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Ivanhoe, Hollister, Cornucopia |
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Nevada, USA |
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Au
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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 Hollister (also known as the Ivanhoe or Cornucopia) gold deposit is located within the Ivanhoe Mining District of western Elko County, north-eastern Nevada, approximately 58 km to the north-west of the town of Carlin. It lies on the current north-western limit of the Carlin Trend and is roughly 15 km to the north-west of the Dee deposit.
Published reserve figures for the Hollister orebodies include:
Geological Resource, Oxide Ore, 1989 - 16.8 Mt @ 1.2 g/t Au = 20 t Au (Coope, 1991).
Geological Resource, Sulphide Ore, 1989 - 75 Mt @ 1.16 g/t Au =87 t Au (Coope, 1991).
Original Identified Resource - 18.4 Mt @ 1.16 g/t Au =21 t Au (Wallace, 2003).
Mining and quarrying have been undertaken in the district for as much as 8000 years. The American Indians quarried the opalite outcrops at the Hollister deposit for tools as far back as 6000 BC. Prospecting for mercury commenced at the beginning of this century with the first production in 1915. From 1915 to 1917 between 50 and 150 flasks were produced. The bulk of production was between 1929 and 1943 when a further 1032 flasks were extracted. Other mines in the district cumulatively yielded more than 1000 flasks. No significant production has taken place since 1943. Since 1963 a number of companies have tested the district for a variety of targets from mercury, to molybdenum, and uranium. Exploration for gold by a number of partners continued from 1980 to 1988. By the end of 1988 a total of 879 holes for almost 100 000 m of drilling were completed. This outlined four distinct zones of mineralisation, the USX, Clementine, Velvet and Butte #1 deposits with 16.8 Mt @ 1.2 g/t Au (Bartlett, et al., 1990).
Mining is by open pit with heap leach treatment for an average recovery of 76.8% (Bartlett, et al., 1990).
By 1994 Newmont Mining had acquired a 75% interest in the property (Min. Jour., 16th Sept., 1994).
Geology
At Ivanhoe the stratigraphic sequence is composed of Palaeozoic rocks, unconformably overlain by a veneer of deformed Eocene to late Miocene volcanic rocks, which are partially covered by nearly flat lying late Miocene to Pliocene tuffs and sediments. The Tertiary veneer is generally less than 200 m thick in the central part of the district, but increases to more than 900 m in adjacent basins. The pre-Tertiary rocks have been significantly influenced by the Antler and Sonoma orogenies and possibly by late Mesozoic thrusting and have a complex structure. The Carlin Trend in this area is sub-parallel to a 20 to 50 km wide NW-SE trending extensional lineament. This lineament is most prominently marked in the Ivanhoe area by 16.3 to 13.8 Ma felsic and mafic flows and ash-tuffs that slightly pre-date or are coincident with the Au-Hg mineralisation (Bartlett, et al., 1990).
The sequence within the Ivanhoe District is as follows, from the base:
Ordovician, Valmy Formation - This unit does not outcrop anywhere in the district, but is encountered in most drill holes at depths of 50 to 150 m. The two most common lithologies are:
• Medium grained ortho-quartzite - mostly massive and featureless with no recognisable bedding other than occasional interbeds up to 0.6 m thick of argillite. It is usually fractured, brecciated and re-silicified near the contact with the overlying Tertiary volcanics. Ortho-quartzite is commonly the first lithology below the unconformity and grades upwards almost imperceptibly into an erosional deposit or residuum locally known as the Tertiary/Ordovician Fragmental Unit. Breccia units are also common at the contact with argillites;
• Grey to black, carbonaceous argillite/mudstone - typically highly carbonaceous, sheared and containing thin interbeds of light grey argillite, dark grey chert and rare tan coloured, calcareous siltstone and mudstone. Bedding is only obvious where the argillite is laminated, although the laminae are usually highly contorted and sheared
Within the prospect area neither the upper or lower contact have been intersected and at least 1000 m of Ordovician sequence is known (Bartlett, et al., 1990).
