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Santa Rita, Chino |
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New Mexico, USA |
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Cu Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
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The Chino/Santa Rita porphyry copper and molybdenum mineralisation is associated with the Palaeocene (63 Ma) Santa Rita Stock, 17 km east of Silver City in the Central Mining District of south-western New Mexico, USA. Approximately 1.5 km to the north of the Santa Rita Stock is the bi-lobate Hanover-Fierro Stock. On the margins of this latter stock the folded limestone and dolomite country rocks are massively replaced by tabular bodies of magnetite on the north end and by sphalerite on the southern margins. Both types of ore are accompanied by pyro-metasomatic calc-silicates. The Continental copper skarn described below is found on the northern tip of the Hanover-Fierro Stock.
(#Location: Santa Rita - 32° 47' 30"N, 108° 4' 6"W; Fierro Stock - 32° 50' 46"N, 108° 5' 30"W).
Mutschler et al., 2004 quoted the total production + reserve/resource for the Central Mining District (Chino/Santa Rita) as: 1400 Mt @ 0.62% Cu, 0.01% Mo. 0.011 g/t Au
Published production and reserve figures for the skarn and supergene porphyry ore include:
230 Mt @ 0.93% Cu (Prod. 1911-62, Gilmour, 1982).
500 Mt @ 0.97% Cu (Res. 1970, Gilmour, 1982).
90 Mt @ 1.8% Cu (Res. 1912, Rose & Baltosser, 1966).
332 Mt @ 0.72% Cu (Res. 1987, USBM).
280 Mt @ 0.69% Cu (Milling Ore - Res. 1992, Am. Mines H'book, 1994).
141 Mt @ 0.30% Cu (Leaching Ore - Res. 1992, Am. Mines H'book, 1994).
Remaining ore reserves and mineral resources - at December 31, 2011 (Freeport-McMoRan, 2012):
proved + probable reserve - mill ore - 179 Mt @ 0.57% Cu, 0.04 g/t Au, 0.009% Mo. 0.47 g/t Au;
(Cu - 78.7%; Au - 78.0%; Mo - 41.8%; Ag - 78.5% recovery);
proved + probable reserve - ROM leach ore - 242 Mt @ 0.31% Cu (43.1% recovery).
inferred resource - mill material - 177 Mt @ 0.45% Cu, 0.013% Mo;
inferred resource - leach material - 179 Mt @ 0.32% Cu.
Total copper production in 1992 was 155 000 t, 96 500 t from milling and 58 500 t from electro-winning [SXEW] (Am. Mines H'book, 1994).
Magnetite replacement ores also occur adjacent to the Santa Rita Stock, while hematite and sphalerite ores surround the smaller Copper Flat Stock, 5 km to the west. Magnetite replacement ore from the Fierro, Hanover, 'north of Santa Rita' and Copper Flats deposits averaged 53.9%, 57.2%, 54.6% and 57% Fe respectively, with 0.03 to 0.065% P2O5 and 0.2 to 0.5% Zn. Sphalerite replacement ores from near Hanover contained about 14% Zn, 1 to 2% Cu, <0.5% Pb, and up to 40 g/t Ag, occurring as roughly tabular bodies from 4.5 to 9 m thick, the largest of which had lateral dimensions of 300 x 25 to 30 m. Sphalerite veins, with non-uniform amounts of Pb, Cu and Ag extend south-westward for about 8 km from the two major stocks (Hernon & Jones, 1968). One such vein system was in the Groundhog Mine which produced 3 Mt @ 12.3% Zn, 3.6% Pb, 1.3% Cu, 75 g/t Ag, the largest shoot of which was 330 x 120 x 7 m (Hawksworth & Meinert, 1990). All three types of ore, namely the 'porphyry disseminated', 'tabular replacement' and 'vein' styles, are considered to be part of a single epoch of primary mineralisation (Hernon & Jones, 1968).
