Manto ore deposits

Manto orebodies are stratabound irregular to rod shaped ore occurrences usually horizontal or near horizontal in attitude.

Manto deposits are an important source of copper, forming one of the world major copper resources in Chile and southern North America.

The term "manto" is from the Spanish word for "blanket" although the geologic "manto" is more like a "blanket roll" than a sheetlike structure.

Classification

Manto ore deposits are defined by a strict stratigraphic control on their distribution, generally within a porous formation within a structural trap site. They are distinct from other copper ore bodies in that they are not associated with shear zones, and an intrusive link to manto deposit formation is not conclusively proven.

Genetic model

The genetic model of manto formation is debated, but consists of the following broad principles;

* The "source" of ore within manto deposits is considered to be interformational, from a sedimentary source within an adjacent sedimentary basin, or from ore fluids driven off from a granite intrusive
* The "transport" of copper into the manto deposit position was likely hydrothermal, either a metamorphic solution or copper-bearing hydrothermal solutions generated by intrusive granites
* The "trap" where the ore materials concentrated is typically a coarse-grained member of a formation, and the manto is usually sited in a stratigraphic or structural pinch-out of this formation although it is now thought that hydrocarbons may have assisted in the migration of metals into favorable trap sites

Morphology

Manto deposits were first described in great detail in Chile, where they sit within sedimentary strata overlying large granitic intrusions, in regions adjacent to porphyry copper deposits.

In Chile, the arid climate and deep regolith development, tended to favor preservation of chalcocite-malachite-azurite assemblages in the manto deposits, leading workers to believe that they were weathered equivalents of primary chalcopyrite deposits of porphyry-copper derivation.

However, some recent work (Wilson & Zentilli, 2006) shows that there may be primary chalcocite and bornite formed within degraded petroleum within trap sites, with copper precipitating from solution by reduction in contact with the reduced carbon. Thus, manto deposits need not be the weathered equivalents of primary chalcopyrite.

Manto deposits may be formed in proximity to intrusives, for instance in the La Providencia mine, Mexico, a porphyry stock is the feeder for some twenty mantos as the pipe intersects favorable layers in the sedimentary sequence. However, these manto deposits are analogous to skarn deposits, and in some cases terminology may be misused.

Examples of manto orebodies include: Magma Mine, Superior, Arizona; Tintic and Park City, Utah; Leadville and Gilman, Colorado and Pioche, Nevada in the United States.

See also

* Chalcopyrite
* Chalcocite
* Porphyry copper
* Ore genesis

References

* Wilson N.S.F., & Zentilli M., 2006. Association of pyrobitumen with copper mineralization from the Uchumi and Talcuna districts, Chile. "Journal of Coal Geology", 65, pp 158-165.
* Evans, Anthony, (1992) "Ore Geology and Industrial Minerals: An Introduction", Blackwell Science; 3rd edition ISBN 0-632-02953-6
*Guilbert, John M. and Charles F. Park, Jr (1986) "The Geology of Ore Deposits", W. H. Freeman ISBN 0-7167-1456-6