Geochemical constraints on high-purity quartz formation in the spruce pine pegmatite, USA: Insights from quartz and coexisting mineral chemistry

This study investigates the geochemical characteristics and crystallization conditions of quartz and coexisting minerals from a high-purity quartz (HPQ) favorable pegmatite at the McKinney Mine, Spruce Pine district, USA, to elucidate crystal-scale processes responsible for quartz purification. Integrated petrography, cathodoluminescence (CL) imaging, LA-ICP-MS trace-element analysis of quartz, electron probe microanalysis (EPMA) of mica and feldspar, fluid-inclusion petrography, and whole-rock geochemistry were used to constrain quartz growth and late-stage modification. Quartz grains display five distinct CL-defined growth domains (Qtz Grn 1–5) characterized by heterogeneous trace-element concentrations (Ti = 3.54–17.55 ppm; Al = 70–258 ppm; Ge/Ti = 0.02–0.66), reflecting combined effects of magmatic differentiation and post-magmatic recrystallization, rather than melt evolution alone. TitaniQ thermometry, using a Ti activity of aTiO 2 = 0.6 appropriate for low-Ti, rutile-absent systems, yields quartz crystallization temperatures of 519–563 °C. Coexisting muscovite (F = 0.18–0.47 wt%; 545–556 °C; 5.4–5.6 kbar) and biotite (Mg# = 0.21–0.23; 335–580 °C; 3.55–5.95 kbar) record evolving conditions during pegmatite crystallization. Feldspar compositions (Or 89-96 , Ab 88-94 ), with low Ti contents in feldspar and micas, constrain crystallization from a Ti-depleted, strongly peraluminous melt. Whole-rock geochemistry indicates peraluminous compositions (A/CNK ≈ 1.49), LREE enrichment (ΣLREE/ΣHREE = 1.8–6.96), and variable Eu anomalies (δEu = 0.17–2.95), consistent with crustal anatexis and plagioclase-controlled differentiation. CL textures, secondary fluid inclusions, and microstructural features indicate localized post-magmatic recrystallization with limited fluid ingress, which contributed to quartz purification. Collectively, the results support a two-stage model involving advanced magmatic differentiation followed by recrystallization-assisted purification, providing a geological framework of geochemical indicators for assessing HPQ potential in peraluminous pegmatite systems.