Copper Creek - Mammoth Breccia

Arizona, USA

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The Mammoth copper rich breccia pipe is located within the Copper Creek mining district in southeastern Arizona, ~70 km NE of Tucson, Arisona, USA (#Location: 32° 45'N, 110° 29'W).

The Copper Creek mining district is known for its abundance of magmatic–hydrothermal breccia pipes, most of which are barren. The district contains more than 500 mineralised breccia pipes, many of which are barren, as well as concealed past-producing porphyry-style, copper-bearing stockworks, and distal lead-silver veins.

Deposits in the Copper Creek mining district formed during the Laramide orogeny, a period of extensive ~80 to 55 Ma magmatism in Arisona and New Mexico, USA, and northern Mexico, characterised by an igneous suite of calc–alkalic rocks, mainly andesitic to rhyolitic composition, indicating a continental margin arc province.

The main Laramide igneous rock units in the Copper Creek mining district include the 61.5±0.7 Ma (40Ar/39 biotite) Copper Creek Granodiorite and the associated 62.8±1.3 Ma (Shafiqullah et al., 1980) Glory Hole volcanic rocks, both of which are cut by younger dykes and irregular intrusions of various porphyry phases, some mineralised.

The Copper Creek Granodiorite and associated porphyries all intrude rocks of the Glory Hole Volcanics. The Glory Hole Volcanics are a heterogeneous sequence of andesitic to latitic welded tuffs, breccias, lavas and flow breccias (Guthrie and Moore 1978), which unconformably overlie a basement of Proterozoic and Palaeozoic sedimentary rocks. The Copper Creek Granodiorite has a variable mineralogical composition and appearance, but in general, is a greenish-grey to brownish-grey, medium-grained, porphyritic rock composed of plagioclase, quartz, potassium feldspar and biotite, with K feldspar and quartz, both of which rarely form phenocrysts, interstitial to the plagioclase. Biotite is present as both small euhedral phenocrysts and as interstitial anhedrons. Pale-green hornblende occurs in areas, while clinopyroxene is present elsewhere. Accessory minerals include titanite, Fe-Ti oxides, apatite, and zircon (Simons 1964).

Plugs and dykes of porphyritic intrusive rocks occur throughout the district, generally trending northwesterly, and have been classified as diorite porphyry and dacite porphyry (Guthrie and Moore, 1978). The oldest of these porphyry phases (although undated) is the diorite porphyry, which is thought to be a late phase of the Copper Creek Granodiorite.

The post-Laramide, early Tertiary was a time of erosion that was accompanied by a lull in arc magmatism (Dickinson 1989). The subsequent Eocene to mid-Miocene (37 to 15 Ma) time saw a renewal in arc magmatism (Dickinson 1989), during which rocks of the Galiuro Volcanics were erupted and widespread extension took place (Dickinson 1989).

The Copper Creek breccia pipes are hosted by both the Copper Creek Granodiorite and the Glory Hole volcanic rocks, and basiaclly consists of angular clasts of granodiorite cemented by quartz, chalcopyrite, bornite, anhydrite and calcite. Drilling indicates that the pipes are situated at the apex of small plugs of grey and dark porphyry, with a porphyry-matrix igneous breccia commonly marking the transitional between porphyry and overlying open-space breccia (Marsh, 2001). The same author also noted that some breccias in the district have slabby clast shapes and a weak, preferential subhorisontal clast orientation where the wall rocks are massive, whereas clasts are more equant and randomly oriented where wall rocks are strongly fractured. Peripheral contact zones of most breccia pipes have a conspicuous sheeted concentric fracturing pattern (spaced at mm to tens of cm) extending a metre or two beyond the breccia into wall rocks (Marsh, 2001).

The concealed Mammoth breccia has the overall shape of an upright cylindrical pipe, with an oval shape in plan view, having NNW-SSE elongated dimensions of ~200 x 100 m. It extends for >870 m vertically, with its top being within 42 m of the present surface. The bulk of the clasts within the pipe are angular to subangular, vary from <5 cm to several metres across and comprise fragments of Copper Creek Granodiorite, although locally there are also clasts of pink porphyry. The breccia is matrix-supported with quartz being the most common matrix mineral.

