A fluid inclusion study of vug minerals in dawsonite-altered phonolite sills, Montreal, Quebec: implications for HFSE mobility

Abstract

Ordovician limestones in the Francon quarr on the island of Montreal, Quebec, are host to three sills of Cretaceous age composed of phonolite that has been extensively altered to dawsonite. An interesting feature of the sills is the presence of abundant vugs containing a wide variety of minerals, including several in which one or more high field strength elements (Zr, Hf, Nb, Ti) is a major component. The most important of these latter minerals is weloganite, a rare strontium zirconium hydrous carbonate, first identified in the Francon, quarry. Four types of inclusions have been recognized in vug minerals: aqueous, aqueous-carbonic, carbonic and solidbearing. Aqueous inclusions homogenize at temperatures mainly between 70° and 170°C and between 230° and 390°C. The homogenization temperatures of primary inclusions cluster around 350°C. Aqueous inclusions and the aqueous phase in aqueous-carbonic inclusions have salinities ranging between 10 and 24 eq.wt.% NaCl equivalent. Primary aqueous-carbonic inclusions have low XCO2 ( 0.7); carbonic inclusions are all secondary. Nahcolite, dawsonite and weloganite occur as daughter minerals or trapped solids. Nahcolite and possibly natron or mirabilite appear to form in frozen inclusions. Analyses of fluid inclusion decrepitates detected high concentrations of Na, Cl, Al, S, and C. The extraordinarily high concentration of Al in the fluid (possibly exceeding 1 wt.%) suggests a pH of approximately 10. Pressure and temperature conditions, estimated from stratigraphic reconstruction and the isochores of primary aqueous fluid inclusions, were 450 bar and 360 to 400°C, respectively. The relatively high temperatures and compositions of primary fluid inclusions suggest that vug filling was the result of mineral precipitation from an orthomagmatic fluid. A model is proposed in which a partially crystallized phonolite melt started exsolving a homogeneous low XCO2 fluid immediately prior to or after intrusion. Sodium, aluminium, chlorine, fluorine, sulphur and HFSE elements such as Zr, Hf, Nb and Ti were partitioned into the hydrous phase, in the case of Zr, possibly to a concentration of 300 ppm. The near horizontal orientation of the sills and the chilled margins, produced by quenching of the magma, created a tight seal that inhibited escape of the fluids. As a result, the phonolite “stewed in its own juices” long after crystallization, giving rise to widespread replacement of primary igneous minerals by dawsonite, and precipitation of this and other minerals in vugs. Once the sills had colled to temperatures between 200 and 300°C, the aqueous fluid exsolved a high CO2 fluid which was trapped as the secondary three-phase type II and type III inclusions. Decreasing temperature is considered to have been the principal control of mineralization, although in the case of the lower temperature minerals, decreased bicarbonate or carbonate ion activity, and a lower dielectric constant, as a result of CO2 exsolution, may have played a role in the deposition of HFSE-bearing minerals.