The Rosebery volcanic hosted massive sulphide ore copper, zinc, lead, silver, gold deposit is located ~195 km NW of Hobart and 90 km SSW of Burnie and 25 km NE of Zeehan on the west coast of Tasmania.
The deposit is hosted by the Central Volcanic Complex of the 250 km long, middle to late Cambrian Mt Read Volcanic Arc on the west coast of Tasmania, Australia. To the north-west of the Henty Fault this complex is composed of rhyolitic to dacitic acid lavas and pyroclastics with a higher andesites content than those to the south-east of this same structure and is the host to major Zn-Pb-Cu-Ag-Au mineralisation. The Central Volcanic Complex is unconformably overlain by the keratophyric tuffs, massive agglomerates, conglomerates, sandstones, shales and limestones of the Tyndall Group. During the Devonian shallow level post-tectonic granitoids intruded the mine area and resulting in metamorphism and recrystallisation of the ores.
In the immediate mine area the host sequence is composed of the following from bottom to top:
• Footwall Pyroclastics - >300 m of mainly pumiceous rhyolitic to dacitic, feldspar-phyric ash flow tuffs, commonly altered to schistose quartz-sericite ± chlorite ± pyrite beneath the ore,
• Tuffaceous Shale (the ore host) - 35 to 200 m thick, a bedded volcaniclastic sandstone and siltstone, generally strongly altered to schistose sericite ± quartz ± carbonate ± chlorite ± pyrite near ore,
• Black Slate - the immediate hangingwall - 0 to 70 m thick,
• Hangingwall Pyroclastics (epiclastics) - 50 to >200 m thick, commonly rhyolitic to dacitic volcaniclastic breccia,
• Mount Black Volcanics (dacitic to andesitic lavas) - >1000 m thick.
This sequence is truncated immediately to the west of the ore zone by the high angle reverse Rosebery Fault.
The orebodies are tabular sheets up to 10 m or more thick, dipping at 45°E. They extend over a strike length of 4 km north-south and down plunge to a depth of more than 1.5 km. They form an annular composite body made up of a series of overlapping lenses surrounding a siliceous core carrying stringer pyrite-chalcopyrite.
There is a vertical zonation of mineralogy from bottom to top of:
• Disseminated pyrite-chlorite,
• Massive pyrite-chalcopyrite,
• Massive sphalerite-galena-pyrite with minor tetrahedrite, arsenopyrite and gold. This ore displays a strong mineralogical banding developed parallel to both lithological contacts and the well developed cleavage,
• Barite-carbonate zone,
• Hematite rich tuff.
Overall the ore minerals comprise 25% sphalerite, 5% galena, and 3% chalcopyrite with 30% pyrite with minor amounts of tetrahedrite, pyrrhotite and arsenopyrite.
Production and reserve figures in 1990 were:
Historic production - 15.3 Mt @ 0.7% Cu, 4.8% Pb, 15.7% Zn, 159 g/t Ag, 2.8 g/t Au,
Reserves - 8.0 Mt @ 0.5% Cu, 3.5% Pb, 10.4% Zn, 109 g/t Ag, 2.8 g/t Au.
According to Large, et al. (2001), production + resources were:
31.7 Mt @ 0.6% Cu, 4.4% Pb, 14.3% Zn, 146 g/t Ag, 2.3 g/t Au.
According to Seymour et al. (2007), the total production + resources were estimated at:
34.03 Mt @ 0.57% Cu, 4.1% Pb, 13.8% Zn, 143 g/t Ag, 2.2 g/t Au.
Remaining reserve in 2007 were (Zinifex, 2007):
11.7 Mt @ 0.42% Cu, 3.83% Pb, 13.0% Zn, 138 g/t Ag, 1.9 g/t Au.
Remaining measured + indicated + inferred mineral resources at 30 June, 2014 were (MMG, 2014):
17.2 Mt @ 0.4% Cu, 3.6% Pb, 11.3% Zn, 123 g/t Ag, 1.7 g/t Au,
Remaining proved + probable ore reserves at 30 June, 2014 were (MMG, 2014):
5.4 Mt @ 0.3% Cu, 3.4% Pb, 9.7% Zn, 115 g/t Ag, 1.4 g/t Au.
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2015)
