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Mineral Park, Ithaca Peak |
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Arizona, USA |
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Cu Mo
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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The Mineral Park orebody is associated with a complex of late Tertiary monzonitic rocks within two main stocks intruding Middle Proterozoic lithologies. Economic Cu mineralisation is only present within the supergene blanket, with hypogene grades of 0.1 to 0.15% Cu and 0.04% Mo. The deposit is located within north-western Arizona, about 125 km to the north-west of Bagdad and lies within the Arizona-New Mexico Basin and Range Province.
Geology
The geological succession in the Mineral Park area is as follows, from the base (Wilkinson, etal., 1982):
Proterozoic Cerbat Complex, represented by:
* Middle Proterozoic Metamorphics - the oldest rocks in the mine area, comprising quartz-feldspar gneiss, biotite schist, amphibolite and quartzite. Amphibolite schists are locally the most abundant lithologies of the sequence, with lesser quartz-feldspar gneiss, while biotite schists are minor and quartzites are the least developed.
* Granite Gneiss - the Middle Proterozoic metamorphics were intruded by a batholithic mass, now represented by gneisses which vary in composition from biotite-quartz-monzonite to biotite granite. These are the dominant rock types in the Mineral Park area and have been dated at 1760 Ma.
Both of these Proterozoic suites are cut by 1515 to 1606 Ma pegmatites. The granite gneiss apparently intruded an already folded sequence of rocks. The contact zone between the Batholithic granite gneiss to the north-east and the older metamorphics to the south-west divides two structural domains as defined by foliation direction and styles of deformation. This domainal and lithological boundary trends at 330° and passes through the main mineralised Ithaca Peak Stock as well as the Alum Wash mineralised centre some 3 km to the north-west. The next youngest rocks are:
* Diana Granite - a weakly foliated, porphyritic granite with large orthoclase phenocrysts set in a groundmass of orthoclase, microcline, oligoclase, quartz and biotite. This granite, which has been dated at 1350 Ma, is in turn cut by 1100±161 Ma pegmatites.
* Hornblende Meta-diorite - intrudes all of the Proterozoic lithologies, and comprises a medium to coarse grained, porphyritic-aphanitic, typically non-foliated rock of dioritic composition. It may be of 1300 to 1400 Ma age.
Late Cretaceous igneous rocks, including biotite-quartz-monzonite porphyry, biotite-quartz-diorite porphyry and rhyolite dykes. This group of intrusives is localised within the Mineral Park mine area, where an age of 71.5 Ma has been obtained from a biotite from the porphyry. The biotite-quartz-monzonite porphyry and the biotite-quartz-diorite porphyry are discrete intrusions, although they appear to represent the same intrusive event. They occur as two main masses, the Ithaca Peak and the Gross Peak Stocks, as follows:
* Ithaca Peak Stock - a single intrusion of quartz monzonite that passively intruded the Proterozoic Cerbat Complex. It shows concentric zoning which ranges from quartz porphyry at the centre to biotite-quartz-monzonite on the periphery. The quartz porphyry forms an elliptical zone 600 x 425 m, characterised by large (up to 1.5 cm) quartz eyes, plagioclase, biotite and occasional large (up to 4 cm) K-feldspar phenocrysts in an aplitic groundmass. Near the centre of the quartz porphyry mass is another 210 x 150 m zone of crenulated sinuous quartz veins and large quartz pods up to 40 x 12 m in size. The outer zone of biotite-quartz-monzonite porphyry is a medium grained porphyritic-phaneritic to porphyritic-aphanitic rock with plagioclase, biotite, quartz and K-feldspar phenocrysts in a quartz and K-feldspar matrix.
Variations in the original texture, superimposed on the compositional differences, define three concentric zones, namely 1). a core of porphyritic-aplitic quartz porphyry; 2). a surrounding porphyritic-aplitic biotite-quartz-monzonite porphyry; and 3). an outer seriate granitic-biotite-quartz-monzonite porphyry forming an incomplete ring on the eastern and south-eastern margin. Outer contacts with the Proterozoic rocks are sharp with few xenoliths except right at the margin.
