Mineralogy and geochemistry of multi-coloured sapphires at the Portezuelo de Pajas Blancas' deposits, northern Chile: revealing crystal growth processes
Gustavo Miranda-Díaz, Andrew Menzies, Karl Riveros-Jensen, Gerhard Heide, Lena Bußmann, Birk Härtel, Roald Tagle, Eduardo Medina, Wolfgang Griem
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引用次数: 0
Abstract
Portezuelo de Pajas Blancas' primary metasomatic deposits host sapphires that differ in size, colour, and colour distribution through different lithological units. The discovery of sapphire-bearing secondary aeolian placer deposits enabled a detailed analysis of sapphire types to determine the causes of this heterogeneity. Representative single sapphires from the primary and secondary deposits were analysed using various techniques, including SEM-CL, OM-CL, Micro-XRF, Raman spectroscopy, and polarised transmitted light multi-focus optical microscopy. Based on the results, seven sapphire types were identified according to their colours and colour distribution. Fe and Ti concentrations mainly contribute to the colour and micro-textures occurrence. The primary growth textures include cores, progressive, oscillatory, and diffusion zoning, whereas the secondary alteration features correspond to micro-brecciation, re-arrangement, and overgrowth. Inclusions such as euhedral andalusite, anhedral anhydrite, and anhedral rutile were identified in the sapphires. The formation of Portezuelo de Pajas Blancas sapphires can be divided into five genetic stages: (i) core growth development; (ii) chemical imbalance due to open system behaviour that produces progressive and oscillatory zoning; (iii) low-temperature deformations that produce micro-brecciation; (iv) re-arrangement of pre-formed sapphire fragments and subsequent sapphire overgrowth; (v) diffusion zoning that produce Fe- and Ti-enriched outer rims. In conclusion, the Portezuelo de Pajas Blancas' sapphires were formed through at least five stages of oxidised contact metasomatism at varying temperatures and low pressures, where a heterogeneous forming condition predominates during sapphire formation.
期刊介绍：
The International Journal of Earth Sciences publishes process-oriented original and review papers on the history of the earth, including
- Dynamics of the lithosphere
- Tectonics and volcanology
- Sedimentology
- Evolution of life
- Marine and continental ecosystems
- Global dynamics of physicochemical cycles
- Mineral deposits and hydrocarbons
- Surface processes.
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