Las Cruces

Seville, Sevilla, Spain

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The Las Cruces supergene enriched cupriferous pyritic volcanic hosted massive sulphide deposit is located within the Iberian Pyrite Belt in Huelva, southern Spain, 60 km ESE of Rio Tinto, and 20 km north of Seville (#Location: 37° 30' 11"N, 6° 5' 56"W).

The Las Cruces deposit occurs near the eastern limit of the Iberian Pyrite Belt, which is a 250 km long x 40 km wide corridor of volcanosedimentary rocks and Cu-pyrite deposits that extends eastward from southern Portugal into southern Spain. It hosts to more than 100 mineral deposits, some of which have been exploited for metals from as pre-Roman times. For details of the geology and setting of the Iberian Pyrite Belt see the Rio Tinto record.

Mineralisation at Las Cruces consists of polymetallic massive sulphides, hosted by late Devonian to early Carboniferous dacite volcanic and sedimentary rocks, deposited in a submarine setting within a narrow and relatively shallow intra-continental sea, characterised by bimodal volcanism and sedimentation. This sequence was subjected to tectonism during the late Palaeozoic Variscan (Hercynian) Orogeny, resulting in the general uplift of the region, with variable deformation and faulting, followed by weathering and erosion of the overlying host rocks during the late Palaeozoic and Miocene. Late erosion and weathering, exposed the upper part of the original massive sulphide deposit, resulting in oxidation of the sulphide minerals, formation of an iron oxide gossan, downward transport of leached copper, and precipitation of a supergene secondary enrichment blanket in unoxidised primary massive sulphide at depth. Relatively immobile gold and silver metals remained in the oxidised gossan, with local enrichments. Subsequent transgression during the Miocene by a shallow sea interrupted the weathering and oxidation process and buried the deposit below 100 to 150 m of sandstone and calcareous mudstone (marl). Marine regression during the Pliocene resulted in the subaerial exposure and erosion of much of the region, although the Las Cruces deposit remained concealed below the Miocene the calcareous mudstone cover. The Miocene sandstone unit at the base of this cover sequence overlying the palaeo-erosion surface is a regional aquifer (Niebla-Posada Aquifer). This aquifer comprises a basal conglomerate overlain by semi-consolidated sandstone and averages approximately 5 m thick over the deposit.

The deposit is hosted by a sequence of volcanic and sedimentary rocks, and occurs within a shale unit which generally forms a 10 to 20 m wide envelope around the massive sulphides. Mineralisation occurs in a single massive sulphide horizon that has a general east-west strike and dip at ~35°N, with a gradual change to the west to a dip of ~30 °NW. The overall dimensions of the massive sulphide lens is ~1000 m along strike, by 500 m or more down dip, and is up to 100 m thick, averaging 30 to 40 m. The massive sulphide lens thickens to the west, before being truncated by a roughly north-south trending fault. Along strike to the NE, the grade of the mineralisation is relatively low. Pre- Miocene supergene enrichment formed a generally horizontal zone of supergene enrichment in the up-dip part of the primary massive sulphide.

The primary massive sulphide mineralisation contains massive to semi-massive sulphide minerals with generally >80% sulphide. Pyrite is the dominant sulphide mineral, with lesser, finely intergrown sphalerite, galena and chalcopyrite, accompanied by minor enargite, tennantite and tetrahedrite. The footwall to the primary massive sulphide contains a stockwork of interconnected pyrite veins and veinlets, with local higher grades of copper and zinc. The primary sulphide is not included in the 2012 resource estimate.

The up-dip section of the massive sulphide deposit is cut by the planar pre-Miocene palaeo-surface, with minor irregularities and a gentle dip to the south. The pre-Miocene weathering and oxidation most affected the central and western part of the deposit.

The uppermost sections of the massive sulphide mineralisation has been completely oxidised to an iron oxide gossan, with local enrichment of gold and silver. The gossan is generally 10 to 20 m thick, with grades averaging 5.9 (up to 353) g/t Au and 98 (up to 1733) g/t Ag, with an average of 0.20 % Cu. It is also enriched in lead, mainly in the form of galena. The porous gossan is infilled with variable carbonate emplaced during deposition of the overlying sedimentary marl.

There is strong silicification of the host rocks in the Palaeozoic hanging wall rocks immediately above the gossan, with erratic gold and silver enrichment.

