A New Class of Nonvolcanic, Large, Peraluminous Copper-Oxide Greisen Deposits in Arizona, USA

2.1 Peraluminous rock systems and petrochemistry

In the magma-metal series classification [6], peraluminous granitoids are classified as mostly peraluminous, calc-alkalic, hydrous, oxidized, normal halogen, magma-metal series. Peraluminous composition (aluminum-rich and low sulfur) results in extensive, deep-seated greisens with quartz-muscovite ± hematite or magnetite alteration. Peraluminous magma chemistry is defined as the whole-rock geochemistry of plutons where molecular Al₂O₃/(CaO + Na₂O + K₂O) ratios are greater than 1 and are typically higher than 1.1 (Figure 3).

A given peraluminous granitoid suite can commonly be divided into two main differentiation stages: Stage 3 biotite granodiorite associated with copper oxides and, locally, Stage 4 muscovite-garnet aplo-granite aplites and pegmatites associated with tungsten (and minor beryllium). An intermediate stage (Stage 3.5) is commonly present, featuring two micas (biotite and muscovite) and occasionally almandine-spessartine garnet. More fractionated phases occur later as Stage 4 differentiates into rare Stage 5 garnet schlieren fractionates that contain almandine-spessartine garnet and muscovite only (not biotite). Accessory minerals in Stage 4 to Stage 5 differentiates include rutile, niobian rutile, specular hematite, zircon, ilmeno-hematite, and monazite.

Ferric-ferrous ratios (Fe₂O₃/FeO) of plutonic rocks associated with peraluminous copper-oxide greisen deposits are in the oxidized range and typically exceed 0.9. More reduced variants also occur, for example, the Coronation Pluton in La Paz County (Figure 2). The more reduced variants appear to have a more gold-rich association, whereas silver is the main precious metal in the more oxidized copper-oxide greisen model, as indicated by the production data.

The main rock systems in the peraluminous igneous sequences are Stage 3 biotite granodiorites that differentiate into a Stage 3.5 biotite-muscovite-garnet granite. Near their roots, the two-mica-garnet granites can locally produce hydrous granitic phases of muscovite-garnet leucogranite and aplo-pegmatites. The copper-oxide greisen deposits seem to be associated with the earlier, more biotitic Stage 3 to Stage 3.5 phases. Tungsten (beryllium) deposits seem to be associated with the later, more leucocratic, muscovite- and garnet-rich Stage 4 phases.

Isotopic characteristics of peraluminous granitoids indicate a strong crustal component, where strontium initial ratios range from 0.709 to greater than 0.720. Peraluminous plutons invariably contain inherited zircons acquired from melt sources in the lower to middle crust, as discussed below.

Trace element enrichments in the greisens, for example in the Texas Canyon plutonic suite, include elevated niobium, tantalum, tungsten, and lithium [9]. Niobium ranges up to 141.5 ppm (up from 8.2 ppm), tantalum ranges up to 12.8 ppm (up from 0.9 ppm), lithium ranges up to 170 ppm (up from 20 ppm), and tungsten ranges up to 11 ppm (up from less than 1 ppm). The tungsten and beryllium-enrichment may be distributed into Stage 4 hydrothermal tungsten and beryllium minerals, such as scheelite, hübnerite, and beryl in the associated greisen deposits, and into Stage 4 tungsten-beryllium veins.

Similar mineralogy is present in the latest Laramide plutons in the Globe-Miami area of Gila County, south-central Arizona. There, the G porphyry and Y aplite, which are the latest peraluminous phases in the Schultze Granite, show distinct enrichment in tantalum, niobium, tungsten, and lithium [10]. The main phase of the Schultze Granite is probably a Stage 4, weakly peraluminous phase of the metaluminous, porphyry copper-related sequence at the Pinto Valley, Copper Cities, and portions of the Inspiration mines.

2.2 Metaluminous rock systems and petrochemistry

Plutonic rocks that are spatially and temporally associated with classic porphyry copper deposits (such as Morenci, Arizona) are metaluminous, calc-alkalic, hydrous (hornblende-bearing), oxidized (magnetite-sphene-bearing), and have normal halogen content, belonging to the magma-metal series [7]. These metaluminous magmatic suites are much less aluminum-rich than peraluminous suites. A representative suite of plutons associated with Arizona porphyry copper deposits is shown on a K₂O versus SiO₂ plot on the right sides of Figure 3 and Figure 4.

Plutonic rocks that are spatially and temporally associated with lead-zinc-silver deposits, and that are also known as “barren for economic copper” deposits (such as Tombstone, Arizona), are metaluminous, alkalic-calcic, hydrous (hornblende-bearing), oxidized (magnetite-sphene-bearing), and have normal halogen content, magma-metal series [11]. These alkali-calcic magmatic suites are more alkaline than the calc-alkalic suites. A representative suite of plutons associated with Arizona lead-zinc-silver deposits is shown on a K₂O versus SiO₂ plot on the left side of Figure 4. The alkali-calcic igneous rocks should not be confused with calc-alkalic, porphyry-copper-related plutons. Many exploration programs have foundered on drilling copper occurrences associated with alkali-calcic plutons.

2.3 Peraluminous mineralogy

The principal rock-forming minerals in peraluminous granitoids are quartz, muscovite, biotite, and K-feldspar, which typically occurs as perthitic microcline, along with albitic plagioclase. The major accessory minerals are biotite and phengitic muscovite (white micas with fairly large amounts of Mg and Fe). Hematite is a common accessory mineral in the biotite granodiorite. Zircon, ilmeno-hematite, apatite, and minor monazite can also occur in the biotite granodiorite.

Minor amounts of accessory minerals include almandine/spessartine garnet and monazite. Both garnet and monazite are persistent accessory minerals and are commonly concentrated in downstream placer deposits of heavy minerals. Niobium-bearing rutile could become an exploration guide to peraluminous copper oxide deposits, given its widespread presence in biotite aplite phases associated with the giant Gunnison copper oxide deposit near Dragoon in Cochise County, southeastern Arizona.

A distinctive petrologic feature of the Texas Canyon pluton is the presence of niobian rutile. Inspection of the petrochemical data in [9] shows that the niobium and tantalum contents of the Texas Canyon intrusive sequences systematically increase with differentiation. The intrusive sequences reach a maximum niobium content in the muscovite-rich greisens associated with the Gunnison copper-oxide deposits. Whether these geochemical and mineralogical characteristics can be generalized as an exploration guide to other copper oxide deposits remains to be seen, as niobium and tantalum data are not typically obtained in the literature. Interestingly, niobium, tantalum, and manganese occurrences, such as tantalite, columbite-Nb, and columbite-Mn, are mainly associated with differentiated aplites or pegmatites as products of peraluminous calc-alkalic systems, although columbite-tantalite-bearing pegmatites have yet to be identified.

2.4 Differentiation stages of latest laramide peraluminous granitoids

Peraluminous granitoid complexes typically have three differentiation stages:

1. An early biotite peraluminous granodiorite (Stage 3) that typically contains 65% to 72 wt.% silica (Figure 5),

2. A later two-mica (muscovite-biotite) granite (Stage 3.5) that contains 72–78% silica (Figure 6), and

3. A later, more evolved two-mica granite (Stage 4) (Figure 7) with minor amounts of phases (Stage 5) containing almandine-spessartine garnet (Figure 8).

2.5 Stage 3.0 biotite granodiorite

Peraluminous biotite granodiorite is the major voluminous phase that is spatially associated with the copper-oxide deposits. Examples of Arizona biotite granitoids include:

• The biotite-aplite association at the Gunnison deposit in Cochise County,

• The biotite granodiorite dikes at the Red Hills deposit in Pinal County,

• The Manitou biotite granite pluton adjacent to Carlota in Gila County,

• Possibly the biotite granodiorites associated with the Zonia copper oxide deposit in Yavapai County, and

• The biotite granitic aplites associated with the copper oxide deposit at Coronation in La Paz County.

