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Pumpkin Hollow |
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Nevada, USA |
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Cu Au Fe
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Super Porphyry Cu and Au
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IOCG Deposits - 70 papers
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All papers now Open Access.
Available as Full Text for direct download or on request. |
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The Pumpkin Hollow high grade chalcopyrite-magnetite deposit is located ~13 km southeast of the Yerington and Ann Mason porphyry copper deposits and town of Yerington, in Lyon County, Nevada, USA (#Location: 38° 54' 46"N, 119° 4' 48"W)
These deposits are located along the flanks of the copper producing Yerington batholith where the district wide mineral inventory in porphyry, skarn and IOCG deposits is >11 Mt of copper. The Pumpkin Hollow deposits are blind, with a halo of hornfels and un-mineralised skarn being the only surface expression. They were first discovered by US Steel Corp. in 1959 by airborne magnetics. Follow-up magnetic surveys and drilling outlined a number of magnetite-rich bodies accompanied by skarn alteration, commonly containing >40% Fe, hosted by Mesozoic sedimentary and igneous rocks. Subsequent work by Anaconda and other companies outlined copper rich areas within the system containing up to 20 % Cu. As of 2014, five deposits of copper-magnetite mineralisation and associated zones of skarn alteration have bee delineated. These are the North, South, Southeast, East, and E2 deposits.
Geologic Setting
The Pumpkin Hollow deposits lie within the western Great Basin of the Basin and Range Province, near the irregular and gradational eastern physiographic margin with the Sierra Nevada range. The eastern slope of the range is dislocated by a number of major north-trending normal faults, controlling horst blocks that form north trending ranges and valleys which are connected to the main mass of the Sierra Nevada to the south, but diverge from it to the north.
Two of these blocks form the Singatse and Wassuk Ranges that flank the Mason Valley, to the west and east respectively. The Pumpkin Hollow deposits are located in the basin of the Mason Valley, between these two ranges. The prominent Singatse Range has a north-south trend and is ~40 long. It is broadly, a west-tilted fault block composed of pre-Tertiary granitic and metamorphosed sedimentary, volcanic and volcaniclastic rocks, unconformably capped by ~1000 m of westerly-dipping Tertiary rhyolitic tuffs, andesites, sediments, and basalts. The eastern face of the range is an irregular normal fault scarp with an ~500 m downthrow to the east.
The Wassuk Range, to the east, is similarly asymmetric, with a steep eastern, and gradual western slope. Thick sequences of Tertiary rhyolite tuffs on the western slope of the range, east of Pumpkin Hollow, unconformably overlay Triassic granodiorites and dip at ~30°W, reflecting a large west-tilted block. As such, the Mason Valley is a half graben between the two west tilted blocks of the Singatse and Wassuk Ranges.
The Yerington District is situated in the west-central portion of the Mason Valley and is underlain by a sequence of Mesozoic metavolcanic and sedimentary rocks which have been intruded and mineralised by the Jurassic-age Yerington batholith. During the late Cretaceous and early Tertiary, the Mesozoic rocks were deeply eroded, and unconformably overlain by a thick succession of Tertiary volcanic and sedimentary rocks. All units have subsequently been steeply tilted to the west, and are displaced into numerous blocks by easterly-dipping normal listric faults.
The oldest rocks exposed in the Yerington-Pumpkin Hollow district are a sequence of metamorphosed volcanic and sedimentary rocks deposited in an early Mesozoic volcanic arc near the then western margin of the North American continent. From oldest to youngest, the main rock units comprise:
McConnell Canyon Formation - a Triassic andesitic to rhyolitic volcanic sequence;
Malachite Mine Formation - a sequence of interbedded fine clastic sediments, carbonates, tuffaceous sediments and tuffs;
Mason Valley Formation - a limestone and calcareous argillite unit;
Gardnerville Formation - Upper Triassic to Lower Jurassic siltstones, argillites, and silty limestone;
Ludwig Formation - consisting primarily of limestone, gypsum and quartzites;
Artesia Lake Formation and Fulstone Spring Formation - comprising Middle Jurassic volcanic andesites and latites, respectively.
This sedimentary-volcanic package, totals ~3000 m in thickness, and was folded and metamorphosed during the emplacement of Middle Jurassic and Cretaceous-age granitoid intrusives.
