Earth Test 1 Flashcards
What are banded iron formations and how are they formed and why are the deposits important ?
- Form on the floor of shallow oceans about 2 billion years ago in the Precambrian period.
- Formed from the great oxygenation event where oxygen and iron joined in the ocean
- Huge deposits of iron ore formed from chemical precipitates on the floor of shallow ocean basins, in a highly oxidising environment.
- In Western Australia BIF’s are found in the Pilbra regions.
- The iron ores derived are hematite and goethite.
- BIF deposits are important because they show the atmospheric conditions for life.
What is Supergene enrichment and what can it tell us about grades ?
- When some iron ore deposits are weathered after formation, so silica from chert is dissolved increasing the iron concentration
- This can cause ores to become high grades
What are Placer deposits ?
- Gold, tin, diamond, and platinum are found where the velocity of water slows due to bends, river bottoms, and beaches.
- Are accumulations of heavy or durable minerals sorted by currents.
How does regional metamorphism effect the profitability of ores ? and what forms from it ?
- Regional metamorphism of pre existing sedimentary ores enlarges the grain size of the ore mineral, making it more profitable to mine and process than it would have been in its original state.
- The process generates hydrothermal fluids which transport or deposit the mineral in other environments, gold forms like this
How does contact metamorphism effect ores? And what forms from it ?
- Contact metamorphism of pre existing sedimentary ores produce Skarns.
- Beds of rock are altered and fluid is introduced and form new economic minerals.
- Iron, copper, and gold form from this.
What are magmatic ores ?
Ore deposits formed from igneous processes
What are extrusive and intrusive deposits ?
Extrusive - deposits formed on the earths surface
Intrusive - deposits formed beneath the earths surface
What are the extrusive and intrusive deposits ? List examples
Extrusive -
- Black smokers
- VMS
- Hotspring and volcanic fumarole
Intrusive -
- Kimberlite and Lamporite pipes
- Magmatic segregation
What are black smokers ?
- Are hydrothermal vents at the bottom of the ocean which emit black smoke.
- They contain copper, iron, and zinc
- The smoke contains high levels of sulfur
What is VMS and what ores form from it ?
- Forms from hot saline, and metal rich fluids being dispatched from vents and fumaroles
- Hot volcanic gasses condense when they hit cooler surface rocks
- VMS makes silver, copper, and gold
What are hotsprngs ?
- Hotsprings are hydrothermal solutions and rich surface land that form from geysers, Hotsprings, and volcanic fumaroles.
- Heated water can escape to the surface and overflow of precipitates of silica
What are kimberlite pipes ?
- Kimberlite occurs in the crust and the mantle in vertical structures (carrot shape)
- It is the most important source for diamonds
- Composed of deep cone of solidified magma
What are Lamporite pipes ?
- They are found in deep magma sources rich in magnesium
- They are found in the Archaen period in Western Australia, and are geologically widespread
What is volatile transport ?
- Magma crystallises, low soluble minerals crystallise out leaving residual magma
- It is rich in water, and has a low melting temperature.
- It contains silicates, elements, and volatile components
- Pegmatite is the main ore formed
What is gravitational setting ?
- When crystals form they are denser than liquid melt so they sink to the bottom of the magma chamber and form a cumulative layer.
- Economical minerals are concentrated and chromium is the main ore formed.
