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Poston Butte, Florence Copper
Arizona, USA
Main commodities: Cu Mo

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The Poston Butte porphyry copper deposit is located 4 km north of Florence in south-central Arizona, USA.
(#Location: 33° 2' 48"N, 111° 25' 45"W).

Mineralisation was first discovered at Poston Butte in 1969, following a regional reconnaissance by Conoco geologists to evaluate the copper potential of the the Florence Copper Area. The first hole of a follow-up drilling program intersected oxide/silicate copper and supergene enriched copper mineralisation on the southwestern flank of the Poston Butte. Further drilling led to the decision to undertake a systematic multi-hole exploration program and engineering studies to assess the economic feasibility of the property. Between 1969 and 1975, Conoco delineated an extensive, porphyry copper system SSW of the Poston Butte. In 1974, Conoco sunk two shafts and ~1.5 km of drives and crosscuts as a single-level, underground mine to extract ~50 000 tonnes of mineralised material for metallurgical samples and to test geological parameters. Upon completion of the underground work, Conoco removed the shaft infrastructure and secured the openings with steel plates. Development drilling ceased in 1975 and the project became dormant until Magma Copper acquired the property in 1992 and initiated a Pre-Feasibility Study. The results from copper resource modeling, metallurgical testing, material property testing, and financial analysis supported the conclusion that the application of in-situ recovery and SX/EW to produce cathode copper was the preferred method to develop the Florence deposit. Work continued after BHP acquired Magma Copper, but upon completion of positive field testing of in situ copper recovery (ISCR) the project was put on hold due to a period of low metal prices. In 2010 Florence Copper LLC owned by Taseko Mines Limited, completed the acquisition of the property and continued testing ISCR extraction, again with positive results. To March 2023, there has been no commercial production from the Poston Butte copper deposit, other than bulk sampling for metallurgical vat leach and ISCR testing.

The Florence porphyry copper 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 and by Laramide granodiorite porphyry dyke swarms of 62 Ma age. These dykes trend east-west and are developed over an east-west elongated area of ~4 x >5 km. The mineralisation accompanied the Laramide suite of granodiorite porphyry dykes, which range from a few to >300 m in thickness, dip at ~80°N, and are cut by a variety of barren hypabyssal rocks and later Tertiary basalt, andesite and felsic intrusions.   The granodiorite dykes were intruded in pulses, and have been divided into types I, II and III, based on differences in mineral composition and texture. Type 1, which is the most voluminous, was accompanied by hydrothermal solutions that altered the host rock and deposited copper and iron sulphide minerals as disseminations and thin veinlets. Hydrothermal alteration and copper mineralisation were most intense along the edges and flanks of the dyke swarms and intrusive mass.

Young, post-mineralisation faulting dislocated the mineralised system, with much of the Florence deposit being isolated within a horst block, which, with the downthrown fault blocks to the west, was exposed to erosion and weathering. The centre of the deposit was eventually eroded to a gently undulating topographic surface, with a deep basin being formed on the western flank of the horst. Weathering led to copper sulphide minerals being oxidised as detailed below. The bulk of the copper oxide mineralisation occurs along fracture surfaces, although chrysocolla and copper-bearing clay minerals also replace feldspar minerals internal to the granodiorite porphyry and quartz monzonite. A barren or very low-grade (leached cap) zone, dominated by iron and manganese oxides/silicates and clay minerals, caps some portions of the top of the preserved, supergene enriched bedrock. Oxidised ore makes up around 30% of the total thickness of the deposit, varying from 30 to 300 m, averaging 120 m. The vertical extent of this mineralisation is influenced by faults and fractures allowing oxidation deeper into the supergene zone.

The hypogene mineralisation and alteration comprises a dominant 1100 x 900 m elliptical core of potassic alteration (quartz-orthoclase-biotite-sericite) with vertical dimensions of at least 750 m. Salmon-coloured, secondary orthoclase replaces primary orthoclase phenocrysts, rims quartz ±biotite veins, and occurs as pervasive orthoclase flooding. Shreddy, secondary brown biotite replaces plagioclase and matrix feldspars, and occurs in biotite-sulphide veinlets. The potassic alteration zone almost entirely surrounds the Laramide granodiorite dyke swarm. The potassic zone is surrounded by concentric shells of phyllic (quartz-sericite-pyrite over widths of 150 to 750 m), propylitic (calcite-chlorite-epidote) and argillic (montmorillonite-kaolinite ±illite ±halloysite) development. The phyllic alteration is particularly evident in the deep portions of the sulphide mineralisation. Fine-grained sericite selectively replaces plagioclase, orthoclase and biotite, and occurs as thin alteration selvages along quartz ±sulphide veins. The propylitic alteration (calcite-chlorite-epidote) is most evident in mafic dykes and is mainly found in exploration holes fringing the deposit. The argillic alteration is the most noticeable feature of the oxide mineralised zone, and is a late-stage assemblage. The conversion of sericite to clay minerals in plagioclase phenocrysts and along fracture surfaces is selectively pervasive. X-ray diffraction analyses suggests the clay is primarily a mixture of calcium-montmorillonite and kaolinite. These clay-altered plagioclase sites were favourable loci for remobilised copper generated from natural in-situ leaching.

