Upper Mississippi Valley Zinc-Lead District
Location
Southwestern Wisconsin, northeastern Iowa,
and northern Jo Daviess County, Illinois (A-3)
References
(selected) Trowbridge and Shaw 1916, Willman et al. 1946, Willman and Reynolds 1947, Agnew 1955, Heyl et al. 1955, 1959, Bradbury et al. 1956, Reynolds 1958, Carlson 1961, Allingham 1963, Taylor 1964, Klemic and West 1964, Mullens 1964, Heyl and West 1982
Description
Mineral deposits were noted by 16th-century French explorers and were mined long before Illinois, Wisconsin, and Iowa achieved statehood. The district became the largest lead producing region in the United States during the middle 1800s. Lead production declined after the Civil War, but zinc mining boomed as smelting techniques and markets for the metal were developed. Production peaked during World War I, fell abruptly afterward, then rose again during and after World War II. Although large reserves remain, poor market conditions, small ore bodies, and high mining costs have brought another decline; no mine is currently active in the district.
The ore, principally sphalerite and galena, occurs in Upper and Middle Ordovician limestone, dolomite, and shale (fig. 53). Although some early geologists believed the minerals were deposited by meteoric waters, a hydrothermal origin is now generally accepted. Structural factors play a major role in localizing ore deposits.
The regional dip in the zinc-lead district is 15 to 20 feet per mile (1:265 to 1:350, a small fraction of 1°) to the south or southwest. Superimposed on this are several sets of gentle open folds. The largest anticlines and synclines cross the entire district from east to west. Their axes are sinuous and structural relief ranges from about 100 to 260 feet (30-80 m). Northern limbs of anticlines generally are steeper than southern limbs and in places are cut by high-angle reverse or normal faults parallel to fold axes (Heyl et al. 1959, Carlson 1961, Allingham 1963). Smaller folds 1 to 12 miles (1.6-19 km) long and having less than 100 feet (30 m) of relief, lie between or on the backs of major folds. Small folds display three dominant trends: east to west, northeast to southwest, and north-northwest to south-southeast. Joints are prevalent and may have served as conduits for mineralizing fluids. In most parts of the district, the primary joints trend east to west and one or two sets of secondary joints strike north, northeast, or northwest.
Ore deposits are strongly concentrated along synclines. The highest ore is mainly galena and occurs in vertical crevices or gash veins that trend parallel to synclinal axes (fig. 53). At lower levels sphalerite and minor galena are found in "pitch-and- flat" deposits. The pitches are inclined fractures that dip away from a central, downdropped core ground (fig. 53). Geometrically, they are reverse faults that steepen and die out upward; below, most of them merge with bedding of the incompetent shale of the Spechts Ferry Formation (basal Galena Group). Flats parallel to bedding intersect and offset pitches.
In most pitch-and-flat deposits, the limestone of the Quimbys Mill (Platteville Group) and Guttenberg Formations (Galena Group), which respectively underlie and overlie the Spechts Ferry, have undergone much dissolution. These limestones commonly are greatly thinned, brecciated, deformed, and locally reduced to a shaley residuum. Adjacent dolomites have experienced little or no dissolution.
Geologists debate the relative importance of tectonic processes versus solution-collapse in producing pitch-and-flat ore bodies and associated structures. Heyl et al. (1959) proposed that horizontal compression produced not only all the folds in the region but also the pitches, which they viewed as tectonic reverse faults. They theorized that Paleozoic strata simultaneously were shoved northward out of the subsiding Illinois Basin and northwestward from the Forest City Basin against the buttressing Wisconsin Dome. Heyl et al. (1959) accepted that solution-collapse took place, but considered its effects minor and secondary to tectonic stresses. Most other geologists attribute pitch-and-flat structures to solution-collapse localized along preexisting vertical fractures. Some have likened pitch-and-flat structures to fractures that develop in a masonry wall when the foundation subsides (Trowbridge and Shaw 1916, Carlson 1961, Mullens 1964). Supporting this view is the observation that many ore-bearing synclines do not extend below the zone of dissolution. Also, many are better described as troughs than as folds; intervening anticlines are absent. Larger folds of the district are, however, considered to be tectonic structures. Reynolds (1958) and Klemic and West (1964) believed they originated through uplift and tilting of basement blocks; whereas Allingham (1963) suggested that sedimentary strata were crumpled as they slid off the rising Wisconsin Arch.
In Illinois, nine synclines within the zinc-lead district have been named in published reports. Three of these, the Galena, Smallpox Creek, and Vinegar Hill Synclines, are northeast-trending folds of moderate scale; the other six are small and trend northeast or northwest.
