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Ajo, New Cornelia

Arizona, USA

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The New Cornelia Mine at Ajo in southern Arizona, occurs within the Arizona-New Mexico Basin and Range Province of the south-western USA. Mineralisation is associated with a late stage Palaeocene phase in the eastern apex of the larger Cretaceous to lower Tertiary Chico-Shunie Quartz Monzonite batholith which is found predominantly to the west (Dixon, 1966). The mineralised phase has been dated at 63 Ma (Titley, 1982) (#Location: 32° 21' 19"N, 112° 51' 56"W).

Reserve and production figures at Ajo include:

  4.06 Mt @ 1.27% Cu at a 0.8% Cu cut-off - carbonate ore reserve, 1924 (Gilluly, 1946);
  55.07 Mt @ 1.32% Cu at a 0.8% Cu cut-off - sulphide ore reserve, 1924 (Gilluly, 1946);
  2.44 Mt @ 3.66% Cu at a 3% Cu cut-off - sulphide ore reserve, 1924 (Gilluly, 1946);

  115 Mt @ 1.25% Cu (Reserve in 1929, Calumet & Arizona Mining Co. quoted in Gilluly, 1946);
  255 Mt @ 0.8% Cu (Production 1917 to 1962, Gilmour, 1982)
  390 Mt @ 0.71% Cu, 1.6 g/t Ag, 0.125 g/t Au, 0.001% Mo (Production to 1981, Titley, 1992)
  190 Mt @ 0.5% Cu (Reserve 1989, Titley, 1992 and USBM).

  Gold production, 1909 to 1934 - 78 t from 31.48 Mt of ore = 0.40 g/t Au.

The copper deposits at Ajo were known and worked in a small way by Spaniards and Mexicans at least as early as 1750. The Ajo mine was located in November 1854 by a party of Americans from California. At that date the border between Sonora, Mexico, and the territory of the United States had not been determined, and its position was not ascertained until the following year, 1855. Ajo is believed to be, after Santa Rita in New Mexico, the first copper district in the SW USA to be worked by US citizens. Sporadic attempts were made to economically exploit and treat the deposit from 1855 to 1917, when a leach and electrolytic winning plant commenced treating carbonate ore. In anticipation of the exhaustion of this ore, a mill to treat the underlying sulphide ore started production in 1919. Lower copper prices after World War 1 caused a shut down from late 1919 until 1924, when treatment of both sulphide and carbonate ore resumed until 1930, when the leach plant was finally shut down. Sulphide ore was treated from then until the mid 1980s by the New Cornelia Copper Co, and Calumet and Arizona Copper Co. which merged in 1929, and were absorbed into Phelps Dodge in 1931.

Geology

The rocks within the Ajo district, comprise, from oldest to youngest:

Palaeo to Mesoproterozoic Cardigan Gneiss - has an overall quartz-diorite composition, and is possibly the stratigraphic equivalent of the widespread 1.7 Ga Pinal Schist and related granite gneisses. It is grey massive or crudely gneissic to schistose rock, composed of white feldspar, quartz and chlorite after biotite. It includes minor albite-chlorite-sericite-quartz schists and widespread grey porphyritic quartz diorite that forms injected migmatitic bands and contorted folds in the darker gneisses. Some pegmatitic facies with pink microcline, white oligoclase, quartz in crystals, and a little muscovite, sericite and chlorite.
Cretaceous Concentrator Volcanics - are the oldest pre-intrusive rocks in the immediate mine area. Further west they abut the Cardigan Gneiss along a major NNE trending fault and are unconformably overlain by the Tertiary Locomotive Fanglomerate. They range in composition from soft dark grey andesite to white, hard, brittle rhyolite, and are generally flows, flow breccias and tuffs. In the mine area and to the west, the formation is predominantly rhyolite with numerous irregular patches of andesite. The proportion of andesite increases downwards in the sequence;
Cretaceous quartz monzonite of the main Chico-Shunie Batholith, possibly co-magmatic with the Concentrator Volcanics;
Palaeocene Cornelia Quartz Monzonite - is a hard, massive, unaltered, grey, equi-granular quartz monzonite outside of the mine area. Within the mine it exhibits two very different facies, separated by the 40° east dipping, north-south trending, Charlie Fault. In the western section of the mine it is a very hard, massive, porphyritic quartz monzonite with strong silicification and minor sericitisation of feldspar, composed of plagioclase and lesser quartz, with orthoclase and chlorite phenocrysts up to 3 mm in size set in a fine (<0.6Êmm) matrix of quartz and orthoclase. To the east of the fault it is a softer porphyritic quartz monzonite with much sericite and minor clay alteration;
Palaeocene Cornelia Quartz Diorite - which is the border facies of the Cornelia Quartz Monzonite, and is believed to have been emplaced in the apex of the quartz monzonite at the time of intrusion. It occurs as two types in the mine area. On the east side of the mine in the upper levels it is a medium grained, equi-granular, dark grey quartz-diorite, while on the west side it is a border phase of the quartz monzonite and is fine grained. In the lower levels of the pit there is an island of quartz-diorite similar to that on the western side, but very fine grained;
Orthoclase rich pegmatites - Soon after consolidation of the quartz-monzonite (adamellite) and quartz-diorite, north-south fractures developed in the apex of the intrusive mass and these were filled with orthoclase rich pegmatites;
Locomotive Fanglomerate - is an alluvial conglomerate, hundreds of metres in thickness, resting on an erosional surface and containing boulders of all of the lithologies described above. In the south-eastern section of the pit however, it has been cut by a minor intrusion of coarse grained un-mineralised quartz monzonite, which may be a very late phase of the Cornelia Quartz Monzonite;
Ajo Volcanics - are a thick sequence of andesitic breccia flows and tuffs which are interbedded with and overlie the Locomotive Fanglomerate (Dixon, 1966).

Structure

A series of faults cut the sequence in the mine area, and to the west. These include:
i). the post-mineralisation Gibson Fault to the west of the mine which has moved the apex of the main Chico-Shunie Batholith, including the New Cornelia orebody, down and to the south-east;
ii). the NE-SW trending, 60° SE dipping pre-mineral Able Fault on the western margin of the orebody;
iii). the arcuate, post-mineralisation, generally north-south trending, 40 to 65° east dipping, normal Charlie Fault. This structure separates the underlying mineralised monzonite from the more pyritic hangingwall in the immediate apex of the intrusion, which has been displaced to the south-east;
iv). the NE-SW trending, 25° SE dipping, post-supergene Dog Fault, which moved the mineralised rhyolite capping of Concentrator Volcanics and the associated supergene blanket some 350 m to the south-east. The fault is accompanied by extensive brecciation below the chalcocite blanket, while the underlying rhyolite and monzonite is un-brecciated, hard and massive;
v). the 30° east dipping, north to north-east trending Fox Fault which cuts the Charlie and the Dog Faults;
vi). the major north to north-east trending, 60° east dipping George Fault which truncates the supergene blanket on the eastern margin of the pit (Dixon, 1966).

Mineralisation

The mineralisation at the New Cornelia Mine is located within the offset apex of the Chico-Shunie Batholith to the south-east of the Gibson Fault. This apex has an axis trending at around 330° that occupies an area of some 1200 x 2000 m within the open pit. The vertical range of mining up to 1966 was 260 m. The open pit encompasses the entire width of the mineralised quartz-monzonite apex, and adjacent mineralised sections of the Concentrator Volcanics. Generally the better grade is concentrated in the quartz-monzonite, and to a slightly lesser degree in the quartz-diorite fringes. In the rhyolites of the Concentrator Volcanics the mineralisation is restricted to micro-fractures that become barren outwards from the intrusive. At depth the apex of the intrusive comes into contact with rhyolites and andesites, lower in the Concentrator volcanics, which corresponds to the limit of economic mineralisation (Dixon, 1966).