Geology
The Central Mining District lies within a NW-SE trending, 15 km wide, fault bounded range of the Arizona-New Mexico Basin and Range Province. An old north-east trending lineament passes to the south-west from the Santa Rita area, through the Tyrone, Bisbee and Cananea mining Districts in New Mexico, Arizona and Sonora in Mexico. To the north-east the lineament may also pass through the Hillsboro (Copper Flat) District (Rose & Baltosser, 1966).
The main sequence in the Central Mining District comprises, from the base,
• Middle Proterozoic basement of granite, gneiss, schist and greenstone, poorly represented in the district;
Unconformity
• Late Cambrian Bliss Sandstone, 45 m thick - glauconitic and hematitic sandstone, and sandy limestone;
• Ordovician El Paso and Montoya Limestones - dolomite, which is sandy in the lower sections and includes cherty dolomitic limestone in the upper sections, grading up into the,
• Silurian Fusselman Limestone composed of cherty dolomite. The three units, the El Paso, Montoya and Fusselman Limestones, total around 300 m in thickness.
Unconformity
• Late Devonian Percha Shale, 70 to 100 m thick - black fissile shale, followed by grey shale with limestone nodules;
• Carboniferous, commencing with Mississippian Lake Valley Limestone - white crinoidal limestone, grey limestone and shaly limestone 100 to 120 m thick, unconformably overlain by the Pennsylvanian Syrena and Oswaldo Formations - limestone, cherty limestone and a few thin shales, and then limestone, limy shale and shaly limestone, totalling 180 to 240 m in thickness;
• Probable Permian Abo Formation, 0 to 60 m thick - red shale, limestone and limestone conglomerate;
Unconformity
• Cretaceous sediments, commencing with the 20 to 40 m thick probable late Cretaceous Beartooth Quartzite - quartzite and minor sandstone, limy sandstone and shaly sandstone; followed by 300+m (locally to 700 m) of the Colorado Formation sediments beginning with a lower black shale, succeeded by sandstone and shale, and capped by an unconformity;
Unconformity
• Late Cretaceous to lower Tertiary andesitic volcanic breccia and tuff, with some sandstone and shale as much as 100 m thick. These are also terminated by an unconformity, followed by;
• A local lower Tertiary sequence of gravel and sand of the Wimsattville Formation which is up to more than 300 m thick and is restricted within a localised 1 km diameter circular depression, known as the Hanover Hole, to the north-west of the Santa Rita Stock;
• Tertiary, Miocene gravel, pumiceous tuff, andesitic basalt, quartz-latite welded tuff, rhyolite tuff and basalt up to 850 m thick; Miocene to Pliocene consolidated sand, gravel, silt and clay around 300 m thick in basins between the ranges; and
• Quaternary, alluvium.
The intrusive complex within the Santa Rita area commences with,
• Late Cretaceous to early Tertiary quartz diorite present as sills and laccoliths, commonly within the late Cretaceous sediments. Two phases are recognised, an early quartz-diorite with phenocrysts of plagioclase, biotite, hornblende and quartz in a groundmass of fine quartz and feldspar, followed by a late quartz-diorite porphyry with phenocrysts of plagioclase and hornblende in a groundmass of quartz and orthoclase found as sills and local dykes;
• Late Cretaceous to early Tertiary basic dykes and plugs, of andesite and diorite porphyry, and a local gabbro plug;
• The early Tertiary (Palaeocene) Santa Rita granodiorite stock, dated at 63 Ma. This was the major intrusive of the district. It is somewhat variable in composition, but generally comprises phenocrysts of plagioclase, hornblende, biotite and sparse quartz in a fine groundmass (0.02 to 0.1 mm) of quartz, orthoclase and minor biotite. It makes up both the Santa Rita and Fierro-Hanover stocks and some dyke like apophyses. A series of slightly younger dykes of similar composition cut both the Santa Rita and Fierro-Hanover stocks;
• Early Tertiary quartz-monzonite (adamellite) porphyry dykes cut the Santa Rita Stock in a north-south direction. Similar dykes are common in the district. At least part of the Cu mineralisation post-dated these dykes, although most of the Zn and a large proportion of the Cu ore pre-dated them. They are composed of phenocrysts of plagioclase, coarse orthoclase, biotite and quartz in a fine aphanitic groundmass of quartz and orthoclase. A younger NW trending dyke of quartz-monzonite porphyry is also found to the north-west of the Santa Rita Stock.