In the Santa Catalina Mountains, Stage 3 biotite granodiorites constitute the main volume of the forerange gneiss complex. Petrographic mineral quantities of the granodiorites range from 32.9 to 29.9% quartz, 37.5 to 26.6% plagioclase (An_20-30 or oligoclase), 25.7 to 29.7% K-feldspar (microcline), 1.7 to 7.4% biotite, 0.3 to 2% muscovite, and 0.1 to 0.4% magnetite [12]. A photograph of a typical biotite granodiorite that has been sheared by Maricopa thrust deformation is shown in Figure 5.

The biotite granodiorite porphyry, known as the G phase of the Schultze Granite, may be associated with the Van Dyke chrysocolla-azurite copper oxide deposit beneath the town of Miami in Gila County, based on data on Schultze Granite phases [10]. More speculatively, the G phase of the Schultze may be associated with the granite porphyry phase of the Schultze Granite in the Live Oak portion of the Inspiration copper deposit. The Live Oak mine locality is famous for gem silica (chrysocolla) mineral specimens.

The rock system stages in peraluminous, calc-alkalic, oxidized systems can be economically significant. Within the peraluminous differentiation system, copper may be held in biotite and/or magnetite as a compatible element within the Stage 3 biotite granodiorite. In the Texas Canyon pluton of Cochise County, Arizona, the biotite-dominated Stage 3 phase of the Texas Canyon pluton contained 21 ppm copper [9]. In contrast, the slightly later biotite aplite phase, which is intimately associated with the Gunnison copper-oxide skarn system, contains 1 ppm copper or less, according to copper data [9]. Hence, about 20 ppm copper would have been available to be incorporated into the incompatible hydrothermal water component, along with aluminum and silica. This chemistry could then precipitate copper silicates, such as chrysocolla.

2.6 Stage 3.5 two-mica granites

Intermediate phases (Stage 3.5) between the Stage 3 biotite granodiorite and the two-mica, garnet-bearing Stage 4 systems may also be associated with copper-oxide deposits of moderate to large size. The Little Hill copper-oxide deposit is an example in Pinal County, as is the Azurite deposit south of Globe in Gila County, Arizona, which is associated with the top of the two-mica Solitude Granite.

Representative petrography for the Wilderness Granite, which is a Stage 3.5 garnet-bearing, two-mica granite (Figure 6) in the fore range of the Santa Catalina Mountains north of Tucson, is spatially associated with copper-oxide mineralization at the Pontatoc mine. This two-mica granite consists of 42% quartz, 32% oligoclase (An_2.0–2.5), 13% orthoclase, 6% muscovite, 4% biotite, and 3% microcline, with trace garnet [12].

Stage 3.5 brings in muscovite, which may not incorporate copper, and magnetite may decrease drastically. This mineralogical change may reverse the compatibility characteristics of copper, which changes from compatible in biotite and magnetite stability to incompatible in progressively more muscovite-garnet-rich assemblages.

From a field petrographic perspective, the transition from Stage 3 biotite granodiorite to Stage 4 muscovite-garnet leucogranite may be significant for copper-related exploration. In this context, the Stage 3.5 two-mica granites might be the most spatially significant rock system tied to peraluminous copper-oxide greisen deposits.

2.7 Stage 4 garnet-bearing leucogranites

More evolved Stage 4 aplite-pegmatites and leucogranites (Figure 7) occur near the roofs of differentiated peraluminous sill complexes. For example, a large sill-like body of pegmatitic garnet-muscovite leucogranite, informally named the Lemmon Rock leucogranite, occurs in the Santa Catalina Mountains on Mount Lemmon, in the roof of the Wilderness peraluminous sill complex.

An example of a Stage 5 aplo-pegmatite is the spectacular outcrop of garnet schlieren in the southern part of Summerhaven in the Santa Catalina Mountains (Figure 8) [14, 15]. The garnet-muscovite aplo-granite of Figure 8 shows a flow-differentiation pattern, emplaced in an active, deep-seated tectonic environment in the midcrust.

3.1 Ore mineralogy

Iron and copper as “oxide” minerals (in the context of leach processing via solvent extraction – electrowinning or SX–EW) are characteristic of peraluminous copper-oxide greisen deposits. The peraluminous copper-oxide greisen deposits can form large, low-grade bodies (0.2–0.5% Cu) of chrysocolla (Figure 9) that are closely associated with primary specular hematite. The specularite can oxidize into red, earthy hematite. Pyrite is conspicuously absent or sparse.

At the Gunnison deposit in the Little Dragoon Mountains of Cochise County in southeastern Arizona, copper-oxide mineralization occurs to a depth of approximately 490 m (1,600 feet) and lies immediately beneath thin Miocene-Pliocene gravel formations. The mineralization is dominantly chrysocolla, with minor tenorite, other copper oxides, and minor secondary chalcocite. Copper-oxide mineralization is present in the calc-silicate skarns as fracture coatings and vein fillings, mainly in the form of chrysocolla. The remainder of the oxide mineralization occurs as replacement patches and disseminations. Copper-oxide mineralization extends over a strike length of 3,380 m (11,100 feet), has an aerial extent across strike of up to 915 m (3,000 feet), and is more than 275 m (900 feet) thick in places [16].

Unlike the secondary enrichment of classic porphyry copper deposits, where a leached iron cap is typical, a leached iron cap has not yet been observed above the copper “oxides” (chrysocolla, minor malachite, and azurite). Similarly, strong chalcocite enrichment blankets beneath the copper-oxide zone are not observed or are thin and insignificant. There simply was not enough sulfur as pyrite in the peraluminous copper-oxide deposits to produce acid leaching and subsequent secondary copper sulfide precipitation as chalcocite beneath oxidized copper zones.

The most distinctive feature of peraluminous copper-oxide greisen deposits is their “oxide” (in terms of leach processing) mineralogy. Copper mainly resides in chrysocolla, which was probably formed as an amorphous, high-temperature mineraloid/colloid. Chrysocolla occurs as botryoidal forms or fracture fillings (Figure 9). The apparent primary nature of the chrysocolla is supported by its common occurrence with primary specular hematite, which is the main iron mineral in this deposit type.

Pyrite, bornite, and chalcopyrite, which are common in classic porphyry copper environments, are generally absent or have low abundance in peraluminous copper-oxide greisen deposits. The low sulfur content also results in the development of major specular hematite alteration mineralogy. Other copper oxide minerals in the peraluminous copper-oxide greisen deposits include azurite and malachite, but these are relatively minor compared to chrysocolla. Cuprite and native copper can also be present, but only in relatively low amounts. The close association of scheelite, powellite, hübnerite, and goshenite beryl also suggests a mineral assemblage that is typical of silica- and alumina-rich greisens.

3.2 Alteration mineralogy

Early alteration assemblages in peraluminous copper-oxide greisen deposits are dominated by adularia, K-feldspar, and quartz. The adularia-quartz alteration is followed by strong, coarse-grained muscovite-quartz greisen-style alteration. This greisen-style alteration forms large vein envelopes as well as pervasive late deuteric alteration of the peraluminous granitic host rocks.

Greisen-style alteration is typically associated with both biotite granodiorites (Stage 3) and biotite-muscovite granites (Stage 4). Garnet may be an accessory mineral in the biotite-muscovite granites.

Possible later supergene dehydration of primary chrysocolla can occur in the presence of initial acetic acid from rotting fleshy cacti (saguaro or prickly pear). The acetic acid is then reduced to produce various glycolates, such as lazaraskeite from dehydrated chrysocolla [17] and rasmussenite [18].

3.3 Mineral zoning

Initial proximal zonation is probably related to muscovite-orthoclase-quartz greisens with minor pyrite and tungsten-minor beryllium mineralization, which may be coeval with copper oxide deposition. An example can be found in the Burro open pit at Johnson Camp (now part of the Gunnison project) in Cochise County, southeastern Arizona. Minor lead and zinc might be deposited in a halo environment outboard from the copper oxides.

Orthoclase-quartz veins ± magnetite, without pyrite or secondary biotite or copper sulfides, are typically the earliest stage and precede the larger volume, muscovite-quartz-minor pyrite greisens. The greisen mineralization may precede the copper and iron oxide assemblages that constitute the main volume of mineralization.

