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Boleo

Baja California Sur, Mexico

Main commodities: Cu
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The Boléo Copper District is located near the town of Rosalía, in the state of Baja California Sur in Mexico. Historically it had been one of the largest producers of copper in Mexico during the first half of the twentieth century. Between 1886 and 1947 some 0.540 Mt of Cu was produced from 13.62 Mt of ore, at an average grade of 4.8% Cu (Wilson & Rocha, 1991).

The El Boléo deposits were discovered in 1868 by the local land-owner, while the first production, which averaged 20% Cu in the early years, was in 1874. The deposits were mined from 1886 by the French owned Compagnie du Boléo. Production peaked in the years up to 1917, but declined there-after with decreasing grade and thickness. By 1927 much of the exploitable ore had been worked. Drilling during that period however, outlined additional concealed reserves which were worked with difficulty from 1932 until 1940. Mining continued at variable rates in the district from 1938 until 1948, but was increasingly by small operators working the old mines that the Compagnie du BolŽo had abandoned. The Compagnie du Boléo closed its last mine in 1950. Underground workings within the district total more than 588 km of development (Wilson & Rocha, 1955). Known reserves of exploitable copper are low, although 'several' million tonnes of marginal reserves averaging 2% Cu still remain in deep, thin seams (Wilson & Rocha, 1991).

Geology

The stratigraphic succession in the district is as follows, from the base (Wilson & Rocha, 1955; Wilson & Rocha, 1991):

Pre Tertiary,
Quartz Monzonite - biotite quartz monzonite, which is only exposed in small erosional windows. These windows represent areas of high basement relief during the deposition of the overlying volcanics.
Nonconformity
Miocene,
Comondú Volcanics, >500 m thick - mainly non-marine andesitic and basaltic flows, tuffs, breccias, agglomerates, conglomerates and tuffaceous sandstones. At the close of Comondú deposition an irregular surface of strong relief, up to 450 m, was carved into the volcanics. The thickness of the unit may be as much as 1300 m in places, but thins to 65 m in the west.
Unconformity
Lower Pliocene,
Boleo Formation, 50 to 250 m thick, averaging 140 m - This unit hosts the main mineralisation. It is mainly composed of interbedded tuff and tuffaceous conglomerates, ranging from latite (trachy-andesite to trachy-basalt) to andesite in composition. These are intercalated with a local non-marine basal conglomerate, formed mainly on the slopes surrounding islands of Comondú Volcanics.
The tuffs and tuffaceous conglomerates of the Boleo Formation are underlain by, and/or are intercalated with, a quite persistent basal marine limestone, which is mainly thin, generally only 1 to 5 m thick, and impure. In places it rests on the basal conglomerate, but elsewhere is found immediately above the Comondú Volcanics. The limestone occupies the structurally palaeo-high zones, but does not extend very far inland. It is generally very impure, and is manganiferous and ferruginous, grading in places to a calcareous tuff, and to magnesian or dolomitic limestone. Chert lenses are also mapped within the limestone. Gypsum beds, ranging from 15 to 80 m thick are found in places near the base of the Boleo Formation, generally overlying the limestone unit, although in places they rest directly on the Comondú Volcanics. The gypsum may locally wedge out into tuffs. In part it is massive, and in others is weakly banded.
A few thin, 2 to 5 m thick fossiliferous sandstone lenses that have lateral extents of 50 to 200 m, are also found locally near the base of the Boleo Formation.
The bulk of the Boleo Formation is composed of alternating layers of tuff and tuffaceous conglomerate. The beds of greatest economic significance are certain distinctive layers of clayey tuff which contain the mined orebodies of the district. Five such ore beds are known, labelled from 4 to 0, from the base upwards. Each of these five layers is underlain by a conglomerate which grades eastward into tuffaceous sandstone towards the Gulf.
In each of the five cycles, the basal tuff bed grades upwards from clayey tuff, into sandy tuff and tuffaceous sandstone to conglomerate. The upper contact of each conglomerate is generally sharp, overlain by a persistent and widespread clayey tuff which marks the beginning of the next cycle. The tuffs are well stratified and latitic to andesitic in composition. They contain varying proportions of detrital grains and angular lithic, vitric and crystal fragments in a matrix of montmorillonite. The clasts are mainly feldspars, biotite, green hornblende, basaltic hornblende, clinopyroxene and magnetite. They are generally pink to purple, but may be bleached almost white. The conglomerate is generally poorly sorted, with angular to sub-angular pebbles to boulders. The grain size decreases rapidly to the north-east or Gulf-ward direction. The clasts are derived chiefly from the Comondú Volcanics, while the matrix is tuffaceous, similar to the main tuffs. Cross-bedding is common.
Unconformity
Middle Pliocene,
Gloria Formation, up to 185 m thick, averaging 60 m - commencing with a local basal conglomerate which is overlain by marine fossiliferous sandstone, followed in turn by an overlying conglomerate that becomes more marine inland. The basal conglomerate has poorly sorted pebbles to boulders, composed chiefly of Comondœ Volcanics. This conglomerate can only be distinguished where it is overlain by sandstones. Further inland where the entire unit is a conglomerate it cannot be differentiated from the overlying lithologies.
Local Unconformity
Upper Pliocene,
Infierno Formation, 50 to 140 m thick, averaging 55 m - fossiliferous marine sandstone, overlain by conglomerate.
Pleistocene,
Santa Rosalía Formation, 5 to 15 m thick - a thin marine fossiliferous sandstone, overlain by conglomerate.