In the secondary sulphide zone, which underlies the iron oxide gossan, the primary massive sulphide mineralisation has undergone supergene enrichment by the downward migration and precipitation of copper leached from the gossan zone. Secondary sulphide enrichment occurs in a roughly horizontal, ~40 m thick, zone within the up-dip section of the original primary massive sulphide deposit. However, in areas of intense fracturing, secondary sulphide may be as much as 60 m thick. The lower boundary of the secondary sulphide mineralisation is gradational with the primary sulphide zone, although the footwall contact of the secondary mineralisation with the underlying, relatively impermeable, clay-altered shales and volcanic host rocks is usually sharp. This permeability contrast tended to channel secondary mineralisation along the base of the massive sulphides, locally producing lenses of secondary mineralisation that follow the contact zone down dip into the primary sulphides. The upper 1 to 2 m of the supergene sulphide zone comprises a layer of sandy, pyritic material with much lower copper grades than the underlying material, referred to as the Depleted Zone.

Supergene sulphide mineralisation predominantly comprises chalcocite and to a lesser extent bornite and covellite, which has partially replaced pyrite and other sulphides, as well as infilling any open space resulting from fracturing or original porosity. Chalcocite ranges from very fine grained (sooty) dark grey coatings and relatively unconsolidated intergrowths, to disseminated consolidated grains, to consolidated veins and replacement bands. Chalcopyrite content is only minor in the supergene blanket. The supergene blanket constitutes the mineable reserve.

The secondary sulphide mineralisation has been divided into three main, and three minor sub-lenses. The main lenses are the:
• High Copper - High Density (HCH) lens consists of secondary enrichment of dense primary massive sulphides;
• High Copper - Low Density (HCL) lens consists of secondary enrichment of primary semi-massive sulphides, footwall stockworks,
    or footwall lithologies of lesser density;
• High Copper lens Number 4 (HC4) is a spatially distinct lens of HCH secondary mineralisation that follows the contact zone down dip
    into the primary sulphide zone.
The three minor sub-lenses are the:
• High Copper Intrusive (HCI);
• High Copper Footwall (HCF); and
• Medium Copper Low Density (MCL).

The open pit reserves in 2002 were stated at: 16.2 Mt @ 5.94% Cu (Barrie et al., 2002).

Remaining ore reserves and mineral resources at the end of 2011 were (Noble, 2012 for Inmet Mining):
    Open pit resource - 12.663 Mt @ 5.67% Cu;
    Open pit reserve (including dilution) - 13.374 Mt @ 5.40% Cu;
    Underground reserve (including dilution) - 0.472 Mt @ 8.38% Cu;
    Stockpiles - 0812 Mt @ 5.01% Cu;
    Total reserve (including stockpiles) - 14.658 Mt @ 5.47% Cu.
Note: Reserves are included within resources.

The resource of the gossan zone, which is within the pit, is (Noble, 2012 for Inmet Mining):
    indicated resource - 0.958 Mt @ 3.85 g/t Au, 109 g/t Au, 5.81% Pb;
    Inferred resource - 1.767 Mt @ 2.47 g/t Au, 48 g/t Ag, 1.62% Pb,

For additional detail consult the reference(s) listed below.

This summary is largely drawn from "Noble, A.C., 2012 - Technical Report for the Las Cruces Copper Mine, Andalusia, Spain; an NI43-101 Technical report prepared for Inmet Mining Corporation, by Ore Reserves Engineering, 138p."

The most recent source geological information used to prepare this summary was dated: 2012.     Record last updated: 12/1/2015
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.

Las Cruces

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Barrie C T, Amelin Y, Pascual E  2002 - U-Pb geochronology of VMS mineralization in the Iberian Pyrite Belt: in    Mineralium Deposita   v37 pp 684-703
Tornos, F., Velasco, F., Slack, J.F., Delgado, A., Gomez-Miguelez, N., Escobar, J.M. and Gomez, C.,  2017 - The high-grade Las Cruces copper deposit, Spain: a product of secondary enrichment in an evolving basin: in    Mineralium Deposita   v.52, pp. 1-34.
Yesares, L., Aiglsperger, T., Saez, R., Almodovar, G.R., Nieto, J.M., Proenza, J.A., Gomez, C. and Escobar, J.M.,  2015 - Gold Behavior in Supergene Profiles Under Changing Redox Conditions: The Example of the Las Cruces Deposit, Iberian Pyrite Belt : in    Econ. Geol.   v.110, pp. 2109-2126

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