3.4 Depositional model for peraluminous copper-oxide greisen deposits

The sulfur-poor and aluminum- and silica-rich, peraluminous, “cold granite” depositional environment of the peraluminous copper-oxide greisen deposits is very different from that of the hotter, metaluminous, porphyry copper deposits. The peraluminous copper-oxide deposit type incorporates the stability of primary copper-oxides (mainly the copper silicate chrysocolla, and lesser amounts of the copper carbonates azurite and malachite). Sulfides, mainly pyrite and minor chalcopyrite, are present but are not considered as economically significant as the copper oxides.

Fluid releases associated with peraluminous copper-oxide greisen deposits appear to be associated with Stage 3 biotite granodiorite porphyries and aplo-pegmatites (both aplites and pegmatites). Additionally, later-stage (Stage 3.5) muscovite-biotite granite systems may be capable of releasing copper oxide-stable hydrothermal fluids. A chemical model for peraluminous copper-oxide deposits is shown in Eq. (1).

*Note: in the above reaction, x = 1. If extra aluminum or silicic acid exceeds copper, quartz/chalcedony or allophane may form as high-temperature colloids.

Hydrothermal fluids released during the final crystallization of relatively “cold” biotite granodiorite are expected to be moderate-salinity and high-aluminum fluids. In these hydrothermal fluids, copper chloride complexes and copper hydroxide complexes are stable, along with silicic acid and aluminum hydroxide. In this setting, quartz and allophane are intimate associates of chrysocolla. If carbon dioxide is present, primary azurite and malachite may be stable.

The initial hypogene fluids are predicted to be relatively low-temperature (in the vicinity of 350–250°C). Fluid inclusion data is needed to constrain the hydrothermal stages. At this point, we are not aware of any constraining fluid inclusion data.

A simplified model showing a depositional sequence for the peraluminous copper-oxide greisen model is outlined below:

1. Emplacement of peraluminous, biotite granodiorite porphyries (Stage 3) to slightly later, more oxidized muscovite-biotite granites (Stage 3.5), aplites, and pegmatites follows progressive biotite disappearance.

2. Quartz-orthoclase ± magnetite veins and/or low-temperature skarns (tremolite-actinolite, grossular garnet) form next.

3. Next, minor amounts of sulfides (pyrite, chalcopyrite, bornite, and/or chalcocite) are deposited.

4. Next, quartz-muscovite ± pyrite, scheelite, powellite, hübnerite, beryl (goshenite) veins, and greisens are deposited.

5. Primary botryoidal chrysocolla is formed coevally with specular hematite (oxidized to red hematite-goethite), quartz, muscovite, with minor magnetite and minor amounts of azurite, malachite, kaolinite, and allophane in the greisens.

Minor amounts of pyrite are slightly more abundant than chalcopyrite and bornite, and there is only trace sphalerite ± tenorite. The above depositional sequence implies that the copper oxide and iron oxide mineralization may have postdated the sulfide mineralization, as the copper oxide and iron oxide deposition would follow the aluminum- and silica-enriched hydrothermal fluid of peraluminous composition. This paragenetic sequence is speculative and remains to be confirmed.

The above sequence reflects a progressive oxidation from magmatic Stage 3 to Stage 3.5 crystallization to late-stage copper and iron oxides. The hematite mineralogy present in the peraluminous Texas Canyon pluton differs from magnetite-sphene assemblages of the metaluminous sequences associated with classic porphyry copper deposits [9].

Conventional interpretations maintain that the copper and iron oxide mineralization is supergene in character. However, several observations allow for the possible interpretation of hypogene character.

Firstly, specularite and magnetite, which are both primary minerals, co-occur with the copper oxides. The presence of red, earthy hematite, however, suggests that there has been some oxidation of the primary iron oxide mineralogy to supergene red, earthy hematite.

Secondly, primary sulfides occur in the presumed oxide zone with the copper oxide minerals. This co-occurrence is anomalous in that leached, oxidized lithocaps over classic porphyry copper deposits contain no sulfides. For example, at the I-10 deposit (now the northern part of the Gunnison project), the paragenetic order shows that primary pyrite, chalcopyrite-molybdenite, bornite, and possibly later chalcocite are spatially associated with soluble copper oxide minerals [19]. This close spatial association is consistent with late-stage primary oxidation of remaining low-temperature, copper-silica- and aluminum-rich hydrothermal solutions. These solutions deposited oxide minerals in dissolution vugs and fractures associated with the sulfides. Also, the presence of hübnerite and goshenite beryl in the diopside-actinolite-garnet-talc skarn rocks that host the copper sulfides and the iron oxide mineralogy supports a primary origin for the copper-oxide mineralogy.

4.1 Peraluminous copper-oxide greisen deposits in Pinal County

At present, at least 14 mineral districts have been identified in Pinal County (Table 1, with locations of districts shown in Figure 10). Additional details for these districts are described in [5].

4.2 Other Arizona peraluminous copper-oxide greisen deposits

Continued inspection of other Arizona copper oxide deposits has revealed an additional 17 copper-oxide greisen mineral systems (Table 2 and Figure 11) that are spatially associated with peraluminous calc-alkalic plutons and/or dikes in Arizona.

4.3 Texas Canyon – Gunnison project example

The best-researched example of the peraluminous copper-oxide greisen deposits at this time is the Gunnison deposit in Cochise County, Arizona [16]. The Gunnison deposit consists of a large copper-oxide deposit, mainly as chrysocolla, hosted in Lower Paleozoic strata on the north, east, and south margins of the Texas Canyon pluton (Figure 12 and Figure 13).

The geologic map of the Texas Canyon plutonic suite in the Little Dragoon Mountains of Cochise County, Arizona [49] has been modified in Figure 13 to show the ages and magma-metal classes of peraluminous Stage 3 (outlined in blue) and Stage 4 (outlined in dark blue) mineral districts. The petrology and mineral deposit geology of the Texas Canyon-Gunnison peraluminous copper-oxide greisen system are well documented [9, 49, 53].

Mineral districts associated with the Texas Canyon plutonic suite in Cochise County (Figure 13) include the following three deposit types:

1. Zinc-copper deposits are associated with mafic, hornblende-biotite monzodiorites that are classified as metaluminous, alkali-calcic (MAC) and are outlined with an orange line. The Peabody sill of metaluminous, hornblende-rich monzodiorite at the Peabody Mine is intimately associated with zinc-copper replacement mineralization in the Horquilla Formation [49, 65, 66]. The Peabody sill is inferred to correlate with monzodiorite inclusions in the Texas Canyon intrusion, which have yielded U–Pb age dates on zircons in the xenocrysts of 67–63 Ma [50, 52].

2. Copper oxide mineralization is associated with biotite granite that is classified as peraluminous calc-alkalic [PCAo] Stage 3, dated at 57–55 Ma based on zircon age dates [50, 52] and is outlined with a blue line. These deposits are associated with biotite aplite dike swarms and masses located between the copper-oxide deposits and the main Stage 3 biotite-dominated phase of the Texas Canyon pluton.

3. Tungsten and minor beryllium deposits are associated with two-mica granites (the Adams Peak leucogranite and related muscovite-garnet aplite dikes) and are outlined with a dark blue line. They are associated with peraluminous calc-alkalic [PCAo] Stage 4 deposits and are dated at 55–54 Ma, based on an Rb–Sr isochron [53].

Peraluminous copper-oxide deposits in the Gunnison project include the giant Gunnison copper-oxide system that occurs in contact metasomatic skarns on the north and east sides of the Texas Canyon pluton. Previously mined copper-oxide deposits include the Burro Pit at Johnson, the Centurion Mine near Dragoon, and mines at the South Star prospect southwest of Dragoon, which were designated as the Copper Area [49]. Associated copper-oxide skarn deposits that are now part of the Gunnison project [16] include the former I-10 copper-oxide skarn deposits [19] and the Burro and Johnson open-pit copper-oxide deposits [54]. At present, approximately three million tons of copper have been mined or drill-identified, which makes this model a significant exploration target [16]. Room exists for at least a possible additional five million tons of copper.