• Miocene latite and quartz latite dykes, basalt dykes and rhyolite plugs and dykes.
The regional dip of sediments in the Central Mining District is generally gently to the south-west. In the vicinity of the Santa Rita and Fierro-Hanover stocks however, there is considerable disturbance, with the sediments being disrupted and commonly dipping outwards from the stocks. The margins of the Santa Rita Stock dip outwards at 45 to 90°. Faults are abundant in the district, with the main trend being north-east, in two sub-parallel groups with 40 and 75° trends. A second set follows a north to north-west trend, with the Central District and the Santa Rita deposit being localised at the intersection of the two trends. Dykes follow both of these trends (Rose & Baltosser, 1966).
Several breccias are found in the Santa Rita area, generally comprising fragments of granodiorite porphyry, quartz diorite and/or sediments in a matrix of fine rock replaced by quartz, orthoclase, biotite and magnetite, with some sulphides. The breccias generally only contain clasts of the intruded lithology (Rose & Baltosser, 1966).
Within the Santa Rita/Chino open pit, lithologies include the Pennsylvanian Syrena and Oswaldo Formations, the Permian Abo Formation, Cretaceous Beartooth and Colorado Formations and the intrusives of the Santa Rita Stock, as described above. Several phases of intrusives have been recognised in the stock, each differing slightly in texture and composition, and in part separated by septa of sediments, suggest it is a multiple intrusion (Rose & Baltosser, 1966).
Mineralisation & Alteration
Two main types of ore and alteration are known in the Santa Rita open pit. These are:
i). Supergene enriched porphyry type mineralisation (after Cook and Porter 2005) comprising chalcocite, generally accompanied by abundant pyrite within both intrusives and sediments. The main areas of thick high grade mineralisation are principally supergene enrichment ore. The southern-most is largely within the granodiorite porphyry of the Santa Rita Stock, while the others are predominantly hosted by sandstones and shales of the Cretaceous Beartooth and Colorado Formations. A fairly continuous sheet of thinner ore grade rock connects these thick high grade zones.
Prior to mining, supergene mineralisation was present as three northwest trending zones of relatively thick, high grade chalcocite ore. In general, these three zones correspond to mineralisation developed along the northeast and southwest margins of the Santa Rita Stock, straddling the contacts and hosted by both granodiorite porphyry and siliciclastic rocks, while the third occupied a broader central zone over the core of the main stock. The dominant sulphide is chalcocite, with minor covellite, while where oxidised, native copper, chrysocolla, cuprite, malachite and azurite are important minerals. Both sooty and steel-glance chalcocite are present, either as veins or discrete grains composed only of chalcocite, or as coatings on pyrite, and at greater depths as partial replacement of chalcopyrite.
The top of the enriched zone is very variable, with a topographic range of more than 100 m. As a consequence, the preserved goethite-hematite-jarosite leached capping (which was rose coloured in outcrop) also varies from a few to more than 100 m in thickness, whilst the underlying supergene enriched sulphide blanket ranges from a few to over 200 m, although appreciable amounts of chalcocite are present partially replacing hypogene sulphides at depths of more than 250 m (Rose and Baltosser 1966).
Three stages of supergene enrichment have been distinguished at Santa Rita. Evidence for the first stage is from both chalcocite mineralisation buried below the Oligocene (33.4 Ma) Kneeling Nun Rhyolite and clasts of chalcocite ore and leached capping in the basal conglomerate of that unit, immediately to the southeast of the Santa Rita Stock. The second stage, believed to be Oligocene to Miocene in age, is inferred from detailed geological cross sections, prepared from drilling and mining information. These sections reveal that chalcocite mineralisation thickened abruptly northwards, away from the Oligocene volcanic overburden, but is offset by around 150 m across the late Miocene or Pliocene Martin Canyon Fault. This fault is believed to be a 'basin and range' structure, active at around 12 Ma. An immature weathering profile is developed in the footwall of this same fault, in which isolated bodies of un-oxidised (chalcocite), or partially oxidised (chrysocolla and malachite mixed with chalcocite) primary and secondary sulphides of the main supergene chalcocite blanket are embedded in 'oxide copper' mineralisation and leached capping, above the main chalcocite blanket. This observation is taken to imply that post-Miocene supergene processes were active, but relatively inefficient after the Basin and Range Event at Chino, defining a third stage of supergene activity. Supergene enrichment had concluded by the Quaternary when the modern surface was incised through the enrichment profile and the supergene enrichment blanket.
