|
Tonkin Springs |
|
|
Nevada, USA |
| Main commodities:
Au
|
|
 |
|
|
 |
Super Porphyry Cu and Au
|
IOCG Deposits - 70 papers
|
All papers now Open Access.
Available as Full Text for direct download or on request. |
|
|
The Tonkin Springs group of deposits are located approximately 65 km to the north-west of the town of Eureka, 90 km south-east of Battle Mountain, and is within Eureka County in north-central Nevada, USA. They lie 18 km to the north of the Gold Bar orebody and 35 km to the south-east of the Cortez Gold Mine in the Battle Mountain Eureka Trend.
There was apparently no evidence of prospecting or mining in the Tonkin Springs district prior to the 1950's. It has been rumoured that the first discovery was by an individual who found minor gold bearing fractures within Tertiary volcanics in the 1950's. Prospecting pits were dug during the 1960's, mainly within Hg rich, argillic altered Tertiary tuffs near their depositional contact with the underlying Palaeozoic sediments. Significant gold related exploration began in 1966 when an individual, Lyle Campbell, took out claims over blocks of Palaeozoic carbonates containing gold bearing jasperoids approximately 3 km to the north of the present mine. These carbonates were present as a window surrounded by siliceous Ordovician Vinini Formation rocks. During the period 1966 to 1981 several companies optioned these claims and added more claims to the block. By the end of 1981 a substantial inventory of low grade, mostly refractory gold mineralisation had been delineated (Gesick, 1987).
In 1979 a small Colorado based company, Mineral Ventures Inc., undertook a regional drainage geochemical survey, with area selection based on 'geological favorability for Carlin type deposits'. Anomalies were defined and blocks of claims taken out. One of these blocks, the Rob Claims, was staked in early 1980. Mapping and closer spaced sampling was followed by a ten hole drilling program in the spring of 1981. Evidence of gold was found in most holes and ore grades in two, although a commercially viable deposit was not delineated. Another partner was introduced and more detailed sampling carried out, followed by further drilling in the autumn of 1982. The first five holes intersected ore in what is now known as the TSP-1 deposit. The sixth hole found the TSP-4 deposit, while drill holes 9 10 and 12 discovered the TSP-3 body. The 1982 program ended with an indicated inventory of 2 Mt @ 3 g/t Au in the TSP-1 deposit. Drilling in 1983 increased the tonnage at that grade to 2.5 Mt, although it was discovered that the majority of the ore in TSP-1 was refractory. In 1984 an oxide mineral inventory of all of the deposits, plus additional testing resulted in 0.4 to 0.5 Mt of oxide ore with a further 0.3 Mt of probable ore (Gesick, 1987).
Published reserves/resources include:
5.9 Mt @ 2.9 g/t Au = 17 t Au (Reserve, 1983, Bagby & Berger, 1985).
0.8 Mt @ 2.9 g/t Au (?) = 2.3 t Au (Total oxide resource, remainder refractory, Gesick, 1987).
Silver State Mining Corporation optioned the property in late 1984. Heap leach pad construction commenced in August 1985 and the first dore bullion was poured at the end of October, 1985 (Gesick, 1987).
Geology
For detail of the setting and stratigraphy see the 'Battle Mountain - Eureka Trend Geology' record.
At Tonkin Springs a cluster of structurally related gold occurrences is distributed over a trend length of 8 km and a width of up to 3 km. They are elongated in a NNW direction, generally parallel to the overall Battle Mountain Eureka Trend, and are parallel to and within the Miocene North Nevada Rift. The North Nevada Rift is reflected in the topography, magnetic and gravity images. It may also be discerned on geological maps on the basis of the distribution of NNW trending normal faults, and basic dyke swarms with associated basaltic andesite flows (Gesick, 1987).
Gesick, (1987) includes much of the section within the district in the Ordovician Vinini Formation, although lithologic associations might suggest that the host carbonates below the Rooster Thrust belong to the Siluro-Devonian Roberts Mountains Formation and/or the Devonian Wenban Limestone of the autochthonous Eastern or Carbonate Assemblage. The succession within the Tonkin Springs belt is as follows, from the structural base:
Telephone Member - thin to medium bedded, grey, blocky, sandy to silty, shaly-parting carbonates in the lower section; overlain by black, calcareous, carbonaceous shales, micrites and very thinly bedded, slabby limestones in the upper sections. All of the ore found to 1987 was principally hosted by this unit, within or adjacent to low angle structures (Gesick, 1987). This unit is lithologically similar to the Siluro-Devonian Roberts Mountains Formation, although Gesick (1987) includes it within the Ordovician Vinini Member which according to other sources does not contain such lithologies.
Devonian, Devils Gate Limestone - which occurs as a thin sliver plastered along the western side of the Telephone Member exposure and acts as a marker. It occurs at the base of the disturbed zone which comprises the Rooster Thrust (Gesick, 1987). The Devils Gate Limestone is generally known as the Wenban Limestone on the Cortez-Battle Mountain Trend.