The large size of the peraluminous Texas Canyon pluton source may be directly related to the size of the copper-oxide deposits at the Gunnison project. The known volume of magma represented by the present-day exposures of Texas Canyon Granite, as mapped [49], is about 150 cubic km. The main Texas Canyon pluton contains about 21 ppm Cu, whereas the later biotite-aplite phases associated with the Gunnison/I-10 copper skarns contain 1 ppm Cu or less. The copper abundance data suggest that about 20 ppm of copper could have been released into hydrothermal fractionates emanating from the Stage 3 Texas Canyon biotite quartz monzonite pluton. If the entire plutonic volume was depleted by 20 ppm Cu, based on the bulk geochemistry data [53], the pluton could have yielded approximately eight million tons of Cu. This amount of potentially released copper is sufficient to account for the three million tons of Cu based on the drill-indicated plus produced copper.

The hydrothermal fluid release, triggered by the fractionation of the peraluminous Texas Canyon granite (Stage 3) to the biotite-aplite phases (Stage 4), deposited chrysocolla, magnetite, and specularite into the lower Paleozoic Bolsa Quartzite and the lower Abrigo Formation bordering the Texas Canyon pluton. To a lesser extent, the upper Abrigo, Martin, and Escabrosa calcareous formations were also mineralized. Additional chrysocolla occurrences have been described to the south in the Centurion and South Star prospects and are not currently included in the existing resource. The specularite and magnetite were later oxidized to secondary, red, earthy hematite, which is preserved with chrysocolla.

The formation of the Stage 3 Gunnison copper-oxide deposit was quickly followed by a differentiation step that may have been associated with regional stress reorientation related to the plate-scale stress change at about 56 Ma. This stress reorientation formed a late Stage 4, two-mica- and garnet-bearing aplite/pegmatite complex, herein called the Adams Peak leucogranite. The stress reorientation is shown by the change from east-northeast-trending biotite aplites common on the east side of the Texas Canyon pluton [49] to northeast-trending muscovite garnet aplites associated with tungsten-beryllium veins related to the Adams Peak leucogranite.

The late Stage 4 magmato-hydrothermal alteration spread to the northeast and northwest, forming the Bluebird tungsten (with minor beryllium) veins and associated greisens. The Bluebird system also includes the Tungsten King tungsten deposit, which was emplaced in a mineralized vein along the Tungsten King fault to the northwest. This fault forms the western boundary of the Dragoon lower plate terrane. The main Stage 3 to Stage 3.5 Texas Canyon pluton is the inferred source for the Stage 3 copper-oxide mineralization in the Gunnison system. The later Stage 4 Adams Peak leucogranite is the inferred source for the tungsten (minor beryllium) vein system.

The Texas Canyon pluton is well-dated radiometrically by an Rb/Sr isochron (54.6 ± 0.2 Ma) for the Stage 4 Adams Peak Leucogranite and related muscovite-garnet aplites associated with the tungsten-beryllium vein deposits at the Bluebird Mine [53]. The main Stage 3 biotite granite pluton has yielded two U–Pb zircon age dates of 57.9 ± 0.8 Ma [67] and 55.5 ± 1.9 Ma [50]. In addition, a Stage 3 biotite-minor muscovite aplite has yielded a 57 ± 1.0 Ma age date [52].

6.1 Lower plate of the Maricopa thrust

The Maricopa thrust may be the largest Laramide structure in Arizona and adjacent regions. The Farallon plate was flatly subducted under the North American plate during the Eocene (Figure 14). This compressional tectonic activity left southwest-directed foliation in the lower plate in the core of the Santa Catalina-Rincon crystalline complex, as well as in the nearby Picacho, Durham Hills, Suizo, and Tortolita Mountains crystalline complexes.

Lower plate windows into the Maricopa thrust were later exposed by up-arching in broad anticlinal folds during the compressional Galiuro orogeny [11, 64] in the Oligocene-Miocene. The anticlinal uplifts form a northwest-southeast-trending anticlinal zone located northeast of the leading edge of the Maricopa thrust (Figure 11) and southwest of the Transition Zone physiographic province. These anticlinal uplifts contain a series of crystalline cores of peraluminous granites.

The broad zone of anticlinal uplifts resulted from crustal softening due to the influx of mantle heat from the collapsing Farallon slab as Oligocene-Miocene magmatic activity returned from the east and spread westward. The broad folds were also triggered by weak regional compression generated at the subduction zone. The mid-Tertiary uplift was accompanied by extensive gravity-driven tectonic denudation of the former Maricopa thrust. These low-angle normal denudational faults have formerly been called “detachment” faults.

The Maricopa thrust system extends southeastward from the Old Woman, Big Maria, and Little Maria Mountains of southeastern California, through southwestern Arizona, to the Huachuca Mountains in southeastern Arizona, and then into northeast Sonora, Mexico. The western end of the Maricopa thrust system has been referred to as the Maria Fold-and-Thrust Belt [75].

The Maricopa thrust tectonically separated upper plate, nonmylonitic rocks from lower plate, strongly reflective mylonitic rocks [76], as shown in the seismic section (Figure 15). Anschutz (Phillips) drilled the AZ-1 oil exploration drill hole to a total depth of 5,490 m (18,013 ft) to test this seismic feature in the early 1980s. The AZ-1 drill hole penetrated the lower plate at a depth of 3,280 m (10,761 ft), where the drill hole intersected a peraluminous granitoid that yielded a 47 Ma Rb–Sr age date. This peraluminous granite is like the Wilderness suite of granites in the Santa Catalina Mountains [48]. The AZ-1 drill hole also intersected rocks containing reduced K–Ar ages that ranged from 31 to 25.2 Ma. These age dates are like uplift-cooling ages obtained from mylonitic gneisses in the lower plate window in the Santa Catalina Mountains.

The AZ-1 drill hole proved that a mylonitic lower plate containing peraluminous granites and reduced K–Ar ages is present north of the North Star fault in the northern Picacho Mountains and north of the lower plate exposure. The regional cross-sections of the Picacho Mountains and Santa Catalina Mountains (Figure 16) show mid-Laramide metaluminous mineralization in yellow and late Laramide peraluminous mineralization in blue. The regional cross-sections show the position of the upper plate and the mid-Laramide, metaluminous, porphyry copper deposit of the now-offset San Manuel (in yellow). The positions of the lower plate, peraluminous copper oxide greisen deposits, are shown in blue at Newman Peak and North Star Mine in the Picacho Mountains and at Pontatoc and Little Hill Mines in the Santa Catalina Mountains.

Cross-sections of the same areas in Figure 17 show mid-Tertiary, metaluminous, magma-sourced mineralization in purplish-pink at Mammoth near the Santa Catalina Mountains and Picacho Peak. The lower-plate-sourced chlorite breccias are shown in green in cross-sections of both mountain ranges.

The Maricopa thrust and lower plate have been offset down an estimated 3,000 m (10,000 ft) by reverse motion on the Mogul fault in the Santa Catalina Mountains and the North Star fault in the Picacho Mountains. These faults may be segments of the same regional fault. The reverse motion and uplift are inferred to have been driven by compression related to the Galiuro orogeny in the middle Tertiary. The cross-section in Figure 17 also presents a model for the uplift of the lower plate terranes. Dehydration of the lower plate basement crystalline rocks during decompression associated with the rapid Oligocene-Miocene uplifts is inferred to have supplied metagenic, locally K-rich, hydrothermal fluids that formed the well-known chlorite breccias associated with “metamorphic core complexes.” Chlorite breccias were mistakenly explored for gold in the 1980s and 1990s and are not considered by us to be economically prospective targets for base or precious metal exploration.

