The trace element chemistry of arsenopyrite and its potential use as an indicator mineral for gold deposit exploration in Australia

Laser Ablation Inductively Coupled Plasma Mass Spectrometry (LA-ICP-MS) has been used to determine the relative abundance of specific trace elements in 626 arsenopyrite samples from a selection of 79 different national and international gold and non-gold mineral deposits. From the investigation of these samples it was possible to determine inter-relationships between the relative abundance of specific elements in the arsenopyrite crystal and the geological environment associated with the formation of these crystals. On the basis of these relationships it was possible to determine relationships between the style of mineralization, associated host rocks, distance from significant gold mineralization, and the endowment of the mineralizing system. In addition to using trace element inter-relationships to determine these aspects of the mineralized system, an investigation was also undertaken to establish if the distribution of mercury compounds (HgS, HgCl2, HgO, and HgSO4) and the mercury isotope distribution patterns for these compounds can give an indication of the distance from an orebody within the depositional environment.

It was determined that the concentration of iron within the arsenopyrite crystal lattice was essentially consistent. In addition, the relative sensitivities of analyte isotopes were influenced (usually as a result of ICP-MS operating conditions and laser ablation conditions) to the same degree as iron. Therefore, it was possible to use the Counts Per Second (CPS) data for this element to normalize data for all other analytes and to ensure consistency of analytical data throughout the entire study data base of arsenopyrite crystals. This observation was confirmed using solution based analytical data. Consequently, iron-normalized analyte data were used for all interpretational purposes throughout the thesis.

Using Linear Discriminant Analyses (LDA), Principal Component Analysis (PCA), Comparability Index (CI), and analyte ratio protocols it was possible to differentiate arsenopyrite samples which were associated with gold depositional environments from those which were formed under different geological processes. Within the non-gold arsenopyrite sample suite (59 individual samples) it was also possible to differentiate arsenopyrite samples which were associated with different metal assemblages (Pb-Zn, Sn-W, Ag and Cu).

Arsenopyrite samples collected from three significant gold producing provinces in Australia (Yilgarn Craton, Pine Creek Region, and Victorian Goldfields) were investigated. It was determined that the trace element signatures of the relevant samples could be used to unambiguously differentiate samples from each of these provinces. Additionally, it was possible to use the trace element assemblages of arsenopyrite samples to differentiate between gold depositional camps from within the Yilgarn Craton (35 deposits) and the Victorian Goldfields (10 deposits).

The overall host rock type at the site of a mineral deposit (i.e. greenstone or Banded Iron Formation (BIF)) has a significant impact on the trace element chemistry of arsenopyrite. This effect is most significant when comparing samples from the Kambalda region (greenstone hosted) with deposits formed synchronously (and likely of a very similar fluid chemistry) in the Randalls region (BIF). It has also been determined that the differences in lithology within a particular deposit change the localized chemistry of the hydrothermal fluid during the mineralizing process. Within the Mount Porter deposit arsenopyrite formed in BIF has a different chemistry to the arsenopyrite formed in dolerite (a greenstone). Overall it was determined that arsenopyrite samples deposited in, or associated with, greenstones contained significantly more Co, and Ni than their BIF counterparts which in turn contained significantly more Se, Mo, Te and Bi. An investigation into the trace element assemblages of arsenopyrite from the Boorara deposit indicated that arsenopyrite formed in, or associated with, chlorite altered dolerite contained higher levels of Co and Ni than the arsenopyrite formed in carbonated dolerite (leucodolerite). This implies that as the host rock alters it transfers analytes into the adjacent hydrothermal fluid, thus affecting the arsenopyrite being formed in the vicinity.

Comparison of geologically analogous gold deposits, with endowments at least an order of magnitude apart, indicated that, when ablated, arsenopyrite from deposits with the greater endowment produced higher CPS data for Sn (commonly with increased Se and Hg CPS) compared to arsenopyrite from their less well endowed analogues. In addition, when the chemistry of arsenopyrite from gold deposits within the Yilgarn Craton was compared, it became apparent that Giant and Super-Giant deposits contained arsenopyrite yielding higher CPS data for Co, Sb, and Hg than arsenopyrite from the Intermediate and Small deposits.

Using samples from a number of known locations within the Homestake, Mount Porter, and Boorara deposits, it was possible to investigate the variation of arsenopyrite trace element chemistry within a deposit. This is most apparent in the Homestake deposit where the relative CPS for Ni, Se, Mo, and Sb in arsenopyrite changed systematically with respect to the distance from the main fluid focussing region (inferred to be the Main Ledge conduit).

Mercury has been recognised as a gold pathfinder in many geological environments since the 1970's and the investigation into their isotopes has commonly been theorised as a potential exploration tool. As such, an addendum to the main body of research has been undertaken. In this investigation specialized instrumentation has been developed to heat a small (<1 g) crushed rock sample for the purpose of volatilizing mercury from naturally occurring mercury compounds trapped within the rock. Two prototypes were developed and a procedure for the determination of mercury released from specific mercury compounds was trialled. However, it was not possible to investigate this part of the research in detail due to time constraints largely caused by breakdown of the heating apparatus during prototype development. However, using the data obtained from preliminary experiments it was possible to confirm that the various mercury compounds could be separated, identified, and determined. It was also observed that there were differences in the concentration and isotopic distributions of the mercury compounds between the samples. However, it was not possible to quantitatively determine the mercury isotopic ratios of the various compounds due to lack of appropriate precision of the quadrupole ICP-MS. Due to the difficulty in determining isotope ratios, together with problems associated with fine tuning the mercury compound release ramps to ensure compete separation of the individual study mercury compounds, it has not been possible at this stage to establish specific relationships between the distribution of these compounds and the proximity of the sample to gold mineralization.

On the basis of the research detailed in this project it is possible to interpret the trace element signatures of arsenopyrite to understand the geological and geochemical processes which formed this mineral under the different geological environments associated with the ore forming process of the main orebodies.