Super Porphyry Cu and Au|
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The Cananea porphyry copper deposit lies within the eastern section of the Sonora Basin and Range Province of northern Mexico, ~75 km SE of Nogales, ~56 km east of Agua Caliente and 40 km to the south of the US border. It is currently mined in the amalgamated Buenavista open pit mine (#Location: 30° 57' 43"N, 110° 20' 6"W).
Mineralisation within the Cananea district occurs in a number of separate high grade orebodies over a NW-SE trending area that is ~3.5 x 10 km, overlain by a thick, extensive supergene blanket. These orebodies are present as disseminated-stockwork mineralisation closely associated with breccia pipes cutting Proterozoic granites, Mesozoic volcanics and late Palaeocene to early Eocene porphyry plugs (58 Ma). These breccia pipes are accompanied by skarn and manto mineralisation within adjacent lower to middle Palaeozoic carbonates (Bushnell, 1988) and are separated by lower grade disseminated and stockwork mineralisation. Most of the copper production in the mine at Cananea comes from open pit mining of an up to >500 m thick blanket of chalcocite (Meinert, 1982; Wodzicki, 2001). The pit produces both sulphide ore to be milled and SXEW (solvent-extraction electro-winning) heap leach ore.
The Buenavista-Zinc deposit is located immediately to the NW of the main Buenavista Cobre open pit. It comprises a zone of skarn alteration hosted sulphide mineralisation of zinc, copper, silver and lead. The deposit contains ~102.6 Mt @ 1.88% Zn, 0.47% Cu, 17 g/t Ag (Grupo Mexico, 2018).
Published reserve, resource and production figures for the Cananea operation are as follows:
Reserve, 1988 (Bushnell,1988) ........... 1800 Mt @ 0.7% Cu;
Reserve, 1993 (AME, 1995) ........... 1200 Mt @ 0.65% Cu;
Pre-1965 cumulative production (Sillitoe, 1976) ........... 100 Mt @ 2 to 3% Cu;
Cumulative production 1900 to 1979 (Meinert, 1982) ........... 2.28 Mt of contained Cu;
Cumulative production 1900 to 1979, La Colorada Pit (Meinert, 1982) ........... 0.5 Mt of contained Cu;
La Colorada breccia pipe (Valencia-Moreno et al., 2007) ........... 7 Mt @ 6% Cu, 0.4% Mo;
Cumulative production 1900 to 1979, Cananea Pit (Meinert, 1982) ........... 0.46 Mt of contained Cu;
Reserve, 1981, Capote Basin Skarn (Meinert, 1982) ........... 9 Mt @ 4.7% Zn, and 35 Mt @ 0.6% Cu;
Reserve, 1979, Kino-Colorado-Veta areas (Meinert, 1982) ........... 850 Mt @ 0.7% Cu, 0.01% Mo;
Production 1906 to 2004 (Grupo Mexico, 1979, 2004; Briggs et al., 2006) ........... 649 Mt @ 0.66% Cu; plus
Reserves 2004, Sulphide ore ........... 1975 Mt @ 0.61% Cu, 'Oxide' ore ........... 2517 Mt @ 0.27% Cu;
Total resource at Cananea (Singer et al., 2005) ........... 7140 Mt @ 0.42% Cu, 0.008% Mo, 0.012 g/t Au.
Reserves, 2018 @ 0.171% Cu cut-off for sulphide ore (Grupo Mexico, 2018),
Sulphide ore ........... 4102 Mt @ 0.443% Cu, 0.008% Mo;
'Leach ore' ........... 2652 Mt @ 0.163% Cu;
'Stockpiled ore' ........... 1676 Mt @ 0.161% Cu.
NOTE: Much of this summary is drawn from papers published prior to 1996. The following precis of the deposit as now understood is drawn from the Grupo Mexico 2018 Annual Report.
