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Warren Mining District - Bisbee, Lavender Pit, Cochise, Copper Queen |
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Arizona, USA |
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Cu Au Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The producing area within which ore deposits are located at Bisbee (also known as the Warren Mining District) covers an area of approximately 7.5 sq. km.
Bisbee deposits, total production to 1981 - 152 Mt @ 2.35% Cu, 21 g/t Ag, 0.58 g/t Au (Titley, 1992).
Lavender Pit had pre-mining reserve of 120 Mt @ 0.81% Cu (Gilmour, 1982).
Lavender Pit was mined from 1951 to 1974 to produce - 84 Mt of mill ore @ 0.81% Cu and
99 Mt of leach ore @ 0.35% Cu (Briggs, 2015).
Copper Queen underground, below & NW of Lavender Pit produced 48 Mt @ 6% Cu (Briggs, 2015).
Cochise immediately to the NE of the Lavender pit had a resource in 2015 of 254 Mt @ 0.46% Cu (Freeport McMoRan, 2015).
Over the life of the Warren mining district, from 1880 to 2013, 3.59 Mt of Cu, 0.147 Mt of Pb, 0.161 Mt of Zn, 0.013 Mt of Mg, 86.8 t of Au and 3180 t of Ag were recovered from the mines of the district (Briggs, 2015).
Within the Warren district the geology is divided into two sections by a NW-SE trending canyon. To the SW the exposures are principally Palaeozoic rocks with occasional windows of Middle Proterozoic Pinal Schist, while to the NE the rocks are mainly Cretaceous, underlain by a strip of granite (the Juniper Flat Granite) immediately adjacent to the canyon. Dips in the Palaeozoic suggest that the granite is emplaced in a NW-SE trending domal anticlinal axis. The ore deposits of the district are found on the south-eastern end of this anticlinal dome. Some 3 km east of the Juniper Granite an irregular shaped stock-like mass, approximately 1.5 km in diameter, the Sacramento Quartz Porphyry manifests itself as the focus of mineralisation in the district. A major east-west trending fault, the Dividend Fault passes through and displaces the Sacramento Quartz Porphyry (Bryant & Metz, 1966).
The Cochise project is an un-mined (in 2015) supergene chalcocite blanket resource immediately to the north of the Lavender pit which is developed within the pyritic quartz porphyry of the Sacramento Quartz Porphyry and the Pinal Schists. Little detail is available. Its resource figures are listed above. As it is a chalcocite blanket deposit it is assumed to be leachable (S Titley, pers. comm.; Am. Mines H'book, 1994).
Historically the Bisbee mine has been operated by Phelps Dodge Corporation, and has subsequently passed to the ownership of Freeport McMoRan. Mining in the Lavender pit was suspended in 1974, while the adjacent underground mines continued to be worked by tributors removing higher grade Cu-Au ore. Leaching of dumps yielded 1400 t of Cu in 1992 (Am. Mines H'book, 1994).
Geology
The oldest rocks in the district are the Middle Proterozoic Pinal Schists. Locally they comprise steeply dipping (where lithological banding is observed) quartz-sericite schists, with accessory tourmaline and red garnets (Bryant & Metz, 1966). In outcrop the Pinal Schists are variable, ranging from a fairly massive, dark, silt-sized granular metamorphic to a strongly contorted, well foliated dark metamorphic with ptygmatic quartz veining. The Pinal Schists are generally found in areas of low relief (Pers. observ.).
The Palaeozoic commences with Cambrian quartzite of the Abrigo Formation (130 m thick), overlain by middle to late Cambrian limestone and sandy limestone with interbedded shale bands and quartzites (230 m thick). Ordovician to middle Devonian rocks are absent with the Cambrian sequence being unconformably overlain by more than 100 m of upper Devonian Martin Formation black shaly to black crystalline limestone. This unit is in turn followed conformably by the early to middle Mississippian Escabrosa Limestone, 180 to 250 m thick, composed of crinoidal limestone with distinctive black interbeds and chert rich sections. These are overlain by up to 300 m of Pennsylvanian Horquilla Formation limestone, and 140 m of Pennsylvanian to Permian sediments comprising a lower clastic unit and an upper limestone-dolomite unit. The overlying Permian is a black fossiliferous limestone 75 m thick (Bryant & Metz, 1966).