The mesas and terraces along the steam beds are covered by Quaternary non-marine gravel beds that are 1 to 20 m thick (Wilson & Rocha, 1955).

All of the post Cretaceous rocks were deposited on the margins of the Gulf of California, which is still present immediately to th north-east.

The Comondú Volcanics were faulted prior to the deposition of the Pliocene beds, with, in some cases displacements of 50 to 100 m and tilting of 5 to 25° to the north-east. A fair proportion of the Pliocene and Pleistocene structure however, represents original compaction dips developed on top of the irregular Comondœ Volcanics surface, ranging from 30° in the basal limestone to between 3 and 20° in the clastic beds, and decreasing upwards. Superimposed on the original dips are the effects of subsequent tilting of 3 to 10° north-eastward, towards the sea. The sequence is also cut by numerous north to north-west striking and south-west or north-east dipping normal faults with displacements of up to 250 m, but more usually around 80 m (Wilson & Rocha, 1991).

Mineralisation

The main El Boleo copper deposits were contained in the dark, bentonitic, impermeable and soft clayey tuffs of the Boleo Formation. Five main beds of mineralised clayey tuff have been identified and worked. Each is immediately underlain by conglomerates or their seaward tuffaceous equivalents (Wilson & Rocha, 1991). The base of the orebeds and the ore is generally sharp, corresponding to the top of the underlying conglomerates, although the upper limits are irregular and diffuse. The majority of the production came from the No 3 Bed (Wilson & Rocha, 1955).

The mined area, excluding some outlying areas, was around 11 km long and 0.5 to 3 km wide, elongated in an north-westerly direction. The best ore is clustered around the seaward side of the projecting hills and ridges of basement rocks (Wilson & Rocha, 1955).

The thickness of the mineralised zone as exploited, averaged 0.80 m in later years, although in the older mines it reached 1 to 2 m, and exceptionally was up to 5 m thick. The average grade of the ore treated from 1866 to 1947 was 4.8% Cu, although the initial production was >8% Cu, subsequently decreasing to 3.5% Cu. Part of the orebodies were present as elongate, rib-like, sub-parallel 'pipes'. Veins and pockets of mineralisation are also found in the underlying Comondœ Volcanics (Wilson & Rocha, 1991).

The main ore minerals in the sulphide zone were very fine, granular chalcocite, accompanied by chalcopyrite, bornite, covellite and native copper, with lesser pyrite and galena. These sulphides were finely disseminated throughout the host clayey tuff. Microscopic examination indicates that the primary copper minerals, mainly chalcocite, have replaced the tuff, and that veinlets of chalcocite cut across the layers of the rock. Textural evidence suggest that the chalcocite crystallised after the host tuff was 'softened', ie. converted to clay. The primary ore appears as a dark coloured, compact, slightly moist clay with very little visible sulphide. The sulphide ore is generally sooty black to dark grey, which passes upwards into a lower grade, medium to light grey, hangingwall. Most of the best ore is massive clay without any visible structure, although some has a banded appearance. The chalcocite however, is present in the form of bands within the clay. Relict brecciated and porphyritic textures are sometimes recognisable. The ore beds have also been sheared with slickensides being common. Carbonised plant remains are found within the ore beds (Wilson & Rocha, 1955).

The oxide zone carried a large variety of oxides, carbonates, silicates and copper oxychlorides, generally to a depth of 25 m above sea level. The copper minerals were commonly associated with manganese oxides, while cryptomelane and pyrolusite were also found concentrated in separate seams. The gangue was mainly montmorillonite, together with other rock forming minerals, gypsum veinlets, calcite, chalcedony and jasper. The more abundant elements were Cu, Mn, Zn, Co, Pb, S and Cl. In addition Ni and Ag were slightly concentrated in the smelter products. The Zn levels increase markedly towards the hangingwall, averaging 0.8% Zn, but locally being up to 6% (Wilson & Rocha, 1991).

A separate field of manganese deposits is found immediately to the north of the Boleo copper district. The ore occurs both within the same clayey beds as the Cu mineralisation, but also in other clayey tuffs between the main Cu ore beds. The largest mine to 1955 had produced 0.215 Mt of ore (Wilson & Rocha, 1955). A high concentration of manganese is ubiquitous to all mineralised units, with average grades typically 2 to 3% Mn, although locally grades are as high as 20% Mn. In the northwestern part of the district, Mn was mined at the Lucifer deposit with a resource of 0.3 Mt @ 40% Mn (Conly et al., 2011).

The most recent source geological information used to prepare this summary was dated: 2005.    
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:
Conly A G, Beaudoin G and Scott S D,  2006 - Isotopic constraints on fluid evolution and precipitation mechanisms for the Boleo Cu-Co-Zn district, Mexico: in    Mineralium Deposita   v41 pp 127-151
Conly A G, Scott S D and Bellon H,  2011 - Metalliferous Manganese Oxide Mineralization Associated with the Boléo Cu-Co-Zn District, Mexico: in    Econ. Geol.   v.106 pp. 1173-1196


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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