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San Xavier North

Arizona, USA

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The San Xavier North deposit is located on the northern margin of the Pima District of southern Arizona, USA, some 3.5 km to the NNW of the main Mission open pit. The deposit lies within the Arizona-New Mexico Basin and Range Province.

Information on reserves is not readily available, as it is generally included as part of the Mission complex. All ore mined was trucked to treatment facilities at the main Mission complex. Production in 1989 was 2.1 mt @ 0.63% Cu. The 'guestimated' total production to 1994, predominantly from the chalcocite blanket, has been:

    30 to 40 Mt @ 0.8 to 1% Cu (Prod. to 1994, Mine visit, pers. comm., 1994)
      161 Mt @ 0.53% Cu (Production + reserve/resource Mutschler et al., 2004)

The San Xavier North deposit was found in 1955 when ASARCO geologists located two small outcrops of altered and mineralised arkose protruding through alluvial cover. Although leached of sulphides, the outcrops were recognised as representing porphyry style mineralisation. Drilling began in 1957 with testing on a 180 m triangular grid which was completed by 1958. The copper grade was too low to permit mining at the prevailing copper prices and as a consequence further work was postponed. In 1965 after a rise in the price of copper, a 90 m equilateral triangular grid was initiated to define the size and economic viability of the deposit. Some 60 holes were completed to an average depth of 180 to 200 m and open pit mining commenced in 1968 to produce copper bearing silica flux for ASARCO's Hayden and El Paso smelters. In 1973 copper oxide ore production began at a rate of 4000 tpd (King, 1982).

Geology

The geological setting of the Pima District is outlined in the record covering the Mission Complex orebodies. The ore deposit and host rocks are all within the allochthonous upper plate above the San Xavier Fault, as described in the 'Mission Complex' record. A single deeper drill hole passed through the 30 m thick San Xavier Fault at a depth of 600 m and passed into Middle Proterozoic granites and a wedge of lower Palaeozoic carbonates (King, 1982). It has been postulated that the Twin Buttes deposits in the autochthonous block below the San Xavier Fault may be the roots of the Mission Complex in the allochthonous plate, some 10 km to the NNW. It has been further suggested that the San Xavier North deposit 3.5 km further to the NNW may represent a still higher part of the same system (ASARCO, 1991).

The San Xavier North orebody originally only outcropped as two small exposures within a large area of alluvial cover. One outcrop was on the western most rim of the open pit, while the other was some 250 m to the southwest of the pit limits. Neither was over ore. All other information is based on drilling and subsequent mining. The orebody consists of three mineralogical entities, namely, 1). oxide copper zones, 2). supergene chalcocite and 3). hypogene sulphides. All are contained within Cretaceous clastic rocks of the Amole Arkose, a probable equivalent of the Angelica Arkose which is found in the Sierrita Mountains to the south (see the 'Mission Complex' description above) (King, 1982). The Amole Arkose is taken to be a foreland basin equivalent of the deeper Angelica Arkose to the south and is part of the lower Cretaceous Bisbee Group (S Titley, pers. comm., 1994).

Throughout the deposit the clastic rocks are an intimately interbedded sequence of mudstone, siltstone, fine to coarse grained sandstone and an occasional pebble conglomerate. The finer lithologies are only a minor component of the mineralised sequence. Most of these sediments are arkosic in composition. The individual sandstone beds are generally less than 6 m thick and comprise 40 to 65% quartz, 20 to 40% feldspar and 5 to 15% argillaceous matrix (King, 1982). Within the mineralised zone, near the base of the supergene blanket, the arkose is strongly altered with silicification, both pervasive and as quartz veins up to 3 mm thick, and sericitisation of the lithic feldspars. Within the leached and supergene enriched zone the sericite has been modified to white clays (Pers. observ.).

Minor conglomerates are interbedded with the sandstones with poorly sorted to sub-rounded pebbles of quartzite, feldspathic sandstone, siltstone and occasional granite set in the same matrix as the sandstones (King, 1982). Where sighted in the pit, conglomerates occurred as poorly defined, matrix supported bands within the arkose, often as a string of irregularly dispersed, well rounded grit to cobble sized clasts (Pers. observ.).

