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Round Mountain |
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Nevada, USA |
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Au
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Super Porphyry Cu and Au
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The Round Mountain gold-silver deposit is located on the western flank of the Toquima Range in northern Nye County, south-central Nevada, USA, approximately 200 km south of Battle Mountain and 96 km north of Tonapah (#Location: 38° 42' 21"N, 117° 5' 0"W).
SUMMARY
Round Mountain is an epithermal, low-sulphidation, volcanic-hosted, hot-springs deposit located along the margin of a buried caldera. It ranks among the world's largest of this style of deposit, with production + reserves amounting to at least 200 Mt @ 1.5 g/t Au, for 300 t Au. In 2006 the mine was operated by Kinross Gold who owned 50% of the joint venture with Barrick Gold who owned the remainder. In 2005, total reserves + resources were 158 Mt @ 0.52 g/t Au for 82 t Au.
The ore deposit is hosted by upper Oligocene (26.5 Ma) rhyolitic ash flow tuffs that have been impounded within the source caldera, represented by a buried ring fracture that opens to the west. Round Mountain is on the rim of this ring structure. The ash flow tuffs are up to 1000 m thick, but abruptly wedge out to the east against the caldera margin.
Basement below the ash flow tuff comprises Palaeozoic siliciclastic and carbonate sediments of the Transition Assemblage (see the 'Gold Quarry' record), and Cretaceous granite. At least 13 calderas have been interpreted within a 50 km radius of Round Mountain, ranging in age from 32 to 22 Ma.
The majority of the mineralisation at Round Mountain is contained within the tuffs, although drilling has encountered some ore in the underlying Palaeozoic basement. Mineralisation occurs as: i). sheeted veins controlled by oblique faults and joints and is confined to densely welded crystal and lithic tuffs; ii). stratabound disseminated, veinlet and stockwork mineralisation in poorly welded permeable, pumiceous tuffs; and iii). within lithic ash flow tuffs.
The mineralised tuffs at Round Mountain have been intensely altered to a propylitic assemblage of adularia, K-feldspar, albite, chlorite, calcite and pyrite. Veining comprises an early chlorite-pyrite±calcite, through quartz-adularia-pyrite and quartz-pyrite to a late cockscomb quartz-adularia. Gold and silver were deposited as electrum in all stages, except perhaps the first, which is usually barren.
The orebody has been overprinted by an essentially supergene weathering episode in which much of the sulphide has been oxidised to goethite, hematite and jarosite. This oxidation by acid-sulphate waters also produced a suite of clays from the feldspar alteration products, dissolved calcite and filled fractures with soft Fe/Mn oxides, kaolinite, alunite, chalcedony and electrum.
The bulk of the grade is in the hypogene veins, with low grades in the supergene fracture fill. The mineralisation is dated as having extended over a period of around 0.1 Ma, at the end of volcanism and caldera collapse at around 26 Ma.
FULL DESCRIPTION
The Round Mountain mine is named after the original topographic knob, now largely removed by mining, which rose above the surrounding pediment just to the west of the Toquima Range. The knob was protected from erosion by a 'silicified cap' at its summit. Gold was first discovered at Round Mountain in the early 1900's, and supported a minor gold rush in 1906-07. Selective underground mining in the early part of the century produced 10 t of Au. During the same period 6.5 t of Au were also recovered from alluvial and colluvial workings. A feasibility study was carried out in 1972, based on sampling from an underground development. The current operation subsequently commenced in 1977, based on an initial 12 Mt resource (Sander and Einaudi, 1987).
Published production and reserve figures include:
200 Mt @ 1.5 g/t Au = 305 t Au (Production +Reserve, 1984, Romberger, 1986).
18 t Au (Placer Au resource).