The Mammoth breccia pipe has three distinct mineralogical zones, from top to bottom, these are the:
Upper Ore Zone, which extends from 42 to 241 m below the surface and is estimated to contain 5.7 Mt @ 1.37% Cu as chalcopyrite (AMT International Mining Corporation 1999). The breccia in this zone has a matrix of quartz, calcite, dolomite and pyrite, with lesser chalcopyrite, and traces of bornite, molybdenite, and tennantite. Euhedral quartz crystals, up 5 cm in length, fill the open spaces between clasts. Pyrite, chalcopyrite and tennantite are found as coarse irregularly shaped blebs growing from the faces of the quartz crystals into open spaces between granodiorite fragments and quartz. Disseminated pyrite, chalcopyrite and bornite are locally found intergrown within sericitic alteration envelopes to clasts. Calcite fills some of the remaining, innermost open spaces in the breccia. Small amounts of scheelite, apatite, galena, sphalerite and specularite are also present.
Carbonate Zone, that extends from 241 to 579 m below the current surface and, although copper-bearing minerals are widespread, does not contain economic copper grades, only containing small volumes of sulphide minerals, the most common of which are pyrite and chalcopyrite, with traces of bornite and molybdenite. The matrix mineralogy, is dominated by quartz, although there is an increase in carbonate mineral abundance, including dolomite, ankerite and calcite. Islands of bornite occur within blebs of chalcopyrite that are adjacent to both euhedral quartz crystals and pyrite grains. In the deeper parts of the zone (>470 m), hydrothermal chlorite occurs in the breccia matrix with subhedral anhydrite and specularite.
Lower Mammoth-Keel Zone, extending from 579 to 800 m below surface, with an estimated resource of 4.0 Mt @ 1.34% Cu as chalcopyrite and bornite, but also containing potentially economic concentrations of Mo, Au and Ag, as indicated by an increase in grade and thickness at depth (AMT International Mining Corporation 1999). This zone is localised in the brecciated hanging wall of southeasterly dipping grey porphyry dykes (AMT International Mining Corporation 1999). The matrix gangue mineralogy comprises quartz, coarsely crystalline purple anhydrite, coarse-grained, flaky, and light to dark grey sericite, specularite, and traces of ankerite. The quartz crystals are smaller (<1 cm in length) than those higher in the pipe. Calcite oours as zoned crystals and stringers, cutting both chalcopyrite and pyrite, and also fills some of the remaining open space between euhedral quartz crystals. Sericite occuppies open spaces associated with chalcopyrite and anhydrite, while specularite blades occur in both the breccia matrix and in sericitic alteration envelopes. Sulphide minerals range in diameter from <1 to ~5 cm, of chalcopyrite>bornite>molybdenite with trace pyrite, galena, sphalerite and chalcocite filling open spaces between quartz and anhydrite crystals in the matrix of the breccia pipe, although, sulphides are also disseminated within the alteration envelopes along the margins of the granodiorite clasts and in grey porphyry dykes on the margins of the breccia pipe.

The Mammoth breccia pipe is characterised by extensive hydrothermal alteration of the clasts, with primary igneous minerals within the smaller clasts (<20 cm) completely altered, although larger clasts (>20 cm) have unaltered or weakly altered cores that preserve primary igneous minerals. Alteration envelopes in the clasts are well zoned, from highly altered margins, to fresh rock in the cores of larger clasts, with some variation in texture and mineralogy between the three depth zones of the breccia pipe. The most abundant hydrothermal alteration mineral is sericite with varying amounts of kaolinite and local potassium feldspar. Within the Upper Ore Zone, three spatially distinct envelopes which range in width from 1 to 4 cm are recognised in the larger clasts, with disseminated sulphides present within each envelope. The progression from the clast margins to the fresh granodiorite core consist of: i). sericite, ii). sericite±kaolinite, iii). sericite±kaolinite±chlorite. Within the Carbonate Zone, only two alteration envelopes occur in the clasts, i). sericite and ii). sericite±kaolinite±chlorite; while in the Lower Mammoth-Keel Zone, clast alteration is not as pervasive and only the sericite alteration envelope is present.

Remnant hornblende laths are locally recognised within the granodiorite clasts, although most have been altered to biotite, chlorite, or sericite. Most primary biotite has been altered to chlorite and/or sericite. Both K feldspar and plagioclase are altered to fine- to coarse-grained sericite. In some areas, the feldspars have been altered to kaolinite and calcite. Early K feldspar-biotite±magnetite alteration is common in the Lower Mammoth-Keel Zone where it is overprinted by sericitic alteration (Marsh 2001). Alteration of all three mineralogical zones (Upper, Carbonate and Lower Mammoth-Keel zones) is interpreted to be the result of one main hydrothermal event, suggested by the lack of cross-cutting relationships in the matrix mineralogy. However, the breccia matrix alteration minerals truncate the pre-brecciation K feldspar-biotite porphyry-style alteration assemblage locally found in some clasts, similar to the porphyry-style stockwork mineralisation found elsewhere in the district.

40Ar/39Ar dates on hydrothermal sericite indicate an age of 61.0±0.5 Ma for copper mineralisation.

Anderson et al. (2009) report that fluid inclusions suggest alteration and mineralisation was related to a supercritical fluid with a salinity of ~10 wt.% NaCl equiv. condensed to a dilute aqueous vapor (1 to 2.8 wt.% NaCl equiv.) and a hypersaline brine (33.4 to 35.1 wt.% NaCl equiv.). Minimum trapping temperatures are 375°C and trapping depths are estimated at 2 km. Sulphur isotope fractionation of cogenetic anhydrite and chalcopyrite yields a temperature of mineralisation of 469±25°C. Calculated oxygen and hydrogen isotope values for fluids in equilibrium with quartz and sericite range from 10.2 to 13.4‰ and -60 to -39‰, respectively, suggesting that the mineralising fluid was dominantly magmatic. Evidence from the stable isotope and fluid inclusion analyses suggests that the fluids responsible for Cu mineralisation within the Mammoth breccia pipe exsolved from a grey porphyry phase found at the base of the breccia pipe.

Estimated resources are (AMT International Mining Corporation 1999):
    Upper Ore Zone - 5.7 Mt @ 1.37% Cu,
    Lower Mammoth-Keel Zone - 4.0 Mt @ 1.34% Cu.

The most recent source geological information used to prepare this summary was dated: 2009.    
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.

  References & Additional Information
 References to this deposit in the PGC Literature Collection:
Anderson E D, Atkinson Jr W W, Marsh T and Iriondo A,  2009 - Geology and geochemistry of the Mammoth breccia pipe, Copper Creek mining district, southeastern Arizona: evidence for a magmatic–hydrothermal origin: in    Mineralium Deposita   v.44 pp. 151-170

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