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La Caridad

Sonora, Mexico

Main commodities: Cu Mo
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The La Caridad porphyry copper deposit is located some 80 km to the south-west of the Cananea mine, and 10 km to the east of Los Pilares in Sonora, Mexico (#Location: 30° 18' 56"N, 109° 33' 48"W).

Published reserve figures include:

   1500 Mt @ 0.40% Cu, 0.03% Mo (Res. 1992, AME 1995).
    750 Mt @ 0.67% Cu, 0.02% Mo (Res. 1979, Gilmour, 1982).
    700 Mt @ 0.72% Cu (Orig. Res. 0.4% cutoff, Coolbaugh, 1976).
    544 Mt @ 0.67% Cu (Res., 1989, Titley, 1992).
    8.4 Mt of contained copper - remaining reserves, 2014 (Grupo Mexico)

La Craidad is 96% owned by Grupo Industrial Minera Mˇxico SA de CV.

Geology

The La Caridad ore deposit is predominantly a supergene chalcocite blanket. It is centred on a 53 ±0.4 Ma Eocene quartz-monzonite porphyry stock and associated zone of intrusive breccias with which the main hypogene mineralisation is associated. The quartz-monzonite porphyry and its attendant breccias straddle the contact between larger masses of porphyritic quartz-diorite to the west, and granodiorite to the east. The intrusion breccias are elongated in a north-south direction and seem to follow the line of the original contact between the quartz-diorite and the granodiorite. This contact appears to represent a structural weakness that controlled subsequent intrusive activity. The quartz-diorite and granodiorite are part of a related batholith that extends for a considerable distance to the south. A small pre-mineralisation pegmatite body occurs in the centre of the quartz-monzonite porphyry stock. The supergene blanket is developed within all of these lithologies. The larger pluton in turn intrudes early Cretaceous or Tertiary volcanics. Post-mineralisation rocks are mainly rhyolitic tuffs and the La Caridad Fanglomerate which is cemented by hematite and contains fragments of the leached cap-rock to the deposit. Post-mineralisation andesite dykes cut the deposit (Saegart, et al., 1974; Salas, 1991).

The main quartz-monzonite porphyry mass is around 1 km wide and is elongated in a north-west direction through the centre of the deposit, over a length of approximately 3 km. Satellite quartz-monzonite porphyry intrusions occur to the north-east and south-west indicating a subordinate north-east trend of porphyry emplacement. The main lithologies are as follows (from Saegart, et al., 1974):

Quartz-diorite, is a grey-green, fine to medium grained rock that has a 'salt and pepper' texture. Megascopically it consists of approximately equal amounts of feldspar laths and anhedral clots of biotite. Petrographically it comprises 50 to 60% euhedral plagioclase laths and interstitial micro-crystalline biotite (20 to 30%) and quartz (15 to 20%).
Granodiorite, is predominantly light grey in colour and megascopically appears to consist largely of an intergrowth of subhedral plagioclase with interstitial quartz and anhedral biotite. It is medium grained, and has a petrographic composition of 35 to 40% euhedral plagioclase crystals, 20 to 25% micro-perthitic K-feldspar phenocrysts, 25 to 30% interstitial quartz and 10% subhedral biotite.
Quartz-monzonite (adamellite) porphyry, which when unaltered is a light pinkish-tan, but alters to light grey to white in colour. Where relatively unaltered it is composed of phenocrysts of quartz, medium grained euhedral feldspar and euhedral biotite, in a fine grained to aphanitic groundmass. The quartz phenocrysts occur as anhedral eyes 2 to 4 mm in diameter. Where the porphyry is highly altered, it can still be recognised by the quartz eyes, and to a lesser extent by the pseudomorphs of the biotite phenocrysts. The altered central part of the deposit is characterised by a sugary textured mosaic of fine grained quartz and sericite randomly studded by quartz 'eyes'. The phenocrysts, which constitute around 50% of the rock, are set in the micro-crystalline, sugary textured groundmass, and include 30 to 35% plagioclase, 5 to 10% micro-perthitic K-feldspar, 5 to 10% quartz, and 5 to 10 % biotite. The quartz phenocrysts are rounded to enlarged 'amoeboid' forms.
Pegmatites, consist of biotite and quartz, occasionally accompanied by microcline. The main pegmatite in the centre of the deposit is composed of very coarse grained biotite crystals intergrown with massive milky-white quartz. The biotite is generally chloritised.