Tertiary, Fragmental Unit, 0 to 30 m thick - This unit is composed of angular to sub-rounded clasts of Valmy quartzite in a matrix of tuff and small quartzite fragments. The matrix typically contains sand grains, volcanic debris, clay and iron oxides which may be partially or wholly cemented by quartz and chalcedony. The boundary with the underlying quartzites is difficult to determine in drill chips, as it grades into the brecciated quartzite below. This unit is one of the important hosts to gold mineralisation within the deposit. It is at least partially pre-volcanics, but has not undergone significant transport. The presence of iron oxide and cross-cutting sulphides suggests weathering prior to mineralisation (Bartlett, et al., 1990).
Tertiary, Lower Tuff Unit, <1 to >100 m thick - This tuff is bounded below by the Tertiary Fragmental Unit and above by the Basalt. It does not outcrop in the orebody area. Its thickness is determined by variations in elevation of the underlying Palaeozoic basement. It is the major host to gold mineralisation in the deposit. The rock is predominantly an homogeneous crystal poor lithic tuff with rare bedded siltstone and claystone units. Clasts are mainly pumice fragments which range in size up to 5 cm. The rock is mottled orange to yellowish-brown where oxidised and is blue-green or grey in the sulphide zone. Near the contact with the underlying fragmental it may contain sparse to abundant quartz sand grains. Small quartzite clasts are present near the base and it passes gradationally downward into the Tertiary Fragmental Unit (Bartlett, et al., 1990).
Tertiary, Basalt, 40 to 120 m thick - The basalt is actually an andesite to basaltic andesite. It varies in thickness, generally increasing to the west. The bulk of the units is massive and forms reddish coloured, platy outcrops to rubble-crops. In un-altered samples it is sparsely porphyritic to aphyric with rare phenocrysts of plagioclase and possibly pyroxene. Fresh samples are magnetic. Vesicles are found near the upper portions of the flow, near its margins. Evidence for repeated flows are not common. Age dating returned a value of 13.6±0.7 Ma. A very consistent trace element composition over a wide area suggest it is very uniform and represents a single flow from a common source. A red montmorillonitic clay overlies the basalt and in places grades up into a red, laminated, silicified tuff. This may have been a palaeosol (Bartlett, et al., 1990).
Tertiary, Upper Tuff Unit, up to 40 m thick - The Upper Tuff outcrops over the entire length of the Hollister deposit and is defined as all tuffs above the basalt, or the water lain tuffaceous sediments above the Lower Tuff where the basalt is absent. Its thickness is extremely variable due to erosion. All of the higher grade mercury is hosted by the Upper Tuff. It is extremely variable in composition and appearance also, varying from massive, sometimes pumiceous, crystal tuff that is predominantly white or light pink, to a finely laminated, bioturbated, tuffaceous siltstone of similar colour. The glassy ash has been entirely altered to clays within the deposit, although fresh ash is observed in outcrop 1.7 km to the west. Interbeds of quartzite bearing conglomerate and sandstone are common on the eastern margin of the deposit, some with clasts of the same tuffaceous conglomerate. Sedimentary features such as ripple marks, mud-cracks, scour and fill and worm burrows are recorded, but are rare. The Upper Tuff appears to be a shallow lacustrine or fluvial-lacustrine deposit developed on the upper surface of the basalt (Bartlett, et al., 1990).
Tertiary, Flow Banded Rhyolite, up to 100 m thick - Flow banded rhyolite which may represent ash-flows, overlies the Upper Tuff adjacent to the ore deposit, but not over it. Anomalous Au and Hg has been encountered in the rhyolite, although it is not an important host (Bartlett, et al., 1990).
Tertiary, Craig Rhyolite, 50 to 700 m thick - predominantly a flow which outcrops to the east of the deposit area and extends to the south-east. It rests disconformably on the Hollister volcanic sequence which apparently dips underneath the rhyolite flows (Bartlett, et al., 1990).