Some 80% of the exposed pre-Tertiary rocks in the Yerington District are Mesozoic plutonic rocks, and the area is considered part of the Sierra Nevada Batholith. Several large batholiths of granodioritic to quartz monzonitic (adamellite) composition were emplaced during an episode of igneous activity in the Middle Jurassic. The Yerington batholith, which was emplaced at 169 Ma, hosts three porphyry deposits (Yerington Mine, Ann Mason, and The Bear-MacArthur-Lagomarsino) and is associated with several magnetite and copper skarn altered deposits (Minnesota, Ludwig, Castings Copper, Douglas Hill, Bluestone, Mason Valley, Western Nevada Mine, and the Pumpkin Hollow Project deposits).
The Yerington Batholith, which covers an area of ~15 x 15 km, is a composite, I-type, magnetite series, calc-alkaline intrusive complex with a composition that ranges from hornblende-quartz monzodiorite through hornblende-biotite quartz monzonite to late-stage hornblende-biotite granite. The youngest intrusions, which comprise strongly porphyritic granite dykes, are closely associated with porphyry copper mineralisation. These dykes, which occur as NW striking, steeply dipping swarms are controlled by regional fracture patterns. Three separate swarms have been recognised within the batholith, each localised above individual cupolas of a deeper granite pluton.
The 165 Ma Shamrock Batholith truncates the Yerington Batholith to the south and is barren of any significant copper mineralisation or related skarn alteration. Late Cretaceous plutons mapped in the Wassuk Range are also barren. The Yerington Batholith is apparently the only productive pluton in the district.
Deposit Geologic
Much of the Pumpkin Hollow deposit is covered by a thin veneer of Quaternary gravel, which forms the floor to most of the Mason Valley. The western section of the deposit lies within a slight elevation that rises to ~120 m above the surrounding countryside, exposing bedrock, primarily an Oligocene tuffaceous volcanic assemblage. This assemblage comprises the:
Mickey Pass Tuff unit, a package of quartz-latite ash flows and crystal tuffs, overlain by,
Singatse Tuff Formation, which includes lithic-rich tuffs, conformably overlain by,
Bluestone Mine Tuff Formation, composed of unwelded rhyolitic tuffs and ash flows.
This volcanic package has a NNW strike and dips steeply to the west.
In the southwestern section of the deposit area, drilling and limited outcrop indicates that the bedrock is dominated by Mesozoic units, including:
Mason Valley Formation, Late Triassic, medium blue, carbonaceous, skarn altered rocks that are exposed locally, and host scattered copper showings that have attracted small-scale pitting in the past.
Possible Gardnerville Formation, Early Jurassic fine-grained felsitic siltstones and tuffs, overlain by thin bedded carbonaceous, calcareous argillites, tuffs, and limestones, conformably overlain by,
Ludwig Formation, blue-grey laminated limestones.
These pre-Tertiary rocks trend generally to the NE with steep dips to the NW. The pre-Tertiary sequence is cut by east-west striking Jurassic porphyritic quartz monzonite dykes that are synchronous with late-stage phases of the Yerington Batholith. Drilling reveals that most of the of variably inclined limestones of the Mason Valley Formation and calcareous argillites and siliceous shales, siltstones and limestones of the Gardnerville Formation are intruded by Jurassic granodiorite and diorite, and in the Pumpkin Hollow area, a large, centrally located, intrusive body is flanked by a semi-circular rim of altered and mineralised carbonate rocks of these two formations.
The Mesozoic-Tertiary contact is a low-angle structural feature which appears to displace the Mesozoic stratigraphy to the east. Later structural rotation to the west of the entire Mesozoic and Tertiary geological column has taken place along stacked, parallel, normal faults displaying displacements of up to ~1 km. Tertiary low angle normal faults have juxtaposed barren Tertiary and pre-Tertiary rocks above the known mineralised deposits at Pumpkin Hollow.
Mineralisation
Large areas iron oxide-copper-gold (IOCG) mineralisation are hosted by carbonate rocks of the middle to lower sections of the Gardnerville Formation and the upper Mason Valley Formation as well as within the intrusive Yerington Batholith, occur as copper sulphides and magnetite with varying amounts of Au and Ag, and associated skarn alteration.
