Geologic Features
The most important mineral of zinc is the sulphide, sphalerite or "zinc blende." The minerals of the oxide zone are smithsonite (zinc carbonate) and calamine (hydrous zinc silicate), which yield minor amounts of zinc and are especially productive at Leadville, Colorado. Zincite (zinc oxide) and willemite (zinc silicate) are the important minerals in the deposits of Franklin Furnace, New Jersey. The association of most deposits of zinc with more or less lead has been noted.
The ores of zinc are of two general classes, corresponding to the two classes of lead ores (pp. 211-212). Zinc ores of the first type are in veins and replacements in sedimentary rocks at shallow depths, independent of igneous association, and are supposed to have been formed by cold solutions. They are found in the Mississippi Valley, in Silesia, and in many of the smaller European deposits. They were formerly the leading zinc-producers, and now produce about 45 per cent of the world's total. Zinc ores of the second type consist of veins and replacements related to intrusive rocks, sometimes extending to considerable depths, and of more complex composition. They include most of the deposits of the American Cordilleran region (Butte, Cœur d'Alene, Leadville, etc.), of Franklin Furnace, of Australia, of Burma, and of many other places.
The zinc-lead ores of the type found in the Mississippi Valley are of special interest, in that they are sulphide ores of an origin apparently independent of igneous agencies. These ores occur as fissure-fillings and replacements, mainly in nearly flat-lying Paleozoic limestones and dolomites—the Bonne Terre dolomitic limestone of southeastern Missouri, the Boone formation of southwestern Missouri and Oklahoma, the Galena dolomite of Wisconsin and Illinois. They are variously associated with a gangue of dolomite, calcite, quartz, iron pyrite, barite, and chert. Not infrequently they are spread out both in sheets and in disseminated form along carbonaceous layers within or at the base of the limestone.
The source of the primary sulphides has been a subject of much discussion. All are agreed that they were first deposited with the sediments in minutely dispersed form, through the agency of the organic contents of the sediments, and that such deposition was somewhat generally localized by estuarine conditions which favored the accumulation of organic remains. Many years ago, before the evidence of estuarine deposition was recognized, Chamberlin suggested an ingenious hypothesis for the northern Mississippi Valley,—that the organic material had been localized by ocean's currents forming something in the nature of a Sargasso sea. Differences of opinion become acute, however, when the attempt is made to name the precise sedimentary horizon, out of several available horizons, in which for the most part this primary concentration occurred. Judging from the organic contents of the several beds, the primary source may have been below, within, or above the present ore-bearing horizons. If the ore came from the lower horizons, it was introduced into its present situation by an artesian circulation, for which the structural conditions are favorable. If the ore was derived from overlying horizons, downward moving solutions accompanying erosion did the work. If the primary source was within the horizon of present occurrence of the ores, both upward and downward moving waters may have modified and transported them locally. For each of these hypotheses a plausible case can be made; but much of the evidence can be used interchangeably for any one of them. In spite of the wealth of data available, it is astonishingly difficult to arrive at a conclusion which is exclusive of other possibilities. Without attempting to argue the matter in detail the writer merely records his view, based on some familiarity with these districts, that, on the whole, the evidence favors the accumulation of these deposits by downward moving meteoric solutions during the weathering of overlying strata; but that it is by no means certain that a part of the ores has not been derived from lower horizons. The great area of the producing districts in comparison with their depth, the uniform association of the ore-bearing zone with the surface regardless of geologic horizon uncovered by erosion, the failure of the ores to extend in quantity under cappings of later formations, and the known efficacy of oxidizing waters in local downward transfers of zinc and lead, seem to suggest concentrating agencies which are clearly related to surface conditions.
It is of interest to note that in many places in the limestones of Missouri and Virginia, and elsewhere in the Paleozoic rocks, there are sinks of limonite and clay near the surface, which are likewise believed to have originated through downward movement of waters deriving their mineral contents from the erosion and stripping of overlying sediments. Still further, the primary deposition of Clinton iron ores in many parts of the Mississippi Valley and eastward to the Appalachians took place in stratigraphic horizons not far removed from the horizons of lead and zinc deposition. When the peculiar conditions controlling the deposition of the Clinton ores are understood (see pp. 52-53) it is entirely possible that they may throw some light on the genesis of the lead and zinc ores.
Since the ores were introduced into essentially their present locations, secondary concentration has produced an oxide zone of clay, chert, and iron oxide, with varying amounts of zinc carbonate, zinc silicate, lead sulphide, and rarely lead carbonate. This zone is obviously developed above water level, and is seldom as much as 100 feet thick. Zinc, and to a less extent lead, have been taken into solution as sulphates, with the aid of sulphuric acid resulting from the oxidation of the associated pyrite. Zinc has been carried away from the weathered zone in solution faster than lead, leaving the lead more or less concentrated near the surface. Some of the zinc carried down has been redeposited secondarily as zinc sulphide. Evidences of this secondary sulphide enrichment can be seen in many places; yet certain broad quantitative considerations raise a doubt as to whether this process has been responsible for the main portion of the values of the sulphide zone. If downward secondary enrichment had been a dominant process, it might be expected that the ores would be richer in places where erosion had cut away more than half the limestone formation carrying the ore, than in places where it had barely cut into the formation. This is not the fact,—which suggests that erosion in its downward progress has carried a large part of the zinc completely out of the vicinity.
Zinc ores of this same general character are also found in Paleozoic rocks (Knox dolomite) in Virginia and Tennessee. Their manner of occurrence suggests the same problem of origin as in Missouri and Wisconsin, but no decisive evidence of their source has been discovered.
Of the zinc ores associated with igneous intrusions, those of the Butte and Cœur d'Alene districts are described in connection with copper and lead ores on pp. 201-203, 208, and 212-213.
Zinc constitutes about 75 per cent by weight of the recoverable metals of the Leadville district of Colorado. About two-thirds of the zinc occurs as the sulphide and about one-third as the carbonate resulting from weathering of the sulphide. The zinc sulphide is associated with lead, iron, and copper sulphides and gold and silver minerals. In the oxide zone the zinc carbonate is associated with oxides and carbonates of various metals, including those of lead, copper, iron, and manganese. The iron and manganese oxides are mined in considerable tonnage as a flux. It is an interesting fact that, although mining has been carried on in this district for upwards of forty years, only within the last decade has the existence of zinc ores in the oxide zone been recognized. This has been due largely to the fact that the iron and manganese oxides effectively stain and mask the zinc carbonate.