Download Inroduction - Geological Sciences, CMU

218462 : Gemstone Deposits
Introduction
- About the course
- Teaching and learning
- Assignments
- Exam and grading
General mode of gemstone occurrences
Distribution of world gemstone deposits
Origin and occurrences of gemstones
Geological processes of gemstone deposits
- subsurface processes
- surficial processes
Distribution of Principal Diamond Deposits
Distribution of Principal Beryl Deposits
Tectonic - hydrothermal type
Granitic pegmatite - hydrothermal type
Distribution of Principal Corundum Deposits
Ruby
Sapphire
Distribution of Principal Opal Deposits
Precious opal
Common opal
Distribution of Principal Peridot Deposits
Distribution of Principal Jade Deposits
Jadeite
Nephrite
Distribution of Principal Turquoise Deposits
Gems, when present at all, tend to be scattered sparsely
throughout a large body of rock or to have crystallized as small
aggregates or fill veins and small cavities.
The average grade of the richest diamond kimberlite pipes in
Africa is about 1 part diamond in 40 million parts "ore.”
Gemstones occur in most major geologic environments. Most
gemstones are found in igneous rocks and alluvial gravels, but
sedimentaryand metamorphic rocks may also contain gem
materials.
Origin and Occurrences of Gemstones
Igneous Origin e.g.
- diamond - bearing kimberlites / lamproites
- corundum-bearing basalts
Metamorphic Origin e.g.
- garnet-bearing mica schists
- jadeite and nephrite
Hydrothermal Origin e.g.
- emerald-bearing calcite veins of Colombia
Pegmatitic Origin e.g.
- gem-bearing pegmatites of Brazil
Etc.
Geological Processes of Gemstone Deposits
A. Subsurface processes
• magmatic
• metamorphic and metasomatic
• pneumatolytic and hydrothermal
B. Surficial processes
• weathering
• sedimentation