Mapping the Resource Potential Beneath the Chilcotin Flood Basalts (CFB): Volcanic Lithofacies Constraints on Geophysical Surveys (NTS 92I, 92O, 92P, 93A, 93B, 93C, 93F, 93G, 93J, 93K)
- Kelly Russell, University of British Columbia

Project area The Neogene (25-- 3 Ma) Chilcotin flood basalt province, in the Interior Plateau physiographic region of central British Columbia overlies an area of nearly 36,500 km2 (NTS sheets 92O, P, and 93A, B, C, F, G, K, J). Previous studies of the CFB mainly involved reconnaissance-scale fieldwork or geochemical studies. It obscures the distribution of Paleozoic- Mesozoic basement rocks with high mineral potential (e.g., Quesnel Trough) and petroleum prospective Cretaceous- Eocene sedimentary rocks of the Nechako Basin, except in scattered basement "windows". The Chilcotin flood basalts (CFB) reach thicknesses of ~200 m and are partially overlain by late Quaternary glacial deposits. Their distribution is entirely within the region of BC that is most affected by Pine-beetle kill.

The distribution of resources and prospects on the periphery of the CFB province makes the potential for unexploited mineral resources extending beneath the cover compelling. However, there is currently little coherent data on the spatial distribution (e.g., thicknesses), the lithostratigraphy (facies variations) and physical properties (density, porosity, magnetic susceptibility, conductivity) of the CFB deposits. The incompleteness of geoscience information for this unit is a significant impediment to successful exploration for the resources underlying the CFB because the depth through cover (glacial and basalts) to basement targets is largely unknown, and the dearth of rock property data for the basalts hinder unambiguous interpretations of geophysical datasets.

Our goal is to generate a 3-D facies and thickness model for the CFB that can be used to aid resource exploration. Our map products will be used to: (1) extrapolate regional geology, metallogeny and structure beneath the CFB cover; (2) identify more windows to the basement and the identity and geophysical signature of those basement rocks; (3) delineate areas where the CFB is thin and exploration drilling for "blind" deposits becomes feasible; and to (4) provide a 3-D representation of physical property variations within the CFB to allow the basalt signature to be accurately stripped from total field geophysical datasets.

Chilcotin Group - Photo by G. Andrews

Posters and Presentations
2008: Depth to basement beneath the Chilcotin Group basalts, BIZ and QUEST project areas south-central BC (092O, P, 093A, B, C, F, G, J, K, M, N, O)
- Mineral Exploration Roundup Poster (pdf, 3.74MB)

2008: A preliminary physical property characterization of the Chilcotin Group basalts, south-central BC (092 O, P, 093A, B)
- Mineral Exploration Roundup Poster (pdf, 4.42MB)

2007: Can't see the mines for the basalt?: Seeing through the Chilcotin Group basalts
- Mineral Exploration Roundup Poster (pdf, 3.18MB)

Technical Articles
2011: "The thickness of Neogene and Quaternary cover across the central Interior Plateau, British Columbia: analysis of water-well drill records and implications for mineral exploration potential"
- Canadian Journal of Earth Sciences, Volume 48(6) p.973-986

2008: "Cover thickness across the southern Interior Plateau, British Columbia: constraints from water-well records"
- Summary of Activities 2007, Report 2008-1 p.11-20 (pdf, 6.99MB)

2007: "Mineral Exploration Potential Beneath the Chilcotin Group (NTS 092O, P; 093A, B, C, F, G, J, K), South-Central British Columbia: Preliminary Insights from Volcanic Facies Analysis"
- Geological Fieldwork 2006, Paper 2007-1 p.229-238 (pdf, 6.32MB)

2007: "Subaqueous Channel-Confined Volcanism within the Chilcotin Group, Bull Canyon Provincial Park (NTS 093B/03), South-Central British Columbia"
- Geological Fieldwork 2006, Paper 2007-1 p.285-290 (pdf, 6.32MB)

