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Tyrone

New Mexico, USA

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The Tyrone porphyry copper deposit in south-western New Mexico is located some 30 km to the SW of the Chino/Santa Rita deposit and 16 km SW of Silver City (#Location: 32° 38' 45", 108° 22' 22"W).

The site was known as a source of turquoise by Native Americans prior to 1860. Phelps Dodge Corporation progressively acquired mining claims in the area over the period from 1909 to 1916, and began large-scale underground mining in 1916, although these operations were closed in 1921. Mining resumed as an open-pit mine in 1967. The first SX/EW facility was commissioned in 1984 and the concentrator phased out in 1992, with the transition to totally heap-leach SX/EW production. The remaining mineralisation is predominantly supergene chalcocite, and leachable oxide mineralisation, principally chrysocolla (Freeport-McMoRan, 2012).

Total copper production in 1992 was 80 000 t, 8600 t from milling and 71 400 t from SX/EW (American Mines Handbook, 1994). Treatment capacity in 2012 was 45 000 t of copper (Freeport-McMoRan, 2012).

Geology

The Tyrone deposit lies within the Arizona-New Mexico Basin and Range Province and is associated with a multi-phase Paleocene to early Eocene quartz-monzonite porphyry stock, which has been dated at 56 Ma (Titley, 1982).

The succession in the vicinity of the Tyrone deposit is as follows (from Kolessar, 1982):

Mesoproterozoic Granite - medium to coarse grained rock containing large grains of orthoclase or plagioclase when fresh, and abundant irregular grains of quartz. Biotite is not common, and hornblende is minor. Accessory minerals are sphene, zircon, tourmaline, apatite, magnetite, ilmenite and rutile. This lithology belongs to the Burro Mountain Batholith which also includes minor isolated masses of anorthosite, meta-dolerite, diorite, syenite and larger masses of quartz diorite gneiss. Mineralised dolerite dykes which do not cut the Laramide intrusives, are mapped crossing the Proterozoic granite in the mine area.
Cretaceous Rocks, including the Beartooth Quartzite - a thin bedded massive to vitreous, fine to very fine grained sandstone containing thin shale partings; and the Colorado Formation - an olive green and brown calcareous and sandy shale; overlain by Cretaceous andesite and dacites.
Laramide intrusives, largely as a multi-phase quartz-monzonite (adamellite) porphyry series of intrusions which form the Tyrone laccolith. The porphyries invade the contact between the Proterozoic granite and the overlying Cretaceous sediments and volcanics as dykes and plugs which differ in texture, composition and sequence of intrusion. Fresh quartz-monzonite porphyry outside of the orebody area comprises moderate quartz, oligoclase, orthoclase and sporadic chloritised biotite. At least five porphyry types have been differentiated. The oldest and most common porphyry in the mineralised sequence is the intensely altered, fine grained quartz-monzonite porphyry which serves as the main host to mineralisation. Coarser textured phases and breccia intrude this finer variety. One of the late stage, less altered phases is a poikilitic quartz-monzonite porphyry with orthoclase phenocrysts up to 75 mm long. Intrusive breccia bodies up to 500 m long are found within the orebody and comprise rounded to sub-rounded mixed clasts of granite and porphyry in a matrix of finer rock fragments, which have in most cases been mineralised. Some of the breccias have a fragmented pyrite matrix which has been coated with chalcopyrite. Some of the later porphyries, while altered and pyritised, are not apparently mineralised, and may be post-mineral.
Tertiary and Quaternary rocks - Tertiary rhyolites and andesites overlie the Cretaceous andesites and dacites to the north-east of the Mangas Fault. In the mine area the only Tertiary lithologies are represented by the Oligocene to Miocene Gila Conglomerate, followed by Quaternary gravels.

The orebody lies within a triangle, bounded, i). to the NE by the major NW-SE trending Mangas Fault, across which Cretaceous and Tertiary volcanics are exposed; ii). by the NE-SW trending and SE dipping Burro Chief Fault to the NW, which resulted in the juxtaposition of barren Proterozoic granite; and iii). by the east-west hinge line of the Racket-Virginia Fault to the south (Kolessar, 1982).

Mineralisation & Alteration

The Tyrone orebody occurs in both the multi-phase Palaeocene to early Eocene quartz-monzonite laccolith and the underlying intruded Proterozoic granite, within a tilted Pleistocene horst. The preserved quartz-monzonite is a deeply eroded remnant of the original intrusion. The orebody consists of a supergene blanket of erratic chalcocite coatings on pyrite mineralisation. It varies from a few metres to over 100 m in thickness, and dips at 8° to the north-west, towards the Burro Chief Fault. Sulphides are close to the surface near the Racket-Virginia Fault, but are at a depth of 150 to 180 m in the north-east sector of the orebody. The top of the blanket is very irregular, with sporadic remnants of sulphide ore in the oxidised capping. The base of the ore is also irregular, grading into the protore which averages 0.1% Cu. Ultimately the pit was planned to reach a depth of 400 m, with lateral dimensions of 3.5 x 3.5 km (Kolessar, 1982).

