Globe Miami District
Super Porphyry Cu and Au|
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The Globe/Miami District, is located ~140 km north of Tucson and 115 km east of Phoenix in south-eastern Arizona, USA (#Location: 33° 24' 6", 110° 53' 46"W).
It embraces a number of porphyry copper deposits that have been mined since the discovery of rich veins of chrysocolla in the Globe Hills in 1874. The Globe Hills are some 7 km to the ENE of the main Miami Inspiration Mine. Veins in the Globe Hills had produced 12 Mt @ 4.3% Cu, 23 g/t Ag, 0.3% Au and 1 to 10% Pb+Zn to 1978. The more significant of these were the Old Dominion, Arizona Commercial and Superior & Boston. All of these veins were formed by the replacement of breccia and of wall rock along faults and fissures cutting the Middle Proterozoic Apache Group and diabases (dolerites), and the Palaeozoic sequence (Peterson, 1962; Titley, 1989). The stratigraphy and lithology of these units are described below. Similarly the Magma/Superior vein system and limestone replacement mineralisation which is found some 18 km to the WSW of Miami Inspiration produced 26 Mt @ 4.5% Cu, 53 g/t Ag, 0.92 g/t Au as outlined in the description below.
Published production within the Globe/Miami district include:
Production to 1981 - 680 Mt @ 0.69% Cu, 0.42 g/t Ag, 0.007 g/t Au, 0.0014% Mo,
(includes Copper Cities, Castle Dome, Inspiration and Miami, Titley, 1992)
Production to 1978, old Globe District veins - 12 Mt @ 4.3% Cu, 0.3 g/t Au (Titley, 1989)
Figures for other mines in the district not separately described in the Pinto Valley-Castle Dome and Miami-Inspiration records, include:
Castle Dome - 41.5 Mt @ 0.62% Cu (Total Production 1943-53, Gilmour, 1982)
Miami East - underground, 50 Mt @ 1.95% Cu, 0.003% Mo (Initial Reserve 1973, USBM)
51 Mt @ 2.1% Cu (Reserve 1989, Titley, 1992)
The first bulk mining of 'porphyry copper style mineralisation' within the district was on the Castle Dome deposit between 1943 and 1953 when 41.5 Mt @ 0.62% Cu was mined. This mineralisation was present as a supergene blanket and was mined during World War II under a federal government subsidy. Subsequently the Copper Cities deposit approximately 9 km to the ENE was exploited, followed at a later date by the small Diamond H pit, some 2 km to the south-west of Copper Cities. Mining of the large Miami and Inspiration deposits 4 km to the south of Copper Cities was also commenced. Stripping of the Pinto Valley deposit, which constituted the hypogene mineralisation just to the north-east of the original Castle Dome supergene orebody, was commenced in 1972. The Miami-East deposit is the eastern down faulted section of the main Miami orebody (Peterson, 1962; Titley, 1989). The Miami-Inspiration operation was spread over an interval of more than 4 km in a complex of open pits including the Thornton, Live Oak, Red Hill, Blue Bird, Joe Bush and Oxhide pits and the underground block-caving Miami and Miami East mines (Skillings, 1978; Creasey, 1980.
Further to the west-south-west, between Castle Dome/Pinto Valley and Superior another resource has been delineated. This deposit is known as Superior East. Mineralisation outcrops on its eastern margin, but is down faulted by around 1200 m just to the west by a basin and range fault into a basin/graben. The mineralisation is apparently predominantly supergene, developed over the Schultze Granite (E John, ASARCO, pers. comm., 1994). The deposit comprises a zone of buried vein controlled and disseminated bornite-chalcocite mineralisation (Sell, 1995).
The Globe-Miami District lies within the Arizona-New Mexico Basin and Range Province, and the broad Walker-Texas Lineament Zone. It has been suggested that all of the productive mineral deposits of the Globe-Miami and Superior Districts lie within a 10 km wide, generally north-east to easterly trending corridor (Peterson, 1962). This corridor marks a zone of Proterozoic structural weakness that parallels the contact of the Pinal Schists and the Proterozoic granites to the north-west, and also parallels the main foliation within the Pinal Schist. It is also the locus of Mesozoic and Tertiary silicic intrusions, which are interpreted as being associated with mineralisation in the district (Hammer & Peterson, 1968). The main porphyry deposits are therefor centred on the main intrusive mass, while the vein deposits are found within the mineralised corridor, but further removed.
