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Carlin |
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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.
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The Carlin gold deposit is located ~45 km WNW of Elko in Eureka County, Nevada, USA. It lies within the Lynn Structural Window, 16 km to the NW of Gold Quarry.
Summary
The Carlin orebodies are predominantly hosted within silty carbonates in the upper 150 m of the Siluro-Devonian Roberts Mountain Formation silty limestones, calcareous siltstone and dolomitic siltstones, and immediately below the Devono-Carboniferous Roberts Mountain Thrust. These sediments belong to the autochthonous Carbonate Assemblage which is capped above the Roberts Mountain Thrust by the allochthonous Siliceous Assemblage.
The gold at Carlin is present as sub-micron sized particles associated with quartz, pyrite and illitic clays. As, Hg and Sb exhibit the strongest correlation with gold as cinnabar, native arsenic, realgar, orpiment and stibnite. In the un-weathered zone gold is encapsulated within fine arsenian pyrite, cinnabar and quartz, while in the oxidised ore it is present as rounded native gold grains accompanied by fine masses of amorphous iron oxides and illite.
The ore zone occupies an area of roughly 2000x750 m, composed of a series of shoots that are grossly stratabound. These ore shoots are both stratabound and transgressive, being markedly controlled by steep fault structures which have heavily fractured the host carbonates, and by favourable beds.
Alteration accompanying and enveloping the mineralisation takes the form of recrystallised calcite on the peripheries and above the gross ore zone; de-calcification - the removal of calcite and creation of a porous silty rock in the ore zone; decarbonated-argillic - characterised by the complete removal of both calcite and dolomite; siliceous-argillic with both silicification and the development of illite-sericite; and jasperoid - the most intense alteration, principally the development of silica to form intensely silicified beds and fault zones. The alteration has resulted in near 50% of the original rock being leached in the ore zone creating porosity.
Introduction
Carlin was the first large developments along the Carlin Trend and is claimed to have been the first major gold deposit in the world to be exploited by bulk mining methods (Christensen, 1993). It was not however the first recorded 'micron sized' or 'invisible gold' orebody. Invisible gold was recorded at the Mercur mine near Bingham Canyon in the 1880's, while similar fine gold was reported at the Gold Acres mine near Battle Mountain in 1936 just prior to production at that deposit (Coope, 1991).
Mining commenced at Carlin in April 1965 and was suspended in 1986. Gold ore occurred in three principal zones aligned east-west along the long axis of the pit (Christensen, 1989). The final 'pit' extended over a length of around 2.1 km with a maximum width of 0.65 km (Baker, 1987). The ore dips at approximately 30°NNW (Kuehn and Rose, 1992).
To the end of 1996, the Carlin open pit had produced 15.5 Mt @ 8.4 g/t Au. In the same year, underground production+reserves totalled around 11 Mt @ 12 to 14.5 g/t Au, and open pit reserves+resources of 28 Mt @ 1.6 g/t Au. Together these total around 320 t (10 Moz) of contained gold.
Other historic published reserve and production figures include: Production during the main operating period between 1965 to 1986 - 12.5 Mt @ 9.26 g/t Au (Myers, 1993); Proved+probable reserve at restart, August 1992 - 8.25 Mt @ 0.96 g/t Au (Myers, 1993); Initial pre-mining reserve in 1965 - 11 Mt @ 9.8 g/t Au (Coope, 1991); Total Geological Reserve at 27/12/89 - 21 Mt @ 0.92 g/t Au (McFarlane, 1991).
Published reserves and resources at December 31, 2011 were (USGS after Newmont, 2012):
Carlin open pits - Proved + probable reserves - 84 Mt @ 1.3 g/t Au, for 109 tonnes of Au;
Carlin underground - Proved + probable reserves - 7.3 Mt @ 9.67 g/t Au, for 70 tonnes of Au.
Carlin open pits - measured+indicated+inferred resources 33 Mt @ 0.93 g/t Au, for 31 tonnes of Au.
Carlin underground - measured+indicated+inferred resources 3 Mt @ 8.47 g/t Au, for 25 tonnes of Au.
Published reserves 31 Dec. 2012 (Infomine, 2014), probably included in Carlin Underground above:
Leeville underground - Proved + probable reserves - 6.08 Mt @ 7.95 g/t Au
Leeville is between 400 and 650 m below the surface and 1.5 km NW of the Carlin East underground operation.
Although prospecting has been recorded in the Carlin district since the 1870's, the first important gold discovery was in 1907, with the location of alluvial gold in the Lynn Creek, some 2.5 km to the north of the Carlin Mine. Subsequently further placer mineralisation was found in Rodeo, Sheep and Simon Creeks, the latter two of which drain the Carlin ore deposit (Lynn and Rodeo do not). Total production from these placers was at least 0.23 t, possibly as high as 0.3 t. The bedrock source of the gold was traced to a series of narrow quartz veins and stringers in shatter zones sporadically distributed within the drainage areas. The best of these was the Big Six Mine in the headwaters of Lynn and Sheep Creeks, some 2.25 km to the NNW of the Carlin Mine. Despite attempts continuing to the present only limited production has come from these veins (Coope, 1991).
Geological mapping by the USGS and Nevada Bureau of Mines between 1939 and the mid 1950's led to the recognition of the Roberts Mountains Thrust and the Antler Orogeny, and to the alignment of erosional lower plate windows of Eastern Assemblage carbonates that he named the Lynn-Railroad and Battle Mountain-Eureka Mineral Belts. These have later become known as the Carlin and Battle Mountain-Cortez Trends respectively. They also recognised that the known gold and copper mineralisation was concentrated in these carbonates below the thrust. This was presented at a meeting of the Cordilleran Section of the Geological Society of America in 1955 by Ralph J Roberts and R E Lehner. The details of the mapping were later published by Roberts etal., in 1958, and by Roberts in USGS Prof. Pap. 400-B in 1960. During the late 1950's Roberts took many parties of USGS, Nevada Bureau and Mines and private industry geologists to north-west Nevada to demonstrate these relationships. In his professional Paper, Roberts suggested exploration along these trends, at the window margins (Roberts, 1986; Coope, 1991).
Roberts gave talks at the Geological Society of Nevada in Reno and to the Eastern Nevada Geological Society at Ely during the summer of 1961. In the meantime, Newmont geologists and others were using Roberts maps. John Livermore of Newmont visited both the Gold Acres gold mine at Battle Mountain and Getchell, while Alan Coope also of Newmont investigated the Marigold and Buffalo Valley Prospects in the Getchell Trend. On the advice of the Gold Acres manager, Livermore visited the Blue Star Mine north of Carlin. After attending Roberts Ely talk and subsequent discussions with Roberts, Livermore and Coope appreciated the significance of his observations. They recommended exploration in Eureka and Elko Counties to prospect the Roberts Mountains thrust zone for fine grained gold deposits. This included an examination of the Maggie Creek/Gold Quarry Claims and an attempt to purchase the rights to the Blue Star Mine. Roberts assisted in the field to help differentiate between jasperoid in the autochthonous Eastern Assemblage carbonate rocks and silicified shale in the allochthonous Western Assemblage (Roberts, 1986; Coope, 1991).