Drilling, as part of the current exploration of the Casa Grande porphyry copper cluster, intersected a similar set of rocks below a feature referred to by the exploration geologists as the “basement fault” [77, 78]. The “basement fault” is the floor beneath the upper plate of antithetically tilted porphyry copper deposits (Cactus deposit, formerly Sacaton Mine, and others) and the overlying, tilted, mid-Cenozoic clastic rocks [77]. Drilling of peraluminous copper-oxide deposits in the upper plate of the northern Vulture Mountains, to the northwest in Maricopa County, also intersected a regional low-angle fault [79].

These drill penetrations into the lower plate prove that the lower plate exists as a regional, originally planar body beneath the widespread Maricopa thrust. The upper plate of the Maricopa thrust exists to the southwest and to the northeast of the lower plate windows.

The overlying upper plate is relatively undeformed and does not contain rocks exhibiting mylonitic deformation. The upper plate includes metaluminous porphyry copper deposits northeast of the lower plate windows, such as the Bagdad and Mineral Park mines in northwestern Arizona. The upper plate also includes metaluminous porphyry copper deposits southwest of the lower plate windows, such as the Ray, Pima, and Morenci mines.

Low-angle, northeast-dipping thrust faults are present in much of the belt of lower plate windows throughout southwestern, central, and southeastern Arizona. These low-angle faults dip northeastward beneath the Transition Zone and Colorado Plateau physiographic provinces and are correlated with the Maricopa thrust fault, which was intersected in the AZ-1 drill hole and imaged in various seismic lines.

In the northern Plomosa Mountains of northern La Paz County in western Arizona, the northeast-dipping Plomosa low-angle fault (Figure 2) juxtaposes lower plate Orocopia Schist beneath lower plate crystalline rocks. A large, sill-like, 9.6 km (six-mile)-long peraluminous pluton was mapped in the lower plate of the Plomosa Fault between the fault and the underlying Orocopia Schist [41, 42]. This pluton contains the Coronation peraluminous copper-oxide greisen prospect (Figure 1 and 2). Xenocrystic zircons in the Coronation pluton exhibit a very strong Orocopia Schist inheritance [41]. This inheritance strongly implies the Orocopia Schist and its metamorphosed peridotites may have been the copper source for the Coronation peraluminous copper-oxide greisen deposit.

The Plomosa Fault has been imaged in a COCORP seismic reflection profile across Cactus Plain [75, 80]. The presence of Orocopia Schist in the lower plate of the Plomosa Fault requires an underthrust geotectonic model. The lower plate probably underlies the so-called “metamorphic core complexes” throughout west-central Arizona.

In effect, the Plomosa Fault is a lower structural-level analog of the Maricopa Thrust Fault. The Plomosa Fault also may merge with an analog of the Chocolate Mountain Thrust that was mapped as a high-temperature, ductile shear zone between the Coronation Pluton and Proterozoic gneisses, which are also juxtaposed next to the Orocopia Schist [75].

Lower plate windows into the Maricopa thrust contain larger areas of two-mica peraluminous crystalline rocks than the areas referred to as “metamorphic core complexes” in the literature. Mylonitic rock fabric has been considered one of the main criteria for their identification as “metamorphic core complexes.” However, mylonitic fabrics only occupy about 30% of the rocks in the lower plate windows. Mylonitic fabrics are generally absent from upper plate rocks.

In the lower plate windows, radiometric dates on the peraluminous granitoids and surrounding crystalline rocks commonly display reduced ages between 28 and 15 Ma, with K–Ar, fission track, and ⁴⁰Ar/³⁹Ar radiometric dating (Figure 18). These ages reflect rapid cooling during the uplift of the lower plate windows during the Galiuro orogeny [64].

Only Eocene uplift-cooling ages occur as reduced radiometric ages on Precambrian through Laramide rocks in the Transition Zone [82], whereas both Eocene and Miocene uplift-cooling ages are represented in the northwest-trending belt of lower plate windows. Tectonic boundaries of the lower plate windows are typically marked by low-angle faults or later Basin and Range normal faults. No middle Cenozoic clastic sedimentary or volcanic rocks are in depositional contact with lower plate crystalline rocks in the lower plate windows.

The lower plate crystalline rocks contain upper greenschist to middle amphibolite metamorphic assemblages that include higher metamorphic-grade minerals, such as cordierite, sillimanite, anthophyllite, and andalusite. These higher-grade metamorphic minerals are not present in upper plate rocks. For example, sillimanite, anthophyllite, and andalusite have been reported in the contact metamorphic rock in and adjacent to the Texas Canyon pluton and the Gunnison Project peraluminous copper-oxide greisen deposit.

It is economically significant that classic porphyry copper deposits do not occur in the lower plate, other than in one area where the deposit may have been subducted beneath the Maricopa thrust. In contrast, peraluminous copper-oxide greisen deposits with coarse muscovite appear to be exclusive to the lower plate and its tectonic margins (such as Little Hill and North Star). One occurrence at Red Hills is located a short distance into the upper plate, associated with peraluminous granite dikes. Red Hills is number 6 on Figure 10.

It is also geologically significant that no mid-Cenozoic volcanism related to the Galiuro orogeny is present in the lower plate. However, mid-Cenozoic plutons and dikes are common in the lower plate. Examples of mid-Cenozoic dikes and plutons include the Eagle Pass dike swarm in the Pinaleno Mountains, dated at about 24–26 Ma [28], and the Catalina suite plutons, dated at 26–22 Ma, in the Tortolita and Santa Catalina Mountains [44, 45, 48].

The lack of super-crustal volcanism in the lower plate is consistent with the midcrustal depth of emplacement of the peraluminous magmatism and its attendant copper-oxide greisen deposits. Significantly, volcanism of the same age as the plutons in the Santa Catalina Mountains is present in the adjacent Galiuro Mountains to the east and Tucson Mountains to the west. However, the tectonic boundary between these two crustal regimes is marked by a major low-angle fault that we infer to be reactivated denudation on the former Maricopa thrust.

6.2 Depth of emplacement and water content

Peraluminous copper-oxide greisen deposits formed in the middle crust at depths of about 8–16 km between approximately 72 and 43 Ma. These depths are based on geobarometry of associated peraluminous granites [45, 83]. This deep “freezing” of peraluminous magmatism in the midcrust precluded any kind of volcanic expression.

Even at depths greater than 8 km, brittle conditions could occur, probably because of high water pressures. The high water pressures are estimated to be in the range of 8–12 weight percent (wt %) in the peraluminous systems. The high water pressure produced a deep-seated hydro-fracking akin to that in the metaluminous porphyry copper systems, which formed at shallower depths between 5 and 2 km. The peraluminous plutonic suites were probably emplaced at midcrustal levels during thrust burial and crustal thickening induced by tectonics related to the Maricopa thrust.

Water exsolution from the Wilderness Sill Complex and other plutons, such as the Texas Canyon Granite, is associated with the formation of deep, peraluminous copper-oxide greisen deposits. These deposits are associated with differentiated leucogranite phases near the tops of the plutons, especially the sills. Examples include the Pontatoc, Pusch Ridge, and Mt. Lemmon mineral districts in the Santa Catalina Mountains [15] and the Coronation mineral district in the northern Plomosa Mountains [1]. Additionally, the Carlota, Warrior, and Live Oak deposits occur near the top of the granite porphyry (G porphyry phase) [10] of the Schultze Granite in the Globe-Miami area.

Much of the tectonic burial preceded the emplacement of the main stage of batholiths of the Wilderness assemblage. In the Santa Catalina Mountains, tectonic burial occurred between approximately 67 and 57 Ma. Mylonitization at the top of the Wilderness suite batholith was achieved at a shallower level, at about 9 km depth, based on the muscovite-biotite-alkali feldspar-quartz barometer and the phengitic muscovite barometer [83]. Hence, south- and southwest-directed mylonitization near the top of the Wilderness sill complex occurred beneath an upper plate that was approximately 9 km thick in middle Eocene time, at about 50 Ma.

Thrust burial reached its maximum during flat subduction in the Eocene [11] and is now widely recognized in the literature. Rapid uplift of the lower plate windows began about 38 Ma. Rapid uplifts occurred in the cores of broad anticlinal uplifts that were formed under the influence of regional compression during the mid-Tertiary [64].

