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The Tusc gold deposit is some 13 km to the north-west of the town of Carlin in north-eastern Nevada, USA. It is located on the south-western margin of the Carlin Window in the Maggie Creek District, approximately 1.5 km north-west of the margin of the Gold Quarry orebody, 750 m to the NNW of the MAC deposit and about 500 m to the south-east of the Mike deposit. The overall mineralised zone has plan dimensions of 900 x 500 m.
Published reserve figures at Tusc include:
12 Mt @ 2.1 g/t Au = 26t Au (Proven+Probable Reserve, 31/12/92, Christensen, 1993)
This reserve is based on 4 diamond and 300 RC/rotary drill holes. Six further diamond drill holes were planned prior to mining. The reserve estimation was based on 7.6 m composites with 18 to 21 m drill centres (Andrew, 1993). No mining had taken place at Tusc to 1993, although production was scheduled to commence in 1994 (Doyle-Kunkel, 1993). The planned stripping ratio is estimated to be 7:1 waste:ore. The ore is 98% oxide (Andrew, 1993).
Tusc is a structurally controlled gold deposit closely associated with the north-west striking and north-east dipping Good Hope Fault. The same fault appears to also be closely related to the distribution of the other significant deposits of the Carlin Window, namely the Gold Quarry, Mike and MAC occurrences. Sub-microscopic gold is disseminated in the shear zone of the fault and the immediate hanging wall rocks (Doyle-Kunkel, 1993).
The old Copper King mine is located within 250 m to the south-west of the Tusc orebody. Copper King was first discovered on the 1880's. Mineralisation consisted almost entirely of the oxide minerals chrysocolla, malachite, azurite and cuprite which occurred along shear zones in the silici-clastic host rocks. Open pit and underground development included a 60 m shaft and a hundred metres or so of underground development. Mining commenced in 1907 with a few thousand tonnes of ore being extracted. From 1952 to 1958 approximately 15 000 t of 3.4% Cu was mined. Small amounts of gold and silver were also produced. Exploration (not by Newmont) between 1958 and 1972 failed to locate any significant economic resources. In the late 1960's Newmont indicated surface potential for gold and an exploration program in 1985 confirmed the existence of a significant gold resource (Doyle-Kunkel, 1993).
The stratigraphy of the rocks within and surrounding the Carlin Window is described in full in the 'Gold Quarry - Geology' record. For the regional setting see the 'Carlin trend - Geology' and 'Carlin trend - Mineralisation' records.
Mineralisation at Tusc is predominantly hosted by the Siluro-Devonian Roberts Mountains Formation which is locally around 600 m thick. In the mine area these rocks comprise a series of laminated to thin bedded, argillised siltstone, silty limestone and un-altered limestone which occur in the hanging wall of the north-east dipping Good Hope Fault. The 'argillised siltstone' represents a decalcified silty limestone. The pre-ore host sequence is interpreted to have had an 'excellent' porosity and permeability which is attributed to a relatively coarse grain size, well developed laminae and de-calcification. These carbonate hosts strike in a NNW direction and dip at 40° to 80° to the WSW (Arkell, 1991; Doyle-Kunkel, 1993).
Exposures of the upper Devonian (to lower Carboniferous) Rodeo Creek Unit are found in the footwall of the Good Hope Fault. These comprise 200 m of grey to black siliceous mudstones, cherts, shale beds and occasional conglomerates that are tightly folded and strongly contorted into recumbent and isoclinal folds. This structural disruption is attributed to low angle thrusting during the Devono-Carboniferous Antler event, followed by later moderate angle reverse faulting at 45° to 60°. A complicated network of fractures resulted from this deformation influencing the development of a subsequent stockwork system. The siliceous nature of the Rodeo Creek Unit is believed to have been a depositional to diagenetic feature, un-related to hydrothermal alteration (Arkell, 1991; Doyle-Kunkel, 1993).