Rooster Thrust - a disturbed zone of altered, crushed rocks and small contorted slabs of various lithologies (Gesick, 1987).
Rooster Member - cherts, shales, argillites, siltstones and laminated silty limestones. The Rooster Member is capped by remnants of a slab of Nevada Limestone which may have been tectonically incorporated. Prominent gold bearing jasperoids have been located in many places within the Nevada Limestone in the Tonkin Springs district. Ore grade mineralisation is present within the Rooster Member, along with a large, low grade mineral inventory (Gesick, 1987). Again this unit is not characteristic of the Vinini Formation into which it was placed by Gesick (1987). The Nevada Limestone is an equivalent of the upper parts of the Siluro-Devonian Roberts Mountains Formation or the lower sections of the Devonian Wenban Limestone.
Coils Thrust - another disturbed zone separating the Rooster Member and its cap of Nevada Limestone, from the overlying Coils Member.
Coils Member - a very pure, white quartzite, brecciated quartzite, shale, siltstones and laminated shaly limestones.
Permian, Garden Valley Formation - unconformably overlies these rocks outside of the mine area (Gesick, 1987).
All of the rocks within the Tonkin Springs District which Gesick (1987) has included within the Vinini Formation apparently contain greenstone units, some of which serve as local markers (Gesick, 1987). These rocks may also represent Transition Assemblage units between the Western, Siliceous Assemblage and the Eastern, Carbonate Assemblage.
Intrusives - of intermediate composition and probable Mesozoic to Tertiary age. They were initially emplaced in high angle faults but branch into the low angle structures. In the mine area these include porphyritic, propylitised bodies of hard, well jointed, discordant intrusive rocks with a high fluorine content. Intrusive breccias passively interfinger with pre-intrusive breccias with little disruption of the pre-intrusive fabric. Thick intrusives are generally comprised of intercalated sheets of almost pure intrusive and thrust breccia. Most of these intrusives are tabular and quasi-concordant with areal extents of hectares and thicknesses of from <3 to 100 m or more (Gesick, 1987).
Tertiary Sequence - including (Gesick, 1987):
- Late Eocene to Early Oligocene, Rhyolites, 0 to 85 m thick - rhyolitic waterlain and welded tuffs, which vary considerably in thickness. These presumably belong to the Caetano Tuff unit.
- River Channel Sediments - cutting both the Eocene to Oligocene rhyolites and the underlying Palaeozoic sediments.
- Oligocene, Basaltic-Andesites - flows.
- Minor Intrusives - of varying composition, mostly mafic, but including acid varieties, and a few small patches of 'hot-spring sinter'. This activity is believed to be associated with the development of the Miocene North Nevada Rift.
- Post volcanic surficial cover - alluvial and lacustrine sediments related to Basin and Range growth.
Structure
The Tonkin Springs district has a strong structural fabric, largely attributed to the Devono-Carboniferous Antler Orogeny. The main feature is a series of imbricated slabs and slivers which have dimensions of up to several km's in length, up to a kilometre or more wide and may be hundreds of metres in thickness. These thrust slices are elongated in a NNW direction and juxtapose packages of quite different rocks which have been thrust eastward normal to their long axes. In addition to the thrust imbrication, the rocks of the slabs and slivers have also been folded about NNW aligned axes, parallel to the thrust slice elongation, and exhibit well developed axial plane cleavage which has influenced the distribution of gold mineralisation. Subsequent tectonic activity (presumably belonging to the Permo-Triassic Sonoma Orogeny) has been responsible for further low angle dislocation (Gesick, 1987).
Folding, interpreted as 'intra-slab', has led to sharp changes in attitude across thrust planes. Imbrication is evident on all scales, while ENE trending tear faults have adjusted differential stress. Broad, gentle, WNW trending folds are attributed to a possible Mesozoic event, and are elongated parallel to the trends of implied Mesozoic to Tertiary intrusives present as swarms and as individual bodies. This WNW direction is also parallel to the axes of the two largest orebodies, and to carbonate windows exposed from below the Roberts Mountains Allochthon (Gesick, 1987).
Prior to and during the development of the Miocene North Nevada Rift, NNW trending faults, parallel to the axial plane cleavage of the Antler folding, either developed, or were reactivated. Post mineral offsets are recognised on both the NNW and ENE fracture sets, suggesting that they were active during the gold mineralisation. As the Basin and Range activity continued the NNW fault set became inactive with the main displacement being on a NNE trending normal fault set and the reactivated ENE tear faults. Following the deposition of the late Tertiary volcanics there was a regional 5° east tilting (Gesick, 1987).