The Breccias at La Caridad cut all of the principal lithologies. They usually consist of large blocks, tightly fitted against one another, so that the main evidence for brecciation is the occurrence of small 2 to 10 cm angular cavities. Careful observation reveals evidence of rotated fragments and sub-rounded to rounded fragments. Two classes of breccia have been described. The first of these is the intrusive breccias, which consist of irregular zones, or in some instances, pipe-like forms, in which fragments of various rock types are mixed. Rounded to sub-angular clasts of boulder to pebble size are common. The breccia fragments occur in a matrix of rock flour and smaller rock fragments. The large fragments are generally close together, with the matrix material forming a relatively small proportion of the rock mass.

The other main breccia type is the mono-lithic breccia, which only contains clasts of a single rock type. This breccia is extensively developed in the quartz-diorite, granodiorite and quartz-monzonite porphyry. Its distribution indicates two dominant structural directions, a north-west trend associated with the central mass of the quartz-monzonite porphyry, and a subordinate north-east trend through the centre of the ore deposit. The mono-lithic breccias are regarded as structural, rather than lithologic features. The intrusive breccia cuts the mono-lithic breccias and all pre-mineralisation rocks except the pegmatites (Saegart, etal., 1974).

Mineralisation and Alteration

The entire deposit is intensely fractured, with multiple fracture directions, although any linear zones that would indicate persistent structures are difficult to identify. Fractures appear to have provided important controls to the hypogene mineralisation, particularly towards the margins of the deposit. Both pre-mineral and post-mineral fractures influenced the supergene mineralisation. Petrographic studies have revealed that fracture control extends to the microscopic scale, and most of the apparent disseminated discrete grains actually occur as beads distributed along minute veinlets. Prominent fracture orientations generally trend north-east, north-west and east-west, the strongest being north-east (Saegart, et al., 1974).

The hypogene mineralisation comprises pyrite, chalcopyrite and molybdenite in order of decreasing abundance, together with minor amounts of sphalerite, galena and bornite. Pyrite is by far the most abundant hypogene mineral. The hypogene sulphides are associated with the quartz-monzonite porphyry stock, being present as stockwork veining, disseminations and breccia fill. Mineralisation is most intense in the centre of the deposit, where 60% of the pyrite and chalcopyrite occur as discrete grains in the aphanitic matrix and in altered biotite phenocrysts of the quartz-monzonite porphyry. The remainder of the sulphides in this area are present in veinlets and as crystal aggregates in breccia voids. There is a direct relationship between the amount of disseminated sulphide and the pervasiveness of quartz-sericite alteration. Sulphide filled breccia cavities are most abundant in the 'intrusive breccias' and adjacent 'mono-lithic breccias' which are developed along the north-south trend through the centre of the deposit. The amount of mineralisation occurring as discrete grains, or breccia cavity fill, decreases outwards from the centre of the deposit towards its margins where mineralisation is almost exclusively present as randomly oriented veinlets (Saegart, et al., 1974).

The overall hypogene grade is approximately 0.25% Cu. Sulphides comprise 2 to 3% of the rock, with a pyrite:chalcopyrite ratio near the centre of the deposit of around 2:1, increasing outwards to as much as 10:1 on the fringes. In the central two thirds chalcopyrite comprises 0.75 to 1% of the rock, decreasing outwards to an abrupt fall-off at the edge of the deposit. Pyrite in contrast, remains the same from the centre to the fringes. Molybdenite commonly occurs as aggregates of fine grained crystals, accompanied by variable amounts of quartz, in thin fracture fillings in the quartz-monzonite porphyry, although minor amounts of molybdenite occur as discrete flakes throughout the rock. Although pyrite, chalcopyrite and molybdenite commonly occur as intimate mixtures in fractures, most of the molybdenite occurs as 'paint' on fractures containing no other sulphides. Important quantities of molybdenite occur in the eastern section of the deposit where grades average 0.04% Mo. Molybdenite also occurs as significant, but sub-economic quantities within the granodiorite to the east of the deposit, but is only present in minor amounts in the western section, generally being <:0.01% Mo (Saegart, et al., 1974; Salas, 1991).

