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Ray Part 1 - Geology & Structure

Arizona, USA

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SUMMARY

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 1,   Background & Geology

The Ray porphyry copper deposit is located in south-central Arizona, some 25 km to the south-south-west of the Miami district. Only a very minor amount of the ore grade hypogene mineralisation is actually within the associated intrusives, the early Palaeocene (Tertiary) Granite Mountain Porphyry.

The bulk of the hypogene mineralisation is hosted by Middle Proterozoic diabase (dolerite), with much of the remainder within the early Middle Proterozoic Pinal Schists. A large part of the production has been from supergene chalcocite blanket enrichment, predominantly within the Pinal Schists.

The deposit lies within the Arizona-New Mexico Basin and Range Province, and the broad Walker-Texas Lineament Zone.

Published production and reserve figures include:

Reserve, 1992, - 1140 Mt @ 0.63% Cu (Amer. Mines Handbook, 1994).
Production to 1981 - 297 Mt @ 0.85% Cu, 0.91 g/t Ag, 0.007 g/t Au,  0.0012% Mo (Titley, 1992).
Silicate ore - Reserve tonnage 1960 - 93 Mt @ 1.12% Cu (Gambell, 1978, recovered grade est., from notes by Gilmour, 1991).
and
Reserve 1911 - 50 Mt @ 2% Cu (Metz & Rose, 1966 - supergene ore).
Reserve 1919 - 115 Mt @ 1.5 to 2% Cu (Ransome, 1919 - mainly supergene ore).
Production to 1966 - 170 Mt @ 0.8-0.85% Cu (Gilmour, 1982).
Reserve 1981 - 590 Mt @ 0.8% Cu (Cornwall, 1982).
Reserve 1989 - 582 Mt @ 0.69% Cu (Titley, 1992).

Some 85% of the remaining reserve (1994) is hypogene mineralisation which is localised almost completely within a Proterozoic diabase (dolerite) dyke where it encroaches into the mineralised system. The remainder of the ore reserve is within the chalcocite blanket which is almost exclusively within the early Middle Proterozoic Pinal Schists (E John, pers. comm., 1994).

The mining rate as of November 1994 was 360 000 tpd of ore and waste at a waste:ore ratio of 3.5:1. Ore treatment is at two mills, both with a capacity of around 30 000 tpd. One, which was commissioned in 1991, is on site at Ray, while the other is at Hayden some 10 km to the south where ASARCO has a major copper smelter (E John, pers. comm., 1994).

Total copper metal production at Ray for the year of 1992 was 125 000t (Amer. Mines H'book, 1994). Molybdenite is present within the ore, but is not recovered. Both chalcocite blanket and oxide ore are leached but separately. Water added to the sulphide leach pile to extract copper also produces acid which is then utilised to leach the oxide ore (E John, pers. comm., 1994).

Geology

The host succession in the Ray district is as follows, from the base, after Ransome (1919), Metz & Rose (1966), Phillips, etal., 1974 , Cornwall, 1982, and visits in 1991 & 1994.

