An association between organic matter and localised, prehnite-pumpellyite alteration, at Builth Wells, Wales, U.K.

Abstract

The Builth Volcanic Group of the Builth Wells Ordovician Inlier, Wales, is a sequence of basic/intermediate lavas and volcaniclastics which shows a unique association between pyrobitumen and localised (domainal) calc-silicate alteration. The pyrobitumens occur in calcite veins and disseminated throughout alteration domains which form haloes to these veins. The pyrobitumens were emplaced by Ordovician hydrothermal fluids which were also responsible for the domainal alteration. Combined petrographic, gas chromatography and stable isotopic studies show that light organic gases, C 2-C 5, were liberated into the hydrothermal fluids by devolatilisation of liquid petroleum to form the pyrobitumens. These light organic gases mixed with methane, characterized by δ 13 C ≈ − 13‰ , which was already present in the fluids. This methane was most likely mantle-derived or produced from inorganic synthesis. The organic content of the fluids exerted an important control on the inorganic secondary assemblages. A similar control of inorganic assemblages by organic matter, might possibly occur deep within modern petroleum-bearing hydrothermal systems, such as the Guaymas Basin and Escanaba Trough hydrothermal systems.