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The geology and magnetic characteristics of precious opal deposits, southwest Queenslands

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Created 13/01/2025

Updated 13/01/2025

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Precious opal in southwest Queensland is found within a weathered profile developed in sedimentary rocks of the Cretaceous Winton Formation. This kaolinitic profile is the product of two periods of deep chemical weathering. The first weathering event is Maastrichtian to Early Eocene, and formed a tri-Iayered profile (Morney profile) in excess of 90 m in thickness. At this time ironstone was chemically precipitated within the basal layer. A period of sedimentation along river systems, with fragmentation and minor erosion of this profile along interfluves, was followed by a second weathering event about the Late Oligocene. A morphologically distinct profile formed (Canaway profile), consisting of an indurated crust and mottled zone, which grades down into a varying thickness of the residual older profile. During the second weathering event silica migrated as an aqueous sol, and was precipitated within voids in ironstone host rocks. The gemmological properties of Queensland boulder opal are described in relation to the weathering history and bedding characteristics of the opal deposits. Rocks which contain scattered opal deposits are poorly exposed in scarp-bounded mesas and flat-topped landforms, or are concealed below a pedimented land surface. Ironstone host rocks are remanently magnetised, and an exploration method based on ground magnetic surveying, using portable and vehicle-borne magnetometers, is discussed.

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Title The geology and magnetic characteristics of precious opal deposits, southwest Queenslands
Language eng
Licence notspecified
Landing Page https://devweb.dga.links.com.au/data/dataset/16a70a00-41a8-4adb-8aae-703b9863042e
Contact Point
Geoscience Australia
clientservices@ga.gov.au
Reference Period 20/04/2018
Geospatial Coverage {"type": "Polygon", "coordinates": [[[141.0, -29.0], [146.0, -29.0], [146.0, -21.0], [141.0, -21.0], [141.0, -29.0]]]}
Data Portal data.gov.au