Asian rubies come always with marble and salt

02/03/04

Ruby deposits are the primary gem source in Central and South-East Asia. They are highly prized and have a special character: the rubies always occur as inclusions in marble. Geologists from the IRD and CRPG/CNRS (1) have investigated the tectonic and geochemical mechanisms involved in their formation and established a new model of how they generated. It involves feeder fluids resulting from solution of layers of salts present in the marble formations. These hot fluids were brought into circulation at the time of the Himalayan chain orogeny and provided the conditions for mobilization of the constituent elements of ruby and its crystallization within the marble. This genetic model gives the partner countries valuable information on the structure and geochemistry of ruby formations and tools necessary for improved targeting of prospection for this rare and highly sporadic type of mineral concentration.

Ruby is mineralogically the chromiferous variety of corundum gemstone, in other words an aluminium oxide in which some of the aluminium ions have been substituted by chromium. Chromium contributes, along with vanadium, another metal constituent of ruby, to the crystal's red colour.

The most prized ruby deposits are those of Central and South-East Asia, like in the celebrated Mogok deposit in Myanmar (ex-Burma), from which the highest gem-quality rubies are extracted, reputed for their intense "pigeon blood" colour and their transparency.

In spite of their commercial interest, these deposits have attracted little geological research. However, they possess a special feature that have stimulated the interest of the IRD and CRPG/CNRS scientists for several years. The ruby crystals they hold are always present as inclusions in marble formations, which are calcareous rocks altered by high temperature. Research projects have been launched to determine in particular the origin of certain major constituents of the ruby which are usually absent from marbles--mainly aluminium, chromium and vanadium--, unravel the mechanisms of formation of these deposits, their age and their significance in the functioning of deep zones of the Earth's crust. Combination of field data and results of laboratory geochemical analyses on samples taken from different deposits recorded from Afghanistan to Viet Nam, the team succeeded in establishing a new genetic model, valid for the whole of these deposits where ruby is associated with marble (2).

Ruby crystals form at high temperature, between 620 and 670°C. Dating of minerals contemporary with their growth, such as zircon and mica, performed in the different deposits gave the age of the ruby crystallization. Depending on the deposits, this was found to be 40 to 5 million years B.P., in the Cenozoic. The ruby is thus an excellent geological marker of the collision between the Indian and the Eurasian plates which set off the Himalayan uplift.

Analysis of liquid inclusions trapped by the rubies during crystallization revealed the participation of feeder fluids riches in salts and carbon dioxide, source of which has been defined by using their chemical composition. They result from the high-temperature solution of salts contained in evaporite-bearing beds (3) which are found in the impure marbles of Central and South-East Asia, rich in clays and organic matter. These fluids were subsequently set in motion under the influence of tectonic pressures linked to the collision of continental plates.

Their interaction with the marbles caused chemical reactions which freed aluminium and chromophor elements of the ruby, like le chromium or vanadium. These elements, initially held in only trace quantities in these rocks, were shown to be mobile in sufficient quantity to produce ruby in this geological setting. Dissolution of salts from the evaporite beds led within the marble to the creation of cavities in which very pure rubies, with well developed faces, were able to develop. Contrary to what theoretical models elaborated up to now would suggest, the proposed model shows the involvement of salts and mineralizing fluids of metamorphic origin, the mineralization of the rubies taking place in the heart of the marble formations.

This model, which is new for natural ruby, proves to be close to the molten salts method used in industry for aluminium production.

The presence of evaporites is therefore a key element for explaining these ruby mineralizations. It is a sign for the carrying rock of a primary environment of a particular nature, the "lagoon" type where sedimentation took place in close relation with the ocean.

Their existence inside the marble formations is still, moreover, rare, which fits the very small number of deposits recorded and their small geographical spread. For the partner countries that are mining these deposits, this model brings information about the history, the structure and the geochemistry of these ruby formations, knowledge which is essential for conducting new prospecting campaigns in these regions of Asia.

Marie Guillaume – DIC

(1) This research brought together the IRD research unit UR 154 " Déformation de la lithosphère continentale en zones de convergence et transferts de matière ", the Centre de recherches Pétrographiques et Géochimiques / CNRS of Nancy, the mixed research unit UMRG2R of the Henri-Poincaré University of Nancy and unit UMR 5025 LGCA (Laboratoire de géodynamique des Chaînes Alpines) of Grenoble. The research project on the precious stones, developed in the research unit UR 154 of the IRD, is oriented towards the study of ruby deposits in South-East Asia. Since 1998, partnerships have been established in Viet Nam with Institute of Geological Sciences of Hanoi (with which a research programme which has been developed since 2001 was to finish at the end of 2003) and with the Pakistan geological survey and the support of the Cultural Service of the French Embassy in Islamabad.

(2) This work was the subject of a thesis by Virginie Garnier. (See references)

(3) These evaporites correspond to the anhydrite (calcium sulfate) and salts like halite and sylvite (sodium and potassium chlorides).

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