Super Porphyry Cu and Au|
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The Morenci copper-molybdenum mining complex is located in south-eastern Arizona, close to the New Mexico border, 170 km NE of Tucson and 42 km NE of Safford (#Location: 33° 6' 16", 109° 21' 54"W).
Mineralisation at Morenci is predominately associated with a 55 Ma late Palaeocene to early Eocene monzonite porphyry, which is a member of a suite of early to middle Tertiary intrusives, ranging from diorite to granite-rhyolite in composition. The hypogene mineralisation is hosted by Proterozoic granites and schists, Palaeozoic carbonates and shales and Tertiary intrusives. It occurs in three forms, namely:
• disseminated ore within Tertiary porphyry and Proterozoic granite and schists,
• roughly tabular skarn mineralisation in Palaeozoic carbonates and shales adjacent to porphyry contacts; and
• lodes, or veins in fault zones. Virtually all of the disseminated ore mined to date has been from the supergene chalcocite blanket (Hardwick, 1959; Moolick and Durek, 1966; Mine visit briefing, 1991).
The Morenci deposits lie within the Arizona-New Mexico Basin and Range Province.
The host sequence in the district is as follows, from the base (from Hardwick, 1959; Moolick and Durek, 1966; Langton, 1972; and mine visit briefing, 1991):
Middle Proterozoic, including,
• Pinal Schist, unknown thickness - sericite schist and quartzite, with steep south dips and remnants of tight overturned folds. The quartzite contains abundant specularite or powdery hematite. Veins and masses of quartz are abundant. Small bands or masses of amphibolite are also included.
• Granite-Granodiorite Complex - comprising an older Morenci Granodiorite which is usually green, coarse grained and consists of oligoclase, andesine and biotite with orthoclase and quartz; and the younger Metcalf Granite which has a reddish colour due to disseminated hematite, and is a coarse grained granite composed of orthoclase, albite and quartz with minor biotite, locally with dykes and minor masses of red aplite and porphyritic granite.
• Cambrian Coronado Quartzite, 45 to 75 m thick - a thick bedded, tan to maroon quartzite with a coarse, basal, rounded cobble to boulder conglomerate which is found sporadically and may be up to 10 m thick.
• Ordovician Longfellow Limestone, 120 m thick - thin bedded, grey to buff, argillaceous limestone. Considerable detrital quartz and clay are present, with calcareous shales near the base. A moderately thick sandy bed occurs near the base, overlain by thick-bedded limestones carrying irregular bands and nodules of chert.
• Devonian Morenci Limestone, 50 m thick - comprising a lower 22 m thick, black, knobby, pitted argillaceous limestone; overlain by 30 m of fissile brown shale.
• Carboniferous (Mississippian) Modoc Limestone, 50 m thick - thick-bedded, grey, fossiliferous limestone, which apparently conformably overlies the Morenci Limestone. It includes thin beds of coralline limestone and quartzite, a moderate bed of dolomite and a thick bed of crinoidal limestone. The upper surface is severely eroded, with the whole unit being absent in places.
Mesozoic, represented by,
• Cretaceous Pinkard Formation, 0 to 250 m thick - interbedded shale and sandstone, with intercalated calcareous sandstone, and locally thin discontinuous beds or masses of limestone. This is correlatable with the Colorado Formation at Santa Rita, and is of upper Cretaceous age.
Tertiary, represented by,
• Diorite Porphyry, dated at 62 Ma - present as a tonalite porphyry stock that contributed radiating dykes and thick diorite porphyry sills. The sills are relatively un-mineralised, although the stock is weakly pyritic. The diorite porphyry contains phenocrysts of hornblende and labradorite, and is generally found within the upper Pinkard Shale only.
• Quartz-Monzonite (adamellite) Porphyry, dated at 55 Ma - which locally has the greatest area of exposure of the Laramide intrusives and is the principal ore bearing lithology. It consists of small closely packed phenocrysts of orthoclase, albite and oligoclase in a micro-crystalline groundmass of quartz and feldspar. When only weakly altered it is grey, brownish-grey or greenish-grey, but it is generally altered to a light grey to white.
• Diabase (dolerite) - commonly present as dykes composed of augite, labradorite and hornblende which are altered to chlorite and epidote. The texture ranges from finely ophitic to coarsely granular, and it is usually dark greenish-black to mottled. Pyrite and magnetite are abundant, and small quantities of chalcopyrite can be observed.
• Granite Porphyry, the youngest phase of which is dated at around 42 Ma - most of the central part of the intrusive complex consists of granite porphyry containing medium to large, well spaced phenocrysts of orthoclase, albite and quartz. Several ages of granite porphyry are observed. The youngest is weakly mineralised, and contains euhedral quartz phenocrysts as much as 1 cm in diameter. The older granite porphyry usually contains smaller quartz phenocrysts and more closely spaced feldspar phenocrysts. Texturally it is similar to the Quartz-Monzonite Porphyry. Granite porphyry dykes are common in the district.
