Discrimination and Variance Structure of Trace Element Signatures in Fe-Oxides: A Case Study of BIF-Mineralisation from the Middleback Ranges, South Australia

Abstract

Uni- and multivariate statistical analyses of trace element laser-ablation inductively coupled plasma mass spectrometry data for Fe-oxides from banded iron formation (BIF) and BIF-hosted ores in 13 deposits/prospects of the Middleback Ranges, South Australia are presented. The obtained trace element signatures were considered within a petrographic-textural framework of iron-oxide evolution from magnetite through clean martite and porous martite to platy hematite, to evaluate changes in trace element concentrations with respect to the ore enrichment processes. Statistically valid distinctions among different hematite textures were indicated for most trace elements by linear mixed-effects models. Furthermore, the hematite data showed significant intra-class correlations between spot-analyses within individual polished blocks and correlations between polished blocks within individual deposits. The data are thus aggregated within their hierarchical levels. Two linear discriminant function analyses were performed to determine the combinations of trace elements that can distinguish hematite by textures and by location within the Middleback Ranges. Tin, a significant discriminator element in both models, reflects the regional influence of granite-affiliated hydrothermal fluids on the clean martite. This granitic signature, therefore, postdates formation of magnetite BIFs and potentially represents a supergene ore enrichment stage. The combination of Ni, Co, Ti and Nb was discovered to be uniquely attributed to discrimination of the Northern and Southern Middleback Ranges, indicating very specific local settings unrelated to hematite textures. Both local and regional settings impacting on the trace element signatures of Fe-oxides throughout iron ore formation are recognised, suggesting distinct ore enrichment conditions within various segments of the belt.

Appendix A: Laser Ablation-Inductively Coupled Plasma-Mass Spectrometry Data Acquisition

LA-ICP-MS spot analyses of different textural types of hematite and magnetite from fifty-one one-inch polished blocks were performed, measuring the following isotopes: ²⁴Mg, ²⁷Al, ²⁹Si, ³¹P, ⁴³Ca, ⁴⁹Ti, ⁵¹V, ⁵³Cr, ⁵⁵Mn, ⁵⁹Co, ⁶⁰Ni, ⁶⁵Cu, ⁶⁶Zn, ⁶⁹Ga, ⁷⁵As, ⁸⁸Sr, ⁸⁹Y, ⁹⁰Zr, ⁹³Nb, ⁹⁵Mo, ¹¹⁸Sn, ¹²¹Sb, ¹³⁷Ba, ¹³⁹La, ¹⁴⁰Ce, ¹⁴¹Pr, ¹⁴⁶Nd, ¹⁴⁷Sm, ¹⁵³Eu, ¹⁵⁷Gd, ¹⁵⁹Tb, ¹⁶³Dy, ¹⁶⁵Ho, ¹⁶⁶Er, ¹⁶⁹Tm, ¹⁷²Yb, ¹⁷⁵Lu, ¹⁸²W, ²⁰⁸Pb, ²³²Th and ²³⁸U. Analysis was carried out at Adelaide Microscopy (The University of Adelaide) using a Resonetics LR (ASI) 193-nm Excimer laser microprobe coupled to an Agilent 7700cx Quadrupole ICP-MS. Total integration time for each spot was 70 s (s): with 30 s of background measurement followed by 40 s of ablation. Spot sizes of 29 and 43 µm were set depending on mineral grain size and trace element concentrations were calculated by referencing against signals from primary standards GSD-1G and BHVO-2G. Accuracy was monitored with reference materials GSD-1G and BHVO-2G by bracketed runs of up to 20 analyses to correct for instrument drift. Time-resolved signals were exported offline and examined for evidence of mineral inclusions and/or other mineral phases. Data reduction was performed using Glitter software (Van Achterbergh et al. 2001). The portion of the signal displaying constant counts as a function of acquisition time was selected for integration using ⁵⁷Fe as internal standard.