Pyrite trace element haloes to northern Australian SEDEX deposits

This research presents results from a systematic laser ablation-ICPMS investigation of the trace metal content of sedimentary pyrite associated with two northern Australian Proterozoic sediment-hosted Zn deposits: the McArthur River deposit (237 Mt @ 9.25% Zn, 4.1% Pb, 41g/t Ag, Gustafson & Williams, 1981) and the Bluebush prospect (5000 Mt @ 0.8% Zn, Large et al., 2005). Previous research, including the recognition of primary geochemical zoning (Lambert & Scott, 1973; Large et al., 2000; 2001), provide the geological context for the McArthur River case study. Twenty-six rock chips were utilised from drill cores within the primary geochemical zones at various distances from the McArthur River deposit: BMR2 (23 km from deposit); Barney Creek3 (15 km); Homestead6 (6 km); F11/98 (300 m) and twelve additional samples from three underground locations on the western margin of the deposit. Texturally similar, fine-grained sedimentary pyrite (Py1) consisting of bedding-parallel layers (327 determinations) and framboidal aggregates (259 determinations) were ablated using a NewWave UP-213 Nd:YAG Q-switched laser ablation system and a 12pm beam. Element concentrations were determined using an Agilent HP4500 quadrapole inductively coupled plasma mass spectrometer for: Ti, Cr, Mn, Co, Ni, Cu, Zn, As, Se, Zr, Mo, Ag, Cd, Sn, Sb, Te, Ba, La, W, Au, TI, Pb, Bi, Th, and U. Detection limits are typically 1-10 ppm for most elements, and range from ‚ÄövÑvÆ0.3 ppm (U, La) to ‚ÄövÑvÆ30 ppm (Cd, Se). Instrument calibration is with the international standard glass NIST612, ablated with a 110 pm wide line at 3 pm/sec and 10 Hz at 10 Jcm-2. The in-house standard STDGL2B-2 is used to calculate the concentrations of unknown minerals (Danyushevsky et al., 2003; 2004). The pyrite trace metal data form three distinct groups with respect to the LA-ICPMS detection limits. Group 1 (>80% of analyses, several orders of magnitude > detection limit) includes: Co, Ni, Cu, Zn, As, Sb, Ba, TI and Pb. Group 2 (-50% of analyses > up to an order of magnitude > detection limit) includes: Ti, Mn, Zr, Mo, Ag, Sn, La, Bi, Th and U. Group 3 (>98% of analyses < detection limit) includes: Cr, Se, Cd, Te, W, Au. The research is focussed on relationships between the Group 1 and 2 trace metals, insufficient data prevents meaningful conclusions being made about the Group 3 trace metals. The concentrations of Co (63-190 ppm), Ni (77-340 ppm), Cu (133-332 ppm), As (504-3484 ppm), Mo (21-60 ppm), Ag (5-13 ppm), Sb (14-52 ppm), TI (11-557 ppm) and Bi (1.01-12 ppm) are many times greater than their corresponding whole‚ÄövÑvÆrock concentrations and suggest pyrite is likely to be the main host minerals for these elements. Barium, La, Sn, Th, Ti, Pb, U, Zn, and Zr are likely present as inclusions within the pyrite. Lead and Zn are likely contained within galena and sphalerite inclusions and these contain significant concentrations of most trace metals. At the McArthur River deposit, down-hole and spatial plots of the trace metals for each sample site display complex distributions related to the interpreted redox conditions. Pyrite trace metals displaying the clearest distal-proximal increase trends include: Ag, As, and TI. Pyrite trace metals that decrease in abundance towards the deposit include: Bi, Mn and Ni. Statistical correlations between these and other trace metals displaying weaker trends yield a large number of trace metal relationships which are potentially useful as geochemical guides to ore. Pyrite trace metals relationships between TI:As, TI:Ag, TI:Mo, Bi:As and Co:Ni exhibit the clearest spatial relationships from the BMR2 drill hole (23 km from deposit) towards the McArthur River deposit and are proposed as potential geochemical guides to ore for northern Australian SEDEX deposits. At the Bluebush prospect, the host consist of organic-rich siltstone and carbonates of the Riversleigh Siltstone Member of the McNamara Group and were likely deposited into relatively deep (below wave base) within a 3rd order, fault bounded (rifted) basin. Iron minerals including pyrite, siderite, chlorite, and minor magnetite and hematite form distinct sulfide, carbonate, silicate and oxide zones, reflecting a basin-wide change from reduced to oxidised conditions. These zones are interpreted to reflect