Evolution of tungsten sources in crustal mineralization from upper archaean to tertiary: preliminary results

摘要

New lead isotope compositions of 31 worldwide scheelites with ages ranging between Upper Archaean and Tertiary are discussed to provide a preliminary model on the evolution of metal sources in crustal tungsten minealization. Our data indicate that the period between 2.7 and 1.5 Ga was characterized by an increasing crustal component in the lead of the scheelites suggesting their emplacement in a progressively more cratonized crust. From 0.75Ga onwards scheelites are genetically associated with Phanerozoic-type orogenic cycles and display a steadily increasing mantle component. Ageing of oceanic crust undergoing subduction could explain such a trend and be relatd to the evolutionof plate configuration after the breakdown of Rodinia occurred between 1.0 and 0.5 Ga. Our data suggest also that several W mineralization pulses recycle older metal concentrations and that the lower crust plays a subordinate role as a reservoir of tungsten.