Mineralogy and petrogenesis of pegmatites in the Spruce Pine District, North Carolina, USA

Abstract

Granitic rocks (mainly granodiorites) of the Spruce Pine Plutonic Suite occur as small plutons and numerous pegmatite bodies intruded into amphibolite-grade schists and gneisses of the Ashe Formation. Intrusion ages range from 377 to 404 Ma and are near the peak of amphibolite-facies metamorphism in the country rocks. The granodiorites have a mortar texture with coarse (~ 1 cm) grains of feldspar, quartz and muscovite enclosed in a finer grained matrix of the same phases. Large (10-30 cm) crystals of perthitic K-feldspar and plagioclase occur in the granodiorite. Pegmatites occur within the granodiorite and as isolated intrusions within the country rocks. Contacts between pegmatites and host granodiorites range from sharp, cross-cutting to completely gradational. Some of the plutons contain up to 50 % pegmatite. Pegmatite mineralogy is similar to the granodiorites; major amounts of feldspars and quartz, accessory amounts of muscovite, garnet, and epidote. Secondary minerals, formed by subsolidus recrystallization, include epidote, muscovite, quartz, and grossular garnet. A few of the pegmatites are more fractionated and contain tourmaline and beryl. Compositions of feldspars, muscovite, and garnet show some regional variation, but are generally similar for pegmatites and associated granodiorite. About half of the pegmatites are unzoned. Zoning in the pegmatites is simple with a quartz core, an intermediate zone of graphic granite, plagioclase and muscovite and an outer finer grained zone that locally shows mineral layering parallel to the contact. The abundance of pegmatitic textures relative to granitoid textures in the Spruce Pine Plutonic Suite is related to the depth of crystallization. Spruce Pine granitoids crystallized at depths of 20-30 km (0.7 to 1.1 GPa ) based on feldspar thermobarometry and experimentally determined melting relations. Skeletal crystals of quartz and feldspar along with plumose muscovite clusters are common in the Spruce Pine pegmatites. Water content of magmas at such high pressures can be high. The subtle transition from granitic to pegmatitic texture so common at Spruce Pine (and in other deep-seated systems) was probably related to the development of zones of water-enriched magma and delayed nucleation in the hydrous melts. Deformation (evidenced by mortar texture) of the Spruce Pine Plutonic Suite was related to later regional deformation.

Geochronology of Uraninite Revisited

“…Moreover, these ages are consistent with the uranium mineralization associated with the pegmatite intrusion in the region at 1030-1070 Ma (Table 1) [35]. The calculated U-Pb and Pb-Pb dates for Yancey uraninite (Figure 4c,d, Table 3) corroborate both the reported Rb-Sr and U-Pb ages of 377-404 Ma for the pegmatite intrusions associated with the Spruce Pine Plutonic Suite (Table 1) [34]. The pristine aliquot for the Yancey uraninite yields a concordant calculated age of 374.1 ± 7.2 Ma, whereas, the altered aliquot shows U gain (Figure 4d, Table 3).…”

“…Yancey is a Paleozoic pegmatite-hosted uraninite from the Spruce Pine pegmatite, Yancey County, North Carolina, which forms part of the Spruce Pine thrust block. The pegmatite crosscuts Precambrian interlayered mica and amphibole gneiss and schist of the Ashe Formation in the Blue Ridge of the Appalachian Mountains [33,34]. Dating of the Spruce Pine pegmatites has included whole rock Rb-Sr ages on two pegmatites with reported dates of 404 and 392 Ma, and U-Pb dating of zircons from the pluton yield a date of 377 Ma [34].…”

“…The pegmatite crosscuts Precambrian interlayered mica and amphibole gneiss and schist of the Ashe Formation in the Blue Ridge of the Appalachian Mountains [33,34]. Dating of the Spruce Pine pegmatites has included whole rock Rb-Sr ages on two pegmatites with reported dates of 404 and 392 Ma, and U-Pb dating of zircons from the pluton yield a date of 377 Ma [34]. The Bancroft and Cardiff samples are both from the Bancroft region of Ontario, Canada.…”

P-T-t evolution of granulite facies metamorphism and partial melting in the Winding Stair Gap, Central Blue Ridge, North Carolina, USA

“…The fact that this thermal event was weaker than the Salinic/Taconic metamorphism and was not accompanied by major deformation suggests that it may have been related to hydrothermal activity caused by the intrusion of magma (at depths >20 km), rather than by the major deformation recorded in the Inner Piedmont during this orogeny. This is consistent with the intrusion of granitoids such as the Yonah Mountain tonalite located 33 miles to the southwest of WSG and intruded at 346 ± 17 Ma (Mapes, 2002), or the Spruce Pine Pegmatites intruded at 7–11 kbar (Swanson & Veal, 2010) 361 ± 2 Ma (Mapes, 2002). Following this thermal event, the Cartoogechaye terrane continued to be exhumed as the Dahlonega Gold belt continued to be underthrust beneath it, possibly facilitated by the emplacement of the Tugaloo terrane (Hatcher et al.…”

Trace Element Concentrations and Mineralogy of Quartz Vein Deposits from Southeastern Hubei Province, China

“…However, natural quartz deposits suitable for HPQ production are rare [1,4,6]. Specifically, the quartz crucibles and semiconductor base materials consumed in photovoltaic and integrated circuit industries have been depending on HPQ produced from the Spruce Pine mining area for a long time [3,7,8], posing serious risks for world HPQ supply chain.…”