The trace of the Roberts Mountains Thrust, readily identifiable by the marked lithological difference it marked, was mapped throughout the Lynn Window. Prospecting and sampling of the rocks along the contact was initiated and within a few weeks anomalous gold values in excess of 1 g/t (0.03 oz/t) were encountered along the trace of the thrust. These gold anomalies were found within strongly silicified (jasperoid) and barite veined exposures approximately 4.5 km to the south-east of the Blue Star Mine. A return visit confirmed these initial values with grab and channel samples assaying from 1 to 6 g/t Au. Seventeen consecutive claims were taken out over what was to become the western section of the Carlin Pit. A series of costeans were then cut by bulldozer. One of these, centred on a quartz-porphyry dyke which was thought to be related to the mineralisation, returned low values from the dyke, but 25 m of 6 g/t Au in the adjacent mildly hornfelsed sediments (Coope, 1991).
A neighbouring claim immediately to the east was optioned, mapped and sampled. The geology was complex due to intense silicification. An iron stained and slickensided fault on the margin of the claim returned values of 2 to 2.5 g/t Au consistently, while float of grey, finely porous silty limestone from which all of the carbonate matrix had been leached returned values of around 6 g/t Au. Due to the high option payments, this lease was drilled first. Drilling commenced in September 1962. The third of a line of eleven holes planned across the strike of the slickensided fault intersected 30 m @ 35 g/t Au in sheared and altered carbonates below the zone of anomalous float. This led to a float sampling program which delineated the ore deposit. A detailed drilling program ensued, with a reserve of 11 mt @ 9.8 g/t Au being outlined. The Carlin Mine commenced production in April 1965 at the rate of 2000 tpd. The total project costs through to production were $US 10 m (Coope, 1991).
Geology
The succession within the Carlin mine area comprises the lower plate autochthonous Eastern or Carbonate Assemblage, and the overlying, but generally older, allochthonous Western or Siliceous Assemblage. These two plates are separated by the regional package of thrust surfaces known as the Roberts Mountains Thrust. Both assemblages are unconformably overlain by Tertiary cover.
For details of the setting and stratigraphy of the Carlin Trend, see the separate Carlin Trend - Geology record.
The stratigraphy in the mine area is as follows, from the base, commencing with the lower autochthonous Eastern or Carbonate Assemblage:
* Siluro-Devonian, Roberts Mountains Formation, 300 to 360 m thick - yellow-red-brown to dark grey, medium to coarse grained silty limestone, calcareous siltstone and dolomitic siltstone. The unit is very carbonaceous in the East Pit. It is distinguished within the Carlin pits by planar, well defined laminations and contains interbedded 2 to >50 cm thick medium to coarse grained bioclastic silty limestone beds which are commonly selectively silicified adjacent to faults structures. The upper contact with the overlying Popovich Formation is conformable. Carbon flooding of the Roberts Mountains Formation is localised below this upper contact (Myers, 1993; Bakken & Einaudi, 1986). The upper 10 to 15 m of the unit has been known as the "maroon marker bed" and is described as a maroon weathering silty to muddy limestone (Lewis, 1991). This maroon bed may represent the oxidation of the zone of carbon flooding below the Popovich Formation contact (Myers, 1993). Locally at the base of the unit, near the contact with the underlying Hanson Creek Formation, medium to thick beds of coarse grained, silty to sandy limestone occur (Myers, 1993). The Hanson Creek Formation is found to the south of the Carlin pits.
Where sighted in the East Pit, the un-oxidised Roberts Mountains Formation is a strongly carbonaceous and very black siltstone to mudstone. Carbon readily rubs off onto hands when touched. It is very well bedded on a 1 to 5 mm basis, contains minor visible pyrite and has some calcite veining, although the latter is not prominent. In places fine pyrite is abundant, while elsewhere well developed pyro-bitumen is present on fractures and as zones within the sediments. Adjacent to the areas of pyrite and pyro-bitumen the section has apparently been de-calcified and is within the oxidised section of the orebody. Within the oxidised zone the sediments are an un-note worthy pale grey to buff, light and porous siltstone. Those beds that are blackest are apparently more clayey than the less carbonaceous sections. As a consequence the carbonaceous content has not been oxidised. The more porous beds are preferentially oxidised. It has been suggested that the carbon is also present as a front pushed outwards and upwards by advancing hydrothermal solutions associated with the emplacement of ore. As such the carbon tends to be better preserved on the peripheries of the alteration (pers. observ., 1993).
Three main types of beds can be texturally distinguished in the mineralised upper Roberts Mountains Formation within the Carlin pits, namely, 1) laminated argillaceous dolomitic siltstone with interbedded calcareous siltstones, which comprise around 90% of the mineralised sequence. These silty carbonates, where un-altered outside of the ore zone, contain 30 to 50% calcite, 15 to 35% dolomite, 20 to 30% quartz silt, 5 to 15% illite, 2 to 4% K-feldspar silt, 0.2 to 1% pyrite and 0.2 to 0.45% organic matter. The silt sized quartz and feldspar grains are less than 0.05 mm across and are set in a matrix of euhedral to anhedral calcite, dolomite and clay minerals. They comprise centimetre thick laminated platy argillaceous beds with intercalated coarser grained calcareous siltstones, generally <2 cm thick, containing more quartz silt and lesser clay than the adjacent laminated beds. Pyrite is concentrated in laminae at the base of individual beds. In general they are devoid of fossils, with the exception of graptolites; 2) relatively massive carbonate beds which are apparently not present in significant quantities; 3) medium bedded, medium to coarse grained bioclastic silty limestone. These coarser beds are composed largely of bio-clastic debris comprising clastic fossil rich calcareous grainstones and packstones that range in thickness from 2 to >50 cm and are continuous along strike for several tens of metres. They are composed of 0 to 30% bioclastic debris, 20 to 40% sub-angular to sub-rounded 0.05 to 5 mm quartz veins and 0 to 30% micritic limestone clasts that are 0.1 to 10 mm in diameter, all set within a matrix of very fine grained calcite and clay minerals. These beds account for about 10% of the section and were selectively leached, brecciated and replaced by silica (Bakken & Einaudi, 1986; Kuehn & Rose, 1992).