After about 30 Ma, rapid uplift took place during the westward migration of the mid-Cenozoic volcanic arc complex through the Tucson region during the Galiuro orogeny. The tectonic burial model satisfies all of the geobarometric constraints, which, in other narratives, have problematic aspects, especially the widely used crustal extension model.

Figure 19 shows the model of thrust burial and subsequent uplift and denudation superimposed on geobarometry [83]. The figure also shows the suggested shallower geobarometry (4–2 km) for the Marble Peak skarn and the Rice Peak epizonal pluton in the Santa Catalina Mountains [84].

Between the anticlinal uplifts, synclinal troughs formed that were filled with coarse clastic sedimentary rocks and volcanics, such as the Galiuro Volcanics. The Galiuro Volcanics are a metaluminous, alkali-calcic, trachyandesite to high-silica rhyolitic ignimbrite field, aged from 28 to 22 Ma, in the type area of the Galiuro orogeny [61]. The Galiuro Volcanic Field is in the Galiuro Mountains, between the San Pedro Valley northeast of the Santa Catalina Mountains and the Pinaleno Mountains, as shown in Figure 18.

Uplifts were also, in part, triggered by thermalism introduced by the return of a hot asthenosphere wedge to an area of delaminated crustal lithosphere. The thermalism produced a softening of the crust that enhanced the compression and uplift effects. The regional uplifts were accompanied by major gravity-driven denudational faulting, which is widely referred to in the literature as extensional “detachment faulting.” In most cases, these denudational reactivations are associated with renewed faulting on the former Maricopa thrust system.

6.3 Laramide age of emplacement

Peraluminous copper-oxide deposits were formed in the latest part of the Laramide orogeny after the deposition of the classic porphyry copper deposits. In terms of relative ages, the latest Laramide-aged peraluminous granitoids typically intrude older metaluminous quartz diorites to granodiorites that are associated with porphyry copper mineralization. One example is the two-mica Wilderness Granite, which intruded the metaluminous Leatherwood suite in the Santa Catalina Mountains at the Marble Peak copper skarn deposit [48]. Another example is the Sierrita peraluminous granite and related pegmatites, which intruded the Ruby Star granodiorite associated with the Pima district porphyry copper cluster in the Sierrita Mountains of Pima County [10]. Additionally, in the Grayback area of the northern Tortilla Mountains, the Ninetysix Hills plutons consist of an early metaluminous phase intruded by later peraluminous phases [28, 30, 32].

In terms of absolute ages, abundant radiometric data clearly show a mid- to late Laramide age of emplacement of the peraluminous plutons (Table 1 and 2). One of the best radiometrically dated examples is the Texas Canyon pluton, which has U–Pb dating of zircons at about 58–56 Ma on the Stage 3 biotite quartz monzonite phase [50, 52]. The main phase of the Texas Canyon pluton is cut by a suite of biotite aplites that display an intimate association with the Gunnison copper-oxide greisen deposits [9]. The Stage 4 Adams Peak leucogranite and related muscovite-garnet aplite dikes and greisens yielded a 54.6 ± 0.2 Ma whole-rock Rb–Sr isochron [53].

Additional radiometric dating of the peraluminous copper-oxide greisen deposits includes:

• The 68 Ma U–Pb date for the Little Hill pluton [22],

• The 69 Ma age for the Signal Peak Granite in the Sacaton Peak complex [85],

• The 59 Ma age for the Newman Peak Member of the peraluminous granite complex in the southern Picacho Mountains [24],

• The U–Pb geochronology for the Wilderness Suite in the Santa Catalina Mountains, with numerous U–Pb dates between 38 and 58 Ma [44, 45, 47], and

• The Relleno peraluminous granitoid suite in the Pinaleno Mountains, with similar ages [86].

On a regional basis, the transition from upper crustal, conventional porphyry copper deposit formation (Morenci assemblage) to midcrustal peraluminous copper-oxide greisen deposit formation (Wilderness assemblage) is time-transgressive from west to east [11]. In eastern California and western Arizona, Wilderness-type peraluminous calc-alkalic plutons are about 73–71 Ma, based on U–Pb dating of rocks in the Old Woman Mountains, and are 76–65 Ma in the northern Plomosa Mountains [41], and 70 Ma in northwestern Arizona in the Cyclopic-Garnet Range area [87]. In southeastern Arizona, Wilderness suite plutons are 53–38 Ma in the Santa Catalina Mountains and 55 Ma as the Relleno suite in the Pinaleno Mountains [86].

6.4 Magmatic water and volcanism

The peraluminous petrology of the Texas Canyon pluton strongly implies a crustal anatectic origin related to a “water flooding” event. The water flooding could have occurred during the flat subduction event that affected southern Arizona between approximately 62 and 43 Ma. The flooding of large amounts of water could have been produced by dehydration of the deep crust, especially by dehydration of underthrust Franciscan metagraywacke and serpentinite. During the Paleocene-Eocene, regional isostatic uplift was enhanced by additional buoyancy produced by tectonic coupling of the northeasterly flatly subducting Farallon plate with the relatively southwest-moving North American upper plate.

Magmatic water can be emplaced along a spectrum that ranges from midcrustal, nonvolcanic fluid releases to uppermost crustal, dry volcanism. The midcrustal fluid releases are associated with peraluminous greisenous metallogeny (such as copper, tin, tungsten, and beryllium). These peraluminous fluid releases are associated with very hydrous magmatism that is not linked to volcanism.

At upper crustal levels, between 2 and 5 km depths, less hydrous plutons are typically metaluminous in character and are associated with metal-bearing, hornblende-bearing magmatism. These plutons produce nongreisenous, more epigenetic base and precious metal deposits. Above 2 km, epithermal metallogeny typically features precious metals and occurs with widespread volcanism that contains hornblende. However, near-surface volcanism that lacks hornblende is typically not associated with robust metallogeny, except for the more vapor-phase volcanism that features mercury.

The midcrustal emplacement depth of peraluminous magmatism is consistent with classic experimental data relating to the water content of pegmatitic granitic igneous rocks [88]. From Burnham’s experimental data, water exsolves from pegmatitic granitic rocks at 8–12 wt% water at a depth of 12–18 km. This experimental data aligns with empirical geobarometry data for the Santa Catalina Mountains [45, 83]. When water exsolves from magma, and temperature and pressure cross the solidus, the magma quickly freezes at midcrustal levels and does not produce volcanism.

In Arizona and other locations, an Eocene “magmatic gap” was based on K–Ar dating of mostly metaluminous plutons and volcanics. However, extensive U–Pb dating has now established that this “magmatic gap” is populated by abundant peraluminous plutonism, but not volcanism. Hence, the term “magmatic gap” should be renamed the “volcanic gap.”

In the classic stratovolcano porphyry copper model [89], conventional porphyry copper deposits are shown to have formed beneath andesitic stratovolcanoes. However, most Arizona porphyry copper deposits are not associated with time-equivalent volcanics. When volcanics are present in the same area, dating of the volcanism yields ages that are typically 8–15 million years older than the mineralization of the porphyry copper deposits.

The porphyry copper intrusive sequences typically contain hornblende. Experimental work [90] showed that hornblende-stable granodiorites, similar to those associated with porphyry copper deposits, require about 4 wt% water, which exsolves from the magma at depths of 5–2 km. This exsolution takes place below the interface between the volcanism and the underlying plutons to produce porphyritic rocks but does not reach the surface to produce extensive volcanism.

In Arizona, the only time-equivalent volcanism that spatially coexists with a porphyry copper system occurs at Copper Creek in Pinal County, where U–Pb dating has shown a time equivalency between the Glory Hole Volcanics and porphyry copper-related granodiorites at Copper Creek [91]. Although copper has been produced from breccia pipes at Copper Creek, at least 100 years of persistent and thorough exploration have not yielded a commercial porphyry copper deposit. The lack of a robust porphyry copper system may have resulted from the Copper Creek granodiorite being too dry to produce a good stockwork effect [91]. In effect, an association with time-equivalent volcanism may be a negative indicator in the exploration for metaluminous porphyry copper deposits.

