Ray Part 2 - Mineralisation & Alteration
Super Porphyry Cu and Au|
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The Ray orebody in Arizona illustrates a number of important characteristics of porphyry mineralisation, particularly the influence of host rock composition on ore style, alteration and supergene modification. There is no clear mineralised, central porphyry body, although numerous porphyry masses are evident, possibly representing a tilted porphyry plug/complex. Ore is hosted by both a major dolerite sill and the quartz-mica schists of the enclosing Middle Proterozoic Pinal Schists. Ore within the dolerite is of economic hypogene grade with no associated supergene enrichment, reflecting the reactivity of the host. In contrast the low reactivity Pinal Schists have only sub-economic hypogene mineralisation, but economic supergene ore. Historic production + reserves are in excess of 1200 mt @ 0.6 to 0.8% Cu. In 1993 8.15 mt of ore at 1.6% Cu were treated for 130 300 t of Cu. The mine is operated by ASARCO Inc.
DETAILED DESCRIPTION - PART 2, Mineralisation & Alteration
Continued from RAY PART 1, Geology, Structure & Alteration
Mineralisation and Alteration
The hypogene mineralisation is interpreted as being associated with the 61 Ma (Palaeocene) Granite Mountain Porphyry, although exposures of that intrusive phase are not well mineralised. In addition, the mineralisation is not spatially related to the exposed porphyry, being disposed in a concentric/horse-shoe shaped pattern, which is not focused on a central mass of Granite Mountain Porphyry. The bulk of the hypogene mineralisation is hosted by the reactive Middle Proterozoic Diabase (dolerite) sills with lesser amounts in both the quartzose Apache Group and the over-faulted older Pinal Schists. The mineralogical and alteration zoning, as described below, cuts across lithological boundaries to maintain the concentric pattern. Much of the high grade ore mined to date has been from a chalcocite blanket which is developed in part over the hypogene copper mineralisation, but predominantly within the pyritic halo to the primary ore, extending for up to 1500 m to the west of exposed hypogene ore (Phillips, etal., 1974).
The stratified Oligocene, Miocene and younger volcanics and clastics overlie parts of the sulphide orebody. These post mineral rocks tend to thin and/or pinch out over the Ray deposit, indicating arching along the north-easterly trend of the Granite Mountain intrusives (Phillips, etal., 1974).
Metz & Rose (1966) suggest that the main controls on the location of the ore are related to the intersection of the north to NNW structures and the deep structural weaknesses reflected in the north-east to ENE trending structures, particularly one known as the Porphyry Break which is 900 to 1200 m wide. The most obvious control on supergene mineralisation at Ray was the Emperor Fault. This low angle fault, with its thick gouge zone, apparently caused a perched water table during the enrichment cycle and is responsible for the high grade supergene ore directly below it. Rock above the Emperor Fault has been thoroughly leached. Conversely steep dipping faults such as the Bishop, have been responsible for deep adjacent oxidation. The diabase (dolerite), due to its content of calcite veinlets and hence has not been leached. In places diabase with fresh hypogene sulphides overlies local supergene enriched rocks. This resistance to leaching has been partly responsible for the formation of the copper-silicate ore detailed below (Metz & Rose, 1966).
Fracturing is radially oriented outwards from the centre of the mineralised system. The fracture density is low in the barren core, increasing to a maximum in the main ore zone before decreasing outward again (E John, pers. comm., 1994).
In plan view the hypogene mineralisation zoning is horse-shoe shaped, concentric and symmetrical. The total sulphide content is low in the centre of the system, increasing outwards for a distance of around 1 km to a maximum of 3 to 8% sulphide, and then decreasing to approach zero some 1.8 to 2.4 km from the centre. The ratio of pyrite:chalcopyrite follows a reverse pattern, with the highest chalcopyrite in the centre, decreasing outwards. Changes in the gradient of total sulphide and the pyrite:chalcopyrite ratios defines three zones. These are 1). a low sulphide (1 to 1.5%), but high chalcopyrite core that rarely approaches ore grade, resulting in a low grade or 'barren' core; 2). a zone of changing total sulphide in which the total sulphide content increases rapidly and the ratio of pyrite to chalcopyrite increases, producing the main higher ore grade mineralisation annulus, with a total sulphide content averaging 2 to 4%; and 3). a pyrite halo where the total sulphide content reaches a maximum before declining again, and the pyrite:chalcopyrite continues to increase, thus producing a 'barren' high sulphide outer zone. The pyritic halo averages 3 to 8% sulphide at its best development, with the maximum sulphides being some 500 m outside of the second zone, and dropping off to zero 1.2 to 2 km from the edge of that main copper rich annulus. This pyritic halo carries 0.1 to 0.2% Cu (Phillips, etal., 1974; Mine visit briefing, 1991). It should also be emphasised that from impressions gained on the visits this pattern of zonation, although approximately correct, appears to be somewhat idealised.
