An oxygen isotope and geochemical study of meteoric-hydrothermal systems at Pilot Mountain and selected other localities, Carolina slate belt

U. S. Geol. Surv., Reston, VA, United States

Several epigenetic mineral deposits in the Ca1rolina slate belt are intimately related to meteoric-hydrothermal systems of late Precambrian and early Paleozoic age. At Pilot Mountain, low ¹⁸ O rocks correlate well with zones of strong silicic alteration and alkali leaching accompanied by high alumina minerals (sericite, pyrophyllite, andalusite + or - topaz) and anomalous concentrations of Cu, Mo, Sn, B, and Au. The alteration occurs within andesitic volcanic and volcaniclastic rocks and is associated with a subvolcanic(?) dacite porphyry stock on the southeastern slope of the mountain. Tilting and erosion have exposed an oblique section through the original system, interpreted to expose shallower rocks to the northwest. A 4-km ² central zone of slight ¹⁸ O depletion (delta ¹⁸ O (sub whole rock) = 4.3-6.1ppm) occurs on the broad and resistant (silicifled) western flank of Pilot Mountain, predominantly within quartz-sericite schist and quartz granofels containing pods of high A1 minerals. A magmatic source for much of the sulfur and metal is likely, and a subordinate magmatic water component in the fluid of the central zone is possible. This central zone is surrounded by a >30-km ² peripheral zone of low ¹⁸ O sericite schists, chlorite-sericite schists, and andesitic volcanic rocks (delta ¹⁸ O (sub whole rock) < 3.8ppm), with the lowest values (1.4ppm) occurring in intensely sericitized rocks on the eastern flank of Pilot Mountain, near the apex of the dacite porphyry stock. Rhyolites of the Uwharrie Formation (delta ¹⁸ O = 3.8-6.3ppm) are not as strongly altered as nearby andesites and may postdate the hydrothermal alteration. The fluid calculated to be in equilibrium with the lowest ¹⁸ O quartz veins and country rocks at 300 degrees + or - 50 degrees C would have delta ¹⁸ O approximately -4.5 + or - 2.0 per mil, whereas analyses of radiating pyrophyllite indicate equilibrium with a fluid having delta D approximately -30 per mil, consistent with a slightly ¹⁸ O-shifted, low-latitude meteoric water. Subsequent greenschist metamorphism caused intermineral isotopic reequilibration in several samples and may have modified preexisting alteration assemblages, but it did not destroy the large delta ¹⁸ O anomaly produced by meteoric-hydrothermal activity at Pilot Mountain. Reconnaissance studies of other alteration zones in the Carolina slate belt have so far disclosed the involvement of meteoric-hydrothermal fluids at the Snow Camp pyrophyllite deposit, at the Hoover Hill and Sawyer Au mines, and probably at the Haile and Brewer Au mines.

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