Over-all the Palaeozoic sequence is flat lying, although local steep to over-turned sections are obvious. In outcrop the carbonates are generally light grey in colour with lesser interbeds of fissile shale (Pers. observ.).
The Mesozoic comprises approximately 1500 m of lower Cretaceous, composed of a 0 to 1000 m thick red polymictic conglomerate, the Glance Conglomerate, followed by 550 m of alternating shale and sandstone grading up into 200 m of limestone which becomes more pure upwards. These are in turn conformably followed by 550 m of alternating shale and sandstone. Quaternary fluvial sediments cover the sequence in part (Bryant & Metz, 1966).
The Juniper Flat Granite has a typically granitic texture composed of quartz, orthoclase, plagioclase and biotite, but is locally porphyritic with lesser plagioclase and biotite. These intrude all of the Pre-Cretaceous rocks in the district, and have numerous associated dykes, irregular intrusive bodies and sills of rhyolitic porphyry composition. These granites are generally not altered, but are believed to be co-magmatic with the altered Sacramento Quartz Porphyry (Bryant & Metz, 1966). The age of the Sacramento Stock and Juniper Flat Granite is dated at 163 to 178 Ma, which is in the middle Jurassic (Titley, 1982). In outcrop the Juniper Flat Granite is commonly leucocratic, with few mafic minerals. It is generally a ridge forming lithology with a characterisitic pink colouration in the distance. The feldspars weather in outcrop and are partly altered to very fine mica (Pers. observ.).
The Sacramento Quartz Porphyry is a composite intrusive body, composed from the centre outwards of:
1). an older, intensely silicified, pyritised quartz porphyry,
2). a breccia consisting of an intensely silicified mixture of schist, quartzite, limestone and quartz-porphyry fragments more or less confined to the southern margin of the stock (possibly an intrusion breccia related to the emplacement of the silicified, pyritised quartz porphyry with which it is closely associated),
3). a sericitised, slightly pyritised, feldspar quartz porphyry in the easterly section of the stock, but also prevalent in dykes and sills, and
4). a breccia consisting of a heterogeneous agglomeration of rounded fragments of schist, quartzite, limestone, both types of porphyry and low grade siliceous sulphide (an intrusion breccia) prevalent as large masses in the stock and small patches in the adjacent limestones. The matrix of this latter breccia is composed of fine grained rock fragments usually cemented by calcite and/or silica and heavily impregnated by pyrite and copper sulphides (Bryant & Metz, 1966).
The quartz porphyry of the Sacramento Stock has been thoroughly altered, silicified and pyritised destroying the original composition and texture. It is now composed of anhedral grains of quartz, intergranular pyrophyllite and scattered large rounded and embayed phenocrysts of quartz. Accessory minerals include dickite, alunite and rutile with minor zircon, apatite and bastite with more than 15% sulphides, mainly pyrite (Bryant & Metz, 1966). In hand specimen it is a pale grey very siliceous rock which is strongly fractured and has abundant pyrite present as veins up to 5 mm thick and as irregular disseminated blebs. It has soapy feeling on fracture faces. Fresh pyrite is present in exposed faces within 10 m of the surface with associated jarosite staining. At the surface and on faces between the jarosite zone and the surface there is a strong red-brown staining after goethite both on fractures and pervasively through the weathered rocks (Pers. observ.).
The feldspar-quartz porphyry is a soft greenish-grey thoroughly altered rock with scattered disseminated pyrite, and is distinctive by the absence of intense pyritisation and silicification. It is believed to post-date massive sulphide development. Within the Lavender Open-pit however it has been intensely sericitised with the destruction of the original texture. It now comprises embayed quartz phenocrysts, pseudomorphs of sericite after feldspar and chloritic biotite phenocrysts in a groundmass of micro-crystalline quartz and sericite. Accessory minerals are zircon, apatite and rutile (Bryant & Metz, 1966).
