Post-Acadian metasomatic origin for copper-bearing vein deposits of the Virgilina District, North Carolina and Virginia

Fla. State Univ., Dep. Geol., Tallahassee, FL, United States
U. S. Geol. Surv., United States

The Virgilina mining district, located in the Carolina slate belt of North Carolina and Virginia, has 1- to 3-m-wide vein systems that contain bornite, chalcocite, and hematite, plus minor argentite and gold in quartz, calcite, and sericite gangue. Chalcopyrite is rare and pyrite is absent in major orebodies. Ore-bearing veins are restricted to greenstone tuffs, breccias, and flows of the upper Precambrian Virgilina formation. Major and trace element studies of country rocks show that ore genesis is probably related to metasomatic alteration of greenstone into epidosite and alkali-enriched schistose greenstone. This metasomatic alteration produced extensive loss of copper in the greenstone. Field and petrographic evidence indicates that vein emplacement and country-rock alteration clearly postdate regional metamorphic foliation and prograde metamorphic mineral assemblages. Rb-Sr whole-rock dating of quartz + sericite + epidote vein material from the High Hill mine indicates a 340 + or - 6-Ma age for vein formation. The calculated initial ⁸⁷ Sr/ ⁸⁶ Sr ratio of the vein material (0.7042 + or - 0.0004) is compatible with Cu-bearing ore fluids that equilibrated with surrounding greenstone country rock. The 340-Ma age is younger than the suggested Acadian age ( approximately 380 Ma) for regional metamorphism in the northern Carolina slate belt. Rb-Sr mineral ages for metamorphic rocks in the Virgilina region indicate that rocks remained at elevated temperatures (250 degrees -300 degrees C) until the late Paleozoic ( approximately 290 Ma). Vein formation in the Virgilina district probably resulted from post-Acadian fracturing of hot, deep-seated greenstone units, followed by the generation and migration of metasomatizing fluids that obtained copper during epidotization and alkali-enrichment of greenstones. Migration of these fluids to shalloxver structural levels during retrograde metamorphism resulted in precipitation of copper-bearing vein deposits.

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