Copper Flat, Hillsboro

New Mexico, USA

Main commodities: Cu Mo
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The Copper Flat porphyry copper deposit in the Hillsboro Mining District of south-western New Mexico is located some 60 km to the ENE of the Chino/Santa Rita mine. Mineralisation, dated at 73.4 Ma (Titley, 1982), is associated with an upper Cretaceous quartz-monzonite plug intruding a circular mass of Cretaceous andesites which are interpreted as representing an eroded caldera (Dunn, 1982). Much of the economic mineralisation is hosted by a breccia pipe within the quartz-monzonite, and few supergene effects are observed. Gold is also associated with the Cu-Mo mineralisation, unlike many of the other porphyry deposits of the region. The deposit lies within a horst developed on the western margin of the north-south Rio Grande Rift, within the Arizona-New Mexico Basin and Range Province.

Published production and reserve figures include:

Initial Reserve 1982 - 54 Mt @ 0.42% Cu, 0.012% Mo, 0.1 g/t Au, 3 g/t Ag (USBM; & Noske, 1987),
Reserve 1989 - 63 Mt @ 0.7% Cu (Titley, 1992).


The central section of the Hillsboro district is underlain by a circular block of andesite some 6.5 km in diameter. Most of the periphery of this block is marked by nearly vertical faults along which the andesite has been down-dropped against Palaeozoic sedimentary and underlying Proterozoic rocks. The thickness of andesite in this structure is <1000 m, but is underlain by Palaeozoic sediments. The andesite is generally a fine grained porphyritic rock with phenocrysts of plagioclase and amphibole in a groundmass of plagioclase and potash feldspar, with rare quartz. Agglomerates or flow breccias are locally present, although the andesite is generally massive. The andesite immediately to the south of the mineralised quartz-monzonite plug is coarse-grained and may represent a shallow intrusive phase. Magnetite is commonly associated with the mafic phenocrysts and accessory apatite is common. An irregular andesite breccia is found along the northern margin of the quartz-monzonite, possibly a pyroclastic, similar to other tuff-breccias intersected in drill holes (Dunn, 1982).

The andesite has been intruded by the: 1). mineralised Copper Flat Quartz-Monzonite in the centre of the interpreted caldera; 2). barren, un-altered Warm Springs Quartz-Monzonite along the southern margin of the caldera, and by; 3). numerous latite dykes that radiate from the Copper Flat Stock, cutting both the andesites and the stock. The Copper Flat Stock forms a topographic depression, or 'flat' within the andesite mass, and was largely covered by alluvium. The entire porphyry copper deposit is contained within the Copper Flat Stock, while the old vein deposits were 0.65 to 2.5 m thick quartz-calcite-pyrite-chalcopyrite veins with minor bornite, gold and silver found along the contacts of the latite dykes with andesites. Placer gold has also been produced from the district (Dunn, 1982).

The Copper Flat Quartz-Monzonite (adamellite) contains a few xenoliths of andesite, and is generally porphyritic with large orthoclase phenocrysts up to 5 cm long. It contains roughly equal amounts of plagioclase and orthoclase, with the plagioclase phenocrysts being smaller, around 1 cm, and the groundmass being predominantly orthoclase. Quartz makes up around 15% of the rock as small phenocrysts and in the groundmass. Hornblende and biotite are also present, while magnetite is common, associated with the mafic minerals and apatite is ubiquitous. Several textural variations have been observed, but not mapped (Dunn, 1982).

The latite in the dykes and plugs is generally fine grained with 5 to 10% euhedral plagioclase phenocrysts up to 5 mm long in a groundmass of plagioclase, potash feldspar and minor quartz. Apatite is also present. Two other textural varieties are recognised, one with rounded quartz phenocrysts up to 2 mm across, and another with no quartz, but with orthoclase phenocrysts up to 25 mm long. Each is a separate phase. Dykes range from 1.5 to 10 m thick. Two plugs of fine grained latite are also mapped (Dunn, 1982).

Post-mineral rocks in the district include Tertiary siliceous pyroclastics to the south-east of the andesite, and basalt flows of Miocene or Pliocene age interbedded with Tertiary sediments. Both sets of volcanics are younger than the faults that bound the andesite mass. The gold bearing gravels of the district are found within Quaternary sediments that overlie the Tertiary sediments and volcanics (Dunn, 1982).

Three principal directions of faulting are recognised in the district, namely a prominent NE-SW direction, WNW-ESE and ENE-WSW. The Copper Flat Stock is located at the intersection of two significant NE-SW and WNW-ESE pre-intrusion faults, while the latite dykes follow these three main directions. The same three directions control fracturing and sulphide mineralisation in the Copper Flat Stock. A weak north-south direction is also indicated by structural studies. Post-mineralisation and post-dyke movement is mapped on most of the same faults (Dunn, 1982).

Mineralisation & Alteration

The central high grade portion of the deposit is contained in a mineralised pipe which was almost completely covered by alluvium. While the eastern margin is outside of the ore, the remainder has higher grades than the surrounding quartz-monzonite, such that half of the ore is within the breccia which only accounts for a third of the volume of the orebody. The pipe is around 400 x 200 m in plan, with the long axis perpendicular to the dominant NE-SW fracture direction. It has a vertical extent of more than 300 m, with veins of coarse pegmatitic material persisting to almost 500 m deep in one drill hole. The pipe comprises a zone within the quartz-monzonite that has been cut by numerous, randomly oriented, irregular veins that are thicker and coarser grained than the narrow fracture controlled veinlets in the surrounding rocks. Part of the north-western section of the pipe contains rotated angular fragments cemented by the same hydrothermal matrix. The clasts are almost always quartz-monzonite although some latite is known where mapped dykes project into the breccia zone. The matrix is composed primarily of varying proportions of quartz, biotite (phlogopite), potash feldspar, pyrite and chalcopyrite. Magnetite, molybdenite, fluorite and calcite are common locally, while apatite is again found as an accessory. The matrix has sharp contacts with the clasts in many instances, while in others the margin is diffuse resulting from an alteration envelope of very fine grained secondary biotite. Other fragments are embayed and corroded on their edges. The breccias have been sub-divided into biotite and quartz-feldspar breccias on the basis of the predominant gangue mineral in the matrix. The former always forms high grade chalcopyrite ore, while the latter has variable amounts of mineralisation. On the eastern barren margins of the breccia pipe the quartz-feldspar breccia consists principally of pegmatitic potash feldspar with subordinate quartz, pyrite and trace chalcopyrite (Dunn, 1982).