Mineralisation and Alteration
Two styles of mineralisation are represented, namely 1). disseminated Cu, and 2). limestone replacement mineralisation peripheral to the disseminated Cu (Bryant & Metz, 1966).
The disseminated copper mineralisation, particularly that of the Lavender Pit, is localised within the Sacramento Stock and is controlled by horse-tailing of the Dividend Fault. Significant mineralisation is restricted to the south, or down-thrown side of that fault (Bryant & Metz, 1966). Mineable primary mineralisation is largely massive pyrite-bornite with minor chalcopyrite, distributed erratically as isolated pods and lenses in the highly silicified and sericitised breccias of the Sacramento Stock. Most of the ore extracted is present as supergene enrichment (Einaudi, 1982). Oxidation, leaching and re-deposition of Cu prior to sedimentation of the overlying Cretaceous sequence produced a supergene chalcocite blanket which dips to the east and is from 15 to 120 m thick. The upper surface is undulose and conforms to the base of the unconformity below the Cretaceous sediments. Copper has been thoroughly leached from the oxide capping, with a sharp transition to the supergene sulphides with no mixed oxide-sulphide zone (Bryant & Metz, 1966).
Within the orebody there are four types of ore, in which the style of mineralisation and alteration is controlled by the different host rock types of the Sacramento Stock described previously, namely (from Bryant & Metz, 1966),
1). The intrusive breccia - where the ore occurs as supergene replacement of hypogene sulphides (principally primary Cu minerals) which are disseminated both in the matrix and along fractures within the fragments. The main ore mineral is sooty chalcocite, with lesser covellite, nucleated, where observable, on chalcopyrite, bornite and sphalerite. Grade is high but erratic;
2). The possible intrusion breccia - where sporadic, relatively small, irregular lenses of very rich chalcopyrite and bornite occur, grading outwards into disseminated mineralisation. There is little supergene enrichment in this style;
3). The argillised younger feldspar porphyry - which is localised in the eastern part of the pit adjacent to the intrusive breccia. It is characterised by intense argillisation that may be related to the supergene process. The ore consists of disseminated shattered grains of pyrite which have been replaced by sooty chalcocite. The primary mineralisation is generally low grade and only of significance in the vicinity of intense fracturing; and
4). The intensely sheared and broken siliceous older porphyry - which to date has been the least productive. It is hard, tough, thoroughly silicified and resistant to fracturing and erosion. It is almost completely altered to an advanced argillic assemblage of quartz-pyrophyllite-sericite and contains 15 to 18% sulphides, practically all pyrite. Because of its impervious nature there is little supergene enrichment. Apart from some high grade pods, this mineralisation style is largely uneconomic. In outcrop however on the northern side of the Lavender Pit this rock is strongly fractured with abundant pyrite veins as described above. The observed outcrop does not conform completely to the description given in Bryant & Metz (1966).
Overall mineralisation in the disseminated Cu orebody is very erratic and as such difficult to mine. It is also difficult to treat due to the fractured nature of the pyrite which has chalcocite developed in hairline fractures and on faces (Bryant & Metz, 1966).
Within the Lavender pit the pyritic, silicified quartz porphyry was obviously pyritic with strong jarosite staining from near the surface to the bottom of the pit. In contrast the other main rock types of the Sacramento Stock were heavily stained by iron oxides to the bottom of the pit (Pers. observ.).