Proterozoic, composed of,
 Pinal Schists, of early Middle Proterozoic age, pre 1600 Ma - regionally a sequence of predominantly quartz-chlorite schist, which in the mine area is represented by quartz-sericite schist (E John, pers. comm., 1994).
 Madera Diorite, 1630 Ma in age - a medium grained, mafic rich diorite.
 Ruin Granite, 1430 Ma in age - coarsely crystalline, leucocratic, quartz monzonite which intrudes the Pinal Schists and is post metamorphism. This is equivalent to the Oracle Granite at San Manuel and the Lost Gulch Quartz-Monzonite of the Globe-Miami District.
 Apache Group, which is Middle Proterozoic in age, post 1600 Ma and probably also post 1430 Ma, subdivided from the base into, the
  - Pioneer Formation, >100 m thick - comprising a basal conglomerate, with overlying, well bedded, dusky-red coarse to fine arkose and feldspathic quartzite with shale partings, fine arkose, siltstone and thin bedded shale. The basal conglomerate is known as the Scanlon Conglomerate, which at Ray is a 3 to 6 m thick clean quartz grit with 2 mm grit fragments.
  - Dripping Spring Quartzite, >160 m thick - basal pebble and cobble conglomerate in an arkosic matrix; brown to yellowish-grey medium to coarse arkosic quartzite and arkose; arkose; and grey to brown thin bedded very fine grained feldspathic quartzite. The basal unit is known as the Barnes Conglomerate and is 9 to 12 m thick at Ray, with well rounded siliceous pebbles from 1 to 1.5 cm in diameter.
  - Mescal Limestone, 80 m thick - white to grey-brown, thin bedded, fine grained granular limestone, with lesser dolomite and local chert. Locally contains significant asbestos and has algal structures near the top.
 Disconformity
  - Basalt, around 55 m thick - lava flows and locally breccia flows of brown aphanitic basalt, which is locally vesicular and amygdaloidal. Original plagioclase, pyroxene and olivine are altered to clay, oxides, serpentine, carbonates and iddingsite.
 Disconformity
 Troy Quartzite, around 350 m thick and also Middle Proterozoic in age, post 1600 Ma- composed of a basal alternating conglomerate; poorly sorted arkose and siltstone; and an upper light grey, medium bedded fine to medium quartzite.
 Diabase (Dolerite), dated in the Ray district at 1140 Ma - occurring as large sills, dykes and irregular masses, composed of brown, grey and green rock made up of coarse to aphanitic plagioclase and pyroxene with lesser amphibole, biotite and oxides. This unit contained around 85% of the remaining ore reserve in 1994.
 Unconformity
Palaeozoic, composed of,
 Cambrian Abrigo Formation, of unspecified thickness which is included within the Troy Quartzite above. Includes quartzites, siltstone and shale. In the district it generally comprises a lower poorly sorted grey to red-brown mudstone; arkosic sandstone; pebble conglomerate and greywacke; and an upper medium to fine quartzite and feldspathic quartzite which is green, yellow and brown.
 Disconformity
 Devonian Martin Limestone, around 110 m thick - generally within the district it comprises, from the base, a greyish-yellow limestone with frosted sand grains; 3 to 10 m of medium bedded, dark-grey crystalline limestone; thin bedded grey dolomite and sparse sandstone interbeds; alternating limestone, dolomite, sparse sandstone and shale; thin to medium bedded limestone (locally with crinoids and brachiopods); and greyish-yellow fissile shale.
 Carboniferous/Mississippian Escabrosa Limestone,
0 to 60 m thick - generally comprising a very light grey to dark grey limestone, sub-divided into a lower thick to medium bedded limestone; and an upper medium to thin bedded limestone with abundant chert.
 Unconformity
Mesozoic, comprising,
 Dykes of Intrusive Period I, dated at 70 to 85 Ma in age - basaltic and andesitic.
 Tortilla Quartz-Diorite, dated at 70.4 ±1.3 Ma in age - occurring as a 5 x 3 km mass to the south of Ray and as a series of small stocks in the district. It ranges from a pyroxene-hornblende diorite to the more common biotite-pyroxene-hornblende quartz-diorite and granodiorite and is typically fine to medium grained and partly porphyritic with andesine, hornblende, pyroxene (augite), biotite and magnetite-ilmenite in an interstitial matrix of quartz and K-feldspar. Pyrrhotite, minor magnetite and streaks of chalcopyrite are found within this intrusive. It is a pre-ore phase, and is only mineralised in small sections where it reaches the margin of the ore zone.