Initial contact metamorphism altered Gardnerville sedimentary rocks to diopside-garnet, calc-hornfels and siliceous hornfels, and converted the limestone of the Mason Valley Formation to calcite±dolomite±tremolite marble, with a diopside-plagioclase endoskarn within the intrusive rocks. Later metasomatism formed diopside-garnet-magnetite ±sulphide replacement zones. Fracturing and brecciation along and near diorite contacts enhanced hydrothermal fluid movement, metasomatism and alteration within the diorite, beds of carbonate rocks and along fractures at a high angle to bedding in hornfels. Higher copper grades are generally found on the fringe of magnetite zones, near a mineralised limestone contact (the marble front) or within bodies associated with late stage retrograde actinolite-epidote-garnet-magnetite skarn alteration.
The deposition of pyrite, pyrrhotite and chalcopyrite, minor tennantite and sphalerite accompanied retrograde skarn development, and may also be synchronous with the intrusion of altered and weakly mineralised andesite and quartz monzonite porphyry dykes. A general zonation involving a decrease in magnetite and increase in copper content away from the intrusive is evident, producing proximal magnetite and distal copper zones.
Although five separate deposits have been defined, Bryan et al. (2013) suggest there is a strong possibility that all deposits were originally part of a single body peripheral to and within the intrusive that have subsequently been displaced and separated to their present locales by a complex post-mineralisation tectonic history. All of the deposits are 'blind' and were discovered by drill testing of magnetic anomalies associated with large magnetite-rich bodies.
The uppermost oxidised parts of the North and South deposits are within 30 m of the surface, although the bulk of the mineralisation lies 100 to 120 m below barren hanging wall rock. The top of the Southeast Deposit is covered by 100 to 150 m of barren granodiorite porphyry. The East Deposit is lies beneath 450 m of Tertiary volcanic rocks, Quaternary gravels and barren hornfels units while the E2 Deposit is covered by 300 m of Tertiary volcanic rocks and Quaternary gravels.
The South and Southwest deposits represent areas of extensive magnetite development with associated low grade copper mineralisation, whilst the Northwest, East, and E2 deposits, contain localised zones of much higher grade copper mineralisation. The North Deposit is a low to moderate grade copper deposit with weak magnetite content and is primarily within hornfels of the Gardnerville Formation.
The five main deposits may be summarised as follows:
North Deposit (formerly North and Northwest deposits)
Representing the northern zone of mineralisation, ~450 m north of the South Deposit. It has strike dimensions of ~1000 m, thickness of 600 to 275 m and down-dip extent of 450 m. In 2013, mineralisation remained open in several directions. The top of the deposit is truncated by a zone of south-dipping listric faults which have emplaced post-mineral Tertiary volcanic and sedimentary rocks over the deposit.
The deposit is centred on a sub-horizontal, pipe-like, copper-rich, magnetite-poor, skarn altered, breccia body, hosted by hornfels of the Gardnerville Formation (previously the Northwest Deposit). This skarn altered breccia grades into a zone of magnetite-rich, copper-poor mineralisation hosted by granodiorite endoskarn and marble (previously the North Deposit). Retrograde alteration and sulphide deposition in the North Deposit, which overprints earlier prograde alteration, was focused along fractures at angles to bedding, and diminishes away from the breccia core. Chalcopyrite tends to be the last sulphide phase deposited.
South Deposit
The South Deposit, which was the first discovery at Pumpkin Hollow, is roughly tabular, with a NE strike over a length of 1100 m, thickness of 120 to 300 m, and persists for ~550 m down-dip to the NW. The top of the deposit varies from within 10 m of the surface on the SW margin, to a depthg of 140 m on the NE side, below a low-angle normal fault that places unaltered Tertiary volcanic rocks and conglomerate over the deposit. It is a magnetite-chalcopyrite body, closely associated with the intrusive contact of granodiorite with limestones of the Mason Valley Formation. The mineralisation can be divided into two parts: i). a footwall, proximal zone, tha, with a low-grade iron, but with higher copper content.
Southeast Deposit
The Southeast Deposit, which is 600 m SE of the South Deposit, has a NE trending strike length of >500 m, is 100 m wide with a steep NW dip, extending from ~60 m below the surface to a depth of ~200 m. The deposit is fault bounded, both above and below, by low-angle normal faults. Mineralisation occurs as a lens of chalcopyrite-magnetite-garnet-actinolite, developed within skarn altered limestones of the Mason Valley Formation. This zone is unique at Pumpkin Hollow, in that it has elevated magnetite grades which may locally reach 75% Fe. Magnetite-garnet-epidote skarn alteration accompanies the mineralisation, with chalcopyrite increasing toward the footwall marble front. Strong similarities between the Southeast and South Deposits suggest the former may represent the upper portion of the latter, displaced 600 m to the SE along the lower listric fault.
