UNUSUAL CALCRETE FORMATION ON PRECAMBRIAN MARBLE FROM THE MERELANI TANZANITE DISTRICT, SIMANJIRO, TANZANIA

The Merelani District in northern Tanzania is known for its deposits of unique gemstones, including Tanzanite and green Grossularite. Regional bedrock exposures and mines that occur along the north-plunging, faulted Lalatema antiform, are defined by high-grade pelitic and graphite-bearing metasedimentary units with interbedded calcitic to dolomitic marbles. Original sedimentary ages are Usagaran (ca. 1900Ma), while peak metamorphism is roughly correlated with intrusion at 840Ma and 600Ma (Pan African).

The particular Merelani calcrete, just north of the mining area, formed directly upon lower-lying surfaces of marble. There is no evidence to suggest that the deposition originated within a caliche-like soil profile. Calcrete is observed encrusting variable indentations of eroded marble with cemented surface debris. Abundant garnet clasts to 7mm exist along with quartz, micas, calcite, amphiboles, opaque oxides, rock fragments and pisolites in a solidified slurry of micritic infilling. Irregular micritic to microsparitic layers coat the pisolites. Diverse textural relationships among clasts, pisolites and finer carbonate matrix material indicate a complicated history of precipitation, dissolution and reprecipitation. Small pockets or caves in the marble display mammilary surfaces formed by black hydroxide coatings over micrite. Laminations of micritic material within and around pisolites bear resemblance to microbial features. However, no rhizolithic (plant root) structures were observed.

Detailed petrographic and optical-scan mapping of the calcrete provides a systematic description of the fabrics. Cathodoluminescence (CL) analysis of Merelani samples shows interesting variation in carbonate chemistry among different components. Variation is probably related to changes in vadose water chemistry and seasonal to longer-term environmental fluctuations during the time that the Lalatema fold has been exposed. No specific age data is available to indicate the age of calcrete formation. Even though Malisa (1987) describes both "nodular" and "honeycomb" types of calcrete in the broader Merelani District, nothing with characteristics of the latter type were observed in this study.