A quantitative method for determination of trace metal concentrations in sedimentary pyrite

A comparative in situ LA-ICP MS trace-element study on pyrite from three different, variably auriferous, Archaean to Palaeoproterozoic palaeoplacer deposits in the Ouro Fino Syncline (Quadrilatero Ferrifero; Brazil), the Elliot Lake area north of Lake Huron (SE Canada) and several deposits within the Witwatersrand Basin (South Africa) revealed systematic differences between morphologically different pyrite types and between the various palaeoplacer deposits. Especially the Ni and Au concentrations as well as Co/Ni and Mo/Ni ratios were found to be systematically different in detrital compact, detrital porous and post-sedimentary/hydrothermal pyrite grains from different source areas. High Co/Ni ratios and low Au concentrations are typical of post-sedimentary pyrite, which is hydrothermal in origin. In contrast, relatively low Co/Ni ratios and high Au contents characterise detrital porous banded and concentric pyrite grains (Au > 1 ppm), which are syn-sedimentary in origin. In the Elliot Lake area and the Witwatersrand Basin, detrital compact rounded pyrite is characterised by high Co/Ni ratios, which is in agreement with derivation from a hydrothermal source. Low Au concentrations in this pyrite type support the contention of the gold and the pyrite in these deposits coming from different source rocks. In contrast, derivation from an originally diagenetic pyrite is suggested for the detrital compact pyrite in the Ouro Fino Syncline because of low to intermediate Co/Ni ratios. High Au contents may indicate a genetic relationship between pyrite and gold there. Systematic differences exist between the three areas with respect to Au, Ni, Co, Mo and Cu distributions in detrital pyrite, which reflects differences in the provenance. A predominantly mafic/ultramafic source is indicated for the Ouro Fino, a felsic source for the Elliot Lake, and a mixed felsic–mafic provenance for the Witwatersrand pyrite populations. Independently of pyrite type, the higher Au endowment of the studied Witwatersrand and Ouro Fino conglomerates are also reflected by an overall higher Au concentration in the respective pyrite grains compared to the relatively Au-poor samples from Elliot Lake. In general, a strong positive correlation between Au and Pb levels in the various pyrite grains is noted. Analogous to Pb, which is well known for not being easily accommodated in the pyrite crystal lattice but occurring as discrete PbS phases, Au is considered to be present mainly in the form of discrete Au phases in minute pores and interstices of the pyrite grains rather than within the pyrite lattice.

Trace-element characteristics of different pyrite types in Mesoarchaean to Palaeoproterozoic placer deposits

Arsenic is one of the most prevalent toxic elements in the environment. The toxicity, mobility, and fate of arsenic in the environment are determined by a complex series of controls dependent on mineralogy, chemical speciation, and biological processes. The element was first described by Theophrastus in 300 B.C. and named arsenikon (also arrhenicon; Caley and Richards 1956) referring to its “potent” nature, although it was originally considered an alternative form of sulfur (Boyle and Jonasson 1973). Arsenikon is believed to be derived from the earlier Persian, zarnik (online etymology dictionary, http://www.etymonline.com/index.php?term=arsenic ). It was not until the thirteenth century that an alchemist, Albertus Magnus, was able to isolate the element from orpiment, an arsenic sulfide (As2S3). The complex chemistry required to do this led to arsenic being considered a “bastard metal” or what we now call a “metalloid,” having properties of both metals and non-metals. As a chemical element, arsenic is widely distributed in nature and can be concentrated in many different ways. In the Earth’s crust, arsenic is concentrated by magmatic and hydrothermal processes and has been used as a “pathfinder” for metallic ore deposits, particularly gold, tin, copper, and tungsten (Boyle and Jonasson 1973; Cohen and Bowell 2014). It has for centuries been considered a potent toxin, is a common poison in actual and fictional crimes, and has led to significant impacts on human health in many areas of the world (Cullen 2008; Wharton 2010).

The potential issues associated with elevated As concentrations in water supplies have led to a large body of published research in the last few years related to:

The Environmental Geochemistry of Arsenic — An Overview —

Textural and chemical characterization of pyrite was used to reconstruct the hydrothermal evolution of the Bracemac-McLeod Archean volcanogenic massive sulfide (VMS) deposits (~6 Mt) in the Matagami district, Abitibi, Canada. The mineralization, hosted in a bimodal volcanic sequence, is divided into a (1) Zn-rich zone, concordant to the Key Tuffite— a marker horizon at the district-scale, 2) a Cu-rich zone which locally crosscuts the stratigraphic pile, and 3) some localized magnetite-rich zones replacing sphalerite and pyrite. Detailed petrographic and laser ablation-inductively coupled plasma-mass spectrometry (LA-ICP-MS) studies of whole pyrite grains (including inclusions and zonation) from the Key Tuffite and from the different ore zones of the deposits, has revealed five pyrite types. Nodular pyrites (I) in the Key Tuffite and subidiomorphic pyrites from the low temperature (250°C) Zn-rich zone (II) have the same chemical signature, suggesting that both precipitated under similar physicochemical conditions. A wide range of trace elements (Cu, Zn, Ag, Sn, Sb, Te, Au, Pb, Bi, Tl) are present in both, but compared to the other types of pyrite, they are significantly enriched in Sb and Tl. Subidiomorphic pyrites from the copper-rich zone (III) have a different signature, enriched only in Se ± Co, In, Pb, and Bi. These pyrites commonly overgrow the nodular pyrites and are related to later higher-temperature fluids (300°C). Pyrite preserved during a later magnetite-replacement (>350°C) stage (IV) also are enriched in Se, but are depleted in all other elements. A similar signature is found in idiomorphic metamorphic pyrite (V), which occurs at the district scale. During high temperature recrystallization of pyrite IV and V, only the contents of Ni, Co, As, and Se are mostly preserved, whereas other base metals are expelled from the pyrite structure. Consequently, only Ni, Co, As, Se, Sb, and Tl were useful to reconstruct the hydrothermal evolution of the VMS system and used to build a suite of discrimination diagrams. Specifically, the ratio Se/Tl can discriminate pyrites from mineralized Zn-rich zone (Se/Tl 10,000). The innovative approach of this study, based on LA-ICP-MS acquisition of the chemistry of entire pyrite grains, makes our results directly applicable for exploration. Consequently, pyrite concentrate samples, analyzed for As, Tl, and Se by more classical mass-spectrometric techniques, have the potential to be used for vectoring to ore by using the proposed discrimination diagrams.

Deciphering the Hydrothermal Evolution of a VMS System by LA-ICP-MS Using Trace Elements in Pyrite: An Example from the Bracemac-McLeod Deposits, Abitibi, Canada, and Implications for Exploration

F. Ivor Roberts

Papers

Trace element chemistry of pyrite: A useful guide to the occurrence of sulfide base metal mineralization