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 mantle although the geologic manto is more like a mantle roll than a sheetlike structure.
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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.
The genetic model of manto formation is debated, but consists of the following broad principles;
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.