Gravity Survey Across the Agate Creek Volcanics, North Queensland

A gravity survey was carried out across the ‘Agate Pocket’ to help investigate the relationships between various units of the Agate Creek Volcanics, and the structure of the underlying basement rocks. The volcanics are the source of high quality agates and other nodules. A key factor operationally was the careful placement of the traverse across elongate topographic features to enable two-dimensional computer processing of topographic effects. Interpretation of the data hinges on the analysis of the effect on the density profiling process of the anomalies caused by geological bodies. A large ridge of rhyolitic rocks provides key information in the form of a bulk density value of 2.45 t.m.^-3, deduced by iterative analysis of Bouguer density profiles. The agate-bearing andesitic volcanics are interpreted to be about 400 m thick. They overlie a homogeneous basement, are fault-bounded on the southwestern side, and abut against the roots of rhyolitic volcanics on the northeastern side.