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Mission Complex (Palo Verde, San Xavier South & North, Pima, Eisenhower ) |
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Arizona, USA |
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Cu Mo
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Super Porphyry Cu and Au
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The Mission Complex comprises a series of orebodies, originally mined individually. These have included the Pima, Palo Verde, Mission and Eisenhower deposits, all now within a large single pit complex. The nearby San Xavier North deposit has been exploited in a separate open cut. All of the mines of the complex are now operated by the Grupo Mexico subsidiary ASARCO Inc.. Historically the district was an underground producer of lead-silver and copper until 1955 when the disseminated Cu orebodies were outlined (Titley, 1989).
(#Location: Mission pit - 31° 59' 18"N, 111° 4' 5"W).
In 1950 the Banner Mining Company commenced production at Mineral Hill, an underground mine developed on a 2% Cu skarn zone. Magnetic surveys followed a magnetite rich skarn zone from Mineral Hill where it outcropped, to the Daisy and then the initial high grade Pima orebody, which was concealed by up to 60 m of younger cover. This mineralisation was localised within a structural zone. ASARCO staff visited the Daisy Shaft during the 1950's when Banner were looking to sell the property. They recognised the porphyry and alteration that implied the possibility of accompanying 'non-magnetic' pervasive mineralisation in addition to the 'shear controlled' magnetic zone at Daisy. A 'pegging battle' ensued between Banner (later to be incorporated into Cyprus-Pima) and ASARCO. Drill testing and the subsequent discovery of the main Pima and Mission orebodies ensued, as well as the other components of the current Mission Complex (Titley, 1982; Titley, 1989; Mine Staff and S Titley, pers. comm., 1994).
The deposits are within the Arizona-New Mexico Basin and Range Province in southern Arizona, to the south of the Colorado Plateau.
Published production and reserve figures include:
Mission Complex, including - Mission, Palo Verde, San Xavier South, San Xavier North, Pima, & Eisenhower - predominantly skarn ore-
Production to 1994 - 600 Mt @ 0.67% Cu (Mine visit briefing, 1994),
Reserve 1992 - 575 Mt @ 0.67% Cu, 4.8 g/t Ag (Am. Mines H'book, 1994),
Reserve 1989 - 302 Mt @ 0.67% Cu, 4.5 g/t Ag (Titley 1992),
Mission orebody - Production 1959-78 - 100 Mt @ 0.63% Cu, 2.3 g/t Ag, 0.01% Mo (Titley, 1992),
Pima orebody - Production 1955-77 - 180 Mt @ 0.48% Cu, 2.1 g/t Ag, 0.004% Mo (Titley, 1992).
Production from the various ore sections within the Mission Complex open pit in 1989 was as follows, Mission - 5.0 Mt @ 0.71% Cu; Pima - 2.75 Mt @ 0.84% Cu; San Xavier South - 0.96 Mt @ 0.37% Cu; and Eisenhower - 2.18 Mt @ 0.50% Cu (ASARCO, 1991).
In 1994 the current had dimensions of 2.5 x 2 km at its widest and longest (Mine Staff, pers. comm., 1994). Total metal production at the Mission Complex in 1992 was 105 000t of copper and 57 t of Ag (Am. Mines H'book, 1994). Molybdenum, although present, was not being retrieved (Mine Staff, pers. comm., 1994).
Geology
The sequence within the Mission-Twin Buttes-Sierrita/Esperanza district, may be summarised as follows (from Barter & Kelly, 1982; Jansen, 1982; Titley, 1982b; Titley, 1989), from the base:
Mesoproterozoic basement, represented by 1400 Ma xenolith rich exposures of granite. The xenoliths are mainly of the early Mesoproterozoic Pinal Schists. In outcrop south of Mission this granite was seen to comprise a coarsely crystalline, mafic rich (around 20%) rock. It has rounded quartz grains from 3 to 10 mm in diameter and a preferred foliation within the mafic minerals. Abundant xenoliths of dark chloritic rock are found within it, while patches of pegmatoid containing coarse pseudomorphs after pyrite are obvious. It is very easy to distinguish from the Laramide intrusives.
