GEOLOGY OF THE RIDGEWAY GOLD DEPOSITS
RIDGEWAY, SOUTH CAROLINA
INTRODUCTION
Kennecott Minerals Company (KMC) operates a 15,000-ton-per-day open-pit gold mine located approximately 5 miles east of the town of Ridgeway, and 25 miles north of Columbia, South Carolina (Figure 1). The Ridgeway gold mine is one of four gold mines which were put in production in South Carolina in the 1980’s. It is currently the only operating gold mine in the Eastern United States.
The Ridgeway mine produces gold bullion from two bulk-mineable, open-pit deposits located one mile apart. Low-grade oxide and sulfide ore produced from the siliceous deposits is blasted and hauled to a central mill complex. Ore is milled to minus 200 mesh, and gold is extracted by a carbon-in-pulp, electrowinning process. The mine operates 24 hours a day, and employs over 100 people. The mine began producing gold in December 1988 and total production will exceed 1,000,000 ounces by the end of 1995. Both pits are currently being developed in order to achieve an ultimate production goal of 1.5 million ounces of gold bullion by the year 2000. The North Pit is 2,000 feet long, 1,400 feet wide, and 320 feet deep. The South Pit is 2,600 feet long, 1,500 feet wide, and 360 feet deep. The South Pit is being mined at full scale, and is expected to be mined out by June 1996. Activity in the North Pit is currently focused on the stripping of oxide ore and waste at the 420-440 level along the northern half of the pit. Future development plans for the North Pit include deepening the pit from the 120 elevation to -40 elevation. This paper presents a field geology-oriented framework for understanding the stratigraphy, structure, mineralization and genesis of these deposits.
BRIEF HISTORY OF DISCOVERY AND DEVELOPMENT
The Ridgeway gold deposits are essentially virgin discoveries which had been known to only a few prospectors before the 1960’s. Both deposits have small prospect pits along the west sides of the pits, and a few other pits have been found scattered along the Ridgeway trend for several miles east and west. Several people participated in the "rediscovery" of the deposits in modern times, and include:
• Prospector John Chapman, whose gold panning efforts recognized gold in the vicinity of the deposits and who passed this information along to the South Carolina State Geological Survey;
• Geologists Henry Bell and Henry Johnson, whose geochemical investigations of streams and auger borings peripheral to the deposits established the presence of gold, tin, and other anomalous metals;
• Geologist Irving T. Kiff, who in the 1960’s recognized the uniqueness of Ridgeway’s sericitic alteration while on a field visit to the area. Kiff later worked at the Haile Mine in the 1970’s as a consultant to Cyprus Mines under the direction of Joe Worthington. Kiff determined that the Haile Mine lithologies and alteration patterns were identical to those seen at Ridgeway, and he brought this knowledge to the attention of Amselco (Selection Trust) geologist William H. Spence in 1979.
• Geologist William H. Spence, who convinced Selection Trust exploration manager William G. Hancock to initiate a southeastern gold exploration program based on the volcanogenic models of Worthington and Kiff (1970) and Spence et. al. (1980). In 1980 Amselco initiated land acquisition on the Ridgeway North and South gold deposits.
• Geologist Bob Carpenter, who focused the attention of a joint AMAX-Phillips exploration program on the Ridgeway North deposit which led to the acquisition of key properties in 1981. The AMAX-Phillips share of the North deposit was eventually acquired by Amselco Minerals in 1985.
Geologists from Amselco Exploration Co. and its successor BP Minerals evaluated the Ridgeway deposits from 1979 through 1985. A corporate decision to develop the deposits was announced in 1986, mine development began in 1987, and the first gold was poured in December, 1988. In 1988 Amselco Minerals was merged with Kennecott Exploration which continued exploration activities in the Ridgeway region until 1993. This work led to the discovery and evaluation of numerous other alteration zones in the region.
REGIONAL GEOLOGIC SETTING
The Ridgeway region is located in the Piedmont Lowlands physiographic subprovince of Hack (1982). The Piedmont Lowlands is characterized by a monotonous topography of low rounded ridges and ravines largely underlain by saprolite on crystalline rock. Relief is generally less than 100 feet, vegetation is dense, and rainfall in the region averages 40 to 50 inches per year. Bedrock is generally weathered, with the resulting saprolite commonly exceeding 100 feet in thickness. The above factors, combined with the extensive Coastal Plain sand and clay cover present along the east and south edges of the district, have made it difficult to map the stratigraphy and structure of bedrock.
