Some aspects of the hydrothermal reactions of tin during skarn formation
Titel:
Some aspects of the hydrothermal reactions of tin during skarn formation
Auteur:
Eadington, P.J. Kinealy, K.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 30 (1983) nr. 3-4 pagina's 461-471
Jaar:
1983-12
Inhoud:
The reactions of tin under hydrothermal conditions in skarns (calcsilicate replacements of carbonate rock) may result in the precipitation of discrete tin minerals such as malayaite (CaSnSiO5) or cassiterite, or tin-rich calcsilicate minerals by cation substitution. Thermodynamic calculations show that cassiterite has low solubilities (< 118 ppm, 10-3m) between 250 and 350°C and near neutral pH. These conditions, which are indicated by the occurrence of calcite with pyrrhotite or pyrite, are the most favourable for the precipitation of cassiterite. At higher temperatures and pH, magnetite displaces the iron sulphides as the stable iron mineral, and the solubility of cassiterite as aqueous hydroxy complexes is increased by several orders of magnitude. Hydrothermal solutions that are ore-forming at 250 to 350°C and neutral pH are undersaturated with respect to cassiterite at higher temperatures and pH. Synthesis experiments and the mineral associations of natural malayaite, the most common tin silicate, show that its occurrence is favoured by hydrothermal reactions at low iron and magnesium activities. Malayaite has been synthesised at temperatures between 240 and 830°C, but geological occurrences suggest that it crystallises after garnet and clinopyroxene. At high activities of iron and magnesium, calcsilicate minerals form in preference to malayaite. High tin concentrations in calcsilicate minerals have been reported for andradite (up to 5.8%; McIver & Mihalik, 1975), epidote (2%) and actinolite (0.3%; Mulligan & Jambor, 1968). Electron microprobe determinations of tin in minerals from the 'number two anomaly' calcsilicate horizon, Mt Lindsay, Tasmania, indicate high tin contents in ferrohastingsite (up to 0.4%) and grossularite-andradite (up to 0.1%) and significantly lower tin contents in tremolite (up to 0.06%), biotite (<0.03%, diopside-hedenbergite (<0.03%) and vesuvianite (<0.03%). These data, together with Mossbauer measurements reported for tin-rich andradites, are consistent with the coupled substitution: 2Fe3+ [??] Fe2+ + Sn4 + , being an important factor in the occurrence of high tin concentrations in some calcsilicate minerals. The occurrence of lattice-bound tin is favoured by hydrothermal reactions at high oxygen fugacities between the haematite-magnetite and nickel-nickel oxide oxygen buffers. The formation of cassiterite-bearing skarn requires cooling under conditions of low oxygen fugacity that leads into the cassiterite stability field without prior precipitation of lattice-bound tin. Remobilisation of lattice-bound tin at temperatures below about 350°C may also precipitate cassiterite.
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