ORIGIN OF OPAL CEMENT IN PENNSYLVANIAN MINNELUSA SANDSTONE

In the Northern Black Hills of South Dakota, the Pennsylvanian Minnelusa Sandstone has an opal cement in two outcrops along Boulder Creek in the Boulder Park area. The occurrence of this rare type of cement is inconsistent with the traditional explanations for its origin, including silicification through deep-leaching in tropical soils, and dissolution of volcanic ash or opaline fossils. Based on the results of field mapping, thin-section petrography and x-ray diffraction analyses, an entirely new mechanism of opal cementation is proposed.

Opal cementation appears to be associated with Tertiary intrusions in the Boulder Park area. It is hypothesized that magma-derived carbon dioxide reacted with heated groundwater to form carbonic acid. This hot acidic water circulated in convection cells and in the process dissolved silica from rocks underlying the Minnelusa (Precambrian basement and Cambro-Ordovician siliciclastics). Where this hydrothermal fluid passed through the Minnelusa, along ascending fracture zones, it dissolved out dolomite from the sandy dolostone interbeds within the sandstone sequence and metasomatically precipitated opal in its place. What was originally a sandy dolostone had become an opal-cemented sandstone. The dolomite was also partially replaced by iron oxides with different oxidation states producing vivid color zonation in the opalized rocks.