Geologic setting of the Hillabee metavolcanic complex and associated strata-bound sulfide deposits in the Appalachian Piedmont of Alabama

Abstract

The frontal crystalline thrust sheet of the southern Appalachians in Alabama is a sequence of lower greenschist facies rocks known as the Talladega slate belt. This sequence is composed dominantly of metapelites and metasandstones, but it contains a distinctive metavolcanic complex, the Hillabee Greenstone, at the stratigraphic top. Sulfide deposits occur throughout the 170 km outcrop extent of the Hillabee. The metavolcanics are bounded above by thrust faults which have emplaced nappes containing medium to high-grade rocks of the Ashland and Wedowee sequences. The Hillabee Greenstone is as much as 2.6 km thick in some areas, but is only a few 100 m thick in most places because of the stratigraphic position of the thrust faults. Low K tholeiitic rocks constitute the bulk of the metavolcanic rocks; calc-alkaline quartz dacites are common. The Hillabee Greenstone appears to be the distal continentward portion of a volcanic arc system which developed during the middle Paleozoic in the southernmost Appalachians. The basal, dominantly pyroclastic part of the volcanic sequence is interlayered with the upper part of the Talladega Group, which locally contains a Lower to Middle Devonian fossil assemblage. In the north-central part of the belt near Pyriton, strata-bound massive sulfide deposits are common very near the base of the volcanic sequence; the deposits appear to be distal strata-bound Zn-Cu-pyrite type. Other strata-bound deposits occur in mafic rocks immediately above siliceous metavolcanic rocks in the broad central part of the belt, which appears to be dominated by basalt flows. Disseminated sulfides are common in this central region as well.