Alteration and mineralization in the Ruth porphyry copper deposit near Ely, Nevada

Abstract

The Ruth-Ruth Extension porphyry copper deposit in the Robinson mining district, east central Nevada, represents the eastern extremity of a once-continuous, large sulfide system that has been fragmented by normal faulting in early mid-Tertiary time. Mineralization and alteration are confined to a miogeoclinal sedimentary sequence ranging in age from Devonian to Permian and to Early Cretaceous quartz monzonite porphyry apophyses of a larger pluton that underlies the district. Prior to intrusion the sedimentary rocks were folded into a northwest-trending, locally overturned anticline. Bedding plane thrusting greatly reduced the thickness of the fiat-dipping upper limb of the anticline, and at least six tectonic units separated by fiat faults can be distinguished. Ore-grade hypogene copper mineralization only occurs within the steeply dipping lower limb of the anticline.Alteration and mineralization are spatially and temporally related to the emplacement of an early quartz monzonite porphyry pluton. In the mine area this intrusion is cylindrical and plunges 25 degrees west with smaller offshoots penetrating the overlying sedimentary rocks. At the lower west end of the cylinder this early quartz monzonite porphyry was intruded by, and graded into, a weakly altered and weakly mineralized late quartz monzonite porphyry. A granodiorite porphyry sill exposed in the southwest wall of the Ruth pit postdates mineralization. Intrusion of the early quartz monzonite porphyry resulted in the formation of biotite-andalusite hornfels in the Mississippian Chainman Shale and massive garnet skarn in Pennsylvania Ely Limestone. In contrast to the sulfide-rich garnet-pyroxene skarns elsewhere in the district, the predominant skarn type at Ruth is a massive garnet skarn low in sulfides. Following crystallization of this early porphyry phase a major fracturing event shattered the porphyry and surrounding hornfels but left the massive garnet skarn unaffected. At this time an extensive zone of biotite-orthoclase alteration formed in early porphyry and in the directly surrounding hornfels, which subsequently was largely obliterated by pervasive quartz-sericite alteration. Limestone within the zone of quartz-sericite alteration was replaced by an assemblage of quartz, chalcedony, and pyrite, locally termed silica-pyrite rock which at higher elevations gives way to pyritic marble. Chainman hornfels adjacent to Ely skarn contains epidote, actinolite, diopside, garnet, calcite, pyrite, chalcopyrite, and magnetite along fractures. At the Ely-Chainman contact there is a magnetite-rich zone which also contains pyrite, pyrrhotite, and chalcopyrite.Early quartz monzonite porphyry outside the quartz-sericite zone of alteration has been argillized. An inner zone with predominantly kaolinite and an outer zone--the clay-calcite zone with montmorillonite, kaolinite, and calcite--have been distinguished. Limestone within the kaolinite zone was converted to a pyritic marble, whereas in the clay-calcite zone it was only recrystallized. The marble halo extends for 460 m south of the Ruth deposit and contains minor amounts of wollastonite and tremolite in addition to calcite and chert. Within the marble halo minor structurally controlled lead-zinc-silver mineralization occurs. Early quartz monzonite porphyry apophyses and Permian Rib Hill Sandstone above the top of the porphyry cylinder contain quartz, kaolinite, alunite, zunyite, pyrite, and minor sericite, minerals characteristic of advanced argillic alteration.Early quartz monzonite porphyry in the core of the hydrothermal system shows biotiteorthoclase alteration and contains chalcopyrite mineralization. An increase in quartz vein density at the upper margin of the cylinder resulted in formation of a zone of pervasive silicification at the interface of the quartz-sericite and biotite-orthoclase alteration zones. This high silica zone contains high-grade chalcopyrite mineralization. Ore-grade hypogene copper mineralization (>0.4%) is largely restricted to the early quartz monzonite porphyry in the Ruth and Ruth Extension deposits and forms a 25 degrees west-dipping tabular ore zone. Following the crystallization of the late quartz monzonite porphyry, the early quartz monzonite porphyry within the biotite-orthoclase zone was intensely argillized. The unfractured late quartz monzonite porphyry is only weakly affected by this event.A pronounced vertical alteration zoning in the hydrothermal system can be demonstrated by the distribution of alteration assemblages in Ely Limestone. In areas above the hypogene ore zone, silica-pyrite rock, pyritic marble, and marble are found as one moves upward and outward from the center of the system. Laterally away from the ore zone, garnet skarn and marble alternate, but silica-pyrite rock is absent and pyritic marble is only rarely found. The asymmetric distribution of mineralization and alteration assemblages with respect to the porphyry cylinder and the inclined attitude of the ore zone and alteration zones are not the result of postmineral low-angle faulting and rotation. The alteration and metal distribution patterns indicate that hydrothermal fluids entered the fractured porphyry at its lower west end and migrated toward the top of the cylinder to the east. Middle Tertiary normal faulting exposed the high-grade hypogene mineralization to oxidation and leaching, producing a mature chalcocite blanket.