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Superior, Magma Mine |
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Arizona, USA |
| Main commodities:
Cu Au Ag
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Magma Mine at Superior in south-central Arizona is developed on mineralisation within the transgressive Magma Vein and on replacement mineralisation within Palaeozoic limestones. Mineralisation at Magma is neither strictly 'porphyry style', nor are skarns developed. The mine is approximately 18 km to the south-west of the Miami Inspiration Mine in the Globe-Miami district, and is 17 km to the north-west of the Ray deposit.
Published production and reserve figures include:
Production to 1978 - 26 Mt @ 4.5% Cu, 53 g/t Ag, 0.92 g/t Au (Titley, 1989)
Reserves 1976 - 13 Mt @ 4.5% Cu, 24 g/t Ag, 0.89 g/t Au (USBM)
The Magma/Superior mine was operated by the Magma Copper Company - Superior Division, which was subsequently purchased and passed to BHP Billiton. Total copper metal production at Superior in 1992 was 11 100t (Am. Mines H'book, 1994). The mine was worked from the mid 1960's to its closure in 1996 (Paul & Manske, 1999).
Geology
The host sequence and intrusives in the Globe/Miami and adjacent Superior District are detailed in the 'Globe/Miami District' record.
Recent work has differentiated some of the intrusives thought earlier to have been of Laramide age (as shown by Hammer & Peterson, 1968). A small medium to fine grained quartz-diorite stock to the north-east of Magma which is composed of plagioclase and variable hornblende, pyroxene, biotite and sporadic quartz was regarded as being of Mesozoic to Tertiary age by Hammer & Peterson (1968). Similarly they regarded diorite porphyry occurring as elliptical bodies, thin dykes and sills composed of plagioclase and locally hornblende in an aphanitic groundmass, which are partially to completely altered to clay, sericite and calcite, as also being of this age. Both may however be either Tertiary of Middle Proterozoic in age, after Creasey (1980).
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) marking 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. This zone is the locus of Mesozoic and Tertiary silicic intrusions which are interpreted as being associated with mineralisation in the districts. The mineralisation is developed at the intersection of this feature with a second major north-west trending structural zone (Hammer & Peterson, 1968).
Mineralisation
The Magma Fault and related parallel mineralised fractures in the vicinity of the Magma Mine strike in an east to north-east direction, and do not displace the dacite or any other Cainozoic formations (see the geological description in the 'Globe/Miami District' section above). These mineralised fractures are however, consistently cut and offset by a set of north to north-west trending faults which also displace the Cainozoic units and are part of a state wide structural belt. A major north-south structure, the Concentrator Fault, is developed close to the mine and forms a major scarp. The east-west Magma and offshoot Koerner Fault carry the richest mineral deposits of the district and the majority of the Magma ore (Hammer & Peterson, 1968).
Three forms of mineralisation are recognised in the Magma area (Hammer & Peterson, 1968), namely:
1). The Magma Vein, which has accounted for the majority of the copper production, occurring in the east striking Magma Fault and its local splits and branches. Other east striking veins in the district have produced minor ore, mainly oxidised manganese and silver ores. The bulk of the mining in the Magma Vein was from the "Main Orebody" a shoot which plunges steeply to the west, paralleling but not reaching the north-south Main Fault, an offshoot of the Concentrator Fault. This orebody has been stoped continuously over a vertical interval of nearly 1500 m. Other offshoot shoots and parallel zones of discontinuous shoots are also mined. The Magma vein occupies a strike interval of the Magma Fault that is near 3 km in length. It dips at 65°N, near the surface, but then reverses to 78°S at depth, with normal movement, south side down. The Koerner Vein, where mined is about 300 m to the south of the Magma Vein. The Magma Vein is a quartz-sulphide body that has replaced gouge and sheared wall-rock. It ranges from less than 25 cm to more than 15 m in thickness. The maximum length of continuously stoped ground is over 700 m. It is frequently confined to a single fault with sharp, well defined gouge planes as bounding structures, but locally has splits and branches.
Hammer & Peterson (1968) point out that the distribution of shoots within the Magma Vein bears an incidental relationship to the sections of the vein in which one or both walls are diabase (dolerite). From plans in this paper it seems that more than 75% of the stoped ground is in vein with one or both walls of diabase.