Precis, 2018 - The host sequence comprises Lower Palaeozoic calcareous sedimentary rocks, lithologically correlated with the similar section in southeastern Arizona, discordantly overlying Proterozoic granite basement. The entire section has been covered by a thick sequence of intermediate to felsic tuffs and flows of Triassic (?) and Jurassic age. These volcanic units are, in turn, overlain by 1500 m of andesite and dacite flows of Laramite age, accompanied by a series of coeval Laramide quartz monzonite intrusive pulses dated at 69 ±1 to 64 ±3 Ma.
Mineralisation in the district covers an area, of ~30 km2, and commenced with a pegmatitic stage with an associated assemblage of bornite-chalcopyrite-molybdenite, followed by an extensive flooding of hydrothermal fluids with quartz-pyrite-chalcopyrite. There is a generalised phyllic quartz-sericite alteration found throughout much of the Cananea district.
A large and economically important blanket of supergene enrichment composed of disseminated and stockwork chalcocite is developed below a layer of iron oxides. This layer follows the topography and has an average thickness of 300 m, but is up to >500 m. The chalcocite blanket is underlain by a mixed zone of secondary and primary sulphides which pass down into hypogene mineralisation, mainly composed of chalcopyrite, which extensively underlies the deposit. Molybdenite occurs throughout the deposit although its grade tends to increase with depth.
Deep drilling into the core of the deposit has confirmed that copper grades increase significantly with depth. This district is also noted for the presence of high-grade breccias occurring in clusters that follow the structural trend in the district. The known dimensions of the deposit in 2018 was 5 x 3 km and it is projected to persist over a vertical extent of >1 km. The deposit is now predominantly mined from the single large Buenavista Cobre open pit.
In 1982, the deposit was seen as follows. Skarn and manto mineralisation accounted for 6.5% of past production and 1.4% of reserves in 1981, while breccia pipes and supergene enriched zones made up the remainder of the past production and current reserves (Meinert, 1982). Most of the reserves consisted of supergene mineralisation (Salas, 1991). The most significant deposits had been worked in the La Colorada and Cananea-Sonora Hill pits, and in a series of more than fifteen underground mines (Velasco, 1966; Meinert, 1982; Bushnell, 1988). To 1965, more than 100 Mt of ore had been mined at grades which averaged between 2 and 3% Cu (Sillitoe, 1976; Salas, 1991). The La Colorada and Cananea-Sonora Hill pits hade respectively exploited breccia pipe, and disseminated supergene mineralisation, with underlying primary breccia pipes. These two pits accounted for around 20% and 30% of the districts production respectively (Meinert, 1982).
GEOLOGY - The host succession within the Cananea district may be summarised as follows (from Velasco, 1966; Meinert, 1982; Bushnell, 1988; Salas, 1991):
o Middle Proterozoic - the oldest rocks recognised in the district are present as the Cananea Granite, which has been dated at 1440±15 Ma. This body is composed of a range of granitoid lithologies whose upper contact is roughly conformable with the bedding of the overlying Capote Quartzite.
o Cambrian Capote Quartzite, approximately 150 m thick - a well bedded quartzite composed of quartz and sericite with abundant pyrite. According to Meinert (1982) it comprises a lower unit commencing with a basal coarse (up to 2 cm) quartz-pebble conglomerate, overlain by fine grained poorly sorted argillaceous quartzite and medium grained moderately sorted, poorly cemented quartzite. This is followed by a middle unit of alternating bands of medium grained, well sorted quartzite and thin layers of shale to argillaceous very fine grained siltstone. The upper unit is a well sorted, very fine grained arkosic quartzite. The Capote Quartzite has been correlated with the widespread Cambrian Bolsa Quartzite of southern Arizona and New Mexico;
o Cambrian to Carboniferous, >800 m thick - composed largely of limestones. The sequence conformably overlies the Capote Quartzite and commences with the:
• Esperanza Limestone which is around 100 m thick and in the Cananea district is thin bedded, highly altered and mineralised. This unit is correlated with the Cambrian Abrigo Formation and is described as medium bedded (0.2 to 1 m), thinly laminated (0.1 to 5 mm), grey-brown micrite to micro-sparite with minor dolomite, abundant thin interlayers of argillaceous micrite and shale, with common detrital silt layers and calc-silicate hornfels.
