Report 108
ABSTRACT
Regolith-landscape evolution and geochemical dispersion about
the Bronzewing Gold Deposit, WA
Varga, Z.S., Anand, R.R. and Wildman, J.E.
The Bronzewing deposit is in the Archaean Yandal Greenstone Belt.
Gold mineralisation occurs within a sequence of mafic volcanics
(basalts, dolerites) and minor sediments, which are intruded by
felsic porphyries. The mineralisation is associated with a dense
stockwork of quartz veining, and alteration of the host sequence,
and is accompanied by pyrite, pyrrhotite and minor chalcopyrite
and scheelite. Gold, W and Cu are the most significant indicators
in mineralised bedrock. Regolith Regolith-landform relationships
over the 1000 km2, centred on the Bronzewing deposit, were mapped
at 1:50 000. This was based on interpretation of aerial photography,
image enhanced Landsat TM images and field traverses. The Bronzewing
Au deposit was covered by a blanket of soil and colluvium. Mapping
regolith relationships and distributions using drilling and pit
faces, has revealed the details of the sub-surface regolith and
palaeolandscape, from which the weathering history and likely origins
of anomalies in the residuum and transported cover can be deduced.
A wide range of sediments overlie older, residual regolith. Colluvial
and alluvial sediments are 20-30 m thick in the Discovery pit and
directly overlie saprolite. In the southwest of the Central pit,
the sediments are 15-20 m thick and overlie lateritic residuum in
places. The sediments thin towards the northeast to less than 5
m thick in the Laterite pit. Alluvium is most likely derived from
the south, and the colluvium from the east where there is a subdued
breakaway. The palaeochannels have been infilled with kaolinite-smectite
sediments derived from the erosion of pre-existing red soils and
saprolites. On a regional scale, palaeochannel sediments reach 120
m in thickness. Dolocretes, calcretes, pisoliths and megamottles
have developed in these sediments. Mobilisation and segregation
of Fe by a combination of roots and reduced groundwaters in porous,
vegetated red clays was probably responsible for the formation of
the megamottles. Dolocretes are likely to be relict forms equivalent
to those in deep sediments in the Roe palaeodrainage. They are confined
to the base of the channel above the lateritic residuum and may
have formed by evaporation of Mg-rich lakewaters so their formation
appears to be related to a period of high evaporation rates. The
dolocretes differ from valley calcretes of the region, which are
surficial deposits in major, active drainages. Considerable palaeotopography
(60 m) around Bronzewing suggests that lateritic residuum did not
form a simple, extensive, peneplained surface but a discontinuous
cover on an undulating plateau. The pre-Eocene landscape at Bronzewing
was not only mantled with lateritic residuum but also with thick
blankets of kaolinite-hematite red soils. Ferruginous duricrusts
and red soils developed in different sites in response to contrasting
geological and topographic conditions. Duricrusts were developed
on mafic and ultramafic rocks and red soils were probably restricted
to felsic lithologies on well-drained upper slopes. Where fully
preserved, the residual profile beneath the colluvium and alluvium
has a 2-5 m thick lateritic residuum consisting of lateritic nodules
and some pisoliths, set in a silty clay matrix. The nodules were
formed by the fragmentation and collapse of the underlying ferruginous
saprolite. The ferruginous saprolite, a few metres thick, grades
downwards into saprolite. Fresh rock is encountered at 80 to 120
m depth. Geochemical dispersion in the regolith of the residual
materials (ferruginous saprolite and lateritic residuum) showed
significant Au, W and Cu anomalies in the vicinity of the Bronzewing
deposit. The lateritic duricrust and nodules and pisoliths of the
Laterite and Central pits, where developed over primary mineralisation,
contain significant amounts of Au. Elements associated with the
Au mineralisation in the lateritic residuum of the Laterite pit
are Ag, Ba, Ce, W, Mo, As, Sb and Cu. The Au anomalies are not as
consistent in the Discovery pit, and the lateritic residuum of both
the Central and Discovery pits are enriched in Cu and W close to
the primary mineralisation. Wide spaced sampling would be adequate
to sample areas of buried lateritic residuum and ferruginous saprolite.
The buried lateritic residuum contains Au to ore grade, which sometimes
extends into the colluvium. This relationship is particularly apparent
in the Central pit area where extensive Au anomalies occur across
the unconformity. Size fractionation of the gravelly colluvium that
occurs within a metre of the residuum/colluvium interface, indicates
that Au is concentrated in the plus 2000 µm fraction and depleted
in the minus 75 µm fraction, relative to the bulk sample.
The enrichment in the coarse fractions represents clastic dispersion
of lateritic detritus. There is no evidence to show that hydromorphic
dispersion has accumulated Au in the fine fraction. Gravelly colluvium
is a useful sampling medium in situations where lateritic residuum
or ferruginous saprolite are missing. Mottles extracted from the
palaeochannel sediments contain no significant enrichment of pathfinder
elements, and Au contents are below detection. No significant concentrations
of Au or pathfinder elements are present in bulk soils collected
from 0.3-0.5 m depth; the Au content barely exceeded the detection
limit of 5 ppb. No dispersion of Au into the soil was detected by
partial extraction of the soil fine fraction (<250 µm).
Last updated: Sunday, August 05, 2001 14:04:21
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