The Mong Hsu ruby deposit in Myanmar is a marble-type deposit. It is well-known for attractive crystals with dark violet/blue to black cores and ruby rims, as well as for trapiche rubies. These exceptional rubies have a texture composed of six growth sectors separated by six dendrites running from a central core to the six corners.

Fluid inclusions (FI) have been studied in both types of ruby, i.e., trapiche and non-trapiche crystals. Microthermometry combined with Raman spectrometry investigations of primary and secondary FI proved the existence of CO2-H2S-COS-S8-AlO(OH)-bearing fluids with diaspore and native sulfur daughter minerals. The carbonic fluid belongs to the CO2–H2S system with CO2 as a dominant component. Minor COS (1–2 mol.%) and diaspore indicate that H2O (∼1 mol.%) was present in the paleofluid. The amount of H2S in FI from trapiche ruby is between 6.9 and 7.4 mol.% for the dendrites and from 7.2 to 15.7 mol.% for the growth sectors. The H2S content is approximately 10 mol.% in FI from non-trapiche rubies.

The presence of dawsonite and previous crushing data carried out on well-cleaned crystals of non-trapiche ruby indicate that an ionic salt melt was present at the time of ruby formation. The non-detection of this melt in trapiche ruby is attributed to the small size of the remnants of molten salts, making it difficult to observe them under the microscope. Following this hypothesis, the FI assemblages in rubies would correspond to the trapping of two immiscible fluids, i.e., a carbonic phase in the CO2–H2S–COS–S8–AlO(OH)-system and molten salts. Such fluid phases are considered to be the product of metamorphism of evaporites during devolatilization of carbonates and thermal-sulfate reduction.

The formation of trapiche ruby is discussed in terms of possible variations of the composition of the fluid, temperature, and pressure conditions. The variation in density of the carbonic fluids from the dendrites (0.70 < ds < 0.78) to the growth sectors (0.56 < ds < 0.68) records variation of fluid pressure in the metamorphic system. This variation led to episodically local fluid overpressure and hydraulic fracturing in the marble. During such episodes, changes in driving-force conditions allowed for the formation of the trapiche texture in ruby: the development of dendrites and growth sectors occurred under high and low driving-force conditions, respectively. Non-trapiche ruby in veinlets formed concurrently under thermodynamic conditions similar to those registered for the growth sectors in trapiche ruby.

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