Palaeochannels near West Wyalong, New South Wales: a case study in delineation and modelling using aeromagnetics

Abstract

In November 1998, the Australian Geological Survey Organisation (AGSO) and the New South Wales Department of Mineral Resources acquired high-resolution airborne magnetic and gamma-ray spectrometric data along lines spaced 50 m apart and 60 m above ground level north of West Wyalong, New South Wales. In addition, AGSO post-processed a proprietary dataset of similarly high-resolution from an area immediately to the south, between West Wyalong and Temora, along the Gilmore Fault Zone. Anomalies interpreted to be caused by palaeochannel deposits have been identified in these datasets. Modelling of the airborne magnetic data was used to define the geometry of the palaeochannel deposits. These models were constrained using data from drill holes and downhole magnetic susceptibility measurements. Mapping palaeochannel deposits is of potential significance for mineral exploration and land and water research. Historically, alluvial gold, often palaeochannel-hosted, has accounted for a large proportion of Australia's gold production. The study area is part of the historic Wyalong Goldfield and is part of the Bland Creek palaeovalley. Palaeochannel deposits may also be reservoirs of saline groundwater, and therefore are important in developing hydrogeological models for dryland salinity hazard assessment. Due to their depth of burial (often concealed beneath surficial sediments, to depths of 150 m), the palaeochannel deposits in the study area are mappable using either airborne magnetic or electromagnetic data, but not using gamma-ray spectrometry. The palaeochannel deposits contain detrital ferruginous maghaemitic pisoliths (which have extremely high magnetic susceptibility) concentrated in lenses in sand and clay. The resolution of the aeromagnetic datasets is compared with results obtained from surveys with different acquisition parameters. These tests suggest that an airborne geophysical survey flown with line spacing of less than 100 m can reveal important information about regolith materials, including channel-fill deposits such as in the sedimentary fill of the Bland Creek palaeovalley.