Open File Report 10
Geochemistry, petrography and mineralogy of ferruginous lag overlying
the Beasley Creek Gold Mine - Laverton WA
Two fractions (0.2 to 4.0 and 10 to 50 mm) of the black ferruginous
lag that used to overlie the Beasley Creek gold mine have been studied
physically, petrographically, mineralogically and geochemically.
The fine lag was split into magnetic and non-magnetic components.
The coarse lag fraction consists of goethite and hematite with
minor quantities of mica, kaolinite and quartz. It contains ferruginised
lithorelics with remnant and pseudomorphed minerals and fabrics.
These are related to both primary and authigenic features of the
underlying saprolite. These relics occur as islands in several cycles
of secondary goethite and hematite, which have obliterated much
of the original fabric. In many instances, hematite can be shown
to be a dehydration product of goethite. Later history of the coarse
lag is shown by skins and nodules of ferruginous clay, which have
undergone several cycles of solution, clay precipitation and permeation
by iron-bearing solutions. Careful study of original fabrics could
be used to aid the geological mapping of lag-covered areas.
In addition to black ferruginous nodules and red-brown and yellow-brown
nodules, analogous to the coarse lag, the fine lag contains minor
components of calcrete, quartz and a cellular ironstone, as well
as very small quantities of silica-cemented red, aeolian sand and
organic debris. The fine lag shows similar fabrics and components
to the coarse lag but its finer and more fragmentory nature and
wider dispersion make elucidation of the original rock type more
The orebody and its host, the weathered black phyllite, are depicted
by positive anomalies in Au, As, Ba, Co, Cu, Mn, Mo (weak), Pb (possible),
Sb, W and Zn in the lag. The cellular ironstone component is strongly
anomalous in the target elements As, Au, Co, Cu, Mn, Sb, Se and
Zn. Similar anomalies in Mg, Ca and Sr mark the occurrence of calcrete,
which occurs near the hill crest and coincides with the orebody.
Gold is the best indicator and shows strong anomalies (1000 ppb
over a 10 ppb background) which are 600 - 900 m in width. Other
elements show narrower dispersions. Superimposed on these broad
gold anomalies are narrow, subsidiary peaks (10 000 ppb), specifically
in the coarse lag, which accurately locate the ore. The wide dispersion
of gold reflects chemical dispersion in the saprolite and lateritic
duricrust, prior to mechanical dispersion of the lag on the surface.
This is supported by the association of gold grains with vesicles
and late goethite and even clay phases in the lag.
The fine lag is the most useful sampling medium and is best suited
as a regional tool. The coarse lag, which is more tedious to collect,
has use in follow-up work. No advantage is gained in analysis of
the magnetic component, as it fails to give anomalies in Co, Cu,
Se and Zn and the Au anomaly is not as distinct. This occurs because
the important, non-magnetic, cellular ironstone or gossan component
Geomorphology, petrography, maghemite, lag geochemistry, magnetic
lag, mechanical dispersion, phyllite, duricrust, primary fabric,
secondary fabric, Permian glacial, mica relic, accordion fabric,
Last updated: Tuesday, January 04, 2000 10:26 AM