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