Geochemical discrimination of volcanic rocks associated with borate deposits: an exploration tool?

Abstract

The Miocene borate deposits of western Turkey are associated with extensive medium- to high-K calc-alkali ignimbritic volcanism and a differentiated comagmatic alkaline trachybasalt–trachydacite lava suite. Ignimbritic air-fall and reworked pumiceous clastic materials are intimately associated with lake sediments that host the borate deposits. Local ignimbritic volcanism is considered the primary source of the B for the Kirka borate deposit in this area. Comparison of the geochemical composition of Turkish ignimbrites associated with borates (`fertile' ignimbrites) with those that do not (`barren' ignimbrites), exhibit a number of features that might prove useful in the exploration for borates in similar volcanic domains. In particular, `fertile' ignimbrites are (a) generally a high-K calc-alkali suite, well-evolved and fractionated (K/Rb is low, <200) with a high-silica rhyolitic bulk composition, (b) exhibit a combined high content of B, As, F, Li and Pb, with high B/La (>1) and B/K (>0.001) ratios, and (c) a mildly fractionated REE pattern (La N/Yb N∼2) and large positive Eu anomaly (Eu/Eu *∼0.1). Other apparent discriminants involving both compatible and incompatible elements (relative to major silicate phases) are largely a function of different degrees of partial melting and fractionation. It is suggested that the initial source of the B (and other associated elements) was from LIL-rich fluids released by the progressive dehydration of altered oceanic crust and pelagic sediments in a subduction zone. The absence or presence of sediments in a segmented subduction zone may influence the variable lateral distribution of borates in active margins on a global scale. Once the crust has become enriched in B via previous or contemporary subduction-related calc-alkali magmatism, the effect of tectonic environment, climate and hydrothermal activity influence the local development of the deposits.