Palaeozoic sediments, totalling around 1500 m in thickness, commencing with Cambrian quartzite, Devonian to lower Carboniferous sequences of predominantly limestones, followed by Upper Carboniferous to Permian limestone, quartzite and marl. These are:
Cambrian,
• Bolsa Quartzite, 50 to 90 m thick - quartzite with a local basal conglomerate.
• Abrigo Formation, +170 m thick - interbedded quartzite shale and siltstone, overlain by a comparable thickness of interbedded limestone and siltstone.
Devonian,
• Martin Formation, approximately 70 m thick - dolomite, limestone and local black shales and siltstone. This unit commonly constitutes a décollement surface regionally, generally along the black shale units
Carboniferous - Mississippian,
• Escabrosa Limestone, 60 to 140 m thick - limestone, partly cherty.
Carboniferous - Pennsylvanian,
• Black Prince Limestone, 0 to 10 m thick - chert and limestone pebble to cobble conglomerate with local ferruginous shale.
• Horquilla Limestone, 170 to 300 m thick - limestone with siltstone beds and a basal quartzite.
Carboniferous to Permian transition,
• Earp Formation, 70 to 110 m thick - siltstone and sandstone, partly limy and dolomitic, with both limestone and shale interbeds.
Permian, which in the Mission Mine area, includes:
• Colina Limestone, 70 to 115 m thick - limestone with thin beds of siltstone, anhydrite and local quartzite.
• Epitaph Dolomite, 250 m thick - composed of,
- a stratigraphically lower 10 to 20 m thick limestone which is commonly altered to a brown garnet tactite in the mine area; followed by
- 22 m of light to dark green siltstone and marl, the limestone members of which are altered to a distinctive light brown garnet and diopside tactite; succeeded by
- a probable original mixed clastic and carbonate unit, 6 to 10 m thick, which in the mine area is represented by a green-brown epidote-garnet tactite; and
- an uppermost unit consisting of 12 to 20 m of anhydrite.
Within the pit, where not altered, The Epitaph Dolomite is generally a grey to dark grey, poorly bedded, dolomitic limestone (Pers. observ.). It also commonly constitutes a décollement surface regionally, possibly due to its anhydrite content (S Titley, pers. comm., 1994). Anhydrite comprises possibly 10 to 20% of the Epitaph Dolomite (Mine Staff, pers. comm., 1994).
• Scherrer Formation, 80 m thick - regionally this unit comprises four alternating units of sandstone and limestone and is 210 m thick. In the mine area three members are recognised, from the base upwards, namely,
- a lower 50 m thick clean, light coloured quartzite which commonly has a faulted contact with the Epitaph Dolomite, but elsewhere is conformable. At a 0.3% Cu cut-off, ore is seldom developed in this quartzite, although it usually carries good Mo levels.
- a probable ex-dolomitic limestone to silty dolomite present in the mine area as a 10 to 15 m thick compact light to dark green diopside tactite. It is generally well mineralised.
- The uppermost unit is a conformably overlying 10 to 15 m thick grey to brown quartzite.
• Concha Limestone, 25 to 50 m thick - which regionally is a 165 m thick unit, dominated by cherty, dark grey, finely crystalline limestone units. In the mine area it is represented by 25 to 50 m of garnet tactite which contains local zones of hematite. This tactite is characterised by white fibrous wollastonite and green andradite garnet. Dark patches of retrograde alteration are obvious where the garnet is being broken down into hematite and silica (Pers. observ.). The upper part of the formation is a thick bedded marble with zones of wollastonite.
• Rain Formation, >120 m thick - limestone with minor sandstone beds.
