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Subject: geochemistry × Type: PhD thesis × Author: Gallacher, Andrew David × Start date: 2001 × End date: 2001 ×

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    Geochemistry of sedimentary opal, Hebel, Southern Queensland
    Gallacher, Andrew David ( 2001)
    Previous studies on Australian opal were geologically uncontrolled and analysed specimens were commonly from an unknown source. This research is the first integrated geologically-controlled study of sedimentary opal in Australia with the aim of refining models for opal genesis. The work was undertaken in the recently-discovered Hebel field of southern Queensland. Mapping, underground sampling and RAB drilling specifically for this project were undertaken. After petrography, weathered host rocks and opal were analysed by XRF, XRD, INAA, LA-ICP-MS mass spectrometry. Geologically-constrained precious and non-precious opal samples were analysed by using mass spectrometry, EMPA, IR, NMR, SEM and bioluminescence. Opal is hosted by porous, low density kaolinite-bearing Early Cretaceous argillaceous sediments overlain by Tertiary rudites and arenites. The sequence is highly weathered and mapping, mineralogical, geochemical and isotope studies show that weathering becomes less intense with depth. Deep weathering took place in the Late Cretaceous and silicification associated with tropical weathering took place in the Early Tertiary. Later Tertiary cooler more arid weathering features overprint the earlier weathering. Geochemical and isotopic studies show that during the long period of weathering, there was constant re-equilibration in an open dynamic evolving system as it still is today. Opal occurrences are stratigraphically controlled by smectitic-bearing clay horizons. There are possible local structures such as faults, joints and bedding planes which allowed the plastic deformation of opaline gel by dehydration into the enveloping smectitic sediments. Opal underwent post-consolidation brittle deformation. Continued long term weathering produced bleaching of and trace element stripping from of the outer rims of opal. This study confirmed structural differences between nonprecious and precious opal and showed that opal operates as a molecular sponge. Hence it was not possible to obtain a RblSr age of formation of opal but it was possible to show, on the basis of 0 and H isotopes, that there were several periods of isotopic overprinting, probably related to different episodes of weathering. NMR, IR and EMP A studies showed that the molecular structure of opal is governed by impurities, water occurs as OH- and possible short chain aliphatic organic compounds occur in the structure. Short chain aliphatic compounds are of biological origin. SEM studies show bacterial microfossil silica pseudomorphs. No DNA was detected in opal. Opal formed from long-term intense tropical weathering of porous permeable clastic sediments. Descending silica-laden fluids were dehydrated by expanding-lattice clays and formed as gel in structural sites in favourable stratigraphic horizons. Ubiquitous microorganisms probably played a role in opal deposition, possibly providing the locus for structural distortion during colloidal precipitation thereby promoting the formation of nonprecious opal. It is suggested that precious opal has formed from colloidal silica precipitation in the absence of bacteria thereby explaining the rarity of precious opal.