Further evidence for post-Miocene supergene processes is illustrated by the north-south cross section from the southeastern part of the deposit. In this area there is a lateral zonation within the supergene blanket with respect the outcrop of the overlying Oligocene rhyolite. This zonation reflects the progressive unroofing and oxidation of the pre-Oligocene supergene blanket. The supergene mineralisation is developed in a relatively homogenous, highly altered (quartz, sericite, K feldspar and pyrite) host of shale and diorite, and hence the results of post-volcanic weathering can be isolated. Chrysocolla predominates furthest to the north, along the southeastern margin of the stock. Further south, towards the more recently exposed parts of the supergene blanket, there is a transition through a mixed oxide-sulphide zone, to unoxidised chalcocite. The oxidation is the result of postvolcanic weathering of a mature enrichment blanket. The leached capping thickens to the west, in the downthrown block and the chalcocite ore was largely not influenced by oxidation.
Two supergene alunite K/Ar dates have been determined from the Chino mine area. The younger of these is Late Oligocene (25.6 ±0.7 Ma), which was taken from a monomineralic veinlet preserved within hematitic leached capping less than 30 m below the modern surface. The sample is considered supergene on the basis of field occurrence alone, and is interpreted to represent the earliest activity in the second stage of supergene enrichment, following removal of Oligocene volcanic cover. The older date of early Oligocene 34.3 ±0.9 Ma is more problematic. There is strong evidence that during the Early- Oligocene, the Santa Rita District was covered by in excess of 100 m of volcanic cover (Hernon and Jones, 1968). This sample was taken from 210 m below the modern (pre-mine) surface. It is isotopically heavy (δ34S = 1.7‰) compared to the typical hypogene sulphides in the Chino mine (δ34S = -2.1‰; Field, 1966), although the difference is not extreme. The contribution of isotopically heavy sulphur from hypogene sulphates cannot currently be quantified, although it is reasonable to conclude that some is taken up by supergene sulphates during weathering. The older alunite may be the product of subvolcanic diagenesis or hydrothermal activity involving meteoric waters trapped by the virtually instantaneous deposition of volcanic tuffs, and heated by prolonged volcanic activity. Alternatively it could be a hybrid date resulting from growth of Miocene alunite on Eocene grains or partial re-equilibration of very fine grains during different periods of supergene activity.
ii). Skarn (Limestone replacement) ores in which chalcopyrite is the main ore mineral, with major amounts of magnetite, pyrite, quartz and calc-silicate minerals. Massive skarn (tactite) alteration outcrops to the north of the open pit, and extends below the surface on the eastern and western margins of the stock within the sub-surface sections of the mineralised zones.
This ore type primarily occurs within limestones of the Pennsylvanian Syrena and Oswaldo Formations and in limy shale of the Permian Abo Formation on the northern section of the open pit, near the contact with the stock. A considerable amount of this mineralisation is ore grade, being primary with little or no supergene enrichment. Chalcopyrite occurs in veinlets and as disseminated grains with magnetite, quartz, pyrite, garnet, epidote, chlorite, pyroxenes and amphiboles. Sphalerite and minor galena are found in limestones near the outer margins of the Cu zone. Alteration in the purer Pennsylvanian and Mississippian (Carboniferous) limestones comprises magnetite, quartz, garnet and pyrite, while in the shaly limestones and calcareous shale, considerable amounts of epidote, tremolite, actinolite and chlorite may also be present. Less abundant alteration in the calcareous sediments include hematite, orthoclase, biotite and sericite, while minor amounts of siderite, apatite, biotite and sericite are also found. Remnants of carbonate from the original limestone are considerably recrystallised, but are not common except near the outer edges of the ore. This mineralogy is similar, but not identical to that of the copper poor magnetite ores of the Fierro-Hanover Stock, described below (Rose & Baltosser, 1966; Hernon & Jones, 1968).