The geology and stratigraphy of the Miami/Globe and Superior District is as follows (from Hammer & Peterson, 1968; Creasey, 1980):
Proterozoic, composed of,
Pinal Schists, of Palaeoproterozoic age, 1.7 Ga - quartz-muscovite schist and quartz-muscovite-chlorite schist, with some feldspar and locally andalusite and sillimanite. The metamorphic grade ranges from phyllite to foliated schist to granular gneiss. Lenses and veins of quartz are common.
Solitude Granite and Willow Spring Granodiorite, possibly around 1600 Ma in age. In the Inspiration pits this is grey, fine grained and contains large phenocrysts of orthoclase. It grades into a quartz-monzonite (adamellite) where orthoclase is more abundant and also becomes a fine grained granodiorite. Olmstead & Johnson (1966) claim it is younger than the Pioneer Formation, although Creasey (1980) dates it as older.
Lost Gulch Quartz-Monzonite (adamellite)and Ruin Granite, possibly around 1400 Ma in age.
Apache Group, which is Mesoproterozoic in age (but is intruded by 1.04 to 1.12 Ga dolerite sills at the Ray deposit), is subdivided from the base into, the
* Pioneer Formation, 120 m thick - basal pebble conglomerate in an arkosic matrix, dusky-red coarse to fine arkose and feldspathic quartzite with shale partings, fine arkose, siltstone and thin bedded shale. The basal conglomerate is known as the Scanlon Conglomerate at Inspiration.
* Dripping Spring Quartzite, 200 to 250 m thick - basal pebble and cobble conglomerate in an arkosic matrix, brown to yellowish-grey medium to coarse arkosic quartzite and arkose, arkose and grey to brown thin bedded very fine grained feldspathic quartzite.
* Mescal Limestone, 100 m thick - white to grey-brown, thin bedded, fine grained granular limestone, with lesser dolomite and local chert. Locally contains asbestos and algal structures near the top.
* Basalt, 0 to 100 m thick - lava flows and locally breccia flows of brown aphanitic basalt, which is locally vesicular and amygdaloidal. Original plagioclase, pyroxene and olivine are altered to clay, oxides, serpentine, carbonates and iddingsite.
Troy Quartzite, 120 m thick, also Mesoproterozoic in age, post 1600 Ma - composed of a basal alternating conglomerate; poorly sorted arkose and siltstone; and an upper light grey, medium bedded fine to medium quartzite.
Diabase (dolerite), dated at 1079±30 Ma - occurring as sills and dykes composed of brown, grey and green rock made up of coarse to aphanitic, plagioclase and pyroxene with lesser amphibole, biotite and oxides.
Palaeozoic, composed of,
Cambrian Sediments, 0 to 110 m thick - comprising a lower poorly sorted grey to red-brown mudstone; arkosic sandstone; pebble conglomerate and greywacke; and an upper medium to fine quartzite and feldspathic quartzite which is green, yellow and brown (the Bolsa Quartzite).
Devonian Martin Limestone, 110 to 140 m thick - greyish-yellow limestone with frosted sand grains; 3 to 10 m of medium bedded, dark-grey crystalline limestone (the host to replacement mineralisation in the Magma Mine and other localities in the district); thin bedded grey dolomite and sparse sandstone interbeds; alternating limestone, dolomite, sparse sandstone and shale; thin to medium bedded limestone (locally with crinoids and brachiopods); and greyish-yellow fissile shale.
Carboniferous/Mississippian Escabrosa Limestone, 150 m thick - comprising a very light grey to dark grey limestone, sub-divided into a lower thick to medium bedded limestone; and an upper medium to thin bedded limestone with abundant chert.
Carboniferous/Pennsylvanian Naco Limestone,
0 to 425 m thick - thin bedded light grey to white to very pale blue limestone with local chert nodules and interbeds of calcareous shale. Red shale at the base. Locally fossiliferous.
Tertiary, composed of,
Quartz-Monzonite Porphyry (adamellite), present as small stocks - characterised by light coloured phenocrysts of quartz, K-feldspar and plagioclase in a fine matrix. The Schultze Granite, the main phase of which is dated at 61.2±0.5 Ma, is a member of this suite, and is found in the core of the Miami District. The Schultze Granite is a composite pluton comprising at least three phases. The earliest is a granodiorite, the intermediate or main phase is a porphyritic quartz-monzonite, while the youngest is a series of porphyries that were not all intruded at the same time. The porphyritic quartz-monzonite consists of large phenocrysts of K-feldspar in a medium to coarse grained groundmass of plagioclase, quartz, K-feldspar and biotite. The typical late stage 'granite porphyry' associated with mineralisation has phenocrysts of K-feldspar, plagioclase and quartz in a fine grained groundmass
Whitetail Conglomerate, 0 to 200 m thick - fluvial conglomerate with angular to sub-angular clasts of local older rocks. This unit is believed to be of Oligocene age.
