Advances in Trace Element “Fingerprinting” of Gem Corundum, Ruby and Sapphire, Mogok Area, Myanmar

F Sutherland,Khin Zaw,Kyaw Thu,Tzen-Fui Yui,Sebastien Meffre

doi:10.3390/min5010061

Abstract

Mogok gem corundum samples from twelve localities were analyzed for trace element signatures (LA-ICP-MS method) and oxygen isotope values (δ18O, by laser fluorination). The study augmented earlier findings on Mogok gem suites that suggested the Mogok tract forms a high vanadium gem corundum area and also identified rare alluvial ruby and sapphire grains characterised by unusually high silicon, calcium and gallium, presence of noticeable boron, tin and niobium and very low iron, titanium and magnesium contents. Oxygen isotope values (δ18O) for the ruby and high Si-Ca-Ga corundum (20‰–25‰) and for sapphire (10‰–20‰) indicate typical crustal values, with values >20‰ being typical of carbonate genesis. The high Si-Ca-Ga ruby has high chromium (up to 3.2 wt % Cr) and gallium (up to 0. 08 wt % Ga) compared to most Mogok ruby (<2 wt % Cr; <0.02 wt % Ga). In trace element ratio plots the Si-Ca-Ga-rich corundum falls into separate fields from the typical Mogok metamorphic fields. The high Ga/Mg ratios (46–521) lie well within the magmatic range (>6), and with other features suggest a potential skarn-like, carbonate-related genesis with a high degree of magmatic fluid input The overall trace element results widen the range of different signatures identified within Mogok gem corundum suites and indicate complex genesis. The expanded geochemical platform, related to a variety of metamorphic, metasomatic and magmatic sources, now provides a wider base for geographic typing of Mogok gem corundum suites. It allows more detailed comparisons with suites from other deposits and will assist identification of Mogok gem corundum sources used in jewelry.

Highlights

The Mogok area, Myanmar, forms a renowned gem tract notable for its ruby and sapphire, which are recovered from both host rocks and transported deposits [1,2]
Recent studies include detailed trace element based surveys of ruby and sapphire deposits [5,6]. These underpin the present follow up study, which concentrates on the trace element and O isotope characteristics of additional samples
The corundum includes low-V (90ppm) types (Figure 2), which pass into extreme V values (>1000 ppm), as was found in the initial study which characterised the Mogok region as a V-enriched corundum province [6]

Summary

Introduction

The Mogok area, Myanmar, forms a renowned gem tract notable for its ruby and sapphire, which are recovered from both host rocks and transported deposits [1,2]. The gem mining areas recently became more accessible after 15 years of government restrictions on visits by outside personnel, which has stimulated and increased research activities on gem deposits [3,4]. Recent studies include detailed trace element based surveys of ruby and sapphire deposits [5,6]. These underpin the present follow up study, which concentrates on the trace element and O isotope characteristics of additional samples.

Methods

Results

Conclusion