Mina Justa


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The Mina Justa copper deposit is located within the Ica Department of the southern Peruvian coastal belt, approximately 400 km southeast of Lima, around 35 km SSW of the town of Nazca and 25 km north of the Pacific Ocean port of San Juan de Marcona.   It is one of five Cu-Au occurrences peripheral to and part of the same hydrothermal system as the 1.4 Gt Marcona magnetite deposit centred a few km to the south and west (#Location: 15° 9' 44"S and 75° 3' 50"W).

The Marcona district and the Mina Justa deposit is near the southern end of an approximately 400 x 70 km belt along the Peruvian coast, stretching from Lima to south of Chala, that hosts numerous IOCG style mineral occurrences. The district straddles the intersection of the northeast trending Nazca Ridge where it subducts below the continental margin, coinciding with the southern limit of the major Peruvian 'flat-slab' domain and the northern limit of the Central Volcanic Zone.

The oldest rocks in the district are a series of Precambrian gneisses and schists of the Lomas Formation, overlain by lesser Palaeozoic sediments and volcanics. In the Marcona district, the Palaeozoic dolomites, limestones and clastic sediments are divided into the San Juan Formation and the overlying Marcona Formation which is distinguished from the lower unit by the presence of a quartzite beds and thin chert laminations within the carbonates and has been metamorphosed to mid-greenschist facies chlorite schists, phyllites and dolomitic marbles. The Marcona Formation is intruded along the western margin of the district by the 425 ±4 Ma San Nicolas Batholith composed of a core of monzogranite passing outwards to granodiorite and diorite.

The extensive overlying Triassic to Jurassic age volcano-sedimentary rocks are locally represented by the mid to late Jurassic Rio Grande, Jahuay and Yauca Formations. The Rio Grande Formation and the overlying Jahuay Formation are mostly andesitic volcanics, sediments and limestones. The Yauca Formation is largely fine grained sediments and subordinate sandstone. The Jurassic sequence is unconformably overlain by the Miocene Pisco Formation ignimbrites in the east, and by Quaternary marine terraces and aeolian deposits in the west.

A wide range of intrusive rocks have been recognised in the district. The San Nicolas Batholith pre-dates the Jurassic sequence, but intrudes all of the older sequences. A number of intra- and post mineral intrusives and dykes occur in close proximity to the Marcona and Mina Justa mineralisation. At Mina Justa, the volcaniclastic hosts are intruded by pre-mineral, sill like, sub-horizontal, sub-volcanic hornblende-plagioclase porphyritic andesite. Late stage 20 to 50 m thick, NW-trending and 60 to 70° NE dipping andesitic porphyry (ocöite) dykes cut the Fe ores in the Marcona mine and the Cu mineralisation in the Mina Justa deposit. Barren ocöite dykes comprise 15% to 35% by volume of the mineralised zones at Mina Justa. Dykes, stocks and plutons of the Tunga andesites also intrude the Marcona, Rio Grande and Jahuay Formations.

Dolomite members of the Marcona Formation and limestone bands in the Rio Grande Formation appear to be the preferred host rocks for the Marcona magnetite deposits, while andesitic volcaniclastic rocks (crystal tuffs, epiclastic sandstones and siltstones, medium to coarse andesitic lithic volcaniclastic rocks, and fine-grained andesites with minor lenses of fossiliferous limestone) and a pre-mineral sub-volcanic porphyritic andesite intrusion in the Rio Grande Formation contain the Mina Justa Cu mineralisation. Both formations have a northeast strike and dip homoclinally at 45 to 65° NW, although they are separated by a substantial time break.

Two dominant fault systems have been recognised at Mina Justa, namley:
i). The 'Repetition Fault System' - a NE trending, shallow (10 to 30°), to steep (70°), ESE to SE dipping set that is the main ore control, particularly where steep splays or 'horsetails' split off the main fault and result in 'bulges' in the ore zone.
ii). The 'Huaca Fault System' - a NW trending, steeply NE dipping normal fault set which forms prominent NW-SE scarps with displacements of up to 400 m down to the east and lesserl lateral movement. Local offset at the Mina Justa ore is estimated to be in the order of tens of metres or less. This set largely accommodates the ocöite dykes.

