Interpreting aerial gamma-ray surveys utilising geomorphological and weathering models

Applying weathering and geomorphological data can improve the interpretation of aerial gamma-ray surveys. Data from two potentially mineralised areas in eastern Australia are used to demonstrate the new approach. At Develin Creek, NW of Rockhampton in Queensland, Permian andesitic basalts host VMS-style Cu-Zn deposits. Topographic and SPOT data were used to generate a digital elevation model (DEM) over an area of approx. 125 km ². The Universal Soil Loss Equation (USLE) was then applied to model soil erosion and deposition. Surface estimates of the distribution of clay, iron-oxides and vegetation were derived from Landsat TM data. Multivariate linear regression and analysis of residuals (i.e. the difference between predicted and observed maps for each radioelement) were then used to look for anomalies that may indicate near-surface mineralisation. This analysis indicated no surface indicators of undiscovered mineralisation, a finding in accord with extensive ground geophysics and mapping. At Barry, SW of Bathurst, NSW, Ordovician basalts potentially host porphyry-related gold deposits. These deposits are most probably seen in aerial gamma-ray survey data through elevated K in alteration haloes. A DEM was created over a study area of approx. 180 km ² and the soil erosion/deposition characteristics modelled using the USLE. Laboratory analyses of rock and soil samples showed that weathering of the basalt (median radioelement contents of 1.4% K, 0.7 ppm eU, 1.6 ppm eTh) produced a soil with decreased K (0.8%) but increased eU (1.5 ppm) and eTh (7.0 ppm). These values were used along with the erosion model to predict the surface K distribution, which was then compared to the observed distribution using a linear regression model. Areas with high K residuals are considered prospective for Au mineralisation.