Mineralogy, geochemistry, and geochronology of the Yogo Dike sapphire deposit, Montana
Creators: name: Gauthier, Guylaine
Publisher: name: University of British Columbia
Year: 2009

description: The Yogo dike is located in the central alkalic province of Montana, on the eastern flank of the Little Belt Mountains, about 95 km southeast of Great Falls. It has been sporadically mined since the end of the last century for its gem quality blue sapphires. The intrusion is classified as an ouachitite of the ultramafic lamprophyre group. It is composed of phlogopite and clinopyroxene megacrysts set into a matrix of phlogopite, clinopyroxene, calcite, analcime, magnetite, and apatite. Accessory minerals include sapphire (gem quality corundum) and pleonaste (Mg-Al-Fe spinel). The rock also contains ocelli composed of calcite, dolomite, ankerite, barite, analcime with or without pyrite, and amphibole. Geochemical analysis of the Yogo dike revealed that it can not be precisely classified. It shows characteristics similar to ouachitite, damkjernite and aillikite, which all belong to the ultramafic group of lamprophyres. Mineralogically, the rock is more akin to ouachitite. It is ultramafic in character and is similar in many ways to a carbonatite. It is poor in silica and rich in Ca, Mg, Cr, Ba, and Sr. Some of the samples studied are alkaline while some are not. The geochemistry of the Yogo dike was compared to the Yogo Peak intrusion located some 16 km northwest of the dike. Some authors have proposed that the two are genetically related but Pearce element diagrams show that they are not. Phlogopite and pyroxene crystals are typically zoned. Phlogopite megacrysts often show pyroxene and apatite inclusions. The phlogopite crystals have Mg-rich cores and Fe, Al and Ba-rich rims. The pyroxenes can show complex zoning; cores are typically diopsidic or augitic, and rims are progressively richer in Al (up to 11 wt.% AI₂O₃), Fe, and Ti, and poorer in Mg and Si. Some of the pyroxenes are Crrich, with up to 1.65 wt.% Cr₂O₃ , and all are poor in Na. Magnetite is sometimes zoned with Al-rich rims. Sapphires are Fe-poor. Their blue colouration is due to intervalence charge transfer phenomena between Fe²⁺ and Ti⁴⁺. The purple colour of some Yogo sapphires is due to a small amount of Cr-content. A phlogopite separate from the Yogo dike yielded a K-Ar date of 48.2 +1.3 Ma. The depth at which the Yogo magma originated is hard to bracket due to lack of assemblages to which geothermobaromety can be applied. A garnet-clinopyroxene-plagioclase granulite xenolith was found. The garnets are pyrope-rich with subordinate almandine, grossular and andradite components. The pyroxenes are Al-rich and Na-poor. Geothermobarometry applied to this xenolith yielded a pressure estimate of between 12 to 20 kb (~40 to 70 km depth) and temperatures around 850°C. Based on this study as well as previous studies, the sapphires are thought to be xenocrysts in the dike. Arguments in favor of this statement are: (1) the Yogo magma is not corundum normative, thus preventing the crystallization of Al-rich phases as phenocrysts; (2) crystals are highly resorbed and always rimmed by pleonaste spinel, which indicates a disequilibrium reaction between the magma and the sapphires; (3) two sapphire-bearing xenoliths have been found during previous studies by Dahy (1988, 1991), a sapphirepyroxene- plagioclase xenolith and a pyroxene-phiogopite-sapphire autolith; (4) a few inclusions found in the sapphires indicate a different paragenesis than the dike rock; (5) the Mg-rich pleonaste spinels found as accessory minerals in the rock seem to indicate that Al-rich material was mixed with the Yogo magma. This type of spinel is often found with sapphires and is usually the result of moderate to high temperature metamorphism of Al-rich phases. The Al-rich material was probably sapphire-bearing. Point (3) should be enough to prove the xenocrystic nature of the sapphires, but the sapphires in the xenolith were identified only by eye. All minerals analyzed show a drastic increase in Al content in their outside rims. This suggests contamination of the magma during its ascent by Al-rich, corundum-bearing material. The high temperature of the ultrabasic magma partially resorbed the corundum and reacted with it, forming pleonaste. Al-rich phases (such as phlogopite and Al-rich pyroxene) also reacted with the new magma to produce pleonaste spinel. The grade of sapphires in the dike ranges between 5 carats (1 ppm) to 100 carats (20 ppm) per ton; few areas are barren. Sapphires range in size from about 0.33 carats to 8 carats.