• Tertiary Volcanics - Tertiary lavas encircle the district. These include rhyolite, basalt, andesite, rhyolitic tuff and perlite. The final eruption deposited a rhyolite tuff breccia. Plugs and vents are exposed in the basalt areas in the north and north-east of the district, while the flows appear to have been diverse and the sequence is nowhere present in its entirety. The oldest volcanics in the district are 33 Ma in age.
Quaternary, represented by,
• Gila Conglomerate - Pliocene tuffaceous conglomerates and related clastics.
• Alluvial cover.
The Tertiary intrusives are believed to have been emplaced as a sequence of pulses and are of relatively similar age and source. Locally the different phases have abrupt contacts and the intrusive relationship of dykes and apophyses is evident. Many relationships however are obscured by alteration. The distribution of the main intrusives, associated dykes and veins is controlled by the Laramide Fault directions which are predominantly north-east or east striking (Moolick and Durek, 1966).
The diorite porphyry does not cut any other intrusive body, and may be older than, or grade into, the quartz-monzonite (adamellite) which forms a centrally located, laccolithic, body within the Tertiary intrusive complex. The hood, or cupola of the quartz-monzonite stock is preserved in the Morenci Mine area. This quartz-monzonite porphyry is the main host to ore. It was in turn subsequently invaded by successive intrusions of granite porphyry, which opened and reopened fractures in the quartz-monzonite porphyry. The granite porphyry occurred largely as dykes which were predominately limited to the quartz-monzonite porphyry mass, parallel to its elongation, intruded into tensional fractures related to arching of the intrusive mass. The diabase (dolerite) appears to have been emplaced prior to the final granite porphyry. The main Tertiary intrusive complex is elongated in a north-easterly direction and has dimensions of the order of 15 x 1.5 to 6.5 km, with dykes extending beyond the main mass at either end. The quartz-monzonite is apparently both stock and laccolithic, passively intruding Proterozoic granite to the north and locally at least, intruding as thick sheets which arch the enclosing sediments to the south. In so doing it has engulfed some of the sediments, leaving isolated blocks, and formed domes in the sequence by intruding between the Proterozoic basement and the sedimentary cover (Moolick and Durek, 1966; Langton, 1972).
Three small breccia pipes are present in the district. Two are to the north-east in the younger granite porphyry, with diameters of 450 and 750 m respectively, while the third to the south is around 350 x 60 m. All are associated with quartz-monzonite to granite porphyry contacts, and all are elongated in a west to north-west direction. The third contains chalcopyrite mineralisation at depth (Moolick and Durek, 1966).
Mineralisation and Alteration
Intense fracturing in the orebody has broken the quartz-monzonite porphyry into fragments a few cm's in diameter. The fractures are erratic in trend and fairly abundant in all directions. North dipping fractures strike between 45 and 65° and are the most persistent and mineralised, resulting in pronounced sheeting on the west and east sides of the mine, and veins in the south and centre. Subsidiary fractures strike at 105°, 140°, 12° and 35° (Moolick and Durek, 1966). Imbricate structures are evident to the east of the mine with Proterozoic granite locally overlying Cambrian sediments (Langton, 1972).
The mineralisation extends for some 500 m from the southern contact of the intrusives into the sediments, with local chalcopyrite and sphalerite, and supergene mineralisation confined to pyritic veins in non-carbonate rocks. The northern edge is marked by a gradual decrease in intensity of mineralisation, accompanied by deep oxidation that has destroyed much of the enrichment in the Proterozoic granite host. To the west the mineralisation weakens rapidly and becomes increasingly pyritic. The eastern edge is bounded by a pyritic interval and by a fault zone. East of the fault there is only weakly disseminated chalcopyrite with minor pyrite, and a thin oxidised cap with little supergene enrichment (Moolick and Durek, 1966).
The hypogene mineralisation consists of small veinlets and disseminations of pyrite, chalcopyrite, molybdenite, sphalerite, rare galena, and traces of gold and silver. Chalcopyrite is difficult to observe in most of the protore. Molybdenite generally occurs as thin films on fractures devoid of other sulphides, but also occurs as flakes and parallel streaks in small quartz veinlets. Although widely distributed in both the sediments and intrusives, molybdenite is most abundant in the granite porphyries and to a lesser extent the Proterozoic granites. Sphalerite is present in the protore in almost equal concentrations to copper, but is totally replaced in the enriched blanket. Gold and silver are sometimes enriched two to three times in the upper part of the zone of enrichment. Both are more abundant in the porphyry than in the granite, and have an inverse relationship with molybdenum (Moolick and Durek, 1966).
