MINERALOGY OF SPODUMENE PEGMATITES AND RELATED ROCKS IN THE TIN-SPODUMENE BELT OF NORTH CAROLINA AND SOUTH CAROLINA, USA

Abstract

The Tin–Spodumene Belt (TSB) along the border of North and South Carolina, southeastern USA, is composed of hundreds of en echelon dikes intruded into amphibolite-facies mica schist and amphibolite. The spodumene pegmatites are unzoned and consist of medium- to very coarse-grained spodumene, plagioclase, K-feldspar, quartz, muscovite, and accessory amounts of beryl, manganoan fluorapatite, triphylite, and cassiterite. The pegmatitic fabric is replaced by a fine-grained saccharoidal albite– quartz fabric that also heals fractures in spodumene and feldspar. Plagioclase (Ab95–96) in the pegmatite is only slightly less sodic than that (Ab98–99) in the replacement albite–quartz assemblage. Rare, nonperthitic K-feldspar is slightly sodic (Or95–99). Spodumene typically contains about 1% iron oxide, giving the mineral a light green color. Muscovite contains 1–2 wt% FeO and a trace of sodium (0.2–0.6 wt% Na2O). A Mn-bearing fluorapatite (altered to low-F and low-Mn apatite) and triphylite (altered to fairfieldite) occur within the pegmatite fabric and appear to be primary phosphate minerals. Secondary phosphate minerals, including several species first described from the TSB, are associated with fracture fillings in sheared pegmatite. Pre-tectonic plagioclase – quartz – muscovite – garnet (Alm71Sps26Prp3) pegmatites are folded and sheared. Muscovite and Mn-bearing fluorapatite compositions are similar to equivalent phases in the spodumene pegmatites. The spodumene pegmatites are sheared, especially in proximity to regional shear-zones, but are not folded. Post-tectonic plagioclase – quartz – K-feldspar – muscovite – biotite pegmatites have Mn-poor fluorapatite, unlike the spodumene pegmatites. The pre-tectonic pegmatites may represent a precursor to the spodumene pegmatites.