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Miami Inspiration |
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Arizona, USA |
| Main commodities:
Cu Mo
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Super Porphyry Cu and Au
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The Miami Inspiration group of orebodies are found within the Globe-Miami District, ~140 km north of Tucson and ~115 km east of Phoenix in south-eastern Arizona, USA (#Location: 33° 25' 12", 110° 44' 24"W).
For detail of the setting and district geology, see the separate Globe-Miami District record.
The first prospecting expeditions visited the Miami district in the 1860s. Copper was mined underground up until after World War II, when the first open-pit mining commenced. Miami was among the first to employ 'vat leaching' (1926) and precipitation plants to recover oxide minerals, which was undertaken in conjunction with a flotation concentrator, that processed sulphide minerals. An SX/EW plant, commissioned in 1979, replaced vat leaching by the mid-1980s and ultimately the concentrator in 1986. The rod mill was commissioned in 1966 and the refinery in 1993. The refinery was permanently closed in 2005. In the 2013, the mine exploits leachable oxide and supergene sulphide mineralisation. (Freeport-McMoRan, 2012). The current Miami operation also includes the older Bluebird, Copper Cities, Globe and Van Dyke mines.
This description is taken largely from Olmstead & Johnson (1966), and deals with the Inspiration deposits, prior to their amalgamation with the Miami operation.
Geology
The mineralisation at Inspiration occurs along the northern margin of, and is associated with the Palaeocene Schultze multiple phase 'granite'. The ore is located both within the Schultze Granite and the intruded early Middle Proterozoic Pinal Schists. In the Inspiration area the Pinal Schists comprise a sequence of coarse grained quartz-muscovite schist, fine grained quartz-sericite-chlorite schist and some fine grained amphibole schist, and is composed predominantly of quartz and sericite with muscovite, chlorite and biotite. Texturally it ranges from micaceous quartzite to well foliated and crenulated schist to poorly foliated gneiss. Many facies can be recognised within the mine area based on texture and mineral content. Generally the schist is calcareous, and near the orebody shows strong sericite alteration. The general schistosity strikes at about 50°, and dips steeply to the south-east. The outlines of the orebody at both Inspiration and Miami, and the intrusion of granite porphyry closely parallels the local schistosity (Olmstead & Johnson, 1966).
The Schultze Granite at Inspiration is predominantly a porphyritic quartz-monzonite (adamellite), although in the mine area it includes granodiorite, quartz-monzonite and porphyritic quartz-monzonite. Generally it has a granitic texture and is greyish-yellow in colour. It has a coarse grained matrix of quartz, oligoclase and biotite which often encloses phenocrysts of orthoclase from 2.5 to 10 cm long. It has a variable content of plagioclase and orthoclase accounting for the composition range above, which may be a result of K-silicate alteration.
Granite porphyry found in the mine area is regarded as a separate, later phase of the Schultze Granite. Roughly half of the ore at Inspiration is found within the granite porphyry, extending outwards into both the Pinal Schists and the granite porphyry from their common contact. Mineralogically the granite porphyry is distinguished from the Schultze Granite by conspicuous euhedral quartz phenocrysts cemented in a micro-crystalline matrix. It also contains easily discernible orthoclase, plagioclase and biotite. Orthoclase exceeds plagioclase by two times or more, with orthoclase commonly enclosing plagioclase crystals. Microscopic studies indicate orthoclase replacing plagioclase. This orthoclase alteration occurred prior to the development of sericite and clay. The borders of biotite flakes are often altered to sericite and chlorite. The rock weathers to a light grey poorly cemented product. The granite porphyry occurs as a series of plugs fringed by Schultze Granite, and as sill-like tongues protruding into the Pinal Schists (Olmstead & Johnson, 1966).
Structure
A series of NNE to NE, east-west and NW-SE faults are obvious in the Miami-Inspiration area. NW-SE structures which dip at 40 to 45° SE are the most continuous in the area and show the most recent movement, although they may not necessarily be the youngest. These include the Pinto and Bullpup Faults. North-south striking faults with dips of 50 to 60° E show the next most recent movement, cutting and off-setting the Pinto and Bullpup faults. Over 250 m of strike-slip movement, which is older than the north-south set, can be seen on the Joe Bush Fault. Normal faulting is seen parallel to the foliation in the Pinal Schist, and is probably early in the structural history. Thrust faulting from the south also occurred early in the structural history of the orebody as the Chlorite and similarly oriented faults of this generation are offset by most other structures in the mine area. Reverse faulting on these structures is indicated by older rocks overlying younger, and repeated mineralogical zoning. This includes instances of Pinal Schist overlying Gila Conglomerate and primary sulphides being found above oxide ore. The South-western Fault which strikes east-west and dips 55°S, and the Sulphide Fault which parallel it and dips at 70 to 80°N, are apparently pre-mineralisation with little post ore movement (Inspiration Consolidated Copper Co, 1987).
