Poston Butte, Florence

Arizona, USA

Main commodities: Cu Mo
Our International
Study Tour Series
The last tour was
OzGold 2019
Our Global Perspective
Series books include:
Click Here
Super Porphyry Cu and Au

Click Here
IOCG Deposits - 70 papers
All available as eBOOKS
Remaining HARD COPIES on
sale. No hard copy book more than  AUD $44.00 (incl. GST)
Big discount all books !!!

The Poston Butte porphyry copper deposit is located 4 km north of Florence in south-central Arizona, USA.

The deposit is totally covered by post mineral conglomerate and recent alluvium, with the only exposure a small outcrop of leached capping.   The country rock comprises Precambrian quartz monzonite porphyry belonging to the Oracle Batholith, which has been intruded by both Precambrian dolerite (diabase) and by Laramide granodiorite porphyries of 62 Ma age.   The mineralisation accompanied this latter intrusion and was cut by a variety of barren hypabyssal rocks and later basalt, andesite and felsic units of Tertiary age.

Both hypogene and supergene mineralisation are present in the deposit. The hypogene mineralisation and alteration comprises a dominant 1100x900 m elliptical core of potassic alteration (quartz-orthoclase-biotite-sericite) with vertical dimensions of at least 750 m, surrounded by concentric shells of sericitic (quartz-sericite-pyrite over widths of 150 to 750 m), propylitic (calcite-chlorite-epidote) and argillic (kaolinite-montmorillonite-sericite) development.

Alteration in the granodiorite appears to be more veinlet controlled than in the surrounding quartz monzonite.   The granodiorite is almost entirely within the confines of the potassic alteration zone.   Grades of copper in the quartz monzonite porphyry adjacent to the granodiorite porphyry are usually at least twice those in the younger intrusive.   Hypogene mineralisation is almost entirely chalcopyrite, pyrite and molybdenite with minor chalcocite and covellite and rare bornite, galena and sphalerite.

Oxidised ore makes up around 30% of the total thickness of the deposit, varying from between 30 and 300 m, but averaging 125 m in thickness.   The vertical extent of this mineralisation is influenced by faults and fractures allowing oxidation deeper into the supergene zone.   The most common copper mineral is chrysocolla adsorbed onto the surface of residual clays after plagioclase, while massive chrysocolla also occurs as vein fillings and on fractures.   Minor oxide minerals are cuprite, native copper, azurite, brochantite and shattuckite.   Due to the low total hypogene sulphide content, the grade of the oxide zone is similar to that of the hypogene ore.

Although chalcocite mineralisation has been encountered over thicknesses of up to 100 m, only a thin secondary enriched zone that generally does not exceed 12 m has been outlined, although this zone is locally better developed along faults.

Tonnage figures include:
Reserve 1978 - 450 Mt @ 0.39% Cu (USBM).
Reserve 1989 - 725 Mt @ 0.4% Cu (Titley, 1992).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1982.    
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:
Nason P W, Shaw A V, Aveson K D  1982 - Geology of the Poston Butte Porphyry Copper deposit, Pinal County, Arizona: in Titley S R 1983 Advances in Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson    pp 375-385

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.

Top | Search Again | PGC Home | Terms & Conditions

PGC Logo
Porter GeoConsultancy Pty Ltd
 International Study Tours
     Tour photo albums
 Ore deposit database
 Conferences & publications
PGC Publishing
 Our books  &  bookshop
     Iron oxide copper-gold series
     Super-porphyry series
     Porhyry & Hydrothermal Cu-Au
 Ore deposit literature
 What's new
 Site map