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Accha

Peru

Main commodities: Zn Pb
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The Accha nonsulfide zinc deposit is located in the southern Altiplano of Peru, 70 km south of the city of Cuzco and lies within the 30 km long Accha-Yanque zinc belt.

The Accha-Yanque zinc belt is a significant zinc-rich metallogenic province hosting a number of economic mineral deposits including porphyry copper, skarn and several nonsulfide-type occurrences, showings and deposits which are situated in a belt, peripheral to the northern, northeastern, and northwestern edge of the Oligocene to (Miocene?) Yauri-Apurímac batholith. Mineralisation in these zinc occurrences is hosted in both sedimentary and tectonic breccias in the limestones of the Middle to Upper Cretaceous Ferrobamba Formation.

Stratigraphic units of geologic importance in the mining district are as follows:

Tertiary magmatic rocks
The Tertiary Yauri-Apurímac batholith consists mainly of intrusive bodies, with a minor porphyry stocks and tuffitic rocks. In the Accha area it is dioritic to granodioritic. The most common magmatic rocks are: (a) hornblende-feldspar porphyry stocks; and (b) feldspar porphyry and quartz-feldspar porphyry. Eocene-Oligocene
San Jerónimo-Puno Group detrital succession - > 1500 m thick - The sedimentary rocks of this Group generally outcrop at Accha as a coarse conglomerate with polymictic clasts, consisting of intrusive rocks, limestone and quartzite. The matrix of the conglomerate is a red mud-supported sandstone. The San Jerónimo Group is informally named the 'Puno Group' locally;
Middle-Upper Cretaceous
Ferrobamba Formation - > 600 m thick - which comprises mainly carbonates (limestone>>dolomite) with thin shaly intervals, subdivided into:
    (f) medium-bedded micrite limestone, characterised by patchy yellow dolomite and/or ankerite
         alteration. The uppermost section of this unit is usually absent, except where unconformably
         overlain by the San Jeronimo Group sediments;
    (e) thin- to medium-bedded dark limestone with massive interbeds containing chert nodules;
    (d) laminated cherty limestone with massive interbeds of micrite;
    (c) massive- to thick-bedded micritic limestone (subdivided into c1 and c2);
    (b) thin-bedded, commonly dark, laminated, locally shaly limestone with massive interbeds (sub-divided
          into b1 and b2). This unit is locally brecciated and is the main host to zinc mineralisation;
    (a) thin-bedded to laminated limestone with interbeds of massive limestone and limestone breccia;
Upper Jurassic
Yura Group Mara Formation - 50 to 100 m thick - Red-bed shale and conglomerate;
Middle Jurassic
Yura Group Soraya Formation - > 8000 m thick - Quartzite and shale;

The primary ores occur as an intrusive-related carbonate replacement deposits that are at least in part structurally controlled and are basically similar to other distal skarn deposits occurring in the region.

The ore minerals originally comprised sphalerite-pyrite > galena, locally with associated halos of silica and ferroan dolomite. The dolomite is interpreted to have been hydrothermal in origin, and may have locally contained a high content of manganese, contributing the high content of Mn oxides concentrated in the secondary deposits. The current deposit is almost entirely oxidised. The mineralised zone occupies the hinge of an antiformal dome that has been exposed by erosion. The southern limb of the fold dips at around 55° SSW, while the northern limb is truncated by faulting. The non-sulfide mineralised zone is 5 to 20 m thick, and occurs continuously along strike to the west over an interval of at least 700 m.

Thicker zones of mineralisation are found in the (a) and (b) carbonate units of the Ferrobamba Formation, hosted within strata-bound, brecciated and laminated limestone. The dominant host to mineralisation is a carbonate-clay matrix-supported breccias, while ore also occurs locally in very thin, quartz-rich conglomerate layers. The total thickness of the brecciated interval varies from 50 to 100 m, whereas individual breccia zones are continuous over thicknesses of 5 to 20 m. The breccias are polymictic, comprising angular limestone clasts and rarely quartzite. Most are poorly sorted with little or no apparent grading.

The non-sulfide mineralogy of the Accha deposit consists mainly of smithsonite and hemimorphite replacing both primary ore minerals and carbonate host rocks, with hydrozincite having only been detected in near surface samples. The smithsonite is found in zoned concretions with goethite, Mn (hydr)oxides and Zn clays, as well as replacive cement in the limestone intervals. A sauconite-like, zincian smectite, is variably concentrated throughout the deposit, occuring locally as a replacement of detrital feldspars and/or detrital fragments in marly sediments or in infills of karst cavities. It also replaces both hemimorphite and smithsonite deposited during earlier stages.

The indicated resource in 2009 was 5.1 Mt @ 8.2% Zn, 0.9% Pb (Boni, et al., 2009)

The most recent source geological information used to prepare this summary was dated: 2009.    
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.


  References & Additional Information
   Selected References:
Boni M, Balassone G, Arseneau V and Schmidt P,  2009 - The Nonsulfide Zinc Deposit at Accha (Southern Peru): Geological and Mineralogical Characterization : in    Econ. Geol.   v104 pp 267-289


Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge.   It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published.   While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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