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A zone of high alumina hydrothermal alteration (HAHA) is documented within granitoid rocks of the Big Indian Polyphase Intrusive suite (BIPI suite), which forms part of the Late Devonian-Early Carboniferous South Mountain Batholith (SHB). The alteration, which la confined to a NE-trending shear zone, is characterized by the quartz-deficient mineral assemblage andalusite-sillimanlte-spinel-oiuscovite-cordierite-apatite-pyrite. The HAHA zone represents the first reported occurrence of this alteration type within the South Mountain Batholith.  Field observations indicate that the HAHA zone is only one of a series of successive and superimposed metasoraatic-hydrothermal events which, together with U-Cu-Ho + U-Cu mineralization, exhibit a transition in style from early pervasive to late fracture-controlled. This change in style within the BIPI suite is considered to reflect a progressive Increase in fluid pressure.  Petrography and mineral chemistry suggest that the HAHA zone formed in response to the interaction of the leuco-monzogranlte host with a Na(+Al)-rich, siliea-undersaturated fluid. Although speculative, alumina may have been a mobile element transported as an alkali-complex via the fluid. High temperature mineral assemblages (gndalusite-aillimanite; andalusite-spinel) Indicate that the initial temperature of the aqueous fluid phase was ≥620 o C for 2 kb pressure. It la proposed that the fluid phase represents a residual magmatic-fluid which was accessed by deep-seated fracturing.   RESUME  On documente une zone d'alteration hydrothermale hautement alumlneuse (AHHA) a l'interieur des granitoides de la suite intrusive Polyphasee de Big Indian (suite IPBI) qui fait partie du Batholite de South Mountain (BSM) d'âge tardidevonien a eocarbonifere. Confinee a une zone de cisaillement A tendance NE, l'alteration est caracterisee par un assemblage de mineraux, deficient en quartz, a andalousite-sillimanite-spinelie-museovite-cordierite-apatite-pyrite. La zone AHHA represente la premiere mention de ce type d'alteration a l'interieur du BSM.  D'apres les observations sur le terrain, cette zone AHHA n'est qu'un d'une serie d'evenements metasomatiques et hydrothermaux successifs et superposes qui. ainsl qu'une mineralisation en W-Cu-Mo+U-Cu. exbibent un style d'abord penetrant et enfin filonlen. On considere que ce changement de style A l'interieur de la suite IPBI represente un accroissement progressif de la pression des fluides.  La petrographie et le chimisme des mineraux tendent a montrer que la zone AHHA e'est etablie en reponse a l'intersction de l'encaissant leucomonzogranitique avec un fluide sous-sature en silice et riche en Na±Al. Bien que cela reste speculatif, l'alumine pourrait avoir ete un element mobile transports par le fluide a l’etat de complexe alcalln. D'apres les assemblages de mineraux A haute temperature (andalousite-sillimanite; andalousite-spinelie), la temperature inltiale de la phase aqueuse fluide etait ≥620 o C pour une pression de 2 kb. On propose que la phase fluide represente un fluide magmatique residuel extrait par une fracturation en profondeur.  [Traduit par le journal]

An occurence of metasomatic aujminosilicates related to high alumina hydrothermal alteration within the South Mountain Batholith, Nova Scotia