By Suzette

Tasmania has a rich history in ore geology and particularly West Tasmania is well-known for its mining industry. The formation of numerous ore bodies in this region were all related to three main geological events: the movement of hot fluids by volcanism in the Cambrian forming the primary minerals, the activity of the Great Lyell Fault exposing and oxidising some of the minerals, and a major orogeny in the Devonian causing the remobilising of the metals into veins and larger crystals. A simplified geological map can be seen below.

1
Figure 1: simplified geological map of the West Coast region of Tasmania.

The Spion Kop Lookout (nr 13 on Fig. 1) is a perfect spot to start understanding the geology in this area as the mountains reveal some of the major faults and exposed rocks in the area. The geology in this area consists of rocks that were formed on the seafloor: submarine lavas, volcanic ash, sediments. About 500 million years ago (Cambrian) volcanoes erupted and formed a thick pile of submarine lavas and ashes (Mount Read Volcanics; i – green colour in Fig. 1). Hot fluids, that were moving through this volcanic pile during the volcanism, were weakening the volcanic rocks while also depositing the ores (pyrite, copper, silver sulphides, gold). After the volcanism, a rift valley was formed in the Late Cambrian-Early Ordovician. This lead to extensional faults (Great Lyell Fault; Fig. 2). Material that filled up the rift valley is comprised of boulders and sand (Owen Conglomerate), which was derived from erosion of quartzites and schists. The Mount Read Volcanics and the Owen Conglomerate where covered by limestone and sandstone in the Ordovician-Devonian. In the Devonian (390 million years ago) a major deformation event caused the formation of the North Lyell Fault and folding and faulting of the Mount Read Volcanics and Owen Conglomerate (Figure 2). During this orogeny the entire rock column was folded, faulted, and heated, causing the schists of the original Mount Read Volcanics to harden again and recrystallised the ore minerals into larger crystals and into veins. Subsequent erosion re-exposed the schists and ore bodies.

2
Figure 2: Major faults and exposure of the Owen conglomerate and the Mount Read volcanics. (http://thelivingearth.com.au/)

An example of one of the ore bodies is the Iron Blow, known for the high abundance of pyrite. The Iron Blow ore deposit (nr 12 on Fig. 1) was formed when hot fluids rich in sulphur vented on the sea floor. The location of the Great Lyell Fault along the Lyell Schists (originally Mount Read Volcanics) is likely to be the reason why so many ore bodies are concentrated in the area of Mount Lyell (Fig. 3). The Iron Blow was the first major mine at Mount Lyell in West Tasmania. The potential of the outcropping ironstone for mining was discovered in the early 1880’s and was unsuccessfully started as a mine for gold. The mine was bought by mining investors from Melbourne in 1891 and the unexposed ore body near the mine was successfully mined for copper for over 120 years. The open pit of Iron Blow mine itself was only mined till 1929.

3a
Figure 3: Profile through the Iron Blow ore deposit. Corbett, 2014.

In West Tasmania, near Zeehan there are also many silver-lead deposits. These ores were formed during the late Devonian vein mineralisation. Before 1900 many small silver-lead-zinc-antimony mines were operative in the area; the ores are indicated in Fig. 4. Around 1900 a high-grade (2000 ounces silver/ton ore) silver-lead-antimony ore was discovered. This ore (Silver Spray Mine) became one of the most important mines in the area (Haberle, 2013). In order to easily transport the mined ore, a tunnel was made through which a tramway was build. We walked through the tunnel and visited some of the small surface mines, finding minerals such as galena.

A good overview of the mining in West-Tasmania is given at the West Coast Heritage Centre in the town Zeehan. The centre tells the story of the history and development of the West Coast, which was strongly influenced by the mining industry. The museum includes mining memories and machinery, a mineral collection, and photographic galleries.

Figure 4a: Ore deposits around Zeehan
figure 4b: one of the minerals we found

In addition to all the sulphide-rich ore deposits, some serpentine-bearing complexes were exploited for asbestos and the rare mineral stichtite in the 20th century. The ultramafic serpentinite rocks are hosting relatively high contents of Platinum Group Elements. In West-Tasmania, a series of dismembered ophiolite sequences are present. The ultramafic rocks were probably formed as cumulates in a magma chamber with boninitic-low titantium melts. During the early Cambrian, the lavas and cumulates were obducted by thrusts onto a continental terrane (Fig. 5).

5
Figure 5: Schematic view of the dismembered ophiolite for the geological setting of West Tasmania. Williams, 1978.

We visited ‘Serpentine Hill’, which is a quarry with exposed serpentinites. The serpinitites are the alteration product of peridotites and pyroxenites. In this quarry we found the mineral stichtite (Mg6Cr2(CO3)(OH)16.4H2O), which is a magnesium hydroxycarbonate and probably was formed by the alteration of chromite-rich dunites (Brown, 1986). We also found fibrous magnetite (Fig. 6).

Figure 6a: stichtite (pink)
Figure 6b: fibrous magnetite

 

References:

CORBETT, K.D. 2014. Chapter 4.7.6 The Mt Lyell field. in CORBETT, K.D., QUILTY, P.G and CALVER, C.R. editors, 2014. Geological Evolution of Tasmania, pp 201-272. Geological Society of Australia Special Publication 24, Geological Society of Australia (Tasmania Division).

HABERLE, C. 2013. Zeehan Silver Spray Mine and Tunnel published online http://www.think-tasmania.com/spray-mine/

BROWN, A. V. 1986. Geology of the Dundas–Mt Lindsay–Mt Youngbuck region. Bulletin Geological Survey Tasmania 62.

WILLIAMS, E. Tasman fold belt system in Tasmania. Tectonophysics, 1978, 48.3: 159-205.