These siliceous rocks are underlain by a 100 to 200 m thick sequence of laminated to massive, grey to black, silty limestone and siltstone, with interbeds of massive micritic limestone. This assemblage has been tentatively correlated with the Devonian Un-named Formation, which is in turn probably equivalent to the Popovich Formation of the Lynn Window. It may however represent the James Creek Member of the Rodeo Creek Unit. The contact between the silici-clastics of the Rodeo Creek Unit and these underlying carbonates is defined by an interpreted splay of the Roberts Mountains Thrust (Arkell, 1991; Doyle-Kunkel, 1993).
The main structure in the Tusc area is the Good Hope Fault which strikes at approximately 315° but has a highly variable dip. It is exposed over a length of 2700 m before passing under Cenozoic cover on both ends. Numerous small scale synthetic and antithetic structures, mainly faults and folds are associated with this feature. The main Good Hope Fault zone juxtaposes carbonates of the Roberts Mountains Formation within the Carlin Window with silici-clastics overthrust from the south-west. Over 200 m of reverse throw is indicated by stratigraphic relationships. Dextral strike-slip of more than 300 m is also implied (Arkell, 1991; Doyle-Kunkel, 1993).
To date drilling and mapping has suggested that there are at least three sub-parallel splays of the Good Hope Fault. The West splay is separated from the Main Fault by 15 to 60 m of intensely crushed and sheared rock to the south-west, while the two other splays are found within 90 to 120 m to the north-east. Possible 15 to 30 m of reverse offset is indicated for each of the splays. Barite veins and jasperoid bodies which show evidence of multiple brecciation, are commonly exposed at the surface within the Good Hope splay zone (Doyle-Kunkel, 1993).
Several fault systems cross-cut and off-set the Good Hope fault system. A splay of the Roberts Mountains Thrust forms the contact between the carbonate and siliceous lithologies within the orebody area. It is also the focus of minor amounts of low grade mineralisation that extend into the footwall of the West Splay of the Good Hope Fault. The Seldom Seen Fault bounds the Tusc orebody to the north-east. This fault, although parallel to the Good Hope structure, accommodates normal displacement rather than reverse movement. The normal, north-east trending, steeply dipping Copper King Fault system limits significant mineralisation to the north-west. The normal K-W Fault system, which has predominantly vertical displacement and is also north-east striking, dips steeply to the north-west and limits significant mineralisation to the south-east (Doyle-Kunkel, 1993).
It is apparent that nearly all of the major faults at Tusc were formed prior to the emplacement of ore. Most have undergone multiple episodes of reverse and normal movement in response to compressional and extensional episodes from the oldest, the Devono-Carboniferous Roberts Mountains Thrust, to the late Tertiary Basin and Range movement (Doyle-Kunkel, 1993).
Four alteration types have been recognised as being spatially associated with the Tusc orebody. The first three, decalcification, argillisation and silicification are believed to be hypogene in origin, followed by supergene acid leaching and oxidation which was accompanied by alunite veining. These have the following characteristics:
De-calcification (or de-carbonatisation) - is the main alteration style exhibited at Tusc. Carbonate removal was one of the earliest hydrothermal manifestations, predating the introduction of gold. Intense decalcification forms a zone extending 150 to 250 m into the hangingwall of the Good Hope Fault. It is accompanied by an increase in both the porosity and permeability of the affected rock, and a general decrease in its bulk density. Almost all of the original 50 to 60% calcite of the silty-limestone has been removed. The altered rock is a porous siltstone (Doyle-Kunkel, 1993).
Argillisation - this is the next most widespread style of alteration at Tusc. The predominant clay alteration products are kaolinite and illite-sericite, with traces of montmorillonite. Both kaolinite and illite appear as white to red iron stained clay veinlets and fracture coatings which are commonly associated with large structures. There is an apparent vertical zonation of kaolinite and illite-sericite, with kaolinite predominating at depth. Argillisation is therefore interpreted to be both structurally controlled and principally hypogene in nature. Advanced argillisation is spatially associated with both decalcification and gold mineralisation (Doyle-Kunkel, 1993). It is uncertain how much of the argillisation is hypogene and how much is supergene.
