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Christmas |
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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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All papers now Open Access.
Available as Full Text for direct download or on request. |
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Skarn and porphyry style mineralisation at Christmas is associated with a Palaeocene, 62 Ma aged, elongated multi-phase, plug like granodiorite and granodiorite porphyry with plan dimensions of some 900 x 450 m. The stock and an associated dyke swarm of rhyodacite porphyry intruded a 1000 m thick sequence of Proterozoic, Palaeozoic and Cretaceous sedimentary rocks along an east-west trending fault zone. The deposit lies within the Arizona-New Mexico Basin and Range Province.
Published production and reserve figures include:
Reserve, open pit, 1984 - 25 Mt @ 0.63% Cu (USBM)
Reserve, underground, 1984 - 18 Mt @ 1.82% Cu (USBM)
Underground Production, 1905-66 - 4.5 Mt @ 2% Cu (Einaudi, 1982)
Open-pit Production, 1965-76 - 16 Mt @ 0.6% Cu (Einaudi, 1982)
Production to 1981 - 19.2 Mt @ 0.74% Cu, 2.95 g/t Ag, 0.069% Mo (Titley, 1992)
Reserve, open pit, 1989 - >6.4 Mt @ 0.63% Cu (Titley, 1992)
Reserve, underground, 1989 - 18.1 Mt @ 1.82% Cu (Titley, 1992)
Geology
The host sequence in the Christmas district comprises, from the base (Koski & Cook, 1982),
Proterozoic basement -
Pinal Schist - of early Middle Proterozoic, pre-1600 Ma age.
Ruin Granite - of Middle Proterozoic age, around 1400 Ma, present locally as plutons.
Apache Group, 275 to 400 m thick - regionally a late, post 1600 Ma Middle Proterozoic terriginous suite of conglomerate, shale, interbedded basalt, dolomite, limestone and quartzite.
Diabase (dolerite) Dykes, usually 1140 Ma in age regionally - occurs within the Apache Group sequence.
Troy Quartzite, 0 to 300 m thick - predominantly quartzite. This unit which is post 1600 Ma in age and pre 1140 Ma is followed by an approximately 500 Ma erosional break.
Palaeozoic sediments, comprising,
Cambrian, a sequence which totals 0 to 100 m in thickness, composed of the Bolsa Quartzite and Abrigo Formation,
Devonian Martin Formation carbonates which are 350 to 600 m thick,
Carboniferous Escabrosa Limestone and Naco (or Horquilla) Limestone - the Naco Limestone is generally 260 to 300 m thick in the mine area and is characterised by thin to medium beds of fossiliferous cherty grey limestone and thin but persistent interbeds of finely laminated calcareous shale and siltstone.
For more detail on the lithologies of these Palaeozoic formations see the descriptions of the geology of the nearby 'Ray' mine and 'Globe/Miami District'.
Cretaceous sequence, comprising the,
Pinkard Formation, shallow water, late Cretaceous sediments.
Williamson Canyon Volcanics, 250 to 600 m thick, dated at 76 Ma - a monotonous, westward thinning sequence of dark-grey to greenish-grey basaltic volcanic breccia and flows with subordinate clastic sediments. The sequence is thinnest along the ridge south of the mine and thickest to the east of the Joker Fault
Late Cretaceous intrusives, comprising,
Hornblende Andesite Porphyry, dated at 82 to 76 Ma - present as greenish-grey dykes sills and plug-like masses which are the oldest rocks intruding the Williamson Volcanics. These may be exhumed feeders for the upper Williamson Volcanics.
Hornblende Rhyodacite Porphyry, dated at 70 to 72 Ma - present as stocks and inter-connecting east-west trending dykes, also intruding the Williamson Volcanics.