41 Mt @ 0.92 g/t Au (Reserve, Type I ore, 1986).
125 Mt @ 1.30 g/t Au (Reserve, Type II ore, 1986).
6.7 Mt @ 1.4 g/t Au (Reserve, Type III ore, 1986).
0.75 Mt @ 12.6 g/t Au, 13.1 g/t Ag (Production, 1906-69).
21.75 Mt @ 1.4 g/t Au (Production, 1977-86).
274 Mt @ 0.8 g/t Au (Reserve, 1993, 0.2 g/t cut off, Round Mt Gold Co., 1994).
140 Mt @ 0.52 g/Au = 73 t Au (Proven + probable reserve, December, 2005, Barrick Gold, 2006).
18 Mt @ 0.52 g/Au = 9 t Au (Measured + indicated + inferred resource, December, 2005, Barrick Gold, 2006).
Published ore reserves and mineral resources at 31 December, 2015 (Kinross Gold Reserve and Resource Statement, 2015) were:
Proven + probable reserves - 66.145 Mt @ 0.7 g/t Au, and 2.37 Mt @ 11.4 g/t Ag, plus
Measured + indicated resources - 42.158 Mt @ 0.5 g/t Au, and 11.239 Mt @ 7.4 g/t Ag, plus
inferred resources - 16.205 Mt @ 0.4 g/t Au, and 2.377 Mt @ 5.9 g/t Ag.
for a total of ~74 t of contained Au.
These reserves apparently only relate to the oxidised ore and do not include any primary ore grade sulphide mineralisation. The orebody has overall dimensions of the order of 1800 x 1500 m in plan, is at least 400 m thick and is elongated in a north-westerly direction.
In 1988 the mine was being operated as an open pit, with a waste:ore ratio of 3.8:1. In that year 56 Mt of waste and ore were mined, of which 11.7 Mt was ore and 5.5 Mt was crushed. The material mined is divided into three classifications, namely i). >0.5 g/t Au is crushed before being sent to the leach heap; ii). 0.27 to 0.5 g/t Au is called 'lean ore' and sent directly to the leach heaps; while iii). material with <0.27 g/t Au is waste. The mill was upgraded to handle 30 000 tpd in 1989. The heap leach operated on a 60 day cycle for crushed ore, and a 100 day cycle for run of mine lean ore. The overall recovery in 1989 was 67%, which was from type I ore, although it was expected that this result could be improved to 80% in the type II ore lower in the mine (the ore types are described below). The output of the mine has progressed from 4.3 t Au in 1985, to 6.6 t Au in 1988. The expansion in 1989 increased production to 9.95 t Au, with 6.5 t Ag. This grew to 13.4 t in 1990, but declined thereafter, being 11.6 t in 1993. The mine ships 2:1, Au:Ag dore, reflecting the fineness of the gold in the mine (Sander and Einaudi, 1987; Round Mt Gold Co, 1994). In 1994 14.3 t Au was produced (AME, 1995). In 2002, 23.5 t Au was produced (Kinross website, 2006)
Geology
The Round Mountain orebody is situated on the western edge of the Toquima Range, between the Toquima Caldera Complex to the north and the Manhattan Caldera to the south. Ore is hosted by the variably welded, rhyolitic Jefferson Tuff of Round Mountain. This tuff varies from 76.3% SiO2 and 7% phenocrysts at the base, to 73.8% SiO2 and 30% phenocrysts at its top. It yields a K-Ar age of 26.7±0.6 Ma and is a single ash flow cooling unit that may be an outflow product of the Mount Jefferson Caldera. The Mount Jefferson Caldera is part of the Toquima Caldera Complex, which comprises three nested calderas ranging in age from 27.2 to 23.6 Ma. Their associated ignimbrites apparently amount to a minimum volume of 900 cubic km. The complex is noted for its lack of mafic to intermediate volcanism and its limited post-collapse resurgence (Sander and Einaudi, 1987; Fifarek and Gerike, 1990).
Pre-Tertiary basement rocks in the area consist of lower to middle Palaeozoic sediments and meta-sediments intruded by Cretaceous granitoids. The deposit is immediately to the east of the western range bounding fault system, and is possibly over the margin of another, mostly buried, caldera which is several million years older than the Jefferson Caldera. This latter, older, caldera is inferred from a moderately welded megabreccia, dated at 32.5±0.7 Ma, that underlies the host tuffs at Round Mountain, some 6 km outside the margin of the Jefferson Caldera. An older sheeted dyke complex dated at 37.4 to 34.4 Ma forms a north-east trending zone some 2 to 3 km to the south-east of the deposit (Sander and Einaudi, 1987; Fifarek and Gerike, 1990).