* Devonian, Popovich Formation, 100 to 120 m thick, although where exposed it is commonly thinner due to faulting and to differences in the original sedimentary thickness. The Popovich Formation is predominantly made up of massive limestones which are primarily of micritic to silty limestone. Throughout the Carlin Pit it is characterised by massive, blocky outcrops and a high concentration of calcite veins (Lewis, 1991; Myers, 1993). It may be divide into three sub-divisions as follows:
i). a lower section composed of massive beds of micrite and limestone with occasional silty limestone interbeds. This section is generally highly fractured with numerous calcite veins. The transitional contact with the underlying Roberts Mountains Formation has increasing amounts of silty limestone as the contact is approached (Myers, 1993);
ii). a middle section comprising medium to thin bedded (2 to 12 cm), wavy, laminated, fine grained silty limestone (Myers, 1993);
iii). an upper section comprising dark-grey to yellow-red-brown calcarenite and massive bio-clastic limestone. It is generally very calcareous and highly fractured. A basal 3 m thick breccia zone is made up of matrix supported rounded clasts of laminated silty limestone. De-carbonatisation and silicification of calcarenite is common near the contact with the overlying Rodeo Creek Formation. Near the base massive beds of very calcareous micrite and limestone are found which are highly fractured with numerous calcite veins (Myers, 1993);
The upper Popovich Formation grades into calcarenite near the contact with the overlying Rodeo Creek mudstone. Within the West Pit this contact is gradational, while elsewhere at Carlin the two units are separated by a fault which was historically mapped as the Roberts Mountains Thrust (Myers, 1993).
Where sighted in the West Pit, the contact with the Rodeo Creek Formation appeared conformable and gradational. It was also noted that the Popovich Formation is a massive grey-green limestone with variable amounts of calcite veining. Interbeds of siltstone increase downwards as the limestone became more silty, although these were finer grained than in the silty limestone of the underlying Roberts Mountains Formation. The zones of silty limestone and siltstone interbeds exhibit finer bedding, generally on a 1 to 10 mm basis, within the otherwise massive limestone unit. These silty lower sections are closer to being a mudstone and become very thinly laminated, contain finely disseminated pyrite and are very carbonaceous towards the base of the unit. Intra-formational breccias up to 1 m or more in thickness and slump folds were obvious locally within the limestones (Pers. observ., 1993).
* Devonian, Rodeo Creek Unit, 85 to 120 m thick - composed predominantly of argillite and mudstone with interbeds of siltstone and sandstone. It is characterised by regular 2 to 7 cm thick beds of mudstone, argillite and siltstone. In detail it comprises grey to black argillite and mudstone interbedded with grey and yellow-red-brown fine grained siltstone. Planar beds which are laterally consistent are common although these may be locally folded near fault structures. Small quartz veinlets are common, cross-cutting beds perpendicular to lithological boundaries. Interbedded 2 to 7 cm thick sandstones may be silicified and mineralised, and are more common near the contact with the underlying Popovich Formation (Myers, 1993). Where sighted within the West Pit it comprised intercalated, well bedded sandstone, siltstone and chert. Bedding was on a 1 mm up to 2 to 3 cm spacing. Some more massive sandstone beds up to 1 m thick were obvious (Pers. observ., 1993).
* Roberts Mountains Thrust - marks the boundary between the Eastern Autochthonous Carbonate Assemblage of the lower plate from the Western Allochthonous Silici-clastic Assemblage of the upper plate. The thrust varies from up to 10 m of fault gouge to none (Baker, 1987). Where sighted in the West Pit during the visit it was from 0.2 to 1 m thick and comprised a zone of sheared sediments (Pers. observ., 1993).
* Ordovician, Vinini Formation, unknown thickness - generally composed of cherts, graptolitic siltstones, mudstones and occasional silty limestone. It is typically very structurally deformed with chaotic, incoherent bedding and is distinguished by the presence of chert interbeds and local occurrences of basalts. The basal section of the formation may contain sedimentary breccias, calcareous sandstones, black shales and cherts (Lewis, 1991). Within the mine area three facies sub-divisions are described by Myers (1993). These are:
i). A thin section, 15 to 25 m thick, which is locally present near the base of the sequence and is composed of fine grained to very fine grained silty, laminated limestone to limy mudstone with 2 cm thick platy beds. It typically occurs between zones of thin bedded to massive mudstone (Myers, 1993).
ii). Thin bedded to massive mudstone with occasional weakly calcareous mudstone found in the lower to middle part of the formation. Bedding is highly contorted and chaotic with flaser and boudinage structures common. It is often strongly carbonaceous with fine grained pyrite visible along bedding planes. Carbonaceous mudstone typically has a noticeable waxy lustre (Myers, 1993).
iii). Yellow-red-brown mudstone and siltstone with lenses of black and green chert and rare interbeds of pillow basalt. Bedding is highly contorted and chaotic with 2 directions of shearing and flaser structures. This facies predominates in the top half of the formation (Myers, 1993).
* Jurassic to Cretaceous, Dykes, 149 to 121 Ma (although the earlier dates are dubious) - these are mapped throughout the Carlin Mine and usually occupy faults which displace the host rocks. Occasionally they may occur as sills within the sediments, following bedding. All of the dykes present within the pit are severely altered, mostly strongly argillised, and are erratically mineralised. At least two separate intrusive phases are indicated, and both cut all other lithologies within the pit (Myers, 1993). The less altered examples are a fine grained porphyry with very strong pyrite. Dykes are commonly 1 to 2 m thick in the pit (Pers. observ., 1993). Some dykes may be up to 15 m thick. Weakly altered dykes have been identified as granodioritic to dacitic in composition, with locally abundant biotite phenocrysts. In thin sections these have quartz, biotite, bladed hornblende and scattered orthoclase phenocrysts in a groundmass of crypto-crystalline quartz and plagioclase (Radtke, 1985).
* Cenozoic, Cover - sediments similar to the Tertiary Carlin Formation are known locally away from the orebody, as are areas of alluvium. The Carlin orebody is largely exposed.
Structure
The Carlin gold deposit is spatially associated to both the Tuscarora Mountains Antiform and certain sets of high angle normal faults. Regionally the Carlin deposit lies near the crest of the NNW striking Tuscarora antiform. High angle normal faulting which has broken the Tuscarora Range into numerous blocks is common in the lower plate below the Roberts Mountains Thrust. The zone of concentrated gold mineralisation occurs in an area where closely spaced faults have intensely shattered the carbonate rocks. None of the through-going regional high angle faults, with some possible exceptions, have apparently influenced the localisation of ore other than to contribute to the structural preparation of the deposit. Field studies indicate that an apparent relationship with the Roberts Mountains Thrust is coincidental and that the thrust has no influence on the localisation of mineralisation. The main orebody is developed within the upper Roberts Mountains Formation below the Popovich Formation, while the Roberts Mountains Thrust is above both the Popovich and overlying Rodeo Creek Unit (Radtke, 1985). It is possible however that the ore bearing position may have been influenced by competence differences which were preferentially deformed during movement on the parallel Roberts Mountains Thrust.
