Bulletin 89: Geology of Tertiary and Quaternary Gold-Bearing Placers in the Cariboo Region, BC
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Bulletin 89 discusses stratigraphic, sedimentologic and geomorphic data collected from man-made and natural exposures at 6l placer gold mining and exploration sites in the Cariboo region of British Columbia that have been used to develop a geologic classification of productive placer settings. These are: Tertiary and pre-Late Wisconsinan paleochannel and paleofan settings; Late Wisconsinan glacial and glaciofluvial environments and; Holocene high and low terrace; colluvial and alluvial fan settings. Differences in the geologic characteristics of the placer deposits in these settings have direct effects on mining and exploration activities. Placer deposits that underlie till of the last glaciation are invariably the most productive as they accumulated over relatively long periods of time. The precise age of many of these pre-Late Wisconsinan deposits is unknown due to the lack of dateable materials. Although many are believed to be Pleistocene interglacial deposits, some may be Tertiary. The main difficulty in locating, evaluating and mining these deposits is a result of the overlying Quaternary sediments, which obscure and commonly deeply bury them.
Five main auriferous lithofacies, distinguished by differences in grain size and sedimentary structures, have been recognized. In order of decreasing importance these include (with inferred depositional environments): a) massive to crude, sub-horizontally stratified gravel with weak imbrication (shallow, gravel-bed stream deposits; mainly longitudinal bar deposits); b) clast-supported, matrix-filled gravel and gravelly diamicton with coarse-clast clusters (sediment-rich flood-flow and noncohesive debris-flow deposits); c) large-scale, trough crossbedded gravels (channel-fill deposits); d) planar cross bedded gravels (transverse bar deposits); and e) massive, fine-grained; matrix-supported diamicton (cohesive debris-flow deposits). Regardless of the dominant lithofacies, gold concentrations are invariably high along well developed erosional unconformities, particularly those over bedrock or clay-rich sediments.
Placer deposits known to be of Tertiary age occur mainly west of the Cariboo Mountains in the Quesnel Trough. They are typically buried by thick sequences of Quaternary sediments. Sedimentary structures, such as well developed stratification and imbrication, and the high textural maturity of the gravels, indicate deposition in well developed fluvial systems, usually large, braided or wandering gravel-bed rivers. The distribution of these ancient channel systems is apparently largely structurally controlled, not having been influenced by glaciation. Sites along major thrust faults separating the Quesnel, Barkerville and Cariboo terranes, specifically the Spanish, Eureka and Pleasant Valley thrusts, are good exploration targets for buried Tertiary deposits. Mining of these normally cemented gravels has been productive only in a few locations in the Cariboo. Locally, however, they can contain significant gold concentrations, warranting further exploration.
Four different types of pre-Late Wisconsinan paleochannel placer settings, distinguishable mainly by paleostream gradient, present topographic position and deposit size, are recognized:
· high gradient narrow valley settings referred to as paleogulches,
· abandoned, high level valleys of intermediate size and channel gradient,
· broad (hundreds of metres wide), abandoned trunk valleys with relatively low channel gradients, and
· channels buried in modern alluvial valleys with stream gradients similar to the modern channels.
The first two settings are dominated by low sinuosity single-channel, autochthonous placer deposits whereas the latter two are characterized by both autochthonous and allochthonous placers deposited in braided streams and, to a lesser extent, wandering gravel-bed river environments. Gold-bearing deposits typically consist of imbricated, moderately to well-stratified and sorted, pebble to 'cobble gravels interpreted as fluvial channel lag and longitudinal bar deposits. More poorly sorted and stratified deposits, interpreted as high discharge, sediment-rich, flood and debris-flow deposits, also occur but are less common.
Buried trunk valley and high level valley systems may have little relation to modern drainage patterns. These placers are most commonly exploited in areas where they have been partially exposed by erosion, as thick overburden is a major obstacle to mining. Paleochannel placers buried below modern alluvium have the additional problem of water drainage. These mining problems are somewhat offset by the potential richness and large volume of these placers.
