The Cleveland Stratabound Tin Deposits, Tasmania, Australia: A Review of Their Economic Geology, Exploration, Evaluation and Production

Abstract

The tin field of western Tasmania, Australia, contains several large stratabound tin deposits associated with massive iron-sulphides in bedded carbonate host-rocks of Upper Proterozoic to Early Cambrian age. Past and present producers include Mt Bischoff, Renison and Cleveland. They all are closely associated, both spatially and temporally, with altered high-level granitic stocks and/or quartz porphyry dykes, emplaced on major faults/lineaments. These stocks and dykes form a late phase of the Late Devonian Meredith Granite pluton.

The Cleveland deposits were explored during 1962–1964 by geological mapping and an evolving series of diamond drilling programmes. A detailed evaluation followed during 1964–66, involving underground exploratory development, comparative sampling, bulk sampling and metallurgical testing, and further diamond drilling. The deposits were brought into production in February 1968 with ore reserves (indicated plus inferred) totalling 2.85 Mt averaging 1.02% Sn, 0.43% Cu.

Since 1968, the Cleveland Mine has been Australia’s second largest tin producer. Production to June 1984 has totalled 5.5 Mt ore yielding 24,000 t Sn and 9,592 t Cu. Continued exploration has led to the delineation of new reserves, such that the original reserves (measured plus indicated) are now estimated to have been 10.3 Mt averaging 0.78% Sn, 0.33% Cu (using a cut-off grade of 0.35% Sn), of which 5.7 Mt averaging 0.71% Sn, 0.32% Cu remained at May 1983. Mining is by sub-level open-stoping using tackless diesel equipment with decline access to all levels. Milling is by crushing, heavy-medium separation, grinding, sulphide flotation, gravity separation and tin flotation.

The Cleveland tin-copper deposits occur within the Tasman Fold Belt of Eastern Australia in a N-S trending eugeosynclinal trough (the Dundas Trough) which developed in western Tasmania during the Cambrian. Regionally the unfossiliferous metasedimentary/volcanic sequence in the mine area occurs at the base of and in the western portion of the Dundas Trough. Following strong deformation during the Early-Middle Devonian Tabberabberan orogeny, a number of high-level granitic plutons were emplaced during the Late Devonian. The Cleveland Mine is located 4 km north of the northern margin of the largest of these plutons, the Meredith Granite.

The Cleveland deposits are stratabound, occuring as a series of subparallel tabular orebodies of pyrrhotite-pyrite-cassiterite-stannite-chalcopyrite mineralisation within a well-layered carbonate-chert-shale unit (Halls Formation). This unit conformably overlies a turbidite greywacke-shale unit (Crescent Spur Mica Sandstone) and is in turn overlain by a spilitic basalt-pyroclastic unit (Deep Creek Basic Volcanics). The sub-vertical orebodies, which strike NE, attain a maximum thickness of 30 m and a maximum strike length of 550 m. Along strike the tin-sulphide mineralisation passes into unaltered carbonate beds. These stratabound deposits are closely associated, both spatially and temporally, with a plumbing system, the locus of which is occupied by a pipe-like body of quartz porphyry surrounded by a W-Mo-Bi quartz vein stockwork deep in the central footwall (Crescent Spur Mica Sandstone) portion of the mine sequence.

The metal zoning and wall-rock alteration patterns indicate the mineralization is essentially post-deformation and emanated from a central plumbing system. The stratabound deposits formed by metasomatic replacement of carbonate beds by hydrothermal solutions derived from a late phase of the Meredith Granite.

This chapter was prepared in October 1984; the Cleveland Mine has been closed since 1987