Calc-silicate alteration influences a wide range of calcareous sediments surrounding the Santa Rita Stock, with calcareous shales being recrystallised (as detected in X-ray analysis) for up to 2400 m from the stock, while visible alteration to hornfels containing quartz, calcite, actinolite and plagioclase can be detected for up to 1500 m in the calcareous sediments. Hornfels, which fringes ore zones closer to the stock, has a mottled greenish-grey appearance and consists of epidote, quartz, actinolite, diopside and albite with minor garnet. Adjacent to potassium silicate alteration in the stock, the hornfels are cut by biotite-orthoclase veinlets, while in the pyritic zone on the western side of the stock up to 10% pyrite is present, occurring in veinlets with alteration envelopes containing variable mixtures of epidote, chlorite, montmorillonoids, calcite, siderite and specularite (Einaudi, 1982).
In the outermost section of the skarn alteration of limestone beds (as distinct from the hornfels in calcareous sediments), reaction skarn occurs at the contact with the hornfels. Silicification within relatively pure limestone beds is restricted to the fractures of breccia zones, while pale yellow-brown garnet veins with local sphalerite cut bleached marble. Closer to the stock, massive, yellow-brown garnet-diopside skarn with interstitial magnetite and pyrite replaces the marble. Near the contact with potassium silicate altered porphyry, magnetite (2 to 40%) and pyrite-chalcopyrite (4 to 18%) becomes more abundant as disseminations and cross-cutting veins in brown garnetite and in massive garnet-epidote-magnetite skarn. The pyrite:chalcopyrite ratios range from 1:1 to 10:1. The early skarn is overprinted by hydrous silicates and carbonates near contacts with sericitised porphyry. Patches and veins of quartz-pyrite are bordered by late minerals, including actinolite, chlorite, montmorillonoids, epidote, siderite, pyrite and chalcopyrite. This type of alteration is associated with the highest grades at Santa Rita, particularly at Lee Hill, where skarn contains 20 to 75% magnetite and 10 to 25% total sulphides, with a pyrite:chalcopyrite ratio of 10:1 to 25:1 (Einaudi, 1982)
Other styles of mineralisation in the district, associated with the same mineralised system include:
• Magnetite replacement ores, are known in all of the Palaeozoic carbonates, although the main developments are within the Ordovician El Paso and Montoya Limestones and the Silurian Fusselman Limestone. The most important minerals are magnetite and lesser hematite in the vicinity of the Fierro-Hanover Stock, although hematite predominates adjacent to the Copper Flat Stock. The skarns contain 54 to 58% Fe, 0.03 to 0.065% P2O5, up to 1% S, 0.2 to 0.5% Zn and 0.03 to 2% Cu. The main variation in gangue minerals is dependent upon the host with magnesian limestones having wollastonite, serpentine and idocrase, while garnet, pyroxene and ilvaite being characteristic of calcic limestones. The main assemblages encountered in cherty, silica bearing dolomite or magnesian limestone is quartz, serpentine, magnesite, idocrase, augite, wollastonite, talc, tremolite, biotite, actinolite, calcite and apatite. In contrast the purer limestones are altered to andradite with lesser epidote, hedenbergite, ilvaite, rhodonite, calcite and tremolite. Impure limestones were converted to silica-hornfels containing garnet, epidote, chlorite, actinolite, tremolite, quartz and calcite, while the sandy dolomites, glauconitic dolomites and sandstones were converted to fine grained epidote schists containing magnetite in thin layers and disseminations (Hernon & Jones, 1968).
