Geology and Exploration Progress at the Resolution Porphyry Cu-Mo Deposit, Arizona
Carl Hehnke, Geoff Ballantyne, Hamish Martin, William Hart, Adam Schwarz, Holly Stein, 2012. "Geology and Exploration Progress at the Resolution Porphyry Cu-Mo Deposit, Arizona", Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe, Jeffrey W. Hedenquist, Michael Harris, Francisco Camus
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In 1995, the Magma Copper Company discovered a porphyry copper deposit beneath thick postmineral cover 2 km south of the historic Magma mine in Superior, Arizona. Since that time drilling has delineated a large, high-grade, hypogene copper-molybdenum deposit, now named the Resolution deposit, with an Inferred Resource of 1,624 million metric tons (Mt) at 1.47% Cu and 0.037% Mo.
The Resolution deposit is hosted by Proterozoic and Paleozoic quartzite and carbonate units, Proterozoic diabase sills, and Cretaceous sandstone, volcaniclastic rocks, and tuff. Minor Cretaceous to Tertiary hypabyssal intrusions and heterolithic breccia bodies cut this Proterozoic-to-Mesozoic section. The mineralized rocks are concealed beneath an eastward-thickening wedge of Oligo-Miocene Whitetail Conglomerate, which in turn is largely covered by 18.6 Ma welded tuff.
The porphyry copper deposit at Resolution is centrally located within a fault-bounded block with plan dimensions of ∼ 3 × 3 km. The fault-bounded block first developed as a horst, which led to local erosion of Paleozoic strata but was later inverted as a graben, which preserves ∼ 1 km of Cretaceous strata not otherwise present in the Superior area. Within the graben, basal quartz-rich sedimentary rocks containing ∼ 97 Ma zircons are overlain by a ∼ 74 Ma andesitic sequence that was probably derived from outside the graben. Younger units consist mostly of quartz-rich tuffs, whose petrographic similarity and U-Pb ages suggest they are extrusive equivalents of ∼ 69 to ∼ 64 Ma hypabyssal intrusions present at depth within the graben. Crustal extension and tilting across multiple, large Tertiary normal faults since the onset of Whitetail Conglomerate deposition has rotated the deposit approximately 25° to the east northeast.
Copper mineralization at Resolution defines a structurally intact, dome-shaped shell up to 600 m thick, the upper boundary of which is overlapped by an unusually strong pyrite halo containing 7 to >14 wt % pyrite. The deposit shows strong alteration and mineralization zoning and strong telescoping of alteration assemblages. Early potassic alteration, associated with dominantly chalcopyrite-rich mineralization, gives way outward to an epidote-bearing propylitic zone. Strong quartz-sericite alteration largely overprints the upper portion of the potassic zone and is associated with chalcopyrite, bornite, chalcocite, and pyrite mineralization. Structurally controlled advanced argillic alteration, consisting of kaolinite, dickite, topaz, alunite, pyrophyllite, and zunyite overprints the quartz-sericite zone and is associated with pyrite as well as hypogene bornite, chalcocite, and digenite, which have substantially replaced earlier chalcopyrite. Molybdenite occurs in quartz veins both with and without copper minerals but economic concentrations of copper and molybdenum are spatially coincident.
Re-Os ages for molybdenite range from ∼ 65 to ∼ 64 Ma, coinciding with the youngest U-Pb age for hypabyssal intrusions. The 40Ar/39Ar ages of biotite and sericite range from ∼ 64 to ∼ 62 Ma and 40Ar/39Ar ages for hypogene alunite range from ∼ 62 to ∼ 60 Ma. Younger, ∼ 52 to 49 Ma 40Ar/39Ar dates for two hypogene alunite veins, and a ∼ 51 Ma Re-Os pyrite date for a massive pyrite-dickite vein, may indicate a later pulse of hydro-thermal activity.
A clearly defined causative intrusion has not been identified at Resolution but strong foliation defined by secondary biotite within the host diabase sills mimics the dome-shaped copper shell and is inferred to reflect stress due to emplacement of a cylindrical stock below the deepest drill holes. Unusually high hypogene copper grades reflect the presence of favorable diabase and limestone host rocks, lack of dilution by postmineral dikes, and multiple spatially overlapping mineralizing events, including the deposition of early chalcopyrite, later chalcopyrite-bornite, and still later bornite-chalcocite-digenite assemblages. The high grades may also reflect an unusually long-lived flux of ore fluids channeled through the center of the deposit by permeable breccia zones.
