50 Subsurface Geologic Methods 



The bauxite deposits of the southern Appalachian states ^^ are an 

 example of residual accumulations on a partly buried erosion surface. 

 These deposits were formed during the Eocene when the climate was fa- 

 vorable for lateritic weathering. The margins of the Appalachians had 

 been leveled by the Highland Rim peneplain, with a karst topography de- 

 veloping in areas of limestone outcrop. Bauxite accumulated in the sink- 

 holes and was incorporated into the basal beds of the Wilcox formation. 

 The deposits are now located on remnants of the dissected peneplain, at 

 the unconformity between the Wilcox and the older truncated rocks. 



Placer Deposits: Accumulations of placer minerals will often be 

 found at or near local unconformities because of their mode of concen- 

 tration. Gravels containing heavy minerals are deposited along the reaches 

 of a stream where there is slack water. Because of higher specific gravity 

 and aided by the jigging action of eddies and swirls, the placer minerals 

 settle toward the bottom of the stream channel. The bottom gravels close 

 to bedrock will have the richest pay streaks. If the bedrock is rough and 

 irregular, natural riffles will trap especially rich streaks. If the stream 

 channel is later filled and abandoned, the contact between the dense bed- 

 rock and the loose gravels becomes a local unconformity. 



The "high level" Tertiary, auriferous gravels of the Sierra Nevada 

 described by Lindgren ^^ are an excellent illustration. Buried Tertiary 

 gravels containing placer gold are exposed high on the sides of Quater- 

 nary valleys. These gravels were deposited in stream channels cut in 

 bedrock during the Eocene, with the richest pay streaks in the deep gravels 

 two to three feet off the bottom. The stream channels were then buried by 

 lean gravels and volcanics which completely filled the valleys. After 

 elevation of the Sierra Nevada in the Pliocene and Pleistocene, new streams 

 eroded canyons an average of 2,600 feet below the early Tertiary stream 

 bottoms. The gold-bearing gravels have been worked by drift mining and 

 hydraulic procedure. 



Hydrothermal Deposits: The influence of unconformities in control- 

 ling ore formation by hydrothermal solutions is that of modification and 

 not primary control. A review of the literature revealed no districts or 

 mines in which an unconformity was considered the primary control in ore 

 deposition. Unconformities, if present in the section, were rarely consid- 

 ered as having any modifying influence at all. 



Bell ^^ mentions the apparent eff"ect of the slope of an unconformable 

 surface on ore deposition in the Hallnor mine, second-largest mine in the 

 east Porcupine area of Ontario. An angular unconformity of considerable 

 erosional irregularity separates a series of younger sediments from older 

 lava flows. A factor that has proved useful in prospecting for new ore 

 zones is the slope of the lava-sediment contact. At the west end of the 



^McKinstry, H. E., Mining Geology, New York City, Prentice-Hall, Inc., 1948. 

 ^3 Lindgren, W., Tertiary Gravels of the Sierra Nevada, U. S. Geol. Surv. Prof. Paper 72, 1911. 

 ^^ Bell, A. M., Hallnor Mine, Structural Geology of Canadian Ore Deposits: Canadian Inst, of Min. 

 Metallurgy Trans., 1948. 