The primary ore control appear to have been the fracturing along the long axis of the orebody which corresponds to the development of 'orthoclase rich pegmatites' as described above. Mineralisation occurs as disseminated blebs scattered throughout the host rocks and as discrete grains along veinlets and seams. Within the quartz-monzonite this is primarily disseminated, but also occurs as veinlets. The disseminated mineralisation is associated with the quartz-orthoclase matrix and rarely with plagioclase and chlorite phenocrysts. The vein mineralisation is associated with quartz, which is the main vein filling, with the sulphide being scattered along the vein (Dixon, 1966).

Alteration and mineralisation styles within the pit are divided by the Charlie Fault. To the west, in the footwall of the fault, the host is a hard, massive, quartz-monzonite with plagioclase, quartz, orthoclase and chlorite phenocrysts in a quartz-orthoclase matrix, containing chalcopyrite-bornite mineralisation with minor pyrite (samples of concentrates average 51% bornite, 16% chalcopyrite and 11% pyrite). These rocks exhibit heavy fracturing in some areas near the fault, while to the south the footwall of the fault is occupied by monzonite breccia. The core of mineralisation corresponds to the zone of fracture controlled orthoclase rich pegmatites.

The hanging wall of the Charlie Fault comprises a complex mixture of quartz-diorite, rhyolite and quartz-monzonite, which are much softer, invariably showing much more intense hydrothermal alteration and contain mineralisation that is predominantly pyrite and chalcopyrite with rare bornite (samples of concentrates average 4% bornite, 42% chalcopyrite and 45% pyrite). The principal products of alteration in the hangingwall of the fault are sericite and lesser amounts of clay which were developed from the plagioclase. Chlorite with minor leucoxene and rutile are alteration products of biotite, while calcite is a late mineral on veinlets. The mineralised rhyolites contain phenocrysts of quartz and orthoclase in a microcrystalline matrix of the same minerals, with bornite and chalcopyrite disseminated on minute seams (Dixon, 1966).

There are two widely separated periods of oxidation and enrichment at Ajo. The first was in the middle Tertiary, prior to the deposition of the fanglomerate, when the rhyolites of the Concentrator Volcanics were oxidised to a depth of about 200 m. Only a minor amount of chalcocite formed in the rhyolite where primary mineralisation was weak. In the small preserved area of rhyolite immediately above the apex, abundant bornite veining yielded a considerable thickness of chalcocite enrichment that merged downwards into the primary bornite at depth. This zone is now above the Dog Fault in the south-eastern part of the pit. After faulting on the Charlie, Fox and Dog Faults displaced the rhyolite capping, primary bornite and chalcopyrite were exposed at the tip of the apex. Oxidation of this exposure yielded an oxide zone with abundant hematite and minor cuprite with occasional chrysocolla, shattuckite and malachite, which changed at depth to the chalcocite of the supergene cap and then to the primary bornite-chalcopyrite. The second period of enrichment developed on the present erosional surface where oxidation produced malachite and chrysocolla to an average depth of 15 m. There was little migration of Cu in the exposed section of the monzonite which carried bornite-chalcopyrite mineralisation, while a small amount of transport took place in mineralised rhyolite (Dixon, 1966).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1995.    
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.


New Cornelia

  References & Additional Information
   Selected References:
Dixon D W  1966 - Geology of the New Cornelia Mine, Ajo, Arizona: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson    pp 123-132
Hagstrum J T, Cox D P, Miller R J  1987 - Structural reinterpretation of the Ajo mining district, Pima County, Arizona, based on paleomagnetic and geochronologic studies: in    Econ. Geol.   v82 pp 1348-1361
Runyon, S.E., Nickerson, P.A., Seedorff, E., Barton, M.D., Mazdab, F.K., Lecumberri-Sanchez, P. and Steele-MacInnis, M.,  2019 - Sodic-Calcic Family of Alteration in Porphyry Systems of Arizona and Adjacent New Mexico: in    Econ. Geol.   v.114, pp. 745-770.

   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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