The structural picture would then appear to comprise one of normal faulting and early thrusting from the south, followed by strike slip faulting and finally by more normal faulting. It is likely that some of the faults showing recent movements are very old faults that were reactivated during the Laramide event with faulting continuing to the present (Inspiration Consolidated Copper Co, 1987).
The NNE trending and 50°E dipping Miami Fault has dropped the Gila Conglomerate and the Miami-East Orebody by 600 to 900 m downwards on its eastern side. Some of the parallel NNE to NE normal faults which dip at shallow angles of 30 to 40°E, offset the ore progressively downwards to the east. The east-west trending, steeply and irregularly north dipping pre-porphyry Sulphide Fault in part forms the boundary between the porphyry and the Pinal Schists. The NW-SE trending Pinto Fault dips irregularly at about 45°NE, and appears to represent both pre- and post-mineralisation movement. There is a zone of high grade mineralisation in the Inspiration orebodies on the footwall of this fault, and the best grade in the Miami workings is at the intersection of the Pinto and Miami Faults. The Inspiration ore is offset by the Pinto Fault (Olmstead & Johnson, 1966).
Mineralisation & Alteration
As known in 1966 within the Thornton, and Live Oak Pits, the Inspiration Orebody had a length of 2500 m and a maximum width of 750 m. The ore attained a maximum thickness of 200 m, but averaged 100 m. It had an arcuate elongated shape which thinned in the middle and on its south-western end. The elongation followed a trend of 72°, with a gentle south-west pitch. The original irregular high grade ore was regarded as a chalcocite ore blanket. Primary mineralisation was believed to have been intimately associated with the Schultze Granite where faults and related crushing aided mineralisation, with some evidence of simultaneous faulting, crushing, stretching and flowage in the granite. Mineralisation followed the introduction of the granite porphyry phase (Olmstead & Johnson, 1966).
The mineralisation within the Miami-Inspiration series of pits is concentrated at the contact of a quartz-monzonite (adamellite) porphyry of the Schultze Granite with the Pinal Schists which occur on its northern contact. The relatively poorly reactive Pinal Schists are permeable to supergene solutions allowing the production of both a chalcocite blanket and good developments of 'exotic' (ie, copper oxides, carbonates and silicates). These blankets are formed by both lateral and vertical transport and precipitation of leached copper. The exotic mineralisation is formed by both the oxidation of the chalcocite blanket, and direct precipitation from the transporting solutions. Similar hypogene grades to those found at Pinto Valley are known and have been mined. Hypogene ore has been mined from the TJ, Joe Bush and Thornton Pits. The extensive oxide mineralisation however has been more profitable to mine following the shut-down of the early to mid 1980's. The current head grade is around 0.4% Cu, of which 0.3% Cu is 'oxide' mineralisation (G Lenzi, Magma Copper, pers. comm., 1994).
Microscopic studies indicate a phase of K-silicate alteration and silicification, largely in the form of orthoclase and quartz, prior to the introduction of at least part of the mineralisation. This took the form of intense orthoclase veining accompanied by secondary biotite in schist and K-feldspar alteration of wall rock bordering veins in the porphyry. In addition there is evidence of alteration involving sericitisation, biotitisation, hydration, argillisation, surface leaching and enrichment. These are profound in and near the orebodies, while epidotisation is more prevalent around the northern and eastern fringes of the ore. Sericite alteration is the strongest, occurring in both the porphyry and schist, often destroying the original rock texture. The distribution of sericite is difficult to differentiate within the schists because of their original composition. Argillisation is difficult to discern also, as it may in part be hypogene, overprinted by the more intense supergene alteration. Propylitic alteration can be seen in outlying areas as occurrences of epidote and manganese oxides. Supergene enrichment created a masking effect through the intense leaching involved (Olmstead & Johnson, 1966; Inspiration Consolidated Copper Co, 1987).
The original hypogene mineralisation was of a low tenor, averaging 0.3 to 0.4% Cu, with the economics being dependent upon supergene enrichment. The primary mineralisation below the chalcocite blanket consists of pyrite, chalcopyrite and molybdenite, traces of gold and silver, and minor occurrences of bornite, sphalerite and galena. The higher grade primary copper mineralisation is found parallel to the contact between the granite porphyry and the schist, extending up to 200 m into the schist side and 60 to 100 m into the porphyry side.