The chief ore minerals are chalcopyrite, bornite, enargite, tennantite, chalcocite, digenite and sphalerite. The principal gangue minerals are pyrite and quartz. Most of the ore mined above the 270 m below surface level was either enriched by supergene processes or oxidised, while small patches of enriched or oxidised ore are encountered to about 600 m depth.
Wall rock alteration is not conspicuous and has received little attention, particularly in the sediments. In the oxidised section of the mine the diabase (dolerite) walls have been intensely argillised. In the primary zone the diabase within the vein is completely altered to quartz and sericite, while in the walls alteration to very fine quartz, sericite and chlorite has taken place. Alteration of porphyry dykes within the vein zone is also intense, mainly as silicification. Porphyry dykes and sills outside of the vein have been well altered to a quartz-sericite assemblage. Pinal Schist and quartzite beds only show alteration for a few cm's beyond the vein, mainly as silicification and sericitisation.
There is a lateral zoning upwards in the Magma Vein from enargite to digenite in the lower sections to Cu-Fe sulphides in the middle section to sphalerite in the upper parts of the vein where Zn>Cu. The Cu-Fe minerals are not apparently zoned outwards from a 'hot centre' but are zoned locally from presumed channel-ways, from high tenor enargite-tennantite or chalcocite-bornite, outwards to low tenor pyritic chalcopyrite. In addition hematite-siderite veins are associated with the low tenor pyrite-chalcopyrite, but absent from the high tenor assemblages.
2). Limestone Replacement Orebodies, which are mainly hosted by the 3 to 10 m thick dark grey crystalline limestone in the lower part of the Devonian Martin Limestone, about 3 to 6 m above the base of the formation. These replacement deposits are found in the eastern part of the Magma Mine and at several other localities in the district. In the upper levels of the mine, to 600 m depth, the most productive pods are aligned along the stronger east-striking fractures, and occur as pods and short discontinuous zones. Below 600 m replacement orebodies in the Martin Limestone are large tabular masses whose outlines are irregular in detail, but are largely east-west in elongation and plunge at 30°E in the plane of the limestone bed. Four bodies were known in 1968, the first of which had an apex at the 600 m below surface level and continued to 1100 m. These bodies range in thickness to as much as 27 m. The longest strike extent of continuous replaced limestone was 300 m in 1968. Similar mineralisation has also been found in the Mississippian Escabrosa Limestone.
These are mineralogically similar to the vein ores but have different textures. Chalcopyrite is the dominant ore mineral and is commonly accompanied by bornite. Enargite, tennantite and chalcocite are scarce, digenite is absent. Bladed specularite and pyrite are the most abundant gangue minerals, and quartz, barite and calcite occur sporadically, generally as crystals lining vugs. Hematite and pyrite are zoned, with more hematite in the upper levels, while it decreases as pyrite increases in the lower levels.
No skarn or hornfels has been developed in the limestone, even adjacent to porphyry dykes and sills, and 'jasperisation' is rare. The limestone is not appreciably dolomitic with less than 1% MgO. Un-replaced Martin Limestone has been recrystallised, normally displaying a uniform, very finely grained crystalline texture, but becomes vuggy and porous under 'advanced attack by hydrothermal fluids'. Alteration of the Escabrosa Limestone is more pronounced with dissolution of limestone and development of stylolites, slumpage and destruction of primary sedimentary features. Veinlets and irregular masses of buff to white calcite have recrystallised in these altered zones, while some of the freshly broken limestone is fetid. No mineralisation has been found in association with the latter alteration.
The favourability of the lower Martin Limestone has been observed in many mines and prospects in southern Arizona, although the reasons are not known. The favourability of the upper and lower Escabrosa Formation has also been demonstrated in the district. The ore zones are clustered near major east striking mineralised veins, but the direct connection to the veins is not necessarily always apparent. Barren sections of veins may be adjacent to well mineralised limestone. Pre-mineral faults that cut the limestone beds in which replacement occurs have a profound effect upon the pattern of ore shoots. Some faults appear to have acted as channelways while others are barriers.
3). Stockwork at the Silver King Mine, which is some 2.5 km to the north-east, has been mined principally for Ag within a supergene zone developed on a stockwork containing sulphides of Zn, Pb and Cu within a plug of diorite porphyry, that was in turn intruded into a larger stock of quartz-diorite. This stockwork of inter-lacing veins from paper thin fracture coatings to 5 mm thick veins was of very limited extent.
The most recent source geological information used to prepare this decription was dated: 1997.
This description is a summary from published sources, the chief of which are listed below.
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