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Helvetia District - Rosemont, Peach Elgin,

Arizona, USA

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Rosemont and Peach Elgin are the largest of the Helvetia group of copper deposits located in the Santa Rita Mountains, some 50 km to the south-east of Tucson, and 32 km to the east of the Twin Buttes and Mission Complex mines. Four individual deposits are known within a 6 x 2 km area. The other two are Broadtop Butte and Copper World.

Background

Copper mineralisation was known in the Helvetia district prior to the American Civil War. In the late 1880's, copper ores were mined in the area and smelted in the Santa Rita Mountains. In 1903, the Helvetia Copper Company was formed and began operations and continued until 1911. Copper production was re-started in 1915 and continued almost without break until 1951. Total production for the district to 1950 amounted to 230 000 t of ore from which 7835 t of Cu, 500 t of Zn and 5.6 t of Ag were extracted. After 1950 activities were restricted largely to exploration and development drilling by a number of companies in the vicinity and over the area of the four deposits now known. The first significant hole was drilled by the Banner Mining Company at Rosemont in the late 1950's with an intersection of 300 m @ >0.9% Cu. Anaconda acquired the property in 1963 and carried out extensive drilling and mapping to define the resource. The leases were incorporated into the Anamax Mining Co when Amax and Anaconda went into joint venture in 1973. Anamax subsequently sold the property to a real estate company in 1986 (Anzalone and Brown, 1992). ASARCO exchanged a tract of land valued at $US 1 million, but originally purchased for $US 250 000, for the leases in 1988 (P Gilmour, pers. comm., 1994). ASARCO estimate the geological resource at Rosemont/Helvetia will exceed 500 Mt (Anzalone and Brown, 1992).

Geology

The four deposits are found within a series of moderate to steeply dipping Palaeozoic and Mesozoic sedimentary rocks that have been intruded by Laramide age igneous rocks. Hydrothermal alteration and zoning of minerals are similar to that found at the Mission and Twin Buttes mines (Anzalone and Brown, 1992).

The Palaeozoic succession is dominated by limestones, dolomitic limestones and quartzites. The Mesozoic rocks are primarily Cretaceous in age and consist of shales, sandstones, arkose, andesitic volcanics and impure limestones. The pre-Cenozoic stratigraphic sequence is as follows (Anzalone and Brown, 1992), from the base:

Middle Proterozoic basement, composed of coarse granodiorite porphyry. Where sighted during a visit in 1991, this was a very coarse, mafic rich granitoid which has been strongly decomposed by weathering.
Unconformity
Palaeozoic sedimentary rocks, totalling around 1830 m in thickness, composed of: Cambrian,
Bolsa Quartzite, 140 m thick - coarse grained quartzite.
Abrigo Formation, 225 to 275 m thick - thinly interbedded shale, siltstone and quartzite.
Unconformity
Devonian,,
Martin Formation, ~120 m thick - dolomite, limestone, siltstone and some sandstone.
Carboniferous - Mississippian,
Escabrosa Limestone, 60 to 140 m thick - thick massively bedded limestone with local chert.
Carboniferous - Pennsylvanian,
Horquilla Limestone, 245 m thick - limestone with thin to massive bedded siltstone and minor shale, and a basal conglomerate.
Carboniferous to Permian transition
Earp Formation, 245 m thick - siltstone and shale with some sandstone and limestone.
Permian, which in the Mission Mine area, includes:
Colina Limestone, 105 m thick - medium to thick bedded limestone.
Epitaph Dolomite, 305 m thick - limestone, marl, siltstone, dolomite, local gypsum and quartzite.
Scherrer Formation, 220 m thick - fine grained quartzite, with lesser dolomite, and minor siltstone at the base.
Concha Limestone, 120 to 175 m thick - thick bedded cherty limestone.
Rainvalley Formation, 0 to 90 m thick - limestone, dolomite and sandstone.
Unconformity
Mesozoic
Cretaceous,
Glance Conglomerate, 0 to 460 m thick - conglomerate containing limestone and granodiorite cobbles.
Willow Canyon Formation, 670 m thick - cobble conglomerate at the base; andesitic volcanics with chert; and arkosic sandstone and siltstone.