Unconformity - which occurs locally in the Mission-Twin Buttes area. This unconformity which is developed above over-turned Permian sediments at Mission, and has an angularity of 60° at Twin Buttes, is unusual. Regionally, Mesozoic sediments normally conformably follow the Permian which is rarely overturned. The anomalous structure is taken to be due to the Permo-Triassic Sonoma Orogeny and may bear some relationship to the major NW-SE trending Sawmill Canyon Fault zone which passes between the Mission and Twin Buttes deposits (Mine Staff and S Titley, pers. comm., 1994).
Mesozoic,
Triassic rocks overlie the Permian with a sharp angular unconformity in the Mission-Twin Buttes area, and include:
• Ox Frame Volcanics, dated at 220 Ma - rhyolite flows, tuffs and tuff breccias with intercalated lenticular beds of sandstone/quartzite and andesite/dacite flows with a few flow breccias.
• Rodolfo Formation, which rests unconformably on the overturned Palaeozoic in the Mission Mine area, and comprise around 300 m of fine grained, compact, siliceous argillites, arkoses and local basal carbonate rich conglomerates.
Jurassic rocks which are only represented by intrusives in the Mission-Sierrita area, although sediments and volcanics are present towards the south-east of the Sierrita Mountains,
• Harris Ranch Quartz Monzonite, dated at 190 to 210 Ma - characterised by medium grained, purple-grey, subhedral perthitic orthoclase (35%), with oligoclase andesine (24%), quartz (23%) and biotite.
• Tascuela Red Beds; and Whitcomb Quartzite, which are absent in the Mission area. The latter comprise up to 250 m (?) of quartzite, rhyolitic volcanics, rhyodacitic tuff and arkose.
• Stevens Mountain Rhyolite, which are believed to be around 173 Ma in age, and are restricted to a zone along the NW-SE trending Sawmill Canyon Fault which passes between the Twin Buttes and Sierrita/Esperanza orebodies to the south of Mission. This unit is absent in the Mission area.
• Sierrita Granite, dated at 140 to 150 Ma - characterised by large equant quartz crystals or masses of quartz.
Lower Cretaceous Angelica Arkose, which overlies Triassic rocks with an angular unconformity. These are probably equivalents of the Cretaceous sediments at Bisbee. At San Xavier North they are known as the Amole Arkose. In the Mission Mine area they are represented by interbedded sandstones and siltstones and massive arkose units which are absent at Sierrita/Esperanza.
Upper Cretaceous ,
• Demetrie Volcanics, up to 2500 m thick - are found to the south in the Sierrita/Esperanza area but not at Mission. They comprise andesite and dacite flows, lahars and pyroclastics, with interlayered rhyolite flows and tuffs.
• Red Boy Rhyolite, 210 to 300 m thick - found locally as remnants near Mission and in the Sierrita/Esperanza area. The unit comprises rhyolitic flows and tuffs which have been dated at 74 to 66 Ma.
Tertiary,
Palaeocene to Eocene Intrusives, comprising three phases, as follows:
• Biotite-quartz-diorite, dated at 67 to 63 Ma - fine grained, near micro-crystalline, equi-granular plagioclase-hornblende-biotite diorite, locally with over 10% quartz. Average compositions are andesine (32%), biotite (24%), hornblende (13%), K-feldspar (8%), quartz (7%) and actinolite (6%).
• Ruby Star Granodiorite, dated at 61 to 58 Ma - which is present as an extensive batholith in the Sierrita Mountains between Sierrita/Esperanza and Mission. It largely comprises an equi-granular, light grey, medium grained, holo-crystalline biotite granodiorite. A gradational porphyritic phase forms a longitudinal core to the batholith.
• Quartz-Monzonite (adamellite) Porphyry, dated at 55 to 58 Ma - which is exposed in or near all of the major orebodies of the district.
Oligocene Helmet Fanglomerate. >3200 m thick - characterised by conglomerate, landslide blocks, volcanic flows, tuffs, breccias, arkose and arkosic sandstone, locally with thin beds of well sorted and layered fine grained sediments.
