Exhalative lithologies along the Dee Range in central coastal Queensland consist of thin (<1m thick), semi-continuous to discontinuous lenses of red to maroon-coloured jaspers, jasper peperites and lesser jasperoidal limestone. Although jasper and jasper peperites are widely distributed along the Dee Range, they are not volumetrically significant. These lithologies crop out in four horizons within the Banded Mineralised Sequence, which is the stratigraphic equivalent of the Banded Mine Sequence at Mt Morgan. The Banded Mine Sequence at Mt Morgan is the host to a world class Au- Cu orebody (350,000 metric tonnes of Cu). The Banded Mineralised Sequence along the Dee Range hosts sub-economic base metal mineralisation, with the Ajax and Upper Nine Mile Creek prospects being the most significant.
Jasper outcrop is
typified by rubble upon the numerous narrow ridge lines, may be spatially associated with manganiferous siltstones and is often partially coated by black manganese oxides. Jasper peperite outcrops consist of contorted to lensoidal jasper domains, usually less than 5cm thick, within quartz-feldspar porphyry bodies. Within drillcore, jasper is most likely to be present as mm-scale wisps, lenses, or clasts within mass flow units, or exhibiting peperitic contacts with quartz feldspar porphyry. Jasper domains within limestone usually display curved sharp margins and may consist of either jasper clasts within limestone, or jasper veinlets within limestone breccias. Jasper sensu-stricto from the Dee Range predominantly consists of cryptocrystalline quartz, very finely microcrystalline iron oxide (haematite?) and microcrystalline haematite. Lesser mineral phases include magnetite/martite, goethite, detrital grains of quartz and feldspar, vein quartz, chlorite, and very rare pyrite.
High magnification (X100 objective) reflected light microscopy reveals an intimate association between quartz and iron oxide within jasper. Fine, near spheroidal crystals of iron oxide exhibiting strong red internal reflections are widely but not uniformly dispersed within cryptocrystalline silica that is isotropic under crossed polars. Circular, to ovoid, to elliptical domains (up to ~100µm across) of dense accumulations of this fine iron oxide are prominent within many jaspers. Often these large 'globules' appear to be comprised of smaller circular, elliptical or irregular iron oxide aggregates or 'sub-globules' generally < 10µm across. Elliptical and more irregular folded or kinked domains may exhibit concentric banding,
consisting of alternating bands devoid of iron oxide (cryptocrystalline quartz) and iron oxide rich (cryptocrystalline quartz and iron oxide). These textures are interpreted to reflect coprecipitation of amorphous silica and amorphous iron oxyhydroxide from a colloidal gel, with subsequent transformation during diagenesis to cryptocrystalline quartz and microcrystalline haematite respectively.
Numerous banded and branched microtextures within jasper, jasper peperite and jasperoidal limestone are strongly suggestive of mineral encrustations of filamentous microorganisms from ancient and modem seafloor hydrothermal and terrestrial geothermal environments and sites of acid mine drainage. Branched silica within microcrystalline haematite is the most prevalent microbial textural sub-type of Dee Range jaspers, with elongate to lenticular haematite microcrystals preserved within elongate to
ellipsoidal void or cryptocrystalline silica domains comprising another significant textural sub-type. Circular to elliptical domains of cryptocrystalline silica or void that may be rimmed by haematite microcrystals are another prevalent texture suggestive of the presence of microorganisms or other symbiont fauna. Living or recently deceased microbial cells may have provided a convenient substrate or template for initial inorganic mineral precipitation during conductive cooling and mixing of a seawater-hydrothermal fluid mixture on the seafloor.
Major element chemistry (mean [SiO2+Fe2O3-total]= 94%, Al2O3 1.75% and Al/(Al+Fe+Mn)= 0.06-0.34) supports petrographic observations that Dee Range jaspers are predominantly quartz and iron oxide and that major element geochemical
inputs are overwhelmingly hydrothermal rather than tuffaceous and/or hydrogenous. The high Fe/Mn ratio (ranging from 1.3-133.5) of Dee Range jaspers is indicative of the redox associated fractionation of Fe and Mn and suggestive of jasper precipitation close to vent orifices. Furthermore, the low Co/Zn (mean = 0.06, median = 0.04) ratio of Dee Range jaspers is typical of a hydrothermal rather than hydrogenous trace element provenance. The relatively lower Ba content of Dee Range jaspers, together with the absence of sulfides suggests that the hydrothermal fluid was too oxidised to transport significant S, metals, and Ba to the seafloor. The immobile element Ti/Zr ratio is similar to that of felsic volcanic lithologies from the Dee Range, suggesting that the detrital fraction is of felsic volcanic provenance. Dee Range jaspers exhibit a relatively higher mean REE content (74ppm) than other ancient Si-Fe exhalites from VHMS environments, while exhibiting similar relative light REE
enrichment, strong positive Ce, weak positive Eu and strong negative Y anomalies when normalised to modern Pacific seawater. Chondrite normalised REE/REY plots display similar trends to seawater-normalised plots but show strong negative Eu anomalies.
Whole rock oxygen isotope values of jasper sensu-stricto ranges from 11.3-15.5‰ and produces geothermometric temperature estimates for precipitation of 129-176°C for late Devonian seawater and 150-206°C for evolved seawater. Temperature estimates corrected for mineralogy indicate likely maximum temperatures of ~150°C and some modification of the δ18O of earlier exhalite horizons by subsequent hydrothermal cycles. Three extremely negative δ34S values (ranging from -14.2 to - 34.2‰) obtained from sulfides from drillcore from the UNMC and Fab prospects are characteristic of a sulfur source involving the bacterial reduction of seawater sulfate and lend support to the existence and activity of microorganisms during the deposition of the Banded Mineralised Sequence. The bimodal sulfur source of Dee Range sulfides is similar to that of the Iberian Pyrite Belt and reflective of the significant proportion of sediment within the volcano-sedimentary pile on the Dee Range.
The geochemistry indicates that Dee Range jaspers were precipitated from a seawater dominated hydrothermal fluid, but elevated temperature estimates and mean REE contents compared to modern and ancient hydrothermal exhalative
sediments may indicate an additional REE source to seawater. This could reflect leaching of REE during convective circulation of seawater, although the relatively low Eu content of Dee Range jaspers indicates that leaching of Eu from the volcanic pile via breakdown of plagioclase feldspar was probably limited by the redox and temperature properties of the fluid. The mineralogy, geochemical characteristics, and outcrop distribution of Dee Range jaspers, along with the tendency for Zn and Pb sulfides to be the most prevalent mineralisation in sub-economic base metal prospects on the Dee Range suggest that the exhalative hydrothermal system is a relatively lower temperature, more diffuse system than that at Mt Morgan.