Biogeochemical mapping of the Cobar Basin using the needles of cypress pine ( <i>Callitris glaucophylla</i> )

Deposit scale biogeochemical surveys have been conducted in many parts of Australia, but there have been few conducted at regional scales. The Cobar Basin in New South Wales hosts a range of mineral deposits and regolith–landform settings. Samples (3300) of Callitris glaucophylla (cypress pine) needles have been collected from road traverses within a 43 000 km 2 area with detailed sampling incorporated over various mineral deposits. Dried and milled samples were analysed by both inductively coupled plasma mass spectrometry following microwave-assisted aqua regia digestion, and directly by portable X-ray fluorescence. The concentrations of most elements in the needles largely reflect variations in underlying lithology including areas with alluvial cover. The concentration of ‘ballast' trace elements Au, Ag, Bi, Pb, As and W are typically higher over the Siluro-Devonian clastic sediments than felsic intrusives and volcaniclastic or Ordovician siliciclastic units, whereas the micronutrients Ni and Co are elevated over ultramafic units. For some major and nutrient trace elements, including Cu and Zn, the pines restrict uptake and limit the response to lithological changes, mineralization or contamination. Minor seasonal variations or site variability in element concentrations do not substantially alter the biogeochemical contrast between regional background and mineralization signatures. The needles display effects of dust or contamination from current or historical mining operations in some areas, but variation in the distance of samples from road edges or road type display no systematic relationship with needle composition, except for Fe and Al. C. glaucophylla needles are an effective sampling media for mineral exploration in the Cobar region and for environmental monitoring.