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Aguilar

Juyjuy, Argentina

Main commodities: Zn Pb Ag
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The Aguilar zinc-lead-silver deposit is located on the margin of the Eastern Andean Cordillera and the Puna high plain, some 120 km to the NNW of San Salvador de Jujuy in Jujuy Province, north-western Argentina (#Location: 23° 12' 39"S, 65° 41' 20"W).

The ore deposit is hosted by strongly folded and faulted, north-south striking lower Ordovician quartzites and hornfels in the contact aureole of the immediately adjacent, north-south trending, 11 x 3 km Cretaceous Aguilar granite.   The orebodies are conformable with the host siliciclastics and parallel to the contact metamorphic halo of the granite in the pyroxene-hornblende and albite-epidote facies.

Basement in the area is composed of Proterozoic greywackes, sandstones and shales which are unconformably overlain by Cambrian shallow marine siliciclastics and by a transgressive sequence of early Ordovician sandstones, shales, minor conglomerates and limestones.   At stratigraphically higher levels, and to the west and north, Ordovician volcanics are evident from a deeper section of the basin.   No late Palaeozoic or Mesozoic rocks are represented.   Two gentle Palaeozoic phases of folding influenced the sequence during this period.   Geologic activity commenced again with the onset of the Andean cycle from the late Jurassic with the imposition of a north-south structural alignment and the introduction of felsic intrusives, including the Aguilar granite, during the Cretaceous.   A thick sequence of Cretaceous continental rocks were deposited to the east of Aguilar.   The major calc-alkaline magmatic cycle responsible for the Andean porphyry deposits commenced in the Oligocene to the west.   The Aguilar Range was uplifted along east-west trending thrusts, which steepen towards the surface, during the late Cenozoic.

The immediate host sediments are Ordovician and are interpreted to have been deposited in a tectonically active shallow marine environment.   The sequence commences with the Despensa Formation, a basal 350 m of grey-green greywacke, arkosic sandstone and siltstone, conformably overlain by the host unit, the Padrioc Formation comprising a 560 m pile of white to light grey, fine to coarse grained arkosic and calcareous sandstone interbedded with balck shales.   Minor limestones are now converted to skarns lenses.   The shale interbeds increase up- and basinwards.   The overlying Cardonal Formation is 340 m of greenish grey to brown sandstone, siltstones and shale with occassional channel conglomerates.   This unit has a tectonic contact with the succeeding Acoite Formation which consists of >3000 m of greenish-grey shales, greywackes and arkosic sandstone with minor limestones and conglomerates.

The late Lower Cretaceous 118±15 Ma Aguilar Granite and the similar Abra Laite granite complex to the west cut the sediments.   The Aguilar Granite is surrounded by a 1.5 km wide contact metamorphic aureole with three zones, progressively outwards of pyroxene, hornblende and albite-epidote hornfels.   In the vicinity of the Aguilar mineralisation the host sandstones have been altered to quartzites.   Undeformed dykes of the Aguilar Granite cut the highly deformed ore deposit.

Ten stratabound Zn-Pb-Ag ore lenses are known within the host quartzite of the Padrioc Formation at Aguilar.   Five styles of ore have been recognised, as follows:
• Disseminated sulphides (fine sphalerite and galena, minor pyrite and pyrrhotite and trace pyrargyrite) in quartzite - these appear first, increasing in density upwards until they define a fine banding;
• Stockwork sulphide veins (galena rich with significant sphalerite pyrite and pyrrhotite), which overlap in the upper part of the disseminated zone where the quartzite becomes shattered and is criss-crossed by fine ansatomosing sulphide veins and veinlets;
• Sulphide fillings in quartzite breccias - as the shattering increases upwards the disseminated and stockwork zones grade into a sulphide filled breccia - the main breccia comprises matrix supported 1 to 100 cm angular quartzite clasts set in a fine matrix of sphalerite and galena with minor pyrite, pyrrhotite and chalcopyrite - a less common calc-silicate diopside-garnet matrix supported breccia with lesser sulphides is also observed;
• Massive conformable, thinly banded sulphides (fine grained pyrite and sphalerite with lesser galena, pyrrhotite and rare chalcopyrite) overlie the breccias with a conformable, gradational to sharp contact.   These sulphides generally have thin bands (<1 cm) and thicker layers of up to tens of metres thick and pinch and swell and along strike;
• Late quartz-sulphide fracture fillings (mainly galena with lesser sphalerite, pyrhhotite and minor chalcopyrite), some of which only cross the disseminated to breccia ore and not the massive banded suphides.

The mineralised zone comprises a series of separated overlapping lenses spread over an interval of 2 km and vertical extent of at least 1 km with a southerly plunge. The geometry of individual lenses is variable and reaches up to 300 m North-South, ~55 m in width and reaches up to 160 m vertically.

Glencore geologists interpret the deposit to represent stratabound mineralisation with sulphide layers at the interface between siliciclastic and shale rocks, with a post-secondary metasomatic overprint between two intrusive stocks.

In addition to Aguilar, another three occurrences, Oriental, Pirita & Esperanza are found over an 8 km interval to the east in the same unit, and in the Aguilar Granite aureole.

Cumulative production from commencement of large scale mining in 1936 to 1992 - 25 Mt of ore yielding 2.2 Mt of Pb+Zn concentrate.
Reserves, 1981:   7 Mt @ 8% Pb, 9% Zn (USBM, 1981),
Head grade in 1992: 6.2% Zn, 5.5% Pb.

Published mineral resources and ore reserves at 31 December, 2014 were (Glencore, 2015 Reserve and Resource Report):
      Measured + indicated resources - 5.1 Mt @ 4.8% Zn, 5.3% Pb, 110 g/t Ag;
      Inferred resources - 3.1 Mt @ 4% Zn, 4% Pb, 100 g/t Ag;
      Proved + probable reserves - 3.4 Mt @ 4.9% Zn, 5.6% Pb, 120 g/t Ag (included in resources)

Production for year 2014 to the end of December was 0.69 Mt @ 4.4% Zn, 4.6% Pb, 93g/t Ag.

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

The most recent source geological information used to prepare this summary was dated: 2014.     Record last updated: 17/6/2016
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.


Aguilar

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Gemmell J B, Zantop H  1992 - Genesis of the Aguilar Zinc-Lead-Silver deposit, Argentina: contact metasomatic vs. sedimentary exhalative: in    Econ. Geol.   v87 pp 2085-2112


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