Abstract
The Zr-Ti-Fe placers along the coast of Rio Negro and Buenos Aires Provinces, Argentina, evolved within the reaches of the Malvinas Current (1st order hydrodynamic regime). Together with some regional 2nd order local currents it is of major influence on the mineral assemblage of these terrigenous linear shoreline deposits. The nearshore-marine placers came into being in a macro- to mesotidal (San Matias Gulf) and meso- to microtidal regime (Bahia Blanca Estuary).The modern-day placers under study result from a transgressive regime with only moderate detrital input by the contemporaneous river drainage systems. The proto-placers are hallmarked among the heavy minerals (HM) by changes in the redox system which affected predominantly the Fe-Ti oxides and lead to pseudobrookites and kleberite. Light minerals (LM) in the present-day placers played a different role which is mirrored by changes in the alkalinity of the intragranular fluid system. As far as the environment of deposition is concerned in these shoreline deposits, the dune belt and backshore are most prospective as to the concentration of magnetic and non-magnetic HM. The HM assemblage of the coastal placers is a mirror image of the Afro-American part of Gondwana and its break-up history during the Mesozoic and Cenozoic. The HM reflect the alternating orogenic and anorogenic geodynamic processes operative along the passive margin of both continents. While the orogenic regimes contributed to the accumulation of metallogenetically irrelevant HM (e.g. alumosilicates, spinel, garnet, amphibole) of the placers, the anorogenic geodynamic regime may be held accountable for the target elements in the placer deposits Zr, Ti and Fe. Fe-Ti oxides (rutile, ilmenite-titanohematites, titanomagnetites) and zircon form placer deposits in their own rights with zircon playing an additional minor role also as ore guide to rift-related deposits. Titanium dioxides (Nb-Ta rutile s.s.s., struverite) and Nb titanite take the most prominent role as ore guides to rare element-bearing pegmatites, alkali magmatic rocks and carbonatites. In conclusion, the HM suite encountered in the placer deposits denotes a hot-spot mineralization at a triple junction of the South Atlantic, the Antarctic and the Proto-Iapetus Oceans. The Zr-Ti-Fe placer deposits are part of the clastic halo or expressed metaphorical as remnants of this hot-spot metallogenesis encompassing anorogenic Nb-Ta deposits related to alkaline-magmatic rocks, Skaergaard-type Fe-concentration and anorthosite-hosted Fe-Ti deposits.
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