Silicification - is only found as elongated pods and lenses within the alteration halo of the Good Hope Fault system and as several bedding plane replacement jasperoids (Doyle-Kunkel, 1993). Silicification is intense and jasperoids are common within faults, particularly those of the Good Hope system, although elsewhere it is only weakly to moderately developed (Ekburg, etal., 1991). The jasperoids within the Good Hope Fault zone are "not subtle features on the landscape" (Andrew, 1993). The pre-alteration sediments of the deposit originally contained an average of 40 to 50% quartz as either detrital sand or silt grains, or as micro-crystalline chert. This was made up of 20 to 30% quartz within the carbonate rocks, while the siliceous lithologies averaged more than 50%. Apparent increases in the quartz content of the carbonate rocks, other than the silicified pods and lenses described above, can be spatially correlated with decalcification and the resulting siliceous residue (Doyle-Kunkel, 1993). Within the Rodeo Creek Unit, argillisation along fractures is the only apparent alteration product. The rock is otherwise unaltered, having a high primary silica content and no carbonates to de-calcify (Arkell, 1991). Barite occurs as thin, discontinuous veins associated with areas of intense silicification (Ekburg, etal., 1991).
Acid Leaching and Oxidation - It has been postulated that advanced argillisation associated with the waning stages of hydrothermal activity may have oxidised areas of wall rock in the upper sections of the deposit. Acid solutions derived from late stage hydrothermal products mixing with meteoric waters are suggested to have percolated downward, resulting in the near complete chemical destruction and oxidation of organic hydrocarbons and pyrite within the deposit. It is further suggested that a large proportion of the deposit has also been thoroughly oxidised by descending acidic supergene fluids. The upper levels of the orebody and the hangingwall of the Main ore zone are both bleached and oxidised (Doyle-Kunkel, 1993). Oxidation occurs from the surface to the base of the orebody. The uppermost portion of Tusc is a tan, bleached zone of limonite/clay alteration and weak to moderate decalcification. This zone is generally barren or carries only very low gold values. Beneath this upper zone there is a hematitic/clay alteration zone characterised by sub-ore grade gold mineralisation, and moderate decalcification, accompanied by moderate hematite and clay formation. Directly above the orebody there is a 1.5 to 3 m thick zone of intense hematite alteration, solution cavities, total decalcification and moderate to strong argillisation. This gossan like cap carries sub-ore to low grade gold mineralisation. These alteration patterns indicate strong supergene leaching and a hint of supergene enrichment of gold in the Tusc orebody (Ekburg, etal., 1991). Black, un-oxidised, pyritic, silty limestone, almost devoid of ore, lies immediately below the Main orebody (Doyle-Kunkel, 1993).
Alunite veins appear to be closely associated with both advanced argillisation and supergene oxidation. An amorphous mixture of alunite±kaolinite±silica is found along open fractures and joints, apparently deposited by descending fluids. Alunite is concentrated in the central core of the deposit and within the Good Hope Fault system (Doyle-Kunkel, 1993).
Sub-microscopic gold is disseminated within the shear zone defined by the splays of the Good Hope Fault, and in the immediate hanging wall rocks to this fault, extending as far to the north-east as the Seldom Seen Fault. Approximately 15% of the ore is found within silici-clastics of the Rodeo Creek Unit, with the remainder being hosted by carbonates, almost all of which belong to the Roberts Mountains Formation (Doyle-Kunkel, 1993).
The major geological factors that control ore deposition are apparently, i). the ground preparation by mechanical and chemical enhancement of porosity and permeability, and ii). the presence of major pre-ore fault structures. Interconnected fracture systems produced within the silici-clastics by the intense folding and faulting have provided sites for stockwork type mineralisation. As detailed above, these rocks are tightly folded and strongly contorted into recumbent and isoclinal folds. Similarly the 30 to 45 m interval between the splays of the Good Hope Fault have been intensely crushed and sheared within both the silici-clastics and carbonates. Other fractures that have influenced the emplacement of mineralisation include bedding planes, especially within the carbonates, and joints. The porosity and permeability of the carbonates in the hangingwall of the Good Hope Fault system have also been enhanced by decalcification prior to the emplacement of gold mineralisation (Doyle-Kunkel, 1993).
