The four most important considerations in the formation of ore deposits are:
- 1. Source and character of ore-bearing fluids.
- 2. Source of the ore constituents and how they were obtained in solution.
- 3. Migration of ore-bearing fluids.
- 4. Manner of deposition.
What is Fluid?
- A substance that deforms continuously whenacted on by a shearşng stress
- A solid will deform to certain point for given stress
- Liquids and gases are fluids (water, oil, air)
- Same substances can act as solids and fluids (e.g. soil standing /avalanche)
Hydrothermal fluids are generally hot, acidic, and very ionrich.
They result in processes such as
Alteration and Leaching
The acidity makes it very easy for feldspars and mafics to be hydrolyzed, resulting in big clay-altered zones.
The acids can leach any metal ions—especially ions that don’t quite fit into their crystal structure (substitutions)—out of minerals, so these can extract trace constituents, like Cu from surrounding rocks.
Where the circulating fluids undergo a sudden change of pressure, temperature, or chemistry, precipitates often form as the mineral-stability changes with PT. For example, where hydrothermal fluids are expelled into cold sea water, sulfides, that had been stable in solution, are suddenly over-saturated and precipitate, giving black smokers.
Where boiling occurs due to decrease in pressure, many ions will partition into the vapor phase and others in the liquid become over-saturated, and you get precipitates.
For a closer examination the ore-bearing fluids are divided into five categories:
- Sea water
- Magmas and magmatic fluids
- Meteoric waters
- Connate waters
- Fluids associated with metamorphic processes
Types of water
a. Sea Ocean water
Water, extensively circulated through the oceanic crust Responsible for;
Redistribution of elements in oceanic crust
- Leaching and precipitation of elements (active role)
- Redistribution of ions in solution in seawater (passive role)
- Alteration and mineralization
Cold seawater seeps down into the crust through cracks in the seafloor. As it gets closer to molten rock, it heats up. In the process of heating, it undergoes several chemical reactions. At first, oxygen is removed from the seawater. Then, as the water seeps further down (and increases in temperature) sulfur and metals such as copper, zinc, and iron dissolve from the surrounding rock into the hot fluid. Eventually, this hot, mineral-rich fluid rises again and gushes out of openings in the seafloor: hydrothermal vents, at temperatures up to about 400°C.
A type of chimney referred to as a “black smoker“. The black color comes from minerals precipitating out as hot vent fluid meets cold seawater
How do these chimneys form?
When the hot vent fluid mixes with cold seawater, chemical reactions occur. For example, the sulfur in some vent fluid combines with the metals, forming sulfide minerals. When the mixing occurs as the fluid exits the seafloor, the minerals precipitate to form chimneylike structures that
2. Meteoric-surface water
It is especially important in supergene processes. Temperature, and consequently solubility of minerals increases as this water percolates down. Meteoric water contains the dominant crustal elements , Na, Ca, Mg, SO4–,CO3– etc, and it can get mixed with magmatic water.
a. Widespread circulation of water and redistribution of elements: alteration+precipitation
b. Many different ore deposits including;
- Low temp. sediment-hosted uranium deposits
- Low to moderate temp. hydrothermal deposits; epithermal deposits of Au-Ag
3. connate-formational water
Water trapped in sediments at the time of their formation (fossil water). Contain Na, Cl, Mg, HCO3, Sr, Ba, and N compounds. They have little direct relationship with orebearing fluids except when the strata containing them are undergoing metamorphism. When activated, they become strong solvents of metals since they contain chlorine. They are thus one of the sources of hydrothermal fluids.
Water that is included within the interstitial pore spaces of sediments
- Kept during sedimentation
- Released during subsequent burial, compaction and lithification
4. Metamorphic water
Water expelled-off from the hydrous minerals during breakdown processes involving dehydration and decarbonation due to metamorphism
- Metamorphism is the result of temperatures and pressures intermediate between diagenesis and melting, associated with
- Hydrothermal/metasomatic fluids are associated with metamorphism, igneous activity, and even sedimentary diagenesis. They are hot fluids that contain lots of ions and alter surrounding rocks and can deposits ores and other vein-fillings.
5. Magmatic water
Magma and magmatic fluid: Magma is naturally occurring mobile melt generated within the earth and capable of intrusion and extrusion, or defined less formally, magma is a rock melt or high temperature molten mush of liquid and crystals. Solidification of magma produce igneous rocks.
Most magma is not homogeneous in composition part may be:
- Rich in ferromagnesian constituent.
- Other rich in silica, sodium or potassium compounds volatiles, reactive xenoliths, or other substances such as metallic elements.
Boiling is process which the volatiles in the magma focuses to carapace
- Focuses fluid flow, perhaps metal transport
- Mechanical energy from fluid exsolution – hydrofracturing?