Where there is a spatial correspondence between volcanism and metaluminous porphyry copper deposits, there is usually an age discrepancy of 5–10 million years between the volcanism and the quartz diorite to granodiorite plutons associated with the porphyry copper deposits. For example, at the Christmas deposit, the Stage 3 biotite granodiorite of the Christmas stock yields U–Pb dates of about 63 Ma [92] and is associated with extensive skarn metallization as well as a large, low-grade copper deposit hosted in the trachyandesitic Williamson Canyon Volcanics. However, the Williamson Canyon Volcanics have been dated at 74–82 Ma [93]. In addition, the Williamson Canyon Volcanics are more alkaline in character and are classified as metaluminous, alkali-calcic, in contrast to metaluminous, calc-alkalic for intrusions associated with the Christmas porphyry copper deposit. Similar age and geochemical discordances exist for volcanism at the Ajo, Silver Bell, Patagonia, and Safford porphyry copper deposits.

Additionally, plutons that are time-equivalent with alkali-calcic volcanism are associated with lead-zinc-silver systems, not porphyry copper systems. Examples of mineralization related to time-equivalent alkali-calcic volcanism and plutonism are:

• The Empire lead-zinc-silver deposits are associated with the Sycamore Canyon stock and Salero Volcanics;

• The Tombstone lead-zinc-silver deposits are associated with the Schieffelin quartz monzonite stock and the time-equivalent Uncle Sam quartz latite volcanics;

• The Charleston lead-zinc-silver districts and associated stocks and Bronco Volcanics;

• The Cerro Colorado lead-zinc-silver vein system associated with time-equivalent dikes and Cerro Colorado volcanics [91]; and

• The quartz monzonite stocks at Washington Camp-Duquesne in the Patagonia area, along with age-equivalent rhyolitic volcanics at Red Mountain and trachyandesite of Meadow Valley, are associated with the Flux lead-zinc-silver mine [94].

The above lead-zinc-silver examples were grouped into the Tombstone assemblage [11]. The mineral deposits of the Tombstone assemblage have yielded moderate lead-zinc-silver production, whereas the various porphyry copper deposits have yielded world-class copper and molybdenum production. The contrast in type and volume of metal production could be related to the water content of the associated magmatism. Tombstone assemblage lead-zinc-silver-related magmatism was less hydrous and more volcanic-related, whereas the Morenci assemblage copper-molybdenum-related magmatism was more hydrous and lacked significant volcanism [11].

The correlation of low water content and volcanism can be extended into the epithermal class of mineral deposits, such as the Comstock Lode deposits in Nevada and the Oatman epithermal vein complex in western Arizona. These districts, and numerous others, are associated with hornblende-bearing andesitic volcanism and biotitic felsic volcanism. Epizonal, hornblende dioritic to quartz-feldspar porphyry intrusions are associated with productive epithermal, mainly precious metal districts. For example, the Mount Davidson hornblende quartz diorite in the Comstock area of Nevada may be the causative intrusion for the Comstock Lode complex, all of which have been emplaced in mid- to late Miocene andesitic volcanics.

Dryer volcanism that lacks hornblende but contains ferromagnesium mineral suites, such as pyroxene and olivine, typically lacks significant epithermal metallogeny. The Bohemia district in Oregon may be an example of a dry to weakly hydrous epithermal mineral system. However, these drier systems may be associated with vapor-phase mercury districts, such as the California Coastal Range mercury belt of mid- to late Miocene age.

The completely dry, bimodal (typically basalt-rhyolite) volcanism of the late Miocene in the Basin and Range province represents an end member of the continuum from midcrustal, nonvolcanic fluid releases to uppermost crustal, dry volcanism. This upper crustal volcanism may be associated with local fumarolic systems that lack any significant base or precious metal metallogeny. For example, this type of magmatism appeared on a regional basis throughout Arizona in the late Miocene during the San Andreas orogeny [64].

6.5 Plate tectonic setting

Between the middle and latest phases of the Laramide orogeny, the plate tectonic setting vastly changed from normal-dip, flattening subduction to very flat subduction [95–97]. Flat subduction was associated with continent-scale underthrusting of the Franciscan mélange wedge as far east as the Four Corners region [98]. At Buell Park on the Apache County, Arizona-New Mexico border, the Franciscan mélange wedge appears as exotic nodules of blue-schist eclogite in serpentinite diapirs. Underthrusting may have reached as far east as the present Mississippi River, which is the eastern boundary of the Great Plains uplift, based on gravity data [99] and crustal thickness data [100].

Subduction of the Franciscan mélange wedge produced massive amounts of dehydration. This dehydration created a “water flood” that rose into the North American upper plate to produce a distinctive suite of peraluminous granitoids, referred to as the Wilderness Suite [48]. The concept of the muscovite-bearing, peraluminous plutons was extended regionally throughout the western USA [101]. The concept of a western North American peraluminous belt was extended to a Cordilleran-wide zone referred to as the Cordilleran anatectic granite belt [51].

From a plate tectonic perspective, peraluminous copper-oxide greisen deposits occur as a late-stage product of flat subduction. During flat subduction in the culminant or latest part of the Laramide orogeny, peraluminous magmatism was produced in very large volumes (Figure 20) by hydrous melting of various kinds of preexisting Precambrian crust. Based on inherited zircon ages extracted from Laramide peraluminous plutons in the Santa Catalina Mountains [44, 47] and elsewhere [50], commonly inherited zircon sources are granites of the Picuris (~1,400 Ma), Mazatzal (~1,600 Ma), and Yavapai (~1,700 Ma) orogenies.

Plate tectonic convergence rates during the emplacement of the peraluminous plutons in the latest Laramide are estimated to have been up to 40 cm/yr [96]. These very high speeds allowed maximum dehydration of the underthrusting oceanic lithosphere, which was likely composed of Franciscan metagraywacke and serpentinized peridotite.

The flat subduction of lower-density metagraywackes and serpentinite (and the attachment of these lower-density rocks to the base of the continent) decreased the overall density of the overlying North American plate. The overall crustal profile thus became more buoyant and allowed a continental-scale epeiric uplift of western North America. This uplift resulted in the formation of the regional Eocene erosional surface and altiplano across the Great Basin and Basin and Range provinces [20, 50, 105].

Peraluminous rocks also have a lower density than metaluminous rocks, which could have contributed to the lower density of the lithosphere overlying the flatly subducting crust. The tectonic thinning has been referred to as “decretion” [106, 107] and more recently as “Laramide lithospheric bull-dozing” [82]. The Altiplano later transtensionally collapsed during the late Miocene creation of the San Andreas Fault margin and its associated San Andreas orogeny [64].

The paleo-convergence rate of the middle Laramide, metaluminous, calc-alkalic, porphyry copper-related plutons was up to 24 cm/yr with a paleo dip of 9° [96]. The low-angle dip and high convergence rate near the end of the middle part of the Laramide orogeny could have contributed to the extremely large size of the metaluminous, calc-alkalic, porphyry copper system at the Morenci mine. Morenci also has the largest Stage 3 biotite granodiorite plutons, which may suggest that the increasingly flat subduction was an important factor in creating the supergiant size of Morenci [5].

8.1 Mantle source of metaluminous porphyry copper deposits

The earlier, classic porphyry copper deposits are associated with moderately dipping subduction (Figure 21) that hydrously melted metaluminous mafic sources in the upper mantle above the subducting Farallon slab [96]. The specific source for porphyry copper deposits may have been a high-alumina eclogite layer in the upper asthenosphere between depths of approximately 130 and 240 km [8]. This depth is based on the application of the potassium-depth (K–h) index (K₂O content at 57.5% SiO₂) determined from K₂O-SiO₂ Harker variation plots [96, 109, 110]. Confidence in the mafic metaluminous source is based on a world-wide empirical correlation between metaluminous calc-alkalic magmatism and porphyry copper deposits [6, 8, 96, 111].