The ore deposit is elongated along the trend of the Good Hope Fault, and is bounded on all sides by faults, as described in the 'Structure' segment above. In summary the bounding faults are the West Splay of the Good Hope Fault to the south-west, the parallel Seldom Seen Fault to the north-east, the NE trending Copper King Fault to the north-west and the parallel K-W Fault to the south-east. Mill (higher) grade ore zones are generally confined to the core of the deposit, while lower grade mineralisation is found concentrically zoned around the high grade core and throughout the Good Hope Fault system (Doyle-Kunkel, 1993). This low grade mineralisation passes outward into barren rocks. The higher grade core (referred to as a 'hydrothermal feeder zone') consists of several coalescing faults which are sub-parallel members of the Good Hope Fault system. This fault zone is characterised by intense argillisation, with intense silicification in the footwall. From this core mineralisation passes out into zones of higher permeability within joints, fractures and faults associated with the Good Hope Fault system and into decalcified hangingwall carbonates (Ekburg, et al., 1991).
The ore of the Tusc deposit is continuous over a length of 750 m parallel to the Good Hope Fault and occupies a width of 360 m (Doyle-Kunkel, 1993). It has a synformal geometric shape, which plunges to the north-west. On the eastern limb of this structure, within the carbonates in the hangingwall of the Good Hope Fault system, mineralisation forms a sub-horizontal, lensoidal, shape which dips to the south-west at 20° to 30°, sub-parallel to the host strata dipping at 40° to 50° in the same direction. The western limb lies within the Good Hope Fault system and comprises a north-west striking tabular zone which cross-cuts stratigraphy at a high angle. In this limb joints and fractures are the primary control on the distribution of mineralisation, with bedding being a secondary influence (Ekburg, etal., 1991). Gold mineralisation within the Good Hope Fault system outcrops at surface, while the eastern limb ore, within the hangingwall of the fault system, occurs 75 m or more below the present surface (Doyle-Kunkel, 1993).
No information is available within the literature on the form of the actual mineralisation.
Three elemental associations have been identified in geochemical data at Tusc. These are, i). Au+As+Sb±g;Ag - a large halo of +500 ppm As surrounds the central alteration and gold mineralisation, while 'upper-level' Sb anomalies are also common. Local concentrations of Ag roughly correlate with Au; ii). Cu+Bi+Mo+Se+Cd - Bi, Se, Cd and Mo form local anomalies with concentrations of 5 to 30 ppm. The Good Hope Fault contains areas of 100 to 400 ppm Cu and locally up to 4% Zn; iii). Te+Tl+Zn - thallium and tellurium anomalies in the 0.5 to 25 ppm range are locally conformable with, or immediately below the orebody. Hg and Pb are notable due to the lack of any strong anomalism (Doyle-Kunkel, 1993).
The deposit was discovered by litho-geochemical sampling. The main jasperoid of the Good Hope Fault, although barren along most of its length, contained up to 6 g/t Au in the discovery sampling. Locally values of 5 to 30 g/t Au may be found within the jasperoid, accompanied by +500 ppm As, anomalous Bi, Se and Cd. The Good Hope jasperoid is significantly developed along thrusts, splays and faults, and are "not subtle features on the landscape". It is banded, brecciated, massive and contains obvious barite (Andrew, 1993).
Beneath the Tusc orebody a zone of strong primary stratabound diagenetic pyrite occurs, containing >10% sulphide. Above and within the orebody pyrite has been completely oxidised. With the exception of local accumulations within the cores of strongly silicified intervals, barite is almost non-existent at Tusc (Doyle-Kunkel, 1993).
The most recent source geological information used to prepare this summary was dated: 1996.
This description is a summary from published sources, the chief of which are listed below.
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