Summary:
The silica sand at the Preston Property is hosted within the Cambrian Wonewoc Formation. At Preston, the Wonewoc Formation sandstone varies in thickness from 50 to 150 feet (15 to 46 m) and is principally medium to coarse grained quartzose sandstone with high-angle cross-stratification. The Wonewoc Formation is overlain by very fine to fine grained sandstone, siltstone and mudstone of the Tunnel City Group, and thin and discontinuous Pleistocene surficial deposits (slightly gravelly sandy loam till).
The Wonewoc Formation and other Cambrian-aged silica sand units in the midcontinental United States formed during a major transgressive event where shallow ocean currents formed a texturally graded shelf. The advanced level of textural maturity in Cambrian quartz grains is thought to be related to a long history of weathering that included wind and marine shoreface abrasion and chemical weathering that preferentially dissolved plagioclase and similarly unstable minerals.
• The position of the Wonewoc Formation is consistent with this unit being situated within topographic ridges of a rugged landscape that is associated with the Driftless Area of southwestern Wisconsin.
• The Wonewoc Formation within the open pit excavation is dominated by clean, white to rusty sand with minor, thin, continuous clay-mudstone bands that are likely associated with inter-tidal and/or transitions in marine. The basal portions of the Wonewoc comprise slightly finder sand than the middle or upper portions of the silica sand unit.
• Overall observation of the mine pit face(s) is that the Wonewoc is locally stratigraphic uniform (the Wonewoc stratigraphy was observed at three different mine face sites located throughout the mine site area.
• The overburden and the current mine faces is minimal (0 to 2 m in thickness). It consists of dark grey to reddish dark grey, clay-rich sandy till with abundant pebbles and minor cobbles; the basal portion of the Tunnel City Group was not observed at the mine site but does occur on the elevated portions of the eastern part of the Preston Property where it consists of mudstone and fine-grained sandstone and siltstone (distinguished by its a higher component of mudstone in comparison to the underlying Wonewoc Formation).
• The Wonewoc Formation silica sand was also observed in archived auger clippings from auger drillholes: PSWB1-12, PSWB2-12, PSWB3-12, and CSL1- 12; here the auger return material was composed of white to slightly iron-stained medium to coarse grained silica sand.
• The basal portion and lowermost contact of the Wonewoc Formation was not observed (in outcrop or in auger return material).
Mineralization
The Paleozoic age bedrock layers of quartzose sandstone in the central mid-continent of North America are known as some of the most mineralogically pure sandstone on Earth with greater than 95% of the sand grains consisting of silicon dioxide (SiO2).
Whole rock chemical analysis (x-ray fluorescence) of the Wonewoc Formation sandstone, which was conducted by the Department of Natural Resources (Brown, 2012), shows that the Wonewoc silica sand consists of: Silicon dioxide (SiO2; 99.20-99.70%); Aluminum oxide (Al2O3; 0.10-0.19%); Calcium oxide (CaO; 0.08-0.21%); Iron oxide (Fe2O3; 0.06-0.03%); Potassiium oxide (K2O; 0.05-0.14%); Sodium oxide (Na2O; 0.002- 0.003%); Magnesium oxide (MgO; 0.01-0.02%); and Titanium oxide (TiO2; <0.01%).
In addition to being composed mostly of quartz, a mineral known for being of high strength and relatively inert, the grains are especially well-rounded, well-sorted, coarsegrained, and poorly cemented. The advanced level of textural maturity in Cambrian quartz grains, including the Wonewoc Formation, remains more uncertain, but is believed to be related to chemical weathering that may have preferentially dissolved plagioclase and similarly unstable minerals, and a long history of abrasion in marine conditions and wind abrasion (Morey, 1972; Odom, 1975, 1978; Dott et al., 1986; Runkel, 1998; Dott, 2003; see Section 8, Deposit Types).