• Martin Formation, of Devonian age, which is 100 to 150 m and overlies the Esperanza Limestone following a time break. It is a thicker bedded, white, grey to dark-grey crystalline limestone which is made up of medium bedded (0.3 to 1 m) grey micrite with minor micro-sparite and dolomite. The crystalline beds have very little mineralisation, except at the upper boundary with the probable lower Carboniferous Chivatera Zone, which is 15 to 30 m thick and particularly favourable to mineralisation.
• The Chivatera Zone, has been correlated with the Mississippian Escabrosa Limestone in Arizona, and is described as thick bedded (0.3 to 1.5 m) argillaceous micrite and micro-sparite with argillaceous or cherty nodules and wavy interlayers, as well as calc-silicate hornfels layers. In the mineralised area, the Chivatera Zone limestones are completely altered to chlorite and epidote with erratic lenses of chalcopyrite, abundant sphalerite, pyrite, minor amounts of chalcocite and bornite and a little quartz and calcite.
• Puertecitos Formation, which follows conformably, and is >300 m thick. It comprises a considerable thickness of relatively thick bedded garnetised limestone, that is mineralised by erratic pyrite, chalcopyrite, sphalerite and minor galena. This unit is equated with the upper Carboniferous (Pennsylvanian) Naco Group which includes a lower facies, the Horquilla Limestone, a thick bedded (1.5 to 2 m) grey to red micro-sparite with less abundant micrite, and argillaceous or dolomitic micrite, with nodules and stockworks of chert, and calcite and clay in the lower sections. The Horquilla Limestone passes laterally and upwards into the main Naco Group which has a central 5 m thick marker unit of well sorted siltstone, overlain by the upper section of that group which has been intensely altered to skarn, but in general are less massive than the lower units. It is typically bedded at 0.1 to 1 mm based on the ratio of garnet to pyroxene, which is originally interpreted as representing relatively pure limestone with minor dolomitic units.;
o Mesozoic volcanics that are approximately 5500 m thick - and comprise the,
• Older Volcanic Group, of Triassic (?) to Jurassic age, which are >1800 m thick and are sub-divided into,
- Elenita Formation - rhyolite, trachyte and andesite flows and volcani-clastics, accompanied by agglomerates and tuffs;
- Henrietta Formation - dacite flows and agglomerate, with rhyolitic and andesitic flows and tuffs.
• Younger Volcanic Group, which overlie the Older Volcanic Group and are >1500 m thick. They are dated at a latest Cretaceous age of 67.4&plusm;3.4 Ma, and locally comprise the,
- Mesa & Maraquita Formations - andesitic and trachytic agglomerate and tuff.
Dips within the Palaeozoic sediments and the Older Volcanic Group generally range from 30° to 60°, while the Younger Volcanic Group dips at 10° to 20°. The block containing the Proterozoic Cananea Granite and the Palaeozoic sediments is bounded by the two most prominent faults in the district, the WNW trending, steeply inclined, Capote and Elisa Faults which have minimum throws of 200 m. Relatively recent, steeply dipping faults and fracture zones strike nearly north-south (Bushnell, 1988).
The sequence above is cut by the following intrusives (from Bushnell, 1988),
o Late Cretaceous to lower Palaeocene Cuitaca Granodiorite, Tinaja Diorite and El Torre Syenite. Age dating of a granodiorite in the north of the district has yielded an age of 64±3 Ma, while a determination on a quartz-monzonite (adamellite) to the west has returned a date of 69±1 Ma. These intrusives are voluminous, are coarse to medium grained, and intermediate to acid in composition. They intrude the Older and Younger Volcanic Groups, but pre-date the Tertiary quartz-feldspar porphyry.
• A less widespread suite of basic to intermediate intrusive rocks present as dykes and stocks associated with the Maraquita Formation and as the Campana Diabase (ie. dolerite). This suite includes numerous dykes, some of which are lamprophyric. Most are roughly parallel to the NW structural grain of the district. These rocks are also younger than the Younger Volcanic Group, but older than the Tertiary quartz-feldspar porphyry.
