“LAG” A NEW CLASSIFICATION SYSTEM FOR CONCEALED MINERAL EXPLORATION

Abstract

Lag is a general term applied to coarse grained (> 2 mm), hard, but partially weathered rockfragments, 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 intothree broad morpho-mineralogical categories; (a) those with a rough, blocky, lithic morphology where fabricsof the parent rock are partially preserved and which evolved predominantly in erosional landforms, (b) asmoother pisoid lag, with a well-developed varnish or polished surface, which is most abundant in deeplyweathered, erosional and depositional landforms, and (c) a detrital lag evolving in a range of situations andwhich is generally more abundant in Quaternary modern drainage landforms. Based on chemistry andmagnetic character, two distinct types of magnetic and non-magnetic lag are readily recognised. The magnetictype may include both pisoid and occasionally lithic and ferrolithic pregnant with maghemite. Analysis forvarious 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 closeto its original source. These unique characters of lag, revealing weak anomalies in covered surfaces and itsabundance in relation to the erosional and depositional landforms, make them a preferred sampling media ingeochemical exploration. The chemical data suggest that the magnetic lag fraction is more useful inreconnaissance exploration, and non-magnetics for follow-up work to locate mineralisation, which mayjustify partitioning a lag sample prior to analysis.