East Deposit
The East Deposit, 2 km east of the North Deposit, comprises several stacked, tabular, bedding-controlled, flat-lying to gently dipping mineralised zones with an aggregate thickness of 12 to 60 m, that cover an area of ~600 x 350 m, within the limestone of the Mason Valley Formation. Mineralisation occurs at depths of from 425 to 670 m below the surface. Higher grade copper mineralisaiton is mainly restricted to dolomitised limestones and skarn-chalcopyrite-magnetite-pyrite breccias immediately adjacent to diorite or diorite endoskarn. Much of the high grade magnetite rock of the East Deposit occurs as diorite endoskarn. Good continuity is evident, with mineralisation possibly continuing and thickening to the west. In 2014, further drilling was required between the East and the North deposits to determine the lateral extent of each. The top of the East Deposit appears to have been displaced to the east by the flat faults which separate the gently dipping limestones of the Mason Valley Formation from the barren hornfels and skarn of the overlying, steeply dipping Gardnerville Formation which us intruded by numerous diorite sills. These formations are, in turn, overlain along another low-angle normal fault by Tertiary volcanic and sedimentary rocks.
E-2 Deposit
The E2 Deposit is located directly south of the East Deposit. It is a steeply NW dipping 12 to 35 m thick lens of high-grade copper-magnetite skarn altered breccia within the Mason Valley limestone, which lies on the hanging wall of an endoskarn sill. The lens has been delineated along a strike length of ~360 m, and is locally continuous for at least 500 m down-dip, from the top of the deposit which is 200 m below the surface to a depth of 720 m. The chalcopyrite-magnetite mineralisation follows the marble front, similar to the East Deposit. A major east-trending rotational fault appears separate the two deposits and results in a significant variation in the deposit orientation. Interpretation of the drillhole intersections suggests the E2 Deposit may be a series of steeply dipping, plunging shoots.
Published reserves and resources as of November 2013 are as follows:
West Model open pit - North, South and Southeast deposits at a cutoff of 0.2% Cu
measured + indicated resources - 484.82 Mt @ 0.45% Cu, 0.034 g/t Au, 1.85 g/t Ag, plus
inferred resource - 125.33 Mt @ 0.40% Cu, 0.034 g/t Au, 1.5 g/t Ag.
proved + probable reserves - 497.3 Mt @ 0.377% Cu, 0.034 g/t Au, 1.646 g/t Ag.
East Model underground - East and E2 deposits, at a cutoff of 0.75% Cu
measured + indicated resources - 45.9 Mt @ 1.45% Cu, 0.31 g/t Au, 7.3 g/t Ag, plus
inferred resource - 10.97 Mt @ 1.11% Cu, 0.07 g/t Au, 2.23 g/t Ag,
proved + probable reserves - 25.07 Mt @ 1.49% Cu, 0.27 g/t Au, 5.83 g/t Ag.
North, South and Southeast deposits stand alone iron resource at an open pit cutoff of 30% Fe
measured + indicated resources - 183 Mt @ 39.3% Fe, plus
inferred resource - 38.7 Mt @ 39.5% Fe.
This summary is largely based on sections of the report: Bryan, R.C., Lips, E.C., Michael, N., Richers, D.M., Scharnhorst, V.J. and Spiller, D.E., 2013 - NI 43-101 Technical Report Open Pit Operations Feasibility Study, Pumpkin Hollow Project Yerington, Nevada, Prepared for Nevada Copper Corp. by Tetra Tech, Inc..
The most recent source geological information used to prepare this decription was dated: 2013.
Record last updated: 7/11/2014
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.
East Underground West Open pit
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Porter GeoConsultancy Pty Ltd (PorterGeo) provides access to this database at no charge. It is largely based on scientific papers and reports in the public domain, and was current when the sources consulted were published. While PorterGeo endeavour to ensure the information was accurate at the time of compilation and subsequent updating, PorterGeo, its employees and servants: i). do not warrant, or make any representation regarding the use, or results of the use of the information contained herein as to its correctness, accuracy, currency, or otherwise; and ii). expressly disclaim all liability or responsibility to any person using the information or conclusions contained herein.
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