10.80350/TEST_DOI_5E58998297508
Geological Survey of New South Wales
2011
Text
0155-3410
Fossil microbes in opal from Lightning Ridge - implications for the formation of opal
2011-06-01
Watkins J.J.
Behr H.J.
Behr K.
Quarterly Notes of the Geological Survey of New South Wales
Opal from Lightning Ridge is amongst the most valuable and widely known in the world. Black opal, characterised by a dark body tone, is the rarest and most valuable. The opal occurs in geode-like nobbies up to several cm in diameter and in seam-like structures in an Early Cretaceous volcaniclastic host rock. The host rock at Lightning Ridge consists of a finely laminated silty claystone that often has a high content of organic detritus. Strong bioturbation by nematodes is common, as are opalised macrofossils. This study reports on the fossil microbe communities discovered within both the host rock and opal in cell numbers up to 107108/cm3. The most common microbes are the aerobic bacteria actinomycetes (Nocardia, Streptomyces, Micromonospora) and myxobacteria. The fossil microbes (mostly preserved as moulds) occur in the form of mycels, mats, biofilms, globular colonies, networks, swarms and as individuals. The cell forms are mostly rod-shaped, ovoid and coccoid and generally range from 25 m but may exceed 100 m. Small globular spores may contain organic residues with strong red fluorescence. All the microbes are autochthonous and are the same age as the opal. The type of fossil microbe communities found in Lightning Ridge opal generally occur in soil or in organic muds deposited under still conditions or in a surface-fouling biomass. The microbes require a nutrient-rich (cellulose and chitin) near-surface aerobic environment with temperatures less than 35 C and near-neutral pH. The microbes produced carbonic and organic acids that aided the biochemical weathering of clay minerals and feldspar to produce silica hydrosol. The kind of environment required by the microbes for life indicates the conditions under which opal was produced. This enables the determination of a new timetable for opal formation involving weeks to a few months and not the hundreds of thousands of years envisaged by the conventional weathering model.