Tertiary intrusives, principally of the Palaeocene Christmas Intrusive Complex, which is dated at around 62 Ma and comprises a composite granodiorite stock and peripheral east-west striking porphyry dykes, all passively emplaced in Palaeozoic sedimentary and Cretaceous volcanic rocks. The elongate stock, positioned near the centre of the Christmas porphyry copper mine, has a plan area of approximately 900 x 450 m, with the long axis sub-parallel to the dykes at approximately 80°. Two elongate faults, the Christmas and Joker Faults bisect and displace the stock by 400 and 580 m respectively, progressively exposing higher levels of the stock and porphyry copper system towards the east (ie. east side down movement). The Christmas Stock is divided into two groups, namely 1). an early veined quartz-diorite, biotite granodiorite porphyry and granodiorite. The quartz-diorite is found on the southern, eastern and northern margin of the stock, while the biotite-granodiorite porphyry forms a younger core to the stock. Local intrusion breccia is found between granodiorite and quartz-diorite porphyries; and 2). late, unveined granodiorite porphyry and dacite porphyry.
Alteration and Mineralisation
The alteration and mineralisation found within the porphyry stock is complex, commencing with stage I potassium-silicate alteration, characterised by a stockwork of orthoclase-quartz-chalcopyrite-bornite veinlets and partial to complete biotitisation of hornblende. This is best developed in the higher level portion of the stock east of the Christmas-Joker Fault system. Quartz-sulphide veinlets and secondary biotite extend 100 to 200 m into the basaltic volcanic rocks. The stage II fracture controlled alteration of quartz-sericite and quartz-sericite-chlorite, accompanied by chalcopyrite and pyrite is superimposed on the outer portion of the stockwork potassium silicate zone (Einaudi, 1982).
Granodiorite west of the Joker Fault also displays potassium silicate alteration accompanied by sulphides, but the intensity of mineralisation and alteration is less intense than in the down-dropped blocks to the east. Cu grades in the western block range from 500 to 1000 ppm. In this area plagioclase is fresh and hornblende is only partially biotitised, with bornite the dominant opaque (around 0.5%) accompanied by magnetite (1%). Two types of sulphide veinlet are described, namely 1). thin discontinuous biotite veinlets that contain accessory quartz, K-feldspar and anhydrite and 2). thicker vuggy quartz lined veinlets that contain accessory K-feldspar, biotite and rare molybdenite. This implies that the western fault block represents a deep section of a porphyry copper system (Einaudi, 1982).
Most of the ore mined up to 1975 was from skarn ore developed within the Devonian Martin Formation, Carboniferous (Mississippian) Escabrosa Limestone and Carboniferous (Pennsylvanian) Naco Limestone of the Palaeozoic succession to the west of the Joker Fault system. Larger porphyry tonnages have however, been delineated and subsequently mined to the east of that fault system (Einaudi, 1982). Tonnages and grades of the skarn and open pit mineralisation are indicated in the reserves tabulation above.
Contact metamorphism and metasomatism have formed hornfels from shale, siltstone and impure carbonates at distances of <750 m from the stock on the north and <450 m on the west. In the same zones cherty carbonates contain wollastonite with local garnet and diopside, dolomite has brucite and calcareous quartzite contains interstitial diopside-quartz with local wollastonite or phlogopite. Skarn occurs in the purer carbonate of the Palaeozoic section and extends farthest from the stock in carbonate beds that are interbedded with siltstone and shale. Thus skarn in the thick-bedded Escabrosa Limestone only extends for tens of metres from the stock, while the skarns developed in the Martin Formation and Naco/Horquilla Formations extends outwards for 120 to 200 m (Einaudi, 1982).
