Application of gamma-ray spectrometry to prospecting for tin and tungsten granites, particularly within the Lachlan fold belt, New South Wales

Abstract

Two major regions of granitoids are present in the early to middle Paleozoic Lachlan fold belt of New South Wales. The larger, in the east (domain 1), is characterized by prominent magnetic anomalies. Its granitoids include a few that are sources of and hosts to several tungsten and mostly separate, minor tin occurrences. Domain 2 is characterized by sparse low-amplitude magnetic anomalies. Tin granites are common and most deposits also have minor wolframite occurrences.Gamma-ray spectrometer measurements have been made over outcrops of most granitoids in the fold belt and converted to equivalent ppm U and Th. All but two of the measured Sn and W mineralized granites contain anomalous U. In domain 1 they contain more than 4 ppm, whereas in domain 2 they contain more than 5 ppm U.The similar elemental properties and behavior in magmas of the Sn, W, U, and Th cause them to independently migrate to and concentrate in K-rich residual melts. Detectable enriched U can therefore distinguish such late-stage granitoids which have fractionated sufficiently to yield Sn or W minerals under the right conditions. U enrichment of the host applies irrespective of the diversity in form of the associated Sn or W deposits, and of whether or not they are intragranitic, within adjacent country rocks, or in placers derived from them.Late magmatic processes are thought to account for thorium depletion in some mineralized granitoids, thus precluding the use of enriched Th as an indicator of Sn or W mineralization. However, several hosts, and greisens, have high U/Th ratios. Other late magmatic alteration such as albitization and feldspar destruction can be detected ratiometrically because of depleted K.Gamma-ray spectrometry is also a useful technique for delineating variants of composite plutons and batholiths, as shown for the Berridale batholith, where some leucogranites, similar to one that is mineralized, can be prospected for W.Numerous granites lacking known Sn and W mineralization have significant U and characteristics similar to those with known mineralization. These are recommended for prospecting.Enriched U and other radiometric characteristics of moderately to well-exposed mineralized and prospective granitoids can also be detected by airborne spectrometer surveys. Profiles over the mineralized Jinderra and Ardlethan Granites and over the unmineralized, but similar, multiphase Nymagee Granite are compared with data acquired over an unsuitable part of the Marulan Granite.The Sn, W, U connection in granitoids is a worldwide phenomenon, and because the radiometric technique can be applied either from the air or on the ground, gamma-ray spectrometry is shown to be a powerful tool for exploring for granitoids that are sources of, or hosts to, tin and wolframite deposits.