Structure
The most prominent structural feature is a north-west trending ridge in the pre-volcanic erosional surface above the Valmy Formation. It is approximately 2000 m long and has a relief of 120 to 150 m over a width of 1500 m. The major fault directions are north-east and north-west with a less well defined east-west trend. The NW structures are the oldest and the NE the youngest. Where exposed outside of the Hollister area they display normal steep sided movement with east side up displacement. Displacement of the volcanic rocks by the NW set is generally 15 to 75 m, while the NE faults show displacement of <20 m (Bartlett, et al., 1990).
A small scale structural control of ore is indicated with linear trends to the distribution of higher grade ore. This is interpreted to reflect zones of higher secondary porosity and permeability developed by structural preparation of the host rocks. There is very little veining or hydrothermal brecciation in the Tertiary in contrast to the Valmy Formation and Tertiary Fragmental Unit which commonly display high angle fractures, open and closed veinlets and breccias (Bartlett, et al., 1990).
Alteration
Three basic alteration events are recognised at Hollister. These include i). early pervasive diagenesis of volcani-clastic sediments and de-vitrification of flows; ii). hydrothermal alteration associated with the mineralisation, including argillisation, silicification and propylitisation; and iii). supergene oxidation. Of these the hydrothermal alteration and post-mineralisation oxidation are the most important. Intense clay alteration and widespread pervasive silicification are the primary features of the Hollister deposit. Propylitic alteration does occur peripheral to the mineralised area, particularly within the basalt (Bartlett, et al., 1990).
Argillisation - argillic alteration within the tuffaceous rocks has been sub-divided into i). a shallow oxidised assemblage characterised by alunite, kaolinite-dickite, quartz-chalcedony, opal±adularia±jarosite±zeolite±carbonate±goethite, hematite and other limonite minerals; and ii). a deeper, generally reduced assemblage of adularia, montmorillonite, quartz-chalcedony, chlorite, pyrite-marcasite±sericite±kaolinite±alunite. There is overlap between the two assemblage where oxidation extends below the silica cap. In general the oxidised assemblage contains more alunite and kaolinite, while the reduced rocks have a greater content of adularia and montmorillonite (Bartlett, et al., 1990).
Silicification - takes several forms, namely i). surface sinter zones - occurring as discrete massive sinter developments, characterised by finely laminated chalcedony with lesser opal, sparse to abundant interlayer cavities, layers of silicified sand and sediment with some mud-cracks and vuggy brecciated chalcedony which contains clasts of laminated silica. The chalcedony varies in colour from mostly white and light grey to tan brown, red, blue and purple. It generally forms a surface layer grading downwards into a silica cap; ii). silica caps - which occur beneath the surface sinter zones, at or near the Lower Tuff/Basalt or Lower Tuff/Upper Tuff contact and is characterised by a layer of vuggy replacement silica. It occurs over the area of most of the deposits and varies from 3 to 35 m in thickness, generally being thicker in the ore zones. It is composed almost entirely of chalcedony with some veinlets and vug linings of drusy quartz. Its texture varies from massive to very vuggy and brecciated and shows well developed replacement structures in the enclosing tuffs. It is interpreted to have formed at or near the water table; iii). contact silicification zones - occurring at the Lower Tuff/Basalt contact and in the underlying Tertiary Fragmental Unit. It is almost entirely composed of dark to light grey chalcedony with a whispy replacement texture in tuffs. The thickness varies from 0.3 to 5 m. In the Tertiary Fragmental Unit silicification occurs as matrix chalcedony surrounding the quartzite clasts; and iv). irregular silicification along structures - occurring as small masses, stringers and rare veinlets of chalcedony within most of the tuffaceous rocks. It has only been seen in core and cannot be related to any particular feature, although it is suspected to be associated with narrow structural zones or areas of other local high permeability (Bartlett, et al., 1990).
Propylitisation - which is most obvious in the basalts, characterised by nontronite (Fe-montmorillonite) chlorite, limonite, carbonate, zeolite and buddingtonite (ammonia-feldspar). Pervasive clay alteration, mostly montmorillonite, is found in the lower tuff forming a considerable halo around the Hollister deposit. Some of this alteration may be early diagenetic modification of the tuffs and deuteric alteration of the basalt (Bartlett, et al., 1990).
