A surface geochemical survey was conducted to help identify, in conjunction with recent geophysical surveys and other data sets, areas within the southern Thomson Orogen that could benefit from a stratigraphic drilling program to test geological models, and to provide information about potential regions associated with mineralisation. The study covered an area of ~185 000 km2 straddling the New South Wales-Queensland border and consists of samples collected during a 2006 Cooperative Research Centre for Landscape Environments and Mineral Exploration (CRC LEME) study in New South Wales and samples from a field campaign carried out by Geoscience Australia in October-November 2013 in Queensland.
For the CRC LEME project, both a Top Outlet Sediment (TOS) and a Bottom Outlet Sediment (BOS) sample were collected at 99 sites located at or near the outlet of large catchments. Top Outlet Sediment samples represented material from a depth interval of 0 cm to 10 cm, whilst the BOS samples ranged in depth from approximately 60 cm to 90 cm. For 79 of those 99 sites, a third Shallow Outlet Sediment (SOS) sample from a depth of 10 cm to 25 cm was also taken. During the 2013 Geoscience Australia field campaign, 41 new catchment-based sites were sampled, from the Eulo and Toompine 1:250 000 scale map sheets in Queensland, with both TOS and BOS samples collected. Geochemical analyses were performed for total, aqua regia, and Mobile Metal Ion® (MMI) inorganic element concentrations in the samples.
For this study samples were sieved into a fine (<75 ¿m) and a coarse (<2 mm) grain size fraction. For the fine grain size fraction, the Eulo-Toompine samples and the CRC LEME samples were combined. For the coarse grain size fraction, only the Eulo-Toompine samples were analysed. The geochemical data were first transformed using a centred log-ratio (clr) transformation, then analysed by Principal Component Analysis.
The dominant element association observed across all of the digestion techniques (total, aqua regia, and MMI) was the grouping of Rare Earth Elements (REEs) and associated elements (Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Yb, and/or Lu) and other elements such as high field strength elements (Zr, Nb, Hf, Th, U, and Ta) in Principal Component One (PC1). The most likely host minerals for REEs include zircon, monazite, baddeleyite, apatite, and allanite. These minerals are resistant to chemical attack and their accumulation likely represents the end-product of extensive weathering.
Some of the resulting Principal Components (PCs) reflect element associations that could be attributed to mineralisation in the area. Using the location of known deposits such as those near Cobar, New South Wales, it was possible to assess the spatial relationship between PCs and mineralisation. The most significant spatial pattern associated with potential mineralisation was observed in PC1 of the MMI analyses and consisted of Au, Ag, Ca, and Sr at one extreme of PC1 and REEs at the opposite extreme. This association is similar to that derived empirically by local mineral explorers to successfully target drill holes for Au and Cu mineral exploration. The eastern, central and western regions of the study area are highlighted on the MMI PC1 map as having the greatest potential for this geochemical footprint. The results from the other PCs provide information on regolith processes (e.g., weathering) or lithology (e.g., mafic/ultramafic rocks).
