The Morenci mine is an unincorporated joint venture owned 72 percent by Freeport-McMoRan Inc., with the remaining 28 percent owned by Sumitomo Metal Mining Arizona, Inc. (15 percent) and SMM Morenci, Inc. (13 percent).
Summary:
The Morenci mine is a porphyry copper deposit that has oxide, secondary sulfide and primary sulfide mineralization. The predominant oxide copper mineral is chrysocolla. Chalcocite is the most important secondary copper sulfide mineral, with chalcopyrite as the dominant primary copper sulfide.
Deposit Geology
The mineral deposits of the Morenci district consist of copper oxide, secondary sulfide, and primary sulfide mineralization associated with a large porphyry copper system. Geologic studies indicate a complex series of Tertiary igneous intrusive rocks were emplaced within Precambrian-age granite and overlying Paleozoic and Mesozoic sedimentary rocks as shown in Figure 6.1. A porphyry copper deposit formed and was associated with the emplacement and crystallization of intrusive rocks. Several cycles of leaching and enrichment of the primary sulfides formed the secondary sulfide enrichment blanket and copper oxide zones currently being mined. Mineralization spans approximately 5 miles in a north-south direction and 4 miles in an east-west direction.
Rock Types
In the Morenci district, rocks ranging from Early Proterozoic schist and granite through Paleozoic and Cretaceous sedimentary sequences are overlain and intruded by early to middle Tertiary igneous rocks. Following a protracted period of uplift and erosion, the Clifton-Morenci area was covered by up to 3,200 feet of Oligocene volcanic rocks with subsequent erosion resulting in thick late Miocene through Holocene basin deposits that filled structural lows to the east and southwest of the Morenci block.
Major host rocks include the Precambrian basement, which consists of granite to the north and northwest and granodiorite in the southwestern and southeastern portion of the district, and Paleozoic sedimentary rocks which are restricted to fault-bound blocks that occur in the southwestern portion of the district and in the Shannon and Garfield mine areas.
Laramide intrusive activity is manifested in the Morenci district by a staged series of Paleocene to early Eocene hypabyssal intrusions. Laramide stocks, laccoliths, and associated dikes and sills constitute a comagmatic, calc-alkaline series of porphyritic intrusions, ranging in composition from diorite to granodiorite to quartz monzonite and granite. These intrusions are separated into at least six texturally and mineralogically distinct stages. Three of these stages are associated with hydrothermal fluids responsible for porphyry copper-style chalcopyrite-molybdenite stockwork and skarn mineralization: dacite, monzonite, and older granite porphyries.
Alteration and Mineralization
Primary hypogene mineralization is associated with emplacement of Laramide-age granodiorite to quartz monzonite stocks and dike swarms intruded into Precambrian granite and Paleozoic sedimentary rocks. Quartz-sericite-sulfide alteration and attendant copper-molybdenum mineralization is temporally and spatially associated with the emplacement and cooling of these intrusions. Low-grade hypogene copper mineralization is manifested as quartz-sericite alteration with pyrite-chalcopyrite-molybdenite stockwork veins that overprinted early quartz-orthoclase and biotite vein assemblages.
The style and sequence of hydrothermal alteration and mineralization in the Morenci district can be characterized from vein mineral assemblages and crosscutting relationships. As in many other well-studied porphyry copper systems (Nielsen, 1968; Phillips, Gambell and Fountain, 1974; Beane and Titley, 1981), alteration and vein assemblages in the Morenci ore body appear to vary systematically from potassic alteration near the core and deep within the deposit to sericite-dominated alteration in the upper and central portions. A propylitic zone is present in the fringes of the deposit. Crosscutting relationships among veins associated with these discrete alteration assemblages reflect the evolution of fluids responsible for copper-molybdenum mineralization. Key characteristics of hypogene mineralization are that the potassicrelated assemblages are sulfide poor and do not contain significant amounts of copper and the later sericite dominant assemblages contain the bulk of the copper, principally as chalcopyrite.
The supergene zone characteristically displays a massive white appearance reflecting pervasive argillic alteration. Textural destruction is commonly so intense that even coarsegrained granitic textures are obscured, making field identification of lithological units difficult.
Anhydrous skarns containing garnet, diopside, wollastonite, marble and hornfels, and hydrous skarns containing tremolite-actinolite, chlorite, epidote, and magnetite developed where the Laramide porphyries intruded Paleozoic sedimentary units.
Most ore mined from the Morenci district and carried in the current operation is the product of supergene oxidation and enrichment processes. Long-lived multiple supergene cycles resulted in an enriched zone localized in the ancestral Chase Creek Canyon. In supergene sulfide zones, chalcocite occurs as thick coatings and complete replacements of pyrite and chalcopyrite.
The form and distribution of supergene mineral assemblages is largely a function of the physical character of the ore body and the nature of the climate and the hydrologic setting at the time of formation. Faults and fractures provide conduits for infiltration of supergene solutions into the host rocks. Supergene profiles typically mirror the current topographic surface. Enrichment and oxidation zones are generally thicker in valleys and thinner at ridge tops.
The predominant oxide copper mineral is chrysocolla. Chalcocite is the most important secondary copper sulfide mineral, and chalcopyrite and molybdenite are the dominant primary sulfide minerals.