Abstract

The Lake Alice District, in Lincoln County, Wyoming, is one of a number of metal occurrences in the Overthrust Belt of southwestern Wyoming. The metal occurrences are contained within the upper portions of the Triassic/Jurassic Nugget Sandstone and the lower portions of the overlying Jurassic Gypsum Spring Member of the Twin Creek Limestone.

The Nugget Sandstone is the uppermost unit of a redbed clastic sequence. It is a texturally mature subarkose which was deposited in a prograding sandy shoreline environment. Sedimentary structures include horizontal parallel to nearly parallel laminated bedding, hummocky bedding, trough cross bedding and massive structureless sandstone. Minor bioturbation can be seen in the upper Nugget. The depositional environments include the swash zone, upper and lower shoreface, and backshore.

The Gypsum Spring Member of the Twin Creek Limestone was deposited on a tidal flat. The lower Gypsum Spring consists of fine to very fine grained sandstones and siltstones and represents sand flat deposition. Sedimentary structures include flaser bedding, wavy bedding, ripples, mudcracks, and bioturbation. The middle units of the Gypsum Spring were deposited in intertidal areas and consist of petroliferous dolomitic limestones, dolomites, limestone and dolomite breccias, algal stromatolites, red and maroon shales, siltstones and siltstone breccias. The upper Gypsum Spring consists of interbedded red and maroon shales, siltstones and siltstone breccias and chert-bearing dolomitic limestone breccias. This sequence was deposited in the mudflat and supratidal environments.

It has generally been considered that the normally redbed host units were bleached to a white color by reducing solutions present at the time of sulfide deposition. While this is probably true to a degree, much of the white units are believed to have never developed a red hematite staining prior to sulfide deposition, and are therefore, not considered primary redbeds.

Metallic sulfides in the Lake Alice District include chalcopyrite, chalcocite, bornite, pyrite, and galena. The sulfides occur as clots up to 15 millimeters in diameter and disseminated grains averaging one to two millimeters. Veinlets of chalcopyrite, bornite, and pyrite are also present in fault zones and along fractures.

A general mineral zonation pattern is observed. From bottom to top the pattern is copper and silver — — > lead — — > zinc — — > iron. This pattern is consistent with that observed in most of this type of deposit worldwide. The zonation pattern is observed at two distinct levels, probably a result of two phases of mineralization with the later phase possibly occuring at the time of thrusting and Laramide deformation.

The genesis of this type of deposit is controversial. The most widely held theory is that they were formed during diagenesis. The most likely source of metals is from oxide coatings in the underlying redbed sequences, oilfield brines, and possibly the underlying Permian Phosphoria Formation. Transporting solutions were acidic and oxidizing. Precipitation of the sulfides occurred when the mobile metal bearing solutions came into contact with the overlying reducing host units or reducing solutions within the host units.