Open File Report 31
Petrography, mineralogy and geochemistry of soil and lag overlying
the Lights of Israel Gold Mine, Davyhurst, Western Australia
Robertson, I.D.M. and Tenhaeff, M.F.J.
The Lights of Israel Mine Site lies in an erosional regime, where
the weathered profile has been truncated to within the mottled zone.
This study area, south of the Menzies Line, in an area of eucalypt
woodland, where soil carbonates are common, provides a useful contrast
with the arid environment of Beasley Creek to the northeast.
Samples of the lag and the generally thin, colluvial soil were
collected along one traverse over the Lights of Israel Mine Site,
prior to mining. The lag, the soil and its components have been
examined petrographically, mineralogically and geochemically. The
coarse fraction (>600 µm) consists of black, goethite-
and hematite-rich nodules (some of which are magnetic), red to yellow,
ferruginous clay granules, quartz fragments and scarce crystals
of tourmaline and gossan fragments. All the ferruginous fragments
are petrographically indistinguishable from the fine lag, which
was formed by deflation of the top layers of soil. Some of the upper
soil layers are rich in carbonates, contain crystals of pedogenic
gypsum and curly crystals of halite.
The iron-rich fragments contain lithorelics, containing microscopic
relics of layer silicates (smectites and kaolinite), set in, and
largely replaced by massive, spongy or vesicular goethite. The clay-rich
granules consist largely of hematite- or goethite-stained kaolinite
and some include goethite-rich lithorelics. Very close to the mineralisation,
fragments of gossan, showing pseudomorphs after fine-grained pyrite,
were identified in the lag. The soil contains a significant, quartz-rich,
component which is largely angular and glassy and appears to be
largely of local derivation. It is most abundant in the 75-710 µm
fraction, where it acts as a geochemical diluent. There is a very
minor aeolian component, which becomes progressively more abundant
in the finer fractions. The silty fraction (<75 µm) contains
less quartz but more iron oxides and clay. Its contained <4 µm
fraction is very clay rich. The sandy and silty fractions contain
a trace of sharp, fresh crystal fragments of tourmaline, which have
a composition indistinguishable from that of local veinlet tourmaline.
The complete soil and the >600, the <75 and the <4 µm
fractions were analysed to assess their value as sampling media.
The quartz-rich 75-710 µm fraction was discarded. Gold in
the fine soil fractions is the best guide to mineralisation by far.
Very weak and equivocal anomalies in As, Sb, Cu and Cd are probably
also ore related. Maxima in K and Rb seem to indicate a phyllic
alteration halo around the mineralisation. The elevated S background
above the mineralisation is problematical; its isotopic composition
is slightly heavier than that which would be expected from meteoric
Gold anomalies are best developed in the calcareous soil fractions,
are small and lack any extensive dispersion halo. Apart from Au,
the multi-element signature is weak and subtle and is likely to
be overlooked by exploration. These conclusions contrast with those
reached at Beasley Creek, north of the Menzies Line, where there
is a strong multi-element signature and the best geochemical medium
is the lag and the coarse, ferruginous fraction of the soil.
Last updated: Tuesday, January 04, 2000 03:59 PM