At Mina Justa two principal zones of copper mineralisation occurring as disseminations, veinlets and breccia fillings of copper-bearing sulphides, accompanied by varying amounts of magnetite and actinolite, are associated with several subparallel, northeast trending, nearly flat to very shallow southeast dipping structural zones, as follows:
i). Main Zone - which can be traced at surface over a 400 m interval on a northeast trend. This structure dips at 10 to 30° SE and has been intersected down dip for at least 1700 m to a depth of more than 500 m.
ii). Upper Zone - at surface, this zone is around 300 m long and is subparallel to, and around 400 m south-east of the Main Zone. It has a similar elongate to oval shape and also dips at 10 to 30° SE. The zone persists over a distance of 1100 m down-dip to a maximum depth of 250 m.

Both the Main and Upper Zones are between 10 and 200 m thick and are 100 to 200 m apart. Both appear to follow curved, listric fault zones with an overall ESE to SE dip and thicken from west to east.

Three principal alteration assemblages are recognised at Mina Justa:
i). K-feldspar - K-feldspar±albite-specularite-chlorite-calcite; as pervasive replacement and minor veinlets.
ii). Actinolite - actinolite±magnetite-apatite; as replacement and coarse actinolite±apatite vein fill,
iii). Magnetite - semi-massive to massive fine-grained magnetite as replacement.

Epidote and calcite veins overprint the previous alteration assemblages. No clear zoning of alteration assemblages has been outlined, although a concentration of actinolite to the southwest and quartz-specularite to the northeast is apparent and a deeper core of potassic alteration is also inferred. Copper mineralisation is spatially associated with stronger actinolite and magnetite development.

Two styles of copper mineralisation are defined at Mina Justa:
i). Structurally controlled - as disseminations and blebs (with varied alteration assemblages, including K-feldspar, actinolite-magnetite-sphene), veinlets and veins (with magnetite ± actinolite ± apatite) and breccia-filling (with magnetite ± actinolite). This is the dominant style with Cu mineralisation occuring in at least three fractured and faulted structural zones;
ii). Stratabound iron-oxide mantos - massive magnetite alteration is the host to manto-type Cu mineralisation, replacing reactive limestone and carbonate-rich beds in the host stratigraphic sequence. The magnetite is an early event and pre-dates the main Cu mineralisation.

The upper 200 m has generally been oxidised, with a downward gradation to a sulphide zone. The oxide zone assemblage is predominantly chrysocolla and atacamite, with minor almagre, neotocite and rare tenorite and cuprite.

The hypogene sulphides are concentrically zoned outward from a core of bornite-chalcocite, through intermediate bornite, chalcopyrite and pyrite zones to peripheral pyrite. This zonation has been interpreted to define the fault conduit via which the mineralising fluid was channelled upwards. The Cu grade is generally proportional to the percentage of copper sulphides, which in turn is proportional to the total magnetite.

Published resource figures include:

    627 Mt @ 0.59% Cu (Global resource, Baxter, et al., 2005);
    346.6 Mt @ 0.71% Cu (Measured + indicated resource, 2006 at 0.3% Cu cutoff, Chariot Resources, 2006);
    127.9 Mt @ 0.60% Cu (Inferred resource at 0.3% Cu cutoff, Chariot Resources, 2006);
    419.2 Mt @ 0.63% Cu (Measured + indicated resource at 0.2% Cu cutoff, Chariot Resources, 2006);
    172.2 Mt @ 0.51% Cu (Inferred resource at 0.2% Cu cutoff, Chariot Resources, 2006);

Summary based on Baxter, et al., 2005.

The most recent source geological information used to prepare this summary was dated: 2006.    
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:
Baxter R, Meder K, Clints R and Berezowski M,  2005 - The Marcona copper project - Mina Justa prospect geology and mineralisation: in    www.proexplo.com.pe/tt2005/tt66_marcona_project.pdf    pp 1-16
Chen H,  2010 - Mesozoic IOCG Mineralisation in the Central Andes: an Updated Review: in Porter T M, (Ed),  2010 Hydrothermal Iron Oxide Copper-Gold and Related Deposits: A Global Perspective PGC Publishing, Adelaide   v.3 pp. 259-272
Chen Huayong, Clark A H, Kyser T K, Ullrich T D, Baxter R, Chen Yuming and Moody T C,  2010 - Evolution of the Giant Marcona-Mina Justa Iron Oxide-Copper-Gold District, South-Central Peru: in    Econ. Geol.   v105 pp 155-185
Zhu, Z.,  2016 - Gold in iron oxide copper-gold deposits: in    Ore Geology Reviews   v.72, pp. 37-42.

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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