Alteration. The earliest observed alteration comprises a K-silicate phase. It is only sporadically preserved as a narrow zone fringing the subsequent central sericitisation, and as lenses within the sericite zone itself. K-silicate alteration is also inferred at depths of more than 500 m. The alteration assemblage is biotite, K-feldspar, quartz, chalcopyrite and molybdenite veinlets, particularly in the granodiorite to the east. Locally, K-feldspar veins several centimetres thick are observed with quartz, biotite, abundant molybdenite and subordinate chalcopyrite. The quartz-diorite around 1 km to the SSW, near the Guadalupe mine, contains abundant hydrothermal biotite accompanied by chalcopyrite. Lenses of K-silicate alteration within the quartz-monzonite porphyry and hangers of quartz-diorite, appear inside the sericite zone. These areas contain more than 1% Cu as chalcopyrite with a low associated pyrite content (Salas, 1991).

The K-silicate alteration is everywhere cut by subsequent sericitic veinlets. Sericitisation, or phyllic alteration is dominant at La Caridad, and is characterised by an assemblage of quartz-sericite-pyrite with tourmaline, chalcopyrite and molybdenite. The main hypogene mineralisation is encompassed by the quartz-sericite phyllic zone. Phyllic alteration in the central core of the hypogene zone is pervasive, obscuring the original rock texture. The central pervasive zone grades outwards, firstly into irregularly developed, elongate north-westerly and north-easterly trending zones. These zones have the same trend as the dominant veinlet and fracture sets, and as such appear to be structurally controlled. Pervasive phyllic alteration is centred within the ore deposit but is generally much less extensive than ore grade supergene mineralisation, except on the western margin. Additional pervasive phyllic mineralisation is also developed west of the ore deposit where Cu is only present in minor amounts (Saegart, 1974; Salas, 1991).

The pervasive phyllic alteration next grades outwards into a poorly defined, irregular argillic zone, although a distinct zoning pattern is not well developed. This argillic alteration consists of the replacement of plagioclase by kaolinite and montmorillonite and of biotite by chlorite, with quartz and K-feldspar being un-affected. Petrographic studies indicate that the argillic alteration was 'encroached upon' by the phyllic phase. Sericite first replaced previously kaolinised plagioclase sites, and then progressively replaced other minerals, except quartz in the groundmass. The argillic/phyllic zones pass outward into a narrow, poorly developed propylitic halo that lacks continuity, represented by the introduction of epidote, chloritisation of biotite, and partial replacement of plagioclase by montmorillonite with associated calcite and pyrite (Saegart, 1974; Salas, 1991).

Tourmalinisation is a minor but significant alteration product at La Caridad. It occurs as aggregates of very fine acicular, black, light green or colourless crystals in breccias, in veinlets and as tiny rosettes distributed throughout the rock (Saegart, et al., 1974). The intensity of phyllic alteration appears to decrease with depth through the zone of chalcocite enrichment to the hypogene mineralisation (Saegart, etal., 1974).

It has been inferred that the K-silicate alteration preceded the phyllic phase, and that the former was more widespread than preserved today, and more extensive than the phyllic alteration. Where preserved, the K-silicate assemblage contains low Cu and high Mo, although the chalcopyrite has a higher Cu content than that of the phyllic phase. The pegmatite occurrences, such as the Guadalupe deposit, are suspected to have been developed contemporaneously with the K-silicate alteration. Age dating of the Guadalupe pegmatite yields values of 54.0±1.6 Ma, around 1 Ma older than the 53.0±1.6 Ma obtained for the sericite alteration (Salas, 1991).

The supergene blanket at La Caridad is a flat tabular sheet which has been modified on the outer edges to resemble an inverted saucer, thickest in the centre and thinning towards the outer economic cut-off margins. Thinner and erratically distributed enrichment extends for a considerable distance beyond the edges of the economic deposit. The thickness cut-off is 15 m. The economic chalcocite blanket has a diameter of approximately 1700 m, averages 90 m in thickness, but reaches up to 250 m. It contains around 800 Mt @ 0.7% Cu, 0.012% Mo, in contrast to the 0.25% Cu of the hypogene mineralisation. The upper and lower margins of the supergene blanket tend to conform to the modern topography, although the downward slope on the fringes is not as steep as the surface. The irregular configuration of the upper surface is apparently entirely a function of leaching, influenced by fracturing density. The lower boundary has been defined on a 0.4% Cu cut-off, and is similarly irregular in detail (Saegart, 1974; Salas, 1991).

