HYDROTHERMAL ALTERATION AND SAPROLITIC WEATHERING: EFFECTS ON THE GEOCHEMICAL CHARACTER OF FELSIC VOLCANIC ROCKS, CAROLINA SLATE BELT, USA

Contents and ratios of trace elements (e.g., REE, Ga, HFSE) and variations in radiogenic isotopes (Nd-Sr) of altered rocks of the Carolina Slate belt (CSB) are used to study the origin of clay deposits and processes leading to remobilization, transport, and concentration of toxic and other elements (As, Se, etc.) during saprolitic weathering. Extensive tracts of volcanic rocks in the CSB host deposits containing clay rocks, silicastones, and iron oxide concretions. The largest deposits at Haile and Brewer contained >650,000 tons of clay attributed to saprolitic weathering of felsic crystal tuffs. The tuffs are intensely sericitized and are associated with deposits of iron, gold, and other base metals that were produced by acidic hydrothermal fluids (ca. 544 Ma). Chemical weathering of the volcanic rocks since the mid-Paleozoic has modified the major and trace element abundances and resulted in a high-quality, white clay rock (~35% kaolinite, ~45% quartz, and ~20% sericite) with distinctive features (e.g., 4-36 ppm Ga, 0-7 ppm Se, 0-22 ppm As). The clay rock is also associated with silicastones (>80% quartz), siliceous-ferruginous sinter (beds of pyrite, quartz, and disseminated sulfides), and concretions (goethite, hematite, lepidicrosite, and kaolinite) formed by saprolitic weathering. Most alkali elements and Fe were removed from the tuffs, in contrast to REE, Th, Hf, and Zr. Variations in the abundances of trace elements (As, Se, Ga, etc.) in the deposits depend on the dominant mineralogy of the residual rocks. For example, compared to the host clay rocks, the iron-oxide concretions are enriched by factors of 50 in Se and 25 in As (60-230 ppm Se and 11-511 ppm As), and the silicastones are depleted in As and Se (factor of 10). Ga, REEs, Hf, and Zr are retained in the clay rocks despite intense acidic hydrothermal alteration and prolonged saprolitic weathering. The distinctive trace element features of the clay deposits result from the overprinting of saprolitic weathering processes on rocks whose mineralogical and isotopic character was previously fixed by the interaction of acidic hydrothermal fluids with the felsic crystal tuffs. Thus, the clay deposits formed originally from hydrothermal alteration of the tuffs and the deposits were subsequently modified as a result of saprolitic weathering.