Red Mountain

Arizona, USA

Main commodities: Cu
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Red Mountain is a hypogene porphyry copper deposit in southeastern Arizona, USA. It occurs at depths of more than 1000 m beneath surface exposures of high pyrite, phyllic alteration. A chalcocite blanket is developed within the low grade pyritic phyllic zone near the surface. The overall alteration/mineralisation system is believed to be centred on a caldera subsidence structure which was associated with explosive volcanism and sub-volcanic intrusive activity. This latter assumption is reinforced by an arcuate zone of dykes, intrusive breccias and subsidence structures (Corn, 1975).

Published resource figures are:   100 Mt @ 0.71% Cu (Total Resource, 1978, USBM)
      570 Mt @ 0.63% Cu (Mutschler et al., 2004)

Mineralisation is dated at around 62 Ma, based on associated alunite. It is developed within both a blind quartz-monzonite intrusive complex of unknown size and extent and has a volcanic litho-cap. The volcanic litho-cap is strongly fractured and exhibits the results of acid sulphate or advanced argillic alteration of a dacite to quartz-latite (rhyodacite) and rhyolite tuff volcanic succession, overlying thick andesites and lower still, conglomerates. The upper tuffs are around 500 m thick, and are estimated at 'around 60 Ma' in age. The underlying andesites comprise an upper 500 m of andesites and trachy-andesites, underlain by a further 500 m of interlayered andesite, felsite and banded hornfels. The pre-ore andesitic volcanic rocks have been dated at about 72 Ma and are considered as late Cretaceous to lower Tertiary in age. This entire sequence has been intruded by sills and dykes of monzonite and quartz-monzonite porphyry which in deep drill holes within the mineralised area account for 25 to 30% of the rock intersected below the upper tuffaceous volcanics (Bodnar & Beane, 1980; Titley, etal., 1989).

Closely contiguous with the intrusive centre are a few mines that have taken complex base and precious metal ores from veins and replacement bodies in volcanic rocks. Mn-Ag mineralisation in carbonate rocks is also known from two sites to the south. Multiple Laramide intrusion in closely spaced centres in the district leaves uncertain the relationship of the base metal mines to any specific intrusion, one of which lies several kilometres to the SSW (Bodnar & Beane, 1980; Titley, etal., 1989).

Red Mountain stands out as the most striking feature in the district, with over 800 m of relief above the surrounding mountains. It is composed of erosionally resistant, highly altered tuffs. It is brightly coloured with jarositic staining from oxidised sulphides in its cap. Virtually all of the rocks exposed on the mountain are part of the late Cretaceous to lower Tertiary volcanic succession, mainly andesites, trachy-andesite and tuffs which extend in outcrop for several kilometres around the deposit. The north-west side of the mountain is cut by a north-east trending fault. The destructive effects of pervasive advanced argillic alteration, coupled with destructive acid weathering are seen in most outcrops and render rock identification in outcrop difficult. Most of the top of the system is in the upper tuff unit and the pyroclastic nature of this rock is sometimes revealed in ghost textures (Titley, et al., 1989).

According to Corn (1975), both the mineralisation and the alteration at Red Mountain exhibit concentric zoning patterns centred on the area of monzonite and quartz-monzonite porphyry intrusions. Surface exposure reflects a zonal pattern of alteration and mineralisation, centred on an area of phyllic alteration (silica flooded, alunite-pyrophyllite1-pyrite bearing rocks) and Cu-Mo mineralisation that is surrounded successively by zones of pyritic-argillic alteration and propylitic andesites. The effects of hydrothermal alteration are evident over an area with a diameter of 11 to 13 km. Vertical zoning in alteration mineralogy appears to be related to a gradual increase in sulphur content from low sulphur K-silicate alteration at depth, through weak K-silicate alteration to sulphur rich, phyllic assemblages near the surface. The lateral and vertical zoning is also reflected in the distribution of Pb, Zn, Mo and Cu minerals both in the zone of pervasive disseminated sulphides and within the exterior veins of the propylitic alteration zone (Corn, 1975; Titley, et al., 1989).

The Red Mountain alteration/mineralisation system exhibits two different types of hypogene Cu mineralisation. Enargite is associated with near surface, high pyrite, phyllic and pyrite-argillic alteration. Chalcopyrite occurs with both weak K-silicate and more intense K-silicate alteration at depth. Although not of ore grade, the enargite mineralisation provided a protore source for the Cu of the 'high level' chalcocite blanket. 'Ore grade' hypogene mineralisation occurs at depths of 1000 m or more beneath the surface. The zoning pattern is characterised by a gradual increase in the grade of Cu with depth within the zone of weak K-silicate alteration, and the upper parts of the main K-silicate zone (Corn, 1975).

Episodic uplift has produced four hematite rich zones reflecting four cycles of chalcocite blanket development and subsequent oxidation and leaching (S Titley, pers. comm., 1994).

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
   Selected References:
Bodnar RJ and Beane RE  1980 - Temporal and spatial variations in hydrothermal fluid characteristics during vein filling in preore cover overlying deeply buried porphyry copper-type mineralisation at Red Mountain, Arizona: in    Econ. Geol.   v75 pp 876-893
Lecumberri-Sanchez, Newton, P.C., Westman, E.C., Kamilli, R.J., Canby, V.M. and Bodnar, R.J.,  2013 - Temporal and spatial distribution of alteration, mineralization and fluid inclusions in the transitional high-sulfidation epithermal-porphyry copper system at Red Mountain, Arizona: in    J. of Geochemical Exploration   v.125, pp. 80-93.

   References in PGC Publishing Books: Want any of our books ? Pricelist
Cook S S and Porter T M, 2005 - The Geologic History of Oxidation and Supergene Enrichment in the Porphyry Copper Deposits of Southwestern North America,   in  Porter T M, (Ed),  Super Porphyry Copper and Gold Deposits: A Global Perspective,  v1  pp 207-242
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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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