CARBONATITE VOLCANISM AS A MECHANISM FOR SILCA MOBILIZATION

Recent work in the Minas Gerais region of Brazil has revealed that calcio-carbonatite melts are not stable and must decompose, probably explosively, near the surface at about 40 to 100 bars. The effects on the paleoenvironment from surge deposition of quick lime and CO₂ have the potential for environmental disaster. However, the presence of massive amounts of quick lime at the Earth’s surface has given rise to new ideas about silica mobilization in such an environment.

The study area is near the Serra Negra region, Minas Gerais, which is dominated by many alkaline volcanics, shows strong evidence for a series of volcanic eruptions that released large volumes of CO₂ gas, lime, and surge deposits over much of Brazil. The deposits from the series of eruptive events created the Bauru fm which is the most extensive Cretaceous continental sedimentary sequence in Brazil. The Bauru fm was initially described as a cross-bedded sandstone and conglomerate, however, is a surge deposit.

Evidence suggests that the quartz veins are formed from the top down through the process of silica mobilization - when lime from an eruption blankets the region and elevates the pH to >8, allowing silica to be become more soluble. The veins originated as faults and joints within the bedrock, and were predominately silicates, or more rarely, heavy fraction surface materials such as diamonds, gold, anatase, and phosphate minerals (ex: gorceixite, florencite etc.).

Quartz crystals present in veins are of high quality and exhibit no growth striations on prism faces. This lack of striations has been exhibited only in lab grown crystals with a high alkaline environment. The lack of striations could be due to the high alkaline environment that was present after the multiple eruptions in the Serra Negra region.

The potential for mineralization of soluble silica in a high alkaline solution could change what we know about quartz deposition, fossilization, and the origin of some chert occurrences.