Oxidation - the depth of supergene oxidation varies from <30 to 150 m. In many places it is apparently related to the present ground water table, although perched sulphides are found within basalt developments that are more than 30 m thick. In these instances the upper an lower margins of the basalt are oxidised, whereas the middle contains either sulphides or mixed sulphides and oxides. The presence of montmorillonite within the basalt may have imparted their impermeability. Supergene clays are also present within the oxidation zone. Pre-mineralisation oxidation is evident within the Tertiary Fragmental Unit and on the upper surface of the Basalt (Bartlett, et al., 1990).
Mineralisation
Within the oxide zone gold occurs as native gold particles that are usually associated with goethite and limonite, and are <50 µm in diameter. Un-oxidised gold mineralisation is more varied, generally occurring as discrete particles attached to or included within sulphides or their oxidised equivalents. It is associated with pyrite-marcasite and with trace amounts of chalcopyrite, cinnabar, stibnite, realgar, tetrahedrite-tennantite and sphalerite. Gold is not encapsulated by silica. Arsenopyrite, covellite, galena, millerite and pyrrhotite have also been tentatively identified (Bartlett, et al., 1990).
Gold grains are distinctly larger in the Tertiary Fragmental Unit and within the Ordovician Valmy Formation, where they cluster and form higher grade patches. In contrast the gold within the tuffs is more disseminated and finer grained (Bartlett, et al., 1990).
Mineralisation and alteration at the Hollister deposit has been dated by K-Ar methods at 15.1±0.4 Ma from adularia in the Lower Tuff. There is some doubt on this age as a sample of the host Basalt 1.6 km from the deposit returned a date of 13.6±0.7 Ma. The latter date is regarded as being less reliable (Bartlett, et al., 1990).
The main Hollister deposits are developed over a basement ridge on the upper surface of the underlying Ordovician Valmy Formation. The main ore controls are believed to be i). intersections of prominent north-east, north-west and minor east-west striking structures; ii). favourable lithological units; iii). impermeable layers within the volcanics; iv). flow margins in the basalt; and v). the disconformity between the Ordovician and Tertiary rocks (Bartlett, et al., 1990).
Some 92% of the ore, as known in 1990, is hosted by the Lower Tuff Unit and Basalt, with the remaining 8% within the Tertiary Fragmental Unit and the Valmy Formation. Only minor amounts of gold are found within the Upper Tuff Unit. The average grade within the volcanics is approximately 1.2 g/t Au, whereas in the Tertiary Fragmental Unit and Valmy Formation it is 1.7 g/t Au. Intercepts within the latter two units however, is very variable with intercepts of 1.5 m @ 116 g/t (Bartlett, et al., 1990). More recent deep intersections within the Valmy Formation have cut 5.6 m @ 52 g/t Au at 227 m, while a twin diamond core hole yielded 73 cm @ 1281 g/t Au from a similar depth (Min. Jour., 16th Sept., 1994). Depending on the position in the ore deposit, one or more controls dominate, creating both subtle and major differences in the ore geometry (Bartlett, et al., 1990).
The Hollister deposit contains four well defined, but open ended, ore zones. These occur within an area of approximately 750 x 2000 m, embraced in turn by a much larger area of anomalous gold mineralisation. Each zone contains one or more pods of thicker and higher grade mineralisation which gradually diminish in grade and thickness laterally from the deposit. Pods range in grade and size from 75 to 150 m in plan diameter and 25 to 50 m in thickness, and may individually contain around 750 000 t of ore. Some pods are associated with recognisable surface vent features and thicker accumulations of massive silica. The deposit as a whole has the characteristics of a stratabound accumulation with good lateral continuity of ore within the volcanics (Bartlett, et al., 1990).
