The Safford Dos Pobres and San Juan porphyry copper deposit is located 280 miles east of Phoenix in eastern Arizona.
It is one of four disposed in a general north-westerly trending, approximately 15 km long, zone in the Safford District of south-eastern Arizona. The others are the Lone Star, Sanchez and San Juan deposits. These deposits also lie within the Arizona-New Mexico Basin and Range Province. See also the separate Lone Star record.
Dos Pobres and San Juan are among the few porphyry style deposits in Arizona with a significant Au content. The deposit was under development by Phelps Dodge Corporation as an underground mine in the early 1980's, but was suspended in 1982 (Financial Times Minerals Year Book, 1984). The Freeport-McMoRan Safford mine has been in operation since 2007, with full production beginning in 2008. The mine exploits two copper deposits that have oxide mineralisation overlying primary copper sulphide ore.
The Dos Pobres deposit largely lies within a series of Cretaceous to Palaeocene sub-aerial andesitic volcanics of the Safford Meta-volcanics. These volcanics were deposited within a narrow NW-SE trending structural trough that originally had more than 3000 m of topographic relief. The mineralisation appear to be linked to a series of Palaeocene to Eocene age intrusives as dykes within the orebody, while a possible stock is indicated by drilling at depth. These intrusives are generally of a quartz-monzonite porphyry composition, ranging from tonalite to granodiorite, and are believed to have been emplaced during two intrusive episodes between 58 and 47 Ma.
The sequence in the Safford district is as follows from the base (Langton & Williams, 1982; Robinson & Cook, 1966),
Middle Proterozoic basement, represented by quartzite, schist and amphibolite of the Pinal Schists, and by related granites and gneiss. The nearest outcrop of these rocks is 25 km to the south-west, although they are also found as xenoliths in the Cretaceous to Tertiary intrusives in the deposit area.
Upper Cretaceous to Palaeocene, comprising,
Safford Meta-volcanics, >1100 m thick - metamorphosed hornblende and pyroxene andesite flows, breccias, agglomerates, wackes and mud-flows. While the bulk of these volcanics are of andesitic composition, some dacites, dacite tuffs and latites have been recognised. Contact metamorphism by the 'parental' Lone Star pluton has been dated at 69.8±2.7 Ma. This unit is also locally known as the 'Older Volcanics'.
Lone Star Plutons, quartz-diorite to granodiorite batholiths, which outcrop as apophyses and in windows through the younger volcanics at three limited localities along the Safford District mineralised trend. Crystallisation has been dated at between 67 and 60 Ma.
Tertiary, represented by,
Productive Porphyries, believed to have been intruded in two separate phases in the Palaeocene to Eocene. They are present within the orebody area as a series of dykes and sills, while a probable parent stock has been intersected by drilling at depth. Compositions range from tonalite, to quartz-monzonite (adamellite) to monzonite to granodiorite porphyry, but generally quartz-monzonite porphyry. The un-altered porphyry tends to have a granodioritic composition, with the quartz- monzonite arising from an increase in orthoclase during the early stages of alteration. The dykes appear to have been emplaced at 52.2±2.0 Ma, while the stock has given an age of 47.8±1.8 Ma.
Baboon Meta-volcanics, 0 to 600 m thick - of Eocene age, composed of propylitised andesitic agglomerates, flow breccias, mud flows, lithic tuffs and conglomerates within the Dos Pobres area. They thicken to the north-west of the orebody to their maximum thickness. The sequence has been hydrothermally altered locally, but not by either the older Productive Porphyries or the Lone Star Plutons. Lower members are intruded by the hornblende andesite dyke system (described below), while upper units contain large xenoliths of the same intrusives. In the Lone Star area they are underlain by 200 m of basaltic flows, while the main Baboon Meta-volcanics units include dacites and are known as the 'Intermediate Volcanic Series'.
Hornblende Andesite Sills, Dykes and Plugs, of Eocene age - which in the Dos Pobres area are found in the ENE-WSW shear zones and appear to be consanguineous with the Baboon Meta-volcanics. Large titaniferous hornblende phenocrysts up to 1 cm long are common. These intrusives are late- to post-ore, un-mineralised and dilute the ore. In the vicinity of the dykes ore grade is lower. At Dos Pobres, dykes of this rock type are a metre or less in thickness in the Safford Meta-volcanics, although they may be up to 60 m thick in the lower sections of the Baboon Meta-volcanics.
Gila Mountain Volcanics, 0 to 900 m thick - of Miocene to Pliocene age, composed of basalt and rhyolite dykes, sills and plugs; basalt flows and tuffs; rhyolite flows, agglomerates and vent debris; and basal conglomerate.
Quaternary Alluvial Cover, 0 to 600 m thick - sands, clay-beds, gravels, siltstones and conglomerates.
The main fault directions in the district are north-south, ENE-WSW and NW-SE. The north-south set are only poorly developed, while the NW-SE en-echelon set are the most pronounced in the district. Two of the most important structures influencing the geometry of the orebody are the Foothill Fault, a major NW-SE trending and 65°SW dipping structure, and the NE-SW trending Dos Pobres Anticline. The ore occurs near the intersection of the two structures. Low angle faults are present in the orebody area with variable strikes and dipping at between 10 and 40°NE. Several ENE-WSW trending faults have had considerable post-mineralisation displacement, although they are also interpreted as having provided the principal channels for the emplacement of the Productive Porphyry dykes. Recurrent movement on these faults has been contemporaneous with the Foothill Fault, and has down-faulted the sequence progressively from the south-east to the north-west (Langton & Williams, 1982).
