Gold Mineralisation, Quaternary Structure and Authigenic Clay Geochemistry, Northeast Ben Ohau Range
1993
Smith, Dean
An area, approximately 70km2, at Glentanner, northeast Ben Ohau Range was studied. The main stratigraphic units in the area are the Mesozoic Torlesse Terrane consisting of quartzofeldspathic TZ2B-3A (schist) greenschist facies and TZ1 (greywacke) pumpellyite-prehnite facies, and the Glentanner Formation which represents a Quaternary fluvial greywacke conglomerate.
Within the Glentanner Formation there are blue/green beds which contain an authigenic green phyllosilicate. Microprobe analyses of the phyllosilicate indicate that it is an Fe rich aluminosilicate which defines a trend from a biotite composition to increasing A1 and Si and decreasing K. Whole rock XRD analyses indicates that smectite is the main clay present in the blue/green beds. The green phyllosilicate represents an Fe2+ smectite, produced during the alteration of biotite in a low oxygen, acidic, high μC02 and permeable environment.
A set of sub-parallel north trending faults within the area represent a northward extension of the Ostler Fault Zone. The major fault of the zone is a N-S striking, west dipping high angle thrust between schist and greywacke basement. East of the main thrust the greywacke basement has faulted into an imbricate fan structure. Folds have developed in the Glentanner Formation and greywacke with the latter appearing to overprint some Mesozoic structures. The fault zone is part of a fold and thrust belt developed to the east of the main divide during the current collisional tectonic regime.
A zone, approximately 15m wide, containing an array of quartz veins within altered schist was discovered on the eastern side of Freds Stream. Vein mineralogy consists of quartz, carbonate (calcite and ankerite), chlorite and subordinate sulphides and gold. Ankerite replacing chlorite is the main mineralogical change in the altered schist. Weathering results in ankerite altering to limonite• giving the altered schist a distinctive red/brown appearance in outcrop. Mass balance analyses on veined and altered schist show that in general there has been an overall enrichment in Ca, Sr and LOI as a result of carbonate deposition during mineralisation. Si is enriched in veined schist and depleted in altered schist. This is a result of Si release during ankerite replacement of chlorite and Si deposition during veining. Trace element analyses of chlorites from the veins show high and low concentrations of Zn and Cr respectively. The preferential incorporation of Zn and Cr into chlorite depends on chemical conditions during mineralisation. Using the six member chlorite solid solution model for chlorites from the veins, conditions during mineralisation were approximately 300°C, logaO2 -31.51 to -33.79, pH 6.33 to 6.46, logaS2 -9.94 to -11.11 and logaH2S -2.31 to -2.48. Carbon and oxygen isotopes from calcites from Freds stream veins range between d13C=-6.44 and -9.96 per mil and d18O=10.29 and 14.43 per mil respectively.
The d18O values show a departure from the field of isotopic signatures for Otago and Alpine Schist calcites and this has been interpreted as an isotopically heavy metamorphic fluid mixing with an isotopically light meteoric fluid. The d13C values are consistent with a metamorphic source for C02. Although the stable isotope signatures do not rule out a magmatic source for the fluid, the lack of any temporal and spatial igneous rocks does. Fluid inclusion homogenisation temperatures from Freds Stream vein quartz range from 236 to 270°C. Using a formation temperature of 300°C this gives a formation pressure of 0.5 to 1.5kb and therefore a formation depth of approximately 3-4km. Gold was transported by reduced sulphur complexes in a reduced high IDS metamorphic fluid. Interaction with an oxidised, low IDS (total dissolved sulphur) meteoric fluid resulted in a drop in gold solubility and hence gold precipitation. Chalcopyrite, galena and pyrite formed as a result of Cu, Pb and Fe release during wall rock alteration and immediate precipitation during interaction with dissolved S in the mineralising fluid. Freds Stream veins formed in a permeable fracture zone created during deformation associated with the current collisional tectonic regime in the South Island. High heat flow close to the surface east of the main divide is related to permeability in the Ostler Fault Zone.
124 leaves : col. ill., maps ; 30 cm.
Three maps in pocket.