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Ridgeway |
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South Carolina, 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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The Ridgeway gold deposit is located approximately 9 km east of the town of Ridgeway, and 40 km north of Columbia in South Carolina, USA and lies within the Carolina Slate Belt which also hosts other gold deposits. The mine produces gold bullion from two bulk-mineable, open-pit deposits located 1.5 km apart and produce low-grade oxide and sulphide ores.
Mining commenced at Ridgeway in 1988. In 1994 the production totalled 3.7 tonnes of Au, with remaining proved + probable reserves of 25 tonnes of contained Au at a grade of 1 g/t Au, and a mill recovery of 79%.
The regional stratigraphy comprises i). A basal felsic and mafic volcanic sequence termed the Persimmon Fork Formation; ii). Overlying turbidite greywacke, siltstone, and mafic volcanics of the Richtex Formation; and iii). A younger, platformal sequence of thin bedded to massive mudstone with lateral and vertical quartz sandstone facies termed the Asbill Pond Formation. Cambrian trilobites of Baltic affinity in the Asbill Pond Formation led Secor et al., (1983) to propose that the Carolina Slate Belt is an exotic volcanic arc terrane accreted to the North American continent by middle Ordovician time.
The Ridgeway deposits are located in the central portion of the Carolina Slate Belt. The Ridgeway gold deposits occur within a transition zone between an older volcanic terrane to the north, Sawneys Creek volcanics, and a younger sedimentary terrane of turbidite greywacke, siltstone and transitional volcanics to the south which are informally labeled Bear Creek turbidites. The youngest unit is composed of thin-bedded siltstones and claystones informally called the Hidden Valley siltstone. Mappable lithological units in the mine area are as follows:
i). The Sawneys Creek Volcanics - Dominantly dacitic to rhyodacitic extrusive, submarine volcanic rock types, including lithic lapilli and crystal tuffs. Some of the fragmentals are agglomeratic (bomb-size), indicating likely vent areas; Minor dacitic to rhyodacitic feldspar and quartz-eye porphyry bodies possibly representing hypabyssal intrusives or subaerial extrusive flow domes; and Minor, interlayered basaltic(?) flows and tuffaceous sediments.
ii). The Bear Creek Turbidites consist of the following lithologies: The northern (older) portion of the is a complex transition zone containing bimodal felsic and mafic volcanic units reflecting submarine ash flow tuffs and debris flows of both felsic and mafic units. The transitional volcanics are interlayered with fine grained, laminated tuffs and siltstones and locally thick-bedded, coarse-grained volcaniclastic greywackes. In addition, massive to laminated, pyritic cherts occur in the transition zone. The southern (younger) portion of the turbidites consists of interlayered siltstone and greywacke. Laminated to massive siltstones dominate volumetrically over greywackes, which vary in thickness from centimetres to tens of metres. These lithologies are cyclical, show depositional features indicative of turbidite deposition. The turbidites display dominantly south-younging stratigraphic directions. Rip-up clasts in the basal traction bed-deposited greywackes are generally fine-grained and consist of siltstone. However, debris flows consisting of large siltstone fragments set in a matrix of quartz and feldspar are locally present. In the South Pit, units consisting of fragments of felsic volcanics, sediments, and chert represent either debris flows or ash flow tuffs.
iii). The Hidden Valley Siltstones are comprised primarily of thinly bedded siltstone and claystone (Figure 8 bottom). The rocks young to the south and are interpreted as a final basin fill clastic sediment.
The Ridgeway mine area is located along an east-west trending portion of the Carolina Slate Belt that is anomalous compared to the generally northeast trend of the belt elsewhere in North and South Carolina. This east-west trend also defines the transition zone between an older volcanic terrane to the north and a younger sedimentary terrane to the south. Northeast plunging antiformal structures project into the mine area from the southwest. These structure developed during Alleghanian dextral shearing and folding. East-northeast trending zones of dextral folding, faulting, and associated mafic dyke intrusions mapped in the mine area are also believed to be manifestations of Alleghanian tectonothermal events.
Bedding in the vicinity of the mine strikes generally east-west and dips moderate to steeply north and south, while a fine-scale foliation which is generally parallel to the bedding plane surface is recognized in the mine area. This compositional fabric is overprinted by an east-west striking, generally north-dipping penetrative slaty cleavage defined by parallelism of fine micas, and in the vicinity of alteration by varying degrees of mineral flattening, silicification, and quartz veining.
The mapped fault separating portions of the Hidden Valley siltstone south and west of the mine area is interpreted to be a thrust. North of the fault, bedding in the Bear Creek and Hidden Valley units strikes east-west and dips moderate to steeply north or south. West and south of the thrust, bedding is shallow dipping to the east and north.
Several shear zones define contacts between and within the gold deposits indicating that both normal and reverse fault movements have modified the structure of the mine area.
