The alteration mineralogy of two outcrops of basaltic lavas in northeast Ireland is studied with respect to the hydrothermal alteration history of basalts. Evidence for contact between basaltic flows and palaeorivers or palaeolakes is reported in both outcrops, which belong to two different formations: the Lower Basaltic Formation (LBF) and the Inter Basaltic Formation (Causeway Tholeiite Member, CTM). The secondary minerals consist of layer silicates (chlorite-smectite, C-S, di- and trioctahedral smectites), zeolites, calcite and silica minerals and there is a repetition of the sequence of alteration mineralogy in each formation. The mineralogy of phyllosilicates from the LBF (C-S) corresponds to hotter conditions of alteration than those of CTM (di/trioctahedral smectites), while the zeolite mineralogy is controlled by the fluid composition. In consequence, the mineralogy and the distribution of alteration minerals suggest that the crystallization of phyllosilicates and zeolites did not develop during burial of lavas a long time after the eruption, but reflects thermal gradients driven by lava heat flow and marked by local perturbation caused by regional water flow through the aquifer zone in the lava during the last cooling of each lava sequence.

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