The individual ore zones are:
USX Zone - this is the original US Steel Corp. discovery area and initial resource. The zone is made up of three sub-zones or pods, the West, East and Rowena. In the West zone the ore is highest grade just below the Basalt/Lower Tuff Unit contact and has more continuity vertically than laterally. Oxidation is deepest in the West ore sub-zone, forming an inverted cone away from that body. The Rowena Zone is a 5 to 10° west plunging pipe like zone some 40 m in diameter and 275 m long. It is completely within the Lower Tuff Unit and entirely sulphide ore, with an average grade of 5 g/t Au. The East zone is over the main basement ridge and has a more 'bedded appearance', with the dominant ore being oxidised, argillised Lower Tuff Unit, minor argillised Basalt and a thin ore zone in the Tertiary Fragmental Unit (Bartlett, et al., 1990).
Clementine Zone - where gold mineralisation is usually confined to the Lower Tuff Unit and to the upper 15 m of the Valmy Formation ortho-quartzites and Tertiary Fragmental Unit. The Basalt is only present on the extreme western margin of the deposit and the combined Upper and Lower Tuff Units are 110 m thick. The ore zone averages 40 m in thickness. Mineralisation appears to be related to palaeo-hot springs localised by the basement ridge and cross-cutting NE and NW structures. Much of the deposit is oxidised. Alteration is characterised by argillisation and moderate silicification of the tuffs. Most of the ore is overlain by a silica cap (Bartlett, et al., 1990).
Velvet Zone - the basalt is again absent at this deposit and the ore occurs within the combined Upper and Lower Tuff Units which are 60 m thick. Gold is generally confined to the Lower Tuff Unit and in the upper 10 m of the argillite basement. The average thickness of the ore zone is 30 m. Mineralisation appears to be related to Palaeo-hot spring activity localised by east-wet and north-south striking structures over a prominent south-plunging nose of the main basement ridge. Alteration is characterised by moderate silicification and argillisation, with more intense silicification of the Upper Tuff Unit above the centres of the two pods of mineralisation (Bartlett, et al., 1990).
Butte #1 Zone - where the geology is very similar to that of the USX Zone. The Upper Tuff Unit contains a well developed Hg bearing silicified tuff and sinter. Two well defined surficial structures near the southern end of the zone are interpreted as sinter vents. These structures are slightly elongated and 2.5 m deep, surrounded by an apron of finely laminated silica. Beneath the laminated silica there is a breccia of hackly chalcedony. The best intersection was next to the vents with 32 m @ 4.1 g/t Au. Gold mineralisation occurs primarily at the Lower Tuff Unit/Basalt contact, extending upwards into the Upper Tuff along a fracture zone. The Ordovician Quartzites are barren. Ore grade mineralisation averages 23 m in thickness and parallels the edge of the basalt over a flat basement profile. Almost all of the ore is oxidised. Alteration is characterised by intense argillisation of the tuffs and basalts. Portions of the Upper Tuff are intensely silicified and contain Hg, with minor Au at the base. The silica cap averages 10 m in thickness (Bartlett, et al., 1990).
Those elements that showed some variation above the background for the deposit within the ore zone include Sb, As, Ba, Cd, Co, Cu, Pb, Mn, Hg, Mo, Ag, Te, Tl and Zn. Within the ore values of some of these metals average 2 to 3.5 g/t Ag, 100 to 800 ppm As, 60 to 150 ppm Sb, 5 to 10 ppm Hg, 400 to 6500 ppm Ba and 9 to 10 ppm Tl. Most of the typical pathfinder elements are elevated above background in drill hole samples around the deposit, including Sb, As, Ba, Mn, Hg, Ag and Te (Bartlett, et al., 1990).
Note: Geological description 1996, reserve figures updated 2003.
For more recent and additional detail consult the reference(s) listed below.
The most recent source geological information used to prepare this decription was dated: 2002.
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.
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Wallace A R 2003 - Geology of the Ivanhoe Hg-Au district, northern Nevada: influence of Miocene volcanism, lakes, and active faulting on epithermal mineralization: in Econ. Geol. v98 pp 409-424
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