• Aplite dykes, occurring as a series of fine grained felsic intrusives that are scattered throughout the district. Some are co-magmatic with the Tertiary quartz-feldspar porphyry, and many, but not all, are earlier than the mineralised breccia pipes.
• Tertiary quartz-feldspar porphyry is present as a series of plugs and stocks associated with the hydrothermal alteration and mineralisation of the district. Quartz, K-feldspar, plagioclase, and locally biotite, occur in varying proportions as medium to coarse grained phenocrysts. Matrices are predominantly fine grained, generally aphanitic, and are composed of quartz and orthoclase. A narrow belt of small plugs of this porphyry approximately coincides with the axis of commercial mineralisation in the Cananea district. Each is generally less than a few hundred metres across, to a maximum of 1 km. A number coalesce at depth. None carry significant ore, although one, the 8-110 stock, was host to some economic mineralisation. The porphyry intrusions themselves must be slightly older than the age of mineralisation detailed below.
MINERALISATION & ALTERATION processes at Cananea have produced prograde skarn which predates the development of retrograde skarn alteration. The retrograde skarn is coeval with the introduction of the Tertiary quartz-feldspar porphyry plugs and the contemporaneous formation of breccia pipes. The emplacement of the main mineralisation accompanies the introduction of the porphyry plugs, and hence the develpment of the retrograde skarn assemblages. This series of processes is described in sequential order below.
The mineralisation in Cananea took place between 58.5±2.1 Ma, the dating of K-Ar age alteration biotite, while sericite in a large mineralised porphyry plug yielded a younger age of 52.8±2.3 Ma (Barton et al., 1995), although a K-Ar age in phlogopite from the La Colorada breccia was dated at 59.9 ±2.1 Ma (Damon et al., 1983).
Significant skarns, mantos* and breccia pipes are localised within a 2 x 4 km horst of Palaeozoic carbonates and lesser quartzites.
Pro-grade Calc-silicates and Hornfels - Calc-silicates were formed first by an early metamorphism that converted impure carbonate lithologies into iron poor garnet-pyroxene±idocrase hornfels. Subsequent metasomatism formed pro-grade garnet-pyroxene skarns along the pre-Cretaceous Elisa Fault contact between carbonate rocks and Mesozoic volcanic rock. The skarn is zoned from an andradite-rich centre, through a interval of both andraditic garnet and salitic pyroxene, to a mineralogic sequence near the marble front which is largely a function of the sedimentary protolith, as follows; chert nodules are rimmed by wollastonite, dolomite is converted to massive phlogopite-magnetite skarn, calc-silicate hornfels is over-printed by veins of skarn garnet and relatively pure marble is replaced by coarse blades of iron and manganese rich pyroxene. Pyrite and minor chalcopyrite are the only sulphides associated with this stage of metasomatism (Meinert, 1982).
Retrograde Skarn - Within the skarns, following the main pro-grade stages of garnet and pyroxene formation, veins and orbicular patches of retrograde amphibole±quartz±calcite occur replacing pyroxene and in some cases garnet. Most of the amphibole is actinolitic and is associated with pyrite and minor amounts of chalcopyrite. The subsequent destruction of pro-grade skarn by alteration related to brecciation, breccia-pipe formation, and the replacement of previously un-altered carbonate rocks by mantos of iron oxides and sulphides, resulted in some of the highest grade bodies. Breccia pipe and manto formation appears to have been largely contemporaneous with emplacement, and subsequent sericitic alteration of, a series of quartz-monzonite porphyry stocks. Where breccia pipes cross-cut skarn, the garnet, pyroxene and amphibole are converted to mixtures of calcite, quartz, chlorite, hematite, siderite and sulphides. Where breccia pipes cross-cut previously un-altered carbonate rocks, mixtures of magnetite, sulphides, chlorite, siderite, calcite, quartz and serpentine form massive mantos (Meinert, 1982).
