PETROLOGY OF CARBONATE-RICH SILICATE ROCKS OF THE BEAR LODGE ALKALINE COMPLEX AND ORIGIN OF CARBONATE MINERALIZATION

The Eocene Bear Lodge Alkaline Complex in the northern Black Hills of Wyoming is a bilobate composite laccolith with hypabyssal intrusions of phonolite, tephriphonolite/phonotephrite, latite, syenite, trachyte, pseudoleucite porphyry, lamprophyre, and carbonatite. A high volume of rock throughout the complex has experienced alteration, mainly potassium-ferric iron metasomatism (fenitization), related to a central carbonatite dike swarm which hosts a large-tonnage rare-earth element resource. The purpose of this study was to characterize the least-altered alkaline igneous rocks of the complex, based on geochemistry and mineralogy, and to investigate the origin of carbonate in these rocks.

Thin sections were examined by polarized light microscopy and cathodoluminescence to determine whether carbonate mineralization is primary or secondary. Whole-rock major and trace element analyses were performed on samples which largely escaped the pervasive fenitization related to carbonatite emplacement. Analyses revealed previously unrecognized silica-poor (30-38 wt.% SiO₂) aphyric rocks compositionally similar to carbonated malignite or damkjernite. The most silica-poor samples generally contain a greater volume percent of carbonate in the groundmass, some showing ocellar textures, possibly related to carbonate-silicate liquid immiscibility. Model estimates and normative calculations support magma differentiation from a silica-poor, carbonated alkalic parent magma to a phonolitic composition with 57-62 wt.% SiO₂. Phonolite textures vary from megacrystic sanidine porphyry to tinguaite. In some phonolites, sodium in nepheline and groundmass aegirine contributes to the high total alkali contents, as opposed to potassium-addition by fenitization.

Calcite and possibly other carbonates that are interstitial to groundmass silicates are likely primary magmatic minerals, whereas lesser amounts of secondary carbonates occur in veinlets and as subsolidus replacement of feldspathoids and pyroxene. The presence of primary carbonate minerals in SiO₂-poor rocks, indicates high CO₂ concentrations in the alkalic magma and permits that carbonatite formed by immiscible separation.