a redox stratified water column and superimposed sea level fluctuations during sedimentation (Krassay et al., 2000). The rocks are enriched Mg, P, Mn, Fe, Ni, Cu, Zn, As, Mo, Sb, TI, Pb, U and REE's relative nonmineralised Riversleigh Siltstone. Zinc mineralisation typically consists of 0.5-1% Zn over many tens of metre intervals in pyritic siltstone, shale and carbonates. To date, no occurrences of bedded sphalerite, typical of SEDEX deposits have been seen at Bluebush. The main form of sphalerite (Sp2) is paragenetically late and typical modes of occurrence include: replacement of carbonate cement and unaltered patches of the host limestone; replacement of fibres in extensional veins; within strain shadows; brittle fracture fillings within recrystallised pyrite beds; and carbonate grain replacement within discordant dewatering structures or stylolites. In contrast to the host sediments that it replaces, Sp2 shows no evidence of deformation or recovery. Minor base metals occur in the oxidised units. The sphalerite (Sp3) is typically intergrown with chlorite, silica and hematite. Mean temperatures estimated from chlorite compositions are 329¬¨‚àûC (n = 52). Whole-rock d13C isotope values range from -5.83%0 to -0.14%0. Through the Carbonaceous unit, d13C values steadily increase, and then decrease towards the relatively oxidised Chlorite/Silica unit. The d13C values shift significantly towards light values through the Chlorite/Silica unit and gradually shift back to heavier values into the Carbonaceous unit. The corresponding 8180 values range from 11.44%0 to 24.42%0. Through the Pyritic unit, 8180 remain relatively consistent, but step-shift to very lighter values into the Chlorite/Silica unit. They trend back towards heavier values up section into the Carbonaceous unit. Whole-rock relationships between Fe, C and S suggest pyrite formation was sulfur limited. Sulfur isotopes of the fine-grained pyrite (Py1) range from -8.55%0 to 37.04%0 d34S. through the Pyritic Carbonate unit, they display a strong trend towards heavier values up-section and are consistent with Rayleigh fractionation in a closed system. The S isotopes through the hangingwall display extreme fractionation trends over short stratigraphic intervals and are best explained by extremely low concentrations of seawater sulfate during the Proterozoic (e.g. Kah et al., 2004; Lyons et al., 2006). Texturally similar, fine-grained sedimentary pyrite (Py1) from 12 regular spaced samples in the BLBD23 drillcore (143 individual determinations) were analysed by LA-ICPMS. The operating conditions were the same as used for the McArthur River samples, however, Bluebush analyses used a 20pm beam. The detection limits for most trace metals were typically an order of magnitude less than at McArthur River. The pyrite trace metal groupings with respect to detection limits at Bluebush are similar to McArthur River. However, pyrite trace metal concentrations are typically an order of magnitude less than at McArthur River. Pyrite recrystallisation also results in a decrease of up to an order of magnitude for most trace metals. Silver, As, Cu and TI remain relatively resistant to remobilisation. Arsenic is a proxy for pyrite at Bluebush and most trace metals display either positive correlations suggesting coupled substitution with the As into the pyrite, or negative correlations that suggest a change in their oxidation state with the changes in basin redox conditions. Downhole plots for the trace metals are complex and reflect numerous geological and geochemical processes including: the basin redox conditions, the higher temperature alteration, basin wide mass flow sedimentation, pyrite recrystallisation and the timing and composition of hydrothermal fluids. These factors impose significant limitations on the application of the pyrite trace metal relationships proposed as potential geochemical guides to ore for northern Australian SEDEX deposits. A genetic model is proposed for Bluebush that invokes an Fe-rich but relatively low S hydrothermal fluid. These fluids are expelled into a highly reduced, sulfur poor basin. Primary base metal sulfides may have been nucleation sites (and sources of sulfur) for pyrite. During diagenesis, the pyrite was recrystallised, the trace metals remobilised and some re-precipitated as widespread, low-grade Zn mineralisation.