Twin Buttes

Arizona, USA

Main commodities: Cu Mo
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The Twin Buttes deposit is located in the Pima District of southern Arizona, some 10 km to the SSE of the Mission Complex orebodies and 10 km to the ENE of the Sierrita and Esperanza mines. The deposit lies within the Arizona-New Mexico Basin and Range Province, and the broad Walker-Texas Lineament Zone, just to the north of the projected Sawmill Canyon Discontinuity, as described in the Sierrita/Esperanza section above.

Published production and reserve figures include:

    Production, 1965-77 - 63 Mt @ 0.70% Cu, 0.008% Mo (Titley, 1989)
    Reserve, 1970 - 475 Mt @ 0.78% Cu, 0.03% Mo (Einaudi, 1982)
    Oxide Production, to 1977 - 42 Mt @ 0.11% Cu (Titley, 1992)
    Oxide Reserve, 1989 - 10 Mt @ 0.73% Cu (Titley, 1992)
    Sulphide Production, to 1977 - 63 Mt @ 0.7% Cu, 3.4 g/t Ag (Titley, 1992)
    Sulphide Reserve, 1989 - 35 Mt @ 1% Cu (Titley, 1992)


The geological setting of the Pima District is outlined in the record covering the Mission Complex orebodies. Note that in contrast to the Mission and Sierrita-Esperanza areas, the sequence is steeply dipping, with the last folding following the Red Boy Rhyolite in the late Cretaceous.

Mineralisation at Twin Buttes is spatially related to an early Eocene (58.6 Ma) quartz-monzonite porphyry intrusive complex on the south-eastern margin of the Palaeocene to early Eocene (62 to 58 Ma) Ruby Star Granodiorite. The quartz-monzonite porphyry complex is joined to the batholith by a corridor of related, but slightly older quartz-monzonite, and is interpreted to be a late phase of the larger granodiorite body. Intrusives in the mine area, all of which are Palaeocene to Miocene in age, include in order of introduction, from oldest to youngest:

* Rhyodacite (or diorite porphyry) occurring as dykes;
* Fine grained porphyry, again present as dykes;
* Porphyritic quartz-monzonite (adamellite), grading locally to an aplite, which occurs as a broad NW trending mass connecting the mine area to the main granodiorite batholith to the west;
* The quartz-monzonite (adamellite) porphyry complex that is spatially related to the mineralisation and which is composed of three main phases, an aphanitic, a xenolithic and an aplitic quartz-monzonite porphyry, which together comprise a steep walled north-west trending stock up to 600 m across; and
* A pipe-like, quartz flooded breccia, containing partially rotated fragments of both the intrusive and country rocks which it penetrates. The clasts are set in a matrix of coarse crystalline vuggy quartz and sparse sulphides. There is minor mixing of lithologies near the pre-breccia contacts (Barter & Kelly, 1982).

Virtually all of the economic mineralisation however, is hosted by country rocks surrounding the intrusive complex (Barter & Kelly, 1982).

Mineralisation & Alteration

Higher grade hypogene mineralisation (>0.4% Cu) is generally confined to the late Carboniferous and Permian carbonate bearing units of the Earp Formation, Colina Limestone, Epitaph Dolomite, and Scherrer Formation, and in the arkosic siltstone, conglomerate and interbedded rhyolitic flows and tuffs of the Rodolfo Formation, Whitcomb Quartzite and the Angelica Arkose which unconformably overlie the Palaeozoic successions. Mineralisation is also found in part within the Proterozoic granite basement. The areas of higher grade mineralisation are segmented by faulting, and are partly separated by the lower grade central intrusive mass of the quartz-monzonite porphyry complex and by the quartz monzonite to the north-west. This separation has led to a mine designation of six orebodies (Barter & Kelly, 1982; Einaudi, 1982).

Three types of calc-silicate alteration are formed in the sediments (Barter & Kelly, 1982; Einaudi, 1982), namely,