North Pit
The geology of the North Pit is complex, and comprises four, east-west trending alteration and lithologic units as follows:
Unit 1 - an altered sequence of interlayered metasediments, mafic volcanics, and cherty silicification exposed along the northern edge of the ultimate pit wall. This unit which is orange/purple/yellow/dark green in colour appears to be a sinistrally fault-displaced from similar rocks mapped to the west of the pit.
Unit 2 - is orange in colour and comprises metasediments, thin bedded tuffs, and minor felsic and mafic(?) volcanic horizons in which bedding is typically preserved even though the rocks have a moderate degree of quartz-sericite alteration overprint). Elliptical-shaped cherty silicified zones with ore-grade mineralisation are present in the eastern half of the unit.
Unit 3 - are magenta in colour, dominating in the centre of the pit, and are the primary host to ore. This fine grained, cherty unit rarely shows bedding features, being dominated by the pervasive S2 fabric, suggesting an origin either as a fine-grained sediment, volcanic tuff, or chemical exhalite. A high grade breccia pipe of unknown origin was present in this unit. The northern edge of Unit 3 is both interlayered and in fault contact with laminated sediments and volcanics of Unit 2 to the north.
Unit 4 - is dark green to aqua green in colour and is located along the south edge of the pit. It is composed of a mafic volcanic sequence and minor sediments. This unit forms the footwall of the deposit, is weakly altered to unaltered, and is cross-cut by an east-west trending and north-dipping, quartz-veined shear zone.
Mafic and felsic dikes and sills are also present throughout the deposit in the North Pit, their intrusion post-dating gold mineralisation and generally accompanied by shearing and/or faulting.
The structure of the North Pit is very complex. Stratigraphic units, while generally trending east-west and dipping steeply south or north, have been variably affected by three deformation events. Near the southern edge of the pit, S2 strikes east-west and has a moderate to shallow north dip. It flattens to a subhorizontal orientation near the centre of the pit, while in the northern half of the pit S2 has a shallow to moderate south dip. The S2 slaty cleavage is the dominant deformational fabric in the pit excluding dykes and sills. In several areas of the pit S2 fabric is parallel to alteration or rock types in several areas of the pit, suggesting fault movement along the fabric.
The footwall of the deposit is defined by a shear zone with a gentle to moderate north dip. Ductile drag folds and slickensides suggest a phase of early reverse movement was followed by normal brittle fault movement. This shear zone is also interpreted to extend east where it forms the hanging wall of a north-dipping mafic dike complex paralleling the east ultimate pit wall. The footwall fault is intruded by the north-south trending felsic dyke, which in turn has a sinistral offset along a northeast trending, oblique normal shear zone. This shear zone is intruded by mafic and felsic dykes, and appears to be dextrally folded as it exits the pit along the east ultimate pit wall.
The north-south trending felsic dyke cutting the centre of the pit occupies a fault which experienced an estimated 30 m of reverse offset (east side down). Other north-south trending, steeply-dipping faults seen in the mine show the same east-side down sense of movement, and may explain the gentle, east-plunging direction of intersection lineations between bedding and S2.
A thin (less than 30 cm thick) mylonite-hosted thrust fault separates mafic volcanics from underlying sediments along the southeast end of the pit. Here, mafic volcanics overlie a sedimentary sequence. The contact appears to be a mylonite. The shallow dip and east trend of this thrust fault suggest it may dip into the pit rather than beneath the footwall.
Late, brittle to ductile, quartz vein-filled shear zones cut the deposit. They are generally accompanied by flexural folding and crenulation cleavage. Dextral movement is indicated by these quartz vein-filled faults in the footwall area along the southeast flank of the pit.
Lithology, cleavage development, pyrite grain size and abundance, and silica content are related to the abundance of gold in rocks of the North Pit. These factors and the alteration patterns mapped in the North pit lead to the following conclusions:
i). The dominant ore-host lithology is a magenta coloured chert which is unique as compared to mafic footwall volcanics and the bedded sediments, tuffs, and volcanics found in the northern half of the pit.
ii). The chert paralleling the north ultimate wall of the pit is associated with mafic volcanic rocks and adjacent, weakly altered sediments.
iii). Ore grade gold mineralisation in the North Pit deposit is often associated with the most siliceous and S2 cleaved zones.
Ore zones have been fault-displaced along the S2 fabric.
South Pit
The Ridgeway South deposit is located along the southern edge of the Bear Creek turbidites near the contact with Hidden Valley siltstone. The mapped units within the South Pit are as follows:
Unit 1 - a yellow, altered sequence of predominantly laminated siltstones and minor greywacke which is exposed along the south wall of the pit. Deformation is minima,l allowing for preservation of relict fine laminations generally less than 2 cm in thickness. Depositional features such as rip-up clasts, ripple marks and cross bedding are recognised in this unit and are similar to those observed in unaltered turbidites outside of the deposit. The unit has a moderate to locally strong quartz-sericite-pyrite overprint and gold is present in sub-economic but highly anomalous values ranging up to 250 ppb. On average the unit contains approximately one percent pyrite as fine disseminated grains in the 5 to 40 micron size range and as coarse euhedra in the 50 to 250 micron range. The southern contact of this unit (and the deposit) forms a discontinuous, topographically resistant, highly siliceous ridge. The siliceous unit is moderate to intensely quartz veined and has 1000 to 10000 ppm molybdenum values spanning the entire length of the deposit footwall. Further north, Unit 1 is heavily saprolitised and does not outcrop. The footwall contact between the deposit and unaltered turbidites to the south is knife sharp and appears to be a fault/shear zone based on attitude reversals within the turbidites across the contact and abrupt geochemical changes.