Rhyolite 10 to 600 m thick - rhyolitic lava flows, grading to perlitic obsidian and devitrified felsite, with minor tuff and tuff-breccia.
Dacite 150 to 600 m thick - a zoned ash-flow sheet, including welded and non-welded tuff. Phenocrysts of plagioclase, quartz, biotite and sanidine are found in a lithic to glassy groundmass. Age dates of 20 Ma have been obtained from this rock type. The dacite and rhyolites are known as the Apache Leap Tuff in the Miami area.
Quaternary, composed of,
Volcanic Rocks, lava flows of rhyolite grading to perlite and pumice with beds of tuff.
Gila Conglomerate, at least 65 m thick - Pliocene fluvial deposits of angular to sub-angular, pebble to boulder in size, set in a coarse grained, poorly sorted arkosic sandstone matrix..
Basalt, occurring as small irregular intrusive bodies, which are generally dense and locally vesicular.
The main central intrusive mass of Laramide age in the district, the Schultze Granite, is the locus of the principal porphyry copper deposits. The Schultze Granite is a composite pluton comprising at least three phases. The earliest is a granodiorite, the intermediate or main phase is a porphyritic quartz-monzonite, while the youngest is a series of porphyries that were not all intruded at the same time. The main intrusive phase has been dated at 61.2 Ma±0.5 Ma, while the age of the later porphyry phases may not be determined because of the extensive alteration and multiple intrusive stages. The age of mineralisation differs across the district by as much as 5 my. From oldest to youngest these are Copper Cities - 63.3±0.5 Ma; regional quartz-sericite veins - 61.1±0.3 Ma; Miami-Inspiration orebodies - 59.5±0.3 Ma; and Pinto Valley - 59.1±0.5 Ma (Creasey, 1980).
The main mass of Schultze Granite which outcrops over a significant area in the Globe/Miami district is generally not mineralised. The porphyries on the Miami, Inspiration, Copper Cities and Castle Dome-Pinto Valley mines is taken represent late stage differentiates of this larger batholithic mass. These porphyries however host very little of the ore and do not occupy significant volumes within the pits. The ore in all of the mines is almost exclusively hosted by the Proterozoic lithologies, the Lost Gulch Quartz-Monzonite at Pinto Valley-Castle Dome and at Copper Cities, and the Pinal Schists at Miami and Inspiration (G Lenzi, pers. comm., 1994).
Mineralisation & Alteration
The ores of the Globe-Miami porphyry system are mostly localised in Proterozoic wall-rocks in the periphery of the Schultze Granite. Mining of the Miami-Inspiration and contiguous orebodies has extracted sulphide ores, mainly secondarily enriched, comprising chalcocite and oxidised copper minerals. The other large systems of Pinto Valley-Castle Dome and Copper Cities are orebodies from which mainly hypogene ores, dominated by chalcopyrite, have been taken. In forming the supergene ore the wall rock character has been important, particularly that of the Pinal Schists, in having both numerous fractures with high pyrite and acid buffers such as abundant primary or secondary sericite to aid the process. Exploration below the Miami-Inspiration chalcocite blanket has revealed subjacent low grade pyritic copper mineralisation (Titley, 1989).
G Lenzi of Magma Copper (pers. comm., 1994) has indicated that the Copper Cities and Castle Dome/Pinto Valley mineralisation are fault offset halves of the same deposit or mineralised system. He similarly implies that the Miami-Inspiration line of deposits is part of a second separate mineralised system. These two systems he claims, have more than 4.5 Mt of contained copper in past production and resources.
Hydrothermal alteration studies at Miami-Inspiration is complicated by the heavy over-print of alteration related to the secondary copper. Similar problems were encountered at Castle Dome. In these areas abundant sericite and kaolinite were delineated and a correspondence with enriched copper established. Hydrothermal alteration at Pinto Valley, which represents the deeper hypogene sections of the Castle Dome system, manifests the same characteristics of mineralogy seen elsewhere in the deposit. These include K-silicate altered wall rocks with veins altered to, or carrying, biotite or orthoclase, overprinted by younger veins carrying quartz-sericite and pyrite. In the Globe-Miami deposits mining has transited from oxidised through enriched to hypogene copper sulphides. In doing so ore grades have ranged from 4% (oxidised), to 1% (enriched) to 0.5% or less in the hypogene zone (Titley, 1989).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1996.
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.
Creasey S C 1980 - Chronology of intrusion and deposition of porphyry copper ores, Globe-Miami district, Arizona: in Econ. Geol. v75 pp 830-844|
Olmstead H W, Johnson D W 1966 - Inspiration geology: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 143-150|
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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