The Carlin mine area is divided into three main structural domains which coincide with the three main pits, the West, Main and East (see Figure 118 and Figure 119). These domains are determined by bounding faults which juxtapose upper plate Vinini Formation silici-clastics against lower plate carbonates. The apparent displacement on these bounding faults ranges from several hundred to several thousand metres. They include the north-west striking Castle Reef Fault to the south of the deposit, the NNE striking Mill Fault to the north of the orebody and the NNW trending Leeville Fault forming the eastern margin of the deposit. Other structures include the north-west striking, dyke filled Midway Fault and the north-east trending Unk Fault (Myers, 1993).
Most of the faults in the mine area are high angle structures, many of which are filled with jasperoid veins or dykes. Cross-cutting relationships between the faults, alteration and mineralisation is quite complex. It has been indicated however, that some of the NNW faults are pre-mineralisation and may have acted as conduits. Some of these faults have been subsequently reactivated by Basin and Range development (Lewis, 1991).
Some of the more prominent faults are:
* The Leeville Fault which is a NNW trending structure of regional significance. It is an important ore controlling feature forming the eastern limit of the exposed host Roberts Mountains Formation in the East Pit. The fault zone itself is made up of numerous high angle normal fault strands ranging from a few metres to over 400 m in length (Lewis, 1991). Regionally the Leeville Fault has been traced for up to 8 km. It dislocates the Tuscarora Antiform and displaces the Roberts Mountains Thrust down to the east, juxtaposing it with the Vinini Formation at the surface. In the East Pit it is a zone some 4 to 11 m wide of gouge enclosing breccia fragments and is bounded by well defined planes that dip at 75 to 80°E. On the north-eastern margin of the ore zone, mineralisation is found on both sides of the fault in lower plate carbonates to the west and in upper plate Vinini Formation to the east with no apparent change in attitude, although the grades differ (Radtke, 1985).
* The Mill Fault has had normal displacement of 100 to 300 m and trends in a NNE direction, with north-west side down displacements. It is truncated by the Leeville Fault, but in turn displaces the Hardie Fault. In the mine area however, it shows field relationships that suggest it has the most recent activation. Elsewhere early displacement indicate that it is pre-ore in age (Radtke, 1985).
* The Hardie Fault is poorly exposed and is of limited lateral length. Never-the-less it is a major structure with a large vertical displacement of more than 300 m. The Hardie Fault strikes at approximately 60° (one of the general NE set) and dips at 50 to 65°N. It is marked by a zone of gouge and breccia ranging from about 1 to more than 25 m in width. The breccia fragments consist of fresh and silicified rocks from both the upper and lower plates. Intense alteration of the rocks within the fault zone suggest that it is a pre-ore structure. This alteration includes zones of intense silicification and the introduction of hydrocarbons (Radtke, 1985).
The characteristics of each of the three domains within the Carlin Mine are as follows:
* The West Pit domain is bounded by the Castle Reef Fault to the SW and the Mill Fault to the east. It is dominated by a large dextral shear zone developed along the WNW striking Castle Reef Fault. Other major structures in this domain apart from the Mill Fault, include another NE striking parallel fault, several NW striking faults and the Roberts Mountains Thrust. With the exception of the latter, all exhibit a normal sense of movement (Myers, 1993).
* The Main Pit domain is developed between the Mill Fault to the north-west and the Castle Reef Fault to the south. Several episodes of faulting are recognised in this domain. The major structures include two NW striking fault controlled dykes, the Main Pit and West Main Pit Dykes. These are cut by later NE striking structures, which are themselves truncated by a series of NNW striking faults. The apparent sense of movement along both the NE and NNW faults is normal with displacements of 5 to 10 m. The Hardie Fault, a NE striking structure, occurs along the north side of the deposit and juxtaposes upper plate Vinini Formation against lower plate carbonates (Myers, 1993).
Two types of fault are recognised in the Main Pit, 1) branching, discontinuous, commonly healed faults that are restricted to altered rocks and generally do not displace alteration, and 2) straight, through-going faults that commonly displace alteration zones and include the major faults described above (Bakken, 1990).
Both types of fault are steeply dipping and have normal displacement. Generally they are oriented in one of three directions that are parallel to the district wide structures, namely, 1) NNW, parallel to the Tuscarora Antiformal axis and the Carlin Trend, with indications that they accommodated both pre- and post-ore movement, but also focused the development of ore; 2) a NE trending set, which are commonly branching, mineralised and occur along or at low angles to bedding. These are interpreted to have been developed during the formation of the orebody in response to carbonate dissolution and the accompanying collapse of overlying beds, 3) NNE striking faults which are predominantly through-going and barren of gold but may contain barite. They are late and displace both the NNW and NE sets, and are most likely related to the Basin and Range episode (Bakken, 1990).
* The East Pit domain is bounded by the Midway Fault to the west and by the Leeville Fault to the east and is characterised by major NE striking structures. Most faults have a normal displacement, although dextral strike slip components are observed on the Midway and Leeville Faults. Many of the NW striking faults within this domain are filled with dykes (Myers, 1993).
In general NE faults are cut by NNE and NNW striking faults. Both the NNE and NNW sets offset ore, alteration zones and veinlet types. In a few cases the NNW set offsets alteration zones zoned around that fault. As of 1986 jasperoid had not been found in the NNE fault set, nor do alteration patterns radiate from these structures. However barite±sphalerite±galena veins fill these faults and are boudinaged suggesting that some barite precipitation post-dates silicification and that fault movement post-dates barite emplacement. NE striking and NNW trending healed faults existed prior to the hydrothermal alteration and were probably not through-going features, but rather discontinuous, branching faults that died out near the top of the Roberts Mountains Formation. Those faults that contain fragments of jasperoid may have been healed, but moved since silicification (Bakken & Einaudi, 1986).
Two styles of folds have been mapped. The first are asymmetric with axial planes striking at between 330 and 10°. These planes dip steeply to the west, while the axes plunge gently to the north. They are consistent with drag folding associated with the Antler thrusting of the Roberts Mountains Thrust, although they may also be sympathetic folds on the eastern limb of the Tuscarora Antiform. Monoclines, the second fold style mapped in the pit, have axial planes with a strike of 50 to 70° and moderate south dip. The axes of these latter folds are near horizontal. They may be the product of drag between parallel faults of the NE set (Bakken & Einaudi, 1986).
Alteration
Both hydrothermal and supergene alteration processes have modified the abundance of all major and minor minerals in the host rock. Pervasive weathering has affected the abundance and distribution of clay minerals, iron sulphides/oxides, carbonaceous material and to some extent the carbonate minerals and porosity. Only the abundance of carbonate and silica retains a systematic mappable distribution that relates to gold mineralisation throughout the Main orebody (Bakken & Einaudi, 1986).
