Data di Pubblicazione:
2010
Citazione:
Giant quartz orebodies: structural, mineralogical and geochemical evolution / M. Moroni, M. Zucali, C. Tiboni, F. Leoni. ((Intervento presentato al 20. convegno IMA2010 20th General Meeting of the International Mineralogical Association tenutosi a Budapest, Hungary nel 2010.
Abstract:
Quartz veins are frequent but giant quartz-only orebodies,mostly mined for metallurgy and ceramic industry, are not so common and their origin and mechanisms of emplacemen are
not completely understood (e.g., see [1] for a geologicalstructural model). At the location of Sondalo, in the Central Alpine Belt (Northern Italy), a huge orebody of pure quartz is emplaced in high-grade continental basement of the Austroalpine Languard-Campo Nappe. The orebody, currently
extensively mined and in development, is open ended at depth.
It has never been studied. The orebody is irregularly tubularshaped, and is in direct, brecciated to faulted contact with basement migmatites, garnet-biotite-sillimanite gneisses and amphibolites as well as with a Permian gabbro intrusion(Sondalo Gabbro)emplaced in the basement. The orebody consists of extremely hard, highly deformed, varicoloured(white, light pink, grey, black) quartz. Muscovite and tourmaline are the rare but coarser accessory minerals near the
margins of the body, although sub-micrometric accessory phases variably disseminated throughout the body were
observed. Fluid inclusions are locally abundant.3D meso- and micro-structural relationships between orebody and basement and thermo-barometric estimates suggest
emplacement of the quartz intrusion later than Permian but before Alpine convergence, at T >750°C and P ≈ 0.7 Gpa.
However unusual, coarse skeletal biotite and orthopyroxene intergrowths in reaction rims of breccia fragments also suggest a remarkably high thermal regime of the quartz emplacement, at T > 670°C (biotite-opx thermometer). During the emplacement
widespread brittle deformation occurred likely associated with hydrofracturing mechanisms. Quartz also bears evidences of
intense plastic deformation with localized annealing, in addition to widespread mm- to cm-spaced late-stage fracturing. At the
mesoscale highly strained portions of the orebody may show impressive colour variations from pure white to black.
Selected trace elements, Al, B, Ba, Fe, Ge, P, Ti were analyzed for evaluating colour variations and provenance of quartz based on mechanisms of Si replacement in the quartz
lattice in pegmatite-like contexts (see [2,3]). The trace element composition of the Sondalo quartz revealed unusually high
values of Si-substituting Ge, P Ti and Al contents and related ratios relative to pegmatite-related quartz, as well as high
contents in Ba. Trace elements show roughly zoned, coherent distribution patterns and show some relationships with colour variations. Coupled Ba and Ti enrichments may also cope the occurrence of phases like benitoite (revealed only in XRD
patterns) among the submicrometric accessory phases. Coupling of structural and geochemical mineralogical investigations of
orebody and wall rock may give additional clues on the still poorly understood nature, mechanisms of formation and emplacement of huge quartz-only bodies in orogens.
[1] Bons, P.D. (2001) Tectonophysics, 336, 1-17. [2] Larsen, R.B. et al. (2004) Contrib. Miner. Petrol., 147, 615-628. [3] Götze, J. et al. (2004) Geochim. Cosmochim. Ac., 68, 3741-3759.
Tipologia IRIS:
14 - Intervento a convegno non pubblicato
Keywords:
quartz; hydrothermal; orebody; trace elements; alteration
Elenco autori:
M. Moroni, M. Zucali, C. Tiboni, F. Leoni
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