Hydrothermal alteration of tourmaline from pegmatitic rocks enclosed in serpentinites: Multistage processes with distinct fluid sources

Abstract

Hydrothermal alteration of primary tourmaline (dravite, oxy-dravite, fluor-dravite,” uvite“, schorl, oxy-schorl, fluor-schorl) and associated plagioclase and quartz is examined in coarse-grained pegmatitic plagioclase-tourmaline rocks, simple granitic pegmatites and zoned beryl-columbite pegmatites, all cutting serpentinite from the Moldanubian Zone, Czech Republic. Tourmaline is replaced by a wide spectrum of minerals—prehnite, pumpellyite-(Al), K-feldspar, chlorite, muscovite, natrolite, arfvedsonite, titanite, epididymite, and hydrous Mg-silicates—and idealized replacement reactions of tourmaline are presented. Secondary mineral assemblages replacing tourmaline and chlorite geothermometry suggest evolution of the PT conditions of tourmaline alteration from T ~ 400–350 °C to 200–100 °C and P < ~1–2 kbar. The replacement processes of tourmaline proceed from the early to late subsolidus stages, and the intensity of tourmaline replacement decreases with decreasing temperature. Alteration typically occurs in small bodies of primitive plagioclase-tourmaline rocks, whereas large, more evolved and locally Li-bearing crosscutting pegmatites exhibit less pronounced tourmaline alteration. Based on the secondary mineral assemblages and types of fluids, four distinct compositional/paragenetic types are defined: Ca,Mg-, Na,K,Mg-, K,Mg- and K,Na-dominant. Sources of fluids include (i) fluids exsolved from pegmatite melt (Na, K, Ca, ± Li), particularly in the more evolved pegmatites, and (ii) fluids infiltrated/diffused from host rocks (Mg, Ca, ± Ni, Cr) at all localities; however, mixing of the fluids is obvious. Boron released from the altered tourmalines into fluids escaped from the system, and the behavior of B in aqueous fluids from granitic pegmatites is discussed. The instability of quartz and tourmaline as well as common late K-feldspar, zeolites, arfvedsonite, and minor smectite suggests alkaline conditions. The alkaline character of fluids promoted by host serpentinite was very likely the main factor affecting tourmaline replacement and the instability of plagioclase, quartz, and other minerals.