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Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe
It has been recognized for the past century that copper deposits, in common with those of many other metals, are heterogeneously concentrated in Earth’s upper crust, resulting in areally restricted copper provinces that were generated during several discrete metallogenic epochs over time intervals of up to several hundred million years. Various segments of circum-Pacific magmatic arcs, for example, have total contained copper contents that differ by two orders of magnitude. Each metallogenic epoch introduced its own deposit type(s), of which porphyry copper (and related skarn), followed by sediment-hosted stratiform copper and then iron oxide copper-gold (IOCG), are globally preeminent. Nonetheless, genesis of the copper provinces remains somewhat enigmatic and a topic of ongoing debate.
A variety of deposit-scale geometric and geologic features and factors strongly influence the size and/or grade of porphyry copper, sediment-hosted stratiform copper, and/or IOCG deposits. For example, development of major porphyry copper deposits/districts is favored by the presence of clustered alteration-mineralization centers, mafic or massive carbonate host rocks, voluminous magmatic-hydrothermal breccias, low sulfidation-state core zones conducive to copper deposition as bornite ± digenite, hypogene and supergene sulfide enrichment, and mineralized skarn formation, coupled with lack of serious dilution by late, low-grade porphyry intrusions and breccias. Furthermore, the copper endowment of all deposit types undoubtedly benefits from optimization of the ore-forming processes involved.
Tectonic setting also plays a fundamental role in copper metallogeny. Contractional tectonomagmatic belts, created by flat-slab subduction or, less commonly, arc-continent collision and characterized by crustal thickening and high rates of uplift and exhumation, appear to host most large, high-grade hypogene porphyry copper deposits. Such mature arc crust also undergoes mafic magma input during porphyry copper formation. The premier sediment-hosted stratiform copper provinces were formed in cratonic or hinterland extensional sedimentary basins that subsequently underwent tectonic inversion. The IOCG deposits were generated in association with extension/transtension and felsic intrusions, the latter apparently triggered by deep-seated mafic magmas in either intracratonic or subduction settings. The radically different exhumation rates characteristic of these various tectonic settings account well for the secular distribution of copper deposit types, in particular the youthfulness of most porphyry relative to sediment-hosted stratiform and IOCG deposits. Notwithstanding the importance of these deposit-scale geologic, regional tectonic, and erosion-rate criteria for effective copper deposit formation and preservation, they seem inadequate to explain the localization of premier copper provinces, such as the central Andes, southwestern North America, and Central African Copperbelt, in which different deposit types were generated during several discrete epochs. By the same token, the paucity of copper mineralization in some apparently similar geologic settings elsewhere also remains unexplained.
It is proposed here that major copper provinces occur where restricted segments of the lithosphere were predisposed to upper-crustal copper concentration throughout long intervals of Earth history. This predisposition was most likely gained during oxidation and copper introduction by subduction-derived fluids, containing metals and volatiles extracted from hydrated basalts and sediments in downgoing slabs. As a result, superjacent lithospheric mantle and lowermost crust were metasomatized as well as gaining cupriferous sulfide-bearing cumulates during magmatic differentiation—processes that rendered them fertile for tapping during subsequent subduction-or, uncommonly, intraplate extension-related magmatic events to generate porphyry copper and IOCG districts or belts. The fertile lithosphere beneath some accretionary orogens became incorporated during earlier collisional events, commonly during Precambrian times. Relatively oxidized crustal profiles—as opposed to those dominated by reduced, sedimentary material—are also required for effective formation of all major copper deposits. Large sedimentary basins underlain by or adjoining oxidized and potentially copper-anomalous crust and filled initially by immature redbed strata containing magmatic arc-derived detritus provide optimal sites for large-scale, sediment-hosted stratiform copper mineralization. Translithospheric fault zones, acting as giant plumbing systems, commonly played a key role in localizing all types of major copper deposits, districts, and belts. These proposals address the long-debated concept of metal inheritance in terms of the fundamental role played by subduction-metasomatized mantle lithosphere and lowermost crust in global copper metallogeny.