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.

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.

Giant quartz orebodies: structural, mineralogical and geochemical evolution

M. Moroni
Primo
;
M. Zucali
Secondo
;
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.
ago-2010
quartz; hydrothermal; orebody; trace elements; alteration
Settore GEO/09 - Georisorse Miner.Appl.Mineral.-Petrogr.per l'amb.e i Beni Cul
Settore GEO/03 - Geologia Strutturale
International Mineralogical Association
http://www.ima2010.hu
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.
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