Placer deposits in paleogulch settings are different from other paleochannel deposits in that they generally consist of poorly sorted, cobble to boulder-sized fluvial gravels with interbedded debris-flow deposits. The sedimentary characteristics of these deposits vary widely from bed to bed. Paleogulch placers also are generally smaller than other buried valley placer deposits but they often contain significantly higher gold concentrations. Historically they have been the richest gold producers in the Cariboo with most operations mining deposits where modern gulch-channel erosion has removed some of the overburden. Present mining operations are focused mainly on remnant paleogulch gravel left by previous hydraulic mining operations. There is good potential for the discovery of new high-grade buried paleogulch channels, especially in high relief areas such as along the upper reaches of Lightning, Antler and Cunningham creeks.
Paleoalluvial fan and fan-delta deposits comprise a significant part of the gold-bearing strata at two of the largest mines in the Cariboo (Spanish Mountain and the Ballarat mines). These placers are large in volume but generally are lower grade than fluvial paleochannel placers of similar age.
They consist of poorly sorted, debris-flow deposits interbedded with lenses of fluvial sands and gravels. Large foreset beds and other structures indicative of subaqueous deposition at the Baflarat mine lead to the inference that sedimentation occurred in a fan delta environment. Like paleochannel and paleogulch placers, paleofan deposits are difficult to locate and mine because of thick overburden.
Glacial and glaciofluvial placers deposited during the last glaciation are relatively rare and lower grade than older fluvial placers. Economically viable quantities of gold contained within till occur locally where glaciers overrode preexisting, relatively rich fluvial placers. Similarly, gold-rich glaciofluvial deposits are mined where older gold-bearing gravel has been eroded by glacial meltwaters. These placers are typically poorly sorted and commonly contain clasts up to large boulder size. Although Late Wisconsinan glacial and glaciofluvial placers generally have relatively low gold grades, they occur near the surface and therefore have minimal overburden removal costs.
Mines exploiting Holocene deposits, particularly terrace placers, are common in the Cariboo. Terrace deposits are dominated by imbricated, moderately to well stratified and sorted, pebble to small cobble gravels with interbedded sand lenses, of fluvial channel and bar origin. Gold generally is distributed throughout the gravels, but may be concentrated in specific facies such as bar-head and channel-lag gravels. Other deposits such as overbank sand facies generally have low gold concentrations. High level terrace gravels, deposited in proximal, aggradational, braided streams shortly after deglaciation, typically are larger in volume, more poorly sorted and lower grade than low terrace placers. The latter are frequently mined on the downstream reaches of streams, like Lightning Creek and the Quesnel and Cottonwood rivers, where mainly fine-grained gold is recovered. Productive Holocene colluvial and alluvial fan placers are relatively rare in the Cariboo. They characteristically consist of interbedded diamicton and poorly sorted gravel deposited under paraglacial conditions. Holocene placers, of all types, are most common where gold has been reconcentrated from underlying Tertiary or interglacial placer sources.
The potential for discovery of new placer deposits is good in the Cariboo. Although there is potential in all of the geologic settings identified, preglacial and interglacial fluvial and alluvial deposits are the best targets. Based on the results of this study, large volume, preglacial, paleochannel deposits in the Quesnel Terrane in the west part of the study region are considered to have the best placer potential. The paleogeography of these channels is believed to be largely related to the distribution of regional faults such as the Spanish and Eureka thrusts. There is also good potential for the discovery of buried placers, especially paleogulch deposits, in the Quesnel Highland in the eastern part of the study area.
Due to burial by glacial and postglacial deposits, air photo and satellite data, geophysical studies (including shallow reflection and refraction seismic, ground-penetrating radar, magnetometer, electromagnetic. resistivity, and induced polarization) and drilling programs are needed to locate and evaluate pre-Late Wisconsinan paleochannel placers. In contrast, most Holocene placers are more readily evaluated and mined due to the absence of a surficial cover; generally low gold grades require that successful ventures rely on detailed sedimentologic and geomorphic information, such as large-scale surficial geology map data, during both the exploration and mining phases. An understanding of the sedimentary origin of existing placers is needed to identify new sites where gold-bearing placers have been deposited and preserved. For example, geologic studies of existing exposures help us understand the paleodrainage patterns of ancient fluvial systems and thereby identify exploration targets. Similarly, stratigraphic and sedimentologic data, providing information on the thickness, depth and geometry of strata, channel orientation and paleoflow direction, are required to locate and trace gold-bearing units. Information presented in this paper improve this database and will assist in the identification of new potential placer sites.
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