The hypogene mineralisation at Morenci comprises from 3.5 to 8% total sulphides, predominantly pyrite with accompanying chalcopyrite and sphalerite, and minor molybdenite. The grade is of the order of 0.1 to 0.15% Cu with zones of higher grade hypogene pyrite-chalcopyrite±bornite mineralisation that are generally >0.3% Cu. The zinc content of the un-enriched primary mineralisation is also of the order of 0.1 to 0.15% Zn. The limited higher grade hypogene ore averages 0.6 to 0.7% Cu. Primary mineralisation is apparently associated with both the quartz-monzonite porphyry and the later granite porphyries (Mine visit briefing, 1991).
The first phase of alteration was a K-silicate stage, involving the development of biotite, quartz and K-feldspar as both veining and pervasive alteration, partially preserved (and poorly known) below the clay alteration associated with the secondary enrichment. This K-silicate alteration is apparently developed in equilibrium with the host rocks, with biotite and K-feldspar being developed in rocks containing the same mineral. The early K-silicate phase was overprinted by quartz-molybdenite veining, followed by quartz-sericite alteration with accompanying chalcopyrite, and then by fissure veins with Au-Ag-Bi which are probably later than the stockwork mineralisation. The sericite alteration appears to emanate outwards from fractures, with sericite selvages, grading outward into montmorillonite clays. Intense kaolinisation is associated with the development of the subsequent chalcocite blanket, masking all previous alteration (Mine visit briefing, 1991).
Appreciable contact alteration has taken place on the northern side of the orebody, for a distance of up 450 to 600 m from the mine and in the Metcalf area at the north-eastern end of the intrusive. In both areas Palaeozoic sedimentary rocks are in contact with the main intrusive and have been intruded by dykes and sills. In general these rocks have been altered to calc-silicate or pelitic hornfels and skarn. The mineral assemblages consist of diopside, epidote, garnet and tremolite with local actinolite, chlorite, idocrase, magnetite, specularite, serpentine and talc. Apatite occurs with pyrite, magnetite and pyrrhotite at one locality. Wollastonite and aluminous silicates are absent. In the limestones fine grained, light green or grey diopside has replaced much of the limestone near the main intrusive. Small pods of sphalerite, pyrite and chalcopyrite or bands of disseminated magnetite are commonly present. In the proximity of dykes, epidote occurs in the limestones in bands from a few cm's to 6 m out from the dyke. Andradite occurs beyond the epidote in granular or massive sheets as wide as 30 m from the dyke. Magnetite occurs widely, but is most conspicuous in masses adjacent to dykes and as selective replacements of limestone beds. Shales are fissile and only slightly altered to grey or dark green hornfels with streaks of epidote and specks of pyrite. Fibrous green amphibole replaces chlorite. Porphyry dykes are only slightly altered with minor chlorite and epidote along fractures and the dyke edges. This pervasive alteration extended for up to a hundred metres from the main intrusive. Small blebs, stringers and disseminations of hypogene pyrite, sphalerite and chalcopyrite are found within the altered sediments, but are rarely at high enough grade to economically exploit. High grade oxidised copper occurring as tabular bodies of malachite, azurite and tenorite were exploited by early mining. These were hosted by skarn altered limestone above or below a non-reactive bed of garnet-bearing shale or quartzite (Moolick and Durek, 1966).
These zones of alteration produce a concentric outward pattern of K-silicate, quartz-sericite, and propylitic alteration, with skarn on the intrusive margins. The chalcocite blanket is embraced by intense argillic alteration and occurs within the limits of the quartz-sericite zone. The higher grade hypogene mineralisation is largely within the inferred K-silicate zone.
The supergene blanket at Morenci is best developed below the central section of the leached cap, where it is 150 to 300 m thick, thinning towards the margins to 50 m, with lateral dimensions of approximately 1800 x 1800 m. It carries from 0.4 to >1%, averaging 0.8% Cu and is underlain by poorly enriched mineralisation grading from 0.2 to 0.4%, averaging 0.3% Cu. This is in turn underlain by the 0.1 to 0.15% Cu protore. The average thickness of 0.8% Cu mineralisation is 120 m in the main pit. The mineralogy of the secondary enriched zone is principally chalcocite which has nucleated on the pyrite, chalcopyrite and sphalerite of the primary zone. Covellite is found at the top of the enriched zone (Mine visit briefing, 1991). In the supergene zone chalcocite has replaced pyrite in varying amounts, dependent upon the degree of enrichment, while covellite tends to appear in the areas of more weakly developed enrichment. Native copper is not abundant, but occasionally occurs in the lower part of the leached capping or in oxidised veins, accompanying cuprite and limonite. Thin plates and nodules of turquoise are found in close association with generally buried diabase (dolerite) dyke systems which cross the orebody from NW to SE (Moolick and Durek, 1966).