The original relative abundance of quartz and carbonate is not the same in the different lithologies of the host. In addition, when altered, the medium to coarse grained bioclastic silty limestone beds are selectively silicified and de-calcified such that they then contain more quartz and less calcite than adjacent altered laminated argillaceous dolomitic siltstones (Bakken & Einaudi, 1986).
Five alteration zones based on the increasing abundance of silica and decreasing quantity of carbonate minerals have been defined for each of the two dominant lithologies. These are:
* Zone 1, Recrystallised Calcite - which is characterised in the coarser bioclastic silty limestone by the development of a coarse grained calcite matrix, and in the laminated argillaceous dolomitic siltstone by a highly calcareous matrix. This zone carries calcite, dolomite, illite, quartz, K-feldspar and pyrite and contains extensive calcite veining. It is the least altered interval occurring on the lateral and hangingwall periphery of the overall alteration zone. The calcite veining is taken to represent precipitation of calcite removed during the de-calcification of the more altered sections of the system. Within the Popovich Formation, which also contains more primary calcite, these white calcite veins commonly amount to 'several percent' of the rock and range from micro-veins of <0.2 mm in thickness and several cm's in length, to veins that are >100 mm thick and are continuous over tens of metres. In general however, these veins are <10 mm thick and are better developed in the more competent, less argillic beds (Kuehn & Rose, 1992; Bakken & Einaudi, 1986).
* Zone 2, Decalcified - with incipient silicification, and calcite depletion such that there is only minor remnant calcite. The laminated argillaceous dolomitic siltstone generally has a moderately carbonatic matrix and is porous and soft in the oxidised zone, while the coarser bioclastic silty limestone is porous with recrystallised calcite in the matrix and has partially silicified fossil debris. These de-calcified rocks are typically more porous due to carbonate removal, and noticeably less dense than the un-altered rocks. The rock is generally composed of dolomite, quartz, illite-sericite, carbonaceous matter and pyrite. Decalcification typically exhibits strong stratigraphic control (Kuehn & Rose, 1992; Bakken & Einaudi, 1986).
* Zone 3, Decarbonated-Argillic - characterised by the almost complete depletion of calcite and of dolomite, and by moderate silicification. The laminated argillaceous dolomitic siltstone generally only has a weakly carbonatic matrix and is porous and soft in the oxidised zone, while the coarser bioclastic silty limestone is porous with a sugary silica matrix, silicified fossil debris, has no matrix calcite but has some remnant dolomite rhombs suspended in a silica matrix. As alteration increases, following the removal of calcite, dolomite is depleted near the faults and permeable interbeds in which silicification is more intense. As the dolomite disappears it leaves a porous, leached low density rock composed of quartz, illite-sericite, pyrite, organic carbon and locally minor dickite-kaolinite1. Accompanying the dolomite depletion, K-feldspar and illite are transformed to sericite and dickite-kaolinite. In zones that were not silicified, these decalcified and decarbonated rocks have commonly compacted to a laminated shale with very little porosity and permeability. In the footwall of the ore, large volumes of rock have been carbonate depleted. While the de-calcified alteration is grossly zoned around the entire mineralised volume, the more intense carbonate removal is zoned around individual interpreted hydrothermal influx channels such as faults and coarse permeable bioclastic rich beds. Thus, at a small scale, the geometric relationships are complex with tongues of locally silicified and decarbonated rock extending along faults and permeable channels into broader de-calcified zones (Kuehn & Rose, 1992; Bakken & Einaudi, 1986).
* Zone 4, Siliceous-Argillic - alteration close to interpreted influx zone, as described in Zone 3, are characterised by silicification, with illite-sericite still present. In zones of higher silicification the latter is altered to kaolinite-dickite. The laminated argillaceous dolomitic siltstone has a completely non-calcareous matrix, well developed fracturing, is less porous than Zone 3 and is moderately hard. The coarser bioclastic silty limestone is composed of dense dark sugary quartz which forms bedded jasperoid, fossil debris is silicified, there is local brecciation and local euhedral rhombs of dolomite remain. The rock in general is made up of quartz, illite-sericite, pyrite and dickite-kaolinite. The presence of remnant encapsulated rhombs of carbonate and clots of mica suggests that silicification locally occurred prior to complete carbonate removal and illite destruction. In all cases silica deposition significantly reduces porosity and increases rock density (Kuehn & Rose, 1992; Bakken & Einaudi, 1986).
* Zone 5, Jasperoid - the most intense alteration, dominated by the deposition of silica to form intensely silicified beds and fault zones. These positions also typically have dickite-kaolinite on shears and fractures in more intensely altered zones. Severe carbonate removal is interpreted to have resulted in a very porous, leached rock. In many areas it has been finely fractured, possibly by solution collapse. The laminated argillaceous dolomitic siltstone has a decreased porosity and is cut by networks of fine grained thin quartz veinlets, with brecciation developed locally and a hard siliceous matrix. The coarser bioclastic silty limestone forms a bedded jasperoid which is commonly brecciated with incorporated fragments of silicified, thinly laminated beds along the fringes of the jasperoid, although fossil debris is no longer identifiable (Kuehn & Rose, 1992; Bakken & Einaudi, 1986).
Two patterns of distribution of these alteration zones are indicated, as follows. 1) In sections cutting the ore, alteration is zoned outwards from mineralised sub-vertical, NE and NNW striking silicified faults and bedded jasperoid (Zone 5), through poorly bedded, locally brecciated, carbonate depleted rock (Zone 3), to moderately well bedded, calcite depleted rock (Zone 2), to well bedded, carbonate abundant rock (Zone 1) to un-altered, silty, dolomitic limestone. 2) Peripheral to ore however, alteration is zoned outwards from well bedded silicified rock (Zone 4) to well bedded carbonate abundant rock (Zone 1). Intermediate zones 2 and 3 are not developed peripheral to ore and only minor trace quantities of gold are present in altered rock of zones 1 and 4 (Bakken, 1990).
Both field and petrographic work indicate that up to 50% of the original volume of rock was lost during alteration to zones 2, 3 and 5. In a few places, collapse of up to 50% can be measured across alteration zones (Bakken & Einaudi, 1986). Kuehn & Rose (1992) similarly suggest that dissolution of 30 to 50% calcite and 15 to 35% dolomite takes place adjacent to and within the ore zone. They add that evidence for significant volume decrease is widespread at both mine and hand specimen scale. This includes evidence of compaction, changes in thickness of altered and un-altered equivalents, collapse breccias, changes in abundance of quartz and fossil fragments, and the flattening of fossils and worm burrows (Kuehn & Rose, 1992). Myers (1993) indicates that collapse is more readily observed on a grain scale than in outcrop in the field.