1). Hornfels, which are developed throughout the Permian section, in both calcareous and siliceous sediments and are extremely variable in composition. They range from a) granular diopside-quartz hornfels with patches of fine grained garnet, developed from silty limestone or silty dolomitic limestone, to b). flinty, aphanitic hornfels with variable amounts of diopside, tremolite, biotite and feldspar, particularly in the Permo-Carboniferous Earp and Cambrian Abrigo Formations, to c). tremolite, biotite, sericite and feldspar bearing quartzite and siltstone beds in the Earp, Permian Epitaph Dolomite and Permian Scherrer Formation;
2). Calcic skarn, formed through metasomatic replacement of limestone in the Permo-Carboniferous Earp Formation, the Permian Colina Limestone, upper Epitaph Dolomite and Concha Limestone. Near the stock and along contacts with interbedded siltstone these beds yield granular to massive, dark reddish-brown andradite garnet and clino-pyroxene (diopside) skarn with interstitial quartz, calcite, chalcopyrite, pyrite and local magnetite. This garnetite is cut by abundant quartz-sulphide and quartz-epidote-sulphide veinlets and patches, especially within 30 m of the stock. Sulphides in the quartz veins are pyrite, chalcopyrite and molybdenite. The most persistent +1.5% Cu zones are within the Colina Limestone skarn that contain 4 to 6% sulphides, with local zones of up to 15 to 20%. Garnetite pervasively replaces limestone beds in the centre of the mine, although some limestone remnants are present within this garnetite farther from the stock. In these cases at the interface brown garnetite with chalcopyrite and pyrite replace limestone or an intervening wollastonite bearing marble with local bornite-chalcocite. In addition this marble contains pale brown and green garnet and idocrase in wollastonite with disseminated chalcopyrite, bornite, sphalerite and local anhydrite, as well as patches of massive sphalerite-chalcopyrite-pyrrhotite-magnetite in marble.
3). Magnesian skarn, formed in dolomitic beds of the Permian Colina Limestone, middle Epitaph Dolomite and middle Scherrer Formation. It is characterised by serpentine and tremolite with high magnetite and high chalcopyrite contents. Forsterite, or serpentine after forsterite are common in magnetite rich zones. Farther from the central zone, un-silicified dolomitic beds contain magnetite-pyrite veinlets with serpentine envelopes.

The quartz-monzonite porphyry complex has been locally subjected to potassium-silicate alteration along quartz veinlets, while sericitic alteration of feldspars and chloritic alteration of mafic minerals is also obvious. Pyrite, chalcopyrite and molybdenite are the principal sulphides, with pyrite:chalcopyrite ratios generally greater than 1, and primary Cu grades ranging from 0.01 to 0.25% Cu. The quartz-monzonite porphyry complex is interpreted as having been emplaced well after the onset of metasomatic mineralising processes. The earlier rhyodacite porphyry occurs as a steep sill like body, 75 m wide, parallel to bedding. It is pervasively biotitised and displays pyrite:chalcopyrite ratios of <1, with 0.1 to 0.5% Cu, and is interpreted to have been emplaced either prior to or during the main period of mineralisation, which was centred on the position now occupied by the quartz-monzonite porphyry complex (Barter & Kelly, 1982; Einaudi, 1982).

Approximately 20% of the ore reserves in 1970 were as oxide ore in the supergene zone, averaging 1.0% Cu, with 0.73% Cu and 0.3% Mo in the primary sulphides (Einaudi, 1982). The mineralised zone had been eroded, oxidised and enriched, with all but the north-western fringes subsequently buried beneath 90 to 250 m of post mineralisation alluvial material.

The depth to which supergene processes penetrated was primarily dependent upon the style of hypogene alteration mineralogy and permeability. Penetration was in siliceous rocks in contrast to adjacent skarns because of the degree of fracturing and the smaller amount of reactive minerals. Supergene processes at Twin Buttes are divided into three zones, namely;

1). Lower zone, where supergene influences have penetrated deeply, to the top of the anhydrite zone. Above this sulphate line, anhydrite has been removed forming a vuggy rock in garnetite, which enhances grade by reducing the volume of gangue. This is particularly important in the Epitaph Dolomite which may contain up to 50% anhydrite, with resultant grade enhancement of up to 100%. Where Permian anhydrite beds are removed completely in this zone, solution collapse breccias result, extending to the surface;
2). Middle zone, where supergene sulphides have been deposited, almost exclusively as chalcocite. The thickness of this zone is very variable, with the quantity of secondary sulphides increasing upwards to the base of oxidation. The percentage increase of Cu grade in the chalcocite zone is usually considerably higher in quartzose rocks than in skarns and probably more in garnetite than in diopside rocks;
3). Upper zone, the base of which may vary by as much as 300 m in the immediate mine area. The base of significant oxide minerals is as high as 7.5 m below the bedrock surface in garnetite of the Main Orebody, and as deep as 250 m below the bedrock surface in siltstone of the Northeast Orebody. Cu, Fe and Mn oxides are abundant in the upper sections, with widespread chrysocolla and neotocite, and local bronchantite, tenorite, cuprite and native copper. There is a crude zoning in the oxide zone, with native Cu and cuprite being found immediately above the chalcocite zone, Above this near sulphide band is a zone of abundant chrysocolla and neotocite or tenorite. Progressing upwards the neotocite decreases and the chrysocolla increases. Clay development attributed to supergene alteration is essentially pervasive in susceptible rocks in the oxide zone.

It has been suggested that the Twin Buttes system may represent the autochthonous roots of the Mission Complex orebodies some 10 km to the NNW on the allochthonous northwards transported plate above the post-mineral San Xavier Fault (Jansen, 1982).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary 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.

  References & Additional Information
   Selected References:
Barter C F, Kelly J L  1982 - Geology of the Twin Buttes Mineral deposit, Pima Mining District, Pima County, Arizona: in Titley S R 1983 Advances in Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson    pp 407-432

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 takes no responsibility what-so-ever for inaccurate or out of date data, information or interpretations.

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