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Safford - Lone Star |
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Arizona, USA |
| Main commodities:
Cu
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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.
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The Lone Star porphyry copper deposit is one of four in the Safford district of south-eastern Arizona.
Geology
The regional geology is as described in the Dos Pobres record.
Mineralisation at Lone Star is closely associated with Eocene age dykes within volcanics of the Safford Meta-volcanics (or Older Volcanic Series). These dykes occur as a swarm within the Safford Shear Zone (Robinson & Cook, 1966).
The Older Volcanic Series is generally a dark grey massive porphyritic andesite and fine textured flow breccia with intercalated tuff bands, overlain by flow breccia and a thin series of grit, sandstone and conglomerate. The intrusives occur as dykes of rhyolite, quartz-latite, dacite and quartz-diorite within the Safford Shear Zone. Some of the dykes probably emanate from a rhyolitic plug, and together with pyroclastics, crystalline tuff and tuff breccia they fill a steep sided, deep-seated vent with dimensions of 600 x 450 m and at least 600 m vertical extent. This vent is about a kilometre to the west of the deposits. Although ranging from a few cm's to 60 m wide, these dykes persist for distances of 600 to 1200 m laterally, with some as far as 3000 m from the source. In some parts of the orebody, up to 25% of the volume is intrusives as 14 to 20 such dykes (Robinson & Cook, 1966).
Both the Lone Star and San Juan deposits are localised in ENE to NE trending extensive sheared zones, termed respectively:
a). the 'Lone Star Shear Zone', which is 1800 m wide and at least 5 km long and embraces the Lone Star Pluton of quartz-diorite, with minor granodiorite and quartz-monzonite (adamellite); and
b). the 'San Juan Shear Zone' which is 900 m wide, has a strike length of around 3000 m and embraces the Lone Star plug of Eocene quartz-monzonite porphyry, granodiorite porphyry and granodiorite (Robinson & Cook, 1966).
Mineralisation
Hypogene mineralisation in the Lone Star orebody occurs both within the Eocene dykes and the Cretaceous to Palaeocene Older Volcanic series (or Safford Meta-volcanics), with a preference for where a large number of small dykes alternate with volcanics, rather than where large dykes, plugs or extensive un-relieved zones of andesite predominate. The zone of shearing and dykes within the Lone Star Shear Zone were strongly hydrothermally altered and pyritised over an area of 3600 x 1800 m, although the maximum plan area of the ore was only 25% of this. Primary minerals are largely pyrite and chalcopyrite, with lesser bornite and molybdenite and rare tetrahedrite, galena and sphalerite. In the main ore zone pyrite comprises 0.2 to 1%, while in the pyritic halo it is 4 to 8%. Chalcopyrite and pyrite both occur as disseminations and as veins. Magnetite is fairly common in quartz veins carrying sulphides. Gold has not been identified in the orebody, but is known to occur in small fractures in a shear zone 600 m to the south-east. Similarly no Ag bearing minerals have been identified (Robinson & Cook, 1966).
Supergene minerals include chalcocite and covellite. Chalcocite may completely replace chalcopyrite, bornite and covellite, while pyrite is usually only coated. Covellite generally replaces chalcopyrite and bornite, and may in turn be replaced by chalcocite. The leached capping was only exposed over a few square metres in a window, although the marginal leached pyritic zone was outcropping to the south-west originally. Overall the main leached cap averages around 0.13% Cu. Oxide minerals in the capping are predominantly chrysocolla, with lesser malachite and bronchantite. Cuprite, tenorite and chrysocolla usually replace chalcocite, while native copper occurs erratically on thin veins or fractures and is usually associated with cuprite or chalcotrichite. Supergene alteration over the buried orebody has taken place to a depth 120 m, while in the exposed pyritic area these are restricted to a depth of 85 m. The oxidation of the altered mineralised volcanics over the orebody has produced a conspicuously coloured cap due to the oxidation of iron compounds and leaching of pyrite to produce goethite and hematite above ore, and abundant alunite and jarosite over the pyritic halo (Robinson & Cook, 1966).
The main alteration zone is some 3600 x 1800 m, enclosing a central 2000 x 1500 m zone of quartz-sericite alteration which adjoins and is partially superimposed upon a large area of secondary biotitisation to the north-east. The orebody is within these two zones, with concentric surrounding chloritic and propylitic haloes. The outer parts of the quartz-sericite, and the chlorite zones are essentially pyritised areas, while the propylitic zone is un-mineralised, except where traversed by shear zones, veins or dykes that may be accompanied by sulphides. The quartz-sericite alteration most severely affects the dykes, plugs and volcanic vent material, but is also evident in the andesites. The biotite zone is mainly within porphyritic andesites of the Older Volcanic Series, with 10 to 50%, but averaging 20% biotite in the strongest interval of alteration. Chlorite alteration is most extensive to the south-west where a zone up to 300 to 600 m wide is developed, with chlorite, which is the most widely distributed alteration mineral, being well developed. The propylitic zone is the most extensive alteration zone and represents the area of lowest alteration intensity. Epidote and to a lesser extent chlorite are the common secondary minerals. The relationship of the propylitic zone to mineralisation is unclear, but may be a regional pre-mineralisation affect as described at Dos Pobres (Robinson & Cook, 1966).
The published reserve figures at Lone Star include:
Reserve, 1981, 966 Mt @ 0.58% Cu (USBM)
Leach Ore Resource, 1992, 1600 Mt @ 0.38% Cu (American Mines Handbook, 1994)
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1966.
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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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
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Robinson R F, Cook A 1966 - The Safford Copper deposit, Lone Star mining district, Graham County, Arizona: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 251-266
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