While this pattern is present in all of the rock types in the mine area, the actual sulphide content and pyrite:chalcopyrite ratio are controlled by the host rock chemistry and vary considerably with lithology. The diabase (dolerite) sills contain both more sulphide, particularly outside of the low grade core, and a lower pyrite:chalcopyrite ratio than the siliceous rocks. Iron rich units of the Pinal Schists show a similar affinity for sulphides and Cu to the diabase (Phillips, etal., 1974).
The hypogene orebody has a generally downward tapering horse-shoe shape, with an outer diameter of 1.5 km and width of several hundred metres. It is open to the north, partly due to thick overburden and by the presence of a large stock of post ore Tea Pot Mountain Porphyry. The geometry of the horse-shoe is also substantially influenced by the Diabase Faultm the mine visit briefings, 1991 & 1994).
The lithologies influence the form and grade of mineralisation as follows:
* The lower diabase (dolerite) sill is the most extensive host, persisting from the core of the mineralised system to beyond its outer margins. Sulphides occur as veinlets and fracture coatings of pyrite and chalcopyrite from <0.5 to 10 mm in thickness. The fractures and veinlets are either coated with sulphides or enclose veins of sulphide with calcite and/or quartz, and are densely spaced. Barren quartz and calcite veins predate and cut the mineralised veins. Chalcopyrite and pyrite are the major hypogene minerals, while bornite is present in minor amounts as incipient replacement of chalcopyrite where chalcopyrite exceeds pyrite. Molybdenite occurs most frequently along the outer edges of the quartz-chalcopyrite-pyrite veinlets and within the higher grade Cu zones. Within the low grade core Cu grades are <0.4% Cu, being as low as 0.1% Cu or less. The ore grade annulus, defined by a 0.4% Cu cutoff, has peaks of more than 1% Cu, but averages 0.7% Cu with 4% total sulphide. Within the pyritic halo the diabase contains around 0.2% Cu, but 10 to 12% total sulphides. The veining is less dense in this zone, although individual veins are generally thicker. The diabase has a similar sulphide content to the siliceous schists of the Pinal Schists, although in contrast the diabase has a higher Cu content, resulting from a lower pyrite:chalcopyrite ratio. The diabase also contains secondary magnetite as both disseminations and in veins, in addition to its primary magnetite. The vein magnetite frequently accompanies the better chalcopyrite mineralisation. The ore within the diabase conforms to the horse-shoe shape of the overall orebody. The NW-SE elongation of this ore grade horse-shoe is normal to the ENE trend of the porphyry belt, although a plot of total sulphide is elongated in an ENE direction (Phillips, etal., 1974; E John, pers. comm., 1994; Pers. observ.).
* Within the quartzose Proterozoic host rocks the orebody has a similar, vertically overlapping horse-shoe shape within the plan view of each of the units. The low grade core averages from 0.2 to <0.1% Cu, while the high copper zone locally averages as high as 0.5% Cu. The pyrite halo lies beyond the outer 0.2% Cu contour. There is usually a break in the total sulphide gradient at the boundary between the low copper core and the high copper annulus. The maximum sulphide content is generally between 3 and 5%, while the minimum outside of the pyrite halo is <1%. In the low grade core chalcopyrite accounts for around 40% of the sulphides, while it is 2% or less in the pyritic halo. The magnetite content also drops in the high copper annulus, increasing in both the core and pyrite halo (Phillips, etal., 1974). The best Mo developments within the mine, locally up to 0.1% Mo, are found within the Quartzites of the Apache Group (E John, pers. comm., 1994).
* The Granite Mountain Porphyry was emplaced and 'solidified' prior to the emplacement of the mineralisation. Drilling to a depth of 1000 m has not encountered a large single stock, nor does it indicate the string of outcropping porphyry exposures are connected. The porphyry was a poorer host to mineralisation than the diabase. Only 17% of the rock within the final limits of the pit as planned in the early 1970's was to be porphyry, with an average and fairly uniform grade of 0.21% Cu. Only 6% of the porphyry in the pit contains more than 0.4% Cu (Phillips, etal., 1974). E John (pers. comm., 1994) considers the Granite Mountain Porphyry to be pre- to syn-ore.
