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:

1.      Recrystallization

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.

THEORIES OF ORE DEPOSIT GENESIS
Origin Due to Internal Processes
Magmatic Segregation	Separation of ore minerals by fractional crystallization during magmatic differentiation.	Pt—Cr deposits Bushveld, S.A. Titanium deposit Tahawas, N.Y.
	Liquid immiscibility. Settling out from magmas of sulfide, sulfide-oxide or oxide melts which accumulate beneath the silicates or are injected into country rocks or extruded on the surface.	Cu-Ni ores of Sudbury, Canada and the nickel extrusives of Kambalda, West Australia.
Pegmatitic Deposition	Crystallization as disseminated grains or segregations in pegmatites.	Li-bearing pegmatites of Kings Mtn. N.C.
Hydrothermal	Deposition from hot aqueous solutions of various sources.	Porphyry Cu-Mo deposits of the W. Cordillera.
Lateral Secretion	Diffusion of ore and gangue forming materials from the country rocks into faults and other structures.	Gold deposits of Yellowknife, B.C. and the Mother Lode, CA.
Metamorphic Processes	Pyrometasomatic (skarn) deposits formed by replacement of wall rocks adjacent to an intrusive.	W deposits at Bishop, CA. Fe deposits Iron Mtn UT.
	Initial or further concentration of ore elements by metamorphic processes.	Homestake Au Mine, Lead, South Dakota.
Origin Due to Surface Processes
Mechanical Accumulation	Concentration of heavy minerals into placer	Placer Au deposits of Alaska and California.
Sedimentary Precipitation	Precipitation of certain elements in sedimentary environments.	Banded Iron Fm. of the Canadian Shield.
Residual Processes	Leaching of soluble elements leaving concentrations of insoluble elements.	Nickel laterites of New Caledonia and Arkansas bauxite.
Secondary or Supergene Enrichment	Leaching of certain elements from the upper part of a mineral deposit and their reprecipitation at depth to produce higher concentrations.	The upper portion of many porphyry copper deposits.
Volcanic Exhalative Process	Exhalations of sulfide-rich magmas at the surface, usually under marine conditions.	Mt. Isa, Aust., Sullivan and Kidd Creek,Canada, Kuroko,Japan.