Two types of Palaeocene intrusive cut these rocks. One is a granodiorite to quartz-monzonite (adamellite) found primarily in the western portion of the project area, while the other is a moderately to strongly altered quartz-latite (rhyodacite) porphyry which is apparently closely associated with the copper mineralisation (Anzalone and Brown, 1992).

Occasional Tertiary lamprophyric dykes penetrate the area (Anzalone and Brown, 1992).

Structure

The structure of the Rosemont/Helvetia project area is complex. The rocks are cut by numerous faults, including thrusts, high angle normal, reverse and tear faults. Considerable folding of the sedimentary rocks is also apparent (Anzalone and Brown, 1992).

A complex assemblage of thrusts, and high angle reverse and tear faults follow the crest of the Santa Rita Mountains locally in the Helvetia area, where they are collectively known as the 'Backbone Fault'. This fault zone forms the western edge of the east dipping block of Palaeozoic sedimentary rocks that host the Rosemont deposit. Post ore faulting, principally high angle normal and thrust faults, have had a substantial influence on all four prospects (Anzalone and Brown, 1992).

Peach-Elgin is the most structurally complex. It occurs in what is known as the Helvetia Klippe. The entire deposit is underlain by a flat fault that places Palaeozoic and Mesozoic sedimentary rocks and Laramide quartz-latite porphyry over Proterozoic granodiorite. The Helvetia Klippe is considered by some to be the offset upper segment of the Copper World prospect (Anzalone and Brown, 1992).

Alteration

The quartz-latite is locally mineralised, and strong copper mineralisation in adjacent garnet-diopside skarns is regarded as being related. The principal alteration products in the porphyry are sericite and clay modification of the feldspar, partial destruction of the mafics and varying degrees of silicification. Limestones in contact with the quartz-latite porphyries have been locally altered to lime-silicate skarns. One of the principal components of the skarn is a garnet tactite composed primarily of andradite, with varying amounts of quartz, diopside, tremolite, serpentine, wollastonite and vesuvianite. Endoskarn is occasionally observed in the porphyry, composed mainly of garnet with lesser vesuvianite and minor epidote (Anzalone and Brown, 1992).

The Escabrosa Limestone is altered to a poddy massive garnetite (andradite) on the Rosemont prospect. The Horquilla Limestone has been modified to be a calc-silicate argillite with wollastonite and lesser garnet. The Earp Formation is intensely silicified with quartz veining, while the Colina Limestone has been altered to actinolite with rare tremolite (S Anzalone, pers. comm., 1991).

Mineralisation

Sulphide mineralisation is considered to have been emplaced after the pyro-metasomatic alteration and occurs in all of the sedimentary rocks. While all of these sedimentary rocks are mineralised to a certain degree, higher grade copper mineralisation tends to favour certain skarn horizons indicating that the original rock type and stratigraphy exerted considerable control over ore deposition. Drilling at Rosemont indicates that the Colina and Horquilla Formations were far more receptive to copper mineralisation than the other lithologies (Anzalone and Brown, 1992).

The Horquilla Limestone carries most of the ore with more bornite than chalcopyrite and some primary chalcocite. The Earp Formation, which is more siliceous, has a low copper grade, but a higher molybdenite content. The Colina Limestone contains high grade copper ore comprising magnetite, chalcopyrite and lesser pyrite ( S Anzalone, pers. comm., 1991).

The principal host at Peach-Elgin is the Horquilla Limestone ( S Anzalone, pers. comm., 1991).

Primary sulphide minerals include chalcopyrite, bornite and pyrite, with chalcopyrite and pyrite dominating overall. Sulphides occur principally as veinlets, coarse disseminations, blebs and clots within irregular lenticular zones, lying in and generally paralleling the lime-silicate areas. Magnetite is found to varying degrees throughout the skarn zone. In the oxidised interval, a considerable tonnage of copper occurs as azurite, malachite, cuprite and chalcanthite. Minor amounts of silver, molybdenite, sphalerite, galena and scheelite occur throughout the deposit (Anzalone and Brown, 1992).

