Gypsum and other sulfate minerals are uncommon in eastern Australian caves. Gypsum, however is considered to be the third most abundant secondary mineral in caves after calcite and aragonite.
The abundance of gypsum in northern hemisphere caves is most likely related to the presence there of strata of gypsum interbedded within the stratigraphic sequences that contain cavernous limestone. Gypsum beds do not occur in the Early Palaeozoic strata of eastern Australia in which our limestone caves are developed.
Very little is known about the origins of eastern Australian cave deposits of gypsum and other sulfates, so alternative sources of sulfur must be found to account for their presence in these caves. Recent work at Jenolan Caves has shown that stable isotopes of sulfur allow the identification of sulfur sources for cave sulfate minerals.
At Jenolan two sources of sulfate have been identified: from chemically-altered bat guano and from weathering of secondary pyrite. Guano-derived sulfates were by far the most abundant, and are presumed to originate from the Large Bentwing Bat, Miniopterus schreibersii. Gypsum from the weathered pyrite source gave low sulphur isotope ratios, while gypsum and ardealite from alteration of bat guano gave much higher sulphur isotope ratios.
Sulfate minerals are known to occur in a number of eastern Australian limestone caves, for example Jenolan, Wombeyan and Walli, New South Wales, and several caves in Tasmania. As part of the study, specimens from known depositional environments from overseas gypsum deposits would also be used, as well as Australian examples. When coupled with stable oxygen isotopes as well, information may be provided on environmental conditions during mineral formation, and sulphur fractionation paths.
The caves at Jenolan, discovered in 1838 by Europeans, are one of the premier tourist attractions of New South Wales located 182 km west of Sydney in the Main Dividing Range. They are developed in the north and south sides of a natural archway in a belt of folded, near vertical limestone 7 km long. Nine caves are regularly shown to visitors, but several hundred of various sizes and shapes are known from the area. In spite of several scientific studies of the caves over the last 150 years, their formation has been poorly understood, and their mineral assemblages largely unknown.
As part of a joint research project between the Australian Museum, CSIRO, Sydney University and the Jenolan Caves Reserve Trust, minerals such as clay, gypsum and phosphates, registered into the collection in 1898, and newly-collected specimens, are being examined with modern methods of analysis. The old specimens were donated by Mr. Voss Wiburd, the Jenolan Caves Caretaker at that time. There has been very little work done on Jenolan Caves minerals since the 1890s, and this current joint study, which started in 1998, is making some important discoveries, being the most comprehensive survey of minerals at Jenolan Caves for over 100 years. Some cave passages are being mapped in great detail to give more information about geological structures and mineral distributions. This study of cave minerals is leading to new ideas about when and how the caves were formed, and how they have changed since their formation.
The old specimens have been re-analysed with the latest methods of X-ray diffraction, X-ray chemical analysis, sulphur and oxygen isotope studies, and potassium-argon dating to extract new information on mineral formation. Recently-collected specimens are also revealing their secrets when analysed using the same methods. For example, rounded, knobbly formations ('The Potatoes') in Lucas Cave at Jenolan, viewed by tourists for over 140 years, have been identified as an unusual calcium sulphate-phosphate mineral called ardealite, encased in gypsum, not the common calcite (calcium carbonate) they were always thought to be. This is the first record of ardealite in New South Wales, but it has previously been found in Western Australia. The Jenolan clays are kaolinite-illite-quartz mixtures which have been dated using potassium-argon (on illite) and fission-track (on zircon) methods, to the period when the Bathurst Granite was being formed, making the caves much older than previously thought - possibly hundreds of millions of years, rather than hundreds of thousands or a few million years.
An unusual feature of Jenolan Caves is that one group of minerals has formed from interaction between geological and biological processes. Water-soluble minerals sylvite (potassium chloride) and niter (potassium nitrate) have been found in large open caverns - the Grand Archway and Devil's Coach House. These minerals have formed from leaching of wombat and wallaby guano by percolating groundwater. Also, reaction of bat guano with limestone and clay has produced a range of unusual phosphate minerals. So far, ten mineral species not previously known or proven from Jenolan have been identified (ardealite, huntite, hydromagnesite, crandallite, ferroan dolomite, montgomeryite, illite, kaolinite-1Md, romanechite, and sylvite) and the identity of eight previously known ones has been verified (aragonite, niter, taranakite, gypsum, hydroxylapatite, hematite, goethite, quartz). The most common minerals are calcite, dolomite, aragonite, kaolinite, illite, quartz, hematite, goethite and gypsum. In a few cases, the actual 1890s collecting sites of some of the minerals have been relocated. More detailed studies examining the distribution and origins of aragonite and sulphate minerals (gypsum and ardealite) at Jenolan are in progress.
Principal Investigators: Ross Pogson and David Colchester (Australian Museum), Dr Armstrong Osborne (University of Sydney), Dr Dioni Cendon (University of Wollongong), Dr Szabolcs Leel Ossey (Eotvos Lorand University of Budapest), Prof Alexander Klimchouck (Ukrainian Academy of Sciences), Ms Jill Rowling (University of Sydney), Dr Horst Zwingmann (CSIRO).
Copyright © Australian Museum, 2004