Ore genesis
Various theories of ore genesis explain how the various types of mineral deposits form within Earth's crust.
The biggest deposits form when the source is large, the transport mechanism is efficient, and the trap is active and ready at the right time.
These processes are the physicochemical phenomena and reactions caused by movement of hydrothermal water within the crust, often as a consequence of magmatic intrusion or tectonic upheavals.
However most metals of economic importance are carried as trace elements within rock-forming minerals, and so may be liberated by hydrothermal processes.
This happens because of: Transport by hydrothermal solutions usually requires a salt or other soluble species which can form a metal-bearing complex.
These metal-bearing complexes facilitate transport of metals within aqueous solutions, generally as hydroxides, but also by processes similar to chelation.
The majority of metal deposits formed by hydrothermal processes include sulfide minerals, indicating sulfur is an important metal-carrying complex.
Ore deposits formed by lateral secretion are formed by metamorphic reactions during shearing, which liberate mineral constituents such as quartz, sulfides, gold, carbonates, and oxides from deforming rocks, and focus these constituents into zones of reduced pressure or dilation such as faults.
Surficial processes are the physical and chemical phenomena which cause concentration of ore material within the regolith, generally by the action of the environment.
Classification of hydrothermal ore deposits is also achieved by classifying according to the temperature of formation, which roughly also correlates with particular mineralising fluids, mineral associations and structural styles.
Particular environmental conditions are needed to transport enough iron in sea water to form these deposits, such as acidic and oxygen-poor atmospheres within the Proterozoic Era.
The vast majority of SEDEX lead and zinc deposits are Proterozoic in age, although there are significant Jurassic examples in Canada and Alaska.
MVT and similar styles occur by replacement and degradation of carbonate sequences by hydrocarbons, which are thought important for transporting lead.
Lode gold deposits are usually hosted in basalt or in sediments known as turbidite, although when in faults, they may occupy intrusive igneous rocks such as granite.
Lode-gold deposits are intimately associated with orogeny and other plate collision events within geologic history.
Intrusive related gold (Lang & Baker, 2001) is generally hosted in granites, porphyry, or rarely dikes.
Intrusive related gold usually also contains copper, and is often associated with tin and tungsten, and rarely molybdenum, antimony, and uranium.
Gold is deposited within iron oxides in the weathered rock or regolith, and may be further enriched by reworking by erosion.
Some laterite deposits are formed by wind erosion of the bedrock leaving a residuum of native gold metal at surface.
The source of platinum and palladium deposits is ultramafic rocks which have enough sulfur to form a sulfide mineral while the magma is still liquid.
This is generally thought to be nearly impossible by pure fractional crystallisation, so other processes are usually required in ore genesis models to explain sulfur saturation.
Uranium deposits are usually sourced from radioactive granites, where certain minerals such as monazite are leached during hydrothermal activity or during circulation of groundwater.
These minerals are sourced from primarily granite bedrock by erosion and transported to the sea by rivers where they accumulate within beach sands.
Skarn deposits form by reaction of mineralised fluids from the granite reacting with wall rocks such as limestone.
Immense quantities of phosphate rock or phosphorite occur in sedimentary shelf deposits, ranging in age from the Proterozoic to currently forming environments.
[11] Phosphate deposits are thought to be sourced from the skeletons of dead sea creatures which accumulated on the seafloor.
Due to the presence of vanabins, concentration of vanadium found in the blood cells of Ascidia gemmata belonging to the suborder Phlebobranchia is 10,000,000 times higher than that in the surrounding seawater.
In the final moments of the collision, the physical conditions are so extreme that these heavy rare elements can be formed, and are sprayed into space.