Miocene Cover, of unknown thickness, but locally up to 3000 m - volcanics, gravels, sands and andesite dykes.
Quaternary Alluvial Cover, 0 to 300 m thick.
Structure
Regionally, a flat lying detachment fault surface, the San Xavier Fault underlies the north-eastern section of the Pima District, with the Mission Complex mines being on the upper allochthonous plate, while the Twin Buttes and Sierrita/Esperanza orebodies, 10 and 12 km to the south and SSW respectively, are on the lower autochthonous block. This surface is also found shallowly below the Mission Complex mines, with most movement being dated at between 24 and 28 Ma (Jansen, 1982; Titley, 1982b; Barter & Kelly, 1982). The upper plate above the San Xavier Fault ranges from a few tens of metres to 600 m in thickness (Jansen, 1982). Movement on this fault appears to represent the post-Laramide, but pre-basin and range detachment extensional phase that took place within the Walker-Texas Lineament zone. It has been suggested that this fault has transported the Mission Complex orebodies from an original position above the Twin Buttes deposits (Mine Staff, pers. comm., 1994). Apart from the possible relationship between ore in the upper and lower plate, the detailed pattern of the geology on the two plates also infers the same movement. The trace of parts of the San Xavier Fault however are still open to interpretation as sections of its western-most extremities have been shown to represent sedimentary rather than faulted contacts (S Titley, pers. comm., 1994).
The Mission Complex orebodies are distributed around the margins of a sill-like tongue of late Palaeocene to early Eocene quartz-monzonite (adamellite) porphyry. Within the pit the northern margin of the flat lying porphyry sill is marked by low angle fault separating it from the Rodolfo and younger Helmet Fanglomerate. The lower, eastern and southern margins are intrusive. This porphyry is found within the core of an east plunging, east-west trending, asymmetric, faulted, post ore antiform/arch whose core consists of Palaeozoic sediments. The flanks of the antiform are occupied by Mesozoic sediments. The units within this post-mineral fold are cut by steep south dipping faults which strike parallel to the fold axis and predate mineralisation. North-west trending faults, fractures and folds influencing all of the sequence up to and including the lower Cretaceous Angelica Arkose are the dominant pre-mineral structures, and have influenced the distribution of grade. These are in turn cut by steeply dipping NNE striking faults which post date the skarn development, while the underlying, flat faults belonging to the Oligocene San Xavier system are interpreted to have subsequently moved the allochthon containing the orebodies northwards (Jansen, 1982; Einaudi, 1982; Titley, 1982b).
The intruded Palaeozoic and overlying Mesozoic sediments comprise the Permian Epitaph Dolomite, Scherrer Formation, Concha Limestone, the Triassic Rodolfo Formation, and the lower Cretaceous Angelica Arkose, which are as described above. The Permian sequence lies immediately above the post mineralisation San Xavier Fault, and is overturned. Below the fault the country rock is composed predominantly of un-altered Mesoproterozoic granite. The Mesozoic sediments are not overturned, with both the Triassic Rodolfo Formation, and the lower Cretaceous Angelica Arkose lying unconformably on the overturned Permian sequence, suggesting the area was influenced by the Permo-Triassic Sonoma Orogeny (Jansen, 1982; Einaudi, 1982; Titley, 1982b).
Within the lower sections of the pit there is a shallowly east dipping thick fault zone, known as the 'Disruption Structure', which separates mineralised skarn from barren, un-altered Epitaph Dolomite. Although it forms a boundary to the ore, structural relations suggest it is pre-mineralisation in age, implying later movement (Mine Staff, pers. comm., 1994).
Distribution of Mineralisation
According to Einaudi (1982), the large Cu bearing skarn bodies and disseminated lower grade ore zones in hornfels, arkose and porphyry are located within a 3 km wide, and 5 km long, north-west trending zone of pyritic mineralisation which encompasses the porphyry mass and the Mission Complex open-pit. However, during the 1994 visit Mine Staff (pers. comm.) advised that pyritisation and alteration continued to the north-east of Einaudi's line by at least 1 km, while to the south-west planning was underway to extend the pit to mine ore as far as the old Mineral Hill shaft.
