Product details

Abstract

1-5 Keith, S.B. and Swan, M.M., 1996, The great Laramide porphyry copper cluster of Arizona, Sonora, and New Mexico: the tectonic setting, petrology, and genesis of a world class porphyry metal cluster, in Coyner, A.R., and Fahey, P.L., eds., Geology and Ore Deposits of the American Cordillera: Geological Society of Nevada Symposium Proceedings, Reno/Sparks, Nevada, April, 1995, p 1-80.

ABS

The term ‘porphyry copper’ embodies more than a casual linkage between porphyry magmatism and spatially-related, large low-grade ‘disseminated’ copper deposits. In the Arizona­Sonora-New Mexico of southwest North America a great cluster of these deposits formed as part of the well known Laramide orogeny. With recent advances in multi-element chemical analysis and isotopic age dates, it is now apparent that the Laramide orogeny is a petrochemically, stratigraphically, structurally, and metallogenically dynamic event. Any true reconstruction of the emplacement of the great porphyry copper cluster thus requires integration of data in a time and space context with empirical petrologic control. Although this use of petrochemistry to constrain tectonics (particularly plate tectonics) has been generally overlooked in recent data synthesis and integration, it may be the most important tool for sorting out the evolution of the Porphyry Copper Cluster.

This paper presents an integration of Porphyry Copper Cluster data by strata-tectonic analysis (Keith and Wilt, 1986) with the Magma-Metal Series Classification (Keith and others, 1991). The resulting model indicates that the Porphyry Copper Cluster is the product of a unique petrology and met­ aluminous calc-alkalic magma-metal seriers produced during the Laramide Orogeny by subduction of a 250-mile-long segment of the Farallon Plate beneath the North American Craton. Emplacement of the calc-alkalic magmas was controlled by deep-seated transcurrent Texas Zone structures and was accompanied by assimilation of oxidized fluorine-rich crustal formational fluids associated with the 1.4  Ga granitoid basement.

Application of the Magma-Metal Series Classification of Keith and others (1991) to well  constrained  petrochemical data (Lang, 1991) for intrusive suites temporally and spatially associated with the porphyry copper deposits (Morenci assemblage plutonic suits of Keith and Wilt, 1986) strongly indicates that porphyry copper deposits of the Porphyry Copper Cluster (45 out of 47 cases with appropriate documentation) are associated with a very specific magmatic crystallization sequence. This sequence is aluminum-poor (metaluminous) and displays calc-alkalic alkalinity. The suite is also titanium, yttrium, and iron-poor – consistent with crystallization under hydrous conditions. The occurrence of hornblende in the early part of the sequence indicates that this crystallization occurred under very wet conditions (greater than 4 to 5 weight per cent water) – also a pre-requisite for porphyry base- and precious­ metal metallogeny. Ferric to ferrous ratios are generally greater than 0.9 and accessory mineralogy features  the occurrence  of a magnetite-sphene assemblage indicative of crystallization under oxidized conditions.

The above petrochemistry is empirically linked with gold­ poor, zoned porphyry copper deposits characterized by copper­ molybdenum central zones with more distal zinc-copper-lead­ arsenic-silver (minor gold-tungsten-selenium-antimony-mercury­ cesium-beryllium-uranium) deposits. No other deposit models have been linked to the aboYe described petrochemistry. Even more importantly, no porphyry copper deposits of this type have been linked to other magma metal series; many of which also occur in Arizona, southwest New Mexico, and northern Mexico.

Petrologic modeling using mass balanced constrained reactions suggests an ultimate source for porphyry copper related plutons in the upper asthenospheric mantle. A likely mantle candidate would be a high alumina phlogopite-bearing quartz kyanite eclogite layer that under volatile present conditions could yield a primary calc-alkalic alkalinity gabbroic partial melt (about 23 percent partial melt). This high alumina gabbro melt rises into the crust where it assimilates crustal fluids (mainly water) in equilibrium with a moderately oxidized to oxidized crust. The model requires assimilation of about 4 weight percent water and minor fluorine,  to oxidize the magma into magnetite-sphene stability, achieve hornblende stability (not achieved under normal subduction conditions), and import a soluble radiogenic and stable isotope signature in equilibrium with the integrated isotopic signatures of Proterozoic crust. This water is acquired  during  a fluid implosion event where hydrous metamorphism of pyroxene to hornblende takes place under igneous conditions. It is also at this time that an initial devolatilization of carbon dioxide and hydrogen gas takes place as the magma achieves charge balance with its wallrocks. The model requires no assimilation of solid state silicate crust. The entire process is volatile driven.

Once, the volatile induced metamorphism has taken place under magmatic conditions in the lower and mid-crust,  the now oxidized magma ascends into the upper crust where a stepped sequence of three additional devolatilization events take place. These devolatilizations are associated with the unmixing of individual rock systems within the sequence from more mafic, less viscous, and dense precursors. Porphyry cop­ per deposits are associated with the third devolatilization event which features the disappearance  of  hornblende  from the sequence. In the region of the Porphyry Copper Cluster, high-angle Texas Zone structural elements were not only crucial guides during the late stage ascent history, but also may have provided fluid reservoir sites and/or access to fluid reservoir sites in the lower and mid-crust where  the critical fluid assimilation and metamorphism occurred.

Comparison of petrologic data from the Arizona-Sonora-New Mexico porphyry copper cluster with other porphyry metal clusters (the 2700 Ma Bourlamaque batholith in Canada and the Porgera Intrusions in Papua, New Guinea), reveals several significant similarities. All three systems show distinct steps in calcium,  magnesium,  and/or iron content as a function of differentiation. Early gabbro differentiates of all three areas are very hornblende rich. All three porphyry metal clusters were emplaced into orogenically metamorphosed crust tapped by throughgoing regional deeply seated structures. Comparison with a sealed in batholith case history (the 92 to 84 Ma Tuolumne Magma Series of central California) provides some intriguing counterpoints. The Tuolumne intrusions are not located near a major known structure, pyroxene relicts persist into the quartz diorite system indicating incomplete metamorphism, and differentiation steps (if they exist) are subtle. No porphyry metal expression is present.

We conclude there seems to be something about the coin­ cidence of large scale, deeply seated fractures, an antecedent metamorphosed basement, big, wet hornblende-rich por­ phyritic intrusions, and the development of world class base, precious and lithophile element porphyry metal clusters. The great Arizona-Sonora-New Mexico porphyry copper cluster is an excellent well documented example of the phenomena.

Key words:

Arizona. copper ores. disseminated deposits. Laramide Orogeny. magmas. metal ores. metallogeny. Mexico. mineral deposits, genesis. New Mexico. porphyry copper. Sonora Mexico. structural controls. United States.