Within the skarn-destructive alteration (or retrograde) zones there is a zonation of sulphides and alteration relative to the locus of the larger breccia pipes. The more intense calcite-chlorite-hematite alteration coincides with the core of the breccia pipes, while sulphides are zoned to form a pattern of:
i). a central core of Cu only, with chalcopyrite which is accompanied by bornite at depth and lesser pyrite, while pyrite with lesser chalcopyrite is found at shallower levels; surrounded by,
ii). a Cu-Zn zone with sphalerite, chalcopyrite and pyrite, but seldom accompanied by bornite; passing out into,
iii). a limited Pb zone consisting of galena with sphalerite, chalcopyrite and pyrite. Pyrite is common throughout extending beyond the zone of abundant Cu, Zn and Pb.
Sulphide minerals occur within the skarns as either veins, disseminated grains or as breccia fill, while the nearby calc-silicate minerals are destroyed by calcite dominant alteration. In almost all cases where sulphide minerals occur in skarn as veins, disseminated grains or breccia filling, the adjacent calc-silicates are destroyed by calcite-dominant alteration. Sulphide mineralisation can occur as calcite-chalcopyrite±pyrite or calcite-chalcopyrite-sphalerite±pyrite veins often with sharp margins. Hypogene sulphide mineralisation may also occur as pyrite, chalcopyrite and locally sphalerite or bornite in the calcite matrix, cementing calcite-chlorite-hematite altered skarn breccia fragments. Characteristically, the Cu sulphides occur only in the breccia matrix and not in the clasts (Meinert, 1982).
Manto mineralisation occurs near porphyry contacts where breccia pipes encounter marble, or skarnoid with a large percentage of remnant marble. The alteration assemblage produced has a similar mineralogy to that described for the skarn destructive breccia related process above, but differs greatly in texture owing to the massive replacement of carbonate rocks by the 'stratabound mantos of iron oxide and sulphides. Generally mantos are restricted to the upper Cambrian Abrigo Formation and the Devonian Martin Formation near breccia pipes, quartz-monzonite porphyries and the contact with the underlying Bolsa Quartzite. In hand specimens and thin section the texture of the original rock is usually preserved rather than brecciated during manto formation. Where the original lithology was thick bedded, manto replacement consists of massive, sometimes mono-minerallic layers, while in thin bedded hosts, the laminations are preserved as layers of contrasting mineralogy. Where skarnoid is interlayered with marble, the resultant manto has interbanded layers high in iron oxides in the ex-skarnoid bands, while the marble is replaced by sulphides and/or siderite. Towards the centre of the manto, magnetite-sulphide replacement becomes more massive, retrograde alteration of skarnoid calc-silicates becomes more intense, the abundance of chlorite and clay replacing marble increases, and the sulphide assemblage changes. Although mixed magnetite-sulphide intervals are encountered it is more usual to encounter alternating layers <1 cm thick of massive magnetite or sulphide and chlorite, clay or siderite. Within the mantos, later veinlets of pyrite and chalcopyrite cross-cut magnetite layers and commonly have siderite envelopes (Meinert, 1982).
Veinlets of calcite and calcite-magnetite which have chlorite envelopes; and veins of pyrite±sphalerite±galena, with or without chlorite envelopes cutting slightly altered limestone, are found on the margins of the manto. Distal veining of magnetite-calcite occurs along, or across bedding, in otherwise un-altered limestone. While marble is altered to chlorite-clay adjacent to veins and in the massive manto, the skarnoids are retrogressed to epidote-chlorite-serpentine. These veins extend for tens of metres beyond the zones of massive replacement (Meinert, 1982).
The breccia pipes at Cananea may be broadly defined as structures of brecciated rock with long vertical and roughly elliptical horizontal or plan dimensions. The elliptical axes of most pipes are generally around 1.5:1, up to an extreme of 5:1. All are nearly vertical, with a fairly uniform trend and plunge of around 70° in a direction of 300°. The breccia pipes generally contain rotated fragments of country rock and a high proportion of inter-fragmental void space containing hydrothermal mineralisation. Features mapped as breccia pipes also include those without pronounced brecciation, which are better described as fracture pipes. Some contacts between breccia pipes and country rock are transitional, passing from breccia, to fractured country rock, to un-broken rock. The only apparent clear-cut control over the distribution of pipes is the disposition of the Tertiary quartz-feldspar porphyries, although some appear to be related to basement structures. With a few exceptions, the elliptical pipes which persist over a significant vertical interval increase in diameter with depth (Bushnell, 1988).
