Petrogenesis of aquamarine-bearing granitic pegmatite: The case of Haramosh pegmatite field (northern Pakistan)

Abstract

The Haramosh pegmatite field in northern Pakistan is renowned for numerous granitic pegmatites, containing economically significant quantities of aquamarine, tourmaline, and topaz. This study investigates the petrogenesis of aquamarine-bearing pegmatite and its genetic relationship with the country rock (Iskere gneiss) and an adjacent intrusion (Jutial leucogranite). The studied pegmatite is composed predominantly of feldspar, quartz, muscovite and tourmaline, resembles the S-type Jutial leucogranite and shows internal zonation with gem-quality aquamarine and topaz forming in the miarolitic cavities. Zircon UPb dating yields ages of 4.3 ± 0.9 Ma for the pegmatite and 10.3 ± 0.2 Ma for the leucogranite, distinct apparently from the Iskere gneiss (1881 ± 27 Ma). LuHf isotopic data suggest a common crustal source for the studied pegmatite and leucogranite, with εHf(t) values ranging from −35.9 to −19.7 (mean = −26.5) and − 29.5 to −24.4 (mean = −26.0), respectively. These results significantly deviated from those observed in the Iskere gneiss (−16.0 to +6.5, mean = −6.0), thereby precluding the possibility of a partial melting model of the country rock. The pegmatite, classified as LCT-type, shows high SiO₂ (71.53–77.17 wt%), Al₂O₃ (13.50–15.82 wt%), Li (14.62–224 ppm), Ta (0.01–4.51 ppm) concentrations and ASI values (1.34–2.37), with higher differentiation-index values (DI = 92–97) and lower K/Rb ratios than the leucogranite. Compositional variations of tourmaline, plagioclase and muscovite also indicate a higher degree of magma evolution in the pegmatite compared to the leucogranite. The plagioclase in the pegmatite displays a decrease in CaO content from the border to core zones, while whole rock Be concentrations exhibit an increase along this gradient, indicating a progressive enrichment of Be during magma evolution within the pegmatite dyke. Based on all results, we propose that the Haramosh aquamarine-bearing pegmatite was likely generated by magmatic fractional crystallization from a leucogranitic parent magma. The formation of aquamarine gems can be attributed to a progressive enrichment of Be during magma evolution process within the pegmatite dykes. The observed widespread occurrence of miarolitic cavities associated with aquamarine gems, coupled with the observed decrease of fluorine content in tourmaline from the pegmatite border to core zones, indicates the involvement of a magmatic volatile phase (MVP) exsolution process during the final stage of magma cooling. This process reduces the solubility of Be in the melt, thereby promoting the crystallization of aquamarine.