Thermométrie sectorielle de la pyrite :
un nouvel outil potentiel pour les études de genèse de minerai

Projet DIVEX-B SC37

Responsable du projet: A.E. William-Jones, McGill University, courriel
Collaborateurs: Sarah-Jane Barnes, UQAC

Introduction

Fluid inclusions have been an important, and in many cases the main source of information on the physical and chemical conditions of hydrothermal ore formation [1]. However, their analysis and interpretation is often difficult, and for some environments, notably those of orogenic and intrusion-related gold deposits, fluid inclusions do not always yield data that reliably describe the conditions of mineralization. Moreover, fluid inclusions cannot easily be used to evaluate changes in the metal chemistry of ore-forming fluids, without which quantitative modelling of ore genesis is simply not possible [2]. In contrast, pyrite, a nearly ubiquitous mineral in a wide range of hydrothermal ore deposit types, is an excellent monitor of the changes in fluid chemistry, especially of the ore metals [3]. Furthermore, it commonly occurs with concentric compositional growth zones, or as successive generations spanning the different stages of ore-forming systems, and can therefore provide information on the evolution of the conditions of deposition.

Recent developments in mineral thermometry based on element distribution between different domains or sectors of single grains [4] provide new opportunities to study ore genesis. This approach has great potential in the study of mineralizing processes as it is independent of the compositional environment and can therefore be applied directly to evaluate the inherent chemical diversity of ore fluids. Sector zoning is relatively common in pyrite, a key mineral in many ore deposits, as well as in pyrite grains from a variety of other igneous, metamorphic and sedimentary environments.

Objective

The main objective of the proposed project will be to investigate the possibility of using sector zoning in pyrite as a geothermometer, with which to determine formation temperatures in hydrothermal ore deposits. If successful, the project will introduce an important new method for ore deposit modelling and evaluation.

Method

In this project, we will calibrate the temperature dependence of inter-sector element partitioning in pyrite, using both experimentally synthesized and well-constrained natural samples. Experiments will consist of hydrothermal synthesis runs in externally heated, large volume steel autoclaves for a temperature range from 300 to 700^oC at saturated vapour pressure (Fig. 1). Marcasite, suspended from a pure iron wire, will act as the source of Fe and S, and the Fe wire will buffer fS₂ and fO₂. Pyrite will be synthesized in an aqueous solution doped with Sb, As, Cu and Se, which are known to be involved in pyrite sector zoning, and seeded with small, well-developed pyrite cubes or pyritohedra. As the trigger(s) for sector zoning are unknown, the pH, ligand and element concentrations will be varied to bring about the development of such zoning in the experiments. Although there is no guarantee that sector zoning can be produced experimentally, this type of zoning has been produced in cordierite, tourmaline and sulfates, amongst others, in similar experiments. The resulting pyrite grains will be mounted, polished and evaluated using SEM back-scattered electron imaging to investigate elemental zonation. This experimental suite will be supplemented with sector zoned pyrite grains from natural localities, including in-house samples of pyrite from the Brioude-Massiac ore province of the French Massif Central and from the Pascua epithermal high-sulfidation deposit in the Andes [3,5]. The major and minor element composition of coeval sectors will be determined by electron microprobe-WDS at McGill, and trace elements by laser ablation ICP-MS at UQAC. These techniques are crucial to obtain the required in-situ spatial resolution.

Figure 1. Overview of the research approach. Sector-zoned pyrite grains will be synthesized in a seeded aqueous solution doped with Cu, Sb, As and Se, over a range of temperatures. The inter-sector element partitioning will subsequently be determined and can be expected to decrease with increasing temperature. This temperature calibration will then be applied to natural grains (e.g., sector zoned pyrite from Brioude-Massiac - right) to obtain its temperature history.

References