Mineralisation and Alteration
Crackle breccia is the dominant fracturing in the mine, affecting the meta-volcanics more than the Productive Porphyry. All of the crackling not related to post-mineral faulting appears to be hydrothermal in origin. Contact breccias intimately associated with the porphyries often host the highest grade ore, and are more pronounced in the upper part of the deposit. Typically the upper sections of the dykes grade into breccias with fragments of volcanics embedded in the porphyry. A later event was the mega-breccia which incorporates 'huge' fragments of meta-andesite, Productive Porphyry and contact breccia, apparently with an appearance that suggests 'floating blocks in the crackle breccia' (Langton & Williams, 1982).
Alteration within the porphyry commenced with early 'late magmatic' biotite, followed by quartz-orthoclase-magnetite-bornite, with quartz and orthoclase growing at the expense of plagioclase in the matrix, and bornite occurring in fractures with quartz and orthoclase. The addition of orthoclase in this phase produced a quartz-monzonite composition from the un-altered granodiorite. This phase is interpreted as having been coincident with the K-metasomatism of the wall rocks. In places the porphyry shows incipient quartz-sericite alteration, with some pyrite and chalcopyrite rather than bornite, and molybdenite being present to a maximum of 50 to 150 ppm.
Alteration within the volcanics, which host up to 75% of the orebody, is more complex. The pre-mineralisation andesitic rocks have undergone an early thermal alteration related to the cooling of the thick pile of volcanics and the underlying Lone Star Batholith. This resulted in an assemblage characterised by epidote and pennine, with accompanying hematite, while dacites were altered to quartz-sericite-calcite-pennine. The second phase was related to the intrusion of the Productive Porphyry and had the same mineralogy with accompanying plagioclase destruction, but was more pronounced, and restricted to within 600 m from the outer limits of the orebody. This was followed by the alteration associated with the main phase of mineralisation and comprised secondary actinolitic-hornblende and brown biotite which has been dated at 56.9±2.2 Ma, and disseminated dust like magnetite. As further potassic alteration proceeded, the actinolitic hornblende was replaced and more biotite grew, resulting in up to 30% biotite in the most altered areas, with the associated disseminated magnetite being accompanied by bornite, minor chalcopyrite, apatite, rutile and occasional granular orthoclase. This represented the main pulse of K-metasomatism, and where more intense also affected adjacent quartz-monzonite porphyry dykes. Crackling became important during this phase, with the wall-rocks being laced with veinlets carrying quartz, orthoclase, apatite, bornite and chalcopyrite (Langton & Williams, 1982).
The bornite:chalcopyrite ratio decreases outwards from the alteration core, with the Cu grade remaining constant at around 0.83% Cu, while the sulphide content is around 3%, but increases outwards. These veinlets probably carry the bulk of the deposit's precious metals within the central section of the deposit. There is an association between bornite and both Au and Ag, as sylvanite [(Au,Ag)Te4], and hessite [Ag2Te] found within most bornite, but not in bornite that is intimately intergrown with chalcopyrite. The biotite zone may be seen up to 500 m from the centre of the orebody. K2O values range from 2.09% at the 0.2% Cu contour to 2.41% at the 0.6% Cu line. Retrograde alteration of the orebody resulted in an increase in Cu accompanied by an increase in the chalcopyrite content, and the re-appearance of epidote in less altered zones. This latest alteration is characterised by veins in fractured zones which often carry quartz and important amounts of chalcopyrite and bornite, with sericite selvages and associated montmorillonite and zeolites. This alteration is restricted to the potassic biotite zone of the orebody, dying out in the earlier propylitic alteration and the quartz-monzonite porphyry (Langton & Williams, 1982).
Subsequent to later tilting, oxidation and erosion during the Oligocene and Miocene resulted in leaching and thorough oxidation of sections of the central ore zone and partial enrichment to depths of up to 400 m. Chrysocolla and cuprite are the dominant oxide minerals to 300 m, although tenorite and bronchantite have also been recognised. Sulphides in the periphery of the marginal zone and in the pyritic aureole are locally found within 100 m of surface. Native copper, chalcocite and covellite are common in the mixed oxide zone between 400 and 500 m below the surface. Oxide ore averages 0.62% Cu in contrast with the 0.72% of the hypogene ore (Langton & Williams, 1982).
Published resource figures include:
Initial Reserve 1978 - 400 Mt @ 0.72% Cu, 0.38 g/t Au, 1 g/t Ag (Gilmour, 1982; Tooker, 1991)
Reserve 1992, Sulphide Ore - 230 Mt @ 0.89% Cu (American Mines Handbook, 1994)
Reserve 1992, Leach Ore - 270 Mt @ 0.46% Cu (American Mines Handbook, 1994)
Remaining proved + probable reserves - at December 31, 2011 (Freeport-McMoRan, 2012):
crushed leach ore - 206 Mt @ 0.43% Cu (67.2% recovery).
(Source: Porter GeoConsultancy, www.portergeo.com.au, 2013)