Bulletin 87: The Geology and Mineral Deposits of the
Hedley Gold Skarn District, Southern B.C.
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The Hedley mining district of British Columbia described by Bulletin 87 lies within the allochthonous Quesnel Terrane of the Intermontane Belt. It is situated at the eastern edge of the Upper Triassic Nicola Group, close to the group's contact with Paleozoic and Triassic oceanic rocks of the Apex Mountain Complex.
The Nicola Group is a westerly thickening, late Carnian to late Norian calcareous sedimentary and arc-related volcaniclastic sequence that was deposited on a tectonically active, west-dipping paleoslope. Sedimentary facies changes and paleocurrent indicators suggest that the sediments in the group were derived largely from an eastern source. The Nicola Group in the Hedley area was laid down across a structural hinge zone that marked the rifted margin of the westerly deepening, shallow-marine Nicola basin.
A sedimentary succession is recognized in the Nicola Group. This includes an upper, widely developed and thick (at least 1200m) unit, the Whistle Formation, which consists largely of alkalic and subalkalic tuffs and tuffaceous sediments. The base of the formation is marked by an extensive limestone-clast-bearing unit, the Copperfield breccia, which reaches 200 metres in thickness and 16 kilometres in strike length. The unit is an important stratigraphic marker horizon in the district, and is believed to represent a gravity-slide megabreccia deposit.
The Whistle Formation is underlain by a succession in which four sedimentary facies are distinguished: from east to west, the thin (up to 200m), shallow-marine. limestone-dominant French Mine Formation; the thicker, siltstone-dominant Hedley and Chuchuwayha formations in the central part of the area; and the thick (up to 2200m), deeper water and argillite-dominant Stemwinder Formation. Conodonts from the French Mine, Hedley and Chuchuwayha formations indicate they are Late Triassic (Carnian-Norian) in age. The sedimentary facies were separated from one another by long-lived faults that marked syn-sedimentation scarp slopes related to rifting along the Nicola basin margin.
The Chuchuwayha, Hedley and French Mine formations are underlain by a sequence of mafic tuffs with minor flows, limestone and chert-pebble conglomerate, the Oregon Claims Formation. The age of this unit and its contact relationship with the overlying rocks are uncertain. The formation may represent the oldest exposed section of the Nicola Group, but it is more likely to be an older basement, and it may represent a western extension of the Apex Mountain Complex.
A newly recognized mid-Jurassic unit, the Skwel Peken Formation, overlies the Nicola Group. It consists of a 1900-metre succession of calcalkaline andesitic to dacitic tuffs with minor amounts of epiclastic sediments, and pyroclastic surge deposits. It was laid down in a non-marine, subaerial to shallow water environment and is believed to represent volcanism related to the mid-Jurassic plutonism represented by the Cahill Creek and Lookout Ridge plutons.
Minor amounts of Cretaceous Spences Bridge Group and the Eocene Springbrook and Marron formations are also present in the area.
Several episodes of plutonism are recognized. The oldest resulted in the quartz dioritic and gabbroic Hedley intrusions that are associated with widespread gold skarn mineralization. Equivocal radiometric U-Pb dating and field evidence suggest they were intruded during Late Triassic to Early Jurassic time (219-194 Ma). The intrusions occur as large and small stocks, as abundant sills, or as rare dikes; the sills are preferentially developed in the thinly bedded Chuchuwayha and Hedley formations.
A slightly younger intrusive episode produced the large, granodioritic Bromley batholith and a related marginal body, the granodioritic to gabbroic Mount Riordan stock. The latter is genetically associated with the Mount Riordan (Crystal Peak) skarn that is being evaluated as a potential industrial garnet deposit. A radiometric U-Pb zircon age of 194.6±1.2 Ma (Early Jurassic) is indicated for the Mount Riordan stock, and a similar age has been obtained from the Bromley batholith.
A subsequent phase of granodioritic to quartz monzonitic magmatism is represented by the Lookout Ridge and Cahill Creek plutons; the latter, which commonly separates the Nicola Group to the west from the Apex Mountain Complex farther east, yielded a U-Pb zircon date of 168.8±9 Ma (mid Jurassic). These high-level plutons are spatially related to a suite of aplites and quartz porphyry minor intrusions that yielded a U-Pb zircon date of 154.5+8-43 Ma (Late Jurassic).
The youngest major intrusion in the district, the Verde Creek stock, is coeval with the Early Cretaceous Spences Bridge Group; it intrudes the Nicola Group in the western part of the district. A rare suite of leucocratic, calcalkaline minor intrusions (or possible volcanic flows) is spatially associated with the Skwel Peken Formation. These rocks contain magmatic garnet phenocrysts with almandine-rich cores and spessartine-rich margins that are chemically distinct from the grossular-andradite garnets in the gold skarns.
Two deformational episodes are identified in the Apex Mountain Complex, the first and predominant of which resulted in tight to isoclinal folds with moderate to strong penetrative axial planar fabrics.
Two structural phases are also recognized in the Nicola Group, neither of which produced dominant penetrative fabrics. These predate both the Skwel Peken Formation and the Cahill Creek pluton, which suggests they are pre mid-Jurassic in age. A younger phase of folding has gently deformed the Skwel Peken Formation. The first phase, which was only locally developed in the Nicola Group, produced minor flexure folds that were probably related to the forcible emplacement of the Hedley intrusions. At Nickel Plate, these structures partly controlled the gold skarn mineralization.