Copper and iron sulphides are disseminated to varying degrees within all of the pre-middle Tertiary rocks within this zone of alteration and mineralisation. Conversely there is no such mineralisation outside of the broader zone of pyritic mineralisation. The individual orebodies of the complex are those volumes within this zone that have sufficient grade and favourable geometry to render them economic (Kinnison, 1966; Einaudi, 1982). While all pre-ore lithologies are mineralised to varying degrees, the skarns are the most receptive, carrying the highest grades (Mine Staff, pers. comm., 1994).
In the Mission orebody mineralised skarn extends eastward for 800 m from the end of quartz-monzonite porphyry sill. The sill was largely emplaced along the unconformity between the Permian and Mesozoic sediments. On the northern limb of the antiform this skarn is continuous with that of the Palo Verde orebody to the north-west. The skarn of the Pima orebody is on the southern limb of the same fold. The Mineral Hill and Daisy orebodies to the south-west are believed to be small scale duplications of the Palo Verde-Mission-Pima deposits, although some of the ore in these deposits is present as massive replacement of limestone by sulphides, carbonates and quartz. Lead and zinc with lesser copper was mined from the old San Xavier underground mine to the south. This mineralisation occurred in structurally controlled pipes and flat ore shoots of galena, sphalerite and chalcopyrite with specularite, magnetite, pyrite, calcite and quartz in a calc-silicate gangue of mainly clino-pyroxene (Einaudi, 1982). The Eisenhower 'deposit' is that section between the Palo Verde and Mission orebodies that was the subject of a separate lease, while the San Xavier South section is a northern continuation of the Palo Verde mineralisation. All of these deposits relate to separate leases and/or mines within the same mineralised system that have been amalgamated into a single open pit (ASARCO, 1991). Massive sulphides may occur at intersections of the skarn with some faults, particularly the vertical fault on the far east of the old Mission Pit (Kinnison, 1966).
Einaudi (1982) suggests that the zoning and distribution of mineralisation and alteration in the Mission Complex orebodies indicate that the quartz-monzonite porphyry is the source, although Kinnison (1966) describes the same intrusives as pre-ore in age.
Mineralisation and Alteration
The majority of ore grade copper mineralisation in the orebodies of the Mission Complex occurs in garnet-diopside skarn/tactite. The highest grades in skarn are found near the transition to marble, occurring in WNW trending fissures which contain massive sulphide pods. The mineralised skarns are developed within the Permian Concha Limestone, Scherrer Formation and Epitaph Dolomite. A lesser quantity of open-pit ore occurs in the Mesozoic argillite-arkose units of the Triassic Rodolfo Formation. The highest grades within this unit are generally at the base where grades of up to 1% Cu, are found, decreasing upwards. The upper section of the Rodolfo Formation is generally <0.3% Cu (Einaudi, 1982; Jansen, 1982;Mine Staff and S Titley, pers. comm., 1994).
The quartz-monzonite porphyry is also mineralised. It is composed of 10% orthoclase, 30% plagioclase and 5% biotite set in a fine grained groundmass of K-feldspar and quartz. Potassium-silicate alteration is only weakly developed, with wisps of secondary biotite and replacement veinlets of quartz and K-feldspar. Weak sericitisation is the most widespread alteration, with quartz-muscovite-pyrite-chalcopyrite veinlets locally accompanied by pervasive replacement of plagioclase (but not K-feldspar) by sericite. Biotite remains fresh or is partly replaced by muscovite-chlorite-apatite-rutile and pyrite (Einaudi, 1982). In the western section of the main Mission Complex open pit the quartz-monzonite porphyry sill is strongly altered with the original texture being almost entirely destroyed. Locally however, a relict porphyritic texture is preserved. Alteration is characterised by intense K-feldspar flooding to produce around 90% orthoclase. Remnant plagioclase crystals have been altered to very fine green sericite, while biotite appears to have been modified to chlorite. Low grade disseminated chalcopyrite occurs within the porphyry (Pers. observ.).
