The discovery of what appeared to be a new zeolite locality in the Tertiary basalts of northwest Tasmania, is actually an extension of a locality recorded by W.F. Petterd in 1893.
Dairy farming, and the growing of legumes and root crops such as peas, beans and potatoes, are important rural industries in Tasmania's northwest. This is mainly due to the deep brown to red soils resulting from the weathering of Tertiary basalts and a high rainfall. The Marawah-Deloraine region has the largest volume of Tertiary basalt in Tasmania which has formed an extensive lava plain (Sutherland, 1989). A number of different zeolite and related species have been recorded from many locatities throughout the northwest region, including the well-known Cape Grim area.
In June, field collecting trips were organised as part of the 1995 Joint Societies Seminar. While on their way from Burnie to Waratah to meet with others attending one of the trips, Ed Richards and Michael Hirst of the Mineralogical Society of Victoria, investigated a number of road cuttings along the Hampshire to Waratah Road. At one of these, 100 metres south of where the road crosses the Hellyer River, they found the zeolites chabazite and phillipsite, as well as other minerals, in vesicular weathered brown basalt boulders on the south side of the road. Mineralisation is patchy, with many vesicles empty. Zeolites were also found in situ in the cutting on the opposite side of the road. This locatity was visited later that week by the other field trip attendees.
Although this was thought to be a new discovery, particularly as the road has only been in use for a few years, it is related to a locality recorded as "...near the railway bridge crossing the Hellyer River..." (Petterd, 1893). This railway bridge is approximately 1.5km east of the current road. Petterd recorded chabazite, phillipsite and mesolite at this locality. The reference in the Catalogue of the Minerals of Tasmania (Anon, 1969), where both chabazite and phillipsite are recorded as occurring at the Hellyer River, with no more precise location details given, is probably the same location.
Interestingly, Petterd records chabazite occurring both as "obtuse rhombohedral crystals" and also the more complex twinned phacolite form. The latter is not evident in recent material collected.
Petterd's description of mesolite as "...a zeolite occurring as small globules of a fibrous structure..." is more likely to be thomsonite or gonnardite, both of which have been recorded from drill cores in the area (Seymour, 1989), although no specimen matching his description has been found at the current locality.
The Minerals
Calcite
Chabazite
Clay Minerals
Goethite
Phillipsite
Pyrite
References
Calcite is not abundant but has been identified in three forms: as colourless transparent acicular ragged crystal groups up to 12mm; massive, completely infilling vesicles; and as irregular greyish translucent rhombohedral crystals to 3mm.
This is the most abundant mineral found in the vesicles. It generally occurs as water-clear bright transparent interpenetrant twinned rhombs to 5mm across, and doesn't appear to readily dehydrate, as often happens with zeolites. It often occurs with no other minerals present, but may be associated with phillipsite or coated by goethite. Among the most attractive specimens are those from vesicles where only a small number of individual twinned chabazite crystals occur.
Some vesicles are filled or lined with either a brown or blue-green clay mineral dehydrating to black and crazing. The exact species has not yet been identified but is probably nontronite.
Many of the vesicles containing the zeolite minerals are lined with goethite, providing an attractive dark background. Goethite also forms as brownish films or as iridescent spheres on chabazite and phillipsite.
After chabazite, phillipsite is the next most abundant mineral found. It invariably occurs as simple twinned "fourlings" to 4mm long and occurs either alone or with chabazite. Unlike the chabazite crystals, the phillipsite does tend to alter on exposure to whitish crazed crytals.
Pyrite is relatively scarce as bright brass yellow cubes to about 2mm completly coating the surface of vesicles.
Anon, 1969: Catalogue of the Minerals of Tasmania. Geological Survey Record, Tasmanian Department of Mines, 9, 110p.
Petterd, W.F., 1893: A Catalogue of the Minerals Known to Occur in Tasmania, With Notes On Their Distribution, Papers & Proceedings of the Royal Society of Tasmania, 1-72.
Seymour, D.B., 1989: Geological Survey Explanatory Report Sheet 36: St Valentines.
Sutherland, F.L., 1989: Tertiary Volcanism in (Bennet, C.F. and Martin, E.L., Eds) Geology and Mineral Resources of Tasmania. Geological Survey Australia, Special Publication 15, 383-386.