* Gross Peak Stock - which is generally highly altered, making phases and internal contacts difficult to recognise. Two phases are suggested, a biotite-quartz-monzonite porphyry, and a biotite-quartz-diorite porphyry. Cross sections indicate that the Gross Peak Stock is keel shaped in an east-west direction, pinching out into two large sill-like projections to the south, but attached to the larger mass in the north. Dykes of biotite-quartz-monzonite porphyry trending into the sill like appendages.
* Rhyolite Dykes - represent the last intrusive event. They occur as aphanitic white to light pink rocks with rare small K-feldspar or quartz phenocrysts. The dykes cut the biotite-quartz-monzonite porphyry, but are in turn cut by the mineralisation.
* Breccia Dykes - several breccia dykes are recognised in the mine, averaging 0.5 m in thickness, but ranging from 1 cm to 6 m. They contain angular to sub-rounded clasts set randomly in a fine grained matrix that has been strongly altered to secondary biotite, K-feldspar and quartz. The fragments comprise all of the rock types of the mine except the Laramide porphyries and are pre-mineralisation. However while the rhyolite dykes cut the monzonite porphyries, and fragments of rhyolite are found in the breccia, none have been recognised from the monzonite porphyries.
Mineralisation & Alteration
Hydrothermal alteration associated with the Mineral Park deposit is structurally controlled and is present in two forms (from Wilkinson, etal., 1982), namely:
1) Selectively pervasive alteration - which, while occurring in large volumes of rock, only affects certain minerals. The earliest event at Mineral Park involved the formation of secondary biotite from hornblende and primary biotite. This took place over an area of 1.7x1.45 km in the Proterozoic amphibolite schist and hornblende meta-diorite, and in the late Cretaceous quartz porphyry and biotite-quartz-monzonite porphyry stocks. In the Proterozoic rocks this alteration was accompanied by quartz and magnetite, while in the Cretaceous porphyries it was less pronounced, replacing primary biotite scattered as small flakes in the groundmass. The most abundant secondary biotite, which is both pervasive and veinlet controlled, is found at the outer margins of the of the low grade copper core. The second type of pervasive alteration involves the replacement and rimming of plagioclase by K-feldspar. This has been observed in all rock types but is less extensive than the development of secondary biotite. Both types of pervasive alteration are overprinted by all of the veinlet controlled stages below.
2) Veinlet controlled alteration - is the dominant style associated with the ore deposit. It occurs both within veins and veinlets, and as halos on their margins. The earliest alteration assemblage of this type encountered comprises pre-sulphide K-feldspar-biotite veinlets. These are usually irregular, contain little quartz and may be tens of metres in length. The biotite in these veins shows evidence of being marginally older than the feldspar. Sparse coarse grained quartz-biotite veins appear next, followed by two molybdenite bearing sets, the quartz-K feldspar-anhydrite-pyrite-molybdenite and quartz-molybdenite-pyrite veinlets. Neither has any observable chalcopyrite. They were followed by chalcopyrite bearing veinlets comprising the assemblage quartz-chalcopyrite-pyrite-chlorite-K feldspar ±anhydrite ±magnetite ±epidote. These chalcopyrite veins consistently carry minor sphalerite. Subsequently quartz-pyrite-sericite ±carbonate veinlets appeared and are best developed in the more felsic rocks, while in the more mafic rocks sericite is less common, and calcite, chlorite and very minor epidote take its place. The final stage of sulphide deposition is represented by a complex series of veins involving quartz-chalcopyrite-pyrite-sphalerite-galena which are of minor significance in the open pits, but are important in the district where they may be up to 16 m thick. Within the Cretaceous biotite-quartz-monzonite porphyry and the Proterozoic Granite Gneiss complex final stage vein assemblages are found of chlorite-epidote-sericite-clay, silicification with white mica selvages and sericite-clay-sphene which replaces the first two.