In general, carbonate mineral abundance decreases and silica abundance increases from the hanging wall to the footwall of the orebody. The same trend is observed towards steeply dipping jasperoid bodies which are discordant to bedding in the immediate footwall of the orebody. The abundance of bedding discordant jasperoid bodies or 'veins' and the volume of intensely to completely silicified rocks increases with depth. In addition, the width over which the transition from unaltered to completely silicified rock takes place decreases with depth in each of the main lithologies (Bakken & Einaudi, 1986).
Adjacent to fault controlled transgressive jasperoid veins, the medium to coarse grained bioclastic silty limestone beds are selectively silicified relative to the enclosing laminated argillaceous dolomitic siltstone. In the footwall of the ore, silicification along these coarser beds extends for tens to hundreds of metres away from silicified fault zones and commonly shows outward lateral zoning through decreasing silica and increasing carbonate, eventually grading into un-altered stratigraphic equivalents. In contrast the transition across bedding between individual zones of alteration is gradational over centimetres (Kuehn & Rose, 1992; Bakken & Einaudi, 1986).
The selective alteration within the medium to coarse grained bioclastic silty limestone beds is lobate with the more intensely silicified rocks occupying the centres of these coarser beds. Quartz veins are usually absent and the process of silicification appears to be pervasive and intergranular. In general, original sedimentary structures are preserved by the silicification within the dark grey, sugary textured bedded jasperoid, although they are obscured by brecciation within less than a metre of transgressive jasperoid veins. The more intensely silicified horizons, the bedded jasperoids, are sporadically brecciated along strike and show textures interpreted to have been formed by collapse. Because individual angular fragments are only partially silicified, collapse is interpreted to have preceded silicification. The collapse brecciation is attributed to de-calcification. However, as distal, non-silicified examples of collapse breccia can be found, it appears that in some locations silicification follows decalcification, brecciation and mineralisation (Kuehn & Rose, 1992; Bakken & Einaudi, 1986).
Based on exposures in the Main Pit, bedded jasperoids are far more abundant than discordant or vein jasperoids. The thickness of bedded jasperoids and the development of internal breccias within them increases towards discordant jasperoids as the coarser bio-clastic beds and adjacent finer argillaceous dolomitic siltstones become completely silicified. The latter finer grained rocks are generally only completely silicified in the immediately vicinity of jasperoid veins. In these zones they have a low porosity and are cut by a network of thin, fine grained quartz veinlets. In the southern extension of the Main Pit, bedding parallel breccias are present in both the bedded jasperoid and in the overlying, partially silicified fine grained laminated argillaceous dolomitic siltstone. Jasperoid beds formed from the coarser bioclastic silty limestones are, in these cases, characterised by massive, silicified, un-brecciated bases that grade upwards through partially brecciated jasperoid into brecciated tops which contain angular clasts of fine, laminated, argillaceous, dolomitic siltstones which have been variably silicified. This implies solution collapse (Bakken & Einaudi, 1986).
The differences in the degree of silicification within the Upper Roberts Mountains Formation are enhanced by oxidation. Within the zone of oxidation the more heavily silicified rocks are darker and less decomposed, whereas those only moderately silicified are buff coloured and contain clay minerals and some calcite (Bakken & Einaudi, 1986). Contrary to the assertion of Radtke (1985) that there were two periods of oxidation, the first being hypogene and the second supergene, Bakken & Einaudi (1986) argue that all of the oxidation was supergene. In general the rocks in the hangingwall of the ore mined to 1986 were both oxidised and un-oxidised (the latter contain pyrite and carbonaceous material). Rocks in the orebody and its immediate footwall to that time were largely oxidised. Less than 10% of the rock in the Main Pit to that stage contained pyrite or carbonaceous matter (Bakken & Einaudi, 1986).
Jasperoid veins exposed in the Main Pit vary from 0.2 to 2 m in width and strike at 40 to 70°, mostly dipping steeply to the south (ie. members of the NE set). In a few cases jasperoid veins that contain trace amounts of gold fill NNW trending, steeply dipping faults. Jasperoid veins usually contain silicified angular lumps of each of the main lithologies of the upper Roberts Mountains Formation that are found in the walls, as well as brecciated jasperoid and barite in a matrix of dark grey micro-crystalline quartz. Cavities in the jasperoid are commonly lined with white, coarse grained quartz and less commonly by kaolinite or barite. Sometimes jasperoid breccias are confined to silica-breccia veinlets in jasperoid that branch and dissipate rapidly into intensely silicified country-rock. In other cases silica-breccia and fragments are stretched and broken or sheared along the axis of the breccia, with the surrounding matrix having been silicified. These instances are taken to imply movement during silicification or multiple events (Bakken & Einaudi, 1986).
Faults containing jasperoid veins are generally not through-going structures. They commonly branch upwards into several discontinuous, partially silicified, branching faults that disappear into locally deformed, thinly laminated beds in the Roberts Mountains Formation. Where the main fault has a core of silicified breccia, this breccia passes into and pinches out within the upward branching faults described above. As suggested in the 'Structure' section these faults may have accommodated solution collapse (Bakken & Einaudi, 1986).
Mineralisation
The gold at Carlin occurs as micron to sub-micron sized particles associated with quartz, with pyrite and with clays (mainly illite1) in siltstones. Arsenic, mercury and antimony exhibit the strongest spatial correlation with the distribution of gold, occurring as cinnabar, native arsenic, realgar, orpiment and stibnite. These minerals occur within the orebody, but with the exception of cinnabar, are not found in contact with gold (Myers, 1993).
Within un-weathered ore, gold is present in two distinct habits, namely 1) particles that are 0.005 to 0.020 µm in diameter and are encapsulated in pyrite, cinnabar and in one sample in quartz; and 2) particles that are 0.020 to 0.10 µm across associated with illite. Gold is not present on, or within the outer 0.005 µm of, the surface of pyrite. These data are taken to indicate that gold, pyrite and cinnabar precipitated synchronously in an illite stable environment (Bakken, 1990).
Within partially weathered ore, where iron oxides predominate, pyrite is present but rare and carbon is observed, the gold varies in grain size between 0.03 and 1 µm. These grains are round and are associated with 1) sub-micron to micron sized poly-crystalline masses of 0.1 µm illite and 0.005 µm grains of maghemite, and 2) 1 to 10 µm masses of amorphous iron oxide with less abundant illite (Bakken, 1990).
Within weathered ore, where visible gold and carbon are both absent, the gold is round and coarser grained from 0.1 to 1 µm. It is commonly associated with 1 to 10 µm masses of amorphous iron oxides and less abundant illite (Bakken, 1990).
These observations suggest that the gold coarsens with weathering, that gold was associated with an iron bearing phase prior to weathering and that illite remains stable during weathering (Bakken, 1990).