Most breccia within the pipes consists of angular to sub-angular fragments of country rock. These commonly range from a centimetre to several tens of centimetres in diameter, although they may range from as small as a millimetre to as large as several metres. There is generally little recognisable rock flour in the breccia pipes, although some may have been altered beyond recognition. The fragments are composed of wall rocks found adjacent to the pipe, from a similar or higher level. In the Capote pipe for example sandstone clasts are encountered 300 m below the lowest sandstone in the walls of the pipe. Several of the pipes at Cananea with much original open space and rotated clasts, do not outcrop at the surface and have known or inferred upper terminations. The marked upward decrease in the diameter of some outcropping pipes implies that they too, ended below the original surface. Rock immediately above the sub-surface roofs of some breccia pipes contain stockwork fracture zones, while the upward projection of others would correspond to anomalous alteration at the surface (Bushnell, 1988).
The Cananea-Duluth breccia pipe is the most extensively mined. It transgresses gently dipping members of the Mesozoic Younger Volcanic Group to the greatest depths mined. The roughly elliptical pipe has plan dimensions of 55 x 315 m near the surface, which increase downwards to 75 x 345 at a depth of 400 m. The corners of the pipe appear to be influenced by intersecting fracture sets in the country rock. There is a distinct contrast in intensity of brecciation and mineralisation between the core of the pipe and its rim. Intensely brecciated rock with fragments that average several centimetres in mean diameter occurs immediately inside the country rock contact. This part of the breccia contains a high proportion of sulphides and is known as the 'ore ring'. Its contact with the surrounding volcanics is generally sharp, though local fracturing of wall rock immediately beyond the pipe can make the contact indistinct. Very locally the 'ore ring' may deviate inward slightly from the outer contact of the pipe. Fragments in the 'ore ring' are predominantly angular to less frequently sub-angular, and are generally chaotically arranged. The inter-fragment 'voids' contain hydrothermal minerals, mainly quartz and fine pyrite, chalcopyrite, tetrahedrite, and open space. The 'ore ring' is usually 1 to 3 m wide along the sides of the pipe, but widens to 10 to 20 m at the 'noses' where brecciation is particularly intense (Bushnell, 1988).
Inside the 'ore ring', the core of the Cananea-Duluth pipe is less intensely brecciated, containing a higher proportion of large blocks of the Younger Volcanic Group which are often gently dipping slabs several metres long. Fragments around them range in size down to several centimetres or less. The slabs are commonly parallel to each other. In at least a few places, fragments across most of the width of the pipe have a low apparent dip similar to that of the un-disturbed country rock. The large size and arrangement of the fragments in the core of the pipe locally result in correspondingly large vugs lined with quartz, carbonate and small quantities of sulphide. These are scattered and not as interconnected as those in the 'ore ring'. Cross structures containing economic sulphides and relatively intense alteration occur in outlying weakly altered or un-altered country rock, and occasionally contain well developed brecciation (Bushnell, 1988).
The main Colorada pipe, within the La Colorada pit, differs from the general pipes described in the district. It occurs at the top of a porphyry stock cutting Mesozoic volcanics, and its shape mimics that of the stock which surrounds most of the pipe. The diameter of the accompanying porphyry increases rapidly with depth below the ore The top of the massive mineralisation within the pipe is more than 100 m below the surface. The Colorada pipe is sub-divided into a) massive hydrothermal mineralisation containing very few rock fragments, and b) a late breccia consisting of clasts of the first style of mineralisation and its wall rock within a finely comminuted matrix. In its upper levels the pipe forms an oval shape 100 x 140 m, with an outer cylindrical shell of massive mineralisation up to 15 m thick, with a shell of the breccia of similar thickness immediately and concentrically inside. With increasing depth the massive mineralisation and breccia expand to occupy the majority of the pipe, which has a diameter of 80 m some 220 m lower. The bottom of the breccia zone, some 280 m below the top of the mineralised zone, is above the bottom of the massive ore, which is around 330 m below the top of the ore. Below the breccia, there is a zone containing abundant seams and disseminations of mineralisation, including 'splotches' or 'bunches' of massive mineralisation. These seams persist at depth below the bottom of the massive mineralisation, forming a cylindrical zone 50 to 60 m or less in diameter, with a barren core (Bushnell, 1988).
