Abstract
Uncommon micrometer-sized aluminium phosphates and sulphates of the alunite supergroup (APS minerals) associated with clay minerals were found in a polymictic Permian granite hosted hydrothermal breccia in the Velence Mts. (Hungary). The mineral assemblage was studied with TEM, XRD, EMPA and FTIR with the aim of revealing the formation conditions and the genetic link to the hydrothermal processes that affected the granite host. Two breccia types were identified in the granite. In a polymictic, strongly silicified breccia type zoned plumbogummite was found in association with rolled tube-shaped halloysite and kaolinite. Calcium, barium and lead zonation of the plumbogummite and the Cl and S contents of the halloysite indicate the hypogene origin of this assemblage. Kaolinite, halloysite and plumbogummite form under acidic conditions at temperatures ≤150 °C in hydrothermal systems. APS minerals belonging to the woodhouseite solid solution series (s.s.s.) were detected in the matrix of argillic polymictic breccia beside the alunite in a kaolinite/dickite, illite rich matrix. Alunite and woodhouseite form under acidic conditions in hydrothermal systems. Kaolinite/dickite and illite also suggest acidic conditions and temperatures of ≥240 °C. The close vicinity of the studied breccias to hydrothermal centres hosted by Palaeogene andesitic volcanic series, the strike of the studied breccia veins as well as the similarities in the mineral assemblages imply that the brecciation is spatially and genetically related to the high-sulfidation epithermal systems of the Palaeogene magmatic-hydrothermal activity. In our model, strongly acidic and hot hydrothermal fluids entered into the granite along E-W trending faults that resulted in silicification, brecciation and the formation of APS minerals in association with clay mineral assemblages.
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