Three types of skarn are recognised (Einaudi, 1982), namely,
1). Endoskarn which is developed within the periphery of the stock adjacent to skarns in the Palaeozoic, and is composed of pale brown grossularite garnet and pale green clino-pyroxene over widths of several cm's to several metres;
2). Calcic skarn in limestones of the Escabrosa and Naco/Horquilla Formations - which within 10 m of the stock (or in xenoliths within the stock) occurs as andradite with minor diopside and epidote, with abundant magnetite and pyrite, lesser chalcopyrite and magnetite pseudomorphs after specularite. Local high chalcopyrite concentrations occur as irregular orbicular structures with calcite and garnet within this interval. These structures are cut by chalcopyrite-epidote-calcite-quartz veinlets and quartz rich pods with disseminated sulphides. Cu grades are generally low in this zone and increase with distance from the stock. In the ore zones granular, yellow-green garnetite, with local diopside, contains chert nodules. Bornite and lesser chalcopyrite occur interstitially to garnet, apparently as a replacement of calcite, and in infrequent quartz veinlets. The outer skarn contact is marked by a zone of garnet veins cutting marble. Erratic concentrations of sphalerite-chalcopyrite, with traces of galena are found along the garnet-marble contacts and in veins;
3). Magnesian skarn in dolomitic units of the Martin and Naco/Horquilla Formations. This skarn type is characterised by forsterite and abundant magnetite (5 to 25%), accompanied by brucite, anhydrite and sulphides. Tremolite-talc masses and nodules of brown and green garnet-diopside-anhydrite-sulphides occur locally. The forsterite, magnetite, anhydrite and sulphides are indicated as being broadly synchronous. Late serpentine replaces most of the forsterite. Although the calc-silicates in this skarn type display no obvious zoning, the sulphides do. The assemblage chalcopyrite-bornite (with local chalcocite) gives way laterally towards the marble contact to chalcopyrite-pyrite-pyrrhotite. Sphalerite abundance increases outward and towards the hangingwall and footwall of the skarn beds. The hangingwall contact of magnesian skarn with dolomitic marble reveals the fracture control of skarn growth, with magnetite-calcite veinlets (plus anhydrite, chalcopyrite and sphalerite) encased in zoned envelopes of forsterite or forsterite-magnetite near the veinlet filling and coarse calcite near the dolomitic marble wall.
The major orebodies are developed in calcic skarn in the upper Naco/Horquilla Formation, and in magnesian skarn in the lower Martin Formation (Einaudi, 1982).
Hornfels are locally extensively veined and mineralised, containing Cu grades of up to 1%, especially where they are in contact with mineralised skarn (Einaudi, 1982).
The principal consequence of supergene alteration in the igneous rocks at Christmas is the near surface oxidation and total leaching of sulphides in the highly fractured, pyritic quartz-sericite alteration zones. The ultimate product of intense leaching is a compact massive aggregate of kaolinite and quartz. Pyrite and chalcopyrite within 20 to 50 m of the surface are commonly coated with a thin film of chalcocite, but enrichment is minimal. Concentration of Cu in the mafic volcanic rocks to the east of the Joker Fault is also minimal, although erratic supergene alteration is observed in fractures to depths of up to 330 m (Koski & Cook 1982).
Approximately 1.5 km to the east of the main Christmas Intrusive Complex stock the outcrop is principally of basalts and andesites belonging to the Williamson Canyon Volcanics. These are cut sporadically by dykes of dacite porphyry belonging to the Christmas Intrusive Complex. According to Einaudi (1982) these outcrops are within the propylitic alteration zone. The well exposed fresh faces in road cuttings bordering the Gila River valley exhibited massive, dark green, andesitic to basaltic volcanics that had extensive epidote development on fracture faces and epidote veins with associated quartz and calcite ranging from a few mm's, occasionally up to 1 m or more in thickness. These veins were developed in several directions, with one preferred in which the spacing varied from 0.25 to 2 m. Some of the quartz-epidote veins passed laterally into epidote breccias. The volcanics are cut by dacite porphyry dykes from a metre to a hundred metres in width. These dykes are very similar in appearance to that described and sighted at San Manuel (see the 'San Manuel-Kalamazoo' description). The intrusives were not as well altered, but in places had similar epidote-quartz, epidote and calcite veining. No signs of copper staining were seen. The volcanics and the dykes had high magnetic susceptibilities, although these values were greatly reduced within the associated veins and their selvages (Pers. observ.).
For detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 1992.
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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Koski R A, Cook D S 1982 - Geology of the Christmas Porphyry Copper deposit, Gila County, Arizona: in Titley S R 1983 Advances in Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 353-374
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