Weld River fossicking area
This 2 km2 area is adjacent to the Tasman Highway at Moorina in northeast Tasmania, and is about 250 km NNE of Hobart and 70 km northeast of Launceston. The area can be reached by the Tasman Highway, either via Scottsdale or St Helens.
The two main collecting areas are shown on the accompanying map. One is reached from Frome Road, which leaves the Tasman Highway opposite the Moorina Golf Course, by walking along the banks of the Weld River. The other is reached via an all-weather track which leaves the Tasman Highway approximately two kilometres south of Moorina.
This area has extensive alluvial deposits which were worked for tin for many years. The alluvial material was derived by weathering and erosion of granite, basalt and other rock types.
Corundum (`Sapphire') Al2O3 - this is one of the most sought after minerals in the area, and is moderately common as subrounded to well-rounded grains, usually small but rarely up to a few centimetres in size. The colour is usually a dark blue, but is sometimes green or parti-coloured. Some dark grains show chatoyancy from included rutile, and may be termed `star sapphire'. Tabular fragments of hexagonal crystals are often seen. The mineral is thought to originate deep in the crust and was brought to the surface in Tertiary basalt. Some pink stones found in the area have been described as ruby.
Hercynite FeAl2O4 - this is one of the more common heavy minerals in the alluvial materials and occurs as subrounded to well-rounded grains up to about a centimetre in size. The colour is usually opaque black in hand specimen, but it may be blue, green or brown in transmitted light. Hercynite was known as `blackjack' amongst tin miners. Well-formed octahedral crystal fragments are often seen. The mineral is thought to originate deep in the crust and was brought to the surface in Tertiary basalt.
Cassiterite SnO2 - this was the major economic mineral in northeast Tasmania, and tin mines were widespread in the area. It is still abundant as a major constituent of the heavy black alluvial sands, occurring as subrounded to well-rounded grains up to a few millimetres in size. Cassiterite is usually a dark brown to black colour, but is sometimes ruby red (`ruby tin'). The mineral originated in the tin-bearing granites common in the area.
Topaz Al2SiO4(OH,F)2 - this occurs as occasional small glassy grains to a few millimetres in size, usually colourless, pale blue or translucent white. It was derived from the tin-bearing granite.
Chrysoberyl BeAl2O4 - this rare mineral has been found as small grains in alluvial materials. It superficially resembles sapphire but is always pale green. Some grains are red under artificial light, indicating that they are the variety `alexandrite'. Some stones exhibit a `cats-eye' chatoyant effect, and are known as `cymophane'. Their origin is uncertain, but may be in deep-seated rocks, brought up with sapphires etc., although some other beryllium minerals have been recorded in some of the tin granites.
Zircon ZrSiO4 - this is one of the more common heavy minerals in the alluvial materials, and occurs as angular to well-rounded grains up to a few millimetres in size. The colour is usually pale pink to reddish brown, and tetragonal crystals are often seen. The mineral has a mixed origin; some is derived from granite, while some of the coarser zircon is thought to originate deep in the crust, and was brought to the surface in Tertiary basalt. This is shown by Tertiary ages obtained from fission track dating.