The limestone replacement deposits, are disposed in a semi-circular arrangement around the Sacramento Stock. Off-shoots radiate outwards like the spokes of a wheel, reflecting the nature of the faults and fractures in the shattered zone surrounding the stock. Copper ore has been found in all of the Palaeozoic limestones, though the most productive has been the upper crystalline to sandy limestone of the middle to late Cambrian, the Upper Devonian and the early to middle Mississippian limestones. These account for a total thickness of around 300 m. The favourable formations are brittle and tend to shatter when deformed, while the less favourable tend to yield resulting in folds or failure along a single break. The favourable units have apparently undergone intense fracturing or brecciation prior to mineralisation, forming cigar, ovoid and/or pipe shaped orebodies, with the long axes being along the intersection of a fracture and a particular limestone bed. In each case there is a large body of low grade (<1% Cu) siliceous pyrite below the productive limestone unit, with ore being above and peripheral to these siliceous, pyritic accumulations (Bryant & Metz, 1966). The massive sulphide bodies are zoned with a gradation from the low grade siliceous-pyrite cores, to an intermediate pyrite-chalcopyrite±bornite ore zone to peripheral pyrite-sphalerite-galena. In some localities specularite occurs between the sulphides and limestone. The limestone ores at Bisbee are very similar to the silica-pyrite zones at Ely and the manto ore at Cananea (Einaudi, 1982).
Adjacent to the pyritic, silicified, quartz porphyry limestone was observed to be intensely pyritised to semi-massive sulphides. While bedding was still obvious in the strongly silicified and pyritic sediments the pyrite was present in a similar style to that in the adjacent porphyry. Further removed from the porphyry the limestones are overall un-mineralised and un-altered, apart from the local development of marble. Zones of sulphide mineralisation are seen in road cutings in the mine area, most associated with old workings. The largest seen had dimensions of 2 x 4 m in a large road cutting face of fresh, grey, un-altered limestone. Mineralisation was present as a mass of sulphide veining, with individual veins being up to 1 cm thick of fine vuggy massive pyrite cutting recrystallised and silicified limestone. The sulphide mass appeared to be localised by a minor fault cutting the limestone (Pers. observ.).
Calc-silicates, dominantly tremolite and chlorite with accessory grossularite, diopside and idocrase, occur as microscopic grains in greenish marble up to 350 m from the stock. They are most prominent in the impure, shaly-sandy limestone of the Abrigo and Martin Formations, and may possibly be a metamorphic product. The contact between the porphyry-breccia stock and the relatively pure Escabrosa Limestone is characterised by a narrow fringe of calc-silicates within marble. Locally up to 60 m from the stock the marble is replaced by a granular aggregate of quartz-pyrite-calcite. The ore extends beyond the zone of contact silicates (Einaudi, 1982).
Much of the ore is closely associated with porphyry dykes and sills, commonly occurring along the margins of the dyke, although it is rare for the porphyries (except in the main stock) to contain sufficient mineralisation to make ore themselves. Closely associated with the replacement bodies are intrusive breccias, occurring between limestone beds, or along joints and fractures, and forming either a wall rock or host to ore (Bryant & Metz, 1966).
The size of individual orebodies is variable, ranging from a few thousand to, in exceptional instances, more than a million tonnes. Possibly two thirds of the production has been from bodies of 25 000 t or less. However within the ore zones intermittent lenses/orebodies may be found over a large area, as in one example where these bodies are found over a 600 m vertical distance within a horizontal area of 600 x 150 m. Much of the early mining was from oxidised orebodies associated with large ferruginous silica masses, the oxidised equivalents of the low grade siliceous masses associated with the primary sulphide bodies described above (Bryant & Metz, 1966). Analysis of sulphide ore suggests it is composed of 15% quartz, 15% Cu-Fe sulphides and 50% pyrite (Einaudi, 1982).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription 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.
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Selected References:
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Bryant D G, Metz H E 1966 - Geology and ore deposits of the Warren mining district: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 189-203
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Titley S R, 1996 - Alteration of mineralized carbonate rocks in the epicrustal environment: in Porphyry Related Copper and Gold Deposits of the Asia Pacific Region, Conf Proc, Cairns, 12-13 Aug, 1996, AMF, Adelaide, pp 3.1 - 3.10
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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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