Sienna ("ocher") deposits in the Cartersville district, Georgia, occur in the contact zone between dolomitic limestone and underlying quartzite of Lower Cambrian age. These rocks, together with interbedded schists, were folded and recrystallized probably near the close of Paleozoic time. Simple fissures and transverse and oblique faults, formed during the late stages of folding, admitted hydrothermal solutions, which deposited concentrated masses of hematite and pyrite, and other ore minerals, in the dolomites and calcareous schists. There is some evidence that large bodies of iron carbonate also were deposited.
Subsequent weathering has removed the dolomite near the surface, and its residuum in many places rests upon the more resistant quartzite. Iron derived from the hematite and pyrite, and possibly from iron carbonate, was transferred by meteoric waters and deposited as hydrous oxides by partial replacement of (1) irregular, structureless bodies of the residuum, and (2) irregular areas in schists both at the base of the dolomite and beneath the uppermost quartzite bed. A relatively thin superficial portion of the uppermost quartzite bed also was replaced. Transfer of the iron has taken place over short distances only, and the present positions of the sienna deposits reflect essentially the positions of the deposits of source minerals.
