The Flowery Gully limestones deposits, Tasmania

The Flowery Gully Limestone Deposit, twenty miles north of Launceston, was surveyed in detail to delineate the most suitable portions for supplying limestone to the Australian Aluminium Production Commission's plant at Bell Bay, five miles to the north east across the Tamar Estuary. Basic requirements for the limestone were that it lie in an area suitable for quarrying and have as low a silica and magnesium carbonate content as possible so that it could be used efficiently in the aluminium extraction process. The preliminary reconnaissance revealed a stratigraphical chemical control and it was found possible to divide the section measuring 1,700 feet in true thickness into three stratigraphical zones, the topmost of which contains the purest limestone and is most suitable for Bell Bay; the middle zone contains isolated areas of high quality stone. The more detailed survey to reveal quarryable stone in these two zones defined one major and one minor deposit in the upper zone and one minor deposit in the middle. These three deposits contain indicated and inferred reserves totalling 1,000,000 tons; of this figure 100,000 tons has an average grade of 0.4% silica and 5.7% magnesium carbonate and the remainder has a grade not exceeding 0.5% silica and 4% magnesium carbonate. The investigation of the depositional environment revealed quiet and consistent conditions of deposition, which information was used to indicate possible areas for exploration for additional reserves when necessary. Numerous bedded chert nodules are present in the high-silica-content areas of the middle zone. The origin of the silica of this zone was investigated chemically and petrologically, but the results were inconclusive. It was discovered however that practically all silica was syngenetic. It is now mainly in the form of quartz grains and in smaller amounts as a component of the small percentage of clay present; however where chert nodules are present the silica of the cherts is in the form of chalcedony. Areas of marked dolomitization were noted, particularly in the upper and middle zones; they probably represent algal reefs (stromatoporoid bioherms). One of them had a lateral extent of at least 1,600 feet. A distinct disconformity was revealed by the detailed mapping at the top of the limestone section.