 Rattler Granodiorite (Intrusive period I), dated at 69.2 ±1.2 Ma in age - ranging from quartz-diorite to quartz-monzonite (adamellite), but dominantly granodioritic in composition. It is cut by dykes of aplitic leucocratic quartz monzonite. It grades from fine to medium and from seriate to porphyritic varieties which have phenocrysts of mainly plagioclase, but also quartz, biotite, hornblende, magnetite and apatite in a matrix of the same composition.
Tertiary, composed of,
 Dykes of Intrusive Period II, dated at 62.6 Ma in age - andesitic and rhyodacitic in composition, generally <30 m, commonly 1 to 5 m thick, and vertical to steeply dipping.
 Tea Cup Granodiorite (Intrusive period II), dated at 62.9 ±1.3 Ma in age - ranging in composition from granodiorite to quartz-monzonite (adamellite). It is medium grained, has granitic, seriate and porphyritic textures, and locally contains aplitic dykes and pegmatitic segregations. Phenocrysts are plagioclase, quartz and locally hornblende up to 12 mm long. The groundmass is of similar minerals plus K-feldspar, biotite, magnetite and accessories.
 Granite Mountain Porphyry (Intrusive period III), dated at 60.6 ±1.5 Ma in age - outcropping in eight areas of mappable size, and six smaller, mostly in Pinal Schists. The outcrops occupy an ENE elongated zone some 8 km long. At the west end this zone is 4 km wide, while at the east end in the Ray mine area it is 600 m wide. It is mostly a granodiorite, but in part is a quartz-monzonite with textures grading from granitic to porphyritic. Phenocrysts are plagioclase, quartz and K-feldspar, while the groundmass also contains biotite, magnetite and accessories.
 Tea Pot Mountain Porphyry (Intrusive period IV), yielding anomalous dates of 63 Ma - this is the youngest of the Laramide intrusives on the basis of field relationships, despite the age date. It is monzonitic in composition, with a similar appearance to the rhyodacite porphyry dykes. It is distinguished by large phenocrysts of pink K-feldspar. This intrusive is post copper mineralisation, although associated breccia pipes have accompanying Pb, Zn and Ag mineralisation in the northern part of the pit. This is the Calumet Breccia with dimensions of 60 x 200 m, which has been shown to extend for at least 250 m below the surface. It comprises clasts of Pinal Schist, Scanlon Conglomerate, Pioneer Shale and diabase (dolerite) as much as 3.5 m across, set in a matrix of powdered rock.
 Dykes of Intrusive Period V, which cut all of the intrusives above and are dated at around 60 Ma - rhyodacitic (quartz-latitic) in composition.
 Whitetail Conglomerate, 0 to 900 m thick, and around 32 Ma (Oligocene) in age - generally occurring as a terrestrial conglomerate deposited on an erosion surface of high relief. It is composed of the underlying older Palaeozoic and Proterozoic lithologies of the district. About a kilometre to the east of the Ray deposit an unusual facies comprises boulders of Apache Group which are cemented by silica. This facies is believed to have been related to siliceous hot springs.
 Apache Leap Tuff, or Dacite, 0 to 250 m thick, and around 20 Ma (Miocene) in age. This unit is post mineralisation although it has an exotic Cu mineralisation along its base, including silicates. It was deposited conformably on the Whitetail Conglomerate and comprises a pink to white rock which weathers to a brown or light grey. Over the Ray deposit it is only 15 m thick, but thickens to the north and to the west.
 Unconformity
Quaternary, which may locally be up to 900 m thick, composed of,
 Gila Conglomerate, 0 to >50 m thick - which is very similar in occurrence to the Whitetail Conglomerate, but may be distinguished by the presence of clasts of Apache Leap Tuff.
 Rhyolite Tuff, 0 to >60 m thick - occurring as a white thin bedded tuff, conformably overlying the Gila Conglomerate.