Ore grade mineralisation is almost completely hypogene, with chalcopyrite and pyrite being the principal sulphides, accompanied by subordinate molybdenite, galena and sphalerite. Rare bornite has been recognised, almost all of which is outside of the orebody. The total sulphide content is low, ranging from 1% in the eastern portion of the breccia pipe and surrounding quartz-monzonite, to 5% in the quartz-monzonite surrounding the mineable orebody to the south and west. Sulphide content within the breccia is highly variable, with small areas containing as much as 20% sulphide. Sulphide mineralisation is almost exclusively restricted to the quartz-monzonite, with an abrupt drop-off at the andesite contact. Minor pyrite extends into the andesite along pre-mineral dykes (Dunn, 1982).

Mineralisation within the quartz-monzonite is mainly pyrite occurring as disseminations and long, continuous, fracture controlled veinlets. Chalcopyrite is present principally as disseminations accompanying primary mafic minerals. In an area to the south-east of the breccia chalcopyrite occurs un-accompanied by pyrite. Molybdenite is most abundant in the quartz-monzonite, occurring in quartz veins or as thin fracture coatings. Mineralisation within the breccia pipe is characterised by large, irregular masses of pyrite and chalcopyrite up to 50 cm across occurring as part of the breccia matrix and is associated with large crystals of quartz, biotite and potash feldspar. Chalcopyrite is most abundant in the biotite breccia, with chalcopyrite being present as large irregular masses or disseminations in biotite books. Within the quartz-feldspar breccia, the highest Cu and Mo content is to the west, with pyrite increasing to the east at the expense of chalcopyrite, molybdenite and quartz (Dunn, 1982).

A central zone of K-silicate alteration embraces the breccia pipe and extends almost to the south-eastern margin of the quartz-porphyry, enclosing the area of chalcopyrite with no associated pyrite. This alteration is manifested as secondary biotite and potash feldspar occurring as large crystals and rock fragment replacements. Secondary K-feldspar occurs as selvages to quartz veins and almost complete replacement of the original rock. Quartz occurs as veinlets and in the breccia matrix, formed with biotite and K-feldspar during the K-silicate alteration. Associated with this style of alteration are lesser quartz, carbonate, chlorite, fluorite and apatite. The bulk of the chalcopyrite falls within this zone, as does the molybdenite, but a decreasing amount of pyrite, the bulk of which is in the sericite zone (Dunn, 1982).

Sericite alteration has affected almost the entire quartz-monzonite stock and related dykes and fragments within the breccia. Sericite replaces plagioclase and occurs in the groundmass, while both sericite and chlorite have replaced mafic minerals. Quartz and carbonate veins and minor kaolinite are associated with sericite alteration. Two periods of sericite alteration are evident, one prior to the brecciation and K-silicate phase which pervaded the stock, including breccia fragments, and is overprinted by the K-silicate alteration; and a second stage which resulted in quartz veinlets with minor sulphides but well developed sericite envelopes which cut the K-silicate alteration pattern. The first quartz-sericite phase is predominantly sericite alteration with associated quartz and chlorite accompanied by pyrite and minor chalcopyrite. This is the major pyrite stage, accompanied by some magnetite. The second is principally a quartz phase with sericite, carbonate and chlorite and associated pyrite, galena and sphalerite, but little chalcopyrite. Extreme sericite alteration is also found in a thin veneer up to 10 m thick over an area of just over 100 m across on the southern margin of the breccia and appears to be a supergene product (Dunn, 1982).

Propylitic alteration is common in the andesite and andesite breccia, but is rarely developed in the quartz-monzonite or latite dykes. Dykes carrying weak quartz-sericite alteration and minor pyrite have been observed cutting un-mineralised andesite with shows only chlorite and epidote alteration (Dunn, 1982).

There is very little supergene oxidation or enrichment, with the maximum base of oxidation at 15 m, generally <10 m. Chalcocite occurs as coatings on chalcopyrite, but is erratic. One small patch of enrichment some 60 m long is known, while sporadic chalcocite is encountered to 60 m depth in places (Dunn, 1982).

For detail consult the reference(s) listed below.

The most recent source geological information used to prepare this summary was dated: 1993.    
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:
Dunn P G  1982 - Geology of the Copper Flat Porphyry Copper deposit, Hillsboro, Sierra 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 313-325
Nadoll, P., Mauk, J.L., Leveille, R.A. and Koenig, A.E.,  2015 - Geochemistry of magnetite from porphyry Cu and skarn deposits in the southwestern United States: in    Mineralium Deposita   v.50 pp. 493-515
Runyon, S.E., Nickerson, P.A., Seedorff, E., Barton, M.D., Mazdab, F.K., Lecumberri-Sanchez, P. and Steele-MacInnis, M.,  2019 - Sodic-Calcic Family of Alteration in Porphyry Systems of Arizona and Adjacent New Mexico: in    Econ. Geol.   v.114, pp. 745-770.

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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