Einaudi (1982) indicates that in the Mission orebody the porphyry contains 0.15 to 0.3% Cu, rarely exceeding 0.4% Cu, with pyrite:chalcopyrite ratios of 3:2 to 3:1. The average grade of the porphyry at Palo Verde is from 0.2 to 0.4% Cu from west to east, while at Pima it averages 0.15% Cu. The only ore present within the sill is on its eastern edge, occurring both as disseminations and on fractures, probably predominantly the latter (Mine Staff, pers. comm., 1994).
Within the Mission Complex mine the main ore mineral is chalcopyrite, with some bornite and sphalerite occurring in the garnet skarn of the Concha Formation. Where present bornite and sphalerite are generally at the base of the garnet skarn at the transition to marble. Within the skarn the chalcopyrite occurs on fractures, as disseminations and as massive sulphide pods/bands up to 50 cm thick (Mine Staff and S Titley, pers. comm., 1994).
Alteration and mineralisation is believed to have taken place in two stages, an early thermal development of diopside in dolomitic siltstone and wollastonite in siliceous limestone, followed by later metasomatic alteration which superimposed the ore bearing skarns. Three main skarn types are recognised (Einaudi, 1982; Jansen, 1982), namely:
1) Fine grained, granular, diopside skarn/hornfels which largely occurs in a 10 to 30 m thick bed in the Permian Scherrer Formation quartzites and in underlying carbonate beds at the contact with the quartzite;
2) Calcic garnet-diopside skarn with variable garnet:pyroxene ratios and an outer wollastonite zone against marble, zoned relative to quartzite-carbonate contacts or fissures. Garnet tactite occurs particularly in the Permian Epitaph Dolomite and the Permian Concha Limestone;
3) Epidote skarn composed of epidote, K-feldspar, quartz and tremolite with lesser garnet and clino-pyroxene, occurs in Mesozoic argillite and arkose of the Rodolfo Formation, at the contacts with calcic skarn. Epidote-garnet skarn is also found within the upper ex-mixed clastic and carbonate member of the Epitaph Dolomite, as detailed in the stratigraphic column above. The Rodolfo Formation is altered to a garnet-epidote skarn within the Eastern Fault zone, a pre-mineral structure which has undergone post-mineral re-activation. The Rodolfo Formation had no original carbonate component, and hence introduction of carbonates along the fault during metasomatism is postulated. This fault locally marks the eastern margin of mineralisation (Mine Staff, pers. comm., 1994)
Within the main Rodolfo Formation clastics mineralisation is hosted by a fractured hornfels, as distinct from a skarn. The hornfels is fine grained and strongly silicified with a densely developed fracture system filled with thin (<1 mm) mineralised quartz veins flanked by thin dark selvages (Pers. observ.). This ore is very hard and must be blended with the other skarns sent to the mill (Mine Staff, pers. comm., 1994).
Where sighted the skarns were very variable and patchy with different textures and mineral compositions at a given site, ranging from prograde skarn, overprinted by veining and by patches and zones of retrograde development. Similarly the form and distribution, presence and absence of sulphides was very also very variable (Pers. observ.).
The descriptions below relate to different sections of the Mission Complex, drawn from separate accounts, as cited, before the amalgamation into a single operation. .
In the Mission orebody section of the Mission Complex, five sulphide mineral associations and zones are recognised (from Einaudi, 1982), namely,
1) Pyrite-chalcopyrite-molybdenite in and near the quartz-monzonite porphyry sill;
2) Chalcopyrite-bornite dominant in skarn at the west end of the orebody near the quartz-monzonite porphyry;
3) Chalcopyrite-pyrite as the dominant skarn assemblage, especially at the eastern end of the pit, some distance from the quartz-monzonite porphyry;
4) Sphalerite which is most abundant at the skarn-marble contact; and
5) Bornite-pyrite-sphalerite-galena-tetrahedrite in late quartz-calcite veins on the eastern margin of the orebody.