The massive mineralisation at La Colorada occurs as zones in which assemblages of quartz-biotite±feldspar and chalcopyrite-bornite-molybdenite each predominate. The texture of much of the silicates is pegmatitic, forming an outer ring composed chiefly of glassy quartz and phlogopite. The sulphides post-date the silicates and tend to occur within and above the silicates. The massive and the brecciated sulphides are found within this outer ring. Chalcopyrite veinlets cut across the earlier silicates and extend into the wall rock which carries earlier pyrite. A second, volumetrically less important sulphide stage of pyrite, Cu [Fe] sulphides, galena, sphalerite and sulpho-salts was deposited during the later brecciation and associated fracturing. Alunite was the last hypogene mineral emplaced. Alteration of rocks around the La Colorada pipe and within its core is sericitic and probably also argillic. Similar alteration accompanied the late brecciation, second stage sulphides and alunite. Hydrothermal biotite and feldspar within the pipe have been altered in part to clay and sericite. A halo of strong silicification is found around the lower levels of the orebody (Bushnell, 1988).
Early alteration and mineralisation at Cananea is different in the quartzo-feldspathic rocks to that in the limestone. In the quartzo-feldspathic rocks, K-silicate alteration is accompanied by disseminated sulphides and is only of significance at depths of greater than 500 m. Thin veinlets and fine fractures contain quartz, orthoclase, sulphides and apatite.
In the limestones, skarn was developed in several parts of the district. The main early skarn is distributed along the Elisa Fault rather than fringing sericitised porphyry, as described in detail above (Bushnell, 1988: Salas, 1991).
Late alteration and mineralisation in different deposits at Cananea shares the characteristics of high overall hypogene pyrite:chalcopyrite ratios of more than 1:1, although this ratio changes with position in the zoning pattern. The sulphides share a common paragenetic sequence of: pyrite-chalcopyrite and/or bornite-sphalerite+tetrahedrite+galena for the bulk of the mineralisation wherever a sequence can be documented.
In the quartzo-feldspathic rocks, intense and pervasive sericitisation is the dominant alteration which is associated with disseminated deposits, as well as with breccia pipes within these deposits, including breccia pipes that do not pass through limestone at depth. It produces a silicate assemblage containing almost exclusively quartz and sericite, with pyrite, chalcopyrite and limited amounts of molybdenite. Common accessory minerals are rutile, apatite, tourmaline and chlorite. Propylitic alteration is marginal to the sericitisation, forming a halo, characterised by chlorite, epidote and pyrite. In disseminated deposits, there are three generations of veinlets, namely, 1) irregular veinlets consisting almost entirely of granular quartz without alteration envelopes; 2) intensely banded veinlets with early fine grained comb-quartz, which has variable sulphide and sericite in the centres, and poorly developed selvages; and 3) veinlets consisting almost entirely of sulphides which may have conspicuous alteration envelopes containing abundant sericite. Sulphides are predominantly pyrite and chalcopyrite with minor amounts of molybdenite, bornite, tetrahedrite, carbonate, hematite, chlorite and alunite. The last sulphides formed are sparse veinlets of chalcopyrite, sphalerite and galena. The development of breccia pipes can be demonstrated to be broadly contemporaneous with the development of the second vein set above, post the first, and prior to the third. Disseminated mineralisation generally surrounds the breccia pipes. In the most extreme cases at Cananea, abundant contemporaneous, fracture controlled disseminated mineralisation and alteration surrounds pipes (Bushnell, 1988). Hypogene disseminated mineralisation within the sericite alteration attains grades of up to 0.6% Cu (Salas, 1991).
