Minerals Review Ruby Deposits: a review and Geological Classification Gaston Giuliani



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ruby deposits[01-22]
Figure 17. FeO-Cr
2
O
3
-MgO-V
2
O
3
versus FeO + TiO
2
+ Ga
2
O
3
diagram based on a database of 2000 
electron microprobe analysis (EMPA) analyses (in wt % oxides) showing the main chemical fields 
defined for different types of corundum deposits worldwide [72]. Different geochemical domains are 
defined (see text). 
The chemical diagram provides sufficient discrimination to distinguish between the different 
types of corundum deposits for the following reasons: (1) the database contains more than 2000 
EMPA analysis representative of all of the types of primary deposits and (2) the magmatic sapphire 
domain (S1) is defined by the geology and chemistry of corundum. The sapphires are hosted in 
syenitic rocks or occur as megacrysts in syenitic xenoliths carried by alkali basalts. Most of the 
sapphires in placers linked to the erosion of alkali basalts (domain S2) overlap the domain S1. The 
main superposition of geological domains is for ruby of metasomatic (-metamorphic) origin that 
overlaps the domain of ruby associated with M-UMR (strictly metamorphic); (3) the diagram uses Ga 
and Mg despite their high detection limits, and the two elements are opposed: Ga is added to Ti while 
Mg is added to Cr and V. This choice corresponds with the fact that Ga and Ti are concentrated in 
magmatic sapphires (syenitic rocks) while Mg, Cr, and V are concentrated in metamorphic rubies 
hosted in Mg-Cr-(V)-rich protoliths. 
A statistical classification by discriminant factors analysis using oxide concentrations (Ti, Mg, V, 
Cr, Fe, and Ga) of more than 900 corundum crystals from the main primary deposits worldwide was 
proposed by [72]. The deposits were classified into different types: (1) for ruby as marble, M-UMR, 
and metasomatites and (2) for sapphires as syenitic rocks, metasomatites, plumasites, and skarns. 
The discriminating factors F1 and F2 calculated for the different types of ruby deposits (known 
classes) with their respective centroid for each class are as follows: 
Factor 1 = 0.271691 × (Cr
2
O
3
− 0.294456)/0.376245 + 0.796762 × (FeO − 0.26294)/0.224015 
+ 0.0206904 × (Ga
2
O
3
− 0.0112047)/0.0124095 + 0.136135 × (MgO − 
0.00545581)/0.00526035 + −0.124367 × (TiO
2
− 0.0274651)/0.0539087 + −0.108332 × (V
2
O
3
− 0.0102233)/0.0111099 
Factor 2 = 0.124957 × (Cr
2
O
3
− 0.294456)/0.376245 + −0.121871 × (FeO − 0.26294)/0.224015 
+ −0.287007 × (Ga
2
O
3
− 0.0112047)/0.0124095 + −0.0527783 × (MgO − 
0.00545581)/0.00526035 + 0.0584802 × (TiO
2
− 0.0274651)/0.0539087 + −0.439517 × (V
2
O
3
− 0.0102233)/0.0111099 
The discriminating factors F1 and F2 calculated for the different types of sapphire deposits 
(known classes) with their respective centroid for each class are as follows: 

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