The second deformational phase to overprint the Nicola Group was the dominant structural event in the district. It resulted in overturned minor and major asymmetric folds, including the large Hedley anticline. The youngest (post mid-Jurassic) fold phase has only been identified in the Skwel Peken Formation; it produced open minor flexure folds.
The Hedley district has important skarn deposits as well as some minor gold-bearing quartz-carbonate veins. The skarns are separable into two types, the most important being gold skarns which are characteristically pyroxene dominant. The second and less common type is garnet dominant and contains some tungsten and copper but little or no gold; the Mount Riordan skarn represents the largest of this second type.
Between 1904 and 1991 the gold skarns produced over 62 tonnes of gold from 8.4 million tonnes of ore mined, of which over 97% was won from the Nickel Plate deposit; lesser amounts were recovered from gold skarns at the French, Canty and Good Hope mines. The overall gold grade of all the gold skarn deposits mined (by under-ground and open pit) in the district is 7.43 grams per tonne.
The gold skarns are genetically and spatially related to the dioritic stocks and sill-dike swarms of the Hedley intrusions. Economic gold skarns are hosted only by the Nicola Group and are structurally, stratigraphically and lithologically controlled; they favour areas where the Hedley intrusions cut the calcareous, shallower marine sedimentary facies rocks of the Hedley and French Mine formations.
Skarn alteration varies from narrow zones less than 10 metres wide to large envelopes hundreds of metres thick. The largest skarn envelope is at Nickel Plate where it outcrops over 4 square kilometres. Prograde skarn development is characterized by an early phase of K-feldspar-biotite-albite alteration, which is replaced subsequently by pyroxene-garnet-carbonate-scapolite assemblages. Mineralogical zoning is present in both the mineralized and barren skarns; this zoning generally consists of coarser grained garnet-rich proximal assemblages and finer grained pyroxene-rich distal assemblages. Gold-pyrrhotite-arsenopyrite mineralization is preferentially developed in the distal, pyroxene-dominant skarn.
Bismuth tellurides, arsenopyrite and high pyrrhotite/ pyrite ratios characterize the auriferous ore, and the gold-sulphide mineralization is generally coeval with widespread scapolitization. The close temporal and spatial association between gold and scapolite suggests that chorine-rich fluids may have been important in the transportation and precipitation of gold.
The proposed model for the Nickel Plate deposit involves the injection of large volumes of magmatic fluid derived from the Hedley intrusions into the calcareous sediments to produce an early, high-temperature mineral sequence of biotite and orthoclase, followed by manganese-poor clinopyroxene and finally grandite garnet. The overall compositional zoning in the Nickel Plate garnets is from grossularitic cores to andraditic margins. Subsequently, sulphides, gold and scapolite were introduced at lower temperatures. Fluid inclusion studies (Ettlinger, 1990a) at Nickel Plate indicate the main pyroxene-garnet skarn formed at temperatures between 4600 and 4800C with average fluid salinities of 18.3 and
9.7 wt% NaCl equivalent for garnet and pyroxene respectively.
The Hedley intrusions, compared to other magmatic rocks related to base metal skarns, have the lowest amounts of total alkalis and silica, and the highest amounts of calcium, magnesium and iron. Low Fe2O3/FeO ratios and the presence of ilmenite and pyrrhotite in the unaltered Hedley intrusions, and abundant pyrrhotite in the ore suggest the skarns formed in a strongly reduced environment.
Skarn overprinting of the intrusions to produce endoskarn was accompanied by an increase in the potassium content and declines in total iron and the Fe2O3/FeO ratios. Initial skarn alteration of the sediments, to produce weakly altered exoskarn, was accompanied by gains in potassium, sodium and iron, with crystallization of K-feldspar, biotite, albite and minor pyroxene. As exoskarn alteration became more intense, the early minerals were replaced by hedenbergitic pyroxene and andraditic garnet, with corresponding losses of potassium, sodium, silica and aluminum and continuing gains in iron. It is believed that the breakdown of the ferromagnesian minerals in the endoskarn was the source for much of the iron enrichment in the exoskarn.
It is postulated that a large thermal cell formed around the Nickel Plate skarn. This probably resulted in the influx of meteoric waters into the base of the system, which mixed with the magmatic fluids and resulted in the deposition of sulphides and gold. Consequently, ore horizons are preferentially developed close to the base and lateral margins of the alteration envelope. By contrast, the upper and interior portions of the skarn tend to be barren. This zoning has relevance regarding future exploration of other, apparently barren, skarn outcrops that may mask mineralization at depth.
A district-wide, east-to-west change in the metallogeny, mineralogy and oxidation state of the skarns is suggested. Pyroxene-dominant and highly reduced skarns containing gold, arsenopyrite and bismuth tellurides occur in the west and central parts of the district, while more oxidized tungsten-bearing and garnet-dominant skarn such as the Mount Riordan skarn occur in the east. This zoning is partly due to the different sedimentary protoliths of the various skarns, which reflects the original sedimentary facies changes in the Nicola Group across the district. It is also related to the different ages and compositions of the associated intrusions.
The district has good exploration potential for new gold-skarn discoveries, and the deposit model and ore controls postulated for the Hedley district are applicable to other areas of the Cordillera.
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