Petrology, Geochemistry, and gemmological characteristics of marble-hosted ruby deposits of Morogoro and Mahenge, Tanzania

Abstract

Petrological, geochemical and gemmological investigations were carried out on 17 rock samples and 80 loose ruby samples from 4 marble-hosted ruby mines in the Morogoro Region, Tanzania. In addition, 46 ruby samples from 5 different deposits including Mogok/ Myanmar, Yunnan/ China, Murgab/ Tajikistan, Luc Yen/ Vietnam, and Mangari/ Kenya were tested for their trace-element compositions and gemmological characteristics. Origin determination for marble-hosted rubies based on their trace-element composition including Li, Mg, Ti, V, Cr, Fe, Ga, Ge, Nb, and Sn was proven feasible by the application of LA-ICP-MS. The detected trace-element compositions indicate that Morogoro rubies are characterized by a Fe-dominated chemical fingerprint compared to other marble-hosted rubies. Consequently, new criteria such as locality distinctive binary and ternary plots were defined and procedures for the application of advanced gemmological testing for origin determination based on inclusion features, UV-Vvis spectrum analysis, and FTIR fingerprinting were refined. Geologically, the ruby deposits of the Uluguru and Mahenge Mts in the Morogoro Region are related to marbles which represent the cover sequence of the Eastern Granulites in Tanzania. In both localities, the cover sequences define a tectonic unit which is present as a nappe structure thrusted onto the gneissic basement in a northwestern direction. Based on geochemistry, the Uluguru Mts deposits are located within calcite-marble whereas deposits in the Mahenge Mts are found in dolomite-marbles. Petrographically, the mineral assemblage found in the Uluguru Mts is characterized by Phlogopite-Amphibole-Carbonate-Corundum-Plagioclase-Spinel-White Mica +/- Scapolite, Tourmaline, Chlorite whereas the combination of Phlogopite-Carbonate-Corundum-Plagioclase-Sapphirine +/- Amphibole, Sphene, Tourmaline, Chlorite describes the mineral assemblage present in samples from the Mahenge Mts. Thermodynamic calculations were carried out based on non-ideal mixing activities of essential minerals. Two ruby formations were distinguished by textural differences which may be the result of an anti-clockwise P-T-t trace where isobaric cooling is followed by isothermal decompression. The first formation appears to have formed during the prograde path (M1) either by a solid-state reaction from diaspore or by the breakdown of margarite into corundum and plagioclase. The peak conditions for M1 metamorphism at ~750°C and 8-9kbar, belonging to granulite facies, occur in relationship to the East African Orogeny at ~620Ma. A CO2 dominated fluid experienced a rapid change in fluid composition which was responsible for the second corundum generation to form and is related to M2 metamorphism >580Ma. Subsequently, the examined units underwent a greenschist facies overprint related to M3 metamorphism at ~580Ma. Finally, a structurally controlled genetic model was proposed where fluids interacted with the marble-host rocks in zones of higher permeability to form a saddle reef type ruby mineralization confined to fold hinges.