The principal hypogene sulphide minerals are chalcopyrite, pyrite and molybdenite, with minor secondary chalcocite and covellite. Alteration in the Laramide granodiorite porphyry is primarily veinlet-controlled, whereas alteration in the Precambrian quartz monzonite includes pervasive, selectively pervasive and veinlet-controlled. Grades of copper in the quartz monzonite porphyry adjacent to the granodiorite porphyry are usually at least twice those in the younger intrusive.

The oxide mineralisation and underlying sulphides are often separated by a thin transition zone of partially oxidised supergene sulphides. The hypogene sulphides below the supergene blanket, due to its depth, low permeability, and relatively non-soluble mineralogy, is not favourable to extraction by by ISCR methods. The oxide zone mineralisation comprises chrysocolla, 'copper wad' neotocite, tenorite, cuprite and native copper, with trace azurite and brochantite and shattuckite.

The most common oxide copper mineral is chrysocolla, occurring in veins and fracture fillings adsorbed onto the surface of residual clays after plagioclase, while massive chrysocolla also occurs as vein fillings and on fractures. The fracture-controlled mineralogy implies that copper was not adsorbed onto the clay surfaces, but rather the copper resides in the octahedral site of the clays. The 'copper wad' occurs an amorphous mix of manganese, iron and copper oxides that occurs as dendrites, spots, and irregular coatings on fracture surfaces. Cuprite is locally smeared out along goethite/hematite-coated fracture surfaces, whilst the chalcotrichite variety of cuprite is also found on fractures or vugs, and is sometimes intergrown with native copper crystals. Minor oxide minerals are cuprite, native copper, azurite, brochantite and shattuckite.

Supergene chalcocite coats pyrite and chalcopyrite which 'dust' fracture surfaces, forming a chalcocite blanket that is very thin and irregular, ranging from zero to 15 m in thickness, although it is locally better developed along faults where it has been encountered over thicknesses of up to 100 m. It is often partially oxidised. In most instances, the transition from the copper silicates and oxides to the sulphide zone is quite abrupt. The grade of oxide mineralisation is generally very similar to that of the primary sulphide mineralisation. Due to the low total hypogene sulphide content, the overall grade of the oxide and sulphide mineralisation are similar, i.e., ~0.36 and 0.27% Cu Total, respectively.

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

NI 43-101 compliant Ore Reserves and Mineral Resources as at 31 December, 2022 (Taseko Mines Ltd Reserves and Resources report, 2022) were:
Mineral Resources
  Measured Resource - 265 Mt @ 0.34% Cu;
  Measured Resource - 64 Mt @ 0.39% Cu;
  Measured + Indicated Resource - 329 Mt @ 0.35% Cu;
  Inferred Resource - 38 Mt @ 0.32% Cu;
  TOTAL Resource - 367 Mt @ 0.35% Cu;
Ore Reserves
  Proved Reserves - 234 Mt @ 0.35% Cu;
  Probable Reserves - 57 Mt @ 0.40% Cu;
  Proved + Probable Reserves - 290 Mt @ 0.36% Cu.
NOTE: The tonnages above are all converted from 'tons' in the Taseko Mines report to 'tonnes'. It is assumed Taseko Mines was reporting short tons (1 short tonne = 0.907185 tonnes). Resources are inclusive of Reserves.

The information in this summary is partially drawn from: Tremblay, R., Weymark, R. and Rotzinger, R., 2023 - Florence Copper Project, Pinal County, Arizona; an NI 43-101 Technical Report prepared by Taseko Mines Limited, 325p.

The most recent source geological information used to prepare this decription was dated: 2023.     Record last updated: 17/7/2023
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.

Poston Butte

  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, its employees and servants:   i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and   ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.

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