Robinson District - Robinson, Ely, Ruth, Liberty, Tripp-Veteran, Emma, Kimbley, Minnesota, Tonopah, Richard, Alpha, Wedge, Taylor
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The Robinson (or Ely) Mining District is located ~4 km west of the town of Ely, White Pine County, in eastern Nevada, 450 km east of Reno and 350 km north of Las Vegas (#Location: 39° 15' 49"N, 114° 59' 39"W).
Mining in the district can be traced back to 1867 when prospector Thomas Robinson discovered silver and gold, along with indications of widespread, low-grade copper. In 1872, more significant copper was found in the area of the current mine, and a year later relatively small scale exploitation of the deposits commenced. Between 1892 to 1907, several gold mines were also opened, but none were successful. In 1900, a considerable tonnage of supergene mineralisation grading from 2 to 4% Cu was discovered on the Ruth claims. A new company, Nevada Consolidated Copper Corporation was formed in 1904 to begin open pit mining as the Veteran pit in 1907, based on the continuation of this supergene mineralisation. In 1932, Nevada Consolidated Copper became a wholly owned subsidiary of Kennecott Copper. While large scale open pit mining continued, underground mining persisted in sections of the deposit until the 1970s when open pits were developed to include the lower grades surrounding the high grade underground ore. Through much of the period the deposit was exploited, it was mined by a number of different companies that were progressively amalgamated, including the formation of Consolidated Copper Mines Co. in 1913, before all were finally consolidated under Kennecott Copper Corporation in 1958. Kennecott closed its Nevada mines, including the Robinson District operations, in 1978. During the mid 1980s the District was owned by Alta Gold who mined strictly for gold. In 1991, Magma Copper bought the Robinson operation and began work on reopening the mine in 1994 to extract the lower grade hypogene ores by open pit, before being taken over by BHP in 1996. BHP closed the operation in 1999 due to low copper prices. Quadra Mining bought Robinson in 2004, and reopened the mine later that year. In 2012, Quadra was acquired by KGHM Polska Miedź S.A.. Overall, from 1872 to the present (2021), with some breaks and numerous changes of ownership, mining of copper, gold and silver and more recently molybdenum, has continued at varying scales for almost 150 years.
The district comprises a generally linearly disposed east-west group of deposits which extend over a distance of around 9 km. These are in turn enveloped by an alteration zone that covers an area of some 13 x 1 km with a similar elongation. Within this interval 12 orebodies had been exploited up to 1963, with productions ranging in descreasing order of size (Bauer, et al., 1966):
Liberty - 165 Mt @ 1.09% Cu; Tripp - 22 Mt @ 0.80% Cu; Ruth - 17.5 Mt @ 1.66% Cu;
Emma - 15 Mt @ 0.91% Cu; Veteran - 6 Mt @ 1.09% Cu; Kimbley - 5 Mt @ 0.97% Cu;
Minnesota - 1 Mt @ 0.83% Cu; Tonopah - 0.5 Mt @ 0.87% Cu; Richard - 0.25 Mt @ 4.24% Cu;
Alpha - 0.12 Mt @ 7.08% Cu; Wedge - 0.025 Mt @ 1.58% Cu; Taylor with 0.006 Mt @ 4.46% Cu .
Published cumulative production and reserve figures for the Ely District are as follows:
205 Mt @ 1.14% Cu (Production to 1960, Gilmour, 1982), or
255 Mt @ 1.1% Cu (Production 1908 to 1963, Einaudi, 1982).
Reserves in 1976 were 82 Mt @ 0.67% Cu.
Published Ore Reserves and Mineral Resources in December 31, 2014 were (after miningdataonline.com viewed February, 2021):
Proved + Probable Reserves - 119.374 Mt @ 0.41% Cu, 0.15 g/t Au
Measured + Indicated Resources - 358.115 Mt @ 0.45% Cu, 0.18 g/t Au for 1.611 Mt of contained copper
Inferred Resources - 11.942 Mt @ 0.38% Cu, 0.18 g/t Au.
Reserves in 2017 were stated at 0.2565 Mt of contained copper (KGHM Polska Miedź S.A.).
In 2019 the remaining operation comprised three large interjoined open pits, Liberty, Tripp-Veteran and Ruth distributed over an east-west interval of ~9 km. This composite pit embraces almost all of the individual historic mines listed above. Production in 2019 was restricted to the central Ruth pit. Plans at that stage were for the mine to operate until 2022 before closure and reclamation.
