CRC LEME
Open File Report 159
ABSTRACT
Regolith architecture and geochemistry of the Byrock Area, Girilambone
Region, North-western NSW
R.A. Chan, R.S.B. Greene, M. Hicks, M. Le Gleuher,
K.G. McQueen, K.M. Scott and S.E. Tate
Stage 3 of the Girilambone (Cobar-Bourke) Project has involved
collaborative work between CRC LEME and the NSW Department of Mineral
Resources in the Byrock area. This work has provided 2713 m of
drilling for regolith study with 60 holes, generally 1-3 km apart
along a major road traverse across the Byrock 1:100 000 sheet area
and more widely spaced across parts of the Glenariff 1:100 000
sheet area.
Regolith-landform mapping conducted in association with drill
hole logging reveals that colluvial and alluvial sediments cover
most of the Byrock area. Islands of in situ regolith are surrounded
by sheetwash deposits on rises grading to depositional plains,
and large areas of stagnant alluvial plains, especially towards
the northwest. Analysis and interpretation of aircore drill samples
has allowed identification of two sequences of sediments, weathered
bedrock lithologies (altered and unaltered), as well as various
types of induration of both transported and in situ regolith. A
16.8 Ma leucitite lava marker flow separates the younger sequence
of sediments (Sequence 1) with inferred arid alluvial, colluvial
and aeolian sediments from the older sequence of sediments (Sequence
2) with inferred lacustrine sediments. Sequence 1 includes magnetic
sediments whose areal extent can be detected on first vertical
derivative airborne magnetic imagery.
Major and trace element geochemical data from the drilling program
indicate a different geochemical terrain to that previously examined
further south. There appears to be a greater abundance of weathered
mafic rocks (probably including dykes and volcanics) and fractionated
granites. There are a number of areas where gold is elevated in
the regolith. These appear to be in a “gold-only” association
and some are spatially associated with mafic rocks, including those
in the western part of the Byrock sheet near the Mt Dijou – Bald
Hills area. In the Lord Carington area, northeast of Byrock, anomalous
gold values (>0.04 and up to 0.15 g/t Au) were intersected over
a 39 m interval in weathered chloritephengite-magnetite schists.
There is potential for vein and lode-style gold mineralisation
associated with mafic rocks in regolith concealed areas.
Detailed work on the soils in the Byrock and adjacent areas has
clearly established the presence of a significant aeolian component.
This is predominantly in the near 70 µm size fraction. To
reduce the diluting effect of this extraneous component, the >100 µm
fraction is the most appropriate fraction for geochemical analysis.
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