The Ridgeway mine is located in the Carolina Slate Belt (CSB), a late Proterozoic (?) to Cambrian age, subduction-related volcanic arc terrane metamorphosed to greenshist and amphibolite grades (Secor, 1988; Maher, et. al., 1991). In South Carolina, the CSB is flanked to the northwest by older(?) amphibolite facies-metamorphosed Charlotte Belt volcanics and intrusives. The regionally extensive, northeast-trending Modoc Fault Zone juxtaposes the southeast edge of the CSB against amphibolite grade gneisses of the Kiokee Belt. The CSB was deformed and metamorphosed to generally lower greenschist grade during the mid-Ordovician Taconic orogeny (Noel, et. al., 1988; Secor, 1988; Maher, et. al. 1991; Offield, 1994). This event is thought to be responsible for the regionally pervasive, northwest-dipping slaty cleavage seen in these rocks. Shearing and intrusive activity characterize the poorly-developed, Siluro-Devonian Acadian Orogeny, as is best exemplified by the Gold Hill /Charlotte Belt boundary which straddles the South and North Carolina boundary (Offield, 1994). A younger tectonothermal event - the Alleghanian orogeny, affected only portions of the CSB, and involved four distinct deformational episodes spanning 268-315 ma. In the Ridgeway region, granitic intrusions associated with the Alleghanian Orogeny include the Columbia, Winnsboro, and Liberty Hill granites, and the Dutchmans Creek Gabbro.
Indicator metamorphic minerals characterizing the Ridgeway area include phengite, albite, chlorite, and epidote (R. L. Barnett, personal communications, 1995). These minerals are indicative of the chlorite zone of greenschist facies metamorphism as defined by Turner (1981). Because these rocks have been only weakly metamorphosed, the prefix "meta-" is not used to describe Ridgeway area lithologies in this paper.
Regional Stratigraphy
Secor and Wagener (1968), and Secor et al.(1986) established the following stratigraphy for the CSB in central South Carolina:
• A basal felsic and mafic volcanic sequence termed the Persimmon Fork Formation;
• Overlying turbidite greywacke, siltstone, and mafic volcanics of the Richtex Formation; and
• 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 CSB is an exotic volcanic arc terrane accreted to the North American continent by middle Ordovician time.
GEOLOGY OF THE RIDGEWAY MINE AREA
The Ridgeway deposits are located in the central portion of the Carolina Slate Belt, approximately 25 miles north of Columbia, South Carolina (Figure 1). Figure 2 is a detailed geologic map of the Ridgeway region refined from over 10 years of mapping, trenching, and drilling by Amselco and Kennecott geologists, as well as input from published sources. Figure 3 is a detailed geologic plan and section map through the mine area.
Stratigraphy
The Ridgeway gold deposits are located along a transition zone between an older volcanic terrane to the north and a younger sedimentary terrane to the south The older volcanic lithologies are informally designated as Sawneys Creek volcanics (Gillon and Duckett, 1988; Duckett et al., 1988). The overlying turbidite greywacke, siltstone and transitional volcanics are informally labeled Bear Creek turbidites. The youngest unit is comprised of thin-bedded siltstones and claystones informally labeled as the Hidden Valley siltstone. These three informal stratigraphic units appear to correlate respectively with the Persimmon Fork, Richtex, and lower Asbill Pond Formations. Lithologies comprising map units in the mine area are briefly described below.
The Sawneys Creek volcanics consist of the following lithologies:
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 (Figure 4 top).
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.
The Bear Creek turbidites consist of the following lithologies:
The northern (older) portion of the Bear Creek turbidites 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 (Figures 4 bottom and 5 top and bottom).
The transitional volcanics are interlayered with fine grained, laminated tuffs and siltstones (Figure 6 top) and locally thick-bedded, coarse-grained volcaniclastic greywackes. In addition, massive to laminated, pyritic cherts occur in the transition zone (Figure 6 bottom).
The southern (younger) portion of the Bear Creek turbidites consists of interlayered siltstone and greywacke. Laminated to massive siltstones dominate volumetrically over greywackes, which vary in thickness from centimeters to tens of meters (Figures 7 top and bottom). These lithologies are cyclical, show depositional features indicative of turbidite deposition as defined by Walker (1984). 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 (Figure 8 top). In the South Pit, units consisting of fragments of felsic volcanics, sediments, and chert represent either debris flows or ash flow tuffs.
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.
General Structural Setting
The Ridgeway mine area is characterized by the following major structural features (Figures 2 and 3):
The mine area is located along an east-west trending portion of the CSB that is anomalous compared to the generally northeast trend of the CSB elsewhere in South and North 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.
The northeast plunging Lake Murray and Irmo antiformal structures (Maher, et. al., 1991) 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 dike intrusions mapped in the mine area are believed to be manifestations of Alleghanian tectonothermal events.
