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Bagdad

Arizona, USA

Main commodities: Cu Mo
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The Bagdad porphyry copper deposit is located 170 km NW of Phoenix and 160 km SW of Flagstaff in western Arizona, USA (#Location: 34° 35' 12", 113° 12' 38"W).

Geology

The orebody occurs within a stock of late Cretaceous quartz-monzonite, dated at 70.9 Ma (Titley, 1982), which intrudes Middle Proterozoic amphibolites, mica schist and slate of the 1800 and 1700 Ma Yavapai Series and associated Middle Proterozoic granitoids. Limited local flat lying remnants of Cretaceous to Tertiary rhyolite tuff are intruded, as are associated rhyolite dykes which also intrude the tuff. The quartz monzonite crops out as a series of stocks and plugs, but only the stock at Bagdad is appreciably mineralised. Dykes of diorite porphyry and quartz-monzonite porphyry are younger than the quartz-monzonite, and some of these are mineralised with Cu, Pb and Zn. The alteration and mineralisation followed the intrusion of the stock, but are presumed to be of the same general age. During the late Tertiary and Pleistocene a surface of considerable relief was developed and a chalcocite blanket was formed over the deposit. Later this surface was partly eroded, truncating the chalcocite blanket, and then buried below the Miocene Gila Conglomerate and its intercalated basalt flows (Anderson, 1950).

The quartz-monzonite (adamellite) stock containing the Bagdad deposit is located essentially at the intersection of two dyke swarms. This is believed to represent two intersecting shear zones which have controlled the emplacement of the stock. The quartz-monzonite host rock of the stock has un-altered porphyritic to seriate textures. Plagioclase occurs as both phenocrysts and matrix, and is largely calcic oligoclase to andesine, while orthoclase is present as equant crystals and is generally interstitial to plagioclase. Quartz usually occurs as small interstitial crystals, and rarely as graphic intergrowths with orthoclase. Biotite, partly altered to chlorite is an accessory, as is hornblende. Minor accessories include sphene, magnetite, apatite and zircon. Aplite dykes up to 1 m thick cut the quartz monzonite. Much of the unaltered quartz-monzonite approaches a granodiorite in composition. However in the altered variety the orthoclase and plagioclase are in nearly equal amounts (Anderson, 1950).

Mineralisation & Alteration

The porphyritic to seriate host quartz-monzonite has been altered within the orebody by hypogene processes to a granular orthoclase-quartz-albite-biotite rock with a fresh appearance. In thin section orthoclase and quartz have been enhanced, plagioclase has been altered to albite, while hornblende and book biotite have been altered to pale brown leafy biotite. Bold outcrops of quartz-orthoclase-sericite rock stand out as ribs and irregular masses in the central stock at Bagdad, although in total they only represent a minor part of the altered quartz-monzonite. The ribs are generally only 3 to 6 m thick and have NE and NW trends, parallel to the main structural elements of the stock. This style of alteration is not restricted to the Cu rich sections, also being found on the western margins of the stock, and appears to post-date the orthoclase-quartz-albite-biotite alteration (Anderson, 1950).

The hypogene sulphides, chalcopyrite and pyrite, locally replaced by supergene chalcocite, are found within a substantial part of the stock. The sulphides occur as minute veinlets defined by 'chains of disconnected granules' with associated quartz and orthoclase, or are present as are disseminations, but are always in contact with quartz and orthoclase. Some quartz-chalcopyrite veins are also present, averaging around 2.5 cm in width, but contain little copper. Molybdenite is limited to quartz-orthoclase-pyrite veins younger than the chalcopyrite veining. The highest grade ore is found in the chalcocite blanket, beneath a leached zone related to the erosional surface below the Miocene Gila Conglomerate. The thin veins and mineralised fractures which carry most of the Cu, follow the direction of regional dyke development and shearing that intersect at the Bagdad Stock and are believed to have been formed by tectonic stress. The supergene enrichment was also controlled by faults parallel to these two directions and the thickest highest grade chalcocite ore occurs where intersecting or closely spaced faults have increased the permeability of the host rock (Anderson, 1950).

Not all of the central stock is copper bearing, and a crude zonal arrangement is evident. A pyritic zone surrounds the high Cu core of the stock, fading in intensity outwards (Anderson, 1950).

The supergene chalcocite ore, which is separated from the overlying Miocene Gila Conglomerate by a leached cap, comprises grains of grey chalcocite which are 1 mm or less in size, present along fractures or disseminated in the quartz-monzonite. The host has been only little effected by the supergene enrichment, with minor clouding of feldspars. Microscopic studies indicate that the chalcocite replaces chalcopyrite in preference to pyrite. Covellite was observed as films on chalcopyrite. In good chalcocite ore pyrite may be partially replaced, or show no signs of replacement. The base of the chalcocite zone is irregular, related to the permeability of the host rocks, with steep prongs of chalcocite occurring along closely spaced faults or fault intersections. The interface between hypogene and supergene sulphides is generally diffuse over an interval of 3 to 6 m, with good chalcocite grades of >1% Cu passing through a transition zone where chalcocite coats chalcopyrite with a grade of around 0.8%, to the hypogene ore with 0.5 to 0.7% Cu. The top of the chalcocite zone is also irregular, marked by the appearance of malachite, chrysocolla, cuprite and native copper. In some faults these minerals cut through the chalcocite zone into the primary ore zone (Anderson, 1950).

The leached capping varies in thickness, locally reaching 100 m. In the leached zone some malachite and chrysocolla are present, but in general the Cu content is appreciably less than in the protore. Very little change is noted in the quartz-monzonite other than the removal of the sulphide minerals, although along the mineralised fractures the breakdown of silicate minerals is indicated (Anderson, 1950).

The total metal production at Bagdad in 1992 was 100 000 t of copper and 4500 t of molybdenum (American Mines Handbook, 1994). The operation includes both conventional milling and SXEW leaching.

Published reserve and production figures include:
    Reserve 1992 - 1074 Mt @ 0.37% Cu, 0.027% Mo (American Mines Handbook, 1994)
    Production to 1991 - 91.3 Mt @ 0.65% Cu (Titley, 1992)
    Reserve 1986 - 385 Mt @ 0.47% Cu, 0.014% Mo (USBM)
    Reserve 1989 - 639 Mt @ 0.42% Cu (Titley, 1992)
Remaining proved + probable reserves - at December 31, 2011 (Freeport-McMoRan, 2012):
    mill ore - 1267 Mt @ 0.35% Cu, 0.02% Mo, 1.74 g/t Ag (Cu - 59.1%; Mo - 70.2%; Ag - 49.3% recovery);
    ROM leach ore - 362 Mt @ 0.12% Cu (25.4% recovery).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1996.     Record last updated: 26/8/2013
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
 References to this deposit in the PGC Literature Collection:
Anderson C A,  1950 - Alteration and metallization in the Bagdad porphyry copper deposit, Arizona : in    Econ. Geol.   v.45 pp. 609-628
Barra F, Ruiz J, Mathur R, Titley S  2003 - A Re-Os study of sulfide minerals from the Bagdad porphyry Cu-Mo deposit, northern Arizona, USA: in    Mineralium Deposita   v38 pp 585-596


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