The geological succession at Round Mountain comprises, from the oldest:
Ordovician or Devonian - phyllite, shale and limestone with some chert and quartzite, of the Transition Assemblage (Sander and Einaudi, 1987; Stewart & Carlson, 1978).
Cretaceous Granitoids - represented by extensively exposed granodiorite and adamellite to the east, while quartz-monzonite of the Shoshone Granite has been intersected in drilling below the orebody (Sander and Einaudi, 1987; Fifarek and Gerike, 1990).
Oligocene Dykes, dated at 37.4 to 34.4 Ma - which occur as a sheeted set of north-east trending dykes some 2 to 3 km to the south-east of the orebody (Fifarek and Gerike, 1990).
Unconformity
Oligocene Megabreccia, dated at 32.5±0.7 Ma - a moderately welded megabreccia that is exposed below the host tuffs at Round Mountain (Sander and Einaudi, 1987).
Unconformity
Oligocene, Jefferson Volcanics, dated at 26.7±0.6 Ma - this is a single rhyolitic ash flow cooling unit that may be subdivided into a:
- Lower, poorly welded portion, which is distinguished by trains of lithic fragments bounding sub-units of the ash-flow. Individual sub-units possessed different primary permeabilities due to differences in pumice size and abundance. These stratabound contrasts in permeability controlled an early, possibly vapour-phase alteration that preserved open space in the tuff;
- Middle, densely welded section, which includes a transitional moderately welded interval at the contact with the underlying poorly welded unit; and
- Upper, thin, poorly welded layer that was capped by interbedded tuffs and tuffaceous sedimentary rocks. The sediments, now removed by mining, are reported to have included beds of siliceous sinter, but also contained a coarse sedimentary conglomerate (Sander and Einaudi, 1987).
Structure
The host tuffs at Round Mountain have not been disrupted by the pre-Basin and Range extension that occurred elsewhere in Nevada. The nearest such terranes are tens of kilometres to the north and south. The tuffs however, have been cut and displaced by WNW trending oblique-slip faults, that in the mine, guide ore related adularia bearing veins dated at 25.2±0.8 Ma to 26.5±0.5 Ma. Modern Basin and Range topography reflects on-going block faulting that began at 10 to 12 Ma. That age corresponds to K-Ar ages of supergene alunite at the mine which occurs on north-east trending faults and joints thought to be related to block faulting (Sander and Einaudi, 1987).
Mineralisation and Alteration
The majority of the ore is contained within the Oligocene Jefferson Tuffs, although 'good' mineralisation has also been encountered by drilling in the Palaeozoic sediments of the basement, which in 1989 was being investigated from a decline excavated for that purpose.
Three types of ore had been defined in 1988. The grades and tonnages of each is listed in the reserve table in the above. These three types are as follows:
- Type I Ore - occurs as sheeted veins with a north-west to NNW, and a less well developed, generally east-west to ENE trend, controlled by oblique faults and related joints. Higher grade shoots are related to the intersection of a low angle set with these high angle faults. Type I ore is confined to the densely welded crystal and lithic tuffs of the middle section of the Jefferson Tuff. This style of ore is predominantly transgressive (Sander and Einaudi, 1987).
The majority of the early production came from this type of ore, including the first production from the open pit as it was stratigraphically higher than the Type II ore. The early production from the open pit exploited zones of north-west trending veinlets characterised by overgrowths of quartz and adularia on quartz and sanidine phenocrysts respectively, in the wall rock. These 'phenocryst overgrowth' veinlets include isolated gold bearing limonite casts after pyrite. Co-extensive veins of chlorite-pyrite contain little or no gold. The tuff hosting this type of mineralisation has been intensely altered to a propylitic assemblage consisting of adularia, K-feldspar, albite, chlorite, calcite and pyrite (Fifarek and Gerike, 1990).
The old workings from the first half of the century were principally underground mines in the northern half of the current pit, and to the north in the Stebbins Hill area. They exploited major low angle veins and stockworks that variously consist of hydrothermal breccia, cockscomb quartz-adularia and iron/manganese oxide-clay±alunite. All of these vein types contain coarse free gold and post date the formation of the 'phenocryst overgrowth' and 'chlorite-pyrite' veinlets described above. Densely welded tuff in the area of these veins has been K-silicate altered to quartz-adularia-white mica, and prior to oxidation, calcite and pyrite. At high levels in the deposit, the tuff was subsequently silicified or argillised where alteration was incomplete. K-Ar age determinations from adularia and sericite yielded dates of 27 to 25 Ma, marginally later than the age of the tuffs (Fifarek and Gerike, 1990).
