CHARACTERISTICS OF MAGNETIC AND NONMAGNETIC LAG IN THE COBAR AREA, NSW

Geochemical patterns from two la anomalies in the Cobar region of cent ral New South Wales are described. The region is semi-arid, deeply wea thered and some areas are covered by variable thicknesses of aeolian a nd alluvial transported overburden. Lag morphology and mineralogy are related to landform. In erosional landforms the surface is covered by lag, which is composed of coarse fragments derived locally from bedroc k and displays varying degrees of ferruginization. together with a ran ge of secondary pisoids. In depositional landforms the lag is less abu ndant and contains more pisoids. Deflation by sheetwash may lead to la g being partially buried by alluvium. In a large proportion of pisoid lag, hematite and goethite have been converted to maghemite, allowing ready separation into magnetic and non-magnetic fractions. The non-mag netic fraction is dominated by the lithic lag whereas the magnetic fra ction is dominated by pisoid lag. The magnetic and non-magnetic compon ents of the 2-11 mm lag fraction, milled to < 75 mu m and subjected to HF-HNO3-HClO4 attack, give rise to distinct geochemical patterns at t argets related to a variant of the Cobar style of Pb-Zn mineralization with variable silicification and to Zn mineralization in a swarm of q uartz and carbonate veinlets. The patterns are influenced by metal sou rce, lag type, surface chemical conditions and landform. Differences i n the correlation between metal contents in the magnetic and non-magne tic lag components, and between trace elements and Fe, indicate variab le loss of Cu and Zn from the magnetic lag at some targets but retenti on of Pb. The presence of a readily extractable base metal component i n the lag has implications for detecting anomalies in transported cove r in the region.