Technical Report NTB 90-22

Pogos de Caldas Report No.4Isotopic geochemical characterization of selected nepheline syenites and phonolites from the Po~os de Caldas alkaline complex, Minas Gerais, Brazil.

This paper presents and discusses the isotopic data from the hydrothermal studies of the Poços de Caldas Natural Analogue Project. The purpose of this study was to elucidate the mass transport of relevant elements and isotopes associated with hydrothermal mineralization and alteration at the Osamu Utsumi uranium mine, particularly as applicable to radwaste isolation concerns in the U.S. nuclear waste program.

Research efforts were focussed 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 studied have a vertically elongated δD, δ18O pattern, which possibly indicates meteoric water/rock interaction. Regression of Rb-Sr whole-rock 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 87Sr/86Sr ratio of 0.70511. The moderate 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 δ18O, if not due solely to surficial processes, also appear to require some assimilation of crustal material. Sm-Nd data for the regional rocks do not define any isochrons, although the nepheline syenite samples conform very well to a calculated model isochron for 78 Ma and an initial 143Nd/144Nd of 0.512359. The regional phonolite samples lie markedly off this isochron. This open system behavior is probably due to the phonolite samples having different initial 143Nd/144Nd values. Even so, all regional samples lie within the "Mantle Array" trend. Their location within the εNdSr space indicates an asthenospheric Mid Ocean Ridge Basalt (MORB)-type source magma also contaminated by continental igneous and metamorphic rocks (e.g. the Precambrian schist 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 δ18O shift.

The δD-δ18O trend of the hydrothermally altered F4 samples most likely reflects the variability of temperature, hydrologic flow and, therefore, water/rock interaction and isotopic exchange in the palaeohydrothermal regime.

Regression of Rb-Sr whole-rock data for subsamples from a nepheline syenite xenolith sample yields an age of 76 Ma and an initial 87Sr/86Sr ratio of 0.70530. 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)i versus 1/Sr mixing diagram, distinct trends are seen for hydrothermal alteration, mineralization and weathering.

Again, the nepheline syenite subsamples do not define a 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 likely 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 nepheline syenite subsamples.