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Michilla - Lince, Estefania, Carolina, Aurora

Chile

Main commodities: Cu Ag
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The Michilla district embraces a number of significant stratabound manto type copper occurrences, which includes the large Lince-Estefania (also known as Susana-Lince), Aurora and Carolina deposits which is located 130 km NNE of Antofagasta in northern Chile.

The host sequence comprises a thick sequence of high-K calc-alkaline to tholeiitic andesiticÐbasaltic lava flows of the Jurassic (186 ±14 Ma to 172.3 ± 0.9 Ma) La Negra Formation which have been folded into a NE to NW trending monoclinal structure. This formation was predominantly emplaced on a thinned pre-Jurassic continental crust, as a consequence of an extreme oblique movement of the Pacific Phoenix Plate in the SSE direction relative to the continental margin.

This La Negra sequence has been intruded by numerous NE trending, SE dipping dykes and small stocks of gabbro-diorite dated at between 168 and 112 Ma. Some of the dykes form breccia bodies with volcanic clasts and an intrusive matrix. Although these breccias are believed to post date the mineralisation, some have a high Cu content and may pre- or syn-mineralisation. Plutonic diorite to granite bodies of the Coastal Batolith plutonic unit intrude the La Negra Formation and have been mainly dated at between 168 and 147 Ma. Both the volcanics and intrusives are covered by semi-consolidated Cenozoic sediments.

Within the deposit area, the host La Negra Formation is exclusively composed of a thick sequence of andesitic lava flows occupying a monoclinal structure which generally strikes at 60° and dips at 45° NW. Individual lava flows are typically between 2 and 10 m thick and vary from breccias at the bottom of the flows, through aphanitic and porphyritic textures in the middle, to amygdaloidal lavas at the top of the flows. The numerous dykes cutting the lavas range from 10 to 30 m in thickness. Small NE trending and SE dipping stocks of gabbroicÐdioritic and dacitic composition intrude the volcanic sequence, almost perpendicular to the bedding.

The dykes have been grouped into two categories based on their composition and temporal relationship to the mineralisation, namely: i). Calc-alkaline diorite dykes which appear to predate the hypogene Cu mineralisation and ii). porphyritic hornblende-bearing calc-alkaline dacitic dykes that locally postdate Cu mineralisation.

The most common and abundant form of mineralisation occurs as stratabound bodies, with the highest Cu content closest to the intrusives. Breccia bodies are generally located around the largest dioritic intrusions and extend laterally from the intrusions to grade into stratbound manto bodies.

Field and petrographic observations, and chemical analyses of the ores suggest the copper sulphides in both stratabound-manto and breccia ore bodies were formed by the same processes. The ore bodies vary in size, ranging from 3 to 20 m in thickness with lateral extensions of up to 120 m along the strike and as much as 200 m down the dip of the volcanic layers.

The principal ore minerals are copper sulphides (mainly chalcocite, bornite, covellite and chalcopyrite) and oxides (predominantly atacamite and chrysocolla). Mineralisation is confined to the permeable zones of the wallrocks and are developed over a vertical interval of 1000 m, with sulphides occuring at deeper levels (below 250 m asl) and oxides dominating nearer the surface (above 500 m asl) with a mixed sulphide-oxide transition.

The mineral paragenesis comprises early pyrite-chalcopyrite overprinted and largely replaced by bornite and chalcocite which are now the dominant sulphides. Where bornite and chalcocite occur together they form intergrowth textures. Minor native Cu and Ag are mainbly associated with chalcocite. Pyrite is rarely associated with the Cu sulphides, and is mainly found within breccia zones and in some aphanitic andesite. It is largely found below 250 m asl while chalcopyrite occurs above that level. Textural relationships suggest hematite postdates both chalcocite and bornite.

Supergene mineralisation is represented by covellite, secondary (bluish) chalcocite (replacing both chalcopyrite and bornite), atacamite and chrysocolla, with covellite replacing chalcopyrite, chalcocite and bornite. The alteration assemblage related to the Cu-oxide mineralisation is calcite and gypsum with minor quartz.

The hypogene Cu mineralisation is associated with an alteration assemblage of chlorite, epidote, albite ± quartz, ± actinolite.

Isotopic dating suggest the age of the main Cu sulphide mineralising event at the Lince-Estefan’a deposits is 159 ±16 Ma, and a magmatic origin for the sulphur of the Cu sulphide minerals, with lower crustal contamination. Other determinations restrict the mineralisation between 154.4±3.9 Ma and 138.1±2.0 Ma.

The Michilla mine (in 2006) is an underground (Estefania) and open pit (Lince) SX/EW operation principally owned and operated by Antofagasta Holdings (through a 74% interest in Minera Mincilla S.A.). Production in 2005 totalled 46 400 tonnes of Cu in cathodes. The quoted reserve in 2006 amounted to 32.1 Mt @ 1.23% Cu, with a further resource of 67.8 Mt @ 1.58% Cu.

Published reserve and resource estimates at 31 December, 2010 were as follows (Antofagasta PLC website, 2012):
    total measured + indicated + inferred resource - 56.2 Mt @ 1.78% Cu; including
    total proved + probable reserve - 5.8 Mt @ 1.53% Cu.

The most recent source geological information used to prepare this summary was dated: 2008.     Record last updated: 17/12/2012
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
 References to this deposit in the PGC Literature Collection:
Oliveros V , Trista-Aguilera D, Feraud G, Morata D, Aguirre L, Kojima S and Ferraris F,  2008 - Time relationships between volcanism–plutonism–alteration–mineralization in Cu-stratabound ore deposits from the Michilla mining district, northern Chile: a 40Ar/39Ar geochronological approach : in    Mineralium Deposita   v43 pp 61-78
Townley B, Roperch P, Oliveros V, Tassara A and Arriagada C,  2007 - Hydrothermal alteration and magnetic properties of rocks in the Carolina de Michilla stratabound copper district, northern Chile : in    Mineralium Deposita   v42 pp 771-789
Trista-Aguilera D, Joaquin Ruiz, Barra F, Morata D, Talavera-Mendoza O, Kojima S and Ferraris F,  2005 - Origin and age of Cu-stratabound ore deposits: Michilla district, Northern Chile: in   6th International Symposium on Andean Geodynamics (ISAG 2005, Barcelona),    Extended Abstracts: pp 742-745


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