Rubies of Asia

Ruby is the red-pink transparent gem form of the mineral corundum (2[Al2O3]), with the intensity of its red colour related to trace amounts of chromium oxide. The coloured gem varieties of corundum include; white sapphire (colourless), ruby (red), pigeon’s blood (deep red), sapphire (blue), golden sapphire (yellow), green sapphire (green), as well as olive green, purple and violet (violet sapphire). Corundum is found as highly valued rubies in Myanmar and Thailand, and as sapphires in Sri Lanka and Cashmere-India, as well as in lesser quality gem deposits in Australia, Tanzania, Cambodia, Russia, Thailand, Malagasy, Nepal, Malawi, and the USA.

Marble-hosted ruby deposits today represent the most important source of coloured gemstones in Central and South East Asia. These deposits are located in the Himalayan belt and formed during the Tertiary collision of the Indian plate in its northward movement into the Eurasian plate (A). 

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Ruby from Luc Yen Mine, Yenbai Province, Vietnam (27 mm  across)

Most SE Asian rubies are spatially related to granitoid intrusions and are hosted in evaporite-entraining platform carbonates that have experienced high-grade metamorphism. All occurrences are located close to major tectonic features active during Himalayan orogenesis, either directly in suture zones in the Himalayas, or in shear zones that guided extrusion of the Indochina block after the collision in South East Asia. Ar-Ar dating of micas syngenetic with the rubies and U-Pb dating of zircon inclusions in the rubies show that these deposits formed during Himalayan orogenesis and associated extensional tectonics active between the Oligocene and the Pliocene, in a belt from Afghanistan to Vietnam. These ruby-bearing marbles lie in the amphibolite facies (T = 610 to 790° C and P ≈ 6 kbar).

Fluid inclusions in the various gem rubies from the Jegdalek, Hunza and northern Vietnamese deposits indicate retrograde metamorphism; 620<T<670° C and 2.6<P<3.3 kbar (B). Whole-rock analyses of non-ruby-bearing marbles show they contain enough aluminium and chromiferous elements to produce all the ruby crystals that they host. As protolith, these carbonates contained Al- and chromiferous-bearing detrital materials, probably clays, which were deposited on the platform as the same time as the mostly carbonate matrix along with locally enriched levels of organic matter. 

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Meta-evaporitic rubies in SE Asia (after Garnier et al., 2008). A) Location and age of various gem ruby fields in SE Asia. B) P-T evolution showing corundum formed during the prograde metamorphic P–T path (dashed thick grey line) and then during the retrograde stage (solid grey line) from the destabilisation of different mineral assemblages as drawn in this P–T sketch: 1 — from margarite, then 2 — from micas, and 3 — from spinel. At this stage, corundum is destabilised at lower temperature and pressure in margarite and then diaspore. Gem ruby formed at a pressure between 2.6 and 3.3 kbar and temperature between 620 and 670°C (pink rectangles). Abbreviations: An = anorthite; And = andalusite; Cc = calcite; Co = corundum; Do = dolomite; Dsp = diaspore; Kfs = K-feldspar; Ky = kyanite; Mrg = margarite; Ms = muscovite; Si = sillimanite; Sp = spinel; W = water.

In addition, the consistency of (C, O)-isotopic values of carbonate in the marbles shows marbles developed in a closed fluid system and were not infiltrated by externally-derived fluids. The carbon isotopic composition of graphite in the marbles underlines its organic origin and that it exchanged C-isotopes with the adjacent carbonates during its metamorphic evolution. Earlier, less evolved examples of this isotopic exchange, with temperatures still in the upper diagenetic realms are seen in Permian Carbonates of the Saraburi region, Thailand (Warren et al., 2014). Moreover, the O-isotopic composition of ruby across SE Asia was buffered by metamorphic CO2, released during devolatilisation of marble, while the H-isotopic composition of associated mica is consistent with a metamorphic origin for water now in equilibrium with the micas. The boron isotopes from tourmalines associated with the rubies indicate a likely nonmarine evaporite source for the boron (C). Thermal breccias or in carbonate-pyrite veins developed within both dolomitic limestones and calcareous black shales. Individual deposits are hectometer-sized at most and display numerous folds, thrusts, and tear faults. All the tectonic contacts are marked by centimetre-to metre-thick hydrothermal breccias that are cataclasites with clasts of black shales and albitites (i.e., massively albitized shales) within a carbonate-albite-pyrite cement. These breccias derived from a fluid-rich pulp. Part of the overpressured fluids escaped and triggered intense hydraulic fracturing in surrounding rocks, especially along tear faults. In each deposit, there is evidence of complex deformation that resulted in polyphase duplex structures. For example, in the Coscuez deposit, the ore-hosting N30°E verging folds and thrusts were a response to movement on the sinistral N20°E trending Coscuez tear fault (C).

Ruby-bearing marbles from Nangimali, in the Azad-Kashmir deposits of Pakistan, contain, besides phengite, atypical mica intergrowths with paragonite, phlogopite and aspidolite (sodium phlogopite). Both phlogopites, although intimately linked and coexisting with paragonite, are fluorine rich, in contrast to the phengite and paragonite. The phengite is either associated with phlogopite or can be isolated. The presence of aspidolite in these ruby-bearing marbles, together with NaCl solid inclusions and the presence of anhydrite, further strengthen the notion that evaporites were involved in the genesis of these gem corundums.

In summary, Southeast Asian rubies formed during the retrograde portion of the metamorphic path, mainly by destabilisation of muscovite or spinel. The metamorphic fluid system was rich in CO2 released from devolatilisation of carbonates, and in fluorine, chlorine and boron, probably released from molten and altering Cretaceous or Precambrian and even Permian salt masses (NaCl, KCl, CaSORuby-bearing marbles from Nangimali, in the Azad-Kashmir deposits of Pakistan, contain, besides phengite, atypical mica intergrowths with paragonite, phlogopite and aspidolite (sodium phlogopite). Both phlogopites, although intimately linked and coexisting with paragonite, are fluorine rich, in contrast to the phengite and paragonite. The phengite is either associated with phlogopite or can be isolated. The presence of aspidolite in these ruby-bearing marbles, together with NaCl solid inclusions and the presence of anhydrite, further strengthen the notion that evaporites were involved in the genesis of these gem corundums.

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Boron isotopic compositions of tourmaline associated with ruby in marbles compared to other typical boron sources and associations (after Garnier et al., 2008).

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Burmese ruby

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