- Type II Ore - developed in the poorly welded, permeable, pumiceous, tuffs of the lower member of the Jefferson Tuffs. The ore in this type comprised the bulk of the reserves in 1988, and was predominantly stratabound. It occurs as veins, stock work and disseminated mineralisation.
- Type III Ore - within lithic ash-flow tuffs.
According to Sander and Einaudi (1987), the most striking characteristic of the altered rocks in the mine area is the variable colour and hardness of the tuff. This arises from the variable mineralogy and the abundance of phyllosilicate minerals replacing the matrix. Because the present day phyllosilicate mineralogy is in part the result of both late hydrothermal alteration that pre-dates the first introduction of gold, and of the later weathering of the orebody, it is not directly related to the distribution or grade of gold mineralisation (Sander and Einaudi, 1987).
Alteration at Round Mountain has been defined on the basis of the mineralogy of the hydrothermal feldspars. The following alteration types are thus defined:
- Fresh Tuff - which is basically un-altered. The plagioclase and sanidine phenocrysts maintain their original volcanic compositions, biotite is fresh and disseminated magnetite has been preserved, although some plagioclase phenocrysts may be partially converted to illite and/or montmorillonite (Sander and Einaudi, 1987).
- K-feldspar (I) - albite - this is the older of the important K-feldspar alteration types. The sanidine phenocrysts have been replaced by pure K-feldspar and the plagioclase phenocrysts have been altered to albite. The feldspar replacement products are pseudomorphs of the original mineral crystals and fragments. Minor amounts of smectite, calcite, chlorite, illite and epidote were introduced during this stage, while feldspars and biotite were altered to chlorite and magnetite to pyrite±chlorite. Volumes of this type of alteration form halo's around north-west trending faults and fractures in the welded tuffs, but spread out to become pervasive in the underlying lower, poorly welded tuffs. This alteration type dies out into fresh tuff across a weakly altered K-feldspar (I) - albite zone that is tens to hundreds of metres wide (Sander and Einaudi, 1987).
The strong K-feldspar (I) - albite zone contains the great majority of the bulk mineable mineralisation within the near surface densely welded tuff where the gold occurs as the 'phenocryst overgrowth' mineralisation of type I ore, described above (Sander and Einaudi, 1987).
- K-feldspar (II) - which is the younger of the important feldspar alteration types. It is defined by the replacement of older albite pseudomorphs of plagioclase by pure K-feldspar, and is imposed asymmetrically on the older K-feldspar (I) - albite alteration. This zone is characterised by strong K-silicate metasomatism, manifested by the addition of potassium and replacement of albite; by the replacement of intermediate K/Na feldspars in spherulites and the matrix by pure K-feldspar; and by biotite being replaced by K-mica and K-feldspar. Sanidine is not affected, retaining their original composition (Sander and Einaudi, 1987).
Volumes of rock within the densely welded tuff that have been affected by this alteration type embrace isolated, high grade, cockscomb quartz-adularia-gold veins, but little bulk mineable ore (Sander and Einaudi, 1987).
However, where this zone occurs within the poorly welded tuffs of the lower member, and is superimposed on the earlier K-feldspar (I) - albite alteration, it contains all of the known type II ore grade mineralisation (Sander and Einaudi, 1987).
Strong silicification on the top of Round Mountain was previously thought to have been due to a silica cap to the hydrothermal system. Textural evidence however, suggests that it was due instead to silicification superimposed as part of the K-feldspar (II) type alteration phase, making it one of the later hypogene events. Quartz however has also replaced some of the K-feldspar (II), alteration textures, making it one of the most recent hypogene processes, and may be either late K-feldspar (II), or a separate event (Sander and Einaudi, 1987).
