Piemontite-bearing rocks from the Olary District, South Australia
Titel:
Piemontite-bearing rocks from the Olary District, South Australia
Auteur:
Ashley, P. M.
Verschenen in:
Australian journal of earth sciences
Paginering:
Jaargang 31 (1984) nr. 2 pagina's 203-216
Jaar:
1984-06
Inhoud:
Well laminated piemontite-bearing rocks occur as a local manganiferous unit within a metamorphosed, dominantly albite-rich volcano-sedimentary sequence forming part of the Olary Block of the Willyama Orogenic Domain. The fine to medium grained rocks contain piemontite, spessartine-rich garnet, albite, phengite, manganoan tremolite and phlogopite, quartz and Mn-bearing hematite. Distinctive mineralogical layering is characteristic and appears to represent original sedimentary bedding. Bulk chemical compositions indicate that the piemontite-bearing rocks represent a Mn (-Fe-Ca-K)-enriched variant of the regionally widespread albite-rich rocks; Piemontite and garnet exhibit wide compositional ranges (Pm3.6-35.1PS2.5-25.1CZ51. 4-75.3 and Sp59.2-82.2 Ad6.6-34.6Gro-16.7Cto-10. 7, respectively) and associated tremolite and phlogopite are highly magnesian, with Mn > Fe. The coexisting metamorphic assemblage of piemontite, garnet, tremolite, phlogopite and hematite indicate that during crystallization, oxygen fugacity was above the hematite-magnetite buffer (10∼22—10∼15 bars). Estimates of metamorphic temperatures are loosely constrained between 400 and 500°C. The manganiferous rocks possibly represent an original exhalative chemical sediment rich in MnO2 and Fe2O3 within reworked felsic-intermediate (-alkaline) tuffs. High oxygen fugacity conditions were maintained during metamorphism, resulting in the piemontite-bearing assemblages.
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