Mineralisation and Alteration
All of the gold deposits exploited to date have been principally hosted by the Telephone Member. All are situated in areas of intensive structural ground preparation, at the intersection of low angle fault breccia and high angle fractures. The ore deposits are made up of discordant, generally high grade zones along high angle fractures, within concordant lower grade envelopes. The steep discordant mineralisation generally trends NNW to north, while the concordant trend has WNW long axes, parallel to the open folds. All of the deposits mined to 1987 had been adjacent to generally concordant intrusives, while younger, thin, discordant dykes of various composition had also been found in all of the mines. The concordant intrusive bodies, which largely occur within semi-concordant brecciated thrust planes, have apparently had an influence on the distribution of ore. Intrusives in the ore zone are strongly altered, usually argillised. Post mineral intrusives have not been observed (Gesick, 1987).
A low angle breccia zone within the Telephone Member, known as the Telephone Thrust, has been identified and traced from the TSP-1 deposit to several satellite bodies. It has been inflated by the presence of an intrusive which splits it into an upper and a lower strand. The TSP-1 deposits is the largest known (Gesick, 1987).
The host lithologies of the Telephone Member are thin bedded sandy, shaly-parting carbonates. The most favourable ore host would appear to be interbedded dolomitic, sandy limestones and calcareous mudstones which may contain continuous mineralisation over strike lengths of more than 300 m laterally. Elsewhere in the district, gold mineralisation is hosted by thin bedded, limy siltstone and interbedded chert-shale-siltstone sequences. Ore is also hosted at the base of the slab of Nevada Limestone (Gesick, 1987).
An extensive halo of alteration and anomalous trace element geochemistry is associated with the Tonkin Springs hydrothermal system. The more obvious alteration features of the district are the large jasperoid bodies within the Nevada Limestone. Less obvious, but more widespread are several stages of silicification in less cohesive rocks, and argillisation and pyritisation of more feldspathic rocks. Elevated Hg, As and Sb are found in a large part of the district. Trace element halos specific to gold mineralisation had not been found in 1987. Elevated concentrations of As, Sb, Tl and Hg are found near ore, but also occur over weakly mineralised and barren ground. Visible As mineralisation as green oxides, or as orpiment and realgar appear to be peripheral to ore. These trace elements are regarded as being guides to mineralised structures and outline favourable areas (Gesick, 1987).
Field and petrographic relationships show the following alteration paragenesis in carbonates: i). decalcification, the surface expression of which imparts a 'sandy' appearance to the host carbonate; ii). decarbonatisation with associated disseminated pyrite, characterised by the removal of dolomite and other carbonates; iii). silicification and development of jasperoids - the early stages of silicification are subtle and are not obvious in hand specimen, while the initial stages of pervasive silicification produce grey to black dense rocks resembling cherts - the more intense stages of silicification related to ore are usually represented at the surface by limited exposures of cobble sized rubble or small chips of jasperoid float, with the most positive, but rare indications being banded bedding plane replacement jasperoid and small blue quartz 'eyes'; iv). silicification, forming silica veinlets with sulphides and gold; v). calcification with associated realgar, cinnabar and pyrite; vi). micro-fracturing; and vii). carbonatisation. Major hydrothermal carbon movement has not been observed, with carbon masses at the redox interface appearing to be supergene. Acid leaching in carbonates is only subtly expressed as gravelly soils containing angular fragments coated by jarosite films (Gesick, 1987).
Feldspar rich rocks show varying degrees of argillic alteration and pyritisation. Near gold mineralisation sericite and blue-green chlorite are common. Drill holes have encountered secondary biotite and epidote in some intrusives. These alteration minerals have however been attributed to a pre-ore higher temperature phase. Some evidence of acid leaching is visible in the upper portions of the deposits and along footwall faults. In outcrop acid leached zones form patches of distinct reddish soils over sulphide halos (Gesick, 1987)
At Tonkin Springs the ore to waste boundaries are generally sharp, although no reliable visual indicators of the transition had been identified (Gesick, 1987).
The hangingwall oxide ore includes a sandy breccia-intrusive mixture and bedding plane replacement open space jasperoid. The un-oxidised mineralisation comprises carbonaceous jasperoid above the pre-mine water table and carbonaceous sulphidic jasperoid below. Carbon masses at the redox interface are generally coarse grained and fissile, filling fractures, shears and partings. This carbon appears to have been mechanically concentrated and is not very active. Within the carbon-sulphide jasperoids, carbon is present as finely divided disseminated flecks and as fracture fillings, and is extremely active (Gesick, 1987).
Gold is present as sub-microscopic particles of free gold in the oxide zone, and as free gold, gold held in carbon and gold in framboidal pyrite in un-oxidised and partially oxidised ore (Gesick, 1987).
Mineralised bodies at Tonkin Springs are tabular silica replacements with the size and shape of individual bodies being influenced by both bedding and structure. Most mined to date are concordant and sub-parallel to the present surface with dimensions of the order of 15:5:1.
The most recent source geological information used to prepare this decription was dated: 1996.
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.
|
|
|
|
Gesick T E, 1987 - Tonkin Springs gold deposits: Their structural setting: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 305-315
|
|
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.
|
Top | Search Again | PGC Home | Terms & Conditions
|
|