Unit 2 - is a more highly deformed and altered orange coloured unit with a well developed north-dipping S2 fabric and bedding transposition. Cleavage is typically at low angles to relict bedding, and these laminations are locally contorted and disrupted into fragments oriented parallel to S2. The unit is characterised by pervasive quartz-sericite-pyrite alteration. The pyrite ranges from 1 to 1.5 modal percent and is typically less than 40 microns in size. While multiple generations of quartz veining are present, the most abundant veins are less than 2 cm in thickness and generally parallel to S2. The contact between units 1 and 2 is gradational and is characterised in the field by zones of increasingly disrupted laminations within the host stratigraphy. Unit 2 comprises a significantly large, albeit lower grade, portion of the ore zone with gold values commonly in the 0.4 to 1 ppm range. Molybdenum values are generally in the 5 to 20 ppm range.
Unit 3 - is a magenta coloured zone characterised by extensive alteration and S2 fabric development. It consists primarily of laminated sediments and fragmental units of diverse origin reflecting either debris flows, subaqueous ash flows related to waning pulses of felsic volcanism, or formation due to tectonic disruption or pressure solution. This unit generally contains the more consistent 1 to 1.5 ppm ore grade gold values within the deposit. It is characterised by a strong quartz-sericite-pyrite overprint which produces an obscure mottled appearance related to irregular concentrations of cloudy fine-grained pyrite. The mottled pyrite, when elongated parallel to the deformation fabric can produce a banded texture similar to and often mistaken for original compositional layering. The original host texture is further obscured by intense sericitisation, quartz veining and anastomosing sub-parallel cleavage traces. Two types of fragments are observed within Unit 3. The first group of fragments are supported in a fine grain matrix, are tapered or flattened parallel to the S2 fabric. These fragments exhibit relict internal features such as fine laminae indicating they are derived from the textural destruction of the turbidite host. The second type of fragments are heterolithic, possessing a fine-grained matrix containing angular to sub-rounded fragments of black to dark grey feldspar (rhyolite?) porphyry, cream-colored chert, whitish sericite(?) lithic lapilli, and blocks of laminated to massive siltstone and greywacke. The gross difference between the two fragmental descriptions is the variation in fragment composition and shape as well as the gold grade. In general, fragmentals within the ore zone contain elongate, flattened, or tapered fragments. The angular, heterolithic breccias appear to exist proximal to the economic ore zone but generally not within it except as infrequent relicts. It may be argued that deformation within the main ore zone has destroyed or obscured these primary fragmental debris flow or subaqueous ash flow units to such a degree that their origin cannot be determined.
Post mineralisation mafic dykes cross-cut the alteration zone and are believed to occupy faults.
The east-west striking lithologies in the Ridgeway South deposit are tightly folded into generally upright, south-verging isoclinal folds. Drag along the hanging wall of the deposit indicates normal faulting occurred either synchronous or after isoclinal folding and S2 development.
The characteristic of "ore grade" mineralization in the Ridgeway South Pitl is alteration-deformation rather than lithologic control. Economic gold grades in the South Pit are associated with zones having all of the following characteristics: development of pervasive deformation cleavage, silicification, multiple quartz veining events, widespread sericitization, and pyrite development.
NOTE: The information in this description is largely drawn from a paper on the Kennecott Ridgeway web site at: http://www.kennecottminerals.com /krmc/KRMC/geopaper.html which is in turn based on text contained in a guidebook
prepared for a Field Trip to the Kennecott Ridgeway Gold Mine on May 18, 1996 by Gillon KA, Spence WH, Duckett RP and Benson CH.
The most recent source geological information used to prepare this decription was dated: 2005.
This description is a summary from published sources, the chief of which are listed below.
© Copyright Porter GeoConsultancy Pty Ltd. Unauthorised copying, reproduction, storage or dissemination prohibited.
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Selected References:
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Alarifi, S.S., Kellogg, J.N. and Ibrahim, E., 2021 - Geophysical Study of Gold Mineralized Zones in the Carolina Terrane of South Carolina: in Econ. Geol. v.116, pp. 1309-1328.
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Ayuso R S, Wooden J L, Foley N K, Seal R R and Sinha A K 2005 - U-Pb Zircon Ages and Pb Isotope Geochemistry of Gold Deposits in the Carolina Slate Belt of South Carolina: in Econ. Geol. v100 pp 225-252
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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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