Appalachian strata-bound deposits; their essential features, genesis and the exploration problem

Abstract

Zinc deposits of Appalachian type were formed in nearly horizontal strata and were subsequently folded and faulted by Appalachian orogeny. In their original nearly horizontal position they closely resemble deposits of the Mississippi Valley with which they are usually classified. With the exception of the Austinville-type deposits, which will not be considered herein, the subject deposits are related to a Lower Ordovician paleoaquifer. The classic examples are Friedensville, Pennsylvania; Timberville (Shenandoah Valley), Virginia; Mascot-Jefferson City district, Tennessee; and Copper Ridge district, Tennessee. Representatives of deposits of this type hereinafter referred to as Tennessee sub-type extend the length of the Appalachian-Ouachita chain from New-foundland to Oklahoma. Except for the superimposed structural variations, these deposits are remarkably similar in their essential characteristics which include Lower Ordovician carbonate sedimentary host, low-iron sphalerite, minor to insignificant galena and pyrite, pervasive dolomitization, sulfide deposition at shallow depth in an essentially lithified section, and sulfide mineralization restricted to a fairly narrow vertical range of about 400 feet, more or less.The recently discovered zinc deposits in the Lower Ordovician of Middle Tennessee are similar in all essential respects to the Tennessee sub-type as described above. Because the Middle Tennessee deposits in the Mid-Continent structural province have not been subjected to deformation of the Appalachian orogeny, they furnish dramatic new evidence of the similarity between Mississippi Valley and Appalachian type deposits.The preorogenic structures of the Lower Ordovician carbonate sediments as modified by the paleoaquifer in the mining districts of East Tennessee appear to be analogous to those of the contemporary aquifer in Tertiary carbonate sediments of the Florida and Yucatan Peninsulas. The Lower Ordovician was characterized by a widespread permeable karst system developed by dissolution of carbonate rocks by meteoric water. Probably in early Middle Ordovician time deposition of sulfide ore began from concentrated saline water which probably contained lime, magnesia, chlorine, silica, sulfur, and ammonia, as well as zinc. The pregnant saline solutions moved into and through the paleoaquifer towards areas of lower hydrostatic potential capturing the discharge areas originally established by the karst system and displacing the fresh water upward.The deposition of zinc sulfide was controlled by a complex of physical and chemical factors which are obscure. Factors which may have been significant are the availability of hydrogen sulfide, the interface of fresh and saline water, and a critical pressure-temperature zone at which a complexing agent would volatilize or which would otherwise trigger sulfide deposition. It appears that the factors controlling deposition were quite sensitive to depth below surface. Probably because of this sensitivity to elevation, host rocks generally considered favorable are mineralized only in local areas. Subtle structures, now obliterated by later tectonism, may have been instrumental in localizing the deposition in the permeable zones only within the critical elevation or pressure-temperature range. In any case, pregnant saline solutions traveled extensively through the aquifer causing widespread dolomitization which with other ore-associated alteration phenomena is much more extensive than the ore itself. Exploration for these deposits should take into account the above factors.The source of the zinc is elusive, and there appears to be nothing in the deposits, their history or environment that is not ambiguous relative to this question. Nevertheless, the diagenesis of regionally adjacent basins of thick argillaceous accumulations is considered to be the probable source of zinc-bearing brines from which the ore was deposited.