Thermométrie sectorielle de la pyrite :
un nouvel outil potentiel pour les études de genèse de minerai
Projet DIVEX-B SC37
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Introduction
Pyrite is a common mineral in the Earth’s crust, ranging from an accessory phase in many metamorphic and igneous rocks to a dominant component of most sulfidic ore deposits. Pyrite is stable over a wide spectrum of physicochemical conditions and environmental settings, including deposition at the surface (e.g., pyrite stalactites in old mine workings, pyrite films in H2S-emitting volcanic springs), formation during sediment diagenesis, and precipitation in high-temperature vents on the ocean floor, in hydrothermal ore deposits, and during metamorphism (cf. Berner 1984; Bowles and Vaughan 2006; Craig 1993; Reed and Palandri 2006). Although most pyrite is near-stoichiometric, in detail pyrite compositions, especially with respect to trace elements, are highly variable and reflects this diversity of host environments. Compositions are controlled by an intimate interplay of physical conditions (e.g., P, T, pH, fO2) and the chemistry of specific environments of deposition. These compositions thus provide a record of the physical and chemical conditions of pyrite formation. Together with its relative abundance, this makes pyrite a potentially useful tool to elucidate the conditions of ore-forming environments where it is commonly difficult to establish the physicochemical conditions of mineralization. For example, fluid inclusion thermometry is the most commoly used method at present, but this is generally applied to inclusions in quartz or other silicate phases that are not necessarily co-genetic with the ore minerals
Unfortunately, at present the use of pyrite analyses in evaluating ore deposits is limited by the multitude of variables that control compositions; this makes it difficult to correlate composition conclusively with the many parameters involved in pyrite formation. However, for growth zoned pyrite grains, a subset of parameters can generally be assumed to be constant, and parameter controls on compositional differences between growth zones can be inferred with considerable confidence. Nevertheless, it remains difficult to differentiate between compositional variations that can be linked to specific chemical environments and/or changing physical conditions. Even when such a correlation can be established, the compositional dependencies for a given parameter have not been calibrated previously and therefore cannot be utilized readily to derive quantitative data.
In order to refine our understanding of the trace element variations between sectors within individual growth zones in pyrite, we have characterized natural pyrites and attempted to synthesize sector-zoned pyrites under varying physical conditions to calibrate compositional dependencies. Sector zoning develops when specific growth surfaces of a mineral preferentially take up or exclude elements. This results in compositionally distinct domains within a single, coeval growth zone, wherein the selective element distribution is controlled by differences in the morphology and charge of crystal growth surfaces; these surfaces represent different slices through otherwise homogenous crystal structures. With higher temperatures, the vibrational energy of crystallographic sites increases, and this limits the element segregation that can be attained at growth surfaces. Consequently, there is a decrease in inter-sector element partitioning with increasing temperature, and this forms the basis of its use as a thermometer (van Hinsberg et al. 2006). Because the host chemistry is identical for the different growth surfaces at all times, sector zoning is entirely independent of the local environment. Therefore, despite variations in the absolute concentrations of elements in the host fluid, the relative compositional differences between sectors will be unaffected, which makes sector zoning a potentially robust thermometer that can be applied directly to settings outside its calibrated environment.
Methods
Natural Samples
In this study we investigated natural pyrite grains from two gold deposits; the Canadian Malartic deposit in Quebec, and the Bawone and Binebase high-sulphidation deposits on the island of Sangihe in Indonesia. Polished thin sections of drill core samples were imaged and quantitatively analysed on a JEOL-8900L electron microprobe. Back-scattered electron imaging was used to locate regions with sector zoning, which were subsequently X-ray mapped for Cu, S, As and Fe. Quantitative spot analyses for Fe, S, As, Cu, Ni, Se and Te were conducted in WDS mode, using a 20 kV acceleration voltage, 50 nA beam current, and a focused beam.
Experiments
In the experimental part of this project we hydrothermally synthesized pyrite at varying conditions of pressure, temperature, fO2 and pH. Four different experimental approaches were investigated.
Experimental approaches used in hydrothermal syntheses of pyrite. Approach 1 centers on the replacement of marcasite by pyrite; 2 on the dissolution and re-precipitation of pyrite in a thermal gradient; in 3, H2S is produced by reaction of Al2S3 with water leading to pyrite precipitation; and in 4, pyrite precipitation is brought about by reaction of solution with native sulphur.
Results
Three main types of sector zoning were observed in the natural pyrites: cross-shaped sector zoning, wedge-shaped sector zoning (most common in colloform pyrite), and trigonal sector zoning. All three types occur in Sangihe pyrite, whereas Malartic pyrite exhibits only rare examples of the cross-shaped type.
Types of sector zoning shown by Cu X-ray maps of Sangihe pyrite: left – wedge-shaped sectors in colloform pyrite; center – cross-shaped sector zoning; right – trigonal sector zoning.
Results indicate that Cu, Se, As and Te show inter-sector partitioning that is significant and systematic. Copper is the predominant element in Sanighe sector zoned pyrite, with concentrations varying by a factor of 4 between high-Cu sectors (lighter grey areas in BSE images and X-ray maps; see Fig. 2) and low-Cu dark sectors. The distribution of As is similar to that of Cu, whereas that of Se shows opposite behavior. Nickel also exhibits variations that are sector zoned, but concentrations in low Cu sectors are generally below the detection limits and therefore cannot be quantitatively evaluated. Variations in Cu between sectors and growth horizons are balanced by corresponding variations in Fe.
Systematic changes in inter-sector partition coefficients of Cu are observed for transects perpendicular to the growth zoning, and such changes are reproducible through different areas of colloform pyrite bands.
Left - normalized transects of Cu concentrations for equivalent points in the |110| and |111| sectors of a Sangihe pyrite; right - corresponding inter-sector Cu partition coefficients. Transect positions are shown on the inset map.
Synthesis experiments were largely unsuccessful. Pyrite formed in the thermal gradient approach, but grains were too small to characterize. In the marcasite-bearing experiments, marcasite grew at the expense of pyrite and no stable assemblage had developed at the end of the run. The experiments with H2S did not produce any precipitates, and in fact resulted in complete dissolution of the pyrite seed crystals. The addition of additional FeCl2 to these latter experiments did not change the results. The final set of experiments produced a stable assemblage of native sulphur and Fe-(hydr)oxide, rather than pyrite. At 350oC, native sulphur was present as rounded globules indicating that it was liquid during the experiment, whereas it was present as angular fragments in the 200oC run product.
Conclusions
Sector zoning in pyrite, formed by differential uptake of elements on different growth surfaces, leads to systematic inter-sector partitioning. Although these compositional differences are independent of the chemistry of the growth environment, inter-sector partitioning coefficients only reflect the physical conditions present at the time of formation. We propose that variations in partition coefficients in pyrite are mainly a product of temperature, which allows changes in these coefficients to be used as monitors of temperature history.
The independence of variations in element partitioning to the chemical environment makes sector zoning a perfect monitor of physical conditions, and consequently allows the chemical variations between growth zones to be evaluated independently. These factors together with the ubiquity of pyrite in crustal rocks in general and hydrothermal alteration zones in particular make pyrite a potentially powerful tool to elucidate the chemical and physical conditions of depositional environments, which will be especially valuable in evaluating the conditions of ore formation.
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