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Open File Report 122

Charters Towers - North Drummond Basin Field Excursion, Field Guide

K. Scott, Li Shu, S. Fraser, I.D. Campbell, R.R. Anand and I.D.M. Robertson

The principal objective of the P417 Project is to improve substantially geochemical methods of exploration for base metals and Au in areas obscured by weathering or under younger cover. The research includes geochemical dispersion studies, regolith mapping, regolith characterisation, dating of profiles and investigation of regolith evolution. The project has sites in both the Mt Isa Region and in the Charters Towers-North Drummond Basin.

Essential to this project is translation of research findings into practical exploration outcomes. Field excursions are an essential link in this technology transfer. The last excursion was in July 1995 and covered the Eastern and Western Successions of the Mt Isa Region, where research sites have deeply weathered profiles on Proterozoic and Mesozoic bedrocks and are partly overlain by variable thickness of recent sediments. This second excursion provides opportunities to examine the weathering of younger Palaeozoic rocks and their Tertiary cover sequences (Suttor, Southern Cross and Campaspe Formations) and the geomorphology of the region, all of which are important to geochemical exploration of the region.

The excursion includes visits to mineral deposits, prospects and field sites. On the first day the excursion visits Red Falls, where the Campaspe Formation can be demonstrated to overlie the Southern Cross Formation, the Featherby Walls, where the sandstones of Southern Cross Formation unconformably overlie the Ravenswood Granodiorite. At both these sites, mottling and nodules are common. In the Waterloo Area a ferruginous profile is developed in the top of the Campaspe Formation. The Waterloo Deposit has been quoted as occurring below 60 m of Campaspe Formation, however, work done in this Project indicates that the thickness of the cover has been overestimated in places and a Pb mechanical dispersion train occurs at the base of the Campaspe Formation.

The second day covers Scott Lode, the Cindy Deposit and the Wahines Prospect. Here, weathered profiles are developed on cover sequences and on the basement. Weathered components of earlier sequences have been included as detritus in later materials, implying a complex and continued weathering history. Concentrations of Fe have developed both at surface and at depth in the cover sequences, which have adsorbed pathfinder elements related to mineralisation in the basement.

The Police Creek and Wirralie prospects and profiles through the Suttor Formation are visited on the third day. Ferruginous duricrusts and silcretes are developed on the Suttor Formation.

At the Police Creek Prospect, the transported cover is generally less than 6 m thick and mottles have developed both in it and in the underlying bedrock. There appears to be both hydromorphic and mechanical dispersion in the cover. The < 75 µm fraction of the soil has a more intense Au anomaly than the <180 µm fraction. Although As and Sb occur in the fine fraction, these elements are concentrated with Fe oxides of the coarse fraction with Mo.

At the Wirralie Deposit, the Au anomalies in the Suttor Formation reflect mechanical transport from the basement. Arsenic and Sb contents are generally low in the Suttor Formation, except where ferruginous sediments occur.


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