Structure

Faulting and tilting are the main structural feature of the mine area. The majority of the faults are normal and of post ore age. They trend north to NNW, are related to the Miocene Basin and Range block faulting episode and have been dated at around 12 Ma. Examples in the mine area are the West End and Bishop Faults. These have had an influence on the orebody, with the west-side up movement on the West End Fault defining the western limit of the orebody (E John, pers. comm., 1994).

In addition however there are major structures related to the pre-basin and range Oligocene to early Miocene period of extension and to the Laramide episode of imbrication. Many of the faults within the mine area have two or more distinct ages of movement, the earliest in some cases having taken place as long ago as the Middle Proterozoic. Movement also generally occurred along a zone of considerable width rather than a single fracture trace (E John, pers. comm., 1994).

The Diabase Fault represents listric rotational movement, probably during the extensional period. The main activity was around 22 Ma, with reactivation at 12 Ma. Movement on this structure is in excess of 500 m. According to E John (pers. comm., 1994) rotation on this structure has resulted in a marked difference in attitude of the rocks across it to the west compared to those to the east. Within the district the regional attitude of dykes of diabase is generally steep, with a north-south to NNW trend. This is consistent with those in the block to the east of the Diabase Fault. To the west however the diabase is flat to gently west dipping, while the Diabase Fault is interpreted to flatten to the west with depth. This change in attitude is taken to be due to listric rotation on the Diabase Fault. If this interpretation is correct, the Granite Mountain Porphyry has also been rotated onto its side to the west (E John, pers. comm., 1994).

As detailed in the 'Geology' section above, the Granite Mountain Porphyry occurs as a number of isolated exposure within the older country rock, occupying an ENE elongated wedge shaped zone some 8 km long. At the western end of this zone where it is 4 km wide, the exposures are more extensive, while at the eastern end, in the Ray mine area, they are more restricted with the point of the wedge being only 600 m wide. Drilling has shown that the outcrops do not connect at depth, and often passes out of the porphyry into Pinal Schists at a shallow depth. If the structural interpretation above is correct it may represent a series of generally north-south trending steep dykes that have been rotated to near horizontal, as has been suggested for the diabase. In this case the large body to the west represents the deeper source section of the intrusive complex, while the ore is localised in the vicinity of the narrower upper sections of the sheeted dyke complex (E John, pers. comm., 1994).

Some of the faults within the mine area associated with both the listric and the basin and range extensional episodes have been compensatory and resulted in local overthrusting with an opposite sense to the dip of the extensional faulting. These have locally thrust propylitic altered barren slabs over supergene mineralisation within the phyllic zone (E John, pers. comm., 1994).

There is ample evidence for an undulose, generally flat lying structure which was a thrust during the Laramide compression and a detachment during the early phase of the subsequent episode of extension. It has resulted in the placement of older Pinal Schist above members of the Apache Group and diabase (dolerite) sills in the mine area. This is the Emperor Fault. It is represented by a thick, low angle gouge zone whose trace encircles Diabase (dolerite) intruding Dripping Spring Quartzite and Pioneer Shale in the central part of the pit. These are overlain by at least 400 m of older Pinal Schist. However allowing for rotation on the Diabase Fault, this structures, while still being reverse faults would have been steeper. Movement on the Emperor and related thrusts is believed to have taken place at around 61 Ma, with reactivation at 31 Ma when it acted as an extensional detachment surface (Metz & Rose, 1966; E John, pers. comm., 1994).

There are also other older major structures developed at right angles to this set and roughly parallel to the regional schistosity. They are believed to be related to deep seated zones of crustal weakness. These faults are not well defined structures. They are present as shattered zones up to 600 m wide which controlled the intrusion of the Granite Mountain Porphyry. One of the largest of these north-east to ENE trending structures passes through the main section of the mine and is known as the Porphyry Break (Metz & Rose, 1966; E John, pers. comm., 1991).

Continued in   RAY   PART 2   MINERALISATION

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.
© Copyright Porter GeoConsultancy Pty Ltd.   Unauthorised copying, reproduction, storage or dissemination prohibited.


  References & Additional Information
   Selected References:
Anonymous  1988 - The Ray unit today: in     E&MJ, Sept, 1988    pp 42-47
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
Porter T M  1998 - The Ray porphyry copper deposit: in   Compilation from published literature and previous visits  (Unpub.)    10p


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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