Abstract

The Silver Reef mining district in southwestern Utah lies across Interstate 15 at Leeds. The district centers on outcrops of the Springdale (Silver Reef) Sandstone of the Jurassic Moenave Formation. This crops out on the northeast trending Virgin anticline and two subsidiary folds to the west. Flank thrust faults repeat the ore zone three times and form the White, Buckeye and Butte reefs of the main district. Within the mining district three members of the Triassic Moenkopi Formation, the Middle Red, Shnabkaib and Upper Red, the overlying Chinle Formation and its Shinarump and Petrified Forest members, the Early Jurassic Moenave Formation with the Dinosaur Canyon and Springdale Sandstone, capped by the Kayenta Formation and the Jurassic Navajo Sandstone comprise the major stratigraphic units. Recent basalt flows, Boulder Alluvium and younger alluvium complete the exposed section.

Ore zones occur in lense-like bodies related to deposits in paleostream channels and carbonaceous plant remains, and in some shear and fault zones of northerly trend, all within the Springdale Sandstone. Ore minerals were oxidized and concentrated as cerargyrite, malachite-azurite, and carnotite-autunite-torbernite above the water table.

The ore cell of the district contains sulfur isotopes suggestive of biogenic origin. Mercury and selenium and also Mo, Zn, Pb, Ni, Co, As and V are pathfinder elements, while gold is in trace concentrations only. Anomalous concentrations of silver occur in the Springdale sandstone well beyond the boundaries of the mining district.

Iron oxide concretions, and some of chalcocite and calcite in Petrified Forest bentonitic shales contain anomalous concentrations of copper, silver and gold. Bentonites such as these far to the southeast of the district are probable sources of the metals in the stream-deposited Springdale sandstone. Folding and faulting of the beds established hydraulic gradients and movement of groundwaters essential to the localization and concentration of the metals in the permeable and syngenetically mineralized Springdale sandstone.