The primary alteration is also markedly influenced by the rock type, as illustrated below.
* In the siliceous Proterozoic rocks an older biotite-K feldspar alteration extends throughout the area of the sulphide system. This was most intense in the low grade core and weakened with distance from the centre. Superimposed on this initial phase is a later period of sericite that obliterated much of the early alteration. The sericite extends throughout the sulphide system, but reaches a maximum intensity near the interface of the high copper zone and the pyrite halo. Overlapping and extending beyond the sericite is an irregular propylitic zone in which epidote is the principal alteration product. In all rock types the frequency of quartz veining decreases outwards from the centre of the sulphide system (Phillips, etal., 1974).
In the diabase (dolerite) there is no major zone of sericitic alteration. The characteristic alteration of the diabase associated with ore is abundant biotite with clay. The biotite is derived from the augite and hornblende of the diabase, via an intermediate stage of actinolite, with alteration being facilitated along the fractures. The mineralised diabase is very hard and blocky (E John, pers. comm., 1994; Pers. observ., 1994).
In the pyrite halo, biotite becomes less abundant. Actinolite predominates, accompanied by lesser biotite. The chalcopyrite content decreases and pyrite becomes considerably more abundant, while magnetite also decreases. Epidote and chlorite also become more common and sericite is seen rarely (E John, pers. comm., 1994; Pers. observ., 1994).
In the biotite zone there is K enrichment, although the biotite is mainly an Fe variety. In the actinolite zone however Mg is enriched, having been moved from the more advanced biotite zone as a front. The actinolite is Mg rich, which means it is actually an actinolite-tremolite (E John, pers. comm., 1994).
Both K-feldspar and sericite are rare in the diabase, although in many instances biotitised diabase is found within centimetres of sericitised schist or 'quartzite'. Biotite occurs as a fine grained mesh in vein envelopes and as thin rims surrounding disseminated magnetite and sulphides. Within the vein envelopes the biotite is an alteration product of pyroxene and hornblende, and is accompanied by the alteration of feldspars to clay. Both early and late barren quartz veins are present in the diabase, while mineralised quartz veins are abundant in the low grade core, outward through the high copper zone. In the pyrite halo quartz decreases markedly. Milky white calcite in quartz-sulphide veins is common near the centres of the veins, with micro-veinlets cutting sulphide grains. Calcite is most common in the high Cu zones, decreasing inwards towards the low grade core and outwards into the pyrite halo (Phillips, etal., 1974; E John, pers. comm., 1994; Pers. observ.).
* Most of the alteration in the Granite Mountain Porphyry is restricted to veins or veinlets and their margins, and pervasive alteration is rare. The dominant alteration assemblage is K-feldspar, quartz, biotite and sericite. Clays and chlorite are less common (Phillips, etal., 1974).
The chalcocite blanket at Ray has historically produced the majority of the ore, although it only accounts for 15% of the remaining reserves. The principal host to supergene ore is the pyritic and un-reactive early Middle Proterozoic Pinal Schists. The original pre-ore Pinal Schists contain around 0.5% pyrite regionally as well as the hypogene 'porphyry' vein pyrite. The same rocks also contain around 0.5% magnetite regionally which may be sulphidised to pyrite by both the hypogene and supergene mineralising system, and then to chalcopyrite and chalcocite. Within the propylitic zone it is possible to see magnetite grains rimmed by pyrite which is in turn coated with chalcopyrite. This is taken to be a supergene process. Sulphide veining which follows both the schistosity of the Pinal Schists and the fracturing, is all believed to be hypogene, related to the 'porphyry' mineralising system (E John, pers. comm., 1994).
In contrast to many other deposits, replacement of pyrite by chalcocite is high, sometimes complete. In some zones, as near the Bishop Fault, veins of steely chalcocite several cm's thick are evident, with relatively little pyrite remaining, and rarely native silver may also be found in association with the chalcocite. These are replacements of the thick phyllic veining developed within the Pinal Schists. In the western pit however the replacement is not as high and sooty chalcocite is found instead. The supergene ore is generally grey in colour as a result of the high sericite content of the host. Grades locally attain levels of 2 to 4% Cu over significant intervals. As the supergene zone passes into the propylitic zone however it quickly drops to around 0.1% Cu (Phillips, etal., 1974; E John, pers. comm., 1994; Pers. observ. 1991 & 1994).