2007: Chasm and Dog Creek lithofacies, Chilcotin Group basalt, Bonaparte Lake map area, British Columbia;
- Farrell, R E; Andrews, G D M; Russell, J K; Anderson, R G.; Geological Survey of Canada, Current Research 2007-A5, 2007; 11 pages (pdf, 10.2MB)

Final Deliverables

Geoscience BC Map 2009-16-1/GSC Open File 6230
Geology, Chasm Provincial Park and vicinity, British Columbia
Chasm Provincial Park is located northeast of Clinton, British Columbia, occupies 3067 hectares. Within the park, which includes the Chasm Canyon, extending approximately 8 km to the south and ranging from 500 - 3000 m wide and 300 m deep, the Chilcotin Group basalt dominates the landscape and bedrock geology.

The Chilcotin Group basalt overlies rocks of the Permian Cache Creek Group, Eocene Kamloops Group and Miocene Deadman River Formation. This Geoscience BC Report/GSC Open File provides the distribution of these units as well as graphic logs, georeferenced magnetic susceptibility data, geochronology sample locations, and location and mineral deposits. All of these data are referenced on the map. It also provides a summary of interim reports on the area. A main focus of this publication is the volcanic facies distribution of the Chilcotin Group basalt, which is well exposed in the Chasm Canyon.

Geoscience BC Map 2010-2-1/GSC Open File 6344
Distribution of the Chilcotin Group, Taseko Lakes and Bonaparte Lake map areas, British Columbia
The Oligocene to Late Pleistocene Chilcotin Group comprises stratified olivine–phyric basaltic flows that underlie much of the Interior Plateau of British Columbia. The distribution of the Chilcotin Group portrayed on published maps is imprecise because of poor outcrop exposure on the plateau, multiple generations of mappers, and lack of consensus concerning definition of the Group. The thickest and most extensive sections are exposed in the valley–margins of major rivers (e.g. the Fraser River). This map contains part of the M.Sc. thesis work by Dohaney (2009) to collate and interpret multiparameter spatial datasets leading to a reassessment of the distribution of the Chilcotin Group within the Taseko Lakes (NTS 092O) and Bonaparte Lake (NTS 092P) map areas.

This new distribution map leads to several important conclusions:

  1. the Chilcotin Group is less extensive than shown in previous compilations for Taseko Lakes and Bonaparte Lake map areas by as much as 48%; by analogy, this implies that elsewhere the areal extent of the Chilcotin Group is also significantly over-estimated;
  2. the numerous areas indicated as not overlain by Chilcotin Group in the compilation constitute previously unidentified basement "windows" that are geologically and economically important;
  3. Chilcotin Group volcanism spanned the Oligocene to the Pleistocene (approximately 30 m.y. duration) and was centered in the central Fraser River area (south of Williams Lake) throughout the Pliocene—Pleistocene;
  4. episodic eruption of Chilcotin Group lavas was likely via a multitude of small–volume, short–lived, monogenetic vents, rather than a series of long–lived volcanic centres or fissures; and
  5. the Chilcotin Group is thickest where lavas ponded in paleo–valleys, a key indicator for mapping the distribution of Neogene channels in the Fraser Basin drainage.
Geoscience BC Map 2010-15-1/GSC Open File 6657
Bedrock Cross-Sections in Chasm Provincial Park
This report contains four photomosaic and associated line drawing cross-sections related to GBC Map 2009-16-1 / GSC Open File 6320 "Geology, Chasm Provincial Park and vicinity, British Columbia." The modified method of Architectural Element Analysis used at Chasm, and inter-relationships of the lavas and paleosols at Chasm, are described in Farrell, 2010 (see thesis below).
M.Sc. Thesis - J. Dohaney
Distribution of the Chilcotin Group basalts, British Columbia - University of British Columbia, 2009 - Available digitally through University of British Columbia (pdf, 7.3MB)