Bedding in the mine region strikes generally east-west and dips moderate to steeply north and south (see cross section in Figure 3). A fine-scale foliation generally parallel to the bedding plane surface is recognized in the mine area and designated as S0/1. This compositional fabric is overprinted by an east-west striking, generally north-dipping slaty cleavage (S2). The S2 fabric is a 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 and indicate both normal and reverse fault movements have modified the structure of the mine area. Mapping by Amselco geologists determined that the Ridgeway area lies at the eastern end of the Cedar Creek Shear Zone. This northeast-trending zone of intense cleavage development was originally defined by Bourland and Farrar (1980), and it hosts quartz-sericite-pyrite alteration along much of its 30 miles of length. The shear zone traces northeast from Irmo, S.C., bending to the east as it enters the Ridgeway area.
Development of the Ridgeway Basin Model
The lithologies and structural features mapped in the Ridgeway area are indicative of a subaqueous, basin-margin depositional environment, which evolved as follows:
An initial, felsic-dominated volcanic terrain followed by development of a basin margin fault;
subsequent transitional bimodal volcanism along with onset of basinal clastic sedimentation;
maturation of the basin as evidenced by waning volcanism and predominance of clastic turbidite deposition fining upwards into a fine grained (quiet water?) clastic sequence.
Regional geophysical evidence supports this interpretation. In Figures 9 and 10 the regional total aeromagnetic and vertical derivative maps, respectively show strong, east-west structures passing through the transitional volcanics and sediments surrounding the Ridgeway North deposit. Some of these structures are believed to represent original basin margin faults which have been reactivated during complex tectonothermal evolution of the CSB. The concept of a fault-bounded basinal model for the Ridgeway area is not unique to the southern Appalachians. In the North Carolina CSB, Moye (1987) proposed a rift-type pull-apart basinal model to explain depositional facies and geophysical signatures of the Abermarle Basin. In the South Carolina CSB, Secor (1988) and Maher, et. al. (1991) have suggested the Richtex Formation is either a thrusted allochthon onto the Persimmon Fork formation volcanics or an unconformity. We propose an alternative interpretation, that the Richtex (Bear Creek turbidites) reflects an evolving basin margin.
Geology of the North Pit
The geology of the North Pit is complex, being comprised of four, east-west trending alteration and lithologic units (Figure 11; Gillon et al., 1995). Lithologically, it is characterized by the following:
Unit 1 (magenta/orange/purple/yellow/dark green colors) is comprised of an altered sequence of interlayered metasediments, mafic volcanics, and cherty silicification exposed along the northern edge of the ultimate pit wall. This unit appears to be left laterally fault-displaced from similar rocks mapped previously by KRMC west of the pit.
Unit 2 (orange color) is comprised of 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 (Figures 6 top, 12 top and bottom). Elliptical-shaped cherty silicified zones with ore-grade mineralization are present in the eastern half of the unit.
Unit 3 (magenta color) dominates the center of the pit, and is the primary ore-host. 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 (Figure 13). A high grade breccia pipe of unknown origin was present in this unit and has been mined out. 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 (dark green and aqua green colors) is located along the south edge of the pit and is comprised 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, their intrusion post-dating gold mineralization 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 the following 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 center of the pit, and 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 dikes 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 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 dike, which in turn is left-lateral offset along a northeast trending, oblique normal shear zone. This shear zone is intruded by mafic and felsic dikes, and appears to be dextrally folded as it exits the pit along the east ultimate pit wall.
The north-south trending felsic dike cutting the center of the pit occupies a fault which experienced an estimated 100 feet 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 1 foot 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.
Ore Controls
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 (Figure 11) lead to the following conclusions:
The dominant ore-host lithology is a chert (magenta color). This rock type is unique as compared to mafic footwall volcanics and the bedded sediments, tuffs, and volcanics found in the northern half of the pit (orange unit). Locations where the chert is in contact with thin layered tuffaceous metasediments suggests it originated as an exhalative chemical precipitate and/or epithermal replacement of previously existing volcanics and sediments.
The chert paralleling the north ultimate wall of the pit is associated with mafic volcanic rocks and adjacent, weakly altered sediments. This alteration zone appears to be the easterly extension of the Watkins tract alteration drilled and trenched by Kennecott Exploration in 1985. The zone also extends for over 1 mile further west of the pit, and its lithologic association suggests the possibility of a bedded exhalative horizon, albeit weakly mineralized.
Ore grade gold mineralization in the North Pit deposit is often associated with the most siliceous and S2 cleaved zones. This suggests remobilization/enrichment of stratabound gold into cleavage dominant zones, or, hydrothermal activity continuing during cleavage development.
