WEATHERING-RELATED MOBILITY OF RARE EARTH ELEMENTS IN HIGH-ALUMINA ALTERED METAVOLCANIC DEPOSITS, SOUTH CAROLINA, USA

Geochemical studies of metavolcanic rocks of the Carolina terrane document processes that remobilized REE during in-situ weathering. Large kaolin deposits (>650,000 metric tons) near Kershaw, S.C., occur predominantly in crystal metatuff. The kaolins are temporally and spatially related to epithermal deposits of Fe and Au that comprise the Haile gold mine property. Chemical alteration of the volcanic rocks strongly modified the original bulk rock composition producing a high-quality white clay resource composed of ~35% kaolinite, ~20% sericite ~45% quartz. The metavolcanic rocks span from 64.9 to 76.8 wt. % SiO₂ with average contents of 14.43 wt. % Al₂O₃, and 3.98 wt. % K₂O. They have total REE from 58 to 113 ppm (ave. ~90 ppm), and are light REE-enriched (ave. 100× chondrite). Silicified, altered and Au-pyrite mineralized volcanic rocks have SiO₂ contents (49–99 wt. %) that are inversely correlated with Al₂O₃ (1.3–27.16 wt. %). Mineralized and altered volcanic rocks and subvolcanic rhyolite porphyry range from 2 to 203 ppm REE (ave. ~76).The kaolin deposits have SiO₂ contents (51–85 wt. %) that are inversely correlated with Al₂O₃(10.1–33.4 wt. %) and contain 60 to 450 ppm REE.

The kaolin deposits are proposed to have formed by a combination of hydrothermal alteration and surficial weathering and their REE signatures reflect the effects of volcanism, epithermal alteration and mineralization, metamorphism, and intense saprolitization. Chemical weathering of the metavolcanic rocks remobilized REE and caused variations in whole rock patterns that reflect differential weathering of primary titanite, allanite, and apatite and precipitation of secondary minerals in the saprolitized volcanics rocks including poorly crystalline and hydrated REE-phosphates, Ba-Al phosphates, diaspore, gibbsite, goethite, lepidocrocite, rutile and siderite. The REE distributions in the kaolin deposits reflect the presence of newly-formed cryptocrystalline monazite-(La), monazite-(Nd), and lesser amounts of poorly crystalline xenotime. The minerals and the chemical processes that produced weathered kaolin deposits in the metavolcanic and subvolcanic rocks at Haile are thought to be analogous to those that formed REE ion adsorption clay deposits in altered granitic rocks of China and Southeast Asia.