Gênese e evolução de espeleotemas em formações ferríferas/lateríticas das cavidades N4WS-067 e S11D-83 da região de Carajás

Abstract

In Carajás region, about 170 km from Marabá city, southeast of Pará, there is extensive deposits of banded iron formation (BIF) Carajás Formation (Archean), covered by lateritic yoke. At interface of these deposits occur numerous of natural cavities / caves, associated with alteration products and / or speleothems housed along the lateritic yoke, which comprises hematite breccia conglomerates cemented by goethite and phosphate. These deposits are part of Serra Norte dissected plateau, Pará State, where there are Plateaus of flat tops to wavy. This work involved mineralogical and petrographic analysis of speleothems through X-Ray diffraction - (XRD), scanning electron microscopy (SEM / EDS) and optical microscopy speleothems along the N4WS cavities (Serra Norte) and S11D (Serra Sul) in order to understand the process of interaction between the host rock and the genesis of speleothems. Mapped speleothems were stalactite, screw, ball, finger, botrioidal and drop. a) stalactite - represented by mineralogically esfeniscidite, leucophosphite and phosfosiderite. b) screw - represented by minerals leucophosphite, esfeniscidite, nitromagnesite, hematite and maghemite. c) Ball - eosforite minerals, leucophosphite and esfeniscidite. d) finger - Mineral leucophosphite, esfeniscidita, gibbsite, magnesian galenite and quartz. e) botrioidal - brushite, monetite, quartz and gypsum, and f) drop - quartz, esfenicidite and gypsum. All forms of speleothems have phosphatic minerals. Microscopically speleothems are encrusted on ferruginous lateritic cover yoke, where the yoke and speleothems interface is rough structure of interleaved continuous layers, speleothems becoming phosphatic as it moves away from the yoke. Banded structure of the layers interspersed on the speleothems basis is attributed to chemical precipitates derived from leaching of lateritic yoke process of alternating periods of clear phosphate and phosphate associated to iron. Origin of phosphates is attributed to interaction of weathering acid conditions products with guano bats present in natural cavities studied, based on the low P2O5 content in the iron formation and host rock around 0.2%. Phosphatic cavernous structures, similar to a skeletal structure, analogous to an organic plot, should be related to phosphate mineralization process where by ion phosphate exchange and the organizational structure consisting of the guano that mineralized skeletal structure. Finally the distribution and genesis of phosphatic speleothems, is related to fecal product that is deposited at the bottom by gravity, the ceiling is related to an appendix that develops in the recesses of cavity generating stalactites in the lower gallery and on the side wall for draining water.