@article{fdi:010078078,
  title = {{G}eology of corundum and emerald gem deposits : a review},
  author = {{G}iuliani, {G}aston and {G}roat, {L}. {A}.},
  editor = {},
  language = {{ENG}},
  abstract = {{T}he great challenge of geographic origin determination is to connect the properties and features of individual gems to the geology of their deposits. {S}imilar geologic environments can produce gems with similar gemological properties, making it difficult to find unique identifiers. {O}ver the last two decades, our knowledge of corundum and emerald deposit formation has improved significantly. {T}he mineral deposits are classically separated into primary and secondary deposits. {P}rimary corundum deposits are subdivided into two types based on their geological environment of formation: (1) magmatic and (2) metamorphic. {M}agmatic deposits include gem corundum in alkali basalts as in eastern {A}ustralia, and sapphire in lamprophyre and syenite as in {M}ontana ({U}nited {S}tates) and {G}arba {T}ula ({K}enya), respectively. {M}etamorphic deposits are divided into two subtypes (1) metamorphic deposits sensu stricto (in marble; mafic and ultramafic rocks, or {M}-{UMR}), and (2) metamorphic-metasomatic deposits characterized by high fluid-rock interaction and metasomatism (i.e., plumasite or desilicated pegmatites in {M}-{UMR} and marble, skarn deposits, and shear zone-related deposits in different substrata, mainly corundum-bearing {M}g-{C}r-biotite schist). {E}xamples of the first subtype include the ruby deposits in marble from the {M}ogok {S}tone {T}ract or those in {M}-{UMR} from {M}ontepuez ({M}ozambique) and {A}appaluttoq ({G}reenland). {T}he second subtype concerns the sapphire from {K}ashmir hosted by plumasites in {M}-{UMR}. {S}econdary corundum deposits (i.e., present-day placers) result from the erosion of primary corundum deposits. {H}ere, corundum is found in the following types of deposits: eluvial (derived by in situ weathering or weathering plus gravitational movement), diluvial (scree or talus), colluvial (deposited at the base of slopes by rainwash, sheetwash, slow continuous downslope creep, or a combination of these processes), and alluvial (deposited by rivers). {T}oday, most sapphires are produced from gem placers related to alkali basalts, as in eastern {A}ustralia or southern {V}ietnam, while placers in metamorphic environments, such as in {S}ri {L}anka ({R}atnapura, {E}lahera) and {M}adagascar ({I}lakaka), produce the highest-quality sapphires. {T}he colluvial {M}ontepuez deposit in {M}ozambique provides a huge and stable supply of clean and very high-quality rubies. {P}rimary emerald deposits are subdivided into two types based on their geological environment of formation: (1) tectonic-magmatic-related ({T}ype {I}) and (2) tectonic-metamorphic-related ({T}ype {II}). {S}everal subtypes are defined and especially {T}ype {IA}, hosted in {M}-{UMR}, which accounts for about 70% of worldwide production ({B}razil, {Z}ambia, {R}ussia, and others). {I}t is characterized by the intrusion of pegmatites or quartz veins in {M}-{UMR} accompanied by huge hydrothermal fluid circulation and metasomatism with the formation of emerald-bearing desilicated pegmatite (plumasite) and biotite schist. {T}ype {IB} in sedimentary rocks ({C}hina, {C}anada, {N}orway, {K}azakhstan, and {A}ustralia) and {T}ype {IC} in granitic rocks ({N}igeria) are of minor importance. {T}he subtype {T}ype {IIA} of metamorphic deposits is related to hydrothermal fluid circulation at high temperature, in thrust fault and/or shear zones within {M}-{UMR} of volcano-sedimentary series, such as at the {S}anta {T}erezinha de {G}oias deposit in {B}razil. {T}he subtype {T}ype {IIB} is showcased by the {C}olombian emerald deposits located in the {L}ower {C}retaceous black shales of the {E}astern {C}ordillera {B}asin. {T}hese are related to the circulation of hydrothermal basinal fluids in black shales, at 300-330 degrees {C}, that dissolved evaporites in (1) thrust and tear faults for the deposits of the western emerald zone ({Y}acopi, {C}oscuez, {M}uzo, {P}enas {B}lancas, {C}unas, and {L}a {P}ita mines) and (2) a regional evaporite level intercalated in the black shales or the deposits of the eastern emerald zone ({G}achala, {C}hivor, and {M}acanal mining district {S}econdary emerald deposits are unknown because emerald is too fragile to survive erosion and transport in rivers.},
  keywords = {{MONDE}},
  booktitle = {},
  journal = {{G}ems and {G}emology},
  volume = {55},
  numero = {4},
  pages = {464--489},
  ISSN = {0016-626{X}},
  year = {2019},
  DOI = {10.5741/gems.55.4.464},
  URL = {https://www.documentation.ird.fr/hor/fdi:010078078},
}