The development of the chalcocite blanket took place in two steps. The first stage developed a mature blanket subsequent to emplacement of the hypogene mineralisation in the early Eocene. The oldest volcanics in the district are dated at 33 Ma, and at least 300 m of tuffs and intermediate volcanics masked the orebody by the late Miocene. These volcanics apparently supplied large quantities of water, carbon dioxide and sulphur dioxide to the district, swelling the water table which, it is interpreted, permitted an even distribution of the chalcocite supergene blanket over 300 m in thickness. During this volcanism the lateral enrichment occurred in a southerly direction across the Morenci blanket and into the permeable quartzites beneath the calcareous sediments. Evidence of continued pyritisation during volcanism exists near the breccia pipes with large veins of barren pyrite cutting enriched pyrite and chalcocite (Langton, 1972).
During the Miocene rapid uplift of several thousand metres took place, accompanied by extension and the development of local grabens. It is suggested that as a result of this uplift the water table was dropped drastically and the mature blanket was stranded, to be subsequently leached, oxidised and eroded, leading to the late Miocene to early Pliocene enriched blanket. This younger immature blanket is typified by covellite and chalcocite partially replacing chalcopyrite. The lower water table was conducive to further enrichment of the lower Morenci body, resulting in the grade decreasing less with depth than is normal in such supergene blankets. During this period lateral enrichment is implied also (Langton, 1972)
The un-eroded remnants of the leached capping at Morenci is commonly 150 to 200 m thick and generally carries less than 500 ppm Cu with no sulphides, in contrast to the hypogene grade of 0.1 to 0.15% Cu. The thickest development of the leached cap is on one margin of the orebody where it reaches a preserved thickness of 300 m but passes directly into un-enriched protore, suggesting lateral movement of the leached metal. Despite this evidence of lateral transport, the secondary mineralisation is usually directly below the leached cap, suggesting an overall vertical transport of leached Cu (Mine visit briefing, 1991).
To the north-west of the mine along the contact of the quartz-monzonite and the Proterozoic granites the Coronado vein hosted a series of rich shoots of high grade mineralisation.
Published production and reserve figures for Morenci/Metcalf include:
Production + Reserve, 2003 - 6700 Mt @ 0.42% Cu (Melchiorre & Enders, 2003).
Milling Reserve, 2000 - 416 Mt @ 0.64% Cu (Phelps Dodge, 2000).
Leaching Reserve, 2000 - 1864 Mt @ 0.22% Cu (Phelps Dodge, 2000).
Milling Reserve, 1994 - 433 Mt @ 0.72% Cu (AME, 1995).
Leaching Reserve, 1994 - 1082 Mt @ 0.32% Cu (AME, 1995).
Milling Reserve, 1992 - 590 Mt @ 0.76% Cu (American Mines Handbook, 1994).
Leaching Reserve, 1992 - 875 Mt @ 0.34% Cu (American Mines Handbook, 1994).
Reserve @ 0.45% cut-off - 665 Mt @ 0.79% Cu (Visit, 1991).
Resource - 2900 Mt @ unstated grade (Visit 1991).
Reserve 1989 - 777 Mt @ 0.775% Cu (Titley, 1992).
Production to 1981 - 661 Mt @ 0.82% Cu, 1.2 g/t Ag, 0.024 g/t Au, 0.0086% Mo (Titley, 1992).
Production to 1972, - 400 Mt @ 0.96% Cu (Gilmour, 1982).
Remaining ore reserves and mineral resources - at December 31, 2011 (Freeport-McMoRan, 2012):
proved + probable reserve - mill ore - 448 Mt @ 0.48% Cu, 0.025% Mo (Cu - 79.1%; Mo - 38.9% recovery);
proved + probable reserve - crushed leach ore - 603 Mt @ 0.58% Cu (77.8% recovery);
proved + probable reserve - ROM leach ore - 3199 Mt @ 0.18% Cu (43.3% recovery).
inferred resource - mill material - 508 Mt @ 0.37% Cu, 0.018% Mo;
inferred resource - leach material - 1893 Mt @ 0.22% Cu.
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.
Enders M S, Knickerbocker C, Titley S R and Southam G, 2006 - The Role of Bacteria in the Supergene Environment of the Morenci Porphyry Copper Deposit, Greenlee County, Arizona : in Econ. Geol. v101 pp 59-70|
Melchiorre E B, Enders M S 2003 - Stable isotope geochemistry of Copper Carbonates at the Northwest Extension deposit, Morenci district, Arizona: implications for conditions of supergene oxidation and related mineralization: in Econ. Geol. v98 pp 607-621|
Moolick R T, Durek J J 1966 - The Morenci district: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 221-231|
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|
| References in PGC Publishing Books:||
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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