Isotopic geochemical characterization of selected nepheline syenites and phonolites from the Poços de Caldas alkaline complex, Minas Gerais, Brazil

Abstract

This paper presents and discusses the isotopic data from the hydrothermal studies of the Poços de Caldas Natural Analogue Project. The purpose of these studies was to elucidate the mass transport of relevant elements and isotopes associated with hydrothermal mineralization and alteration at the Osamu Utsumi uranium mine, as applicable to high-temperature radwaste isolation (particularly in the U.S. nuclear waste program). Research efforts were focused on studying the thermal, chemical and hydrologic nature of the palaeohydrothermal regime associated with a breccia pipe at the Osamu Utsumi mine, and related to the geochemical, geochronological and petrological characterization studies of unaltered regional nepheline syenite and phonolite. The regional rocks studies have a vertically elongated δD, δ 18O pattern, which possibly indicates meteoric water/rock interaction. Regression of RbSr whole-rock isotopic data for the regional nepheline syenite and phonolite samples did not produce isochrons. An internal, mineral-separate isochron regression from a nepheline syenite sample, considered representative of unaltered nepheline syenite of the Poços de Caldas plateau, yields an age of 78 Ma, and an initial 87S 86Sr ratio of approximately 0.7051. The initial 87Sr 86Sr ratios of the regional nepheline syenites are possibly indicative of a mantle source for the alkaline magmatism, with some incorporation of old, high Rb/Sr crustal material. The greater-than-mantle values of δ 18O, if not due solely to surficial processes, also appear to require some assimilation of crustal material. SmNd isotopic data for the regional rocks do not define any isochrons, although the nepheline syenite samples conform very well to a calculated reference isochron for 78 Ma and a fixed initial 143Nd/ 144Nd of 0.512359. The regional phonolite samples lie markedly off this isochron. This is probably due to the phonolite samples having different initial 143Nd/ 144Nd values. All regional samples lie within the “Mantle Array” trend. Their location within ϵ Nd–ϵ Sr space indicates as asthenospheric Mid Ocean Ridge Basalt (MORB)-type source magma also contaminated by continental igneous and metamorphic rocks (e.g. the Precambrian gneiss surrounding the Poços de Caldas plateau). The rocks studied at the Osamu Utsumi mine from the F4 drillcore have experienced varying degrees of hydrothermal mineralization and metasomatism, and deep weathering. The hydrothermally altered rocks have a quite pronounced δD shift, with only a slight δ 18O shift. The δD-δ 18O trend of the hydrothermally altered F4 samples most likely reflects the variability of temperature, hydrologic flow, mineralogical alteration and, therefore, water/rock interaction and isotopic exchange in the palaeohydrothermal regime. Regression of RbSr whole-rock isotopic data for subsamples from a nepheline syenite xenolith sample yields an age of 76 Ma and an initial 87Sr 86Sr ratio of approximately 0.7053. Due to the marked hydrothermal alteration and metasomatism of this sample, the RbSr isotopic system is interpreted as being re-equilibrated and thus the regressed age is the age of the hydrothermal event. Using a ( 87Sr 86Sr) 1 versus 1/Sr mixing diagram, distinct trends are seen for hydrothermal alteration, mineralization and weathering. Again, the F4 nepheline syenite samples do not define an SmNd isochron, but conform very well to a calculated model isochron for 78 Ma and an initial 143Nd/ 144Nd of 0.512365. The SmNd isotopic data also exhibit a possible disturbance by the hydrothermal, metasomatic alteration. A lamproite dyke which crosscuts the hydrothermal alteration in the Osamu Utsumi mine gives an age of 76 Ma, which is essentially the same as the RbSr age of the hydrothermally altered nepheline syenite subsamples.