Abstract: The Chilcotin Group basalts (CGB) are Oligocene to Late Pleistocene, stratified olivine-phyric basaltic lavas that overlie a large proportion of the Interior Plateau of British Columbia. The distribution of the CGB is poorly understood regionally; the current distribution is based on compilations of previously published geological maps that employ a diverse set of lithostratigraphic definitions of the Group. Exposure of the basalts is typically poor, but the thickest and most extensive sections are exposed in the valley-margins of major rivers (e.g., the Fraser River). This study collates and interprets spatial data sets and reassesses the distribution of the CGB with the intent of producing a new, more robust distribution of the CGB within the Taseko Lakes (0920) and Bonaparte Lake (092P) map areas, with the goal of better characterizing their geological history and physical volcanology. The new distribution map demonstrates several important observations: (1) the distribution of the CGB is less extensive than previous compilations by up to 48%; this implies that, regionally, the CGB is probably significantly over-estimated; (2) there are abundant, yet not previously identified "windows" through the basalt that expose underlying rock units which may be geologically and economically important; (3) CGB volcanism spanned the Oligocene to the Pleistocene (—30 Ma) and was centered in the central Fraser River area (south of Williams Lake, B.C.) throughout the Pliocene Pleistocene; (4) the CGB was likely erupted from a multitude of small-volume monogenetic vents, rather than a series of long-lived volcanic centres or fissures; and (5) the CGB is thickest where lavas ponded in paleo-valleys, providing a key to mapping the distribution of Neogene channels in the Fraser Basin drainage.

M.Sc. Thesis - R-E. Farrell
Volcanic facies architecture of the Chilcotin Group basalts at Chasm Provincial Park, British Columbia - University of British Columbia, 2010 - Available digitally through University of British Columbia (pdf, 4.4MB)

Abstract: The Chilcotin Group basalt (CGB) of south-central British Columbia, Canada defines a medium-sized igneous province (ca. 17, 000 km˛), characterized by basaltic lavas, volcaniclastic deposits, and paleosols with minor ash deposits. The CGB has previously been mapped only at reconnasissance scale (1:250 000), and most studies concentrated on geochemical and petrological studies; no stratigraphic relationships or volcanological models were attempted. Chasm canyon exposes one of the thickest successions of the CGB. Here, I explicate the volcanic facies architecture at Chasm to reconstruct the emplacement history and volcanism in the Neogene using geological mapping, cross-sections, and graphic logs. Specifically, seven discrete facies are recognized. The coherent facies are: i) vesicular/amygdaloidal pahoehoe lobes; ii) columnar-jointed, sheet-like lava; and iii) intact basaltic pillow lava. The clastic facies are: iv) paleosols; v) pillow-fragment breccia; vi) hyaloclastite; and vii) lacustrine sandstone. Facies are grouped into broad facies associations including the subaerial facies and interstratified subaqueous and subaerial facies. The subaqueous facies are a minor component in the canyon stratigraphy. The geometry of the lavas is indicative of the eruptive style of volcanism at Chasm, which defines the volcanic facies architecture. Four architectural elements have been observed: i) tabular-classic (TC), which represents a steady continuous supply of subaerial effusive basaltic lavas; ii) compound-braided (CB), which is typical of a shield volcano where anastomosing, branching flow fields result; iii) transitional-mixed, a combination of TC- and CB-type suggestive of bimodal emplacement, perhaps sourced from coalesced shield volcanoes and flank fissures; and iv) foreset-bedded indicative of subaqueous lavas. The exposed rocks record the evolution of CGB volcanism through ten distinct eruptive episodes and intermittent lakes, with periods of quiescence characterized by the paleosol development. Whole-rock Ar-Ar dates were obtained; the duration of volcanism is calculated as 1.28 ± 0.61 m.y. Emplacement is suggestive of shield volcanoes and small fissure eruptions with a northerly flow direction. Laterally extensive paleosols, classified as Brunisolic soils, were examined closely and display a range of morphological features suggestive of the paleo-environment. Lateral variability amongst paleosols have been mapped over a distance of more than 8 km, including a subqueous to subaerial transition.


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