The total sulphide content within the Palaeozoic rocks is relatively low, seldom exceeding 3%. The sulphide content of the Mesozoic rocks is somewhat higher, due primarily to an increase in pyrite (Anzalone and Brown, 1992).

Mineralisation was very obvious in the prospect area. Prominent faces of silicified Cambrian Abrigo Quartzite on the crest of the Santa Rita Range had spectacular splashes of green malachite and chrysocolla that were obvious from hundreds of metres away. These faces are accompanied by sheets of quartz veins from 1 to 20 mm thick, a few of which have thin selvages. The fractures have goethite and jarosite coatings with minor hematite, as well as copper staining and hematitic pseudomorphs after pyrite. Thin skarn bands with associated copper carbonates are developed within the quartzite locally. Within the same quartzite unit, occasional shear zones cutting across the trend of the band have been more heavily mineralised and were the sites of old workings. These contained massive goethitic gossan pods several metres long and a metre across in outcrop. There was abundant malachite, chrysocolla, some chalcocite and goethite on the shear cleavages.(Pers. observ., 1994).

Within the Palaeozoic carbonates there is banded skarn in outcrop, although this is more recessive and masked by scree from the quartzite unit. At the base of the scree there is epidote rich rock with a 'cherty calc-silicate' appearance and abundant dark 'manganiferous' fractures and occasional malachite patches. In other outcrops patches of malachite and chrysocolla were more obvious and sporadically distributed. Elsewhere iron staining is common on fractures, although not all outcrops are mineralised to the naked eye. All of the rocks of this section of the sequence are well fractured, often at a spacing of 1 cm or less, but generally up to 10 cm apart. A number of old diggings were developed within this rock type with abundant malachite and chrysocolla (Pers. observ., 1994).

The Cretaceous arkoses and volcanics weather to red at the surface, are more recessive than the skarns, and have no obvious mineralisation in outcrop (Pers. observ., 1994).

Reserves and Resources

Rosemont
    350 Mt @ 0.6% Cu, 8 g/t Ag, 0.01% Mo, as sulphide ore, + >100 Mt of oxide ore (Reserve, Visit 1991).
    365 Mt @ 0.61% Cu, 0.019% Mo, 7.8 g/t Ag sulphide (cut-off 0.3% Cu), plus
        66 Mt @ 0.53% Cu oxide ore (Reserve 1977, Anzalone and Brown, 1992).
    290 Mt @ 0.64% Cu (Init. Res. 1973, Gilmour, 1982).
    306 Mt @ 0.54% Cu, 3 g/t Ag, 0.02 g/t Au, 0.012% Mo, (Sulphide Res.) + 20 Mt @ 0.55% Cu (Acid Sol. Res., 1989, Titley, 1992).
    Measured + indicated resource @ 0.3% Cu cutoff (Augusta Resources, 2006): 357 Mt @ 0.58% Cu 0.016% Mo,
    Inferred resource @ 0.3% Cu cutoff (Augusta Resources, 2006): 96 Mt @ 0.56% Cu 0.017% Mo.

Peach-Elgin
    13 Mt @ 0.78% Cu (Sulphide Res., 1989, Titley, 1992).
    9 Mt @ 0.75% Cu (Acid Sol. Res., 1989, Titley, 1992).
    23 Mt @ 0.76% Cu, 60% are sulphides.

The most recent source geological information used to prepare this summary was dated: 1994.    
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:
Anzalone, S.A., Brown, R.L.,  1992 - Geology of the Helvetia copper deposit: in   Presentation to the SME Annual Meeting, Phoenix, Arizona, Feb. 24 to 27, 1992, Soc for Min Met & Expl. Inc, Littleton Colorado,    5p.


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