Pinto Valley, Castle Dome

Arizona, USA

Main commodities: Cu
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The Pinto Valley hypogene porphyry copper deposit and Castle Dome supergene orebody lie on the western periphery of the Globe-Miami district in south-eastern Arizona (#Location: 33° 24' 31", 110° 58' 5"W).

For detail of the setting and district geology, see the separate Globe-Miami District record.

The Pinto Valley deposit is almost exclusively composed of hypogene ore and lies below the old Castle Dome supergene orebody. The ore is primarily hosted by the 1400 Ma Lost Gulch Quartz Monzonite and to a lesser extent by the two phase Laramide Schultz Granite. All of these intrusives cut the Paleoproterozoic Pinal Schists, an 1100 Ma dolerite mass and Devonian carbonates within the pit. None of these latter lithologies have been significantly mineralised, although the carbonate has been extensively altered to skarn.

The annular orebody, which plunges to the north at around 45°, surrounds a barren core characterised by a high density of un-mineralised quartz veining. The orebody comprises stockwork veinlet and disseminated sulphide mineralisation, mainly chalcopyrite, associated with biotite rich potassic alteration, which are overprinted by pyritic veins with associated phyllic selvages. Hypogene mineralisation has been dated at 59.1 ± 0.5 Ma (Creasy, 1980).

The Pinto Valley deposit is located in a horst, bounded to the west and east by outward dipping normal faults. The uplift of the horst is believed to have kept pace with chemical weathering, although the age of that weathering is not well constrained by field evidence. Petersen (1962) reported that the top of the supergene mineralisation conformed closely to the pre-mine topography.

Immediately to the west of Pinto Valley, in the Cactus-Carlotta copper deposit, there is evidence of at least two cycles of weathering and enrichment. The Cactus-Carlotta deposit is hosted by a breccia which is believed to be the product of a large landslide of Pinal Schist containing partially oxidised veinlets and disseminations of pyrite from the upper parts of the Pinto Valley mineralised system. The copper at Cactus-Carlotta is thought to have been introduced by supergene solutions derived from weathering of the Pinto Valley sulphide mineralisation, which percolated through the breccia and precipitated as chalcocite partially replacing the original pyrite within the breccia. These supergene sulphides were subsequently oxidised to chrysocolla and 'copper-wad'. The Cactus-Carlotta mineralisation is overlain by the Early-Miocene (ca. 17 to 20 Ma) Apache Leap Tuff. Some copper mineralisation is found in the base of the Apache Leap Tuff, indicating that at least some of the supergene mineralisation is Late Miocene.

Production + reserves/resources at the Pinto Valley and Castle Dome deposits in 1996 totalled:
    1370 Mt @ 0.32% Cu.

Remaining mineral resources and ore reserves at 30 June, 2012 (BHP Billiton, 2012) were:
  Total measured + indicated + inferred resources at Pinto Valley
    sulphides - 191 Mt @ 0.4% Cu,
    low grade leach - 78 Mt @ 0.19% Cu,
    sulphide stockpile - 448 Mt @ 0.11% Cu,
  Total measured + indicated + inferred resources at Pinto Miami
    in situ leach - 214 Mt @ 0.31% Cu,
  Total proved + probable reserves (included in resources) at Pinto Valley
    sulphides - 75 Mt @ 0.4% Cu,
    low grade leach - 13 Mt @ 0.21% Cu.

The most recent source geological information used to prepare this summary was dated: 2004.    
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 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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