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Geology and Exploration Progress at the Resolution Porphyry Cu-Mo Deposit, Arizona

By
Carl Hehnke
Carl Hehnke
1
Resolution Copper Mining, 102 Magma Heights, Superior, Arizona 85273
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Geoff Ballantyne
Geoff Ballantyne
2
Rio Tinto Copper Development, 4700 Daybreak Parkway, South Jordan, Utah 84095
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Hamish Martin
Hamish Martin
1
Resolution Copper Mining, 102 Magma Heights, Superior, Arizona 85273
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William Hart
William Hart
1
Resolution Copper Mining, 102 Magma Heights, Superior, Arizona 85273
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Adam Schwarz
Adam Schwarz
3
Northparkes Mines, Northparkes Lane, Goonumbla, New South Wales 2870, Australia
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Holly Stein
Holly Stein
4
Airie Program, Colorado State University, Fort Collins, Colorado 80523-1482
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Published:
January 01, 2012

Abstract

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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Contents

Special Publications of the Society of Economic Geologists

Geology and Genesis of Major Copper Deposits and Districts of the World: A Tribute to Richard H. Sillitoe

Jeffrey W. Hedenquist
Jeffrey W. Hedenquist
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Michael Harris
Michael Harris
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Francisco Camus
Francisco Camus
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Society of Economic Geologists
Volume
16
ISBN electronic:
9781629490410
Publication date:
January 01, 2012

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