“LAG” A NEW CLASSIFICATION SYSTEM FOR CONCEALED MINERAL EXPLORATION

Lag is a general term applied to coarse grained (> 2 mm), hard, but partially weathered rock
fragments, which are concentrated at the surface through attrition of finer materials. Based on morphology,
mineralogy and the origin of lag from the Cobar region of Australia, lag may be conveniently grouped into
three broad morpho-mineralogical categories; (a) those with a rough, blocky, lithic morphology where fabrics
of the parent rock are partially preserved and which evolved predominantly in erosional landforms, (b) a
smoother pisoid lag, with a well-developed varnish or polished surface, which is most abundant in deeply
weathered, erosional and depositional landforms, and (c) a detrital lag evolving in a range of situations and
which is generally more abundant in Quaternary modern drainage landforms. Based on chemistry and
magnetic character, two distinct types of magnetic and non-magnetic lag are readily recognised. The magnetic
type may include both pisoid and occasionally lithic and ferrolithic pregnant with maghemite. Analysis for
various trace elements indicates a drastic difference in their chemistry and anomaly detection ability.
Magnetic lags contain anomalous Fe, Pb and other heavy metals, while lithics were enriched Cu, Zn and Mn.
Spatially, magnetic lag have broader distribution compared to the lithic fraction, which is concentrated close
to its original source. These unique characters of lag, revealing weak anomalies in covered surfaces and its
abundance in relation to the erosional and depositional landforms, make them a preferred sampling media in
geochemical exploration. The chemical data suggest that the magnetic lag fraction is more useful in
reconnaissance exploration, and non-magnetics for follow-up work to locate mineralisation, which may
justify partitioning a lag sample prior to analysis.