The siltstones and mudstones are argillaceous, with individual mudstone beds being <6 m, and usually <25 cm thick. The siltstones grade into fine sandstones and have thin intercalated laminae of fine arkose similar to the main arkose beds. The thicker mudstone beds have occasional calcareous nodules up to 1 cm in diameter (King, 1982). In outcrop the siltstone is generally verging on a fine sandstone and is very much subordinate to the arkose beds. Where sighted the outcrop was not fresh and commonly displayed strong clay alteration which may have been associated with either the hypogene mineralisation or the subsequent supergene enrichment process (Pers. observ.).

Bedding is difficult to discern within the fresh rock, although it is obvious within the upper weathered faces (Pers. observ.).

A number of quartz-monzonite porphyry dykes intrude the clastics, while a single larger mass of quartz-monzonite porphyry lies to the south of the present pit at depth and has only been encountered in some of the drilling. This quartz-monzonite porphyry has a very similar composition to those at the Mission Complex and at Twin Buttes, and is assumed to be of the same age. The dykes are composed of subhedral phenocrysts of plagioclase, orthoclase and quartz ±biotite set in a fine groundmass consisting essentially of quartz and orthoclase. Closely associated with the quartz-monzonite porphyry are intrusive and intrusion breccias in which clastic rock fragments and occasional porphyry xenoliths are sealed in an igneous matrix (King, 1982). The quartz-monzonite porphyry dykes are in-significant in volume within the pit. Only one was sighted during the visit. In hand specimen this comprised around 20% 1 to 3 mm white feldspar phenocrysts set in a fine grey siliceous matrix, with disseminations and fracture coatings of pyrite and chalcopyrite (Pers. observ.).

Structure

The sediments are asymmetrically folded with axes plunging to the north-west and axial planes trending at 345š and dipping at 70š to the south-west. Within the pit two antiforms and an intervening synform have been identified. Apart from the flat lying San Xavier Fault detachment, all other significant faults with >15 m of displacement are post-hypogene and post-supergene enrichment in age. The largest, located in the south-western part of the pit, has a steep south dip, a WNW trend and has had reverse movement.

Mineralisation & Alteration

Hypogene mineralisation is zoned. Ore grade (>0.5% Cu) hypogene mineralisation is developed entirely within a central core of 1 to 3% sulphide with a low pyrite:chalcopyrite ratio of between 1:1 and 1:3. Sulphide minerals include, in order of abundance, chalcopyrite, pyrite, molybdenite, bornite, sphalerite and galena. Mineralisation occurs as disseminations, as fracture fillings without gangue quartz and as discrete grains within quartz veins. A direct relationship exists between the grain size of disseminated and veinlet sulphides and the grain size of the sedimentary host, ie. the finer the host, the finer the sulphides. Anomalous Ag (>1 g/t) and Mo (>100 ppm) mineralisation is associated with the ore zone. Molybdenite commonly occurs either as aggregates of crystals within late quartz veins or as films on fractures and appears to be younger than most pyrite and chalcopyrite. The highest Mo is consistently in the quartz-monzonite porphyry intersections (King, 1982).

Within the central ore zone the better grade is generally controlled by the proximity to the main quartz-monzonite porphyry mass, although the porphyry rarely attains ore grade. A secondary control on grade is lithology. Statistical studies have shown that the hypogene ores in mudstones and siltstones averaged 0.4% Cu, while the coarser sandstones averaged less than 0.3% Cu. This is apparently due to the higher CaCO3 content of the finer sediments, and a higher concentration of partings, both along bedding and as cross fractures (King, 1982).

Surrounding this central core of low pyrite and higher chalcopyrite, there is a zone of higher pyrite typified by a total sulphide content of 2 to 4%, and a pyrite:chalcopyrite ratio of from 10:1 to 3:1. Pyrite predominates with lesser chalcopyrite, sphalerite, galena and molybdenite. Pyrite occurs as disseminations, as thin fracture fillings, as discrete grains in occasional quartz veins and as prominent veins up to 2.5 cm thick without significant addition of quartz. Mo and Ag average 10 and 0.2 ppm respectively in this zone. The same lithology controls apply with siltstone-mudstones and sandstones averaging 0.2 and 0.1% Cu respectively. There is a 30 m wide transition between the ore zone and the pyrite shell. At San Xavier North the disseminated mineralisation appears to be closely related to the earliest of the numerous multiple cross-cutting vein systems. The mineralised system appears to be a multiple event with veins becoming thicker, richer in gangue quartz and having wider selvages with time (King, 1982).