The Ely group of deposits lies near the centre of the Great Basin in eastern Nevada, just to the west of the Sevier-Cordilleran Foreland Thrust Belt. The deposit is located near the thickest section of the Palaeozoic sequence of the Cordilleran Trough, to the west of the hinge line marking the western limit of the Interior Platform. This sequence, which is around 11 000 m thick in the district, ranges from Ordovician to Permian in age and comprise limestones, dolomites, quartzite, sandstone and lesser black shale, overlain by Tertiary to Quaternary fluvial to lacustrine and volcanic cover. Rocks older than Devonian are restricted to the northeastern part of the district and are not found in the deposit area (Bauer, et al., 1966).
The Palaeozoic sequence in the Robinson District may be summarised as follows, from the base (after Burns, 2006):
Pogonip Group comprising the 170 m thick Kanosh Shale composed of shale, calcarenite, silty limestone, sandstone and calcisiltite; and the overlying Lehman formation which is an ~180 m thick unit of flaggy, dark grey, orange to red mottled, medium-grained limestone and calcisiltite.
Eureka Quartzite which is ~150 m thick; it is a sheet-like quartz-arenite deposited on the eastern shelf of the Cordilleran miogeocline from Canada to California, and is remarkable in its purity of detrital and authigenic quartz and scarcity of bedding, often occurring as a white sugary quartzite;
Ely Springs Dolomite that is ~65 m thick and is composed of argillaceous and sandy dolostone, dark grey dolostone;
Laketown Dolomite - ~580 m of light to medium grey, medium grained massive dolostone;
Guilmette Formation - 765 m of argillaceous limestone;
Pilot Formation - 120 m of calcareous siltstone and silty shale;
Joanna Formation - 105 m of thickly bedded bioclastic limestone;
Chainman Formation - 120 to 450 m that is characterised with four distinct lithologies, siliceous sandstones, black organic limestones, black fossile shales and calcareous siltstones;
Ely Formation - 700 m of light grey to dark grey, bedded sandy limestone, which is the major host rock to mineralisation;
Reipe Springs Limestone - 75 m of light brown to grey, massive-bedded limestone;
Rib Hill Formation - 330 m of tan calcareous sandstone that weathers to a brick red colour, with thin interbedded fossiliferrous fusulinid limestones;
Arcturus Formation - 850 m of tan to grey limestone. The upper portion is a sandy limestone; the middle a pure limestone with white calcite spotting and bryozoan fossils; whilst the lower section is a slightly sandy limestone with colonial coral fossils.
Monzonite to Quartz Monzonite Porphyry - 121 to 123 Ma (K-Ar; Bauer, et al., 1966) porphyritic intrusive that has been strongly altered during mineralisation. The unaltered Weary Flats Pluton is light grey to dark grey composed of plagioclase, K feldspar and hornblende with white plagioclase feldspars that are rarely >2mm and are often argillically altered in a matrix that encloses characteristic large feldspar phenocryst. The quartz monzonite end member has increased quartz, but is otherwise similar. Accessory minerals include magnetite, titanite, apatite and zircon.
Rhyolite - 37 Ma porphyritic rhyolite that contains abundant clear and smoky sub-hedral quartz, cloudy to clear sanidine. Occurs as diatremes, breccias and Tuffs.
Mineralisation is associated with a lower Cretaceous monzonite stock dated at 121 to 123 Ma. This monzonite occurs as both a stock and as associated dykes, sills and irregular bodies, and ranges from monzonite to quartz-monzonite (adamellite) in composition. Both compositional phases contain K-feldspar, plagioclase and hornblende, while the monzonite has subordinate augite and the quartz-monzonite (adamellite) has more quartz. Accessory minerals are magnetite, sphene, apatite and zircon. The quartz-monzonite is porphyritic. This change from a granular monzonite to a porphyritic quartz-monzonite occurs 'upwards' within the mineralised stocks. Post ore rhyolites of Eocene to Oligocene (41 Ma) age cut and overlie the monzonites and occur as plugs, dykes, sills and diatremes as well as flows, agglomerates and tuffs (Bauer, et al., 1966).
The most notable feature of the district is the east-west alignment of monzonite intrusions, alteration and mineralisation, although the trend of faulting is predominantly north to NNW. This east-west trend corresponds to what is referred to as an 'accommodation zone' within the north-south Egan Range. To the north, the strata of the range are tilted to the west, while to the south it tilts to the east. Within the accommodation zone, where the porphyry intrusions are concentrated, the units are compressed into folds and segmented by dozens of small faults related to this rotational event (Burns, 2006). These monzonite bodies range from a few hundred metres across to around 1000 x 500 m. No major faults are mapped along this trend although regional structure suggests movement on a major basement dislocation not cutting the Palaeozoic cover. Pre-monzonite, west dipping thrust planes are mapped in the district, probably related to the Sevier Orogeny, while younger extensional detachment surfaces are also defined (Bauer, et al., 1966).
