Hydrothermal alteration in the lower part of an early Proterozoic greenstone complex at Palovaara, Enontekiö, northwestern Finland

The lower part of the early Proterozoic greenstone complex at Palovaara, northwestern Finnish Lapland, comprises a quartzite formation overlain by intermediate and felsic tuffs and tuffites with some lava flows, mostly intermediate in chemical composition. They are intruded by numerous sills of albite diabase and also by some minor keratophyric dykes. The lithology and geochemistry of the rocks are similar to those of the spilitekeratophyre association and provide evidence that the rocks were thoroughly altered by postcrystallization processes. Geochemically, all the rock types, including the extrusive and subvolcanic varieties, are greatly enriched in Na and depleted in K and Mn. The alteration resembles that observed in recent submarine volcanics, whose mineralogy is largely governed by the water/rock ratio during alteration. Subsequent regional metamorphism had only a minor, if any, effect on the geochemistry of the rocks but adjusted the mineralogy to the conditions of epidote-amphibolite facies. The analytical data on lavas and albite diabases suggest an increase in Na, Si, Al and Ti and a decrease in total Fe, Mn, Mg and Ca, with an increase in w/r ratios from low to moderate values, thus reflecting intensifying albitization of the host rock. Further intensification of the water-rock interaction resulted in a dramatic change in alteration. The most permeable parts of the rocks, such as the interpillow matrices, pillow selvages and some of the fractures plus their wall rocks, acquired an ultramafic mineral composition with a marked increase in Fe and Ca and a decrease in Si and Na. A K-metasomatic event that postdated the general hydrothermal alteration locally raised the K-content of the rocks.