The early pervasive, pre-mineral K-silicate assemblage is extensively developed throughout the pit area, although the late overprinting post-mineral quartz-sericite veining is equally extensive, occurring up to several hundred metres beyond the pervasive alteration. Molybdenum mineralisation at >0.01% Mo forms an elliptical zone which encloses an annular +0.03% Mo zone 200 to 360 m wide surrounding a low grade core. Grades within the 0.03% Mo annulus are variable, with zone exceeding 0.06% Mo being common. The bottom of the Mo zone, which appears to have steep margins, is not known, but may extend for up to 500 below the surface. The lateral extent of hypogene Cu is not well defined because of the overprint of supergene mineralisation (Wilkinson, etal., 1982).
Drilling into the hypogene zone at depth indicates a similar annular zone of Cu mineralisation, with values ranging between 0.05 and 0.15% Cu, averaging 690 ppm. The low grade Cu core within the annulus has <500 ppm Cu and generally coincides with the low grade Mo core. Primary Cu decreases with depth, and bottoms at higher levels than does the Mo. The low grade core zone is not centred on the stock, but is located partly within the main stock and partly within the Proterozoic amphibolite schist. Within the barren core the porphyry stock is heavily quartz-veined and includes the crenulated quartz veins and pods described earlier. Cu mineralisation is more extensive laterally than the Mo. In addition the associations with host rock also vary, with higher Cu grades tending to occur in the more mafic Proterozoic rocks, relative to the more felsic rocks, while the reverse is the case with Mo. Mo is almost totally restricted to quartz veins, whereas much of the chalcopyrite is observed in selvages adjacent to veins where it shows an affinity for mafic minerals, especially biotite (Wilkinson, etal., 1982).
Molybdenite veining exhibits a preference for an east-west fracture trend which does not parallel any pre-Laramide direction, although subsidiary sets are sub-parallel to Proterozoic foliations. These have densities of 0.02 to 0.14 per cm, averaging 0.05 per cm. They are cut by quartz-pyrite-sericite veins which are mainly NW and less frequently NE trending, and are more extensive than the molybdenite system (Wilkinson, etal., 1982).
The supergene blanket at Mineral Park constitutes the economic orebody. The zone of oxidation, as known in 1968, extends to an average depth of 40 m below the present surface. Leaching was thorough, with turquoise the only Cu mineral of significance in the leached cap (Anderson, 1968). Supergene enrichment below this is distributed over a vertical interval of around 200 m and is perched some 30 m above the present water table. Although the base of the supergene blanket is irregular due to variations in fracture density, it generally conforms to the topography of Ithaca Peak.
The supergene blanket resulted from chalcocite coating or replacement of pyrite and minor chalcopyrite. The present form of the chalcocite blanket mineralisation is the result of oxidation and erosion of a previously cycle of enrichment. Since the emplacement of the hypogene ore at around 71.5 Ma, the Mineral Park area has undergone two periods of uplift, in the Miocene and the Pliocene-Pleistocene. The deposit had previously been buried by a widespread Oligocene ignimbrites. The initial enrichment may have been pre-Oligocene, followed by the second stage in the Pliocene-Pleistocene, after the erosion of the ignimbrite (Eidel, etal., 1968).
The chalcocite blanket cuts across the contact between the Laramide stocks and the Proterozoic Cerbat Complex. It occurs as several separate layers with intervening barren to weak Cu mineralisation. A barren pyrite zone is found between the base of oxidation and the zone of enrichment. This barren zone comprises pyrite from which thin earlier enrichment coatings of chalcocite were leached by meteoric fluids. Similar pyrite mineralisation with very low accompanying Cu extends downwards in the centre of the pit (Eidel, etal., 1968).
