Abstract

Surface mapping, core logging, and compilation of underground mine maps in the Carr Fork area have led to the definition of the stratigraphic, structural, and time-space distribution of copper, copper-lead, and lead-zinc-silver ores in the western contact aureole of the Bingham stock. The succession of alteration-mineralization stages in sedimentary rocks is defined, and each stage tentatively is correlated with stages in the evolution of alteration-mineralization in the Bingham stock.Sedimentary rocks in the contact aureole are of Pennsylvanian age and consist of quartzite with lesser amounts of calcareous carbonaceous siltstone and limestone. Compressive deformation from the southeast during Mesozoic time formed east-west trending folds and bedding plane faults across the district; differential yielding resulted in over-turning of folds in the highly deformed Carr Fork area. Following the main period of folding, stress was relieved by northward thrusting; in the Carr Fork area a set of northwest-striking imbricated thrust or right-lateral strike-slip faults formed in the upper plate of the basal Midas thrust. A system of northeast-striking faults, which were later to serve as major ore solution conduits, originated largely as tension fissures parallel to the direction of maximum principal stress. Multiple intrusions of monzonite and quartz monzonite during mid-Tertiary time were controlled to a large degree by the zone of north-east faults and by steep bedding. An Early Stage of contact metasomatism produced: diopside in quartzite and in interbedded, thin silty limestone beds; wollastonite, with minor idocrase and garnet, in thick cherty limestone; and a trace amount of sulfides. The Early Stage is related at least in part to the emplacement of the quartz monzonite of the Bingham and Last Chance stocks and continued during the initial emplacement of the quartz monzonite porphyry. Actinolite alteration of diopside in quartzite and garnetization of wollastonite-bearing marble represent the beginning of Main Stage mineralization and are time equivalent with biotite-orthoclase alteration of igneous rocks. As the stock is approached, actinolite alteration in quartzite and hornfels exhibits textural changes from bedding streaks to orbicules and to envelopes on fractures; closer to the instrusion, actinolite is altered to biotite along fractures. Traces of pyrite accompany actinolite in the outermost zones of bedding streaks and orbicules. Chalcopyrite accompanies pyrite as the fracture filling in the envelope zones. Near the stock, molybdenite veinlets have primarily biotite envelopes. Crosscutting veinlet relations indicate that the biotite zone expanded from the stock onto the outer actinolite zone during a trend toward increasing chalcopyrite: pyrite and molybdenite: Cu-Fe-sulfide ratios. Late pyritic veinlets with actinolite envelopes represent the waning of Main Stage mineralization in quartzite.Two cherty limestone beds, 15 to 60 m thick and separated by 100 m of quartzite, contain the major copper-bearing skarns in the district. Main Stage alteration in limestone consists of andradite-diopside superimposed on Early Stage wollastonite. Actinolite-diopside-garnet-epidote endoskarn occurs in quartz monzonite adjacent to garnetized limestone. In wollastonite, sulfides are chalcopyrite and bornite, with minor galena and sphalerite. Chalcopyrite: pyrite ratios increase from the outer edge of the andradite-diopside zone toward the intrusive contact, where trace amounts of bornite reappear. Chalcopyrite: pyrite ratios also increase with depth. Massive magnetite replaced garnet adjacent to the stock at lower elevations, and hematite is absent. At higher elevations, the skarns have a lower iron-oxide content, hematite appears, and the hematite: magnetite ratio increases to unity near the surface. These relations suggest an increase in oxidation and sulfidation states toward the surface and away from the stock. The Main Stage of ore deposition in limestone skarns culminated in the partial destruction of andradite and diopside. Apparent reactions include: carbonation of andradite yielding calcite and quartz with hematite, magnetite, or siderite; and combined carbonation and sulfidation, yielding pyrrhotite, pyrite, and chalcopyrite in addition to calcite and quartz. In the more diopside-rich skarn of the outer zones, alteration assemblages accompanying sulfide deposition include andradite-actinolite or actinolite in addition to the products of sulfidation and carbonation. These reactions may have been largely the result of declining temperatures; it is unclear whether any new Cu, Fe, and S was introduced at this stage.A Late Stage of alteration produced pyrite, chlorite, montmorillonite, sericite, and talc from earlier calc-silicates and locally redistributed chalcopyrite. Lead-zinc and gold deposits, accompanied by arsenic-bearing minerals such as tennantite and arseno-pyrite, also belong to this time frame. The Late Stage is believed to be contemporaneous with sericite-pyrite alteration of intrusive rocks.The following metal zones and their maximum right-angle distances from the stock contact are recognized: (1) lead-zinc-silver northeast fissure ore (Pb/Cu = 30 to 10) in nonsilicated limestone, up to 1,200 m; (2) lead-copper northeast fissure ore (Pb/Cu = 5 to 0.2) in nonsilicated limestone, up to 900 m; (3) copper ore (Pb/Cu < or = 0.02) in garnet skarn, up to 450 m. These zones do not represent a contemporaneous zonal growth pattern but rather reflect a complex history of expansion and regression of zones dependent on rock compositions, permeabilities, and the evolution of the ore fluid.

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