Similarly in the Palo Verde orebody section of the Mission Complex, the majority of +0.4% Cu mineralisation is within the Palaeozoic and Mesozoic sediments to the east of and below the quartz-monzonite porphyry sills. High grade zones are restricted to skarns in Palaeozoic limestones on the contact with either quartzite or Mesozoic Arkose. The skarn is separated from low grade porphyry by 30 to 60 m of quartzite and hornfels. Studies have outlined the following skarn/tactite zones (from Einaudi, 1982);
Quartzite-hornfels within the Permian Scherrer Formation quartzites, which comprises a lower pure glassy white quartzite; a middle diopside-quartz hornfels; and an upper quartzite with 5 to 20% interstitial biotite, feldspar-mica, diopside-montmorillonite or dolomite and fracture controlled mineralisation. A white hornfels, consisting largely of dolomite, with lesser recrystallised quartz, occurs at the contacts between quartzite and diopside hornfels. Grades within the upper quartzites increase from 0.1 to 0.25% Cu from west to east, with pyrite:chalcopyrite ratios respectively of 25:1 to 10:1. This mineralisation is mainly in fractures. The middle Scherrer Formation diopside-quartz hornfels contain chalcopyrite-pyrite (±quartz, magnetite) veinlets with quartz-actinolite envelopes and have 0.4 to 0.6% Cu;
Magnesian skarn occurring in the core of the mid Scherrer Formation diopside hornfels, is characterised by patches and specks of brown serpentine rimmed with magnetite and set in a pale green, coarse tremolite-calcite matrix with trace talc. Textures, and a comparison with Twin Buttes, suggest that the retrograde serpentine replaced prograde forsterite grains. Chalcopyrite, with only minor pyrite, is disseminated throughout. Grades are generally >1% Cu;
Calcic skarn which occurs largely within the Permian Concha Limestone. This unit has been converted from a dark, locally cherty limestone to a garnet-diopside-wollastonite skarn along the contact with the quartzite, over a width of 20 to 45 m. The garnet is andradite. Sulphides comprise pyrite-chalcopyrite-bornite-sphalerite with no magnetite, although fine hematite dusting in garnet is common. Pyrite and bornite are mutually exclusive at any point. In general, chalcopyrite-pyrite is associated with diopside and garnet-diopside (usually brown garnet), while chalcopyrite-bornite (±sphalerite and pyrrhotite) is associated with garnet-wollastonite (usually green garnet). Pyrite does not occur with wollastonite. The fractures and veins in the inner section of the skarn are rimmed by dark green actinolite halos. Sulphides are present as streaks and disseminations in fresh garnet, diopside and wollastonite. Within a metre or so of the marble contact sulphide abundance increases. Cu grades within the inner 7 to 15 m of the outer skarn zone average around 0.4% Cu, while in the outer 5 to 7 m as the marble contact is approached, grades increase to 2.5 to 4% Cu;
Marble zone, with a patchy silicate mineralogy, predominantly diopside, commonly with minor garnet and idocrase, as well as minor zones of wollastonite associated with silica rich zones or chert beds. Talc-serpentine alteration of diopside is pervasive. Metals are present in veinlets as pyrite-chalcopyrite-magnetite (with serpentine envelopes), and sphalerite-chalcopyrite±pyrite (with white quartz-dolomite±calcite envelopes). Bornite is absent;
Late alteration of wollastonite and garnet to assemblages including calcite, quartz, hematite, siderite and chlorite in the outer skarn zones. This late alteration is seen in breccia zones, and along fractures and fault zones and extends for distances of up to 1 m from strong faults and along fractures into the skarns.