As explained above, one of the most important controls on the development of the supergene mineralisation at Ray is the Emperor Fault. This low angle fault with its thick gouge zone apparently caused a perched water table during the enrichment cycle and is responsible for the high grade supergene ore directly below it. Rock above the Emperor Fault has been thoroughly leached, with these flat faults forming a 'cap' to the main supergene blanket. Conversely steeply dipping faults such as the Bishop, have been responsible for deep adjacent oxidation (Phillips, etal., 1974).
The chalcocite blanket is best developed away from the primary ore. It occurs as an elongate, flat lying sheet as much as 75 m thick, locally up to 120 m thick, extending eastward into the zone of better pyrite in the pyritic halo, away from the better chalcopyrite developments. This sheet continues for up to 1.5 km to the west of the hypogene ore, over a width of up to 750 m, with the highest grade and thickest chalcocite within the higher primary pyrite zones. Overall the chalcocite blanket has dimensions of the order of 3000 x 1200 m and is 60 to 75 m thick. The secondary chalcocite veins are commonly thicker than the replaced hypogene pyrite veins, suggesting outward growth into the wall-rocks (Mine visit briefing, 1991). Transport of leached copper has been both vertical and laterally, following the hydrological gradient (E John, pers. comm., 1994).
The overlying leached capping is up to 150 m thick, although it is has been substantially eroded and generally is <60 m thick. At the base of the leached cap a thin cuprite-tennantite layer passes to some native copper and then to chalcocite, which in turn passes down into primary chalcopyrite where present. The leached capping contains around 100 ppm Cu and is a mottled and patchy 'red, brown, cream and yellow' clay-sericite rock with red hematite, brown goethite and yellow jarosite staining. The original texture of the leached rock is still partially recognisable, with leached mineralised veins being discernible. Au is also leached from the capping, although Mo is not dispersed. Where sighted the capping had been block caved from the mining of the 1940's and 1950's into the lower bench of the current open pit (E John, pers. comm., 1991 & 1994; Pers. observ.).
Copper silicate mineralisation, characterised by abundant chrysocolla, with Cu bearing Mn wad, copper-montmorillonite and halloysite clays, Cu-Fe oxide complexes, malachite, cuprite, libethenite, dioptase, azurite, native copper and chalcocite. It is developed in the low grade core of the deposit, characterised by low total sulphide but a high chalcopyrite:pyrite ratio. This zone partly overlaps the chalcocite blanket and hypogene sulphides, but is largely developed over the diabase. Silicate copper mineralisation is found within the Pinal Schist, Pioneer Formation, Dripping Spring Quartzite, diabase and to a lesser extent the early Tertiary Granite Mountain Porphyry. In the early 1960's some 93 mt of potential ore had been delineated in this silicate zone (Gambell, 1978). The mined tonnage and recovered grade in the 1980's was of the order of 2.5 mt @ 1.12% Cu (P Gilmour pers. comm.).
A similar mineralogy is found in the oxidised zone of the diabase, with a progression from azurite to malachite to chrysocolla. This is not a product of supergene enrichment, as is the case with the silicate mineralisation described above, but is due to in situ oxidation of the sulphide ore of the diabase (E John, pers. comm., 1994).
Other Mineralisation - Ray is one of a string of 'porphyry' deposits elongated in a general east-south-east trend within the district. These include the Christmas orebody approximately 25 km to the ESE (as described below), with two intervening deposits. These are Cilito, which has an upper 60 mt and a lower resource of 200 mt at unspecified grades; and Troy, an uneconomic resource associated with a large circular plug. Troy was represented by a number of higher grade, but small breccia related occurrences, each of a 'few hundred thousand tonnes' that have been worked on a small scale in the past. A third un-economic occurrence, known as Red Hills is located some 2 to 3 km to the north of Ray. It has a grade of approximately 0.3% Cu and is at a depth of 300 to 1200 m below the surface (E John, pers. comm., 1994).
Weak sediment hosted copper and pyrite mineralisation is known within the Dripping Springs Quartzite of the Middle Proterozoic Apache Group. The only occurrence of significance is the Mountain Mine in the Rustler Gulch area, with 0.2 mt @ 1% Cu. 45 g/t Ag (E John, pers. comm., 1994).