Alteration that overprints the feldspar linked assemblage types described above, is related to later stage hydrothermal and supergene processes. It is uncertain whether the late hypogene processes introduced new gold to the system, but together with weathering it has locally remobilised gold, partially replaced feldspar with silica and various phyllosilicate minerals, and obscured the original phyllosilicate mineralogy of the early K-feldspar (I) - albite phase, but not the remnant feldspars. Other effects include, i). complete sericitisation of all feldspars and matrix in cm-scale halos around steep NW faults, ii). strong kaolinisation of plagioclase feldspars and matrix in metre-scale halos around low angle veins, iii). leaching of all hydrothermal carbonate minerals in conjunction with the oxidation of pyrite, and iv). alteration of hydrothermal chlorite replacing biotite and matrix to mixed layer illite-chlorite (in biotite sites) and smectite (in the matrix), also in conjunction with the oxidation of pyrite. Of these four, the first is regarded as hypogene, the second could be either hypogene or supergene, while the last two are more likely to be supergene affects, related to the weathering of hydrothermally altered pyritic rock (Sander and Einaudi, 1987).
The unoxidised veining has a paragenesis from an early stage chlorite-pyrite±calcite, through stages of quartz-adularia-pyrite and quartz-pyrite (following hydrothermal brecciation) to final stages of cockscomb quartz-adularia. Gold and silver were deposited as electrum in all stages except perhaps the first, the chlorite-pyrite veins which are usually low grade to barren (Fifarek and Gerike, 1990).
The lower limit of oxidation is at a relatively constant elevation, although oxidation effects extend to lower elevations in areas of high primary and secondary permeability. Oxidation margins transect both gold grade and alteration boundaries (Fifarek and Gerike, 1990).
Essentially all of the gold produced from the Round Mountain district has come from a 'rhyolitic' tuff in which hydrothermal sulphides (predominantly pyrite) have been oxidised to goethite, hematite and jarosite as a result of weathering. The oxidation process is interpreted to have generated acid-sulphate waters that altered feldspar to clay, dissolved calcite and filled fractures with variable proportion soft Fe/Mn oxides, kaolinite, alunite, chalcedony and electrum. The d34S of the alunite is indistinguishable from that of the hydrothermal pyrite, while the dD is characteristic of meteoric water and the d18O is heavier than that of meteoric water. The alunite is micro-crystalline, powdery and porcelaneous, and yields K-Ar dates of 9 to 16 Ma, considerably younger than the host volcanics and associated hypogene alteration products. The alunite is therefor interpreted to be a supergene product. It is also inferred that the supergene processes mobilised gold from hydrothermal sulphides and redeposited it with Fe/Mn oxides and alunite. Veins of Fe/Mn-oxide-kaolinite±alunite average around 0.18 ppm Au, whereas veins also containing hydrothermal quartz and adularia average several, to several tens of ppm Au. Therefor, although secondary gold contributes to the present production of the mine, it is not certain whether there has been any enrichment resulting from the process (Fifarek and Gerike, 1990).
The most recent source geological information used to prepare this decription was dated: 1999.
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.
Round Mountain
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Selected References:
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Henry C D, Elson H B, McIntosh W C, Heizler M T, Castor S B 1997 - Brief duration of hydrothermal activity at Round Mountain, Nevada, determined from 40Ar/39Ar geochronology: in Econ. Geol. v92 pp 807-826
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Sander M V, 1988 - Geologic setting and the relation of epithermal gold-silver mineralization to wall-rock alteration at the Round Mountain mine, Nye County, Nevada: in Schafer R W, Cooper J J, Vikre P G (Eds), 1988 Bulk Mineable Precious Metal Deposits of the Western United States Geol Soc of Nevada, Reno, pp 375-416
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Sander M V, Einaudi M T 1987 - The Round Mountain gold-silver mine, Nye County, Nevada: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 130-135
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Sander M V, Einaudi M T 1990 - Epithermal deposition of Gold during transition from propylitic to potassic alteration at Round Mountain, Nevada: in Econ. Geol. v85 pp 285-311
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Shawe D R, Marvin R F, Andriessen P A M, Mehnert H H, Merritt V M 1986 - Ages of igneous and hydrothermal events in the Round Mountain and Manhattan Gold districts, Nye County, Nevada: in Econ. Geol. v81 pp 388-407
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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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