Geologic setting of volcanogenic base and precious metal deposits of the West Georgia Piedmont; a multiply deformated metavolcanic terrain

Abstract

The northern Piedmont of Georgia contains numerous gold and base metal occurrences within a sequence of high-grade, multiply deformed metamorphic rocks. These rocks are interpreted as regionally correlative with portions of the Ashe Formation in North Carolina, and sulfide deposits are similar to those seen in the Ashe at Ore Knob, North Carolina, and Great Gossan Lead, Virginia. Gold and sulfide deposits in northern Georgia were sporadically worked prior to the Civil War until approximately 1920. These deposits were previously thought to have formed by replacement from hydrothermal solutions but are now recognized as syngenetic with surrounding volcanic rocks. Mineralization is mainly strata bound within a metamorphosed sequence of mafic to felsic volcanic rocks interlayered with metasedimentary rocks. Banded iron-formation commonly is associated with mineralized zones. Sulfide deposits, previously mined chiefly for pyrite for the production of sulfuric acid, also contain pyrrhotite, chalcopyrite, and sphalerite with trace amounts of gold. These deposits are generally of three types: pyrite + gold, pyrite + base metal sulfide, or barren pyrite. Sulfide ores are massive and disseminated and occur within metamorphosed felsic volcanic rocks, interlayered felsic and mafic volcanic rocks, and interlayered volcanic and sedimentary rocks. Gold occurs in placers and in vein deposits associated with banded iron-formation, siliceous alteration zones, and quartzose schists.Mineralized rocks are dominantly within the New Georgia Group and immediately overlie units of the Sandy Springs Group. New Georgia Group rocks are dominantly metavolcanic and metaplutonic rocks with interlayered metamorphosed volcaniclastic and sedimentary rocks. Algoma-type banded iron-formation, formed during periods of quiescence in the volcanic sequence, is interlayered with amphibolite. New Georgia Group rocks grade upward, through decreasing abundance of metavolcanics, into overlying metasediments of the Sandy Springs Group. Multiple folding has overturned the sequence and older volcanic rocks now structurally overlie metasedimentary rocks.