There is a distinct zonal pattern of mineralisation, with higher grades occurring along the Miami and Pinto Faults, and between the Joe Bush and Bulldog Faults. Very little primary copper was encountered in the Live Oak Mine, although some chalcopyrite was associated with the crush-zone near the Sulphide Fault (Olmstead & Johnson, 1966).
A higher grade molybdenite zone is found nestled inside the primary copper zone, extending not quite as far into the schist, but further into the porphyry (Inspiration Consolidated Copper Co, 1987). The hypogene ore minerals filled small fractures in the host rocks, creating a low grade mineralisation that now assays 0.15 to 0.4% Cu at depth below the supergene blanket. In a few places chalcopyrite-quartz veins are observed to cut earlier pyrite-quartz veins, while the molybdenite-quartz veins cut both of the others (Olmstead & Johnson, 1966).
Supergene enrichment produced a leached capping, an intermediate zone of oxidation and hydration and a supergene chalcocite enriched blanket (Olmstead & Johnson, 1966).
The leached capping varies in thickness from zero to 300 m or more, and averages around 120 m. Near the upper extremity of the leached zone the copper content is often no more than a few ppm (Olmstead & Johnson, 1966). The leached capping consists of pervasive hematite and limonite with spotty remnants of chrysocolla (Inspiration Consolidated Copper Co, 1987).
In the intermediate oxide zone between the leached capping and the supergene blanket, the main mineral assemblage comprises malachite, azurite, chrysocolla and ferric hydroxide minerals. Also present in this zone are minor 'copper pitch', bronchantite, atacamite, lindgrenite, libethenite and extremely minor metatorbernite. In this zone there are primary quartz veinlets with chalcopyrite and pyrite which have not completed the enrichment cycle, nor undergone significant oxidation. The oxide zone is fairly consistent in thickness, averaging around 60 m, although this varies from 60 to 120 m in faults or other permeable zones. The oxide zone, which accounted for half of the ore in 1966, resulted from the oxidation of enriched mineralisation and is much higher grade than the hypogene mineralisation (Olmstead & Johnson, 1966; Inspiration Consolidated Copper Co, 1987; G Lenzi, Magma Copper Co, pers. comm., 1994).
The supergene blanket contains chalcocite, bornite, covellite and digenite replacing chalcopyrite and pyrite. The degree of replacement decreases with depth and varies from the complete replacement of the original pyrite and chalcopyrite by chalcocite, to films of chalcocite on pyrite crystal surfaces. Finely disseminated native copper is found with chalcocite in some places. A consistent chalcopyrite rich zone is found below the chalcocite blanket. This has been attributed to supergene enrichment by Olmstead and Johnson (1966), who believed that the supergene chalcocite replaced mainly pyrite, and hence orebodies are localised in zones of hypogene mineralisation, regardless of the amounts of primary copper present. Chalcopyrite and bornite, they observe, also occurs as thin films or tarnish on pyrite crystal surfaces in this zone below the chalcocite blanket. The mainly supergene ore mined to 1966 averaged around 1% Cu and 0.02% Mo (Olmstead & Johnson, 1966; Inspiration Consolidated Copper Co, 1987). The chalcocite blanket mined to 1987 averaged 60 m in thickness with an average grade of approximately 1.5% Cu. The deepest supergene ore in the Live Oak Pit was at 350 m below the surface in the south-west of the mine. The boundary between the oxide and chalcocite zones is usually occupied by a 6 m thick interval of mixed oxides and chalcocite (Inspiration Consolidated Copper Co, 1987).
Published reserve and production figures include:
Production, 1911-1978 - 760 Mt @ 0.67% Cu, 0.001% Mo (Titley, 1989),
Reserve, 1982 - 222 Mt @ 0.58% Cu (USBM)
Reserve, 1989 - 229 Mt @ 0.43% Cu (Titley, 1992)
Reserve, 1993 - 193 Mt @ 0.45% Cu (AME)
To 1987, approximately 3.7 Mt of Cu had been extracted from the Miami Inspiration mines (Inspiration Consolidated Copper Co, 1987).
Remaining proved + probable reserves - at December 31, 2011 (Freeport-McMoRan, 2012):
ROM leach ore - 60 Mt @ 0.47% Cu (60.5% recovery).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1996.
Record last updated: 27/8/2013
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.
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Selected References:
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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
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| References in PGC Publishing Books: | |
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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
 Abstract
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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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