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El Mochito

Honduras

Main commodities: Pb Zn Ag Cu
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The El Mochito skarn-sulphide-oxide limestone replacement Zn-Pb-(Ag) deposit was (in 1997) the largest operating mine in Central America. It is located in north-western Honduras, ~88 km SW of San Pedro Sula and 220 km NW of the capital city, Tegucigalpa (#Location: 14° 51' 50"N, 88° 4' 49"W).

El Mochito is similar to deposits such as Santa Eulalia and Naica in Mexico, and Tintic in the USA (see the records for these deposits also).

The Mochito deposit lies a short distance to the south of the Motagua Suture that separates the North America Plate in the north from the Caribbean Plate to the south. It occurs on the the eastern side of the Central American section of the North American plate.

The volcanic rocks of the Tertiary Matagalpa Formation are related to subduction along the Middle American trench, and cover much of southern and central Honduras. These volcanic rocks thin to the north, and the underlying Mesozoic shelf sedimentary and Palaeozoic to Proterozoic basement metamorphic rocks become exposed. Mochito is located at the southern end of the Sula graben, which is possibly related to a sinistral offset along the Motagua suture zone during the Late Tertiary (Jensen and Goffaux, 2010).

The deposit is hosted by a 6000 m thick Mesozoic shelf sediment succession deposited unconformably on a the Palaeozoic to late Neoproterozoic basement.

In the mine area, the oldest exposed sequence are the clastic sedimentary rocks of the Late Jurassic Todos Santos Formation, comprising post orogenic molasse sediments which pass upward, via the Cantarranas Formation, a 30 m thick band that is gradational between the Todos Santos Formation and the overlying Atima Atima Limestone. The latter is a 1200 to 1300 m thick, lower to upper Cretaceous, shallow reefal marine carbonate succession, comprising lower and upper limestone beds separated by the 150 to 200 m thick Mochito Shale, a unit of limey shales with intercalated limestone beds. The lower and upper limestones are the main host to ore, generally a dark grey, massive micrites to biomicrites with shale partings (Jensen and Goffaux, 2010).

The Atima limestones are, in turn, unconformably overlain by a late Cretaceous to Tertiary red bed sequence of the Valle de Angeles Formation, and then by Cenozoic volcanic rocks, including andesites and basalts. In the eastern part of the mine, a number of thin, partly altered olivine-diabase dykes occur that appear to have channeled some of the mineralising solutions. Based on the possible age of these dykes, this would suggets that the mineralisation is post mid-Tertiary in age (Jensen and Goffaux, 2010).

Pb-Zn-Ag mineralisation at Mochito occurs in the limey sedimentary package from the Cantarranas Formation, extending upwards into the top section of the upper Atima limestone, over a vertical interval of more than 1200 m (Jensen and Goffaux, 2010).

The ore system comprises 6 major chimney orebodies, San Juan, Nispero, Nina Blanca, Yojoa, Nueva and Main, each of which has an up to 1 km vertical extent, and is rooted in a lower manto system, known as the Nacional and Salva Vida that follows the base of the Atima Limestone in the Cantarranas Formation. The basal parts of the chimneys are broad trunks controlled by ESE and ENE trending faults related to basement structures. In their upper sections the chimneys split up into a system of branches following NNE faults.

The deposit contains skarn alteration assemblages, comprising pyroxene and garnet, with sphalerite, argentiferous galena, pyrrhotite, pyrite, chalcopyrite and arsenopyrite. Deposits along the fault-controlled Nacional-Salva Vida trend grade outward and upward from zinc-iron rich cores to lead-rich and marginal silver-rich zones. Garnet:pyroxene ratios progressively decline away from fault zones toward unaltered limestone and along the NE-SW trend of the deposits. Changes in Zn:Pb, Zn:Cu, Pb:Cu and Cu:Ag ratios along trend appears to reflect the direction of hydrothermal fluid flow (NE to SW) and increased distance from a potential source region (Jensen and Goffaux, 2010).

Silicate mineralisation at Mochito generally comprises an assemblage of medium to coarse grained primary silicates, uncluding grossular to andradite garnet, hedenbergitic pyroxene in addition to quartz, calcite, minor vesuvianite and rhodochrosite as well as a common assemblage of epidote-quartz-calcite-chlorite-iron oxide. Local retrograde minerals after pyroxene (predominantly hedenbergite) are also present, including epidote, quartz-calcite-chlorite-manganese-actinolite-iron oxide. Locally some clays occur, including smectite-nontronite and iron-saponite. Sulphide mineralisation generally consists of sphalerite and galena with minor chalcopyrite. Sphalerite may have inclusions of the last two minerals and was preceded by magnetite, pyrrhotite and pyrite, generally followed by galena and may be contemporaneous with chalcopyrite. Chimneys developed in the lower Atima formation tend to have a general mineralogical zoning with Zn increasing with depth and Pb-Ag in the upper levels (Jensen and Goffaux, 2010).

No significant intrusives are known within 8 km of the orebodies, although NW trending olivine dolerite dykes cut the limestone and appear to control alteration and mineralisation.   These dykes are believed to be related to a late Cenozoic basalt field found in the region.

Production plus reserves in 1997 amounted to ~15 Mt @ 5% Pb, 8% Zn, 0.3% Cu, 250 g/t Ag.

Breakwater Resources (NI 43-101 Technical Report, 2008) reported remaining resources of:
    Measured + Indicated resources - 4.202 Mt @ 5.9% Zn, 2.7% Pb, 76 g/t Ag;
    Inferred resources - 3.330 Mt @ 4.5% Zn, 2.1% Pb, 46 g/t Ag.

Nyrstar NV (reserve release, 2011) quoted remaining resources of:
    Measured + Indicated resources - 5.92 Mt @ 4.78% Zn, 2.23% Pb, 69 g/t Ag;
    Inferred resources - 2.89 Mt @ 5.30% Zn, 2.30% Pb, 46 g/t Ag.

This summary is largely drawn from both the references below and "Jensen, T. and Goffaux, D., 2010 - Breakwater Resources Ltd. American Pacific Honduras S.A. de C.V. Mochito Mine Las Vegas, Honduras, an NI 43-101 Technical Report prepared for Breakwater Resources Ltd, 56p."

The most recent source geological information used to prepare this summary was dated: 2010.     Record last updated: 20/7/2016
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


El Mochito

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Ault K M, and Williams-Jones A E,  2004 - Sulfur and Lead Isotope Study of the El Mochito Zn-Pb-Ag Deposit: in    Econ. Geol.   v99 pp 1223-1231
Vazquez R, Vennemann T W, Kesler S E  1998 - Carbon and Oxygen isotope halos in the host limestone, El Mochito Zn-Pb-(Ag) skarn massive Sulphide-Oxide deposit, Honduras: in    Econ. Geol.   v93 pp 15-31


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