Exotic copper mineralisation - is developed within the Tertiary Whitetail Conglomerate in the Ray District, overlying the pre-Tertiary hosts of the porphyry copper mineralisation at Ray. This young conglomerate is also above the unconformity surface that cuts the Granite Mountain Porphyry which is the proposed 'causative intrusive' at Ray. As such it is later than the emplacement of ore at Ray. The Whitetail Conglomerate is as much as 250 m thick in places and contains detritus from all of the older rocks in the district. At the base of the sequence the clasts are generally angular, while higher they are more rounded and are crudely stratified. It is not known to the south of the Ray orebody (S Anzalone, pers. comm., 1991).
Within the Whitetail Conglomerate there is a block of rock some 750 x 180 m in surface area, and up to 60 m thick which is interpreted to represent a mud flow slump block of Pinal Schist derived from the top of the Ray deposit. This block which is interpreted as having been part of the supergene chalcocite blanket at Ray, may account for 100 mt of the orebody that has been lost. Volumes of higher grade mineralisation have been interpreted as having been leached from the mud flow and concentrated within adjacent coarser clastics of the Whitetail conglomerate. The mud flow has leached capping characteristics and textures, but is now barren. The stated resource totals,
28 mt @ 1% Cu,
This mineralisation is found below the mud flow on its eastern side, while a further block occurs above the mud flow further to the south-west. Hydrological studies indicate that the water flow regime to the east is a loosing stream (ie. the water flow is downward into the underlying coarse clastics), while to the south-west there is a gaining stream (ie. the water flow is upwards from the mud flow). These higher grade accumulations, as defined by the 0.8% Cu contour drawn on drill sections, occur as blankets disposed parallel to the mud flow contact. Additional lower grade layers, defined by a 0.4% Cu contour, parallel the higher grade bands further from the mud flow, both vertically and laterally (E John, pers. comm., 1994).
The higher grade resource of the Whitetail Conglomerate apparently lies within a larger mass of lower grade mineralisation which contains around 2 mt of Cu at a grade of 0.05 to 0.15% Cu. This mineralisation is found in remnants of the flat lying conglomerate within thicker original developments that are interpreted as representing a palaeo channel, capped by younger resistant volcanics. The nearest known block of 'ore' is 5 km and the furthest 15 km from the Ray orebody. The mineralisation is up to 40 m thick and 600 m wide. Mineralisation is present as chrysocolla, malachite and black manganiferous copper wad, minor azurite and variable native copper (S Anzalone, pers. comm., 1991).
The Whitetail Conglomerate contains facies from different sources. Those derived predominantly from the siliceous Pinal Schists are generally barren, while those whose provenance is from the Proterozoic and Palaeozoic carbonate rocks are mineralised as the carbonate is reactive and will neutralise acid solutions and cause them to precipitate their contained copper (S Anzalone, pers. comm., 1991).
Mineralisation which is apparently very similar to the Whitetail deposit was inspected in another younger conglomerate in the open pit at Ray. This comprised an angular breccia to conglomerate with subangular to subrounded clasts up to 20 cm across set in a poorly sorted silty to sandy matrix. Chrysocolla is the most noticeable mineral, occurring as coatings on fractures, open space linings and fillings between clasts, and as replacement of clasts. In addition the fracture surfaces are coated with manganese staining and malachite is found in similar positions to the chrysocolla. The host has a generally red-brown colouration (Pers. observ., 1991).
There was a divergence of opinion as to the mode of origin for these deposits. The mine staff favoured a mud flow which swept the upper section of the orebody into the Whitetail Basin, as described above, while some senior exploration geologists believed the deposit was a result of acid ground water carrying Cu from the Ray orebody and precipitating it at favourable locations within the Whitetail Conglomerate aquifer. The former view has apparently been strengthened by drilling.
For detail, consult the reference(s) listed below.
The most recent source geological information used to prepare this summary was dated: 1998.
This description is a summary from published sources, the chief of which are listed below.
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Favorito, D.A. and Seedorff, E., 2020 - Laramide Uplift near the Ray and Resolution Porphyry Copper Deposits, Southeastern Arizona: Insights into Regional Shortening Style, Magnitude of Uplift, and Implications for Exploration: in Econ. Geol. v.115, pp. 153-175.|
Force E R 1998 - Laramide alteration of Proterozoic diabase: a likely contributor of Copper to Porphyry systems in the Dripping Spring Mountains area, southeastern Arizona: in Econ. Geol. v93 pp 171-183|
Metz R A, Rose A W 1966 - Geology of the Ray Copper deposit, Ray, Arizona: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 177-188|
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