Processes of Ore Formation
Current theories of the genesis of ore deposit can be divided into internal (endogene) and external (exogene) or surface processes. It must be understood that more than one mechanism may be responsible for the formation of an ore body. Example - stockwork porphyry copper deposit at depth (epigenetic) with a syngenetic massive sulfide deposit at the surface. The Table at the end of the document summarizes the principal theories of ore genesis,
Depending upon whether an ore deposit formed at the time of and together with the enclosing rock, or was introduced into it by subsequent processes, they are classed as:
Syngenetic - A deposit formed at the same time as the rocks in which it occurs. Ex. Banded Iron Formation
Epigenetic - A deposit introduced into the host rocks at some time after they were deposited. Ex. Mississippi Valley-type Deposits
Magmatic Deposits: Those deposits, not including pegmatites that have formed by direct crystallization from a magma. Two types:
· Fractional crystallization - Any process whereby early formed crystals can not re-equilibrate with the melt. Includes 1) gravitative settling; 2) flowage differentiation; 3) filter pressing and 4) dilation. Number 1 is the most important and results from the settling of early formed crystals to the bottom of the magma chamber. Rocks formed in this manner are termed cumulates and are often characterized by rhythmic layering. In ore deposits the alternating layers are often magnetite and/or chromite between layers of silicate. Ex. Bushveld igneous complex.
· Immiscible liquid - Typical example is oil and water. In ore deposits we deal with silicate and sulfide magmas. As a magma cools, sulfides coalesce as droplets and due to higher density settle out. Most common sulfides are iron sulfides, but nickel, copper and platinum also occur. Ex. Sudbury, Canada. The settling out of the heavier sulfides results in the peculiar net-textured ores often found in many of these deposits.
Pegmatitic Deposits: Pegmatites are very coarse grained igneous rocks. Commonly form dike-like masses a few meters to occasionally 1-2 km in length. Economic ore deposits are associated with granitic pegmatites since felsic magmas carry more water. Residual elements such as Li, Be Nb, Ta, Sn and U that are not readily accommodated in crystallizing silicate phases end up in the volatile fraction. When this fraction is injected into the country rock a pegmatite is formed. Temperatures of deposition vary from 250-750°C. Pegmatites are divided into simple and complex. Simple pegmatites consist of plagioclase, quartz and mica and are not zoned. Complex have a more varied mineralogy and are strongly zoned. Crystals in pegmatites can be large, exceeding several meters. Three hypotheses to explain their formation:
a. fractional crystallization
b. deposition along open channels from fluids of changing composition
c. crystallization of a simple pegmatite and partial to complete hydrothermal replacement
Hydrothermal Deposits: Hot aqueous solutions are responsible for the formation of many ore deposits. Fluid inclusion research indicates most ore forming fluids range in temperature from 50°C to 650°C. Analysis of the fluid in inclusions has shown that water is the most important phase and salinities are often much greater than those of seawater. The chemistry of ore fluids and the mechanism of deposition of ore minerals remains a subject of hot debate. Arguments boil down to a) source and nature of the solutions b) means of transport of the metals and c) mechanism of deposition.
Metamorphic/Metasomatic Deposits: Pyrometasomatic deposits (skarns) developed at the contact of plutons and host rock. Generally, host rock is a carbonate and new minerals formed are the calc-silicates diopside, andradite and wollastonite. Temperatures involved are thought to be 300-500°C, but pressure is probably quite low. Three stage process:
2. Introduction of Si, Al, Fe, Mg
3. Hydration and introduction of elements associated with volatile fraction
Other metamorphic processes are relatively unimportant, but hydration/dehydration during regional metamorphism may concentrate metals at the metamorphic front. Sodic metasomatism of K-spar is thought to have been important in the concentration of gold at Kalgoorlie. Conversion of feldspar from K-spar (1.33A) to Na plag (.97A) resulted in the expulsion of gold (1.37A) which could no longer be accommodated in the feldspar lattice. Skip over Mechanical-chemical sedimentary processes since they are covered in the other course.
Volcanic Exhalative Deposits: Some ore deposits often show spatial relationships to volcanic rocks. They are conformable with the host and frequently banded suggesting sedimentary processes. Principal constituent is pyrite with lesser chalcopyrite, aphalerite, galena, barite and Ag-Au. These were thought until the late 60’s to be epigenetic, but it is now realized they are syngenetic. They show a progression of types with three distinct end members:
1. Cyprus type - Associated with mafic volcanics and ophiolite sequences. Found in spreading centers and back arc basins. Consist predominantly of pyrite with lesser chalcopyrite. Typified by the Cyprus pyrite-cu ores.
2. Besshi type - Associated with basaltic to dacitic volcanism. Thought to form during the initial stages of island arc formation. Many Besshi type deposits occur in Precambrian rocks and these may have been generated in entirely different tectonic settings. Pyrite dominant, but chalcopyrite and sphalerite very common. Typified by many of the volcanogenic deposits of Canada.
3. Kuroko type - Associated with dacitic to rhyolitic volcanics. Form during the waning stages of island arc volcanism. Pyrite occurs, but is not dominant Usually galena or sphalerite are predominate with lesser chalcopyrite and tetrahedrite. Also significant silver in this type. Typified by the Kuroko deposits.
Although it is agreed ores are associated with volcanism the source of the ore bearing solutions continues to be debated. Many feel ore fluids are of magmatic origin, but others feel they are merely convecting seawater.
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