Observations within the pit confirmed Kings descriptions. The arkose is generally fine to medium grained and contains chalcopyrite which is present as interstitial disseminations throughout the arkose, as fracture coatings and within thin quartz veins occupying the abundant fracture sets cutting the arkose. The disseminated chalcopyrite within the arkose resembles that seen in sediment hosted deposits in that it occurs as interstitial grains which are similar in size to the arkose grain size. In places it appears to follow a preferred banding which may be bedding, although it could equally be a fracture set. The individual chalcopyrite grains within the arkose are generally from 0.3 to 1 mm in diameter. In contrast to sediment hosted deposits however, there appear to be no 'clouds' of sulphides developed. Quite a few possible 'clots' can be seen, comprising irregular accumulations of adjacent interstitial chalcopyrite grains. However it is hard to decide whether these are accumulations on a fracture along which the rock has broken. The other main difference of course is the abundance of veinlet mineralisation, as described below (Pers. observ.).

Disseminated sulphides are far less densely developed within the finer lithologies, with almost all being found along fractures and micro-fractures. The density of fractures however is greater in the finer lithologies, and these are more likely to be fine (Pers. observ.).

Thin plate-like accumulations of chalcopyrite are developed on fractures without associated quartz. These may be up to 1 cm in diameter and are sites of nucleation for subsequent supergene chalcocite (Pers. observ.).

The arkose has been cut by a dense well developed network of quartz veinlets from <1 mm to 3 mm in thickness. These veins have selvages from <1 to >10 mm in width and are developed in around 5 different directions. They appear to be largely quartz veins with quartz-sericite selvages and generally follow the main fracture directions, both within the arkose and the finer lithologies. Fracturing and selvages are best seen in the more oxidised rocks due the weathering contrast they produce (Pers. observ.).

Alteration - Classic phyllic alteration is exhibited by the host sediments. Although potassic and propylitic alteration zones are also recognised, they have not been outlined due to the lack of deep and peripheral drilling. The phyllic alteration in the pyrite shell is present as 5 to 20% hydrothermal sericite, minor epidote, kaolinite and virtually no quartz veining. Sericite partially replaces feldspar grains, while in the groundmass of the mudstones and finer siltstones sericite is closely associated with fine grained hydrothermal chlorite, actinolite and minor epidote. In the central ore bearing core, the hydrothermal alteration is characterised by 15 to 30% sericite, minor groundmass silicification, and a moderate abundance of quartz veins. Plagioclase grains are pervasively altered and are 45 to 90% converted to sericite and minor quartz, while K-feldspars are 5 to 30% sericitised. In the groundmass of mudstones and fine siltstones, chlorite is up to 50% converted to biotite. Quartz grains are sutured and secondary silica is found in the matrix. Late hydrothermal calcite and dolomite form up to 4% of the rock. Although a potassic zone has yet to be defined, a deeper hole passed through a mixed potassic and phyllic interval (King, 1982).

The supergene mineralisation at San Xavier comprises a leached capping, two different oxide copper zones and an economically significant chalcocite blanket. These are as follows;

1). Upper zone of oxide copper which is contained within the capping and was stranded during the main period of leaching and enrichment. Chrysocolla was the dominant oxide mineral, with varying amounts of malachite, azurite, neotocite and melaconite. These minerals occur as coating on fractures varying from 0.5 mm to a maximum of 25 mm, averaging <1.5 mm. In general the black oxides occur on the outside of the fractures and the carbonates and silicates in the centre. Siltstones have higher oxide grades, with Cu minerals mainly on fractures, while the sandstones have a lower grade and slightly higher percentage of disseminations and absorbed Cu oxide on altered feldspars. This zone is developed at the interface between the bedrock and the overlying alluvium. It appears to have been partially eroded, with chrysocolla being dominant at the interface. However with increasing depth chrysocolla declines and the Cu carbonates become dominant. It has been suggested that the chrysocolla was related to the erosional surface, and the originally this zone may have been a carbonate zone completely (King, 1982).