Water contents were estimated to be 4–8 wt% water in the field of hornblende stability [96]. Hydrous melting produced significant volumes of gabbroic magmatism, which fractionated in the upper crust into stages of increasingly more felsic plutonic differentiates. Both calc-alkalic copper-molybdenum porphyry deposits and alkali-calcic lead-zinc-silver deposits are associated with Stage 3 biotite granodiorite and biotite quartz monzonite plutons.

Within metaluminous calc-alkalic, hornblende-bearing, oxidized differentiation sequences, conventional porphyry copper deposits are associated with Stage 3 biotite granodiorite systems. Both the copper and the Stage 3 biotite granodiorites are derived from a biotite-hornblende Stage 2.5 precursor. Additionally, the later Stage 4 quartz-feldspar porphyritic granite rock systems are associated with arsenical copper deposits, mainly as veins with fringing copper-lead-zinc-silver deposits and more distal manganese-silver deposits [11].

Inherited zircon xenocrysts in metaluminous rocks are much less common in igneous rocks associated with metaluminous porphyry copper deposits than in peraluminous igneous rocks. In peraluminous rocks, zircon cores commonly display evidence of hydrous supercritical fluids causing metamictization, which is the process of structural breakdown caused by accumulated radiation damage from the radioactive decay of trace amounts of uranium and thorium within the crystal structure. The supercritical fluids reflect the addition of crustal fluids that altered existing early zircon cores and produced hydrous ferromagnesian reaction products. Examples include hornblende and biotite mantles surrounding pyroxene and olivine (less commonly) cores.

However, the supercritical fluids did not melt the diorites or gabbros that contained the original zircons. Rather, the original gabbros were derived by hydrous melting of the high-alumina eclogite in the asthenosphere of the hanging wall immediately overlying the descending oceanic slab. Hence, the inherited metaluminous zircon mineralogy probably reflects igneous formation in the high-alumina, Stage 1, gabbroic sources.

8.2 Crustal source of peraluminous copper-oxide greisen deposits

The source of copper for peraluminous copper-oxide greisen deposits relates to the chemistry of the associated peraluminous plutons. Compared to metaluminous plutons, peraluminous plutons are more aluminum-rich and silica-rich, as shown in the A/CNK versus SiO₂ variation diagrams (Figure 3).

Peraluminous plutons were formed by hydrous “water flooding” of the lower to mid-crust during flat subduction and decretion (lower crustal erosion) [74] in the latest part of the Laramide Orogeny (Figure 22).

Peraluminous sources are almost certainly crustal in origin, as peraluminous magmas were generated by hydrous melting of granodioritic to granitic or metasedimentary sources. Peraluminous plutons acquired their copper from minimum melt, sialic, plutonic to metasedimentary sources in the lower to middle crust. Melting occurred under middle to upper amphibolite-grade metamorphic conditions. In some cases, such as at the Coronation prospect in La Paz County [1], underthrust metagraywacke sources (such as the Orocopia Schist) were anatectically melted under very hydrous conditions, in the presence of fluids derived from the flatly subducting slab.

A major clue to answering the source question is present in the morphology and geochemistry of zircons in igneous rocks in both peraluminous and metaluminous magma-metal series classes. Zircons in peraluminous igneous rocks have cores of inherited zircons with younger rims and/or tip overgrowths that reflect the age of emplacement (Figure 23). The inherited zircon component is commonly overgrown by hydrothermal zircon and, in some cases, by later magmatic zircons.

Igneous peraluminous zircons were formed in the crust under mid- to upper-amphibolite melt conditions. However, inherited zircons represent survivors of various crustal sources that may have contributed copper to the various bulk peraluminous melts.

Xenocrystic zircon data from the Texas Canyon pluton indicate the pluton acquired zircons from Precambrian Johnny Lyon granodiorite (~1,655 Ma), possibly Pinal Schist (~1,650 Ma), and minor Precambrian diabase sources (~1,100 Ma). Similar xenocrystic zircons occur in the lower plates throughout southeastern Arizona. The xenocrystic zircons may reflect assimilation by melting of the above crustal sources, which could have contributed copper to the peraluminous magma.

Very strong evidence for the Orocopia Schist as a source of the Coronation peraluminous pluton in the northern Plomosa Mountains is present in its zircons (Figure 23). A complete sequence of zircon morphology is documented in the Coronation pluton [41]. The zircon morphological sequence progresses from initial detrital zircons through hydrothermal metamorphism to initial magmatic zircons and finally to fully magmatic zircons (G panel in Figure 23). The Coronation peraluminous granite is genetically related to the Coronation copper-gold peraluminous district of the northern Plomosa Mountains [1].

The zircon morphological sequence in the Coronation Pluton consists of the following types:

1. Detrital. Rounded fragments with original internal textures, including oscillatory growth zoning, no alteration rims, and high Th/U ratios (> 0.1).

2. Initial hydrothermal metamorphic. Bright luminescence indicating high U/Th ratios (>10), with corrosion rims containing original internal textures and older detrital ages associated with high Th/U (>0.1). Not shown are the ages of the rims, which are the age of metamorphism (68–50 Ma).

3. Advanced hydrothermal metamorphic. Thicker oscillatory overgrowths with very high U/Th ratios (>10) on bright rims with intermediate ages, destruction of the original core replaced by chaotic mottling, dark opacity induced by metamictization, and original ages of the detrital grain still preserved in cores.

4. Advanced hydrothermal metamorphic. Complete metamictization of cores exhibiting strong, chaotic, mottled textures with intermediate ages in overgrown rims, which reflect incipient melting.

5. Incipient magmatic. Thicker oscillatory overgrowths with high U/Th ratios (>10), exhibiting magmatic formation ages, while the original ages of detrital grains remain preserved in metamict cores. Oscillatory growth zones reflect growth under anhydrous conditions, where the supercritical water responsible for metamictization is transferred to late hydrous mica crystallization.

6. Incomplete magmatic. The entire grain contains magmatic formation ages, including cores that display relict metamict texture with high U/Th ratios (>10).

7. Complete magmatic. The entire grain exhibits hydrothermal growth zone texture and magmatic formation ages. These grains are inferred to have grown under entirely magmatic conditions, where the water component was distributed into the later, lower-temperature, hydrous mica component. The ages of these grains best reflect the crystallization age of the peraluminous magma.

Zircons from peraluminous plutons contain true inherited xenocrysts (Panel A of Figure 23). The case history of the sequence of inherited to magmatic zircons from the Orocopia Schist to the Coronation pluton provides proof of a crustal source for the Coronation peraluminous pluton. The zircon data show that the NIC unit (here called the Coronation pluton) was a probable melt product of the Orocopia Schist, based on age histograms for the detrital zircon population [41].

Similar zircon inheritances are now well-documented in the Santa Catalina Mountains [44, 47]. The Wilderness Sill Complex is associated with hematitic copper-oxide deposits at Mount Lemmon, the Pontatoc Mine area, and the westernmost Pusch Ridge. Detrital and metamorphic zircons in the Wilderness Sill Complex show abundant 1,440 Ma age dates, which indicate major assimilation of an Oracle Granite source, and show lesser amounts of 1,640 Ma age dates, which indicate a lesser assimilation of Johnny Lyon Granodiorite. These sources are believed to be good candidates for the ultimate source of the copper. The copper was then distilled by fractionation in the Wilderness Sill Complex due to progressive biotite disappearance.

Inherited zircons in the Globe-Miami area indicate that Mazatzal-age sources, specifically the 1,630 Ma Madera Diorite, may have been a melt source for the Solitude Granite, the G Porphyry phase of the Schultze Granite [10], and the Manitou Granite. These igneous bodies may then have fractionated to produce the Carlota, Live Oak-Warrior, and Van Dyke peraluminous copper-oxide deposits.

Cathodoluminescence characteristics of zircons in peraluminous rocks are much brighter on an overall basis compared to zircons in metaluminous rocks. The cathodoluminescence is probably due to the much higher uranium and thorium contents of peraluminous zircons compared to metaluminous zircons.