Temperatures and duration of crystallization within gem-bearing cavities of granitic pegmatites

Abstract

Crystallization within miarolitic cavities in granitic pegmatites begins with the primary igneous minerals – feldspars, quartz, micas, and lesser others – along the margins of pockets and ends with the formation of zeolites, carbonates, and finally dense clay. The euhedral and gem-quality crystals for which many pegmatites are mined form within this interval, and they are commonly suspended in the dense clay with no points of attachment to a substrate. The sequence of mineral assemblages reflects a history of crystallization with cooling that continues well after and to temperatures below the initial magmatic stage. Low-temperature assemblages of zeolites and clays are normally restricted to the miarolitic cavities themselves in otherwise fresh and unaltered pegmatite of igneous origin. This study utilizes feldspar solvus thermometry to delineate the temperature interval over which the euhedral and gem-quality crystals form. Additional data include a survey of Al/Si order in K-feldspar and the mineralogy of the clay-size fraction from miarolitic cavities as further constraints on the conditions of crystallization. The primary feldspars at pocket margins record an average crystallization temperature of 431°C (1σ = 68°C), approximately the same temperature at which crystallization takes place across the massive portions of thin miarolitic pegmatite dikes. The average calculated temperature of closure of perthite exsolution occurred at 338°C (1σ = 60°C). Pairings of glassy, non-perthitic rims on K-feldspar with radial albite of cleavelandite habit on the K-feldspar surfaces record the final stage of crystallization of the primary silicates in pockets at an average temperature of 359° (1σ = 64°C). The glassy rims are not perthitic because they form close enough to the closure temperature of perthite that exsolution does not occur. The zeolites, which include stilbite-Ca, laumontite, and heulandite, are restricted in their stability to less than 300°C, and they follow the precipitation of the gem minerals in all known cases. Thus, the gem-forming stage likely falls between ~ 435°-355°C based on the feldspar solvus temperatures. Cookeite, kaolinite, montmorillonite, and illite enclose zeolites and formed after them. In a thermal model, a dike 1 meter thick emplaced as hydrous granitic liquid at 700°C at a depth of 7 km (ambient temperature of 160°C) cools to 435°C at its center in 12 days, and from 435°C to 355°C – the interval over which the primary silicates crystallize in pockets – in another 14 days. Increasing the host rock temperature to 300°C only extends the crystallization interval of the principal pocket silicates by another 5.2 months. The dike center reaches 275°C, at which temperature laumontite could crystallize, approximately 55 days after emplacement. Clay minerals likely form and perhaps reform below ~200°C, the upper thermal stability limit of stilbite-Ca, as the devitrified products of a remnant and flux-depleted hydrosilicate glass that partially fills the miarolitic cavities. The pegmatite achieves thermal equilibrium with the host rocks at 160°C more than 200,000 years later.