Hydrosilicate liquids in the system Na2O-SiO2-H2O with NaF, NaCl and Ta: Evaluation of their role in ore and mineral formation at high T and P

Abstract

Consideration of the existence of hydrosilicate liquids (HSL) in nature can help in understanding the accumulation and transport of some mineral and ore forming components at the transition from mag mas to hydrothermal fluids. We studied the experimental formation of HSL using a base system Na 2 O-SiO 2 -H 2 O with addition of NaF, NaCl and metallic Ta. The interaction between quartz and aqueous solution, per formed at 1.5 kbar and 600°C and followed either by cooling or by quench, showed that the formation of HSL occurred when initial Na 2 O exceeded 2 wt %. Neither NaF nor NaCl have a significant effect on the forma tion of HSL. The HSL concentrates F, whereas Cl partitions into the aqueous fluid. With addition of Ta to the system, the HSL becomes metal enriched. Natural analogs of experimental HSL can be found among "melt/fluid" inclusions entrapped in quartz and other minerals of miaroles in granite pegmatites and rare metal granites. The HSL is a novel medium enabling extreme concentrations of lithophile ore metals at the magmatic hydrothermal transition.

Homogenization Experiments of Crystal-Rich Inclusions in Spodumene from Jiajika Lithium Deposit, China, under Elevated External Pressures in a Hydrothermal Diamond-Anvil Cell

“…In addition, the compositions of the dissolved daughter minerals observed within the CIs indicate that this type of carbonate aqueous fluid is rich in silicate and alkali metals. Its composition appears to be close to that of hydrosilicate liquids with a low viscosity, high element diffusion, and a capacity for mass transport, enabling extreme concentrations of lithophile ore metals [42,43]. These observations provide useful information for evaluating the formation models of the rare-metal pegmatites, such as those suggested by London [15], Thomas et al [19], and Thomas and Davidson [40].…”

Evidence of a water-rich silica gel state during the formation of a simple pegmatite

“…In such systems the alkalis remain in solution as ions, providing a simple mechanism for the strong separation of the silica gel. For example, Smirnov et al (2012) showed experimentally that a minimum of 2 wt.% Na 2 O was required to produce a colloidal form of silica (a hydrosilicate liquid or HSL in their terminology), and that above that value the amount of HSL formed was proportional to the amount of Na 2 O present. Mustart (1972) demonstrated a possible alternative to meltÀmeltÀfluid immiscibility processes by using a hydrous albite and sodium disilicate melt as a model for peralkaline magmas.…”

“…Recently, Smirnov et al (2012) published a study which addressed an intermediate composition, with approximately equimolar H 2 O and SiO 2 . They showed that at Na 2 O concentrations >2 wt.% a water-and a silica-rich phase, referred to as a hydrosilicate liquid (HSL), with colloidal properties, developed from water-and alkali-rich silicate melts at~600ºC and remained relatively stable to low temperatures.…”

“…Experimental work Smirnov et al, 2012) suggests that this compositional space is represented by a continuum of water-and silica-rich phases ranging from very volatile-rich melts, through hydro-colloid suspensions, to very dense hydrothermal fluids. The importance of such colloidal systems is being increasingly recognized.…”

Water- and Boron-Rich Melt Inclusions in Quartz from the Malkhan Pegmatite, Transbaikalia, Russia

“…The term "gel state" is used by the cited authors and especially by Smirnov et al [40] sensu lato for hydrosilicate liquids derived experimentally from the simple system Na 2 O-SiO 2 -H 2 O. In this system with more than 15% (g/g) Na 2 O the formation of gels is plausible, because other glass formers (B 2 O 3 , Al 2 O 3 ) are missing.…”