2). Lower oxide zone, which is intermixed with chalcocite and occurs at the base of the leached capping. It is a zone of mixed oxide and sulphide (chalcocite) mineralisation. This zone is the product of erosion and partial oxidation of the chalcocite blanket. With the exception of minor native copper and cuprite, the oxide copper minerals of the lower zone are the same as the upper zone, except that the lower zone Cu carbonates (azurite and malachite) are in equal proportions with the silicate (chrysocolla). Again chrysocolla appears to be replacing Cu carbonates (King, 1982).

3). Main chalcocite blanket, which is only developed where the hypogene mineralisation was exposed by pre-faulting erosion to oxidation and enrichment. Typically the chalcocite replaces chalcopyrite in preference to pyrite. Upgrading is at most two-fold, with only a single cycle of enrichment, as implied by a gradual increase of hematitic limonite with depth within the preserved leached capping (King, 1982).

Stratigraphy influences the thickness of chalcocite development. Siltstones and mudstones exhibit a high degree of chalcocite replacement because of the combined effect of their finer sulphides, higher chalcopyrite content and prevalence of fracture controlled sulphides. Because of this high degree of total sulphide replacement by chalcocite, the blanket in siltstone horizons is relatively restricted in thickness. The sandstones in contrast, show a lower percentage of chalcocite replacement, but develop a chalcocite enrichment zone over a greater thickness. This is the result of their greater porosity, larger sulphide grains and relative abundance of pyrite over chalcopyrite (King, 1982).

Within the supergene blanket chalcopyrite and pyrite are generally only partially replaced. The greatest degree of replacement is on fractures where sooty chalcocite has often fully replaced the chalcopyrite and pyrite. Within the arkose fresh chalcopyrite is present locally, generally further removed from a fracture, while those adjacent to a fracture are more likely to have been rimmed with chalcocite. Siltstones interbedded within the arkose are more likely to have enclosed un-replaced chalcopyrite than in the immediately adjacent arkose (Pers. observ.).

Within the supergene enriched zone the sericitised feldspars of the arkose have generally been kaolinised, particularly along the walls of fractures, corresponding to the best chalcocite development. Within the same zone the interbedded siltstones and mudstones have been more strongly bleached to a medium hardness white rock, probably alunite, even where fresh chalcopyrite remains (Pers. observ.).

4). Leached capping - which has proved difficult to rationalise, and does not apparently obey the normal rules. Above the chalcocite blanket mudstone and siltstone beds exhibit a false leached cap that shows essentially no evidence of disseminated sulphides, virtually no goethitic limonite or boxworks after chalcopyrite and only minor hematite. The adjacent sandstones display a 'true capping' in which the boxworks are indicative of the known supergene and hypogene mineralisation below. Typical supergene products seen in the leached capping are alunite, kaolinite, halloysite and dickite. The alunite is extensively abundant in the siltstones and mudstones. Its abundance may be the result of sulphide oxidation and iron leaching in a high Al environment (King, 1982).

In the upper benches of the pit a leached capping developed above the main supergene blanket is exposed, some 10 m or more below the original surface. The enclosing arkose and siltstones are strongly bleached, and in places have a pale pink hematitic tinge. The dense fracturing of the rock is accentuated by iron oxides on the fracture faces and the host immediately adjacent. Most faces are coated with iron oxides, the majority of which is goethite, largely after pyrite. However hematite is also well developed, although sporadically, as coatings on fracture faces up to several tens of cm's in diameter. Scratching of the 'hematitic' face reveals a variety of streaks ranging from bright red hematite to red-brown, poorly hematitic goethite, respectively representing strong ex-chalcocite to mainly ex-pyrite coatings. Bright hematitic occurrences may be found within a metre or less of strongly goethitic faces, emphasising the lack of uniformity in the development of the earlier, subsequently leached supergene mineralisation (Pers. observ.).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1996.    
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:
King J R  1982 - Geology of the San Xavier North Porphyry Copper deposit, Pima mining district, Pima County, Arizona: in Titley S R 1983 Advances in Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson    pp 475-485

   References in PGC Publishing Books: Want any of our books ? Pricelist
Cook S S and Porter T M, 2005 - The Geologic History of Oxidation and Supergene Enrichment in the Porphyry Copper Deposits of Southwestern North America,   in  Porter T M, (Ed),  Super Porphyry Copper and Gold Deposits: A Global Perspective,  v1  pp 207-242
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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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