At Carlin, gold mineralisation is both structurally and stratigraphically controlled. The majority of the ore is contained within altered silty-carbonate of the upper 150 m of the Roberts Mountains Formation, immediately below the contact with the overlying Popovich Formation (Myers, 1993; Andrew, 1993). The ore grade material at this stratigraphic position is normally 45 to 60 m thick, but near some faults is thicker (L McEvers, pers. comm., 1993). High grade mineralisation however, is also controlled by high angle structures, as follows:
* West Pit - gold mineralisation is spatially related to a broad north-west striking, north-east dipping zone of shearing along the hanging wall of the Castle Reef Fault (Myers, 1993). The orebody is a tabular, somewhat vein like mass, dipping steeply north, apparently parallel to the hangingwall of the Castle Reef Fault. Ore grade is truncated to the west by a north-south striking high angle structure. Mineralisation extends continuously for approximately 300 m south-easterly at which point the orebody assumes a pipe-like configuration with a 70° northerly plunge. Ore controls are east-west to WNW trending high angle faults intersecting NW to N trending high angle structures (Baker, 1987);
* Main Pit - the distribution of gold mineralisation is controlled both by north-east and NNW2 striking high angle structures and by the stratigraphic contact between the Roberts Mountains and Popovich Formations, specifically immediately below a massive calcareous bed near the base of the Popovich Formation. High grade mineralisation is found at the intersection of NNW striking structures and the stratigraphic contact. Lower in the sequence, but within the Roberts Mountains Formation, high grade mineralisation is also commonly localised at the intersection of NE and NNW striking faults (Baker, 1987; Myers, 1993). This has led to two forms of orebody, namely somewhat irregular pod like masses, and largely sheet like masses which are basically parallel to the strike and dip of bedding. Ore grade mineralisation occurs over an apparent length of 850 m, with a general 60° trend (Baker, 1987).
* East Pit - this pit has historically yielded the highest grade mineralisation, locally with sections containing up to 60 g/t Au. Two orebodies were mined. The first was an irregularly shaped tabular body, while the second was generally pipe shaped with an overall north-easterly trend. Mineralisation is primarily within the Roberts Mountains Formation calcareous siltstones, but may also occur within the Popovich Limestones adjacent to structures. The high grade mineralisation is localised both at the intersection of different high angle faults within the Roberts Mountains Formation as well as at fault intersections with the stratigraphic contact of the Popovich and underlying Roberts Mountains Formation and along this same contact (Baker, 1987; Myers, 1993).
The combination of the fault and stratigraphic controls result in the orebody taking the form of a series of anastomosing pods and lenses of gold mineralisation whose overall geometry and boundaries are determined by a given assay cut-off. At a 0.5 g/t Au cut-off, mineralisation is fairly continuous over the 2 km strike of the orebody (Kuehn & Rose, 1992). Note however that there are still significant gaps and irregularities. The ore as illustrated on this plan is the exposed outline within the pit floor and walls, not a plan projection. The ore dips at 30° to the NNW below the Popovich Formation.
Three stages of element mobilisation have been recognised, distinguished by processes related to:
i). Hydrocarbon maturation - cross-cutting relationships and fluid inclusion studies indicate that organic metagenesis occurred under conditions of 155±20°C and 0.6 to 1.4 Kbars and was wholly or dominantly a pre-ore event, un-related to the main gold mineralising event. Hydrocarbons were emplaced during or after the late Triassic to late Jurassic development of the NW to NNW, 325°, trending Tuscarora Antiform. Organic maturation is interpreted to have culminated during heating by the Mesozoic granodioritic intrusives. Redistribution of organic matter during the subsequent gold mineralisation is restricted to the physical concentration of solid organic carbon by lithologic compaction or during removal by oxidation (Kuehn & Rose, 1992). Discontinuous calcite±hydrocarbon veinlets are un-related to the gold mineralisation. They occupy NNW trending deformational structures, are found in both altered and un-altered rock and are cut by all other veins and veinlets (Bakken, 1990). These veinlets are found up to several kilometres distant from the ore. Their attitude suggests that they may be related to the development of the Tuscarora Antiform. As well as in the calcite veins, carbonaceous matter is also present in stylolitic contacts, along micro-faults and as coating on fractures and grains (Bakken & Einaudi, 1986).
ii). Gold ore deposition stage - gold mineralisation occupies a range of positions within the alteration zone, occurring most abundantly in de-calcified rocks which still contain appreciable dolomite and are only moderately silicified (i.e., zones 2 and 3). In both oxidised and un-oxidised ore the highest Au concentrations and the major volume of gold mineralisation is located above the uppermost strongly silicified horizon. This is apparent on the hand specimen, outcrop and mine scales. Both the bedded and transgressive jasperoids typically contain lower Au values than immediately adjacent less silicified, decalcified, dolomitic shales and argillic carbonates. Erratic high Au values are also found in intensely de-calcified and/or silicified rocks and less commonly in calcareous rocks (Kuehn & Rose, 1992).
A study of grade distribution has indicated that within the Main Pit, gold values are reasonably continuous over distances of several tens of metres along strike. Grades are also fairly constant within beds of the same alteration type, but vary by a factor of five between adjacent beds of different lithology but the same alteration type. Grades vary however, by up to two orders of magnitude between different alteration types within the same lithology. Contouring of blast hole grades over 10 mining levels (65 m vertically), indicate that the morphology of the orebody is conformable to bedding in the upper levels but becomes more vein-like than stratabound with depth. In cross-section the 0.7 g/t Au contour is shaped like a tilted mushroom, elongated up-dip. Contours radiate from jasperoid veins at the base of the orebody and expand upwards along the boundary enclosing weakly calcareous and incipient silicified to moderately silicified Roberts Mountains Formation. Rock chip sampling suggests that higher grade values of >15 g/t Au occur in incipient to moderately silicified, weakly calcareous Roberts Mountains Formation. Both the laminated argillaceous dolomitic siltstone and the coarser bioclastic silty limestone beds where they occur in these alteration types (i.e., zones 2 and 3) may contain gold values of >30 g/t Au (Bakken & Einaudi, 1986).
Fluid inclusion studies indicate a high concentration of CO2 in quartz veins cutting jasperoids at pressures of 800±400 bars and temperatures of 215±30°C. It is suggested that this CO2 and accompanying H2S lowered the pH to decalcify the carbonate host rocks and alter K-feldspar and illite to dickite-kaolinite as is observed. These data, if correctly interpreted, indicate a depth of 4±2 km for the formation of the ore (Kuehn & Rose, 1992). Andrew (1993) reports that 'isotope data' suggests the ore fluid was meteoric. Kuehn & Rose (1992) interpret data to indicate that the silicification, which is spatially associated with the decalcification, may have resulted from the mixing of meteoric waters with the low pH, CO2 rich media responsible for the de-calcification of the host carbonates. This meteoric/non-magmatic water may have been supplied by the fluids of the hydrocarbon reservoir (probably H2S bearing) with which magmatic waters mixed.
