Jean Clair DUCHESNE, Eric GOEMAERE, Jean-Christophe GRIGOLATO, Hyacinthe VANDERSCHUEREN & Bernard CHARLIER

ALTERED RHYOLITIC ROCKS IN THE VISE BOREHOLES: A GEOCHEMICAL APPROACH

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Abstract

Under a sedimentary cover of calcareous breccias of Dinantian and Frasnian ages, and a conglomeratic level with volcanic clasts, a magmatic rock, presumably belonging to the Ordovician-Silurian magmatic province, was cut across in two boreholes at Visé (Quarry K). Macroscopically the rock is brecciated and locally displays evidence of flow structures. Microscopically, it shows various stages of alteration (devitrification, silicification, carbonatation and sericitation) superimposed on magmatic flow structures, the most evolved assemblage being quartz + illite ± sulphides ± carbonates. 27 whole-rock analyses (major and trace elements by XRF) show large ranges of chemical variations, e.g. 51 to 80% SiO₂, 1.7 to 10% K₂O, 0.03 to 6.6% Na₂O, making impossible the use of classical petrochemical methods of identification. In the classic AFC-AKF projections of Eskola, the most altered rocks appear made up of various proportions of quartz and an illite of restricted composition. This clay mineral is similar in composition to hydrothermal illite from the phyllic alteration of the Bingham porphyry copper deposit and from the Los Azufres volcanic centre. This points to identical conditions of formation ca. 310°C. Although the original magmatic rock has been deeply metasomatised by addition and subtraction of mobile elements, the ratios of some trace elements, such as Zr, Nb, Y, Th, REE and Ti, have been preserved, thus confirming that these elements were immobile during the alteration processes. It is therefore possible to use their ratios to identify the nature of the protolith by means of the Winchester and Floyd (1977) discrimination diagram. It turns out that the Visé felsic rock protolith is effectively a rhyolite. Multi-element diagrams also point to calc-alkaline series of rock, with « Within Plate Granite » characteristics.  This suggests a formaton in a post-collisional extensional environment. Interestingly, analysis of 2 samples from the conglomeratic level above the unconformity shows that the volcanic clastic material has a different immobile element signature than the rhyolite. Thus it could have its origin in another volcanic rock, Ordovician-Silurian or Devonian in age.

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Over : Jean Clair DUCHESNE

Over : Eric GOEMAERE

Over : Jean-Christophe GRIGOLATO

Over : Hyacinthe VANDERSCHUEREN

Over : Bernard CHARLIER