Autometassomatic and hydrotermal processes in the crystallization and recrystallization of calcite, Floresta Azul Alkaline Complex, NE Brazil

Abstract

The syenitic intrusion of the Floresta Azul Alkaline Complex is characterized by the presence of intense fluid activity in late stages of crystallization, leading to the formation of sodalitites and to the genesis of REE-rich accessory minerals. Calcite is a common mineral in the nepheline syenite of the complex, its formation being derived from CO3 enrichment during fractional crystallization of phonolitic magma. Calcite may be found as white, pink to brown anhedral crystals which occur in the interstices of essential mineralogy and are commonly associated with cancrinite and siderophyllite. Calcite shows high contents of the rhodochrosite, siderite and strontionite molecules and the REE are present in its structure. From scanning electronic microscopy studies, with backscattered electron detector and a dispersive energy spectrometer, it has been possible to identify and characterize a number of textures and minerals related to calcite. The cooling of calcite promotes the formation of a rare REE-carbonate, the carbocernaite. This mineral occurs as exolutions, being distributed disseminated in calcite with irregular crystals and with acicular and oriented crystals. The interaction with hydrothermal fluids resulted in the another unusual REE-carbonate, the ancylite. That results from the recrystallization of calcite, mainly at its borders. A leaching process also occurred at the hydrothermal stage, leading to remobilization of Fe-Mn-Sr which precipitation as siderite, ankerite and strontianite. Therefore, calcite is an extremely important mineral in the syenitic intrusion of the Floresta Azul Complex, being possible to describe the autometasomatic and hydrothermal stages with their genesis, compositional variation, present textures and mineralogical associations.