Gold mineralisation can be divided into a main and a late phase. The late phase veins cut jasperoids and other intense silicification in the vicinity of the orebodies. Because of the coherent spatial associations of Au, As and Sb with silicification it has been assumed that the gold was introduced, at least in part, during or in the early stages of silicification and development of jasperoids. Broadly the jasperoids are taken to represent the main phase of gold mineralisation. Mineralised jasperoid breccias locally contain clasts of altered dyke material, entrained sheared barite clasts and polylithic altered fragments which are themselves veined and brecciated. The jasperoids and carbonate rocks are in turn cut by discontinuous to planar milky white quartz veinlets and silica-breccia. They have no preferred orientation and are most abundant in highly silicified, locally brecciated rock in the core of the orebody. Un-oxidised silicified and altered zones also contain late barren white calcite veins and calcite-realgar veins (Bakken, 1990; Kuehn & Rose, 1992). Barite (±galena±sphalerite) veins and veinlets cut the ore, commonly occupying NNE striking faults, and are observed in the more siliceous alteration types. Planar, coarse carbonate (calcite, dolomite, ferroan-dolomite) veins occur in the hangingwall and peripheral to the zone of carbonate dissolution and gold deposition. They occupy NNW trending tensional structures and are most abundant in high carbonate rocks on the margins of decalcified zones. They may represent expelled carbonate resulting from the de-calcification of the host rock (Bakken, 1990). Similarly as the main gold ore occurs within the porous decalcified and only incipiently silicified carbonates on the upper fringe of the most silicified zone, it is implied that the ore is localised at the silicification front.
iii). Subsequent oxidation - an upper zone of oxidation has been imposed on the hypogene ore, resulting in the conversion of pyrite to iron oxides and the oxidation of carbon to give buff-tan rocks which contrast with the un-oxidised grey-black lithologies. Oxidation has also led to the development of kaolinite and late calcite veins (Kuehn & Rose, 1992). Trace amounts of alunite are found in the oxidised zone (Bakken & Einaudi, 1986). Oxidation is preferentially developed in the decalcified host rocks of the orebody, being less obvious in the massive un-altered hangingwall carbonates or the more silicified lithologies of the footwall.
While a range of vein types are described above, it must be appreciated that in the Carlin mine veining, with the exception of jasperoid, rarely exceeds 1% by volume within the Roberts Mountains Formation and 3% within the Popovich Formation. Veining is not apparently directly associated with ore, either in time or space (Bakken, 1990). In general, the vein mineralogy reflects that of the host rocks, with carbonate veinlets in 'un-altered' or incipiently silicified rocks, while quartz veins occur in intensely silicified lithologies. Beds that are only moderately silicified are relatively porous and contain a paucity of veins.
The Jurassic to Cretaceous dykes at Carlin, although intensely altered, are not apparently mineralised. However, good grades may be found adjacent to them over widths of up to several tens of metres. The dykes are restricted to a NW fault set (L McEvers, pers. comm., 1993).
A considerable amount of the pyrite at Carlin is apparently diagenetic in origin, as indicated by the ubiquitous presence of pyrite in un-weathered Roberts Mountains Formation both within the orebody and in un-altered rocks distant from ore. This is supported by most samples showing a correlation of Fe with Al, a predominantly sedimentary constituent (Kuehn & Rose, 1992).
At Carlin the uppermost 10 to 15 m of the Roberts Mountains Formation comprises a maroon weathering silty limestone containing 5 to 20 mm diameter mud-clasts. This is conformably overlain by the thick-bedded limestones of the Popovich Formation. This unit does not apparently host ore, but forms a barren band between the ore bearing section described above and the overlying Popovich Formation (Bakken & Einaudi, 1986). This maroon colour may represent weathering of a zone of hydrocarbon flooding above the ore. This again is described as a hydrocarbon front ahead of the decalcification and silicification zones (Myers, 1993).
The most recent source geological information used to prepare this decription was dated: 2001.
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.
Carlin
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Baker E D 1987 - Carlin mine geology: in Johnson J L (Ed.), 1987 Bulk Mineable Precious Metal Deposits of the Western United States - Guidebook for Field Trips Geol. Soc. Nevada pp 276-280
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Cline, J.S., Hofstra, A.H., Muntean, J.L., Tosdal, R.M. and Hickey, K.A., 2005 - Carlin-Type Gold Deposits in Nevada: Critical Geologic Characteristics and Viable Models: in Hedenquist, J.W., Thompson, J.F.H., Goldfarb, R.J. and Richards, J.P. (eds.), Economic Geology, 100th Anniversary Volume, Society of Economic Geologists, pp. 451-484.
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Emsbo P, Groves D I, Hofstra A H and Bierlein F P, 2006 - The giant Carlin gold province: a protracted interplay of orogenic, basinal, and hydrothermal processes above a lithospheric boundary : in Mineralium Deposita v41 pp 517-525
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Groff J A, Campbell A R, Norman D I 2002 - An evaluation of fluid inclusion microthermometric data for Orpiment-Realgar-Calcite-Barite-Gold mineralization at the Betze and Carlin mines, Nevada: in Econ. Geol. v97 pp 1341-1346
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Holley, E.A., Jilly-Rehak, C., Fulton, A.A. and Gorman, B., 2024 - Trace Element Zonation in Carlin-Type Pyrite: Tracking Ore-Forming Processes at the Nanoscale: in Econ. Geol. v.119, pp. 1139-1169. doi.org/10.5382/econgeo.5089.
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Ilchik R P, Barton M D 1997 - An amagmatic origin of Carlin-type Gold deposits: in Econ. Geol. v92 pp 269-288
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Kuehn C A, Rose A W 1995 - Carlin Gold deposits, Nevada: origin in a deep zone of mixing between normally pressured and overpressured fluids: in Econ. Geol. v90 pp 17-36
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Kuehn C A, Rose A W 1992 - Geology and geochemistry of wall-rock alteration at the Carlin Gold deposit, Nevada: in Econ. Geol. v87 pp 1697-1721
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Large R R, Danyushevsky L, Hollit C, Maslennikov V, Meffre S, Gilbert S, Bull S, Scott R, Emsbo P, Thomas H, Singh R and Foster J, 2009 - Gold and Trace Element Zonation in Pyrite Using a Laser Imaging Technique: Implications for the Timing of Gold in Orogenic and Carlin-Style Sediment-Hosted Deposits: in Econ. Geol. v104 pp 635-668
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Peters S G, 2004 - Syn-deformational features of Carlin-type Au deposits: in J. of Structural Geology v26 pp 1007-1023
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Ramadorai G, Hausen D M and Bucknam C H, 1991 - Metallurgical, analytical and mineralogical features of Carlin refractory ores: in Ore Geology Reviews v6 pp 119-132
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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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