Ore zones have been fault-displaced along the S2 fabric, as shown in Figure 12, as well as in the cross section through the North Pit (Figure 3).
Geology of the 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 protoliths of the Ridgeway South deposit as well as its genesis are equivocal and a subject of continuing debate among several of the authors. Figure 14 is a simplified toe-face geologic map of the South Pit. Alteration/deformation map units shown on this map were developed prior to mining. The mapping and modeling of these units have been utilized throughout the entire mine life of the South Pit and each is described below:
Unit 1 (yellow color) is an altered sequence of predominantly laminated siltstones and minor greywacke exposed along the south wall of the pit. Deformation in this unit is minimal 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 recognized 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 as well) 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 saprolitized 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 (orange color) is a more highly deformed and altered unit than Unit 1 to the south. The north-dipping S2 fabric and bedding transposition are well developed in this unit. 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 characterized 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 characterized 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 (magenta color) is a zone characterized 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 characterized 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 (Figures 15 top and bottom, mottled zones). The original host texture is further obscured by intense sericitization, quartz veining and anastomosing sub-parallel cleavage traces. Tomkinson (1985) characterized the mottled unit as consisting of phyllonites in which pre-existing textures have been transposed and overprinted by a tectonic fabric.
Within Unit 3, two types of fragments are observed. 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 (Figure 15 top and bottom, fragmental zones). 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 (Figure 16 top and bottom).
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 mineralization mafic dikes cross-cut the alteration zone and are believed to occupy faults. The dikes can behave as both aquatards and aquacludes. Three slope failures in the south pit are directly related to a set of mafic dikes trending 240° , 50° NW. The northeasterly trends seen in the contacts of Units 1, 2, and 3 in Figure 14 are related to termination of alteration units along these dike-filled faults. The exact displacement along the faults is unclear although offsets in the footwall suggest both a lateral and vertical slip component.
Structural Geology
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. Kennecott geologists, using detailed trench mapping and fabric analysis from oriented core during exploration, characterized the alteration units based on progressive textural destruction of the original compositional layering. Based on the angle between bedding and the cross-cutting cleavage fabric, deformation domains were mapped moving from possible hinge zones into deformed limb zones of suspected folds. Attenuation and shearing along the limbs of folds were recognized by Tomkinson (1985) and Kennecott geologists as producing the north dipping deformation zones. Movement along the shear planes has destroyed evidence of complete closure on the pit scale leaving only portions of hinge zones adjacent to zones of banded and fragmental tectonic textures.
Ore Controls
The characteristic of "ore grade" mineralization in the current Ridgeway South mine model is alteration-deformation rather than lithologic control. Economic gold grades in the South Pit are associated with zones having all of the following characteristics as exemplified by Figure 17 top and bottom): development of pervasive deformation cleavage, silicification, multiple quartz veining events, widespread sericitization, and pyrite development.
GENESIS OF THE RIDGEWAY GOLD DEPOSITS
The genesis of the Ridgeway gold deposits has been the subject of considerable debate among Amselco and Kennecott geologists over the past 15 years. The models presented by these workers are briefly summarized below:
In the early 1980's, Spence proposed in numerous in-house publications a submarine hot-spring model for the development of the Ridgeway South deposit analogous to the model he developed for the Haile Mine (Spence et al., 1980).
Tompkinson (1985) proposed a shear zone-hosted, synmetamorphic origin for the deposits.
Gillon and Duckett (1988) modified Tompkinson’s model, suggesting mineralization was syntectonic but ultimately epithermal in origin.
Although the authors of this paper have argued vehemently about the genesis of these deposits, we are finally in agreement in regards to the following observations:
Exhalative chemical sediments are present in the deposits.
Mineralization is present in the deposits which predates the S2 slaty cleavage.
It is not clear how much of the primary silica/pyrite/gold mineralization is a direct chemical precipitate or replacement by shallow epithermal processes.
There has been remobilization of gold, silica, and pyrite into the S2 fabric.
This remobilization resulted in enrichment of gold grades in portions of the deposits.
There are felsic volcanics in the South deposit.
ACKNOWLEDGEMENTS
We are very grateful to the many people and organizations who made this paper possible. Kennecott Minerals - Ridgeway Mining Company, Kennecott Exploration Company, and RMT, Inc. provided time and financial support for this study. Discussions with numerous co-workers and other gold explorationists helped clarify issues. Past exploration work by the many Amselco and Kennecott geologists contributed significantly to our understanding of the regional geology. Ken Gillon wishes to thank personally his wife and children for the extensive leave of absence from home. He is also very grateful to Bill and Jeanette Spence and their family for their gracious hospitality during the research and writing stages.
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