More copper occurs in the Proterozoic granite than in the porphyry. The granitic rocks, intruded by dykes and plugs of quartz-monzonite, are exposed prevalently in the northern half of the pit. The Mo content is also higher in the granite. Within the mine area some 20 high grade areas of chalcocite ore are known, each averaging between 2 and 3% Cu. These were the targets of prior underground mining. They trend north-easterly along the larger fractures, most of which parallel the Burro Chief Fault (Kolessar, 1982).

The hypogene mineralisation is principally pyrite, chalcopyrite and sphalerite, with occurrences of molybdenite and trace bornite in the granitic section to the north. Pyrite is the most abundant sulphide, containing small inclusions of quartz and chalcopyrite, while chalcopyrite also occurs in quartz and sphalerite. Sphalerite is commonly associated with pyrite and partially replaces it. Molybdenite occurs as curved shreds accompanying other sulphides, as thin coatings on joints and fractures in the porphyry and as veins in the granite. Hypogene sulphides are generally <6% by weight. The hypogene ore averages 0.1% Cu, while the average mined supergene grade is 0.79% Cu. The ore mineralisation occurs in fractures, as disseminations in the country rock adjacent to fractures and as matrix in breccias. Most of the fractures are less than 2.5 cm thick, and only rarely range up to 30 cm (Kolessar, 1982).

Primary hydrothermal alteration and associated processes may contribute to preparation of the rock for enrichment by development of porosity and permeability as well as introducing the necessary primary sulphides. Argillic alteration occurs erratically, but quartz-sericite is the dominant alteration style. Propylitic alteration occurs peripherally to the north-east at the edge of the Gettysburg Extension, which occurs along the Burro Chief Fault in the north-west part of the West Racket Extension; and near the geographic centre of the orebody between the original pit and the West Racket Extension. The younger porphyries exhibit chloritisation of biotite, with feldspar altering to montmorillonite. The most significant changes are related to the free silica content which may range from <15% in unaltered rock to 40 to 70% in unaltered varieties, particularly on the granite-porphyry contacts. There is very little change in the total K2O content. The preponderance of yellow-brown montmorillonite at the top of the chalcocite zone suggests it is related to supergene enrichment. Nontronite occurs at the transition zone between the bottom of the capping and the supergene enrichment, while alunite is found in localised veins in the granite to monzonite porphyry contact zone, and has been observed in the matrix in one of the breccia zones (Kolessar, 1982).

In the supergene blanket, chalcocite the most important ore mineral, and covellite, replace pyrite, chalcopyrite and sphalerite. Chalcocite occurs most often as a thin coating on pyrite grains, although in some siliceous breccias it occurs as almost pure blebs, possibly replacing chalcopyrite or bornite. The pyrite:chalcocite ratio is approximately 6:1. The ore mineralisation occurs in fractures, as disseminations in the country rock adjacent to fractures and as matrix in breccias (Kolessar, 1982).

In the oxide zone, chrysocolla is the most abundant, but erratically distributed mineral. Tenorite, malachite and azurite, together with rare cuprite, native copper, bronchantite and chalcanthite are present in cappings and near the Burro Chief Fault. The appearance of the leached cap does not vary considerably with lithology, and has characteristic features of all stages from mature to immature leached cappings. Supergene hematite is the most common mineral filling voids after sulphides in the leached capping. Neotocite occurs around the margins of economically important areas (Kolessar, 1982).

Published production and reserve figures include:
    Production + reserves/resources - 1050 Mt @ 0.49% Cu (Mutschler et al., 2004)
    Start-up Reserve. 1975 - 321 Mt @ 0.79% Cu (1969, USBM),
    Production to 1981 - 128 Mt @ 0.6% Cu (Titley, 1992).
    Leaching Ore - Reserve 1992 - 171 Mt @ 0.33% Cu (American Mines Handbook, 1994).

Remaining proved + probable reserves - at December 31, 2011 (Freeport-McMoRan, 2012):
    ROM leach ore - 148 Mt @ 0.29% Cu (61.1% recovery).

For detail, consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1992.     Record last updated: 27/8/2013
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:
Kolessar J  1982 - The Tyrone Copper deposit, Grant County, New Mexico: in Titley S R 1983 Advances in Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson    pp 327-333

 References to this deposit in PGC Publications: 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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