The History of Ruby
Corundum is not uncommon on Earth but the gem varieties of ruby and sapphire
are
relatively rare.
|
| Ruby |
Gem corundum deposits are classified as primary and secondary deposits.
Primary
deposits contain corundum either in the rocks where it crystallized or as
xenocrysts and xenoliths
carried by magmas to the Earth’s surface. Classification systems for corundum
deposits are based
on different mineralogical and geological features.
|
| Sapphire |
An up-to-date classification scheme for ruby
deposits is described in the present paper.
Ruby forms in mafic or felsic geological environments,
or in metamorphosed carbonate platforms but it is always associated with rocks
depleted in silica
and enriched in alumina. Me
Two major geological environments are favorable for the presence of
ruby: me
(1) amphibolite to medium pressure granulite facies metamorphic belts and (2)
alkaline basaltic
volcanism in continental rifting environments.
Primary ruby deposits formed from the Archean
(2.71 Ga) in Greenland to the Pliocene (5 Ma) in Nepal.
Secondary ruby deposits have formed at
various times from the erosion of metamorphic belts (since the Precambrian)
and alkali basalts (from
the Cenozoic to the Quaternary).
Primary ruby deposits are subdivided into two types based on their
geological environment of formation:
(Type I) magmatic-related and
(Type II) metamorphic-related.
Type I is characterized by two sub-types, specifically Type IA where
xenocrysts or xenoliths of gem
ruby of metamorphic (sometimes magmatic) origin are hosted by alkali basalts
(Madagascar and
others), and
Type IB corresponding to xenocrysts of ruby in kimberlite (Democratic Republic
of
Congo). Type II also has two sub-types; metamorphic deposits sensu stricto
(Type IIA) that formed
in amphibolite to granulite facies environments, and metamorphic-metasomatic
deposits
(Type
IIB) formed via high fluid–rock interaction and metasomatism. Secondary ruby
deposits, i.e., placers
are termed sedimentary-related
(Type III). These placers are hosted in sedimentary rocks (soil, rudite,
arenite, and silt) that formed via erosion, gravity effect, mechanical
transport, and sedimentation
along slopes or basins related to neotectonic motions and deformation.
Keywords: ruby deposits; classification; typology; magmatism; metamorphism;
sedimentary;
metasomatism; fluids; stable and radiogenic isotopes; genetic models;
exploration