Late alteration and mineralisation within breccia pipes is dominated by quartz, while white phyllo-silicates are common in many. Tourmaline occurs locally in some pipes and breccias of the district, but is abundant in others. Molybdenite is present in small amounts, although its position in the paragenetic sequence is variable - often occurring with the early quartz, or after pyrite. Small quantities of alunite are found in many pipes, and are typically the matrix in those pipes. Vertical zoning within pipes suggests that quartz and pyrite are distributed throughout the vertical column, while bornite and chalcopyrite give way to sphalerite, tetrahedrite and galena upwards, and molybdenite occurs in the upper sections of the Cu zone. Intense alteration and sulphide mineralisation in and around most breccia pipes at Cananea are continuous to depths whose maxima are unknown, but exceed 500 m. Grades within massive sulphide zones in the breccias are not stated anywhere. It can only be assumed from historic production figures that they are of the order of 2 to 3% Cu.
Replacement deposits in limestones are present where breccia pipes or porphyry margins come into contact with the Palaeozoic carbonates, and are interpreted as being developed contemporaneously with the breccia and disseminated orebodies, as described in the "skarn" and "manto" paragraphs above (Bushnell, 1988).
Supergene mineralisation comprises most of the mineral reserves at Cananea, and consists of enriched mineralisation in 150 m thick seams, on average, developed over a thickness that can be >500 m in thickness. Residual hematite at the surface indicates the presence of a multi-cyclic chalcocite seam at depth. Minor amounts of jarosite and goethite are also present, mainly at the edges of the mineralised body (Salas, 1991). This supergene alteration is apparently developed over both fracture controlled disseminated mineralisation in both porphyry plugs and volcanics, and over breccia pipes. In many of the high grade hypogene mineralised centres the higher grade bands of enriched chalcocite are localised along and near wide, north-south striking, steeply dipping fracture and shear zones. Their trends are often locally interrupted by zones of brecciation, in which case the ore takes on the roughly circular or otherwise irregular shape of the breccia structure. Oxidation reaches a considerable distance below the normal level of surface oxidation in these zones (Velasco, 1966).
The most recent source geological information used to prepare this summary was dated: 2018.
Record last updated: 31/1/2021
This description is a summary from published sources, the chief of which are listed below.
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Bushnell S E 1988 - Mineralization at Cananea, Sonora, Mexico, and the Paragenesis and Zoning of Breccia Pipes in Quartzofeldspathic Rock: in Econ. Geol. v 83 pp 1760-1781|
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Megaw, P.K.M., Ruiz, J. and Titley, S.R., 1988 - High-temperature, carbonate-hosted Ag-Pb-Zn(Cu) deposits of Northern Mexico: in Econ. Geol. v.83, pp. 1856-1885.|
Meinert L D 1982 - Skarn, Manto, and Breccia Pipe Formation in Sedimentary Rocks of the Cananea Mining District, Sonora, Mexico: in Econ. Geol. v77 pp 919-949|
Santillana-Villa, C., Valencia-Moreno, M., Del Rio-Salas, R. and Ochoa-Landin, L., 2021 - Geochemical variations of precursor and ore-related intrusive rocks associated with porphyry copper deposits in Sonora, northwestern Mexico: in J. of South American Earth Sciences v.105, 22p. doi.org/10.1016/j.jsames.2020.102823.|
Valencia-Moreno, M., Ochoa-Landin, L., Noguez-Alcantara, B., Ruiz, J., and Perez-Segura, E., 2007 - Geological and metallogenetic characteristics of the porphyry copper deposits of Mexico and their situation in the world context: in Alaniz-Alvarez, S.A. and Nieto-Samaniego, A.F., (Eds.), 2007 Geology of Mexico: Cele brating the Centenary of the Geological Society of Mexico: Geological Society of America, Special Paper 422, pp. 433-458.|
Velasco J R 1966 - Geology of the Cananea district: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 245-249|
Wodzicki W 2001 - The Evolution of Magmatism and Mineralization in the Cananea District, Sonora, Mexico: in Albinson, T. and Nelson, C.E. (Eds.) 2001 New Mines and Discoveries in Mexico and Central America Econ. Geol. Special Publication 8 pp 241-261|
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