At least three episodes of extension and normal faulting preceding and during the Mid Tertiary basin and range event dissected and rotated the hypogene copper deposits to their current attitude and also resulted in extensive oxidation, leaching and supergene enrichment. 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 current surface, suggesting most supergene activity post-dated this faulting.
Mineralisation and Alteration
The copper deposits of the district occur within the east-west trending zone of alteration defined by outcrops of bleached limestone, jasperoid, leached porphyry and limonitic staining. Most of the copper ore occurs within the altered porphyry, although about 20% of the total production has come from the adjacent metamorphosed sediments. There are three types of ore, namely,
i). Primary disseminated ore which is mainly chalcopyrite and occurs within the altered porphyry and adjacent sediments. The primary ore in the porphyry ranges in grade from the cut-off of 0.4%, up to 2% Cu, with large parts of some porphyry bodies averaging more than 1% Cu (Bauer, et al., 1966). Prior to 1963 about 80% of the ore was of this type, although the importance of replacement ore increased in subsequent years (Einaudi, 1982).
Throughout the district the bulk of alteration within the monzonite is pervasive and only locally related to veining. Alteration is a) Biotite-argillic, which is present at greater depths, and grades upwards into the sericitic zone. The K-feldspar of the intrusive is not affected, while hornblende is altered to biotite, then to phlogopite. Plagioclase is altered to montmorillonite, illite and finally sericite as the the sericitic zone is approached (Bauer, et al., 1966). According to Einaudi (1982), major portions of the stocks contain hypogene ore grade mineralisation associated with biotite-orthoclase alteration. Pyrite:chalcopyrite ratios average around 4:1. b) Sericitic, which corresponds to the change from a monzonite to the porphyritic quartz monzonite in the upper sections of the intrusives. Within this zone, alteration does affect the K-feldspar of the intrusive, producing sericite and quartz (Bauer, et al., 1966). Again according to Einaudi (1982), the porphyry is altered at high levels to a porous quartz-sericite-pyrite aggregate with subordinate, but sub-ore grade chalcopyrite. Areas of pervasive sericite alteration grade into biotite-orthoclase alteration cut by quartz-sericite-pyrite veinlets, ie. the sericite alteration overprints the biotite-orthoclase. c) Propylitic, which occurs outside of the ore grade mineralisation.
Disseminated sulphides first appear in the initial stages of alteration, where the pyrite:chalcopyrite ratio is 1:1. Chalcopyrite, after reaching a maximum in the biotite-argillic zone, diminishes to a trace in the most intense sericitic alteration (Bauer, et al., 1966).
ii). Replacement ore in silicified limestone adjacent to large masses of porphyry. In some cases this ore may be present adjacent to sub-ore porphyry, although in general the porphyryr ore is higher in grade. Copper grades are erratic, although in general they decrease away from the porphyry, extending up to 150 m, but averaging around 60 m from the contact. Xenoliths and salients of limestone within the altered porphyry, and lenses of limestone within shales or contacts of those two lithologies are commonly higher grade than the surrounding rock. Occassional large pods of massive sulphide up to 15 m long are found within the silicified limestone, composed principally of pyrite, some pyrrhotite and lesser chalcopyrite. Metamorphosed shale is not as productive as the limestone, although low grade ore up to 30 m from the contact has been mined in the Liberty and Kimley pits (Bauer, et al., 1966).
Two major altered and mineralised styles occur in this ore type, namely calcic-skarn and silica-pyrite. The calcic-skarn is developed adjacent to biotite-orthoclase alteration in the porphyries, and typically extends up to 100 m outwards from the intrusive contact. Limestone is converted to a skarn composed of garnet, chlorite, epidote, magnetite, specularite and sulphides. Garnet, mainly andradite, tends to occur near the contact with garnet-pyroxene (diopside) near the contact with marble. The contact between the skarn and marble is marked by a 5 to 10 m thick zone of wollastonite-quartz, with minor diopside, idocrase and green garnet. Beyond the skarn, bleaching and re-crystallisation, accompanied by local white clay, occur up to 300 m from known mineralised stocks. The majority of ore in skarn is associated with quartz-calcite-sulphide-magnetite veinlets occurring within 60 m of the porphyry contact. The selvages of these veins vary with the composition of the enclosing skarn. The silica-pyrite alteration is closely associated with sericitic alteration in the porphyry, resulting in massive quartz-pyrite alteration of limestones. Analyses of typical quartz-pyrite yielded 28% quartz, 23% pyrite and 48% calcite. In some cases the replacement is of earlier skarn, while in others direct alteration of limestone is indicated (Bauer, et al., 1966; Einaudi, 1982).
