Tectonic setting, deep faults and mercury mineralization at Almaden, Spain, and Nikitovka, Ukraine: affinities and contrasts

摘要

West of the Tornquist-Teisseyre Zone, between the Black Sea and the Baltic Sea, mercury and antimony deposits show a spatial relationship with late-orogenic extensional processes. This applies to the Alpine and Variscan orogenies and may also apply to the Cadomiati orogeny. The Silurian Almaden deposits and other mercury deposits in early to mid-Palaeozoic host complexes in western and central Europe occur in a number of micro-continents that are thought to have been derived from the continent of Gondwana, although relationships between the micro-continents have been largely obliterated by the Variscan and Alpine orogenic events. In the Almaden region aeromagnetic anomalies indicate the presence of deep faults, which, it is suggested, represent remnants of the pre-Variscan Cadomian tectonic framework. At Almaden the close association between the mercury mineralization and alkaline basaltic volcanism suggests that these faults may tap the upper mantle. East of the Tornquist-Teisseyre Zone mercury is confined to the Pripyat-Dnleper-Donets (FDD) rift and is generally accompanied by antimony and also, locally, by base metals. Together with gold, these metals appear to express a spatially zoned association. Although the degree of surface outcrop is much less than in the Almaden region, the deeper crustal structure is better known from geophysical investigations. The FDD rift is located in the Sarmatian block. Together with the Fennoscandian and Volgo-Uralian blocks it forms the crystalline Precambrian basement of the East European Platform. In contradistinction to the other two blocks, the Sarmatian block appears to resemble the continents of the present southern hemisphere. Again, this would suggest a link between the mineralization and the old Gondwanan continent. The present FDD rift was initiated in the mid-Devonian, although a Proterozoic precursor is debated. The rift Is part of an orthogonal framework, apparently syntectonic, of northwesterly and northeasterly trending sets of grabens in the East European Platform that rotate into each other rather than Intersect. Some of the grabens are demonstrably underlain by early Proterozoic mobile zones that surround Archaean cratons. In the PDD rift the principal phase of mid-Devonian to Carboniferous subsidence was followed by a phase of Pemio-Trlassie uplift, which, in the Donets segment (which contains the Nikitovka district), developed into tectonic inversion. During the late Palaeozoic rifting major listric faults affected the Moho and probably reached into the mantle. In the Donets Basin these faults are interpreted to have been reactivated as thrusts in a dextral deformation regime. Volcanic and plutonic rocks of mafic alkaline and calc-alkaline affinity are abundant and it is generally assumed that there is a relationship between volcanism and mineralization. However, it is not clear which volcanic phase was principally responsible for the mercury-antimony mineralization, although a mantle source is generally, accepted. Processes of remobilization should not be excluded, however, in view of the complex evolution of the rift. The tectonic framework of the rift has provided ample pathways between the upper mantle and the crust during the various stages of its history, Isotopic dating of suitable minerals associated with the deposits should be given high priority. In view of the instability of the continental lithosphere in the early to middle Palaeozoic west of the Tornquist-Teisseyre Zone and Its stability during the middle to late Palaeozoic in the Sarmatian Craton east of the zone, together with the emplacement of the Moho at higher levels that is common to both settings, it is suggested that major disturbances In the mantle provided the driving force for the concentration of mercury and antimony and probably also of other heavy metals.