In the old Pima open pit the Palaeozoic carbonates were altered to assemblages consisting principally of garnet, diopside and tremolite. The Mesozoic clastics and the quartz-monzonite porphyry showed abundant evidence of K-silicate alteration, in the form of K-feldspar, with associated sericite, calcite and sulphides. Phyllic alteration was generally restricted to vein selvages ranging from 1 to 5 cm wide and was characterised by feldspar destructive quartz-sericite-muscovite and sulphides with the sericite veins and alteration halos cross-cutting the K-silicate assemblages in the quartz-monzonite porphyry. Propylitic assemblages of epidote, chlorite, calcite, quartz and sericite, were generally restricted to the south-west of the old Pima pit in Mesozoic clastics, with scattered sulphides. Alteration of the Permian carbonates had produced hornfels and tactite, composed mainly of andradite or grossularite, with diopside, tremolite, magnetite, calcite, quartz and biotite. The distribution of the different hornfels was controlled by the original rock type. Sulphides were distributed within the hornfels as local massive clots, and within and adjacent to veins. Cross-cutting the tactite were veins with associated alteration halos of actinolite, epidote, magnetite and serpentine. Supergene alteration effects were largely limited to the development of clay minerals, iron oxides, copper oxides and sparse chalcocite along the upper benches of the pit. Gypsum and calcite were present as fracture fillings deep in the pit. Alunite of uncertain origin and mixed with clay minerals occurred as fracture fillings along the highest benches to the south-east (Langlois, 1978).
The mineralisation in the old Pima pit consisted of veinlets, disseminated and locally massive types with chalcopyrite being the dominant Cu sulphide. Veinlet and disseminated mineralisation was present in approximately equal proportions and were most abundant in the altered Mesozoic clastic sections, while massive replacement sulphides were largely limited to the altered Palaeozoic carbonate lithologies. Other common sulphides included pyrite and molybdenite with lesser amounts of sphalerite, galena, tennantite-tetrahedrite, chalcocite, bornite and valleriite. Local massive occurrences of sphalerite, magnetite and chalcopyrite were limited to tactite bodies present in the western part of the pit. Chalcocite was present only in limited quantities near the shallow base of oxidation. Pre-ore brecciation and fracturing were major ore controls, as were the original lithologies. Ore was preferentially developed in decreasing order of abundance within Palaeozoic hornfels, Mesozoic clastic rocks, Palaeozoic quartzite, Tertiary porphyry and Palaeozoic limestone. The highest grade ore however was found within the tactites of the Palaeozoic carbonates. Molybdenite mineralisation, although originally extracted at Pima, showed a poor correlation with the Cu content. It is inferred that the Mo content is highest in the silica rich rocks, such as the porphyry and clastics. With increasing distance from the porphyry sill the proportion of tetrahedrite-tennantite and sphalerite increase relative to chalcopyrite, while bornite is highest relative to chalcopyrite in the slightly marble rich sections below the main ore zone (Langlois, 1978).
Oxidation within the Mission Complex orebodies is restricted to a zone generally no more than 15 m below the surface and secondary enrichment by chalcocite is restricted to a thin veneer (Kinnison, 1966). The principal oxide minerals are chrysocolla, tenorite, malachite and azurite (Langlois, 1978).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1998.
This description is a summary from published sources, the chief of which are listed below.
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Mission pit
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Selected References:
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Jansen L J 1982 - Stratigraphy and structure of the Mission 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 467-474
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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
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Kinnison J E 1966 - The Mission Copper deposit, Arizona: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 281-287
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Porter T M 1998 - The Mission skarn porphyry copper deposit: in Compilation from published literature and previous visits (Unpub.) 9p
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Titley S R 1982 - Some features of tectonic history and ore genesis in the 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 387-406
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Titley S R, 1996 - Alteration of mineralized carbonate rocks in the epicrustal environment: in Porphyry Related Copper and Gold Deposits of the Asia Pacific Region, Conf Proc, Cairns, 12-13 Aug, 1996, AMF, Adelaide, pp 3.1 - 3.10
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