Ore controls and related sedimentary features at the Flat Gap Mine, Treadway, Tennessee

Abstract

The Flat Gap ore body is a strata-bound, low-grade sphalerite deposit in Lower Ordovician carbonate beds that have a 37 degree dip. The mineralization is localized in breccia bodies formed in the Mascot and Kingsport Formations when the beds were flat lying. Uplift of the sediments at the end of Early Ordovician time resulted in the development of an unconformity of major significance. Limestone beds, located in the upper part of the Kingsport, served as a paleoaquifer of regional extent. Meteoric waters, circulating along faults and joints in the aquifer, dissolved porous limestone beds to the extent that interbedded and overlying dolomite beds collapsed forming long, narrow breccia bodies. These locally termed fine, rock-matrix breccias, are older than the ore-bearing breccias and appear to be the framework upon which the main ore trends developed.Warm metal-bearing brines, introduced into the aquifer shortly after the initial periods of dissolution and collapse, extensively enlarged, extended, and mineralized the older breccias as well as developed new breccias along joints essentially normal to the main trend. Both chemical and mechanical erosion of the solution channels resulted in the accumulation of debris at the bottom of the breccias. These accumulations, locally termed consist of a poorly-sorted sediment of dolomitized limestone, fine-grained dolomite, and bleached chert fragments as well as ubiquitous sand-size accumulations. Above the trash zones, the breccias are cemented with white crystalline dolomite and sulfides. The sulfides are not restricted to the later breccias but are present in the trash zone as well, occurring as breccia fragments, open-space filling, and possibly replacement. Mineralization appears to have been essentially completed before the deposition of Middle Ordovician Chickamauga sediments.The Flat Gap ore body has two distinct types of ore; one is older and contains galena-bearing, dark reddish-brown sphalerite, the other is galena-free and contains yellow sphalerite. The dark ore is confined to a single, faulted, eastward-plunging, narrow shoot, that is at least 1 1/2 miles in length. Radiating from the dark ore shoot in a net-like pattern are numerous yellow sphalerite shoots. The entire ore body is cut by several large post-ore faults that have horizontal displacements of as much as 1,500 feet.