Shale is metamorphosed to andalusite-biotite hornfels adjacent to biotite-argillic altered monzonite, and to quartz-sericite hornfels along-side sericitic altered monzonite. Silicification of sandstone has produced dense masses of 'jasperoid' surrounding and overlying sericitic monzonite (Bauer, et al., 1966).
iii). Supergene enriched ore which often attains a thickness of up to 100 m, and overall ranges from 1 to 2.5% Cu, with common 4 to 5% Cu sections. Chalcocite replacing primary sulphides is the principal supergene mineral although covellite is present locally. The depth of oxidation ranges from 30 to 120 m, being greatest over the monzonite. Oxide copper is seldom found at the top of the supergene zone, although high grade Cu ore was mined from the limestones in the Alpha and Richard pits (Bauer, et al., 1966).
Chalcopyrite and pyrite are the principal primary sulphide minerals, with sporadic, minor intergrowths of bornite, bornite-chalcopyrite and molybdenite. In mineralised monzonite, the sulphides are disseminated as discrete grains, in veins and along fractures. Chalcopyrite occurs essentially as disseminated grains, and only sparingly in veins, almost always with pyrite. Pyrite is equally abundant as disseminations, in veins and on fractures. The pyrite:chalcopyrite ratio ranges from 1:1 to 10:1, averaging 5:1. Sulphides in sedimentary rocks are both disseminated and massive, being pervasively distributed in all sediments in the heart of the mineralised zone. In addition to pyrite and chalcopyrite, magnetite and specular hematite are also often abundant. Minor fissure and replacement base and precious metals occur peripheral to the Cu mineralisation (Bauer, et al., 1966).
Geology and Mineralisation of the Main Pits
The geology of each of the main pits may be summarised as follows after miningdataonline.com (viewed February, 2021):
• Tripp-Veteran Pit
Monzonite porphyry intrudes the Lower Carboniferous Chainman formation shales, Upper Carboniferous Ely formation cherty limestones, basal Permian Reipe
Springs limestone and Lower Permian Rib Hill sandstone in the Tripp-Veteran open pit.
Carbonate and fine grained clastic wall rocks on the peripheries of the ore deposit have been recrystallised to marble and hornfels respectively, whilst in the shallower and proximal parts, there is a strong hydrolytic overprint of the prograde skarn and potassium silicate assemblages. This has resulted
in retrograde skarn assemblages and intensive and extensive quartz-sericite-pyrite phyllic alteration in the intrusions. Near surface pyrite rich
retrograde and phyllic zones have subsequently been oxidised and leached, with extensive chalcocite enrichment superimposed on the hypogene system.
An intact sequence of upper Ely formation cherty limestones, Reipe Springs limestone and Rib Hill sandstone with a moderate dip and ESE strike is exposed on the western and southern benches of the open pit. The northeastern wall of the pit marks the Footwall West fault, which separates the pit from the Weary Flat structural block - characterised by a north striking, westward facing, steeply dipping sequence of lowermost Ely formation, Chainman shale, and the underlying basal Carboniferous Joana limestone, and Devonian Pilot shale and Guilmette formation. Monzonite porphyry is intrudes both of these stratigraphic domains.
• Liberty Pit
Both supergene and hypogene sulphide orebodies in this pit were hosted in altered porphyry and skarns of the Ely and upper Chainman formations. Rocks on the south side of the pit comprise 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.
• Ruth Pit
In this pit, the Chainman, Ely, Reipe Springs and Rib Hill formations have been intruded by monzonite porphyry. Historically production has been hosted in the monzonite porphyry and surrounding skarns developed within 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 by the west dipping High Grade fault. Rocks above and below this fault are strongly altered, although the best hypogene mineralisation is hosted by the porphyry and, to a lesser extent skarn, below the fault. Grades above the fault are generally lower and a lot more erratic. The Ruth ore body is separated from the Liberty to the west by the Eureka fault, a 35°E dipping structure that has a >900 m down-throw to the east. It is bounded on the east by the 40 to 50°E dipping Queen fault with >150 m of normal displacement. All of
these faults offset intrusive rocks and hypogene alteration and mineralisation and are mid-Tertiary in age.
The most recent source geological information used to prepare this summary was dated: 2006.
Record last updated: 6/2/2021
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
Bauer H L, Breitrick R A, Cooper J J, Anderson J A 1966 - Porphyry copper deposits in the Robinson Mining District, Nevada: in Titley S R, Hicks C L 1966 Geology of the Porphyry Copper Deposits, Southwestern North America University of Arizona Press, Tucson pp 233-244|
Westra G 1982 - Alteration and mineralization in the Ruth porphyry copper deposit near Ely, Nevada: in Econ. Geol. v77 pp 950-970|
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