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TitreFeldspar triclinicity, superheating and mineralization
TéléchargerTéléchargement (publication entière)
AuteurGirard, R; Potvin-Doucet, C; Bédard, L P; Bouchard, F
SourceTGI 4 - Intrusion Related Mineralisation Project: new vectors to buried porphyry-style mineralisation; par Rogers, N (éd.); Commission géologique du Canada, Dossier public 7843, 2015 p. 429-443, (Accès ouvert)
LiensCanadian Database of Geochemical Surveys, downloadable files
LiensBanque de données de levés géochimiques du Canada, fichiers téléchargeables
ÉditeurRessources naturelles Canada
Documentdossier public
Mediaen ligne; numérique
Référence reliéeCette publication est contenue dans Rogers, N; (2015). TGI 4 - Intrusion Related Mineralisation Project: new vectors to buried porphyry-style mineralisation, Commission géologique du Canada, Dossier public 7843
Référence reliéeCette publication est reliée Chapman, J B; (2014). Mineralogy and chemistry of tourmaline in the Woodjam porphyry deposits, British Columbia, L'Association géologique du Canada-L'Association minéralogique du Canada, Réunion annuelle conjointe, Recueil des résumés vol. 37
Sujetsminéralisation; feldspath; spécimens minéralogiques; analyses minéralogiques; cristallographie; cuivre porphyrique; gisements porphyriques; altération hydrothermale; gisements minéraux hydrothermaux; géologie économique; minéralogie
Illustrationsphotographs; photomicrographs; cross-sections; models; ternary diagrams
ProgrammeInitiative géoscientifique ciblée (IGC-4), Étude des gîtes porphyriques
Diffusé2015 06 11
Résumé(disponible en anglais seulement)
The association of hydrothermal alteration and gold mineralization with Archean porphyric felsic dykes swarms is a well-known feature. Examples are abundant in the Abitibi greenstone belt, such as Fenn-Gibbs or Lamaque. It has been frequently observed by the authors, in the course of routine petrographic work, that feldspar phenocrysts from such dykes ("FP" or "QFP") are monoclinic sanidine or anorthoclase, rather than the more usual orthoclase. It is estimated that about 50 % of the "QFP" occurrences are apparently sanidine bearing, suggesting an underlying process. Preservation of the high temperature structure of feldspar requires rapid heat dissipation in order to avoid inversion to triclinic feldspar or perthite. In hypabyssal or mesozonal conditions, it requires abundant fluids percolation. Such fluid inflow necessitates permeability, likely by fracturation, as indicated by the coeval pervasive sericite dominated hydrothermal alteration invading in the wallrock of the dyke swarm. Porphyritic felsitic dykes can be seen as the uppermost expression of larger magmatic chambers, linking it with the eruptive complex Superheating evidences are abundant, such as phenocrysts resorbtion, similar to what is commonly seen in rhyolite and dacite.
Feldspars, under magmatic conditions, are monoclinic and form a complete solid solution between albite and orthose. Their inversion into triclinic structure occurs at about 700°C, where microcline and albite end-members exsolve into perthites. Orthose can preserve pseudomonoclinic structure and variable obliquity. The magnitude of this obliquity is potentially related to the cooling history of the intrusion, and is envisaged as a proxy of the cooling gradient.
Recognizing sanidine or anorthoclase from orthoclase can be done under the petrographic microscope, while accurate measurement of obliquity needs time-consuming 5- axis Federov's stage manipulations. The relation between optical obliquity and d (13 1) - d 131 spacing by XRD are under evaluation, as well is the development of more efficient procedures. The effectiveness of the method is currently tested on a well-documented mineralized system at Osisko's Canadian Malartic deposit. In the event the premise works, it may provide a new tool to test the fertility of porphyritic intrusions.