• Sphalerite replacement ores, which are developed at a greater distance from the stock than the Cu skarns, are characterised by an assemblage of mangano-hedenbergite, bustamite, rhodonite and local ilvaite located on the marble side of andradite garnet zones. This association is apparently rare for porphyry copper related skarns, and may be un-related to the intrusion of the stock. To the south-west of the Fierro-Hanover Stock, and west of the Santa Rita Stock, zones of skarn with accompanying Zn-Pb-Cu mineralisation are found encompassing the zone of 'sphalerite rich vein systems' described below (Einaudi, 1982).
• Sphalerite rich vein systems, such as in the Groundhog Mine, are mined to within 2 km of the Santa Rita Stock. In this mine veins are exploited from a near surface 200 m vertical interval forming a flat pitching zone in the steep Groundhog Fault where it cuts Cretaceous sediments of the Beartooth and Colorado Formation, principally sandstones and shales with minor carbonates, and interleaved diorite sills. The vein system occupies fractures and sills associated with the fault. Deeper in the mine extensive replacement skarn orebodies were mined from 400 to 700 m below the surface, inferred to be within the underlying Carboniferous carbonates. The Groundhog Fault is a regional NNE trending structure. Anastomosing granodiorite porphyry dykes from 1 to 60 m wide, similar in composition to the Santa Rita Stock which hosts 'porphyry copper mineralisation', are found in association with the veining in the fault, occupying a structural zone with a width of 100 to 400 m, over a distance of 3000 m. Three pulses of alteration are recognised, a) an early sericite phase accompanying barren quartz-pyrite prior to porphyry intrusion and mineralisation; b) sericite and chlorite related to main stage mineralisation associated with and following granodiorite intrusion; and c) weak quartz-sericite-pyrite alteration following injection of late quartz monzonite porphyry dykes. Propylitic alteration characterised by albite, chlorite, calcite, epidote and sericite accompanied the granodiorite porphyry dykes. The mineralogy of the veins is basically early quartz and pyrite, followed paragenetically by sphalerite, galena, chalcopyrite and pyrite. The quartz-sulphide veins are enveloped by quartz-sericite-pyrite assemblages. Locally the main shoot grades laterally into stockwork quartz±pyrite veins with associated quartz-sericite-pyrite alteration. The quartz-sericite-pyrite alteration occurs along the Groundhog Fault for more than 4 km. Overlapping alteration on numerous fractures forms wide zones. This alteration is best reflected in the diorite porphyries, locally extending from 0.1 up to 20 m from veining, but is less evident in the granodiorite porphyry where chlorite-quartz-hematite-pyrite-sericite overprints earlier propylitisation adjacent to veins. In sediments, both sandstones and shales are variably sericitised and may contain up to 3% disseminated pyrite and local matrix silicification, although strong sericitisation and disseminated pyrite rarely extends for more than 2 m into the sedimentary wall (Hawksworth & Meinert, 1990).
For detail see the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1994.
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.
Santa Rita Fierro
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Audetat A and Pettke T, 2006 - Evolution of a Porphyry-Cu Mineralized Magma System at Santa Rita, New Mexico (USA): in J. of Petrology v47 pp 2021-2046
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Audetat B, Pettke T, Dolej D 2004 - Magmatic anhydrite and calcite in the ore-forming quartz-monzodiorite magma at Santa Rita, New Mexico (USA): genetic constraints on porphyry-Cu mineralization: in Lithos v72 pp 147-161
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Nadoll, P., Mauk, J.L., Leveille, R.A. and Koenig, A.E., 2015 - Geochemistry of magnetite from porphyry Cu and skarn deposits in the southwestern United States: in Mineralium Deposita v.50 pp. 493-515
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Reynolds T J, Beane R E 1985 - Evolution of hydrothermal fluid characteristics at the Santa Rita, New Mexico, porphyry Copper deposit: in Econ. Geol. v80 pp 1328-1347
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Rose A W, Baltosser W W, 1966 - The porphyry copper deposit at Santa Rita, New Mexico: in Titley S R, Hicks C L, 1966 Geology of the Porphyry Copper Deposits, Southwestern North America, University of Arizona Press, Tucson, pp 205-220
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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, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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