An enrichment factor of 3 is estimated from a hypogene grade of 0.15% to the average 0.45% Cu of the bulk supergene blanket. Smaller tonnages of +0.45% Cu are attributed to higher hypogene protore alone, or to more intense enrichment. The large majority of the mineralised veinlets had a low chalcopyrite content and were only weakly enriched by chalcocite coatings of pyrite, while the few with a high chalcopyrite content were strongly enriched by chalcocite completely replacing chalcopyrite as well as coating accompanying pyrite. A few major veins originally containing sphalerite, argentiferous galena and chalcopyrite were also strongly enriched by chalcocite. Replacement of chalcopyrite by chalcocite is so strong that the little remaining chalcopyrite is usually only found within pyrite grains that have been coated, but not entirely replaced by chalcocite. Many of the fractures occupied by veins within the orebody were reactivated and the contained pyrite was comminuted, thus increasing their surface area for reaction and generation of chalcocite. Sphalerite is partially replaced by chalcocite. Molybdenum levels in the supergene and hypogene mineralisation are similar, while in the oxide zone molybdenite was converted to ferrimolybdite, with no significant leaching or enrichment (Eidel, etal., 1968).
Published reserve and production figures include:
Production + reserves, 1978 - 175 Mt @ 0.53% Cu, 0.04% Mo (Gilmour, 1982),
Production to 1981 - 86 Mt @ 0.35% Cu, 0.03% Mo, 2.3 g/t Ag (Titley, 1992)
Reserves, 1992 - 12 Mt @ 0.27% Cu (Am. Mines H'book, 1994)
Prov.+Prob.+Poss. Leaching reserves, 1994 - 61 Mt @ 0.23% Cu (AME, 1995)
Remaining Ore Reserves and Mineral Resources at 1 June, 2013 were (Mercator Minerals TSX Release 26 June, 2013):
Proved + Probable Ore Reserves - Mill ore - 368.940 Mt @ 0.12% Cu, 0.037% Mo, 2.6 g/t Ag;
Proved + Probable Ore Reserves - Mill ore - 33.442 Mt @ 0.11% Cu.
Measured + Indicated Mineral Resources - 1034.882 Mt @ 0.098% Cu, 0.032% Mo, 2.25 g/t Ag (0.136% Cu eq. cut-off);
Inferred Mineral Resources - 399.275 Mt @ 0.102% Cu, 0.023% Mo, 2.44 g/t Ag (0.136% Cu eq. cut-off).
Alternate Mineral Resources at higher cut-off gades were:
0.20% Cu eq. cut-off
Measured + Indicated Mineral Resources - 1034.882 Mt @ 0.098% Cu, 0.032% Mo, 2.31 g/t Ag (=0.278% Cu eq.);
Inferred Mineral Resources - 189.696 Mt @ 0.104% Cu, 0.031% Mo, 2.59 g/t Ag (=0.247% Cu eq.).
0.30% Cu eq. cut-off
Measured + Indicated Mineral Resources - 209.386 Mt @ 0.14% Cu, 0.046% Mo, 2.44 g/t Ag (=0.353% Cu eq.);
Inferred Mineral Resources - 19.086 Mt @ 0.107% Cu, 0.046% Mo, 2.28 g/t Ag (=0.322% Cu eq.).
0.40% Cu eq. cut-off
Measured + Indicated Mineral Resources - 31.427 Mt @ 0.248% Cu, 0.048% Mo, 2.66 g/t Ag (=0.471% Cu eq.);
Inferred Mineral Resources - 0.092 Mt @ 0.073% Cu, 0.073% Mo, 1.09 g/t Ag (=0.419% Cu eq.).
NOTE: Mineral Resources include Ore Reserves.
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1997.
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.
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Selected References:
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Lang J R, Eastoe C J 1988 - Relationships between a Porphyry Cu-Mo deposit, base and precious metal veins, and Laramide intrusions, Mineral Park, Arizona: in Econ. Geol. v83 pp 551-567
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Lang J R, Guan Y, Eastoe C J 1989 - Stable isotope studies of Sulfates and Sulfides in the Mineral Park Porphyry Cu-Mo system, Arizona: in Econ. Geol. v84 pp 650-662
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Wilkinson W H, Vega L A, Titley S R 1982 - Geology and ore deposits at Mineral Park, Mohave County, Arizona: in Titley S R 1983 Advances in Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 523-541
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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, 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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