The main ore mineral in the supergene blanket is chalcocite, with rare covellite and digenite, together with remnant pyrite and chalcopyrite. The chalcocite generally occurs as a complete to partial replacement of hypogene chalcopyrite and pyrite. The degree of chalcocite replacement decreases with depth. In the upper section of the blanket chalcopyrite is generally completely replaced, while thin films of chalcocite are formed on pyrite grains. In the lower portions of the chalcocite blanket the chalcocite occurs as thin films on chalcopyrite. Chalcocite development has been recorded to as much as 600 m below the present surface. There is strong evidence at La Caridad for lateral migration during the formation of the chalcocite blanket. Ore grade chalcocite extends outward into fringe areas where the underlying primary Cu grade is generally <0.1% Cu. If chalcocite had formed solely by vertical descending Cu bearing media, the enrichment ratio would have been much higher in the fringe areas than in the centre of the deposit. Furthermore there is a paucity of indigenous iron oxides in leached cavities in the surface rocks overlying fringe area chalcocite ore (Saegart, 1974; Salas, 1991).

Leached capping. The chalcocite blanket underlies a preserved leached capping that averages 50 m in thickness, but ranges from 10 to 230 m. The remaining capping is thickest near the centre of the deposit, where the Cu content is highest, averaging 200 ppm. Surface rocks in fringe areas around the deposit contain distinctly higher Cu values, creating a geochemically anomalous halo averaging 400 ppm. There are essentially no oxidised copper minerals in the leached capping. Almost total leaching of Cu occurred because pyrite was sufficiently abundant in the hypogene ore, and the altered siliceous hosts were not reactive enough to neutralise the acid environment. The distribution of molybdenum in the leached outcrop reflects the underlying Cu grades more closely than the Cu content of the capping. In the eastern section of the ore 50 ppm Mo values in the leached cap rocks reflect the high molybdenum rich Cu ore below, while to the west, the average 15 ppm Mo levels overlies the lower grade Mo in this section. Soil sampling over the leached cap returned values of generally <200 ppm Cu, but 30 to 400 ppm Mo. Cu values increase to >500 ppm at the margins of the deposit where the chalcocite crops out locally (Saegart, et al., 1974; Salas, 1991). The leached capping comprises a layer of red and black hematitic leached rock representing an indigenous cap-rock to the deposit, particularly over the centre. This assemblage is assumed to have been derived from supergene mineralisation with a low pyrite:chalcocite ratio. The margins underlie jarositic-goethite cap rocks, which are taken to reflect higher pyrite:chalcocite ratios, suggesting lateral movement of Cu during the oxidation process. Fragments of this leached cap are contained within the overlying 24 Ma fanglomerate, indicating that enrichment took place during mid Tertiary time, prior to its burial by this unit and subsequent volcanics. It was exposed again in the late Tertiary and kaolinisation took place in both the oxidised zone and parts of the chalcocite blanket. The K-silicate halo, with its restricted pyrite content, underlies small goethite patches, in places accompanied by malachite, chrysocolla, neotocite, limonite and ferromolybdate (Saegart, et al., 1974; Salas, 1991).

The most recent source geological information used to prepare this summary was dated: 2000.    
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.


La Caridad

  References & Additional Information
   Selected References:
Saegart, W.E., Sell, J.D. and Kilpatrick, B.E.,  1974 - Geology and Mineralization of La Caridad Porphyry Copper Deposit, Sonora, Mexico : in    Econ. Geol.   v.69, pp. 1060-1077.
Valencia, V.A., Eastoe, C., Ruiz, J., Ochoa-Landin, L., Gehrels, G., Gonzalez-Leon, C., Barra, F. and Espinoza, E.,  2008 - Hydrothermal Evolution of the Porphyry Copper Deposit at La Caridad, Sonora, Mexico, and the Relationship with a Neighboring High-Sulfidation Epithermal Deposit: in    Econ. Geol.   v.103, pp. 473-491.
Valencia, V.A., Ruiz, J., Barra, F., Geherls, G., Ducea, M., Titley, S.R. and Ochoa-Landin, L.,  2005 - U-Pb zircon and Re-Os molybdenite geochronology from La Caridad porphyry copper deposit: insights for the duration of magmatism and mineralization in the Nacozari District, Sonora, Mexico: in    Mineralium Deposita   v.40, pp. 175-191.


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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