CRC LEME
Open File Report 7
ABSTRACT
A mineralogical, geochemical and petrographic study of the rocks
of drillhole BCD1 from the Beasley Creek Gold mine - Laverton, Western
Australia
Robertson, I.D.M. and Gall, S.F.
The mineralogy, petrology and geochemistry of selected samples
from DDH BCD1 were examined in detail to gain an overall understanding
of the lithology, weathering characteristics and geochemistry of
the Beasley Creek gold deposit (Laverton). Vertical drillhole BCD1
intersects the ore zone and footwall rocks to a depth of 100 m.
The footwall consists of fresh, foliated basaltic amphibolites metamorphosed
to the upper greenschist facies, consisting of a tremolite-albite-quartz
assemblage. They contain at least one felsic unit and have been
locally carbonated. There is a suggestion of hydrothermal alteration
near the bottom of the hole where kaolinisation has taken place
in the presence of fresh pyrite. Above this, pyrite is oxidised
to goethite. Feldspars weather progressively to kaolinite and amphiboles
to smectite. These rocks become upwardly friable and their foliation
is cemented with clay, iron oxide and calcite.
The ore zone lies mainly within phyllitic rocks which are variably
kaolinised and ferruginised. Garnet pseudomorphs indicate metamorphism
to the quartz-albite-epidote-almandine subfacies. Progressive ferruginisation
has replaced both minerals and fabrics, leaving only relict muscovite
and shard-like quartz. Vesicles are lined with goethite, which becomes
progressively poorer in Al, hematite and several Mn minerals including
cryptomelane and lithiophorite. The upper part of the weathered
profile has been subjected to calcrete formation where the rocks
have been brecciated and set in a complex carbonate cement. Some
of this has subsequently been dissolved leaving carbonate-lined
vesicles.
The Ti/Zr ratio, together with Al and Cr are useful lithological
indicators. Weathering has progressively depleted Mg, Ca and Sr
and this is complete above 65 m. Mineralisation is marked by anomalous
Pb, W, As and, to a lesser extent, by Be, Fe, Zn, Sb and Co.
Keywords:
Amphibolite, geomorphology, felsic schist, phyllite, mafic rock,
Last updated: Tuesday, January 04, 2000 10:14 AM
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