Summary:
Copper mineralization at Robinson occurs in monzonite porphyry intrusive rocks and associated marginal garnet skarns with varying quantities of pyroxene, magnetite and sulfide which formed in the surrounding upper Paleozoic wall rocks. Early stage potassium silicate alteration in porphyry and skarn mineralization in the wall rocks are typically overprinted by a widespread and commonly intensive high acid alteration event which altered porphyry and some host units to quartz-sericite-pyrite or even advanced argillic assemblages locally. Carbonate host rocks were altered to pyritic marbles or silica pyrite rock containing up to 20 volume percent pyrite locally during this late stage alteration event. Early stage potassium silicate mineralization and skarn was dominated by pyrite and chalcopyrite and is generally low in sulfide with average values ranging from trace to 3%. The high acid alteration event is dominated by up to 10% or 20% pyrite with pyrite:chalcopyrite ratios typically ranging from 10:1 to more than 1,000:1. Where present in the late stage alteration zones, the chalcopyrite may be relict from earlier stage alteration events. Chalcopyrite appears to be the sole hypogene copper mineral, with a few possible exceptions.
Skarn and porphyry ores form much of the current resource and reserve with grades ranging from cutoff (and below) to greater than 1.0%. Metallurgical recoveries in this material are generally good, but are complicated where weathering has partially altered the sulfides or silicate gangue.
Mid-Tertiary extension and normal faulting dissected and rotated the hypogene copper deposits to their current configuration and they have been extensively oxidized, leached and enriched. The oxidation and enrichment is extensive, generally pervasive and intense. Faults and fracture zones locally modify the morphology of the leached and enriched zones, but in many places the upper surface of the original enrichment blankets parallel the surface. In light of this, it seems likely that much of the weathering and enrichment took place after the mid-Tertiary faulting.
Geology and Mineralization of the Tripp-Veteran Pit
Monzonite porphyry, historically known as the “ore porphyry” intrudes Chainman formation shales, Ely formation cherty limestones, Reipe Springs limestone and Rib Hill sandstone in the Tripp-Veteran open pit.
Carbonate and fine grained clastic wall rocks on the distal margins of the ore deposit are recrystallized to marble and hornfels respectively. Shallower and proximal portions are characterized by strong hydrolytic overprinting of the prograde and potassium ilicate assemblages, yielding retrograde skarn assemblages and intensive and extensive quartzsericite-pyrite alteration in the intrusions. The nearer surface pyrite-rich retrograde and quartz- sericite-pyrite zones subsequently were oxidized and leached, and extensive chalcocite enrichment was superimposed on the system.
A moderately dipping, east-southeast striking, intact sequence of upper Ely formation cherty limestones, Reipe Springs limestone and Rib Hill sandstone are exposed on the western and southern benches of the open pit, generally above the 6700 elevation, on average. The northeastern wall of the pit is formed by the Footwall West fault, which separates the pit from the Weary Flat structural block - characterized by a north striking, westward facing, steeply dipping equence of lowermost Ely formation, Chainman shale, Joana limestone, Pilot shale and Guilmette formation. Monzonite porphyry is present within both of these stratigraphic domains.
The northwest trending Tripp-Veteran pit is separated from the Weary Flat structural block on the northeast by the steeply southwest dipping Footwall West fault, an ore boundary. Drill substantiated mineralization is present to the north of the Footwall West fault but lies outside the current pit design.
Geology and Mineralization of the Liberty Pit
Supergene and hypogene sulfide ore bodies in the Liberty pit were hosted in altered porphyry and skarns in the Ely formation and upper parts of the Chainman formation. Rocks on the south side of the existing pit are now intensely weathered leached cap, probably after silica-pyrite altered limestones and calcareous sandstones of the Ely and Rib Hill formations, the few in-situ exposures of Chainman formation on the north are also intensely weathered leached cap.
Geology and Mineralization of the Ruth Pit
Chainman, Ely, Reipe Springs and Rib Hill formations were intruded by monzonite porphyry within the Ruth open pit, and historically mined mineralization was hosted in porphyry and surrounding skarns of the Ely and Chainman formations. The Chainman and lower part of the Ely formation are separated from strongly altered limestone and sandstones of the upper Ely and the overlying Permian units along the west dipping High Grade fault. Rocks above and below this fault are strongly altered, but the best hypogene mineralization lies in porphyry and, to a lesser extent skarn, below the fault. Grades above the fault are generally lower and considerably more erratic. Geologically, the Ruth ore body is separated from the Liberty on the west by the Eureka fault, a 35º east dipping fault with more than 3,000 feet of down to the east movement, and bounded on the east by the Queen fault, a 40º to 50º east dipping fault with 500 feet or more of normal movement. All of these faults offset intrusive rocks and hypogene alteration and mineralization and are mid-Tertiary in age.