A zona zeolítica da Formação Corda, Bacia do Parnaíba

Abstract

Since the occurrence of zeolites in the Parnaíba Basin were revealed, in the middle of the last decade, several works have been published addressing the main traces of geological conditioning, as well as the possibility of processing the ore. These investigations provided an overview of the mineralogical composition of the deposit and identified the presence of zeolites stilbite and laumontite occurring as cement in fluvial and eolian sandstones of the Corda Formation, associated with caucite and smectites. They also provided some data on the chemistry of the whole rock and one of the zeolite species (stilbite). They did not contain information about paragenesis and the hypotheses raised about genetic aspects were inconsistent. There was speculation about the possibility of a hydrothermal influence and the probable existence of a well-defined mineralogical zoning, which could favor eventual exploratory investments. The main objective of this dissertation was to systematically study two representative sections of the Zeolite Zone of the Corda Formation, to define the paragenesis and the spatial pattern of mineralogical distribution, as well as to elucidate the issue of genetic modeling for the accumulation of zeolites. Additionally, we tried to open perspectives for possible fields of application of this material. The work methodology involved an intensive bibliographic research on the geological conditioning of zeolite deposits in sedimentary rocks and on the physicochemical parameters that regulate the stability of stilbite and laumontite. This theoretical foundation was an important subsidy in the interpretation of the data obtained through various analytical resources, managing to assemble a table that provides a generic overview of the diagenetic environment of the Corda Formation, and its evolution, with emphasis on the conditions of pressure, temperature and chemistry of the zeolite zone, and the spatial distribution of the two zeolites, in addition to paragenesis relationships. The study of the isotopic compositions of carbon and oxygen was an important auxiliary tool in this interpretation The focus on Regional Geology admits the need for a review of the Mesozoic stratigraphic order of the basin, with the possibility of dissociating the Corda Formation into two distinct segments, and emphasizes the importance of the role played by basic volcanic rocks in this order, and their spatial and genetic link with zeolites. The local geological context reflects a broad sedimentary system, predominantly eolic and fluvial, developed over basaltic flows. Mineralogically, two distinct universes can be highlighted: a) a detrital one, containing grains of diverse composition in which quartz predominates and, subordinately, basalt fragments, in addition to feldspars, quartzite, chert and limestone, among others; b) an authigenic phase, with the paragenesis involving a ferruginous coating, the smectites (predominantly montmorillonites), and zeolites (stilbite and laumontite) and calcite. It is not possible to define the order of crystallization of zeolites among themselves, nor of these with calcite, as there is evidence that these minerals are cogenetic and formed from fluctuations in the chemical balance of the interstitial fluid, both in fluvial and aeolian facies. The texture of the sandstone, characterized by a predominantly floating and/or punctual packing, the isotopic composition of oxygen and carbon in the calcitic cement and the absence of albitization in the detrital plagioclase, in addition to the degree of preservation of the smectites and the absence of siliceous cement, provided subsidies to characterize a diagenesis dominated by temperature and pressure conditions compatible with surface conditions, with the zeolite process involving an open hydrological regime, developed in an arid and semi-arid environment. Adsorption tests for copper show that phases enriched in laumontite have a relatively high performance, compatible with some activated aluminas, which recommends carrying out more complete investigations of the industrial properties of the zeolite ore from the Corda Formation.