Physicochemical conditions and timing of rodingite formation: evidence from rodingite-hosted fluid inclusions in the JM Asbestos mine, Asbestos, Québec

Fluid inclusions and geological relationships indicate that rodingite formation in the Asbestos ophiolite, Québec, occurred in two, or possibly three, separate episodes during thrusting of the ophiolite onto the Laurentian margin, and that it involved three fluids. The first episode of rodingitization, which affected diorite, occurred at temperatures of between 290 and 360°C and pressures of 2.5 to 4.5 kbar, and the second episode, which affected granite and slate, occurred at temperatures of between 325 and 400°C and pressures less than 3 kbar. The fluids responsible for these episodes of alteration were moderately to strongly saline (~1.5 to 6.3 m eq. NaCl), rich in divalent cations and contained appreciable methane. A possible third episode of alteration is suggested by primary fluid inclusions in vesuvianite-rich bodies and secondary inclusions in other types of rodingite, with significantly lower trapping temperatures, salinity and methane content. The association of the aqueous fluids with hydrocarbon-rich fluids containing CH4 and higher order alkanes, but no CO2, suggests strongly that the former originated from the serpentinites. The similarities in the composition of the fluids in all rock types indicate that the ophiolite had already been thrust onto the slates when rodingitization occurred

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

The Miocene borate deposits of western Turkey are associated with extensive medium- to high-K calc-alkali igniometric volcanism and a differentiated comagmatic alkaline trachybasalt-trachydacite lava suite. Ignimbritic air-fall and reworked pumiceous elastic 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, 1) and B/K (> 0.001) ratios, and (c) a mildly fractionated REE pattern (La-N/Yb-N similar to 2) and large positive Eu anomaly (Eu/Eu* similar to 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. (C) 1998 Elsevier Science B.V