NOTES ON THE PENTIC'PON GROUP A PROGRESSREPOW ON A NEW STRATIGRAPHICSUBDIVISION OF THE TERTIARY SOUTH-CENTRALBRITISH COLUMBIA (82E/L)
By B.N. Church
The name Penticton Group has beenproposed for Eocene volcanic and sedimentary rocks of the Okanagan-Boundary region. The areas of recent studyrequired to delineate these rocksare shown on Figure 1.
Figure 1. Index MP of areas of detailed study.
12 The principalresources of thePenticton Group arecoal, precious metal deposits, uranium, and geothermalenergy.
The Group consists of sixwell-defined formations having an aggregate thickness of about 2 500 metres in thetype area near Penticton (No. 4, Figure 2). At thebase are polymictic conglomerates and breccias referredto as the Springbrook Formation and coevalbeds of the Kettle RiverFormation consisting of graniteboulder conglomerate, rhyolite breccia, and tuffaceoussedimentary rocks. Above this is t.he Marron For- mation composed mainly of thickandesite, trachyte, and phonoliticlava flows,succeeded upward by dacitic and some andesitic domes: of the blararna Formation.This is followed by volcanicbreccias and lacustrine and fluvialsedimentary rocks of the White Lake Formationand, uppermost., the Skaha Formationconsisting of alandslide complex and fang1.omeratebeds. The Group rests unconformably on pre-Tertiarygranitoids, metamorphosed Mesozoicsedimentary and volcanicrocks, and olderschists and gneis:oes.
1 2 3 4 5 6 HATCREEK TERRACE KELOWNA PENTICTON MIDWAY REPUBLIC MTN.
W Z
W z W 0 -.7 2 I
TW wz 0 0 W
Flgure 2. Correlation chart of major Tertiary units.
13 The agerange forthe Group, asdetermined by potassium-argonradiometric methods, is 48.4 Ma (wholerock) to 53.1 Ma (biotite) f1.8 Ma. Pre- liminarytests performed on theprincipal lava members indicate normal magneticpolarity. Overlying the Group areisolated patches of Miocene rhyolite(Olalla rhyolite, 13 Ma) 'plateaubsalt', andyounger 'valley basalt. '
In the Kelowna area'plateau basalt' occurs on Carrot Mountain ( 11.8 Ma) and Daves Creek (14.9 Ma). TheLambly Creek basalt (0.762 Ma) is the onlyso-called 'valley basalt' of theregion.
Northand west of thetype area (see Nos. 1, 2, and 3, Figures 1 and 2) theconstitutent formations interfinger and arereplaced by units of the KamloopsGroup (Ewing, 1981). At TerraceMountain near Vernon the names NaswhitoCreek Formation, Rouleau Rhyolite, Attenborough Creek Formation, and Shorts CreekFormation are applied to Eocene volcanic and sedimentary unitsthat only partly correlate with the Penticton Group (Figure 2). Further west at Hat Creek few comparisonscan be made otherthan to say that the Coldwater Beds partlyresemble the basal Tertiary sedimentary rocks in the Okanagan area.Recent studies suggest that some part of thesebasal successions may be Cretaceous (K. Shannon, pers. corn.).
To thesoutheast near Midway in the BOundary area the EOcene section is incompletebut strikingly similar to the type assemblage (Monger, 1968).
In northern Washington Statenear the Republic area the O'Brien Creek Formation and Klondike buntain Formation, at the baseand top of the Eocene sectionrespectively, are correlated in part withthe Kettle River Formationand Skaha Formation of thePenticton Group (Pearson and Obradovich, 1977). However, thedacite-rich Sanpoil volcanics which underlie part of theRepublic graben are lithologically unlike the Plarron Formation and represent a significantuncorrelated unit.
A checkwith Dr. T.E. Bolton in thespecial projects section of tRe Geological Survey of Canada on thepre-occupation of stratigraphic names hascleared the following list for formaluse in south-centralBritish Columbia:
Area Nam Area Age
Hat Creek LakeFinney Beds (Eocene) kdicine Creek Formation (Eocene) Hat Creek Formation(Eocene)
TerraceMountainNaswhito Creek Formation(Eocene) Bouieau Rhyolite (Eocene) Attenborough Creek Formation (Eocene) Shorts Creek Formation (Eocene)
Penticton Penticton Penticton Group (Eocene)
Ke I owna Lambly Creek (Pleistocene)Basalt
It isconsidered that name Hat CreekCoal Formation (Eocene) is sufficientlydifferent from Hat Creek beds (Oligocene)of Wyoming and Hat Creek Basalt(Recent) of northern California.
14 structuralcontrol of thevarious outliers of thePenticton Group relates to some east-west-trendingsynclines and to a pattern of north-south gravityfaults andpronounced conjugate shears of northeast and northwest orientation. These folds and fracturesare viewed as essential elements in a north-southdirected stress scheme thought to be responsiblefor the many graben-likestructures and overallbasin and range-likafabric of the region.
The Hat Creekbasin is a typicalgraben. In this example thecentral zone of thevalley has beendowndropped on a series of north-southtension faultstrending subparallel to the direction of regional maKiman stress, thewalls of this grabenhaving been offset by northwest an,3 northeast- trendingconjugate shears.
Vertical movementon thegraben faults is commonly inthe range of hundreds of metres. As viewed on ShortsCreek near Vernon, theTertiary bedshave been downfaulted inexcess of 700 metres against Bolder limestone and granite; at Hat Creek thevertical displacement is locally in excess of 1 000 metres. As a netresult of this movement, cormnon1:y thebase of thesecontinental beds is displaced well below mean sea levelelevation.
Lateral movement on theshear faults is notreadily documented although it is suspectedthat the southern Okanagan andBoundary areas may have resided on a northeast-trendinggeothermal lineament simi1a:c to the present Reno, Nevada-Billings, Montana volcanic-geothermal belt (Grim, 1977). Thiscould explain the marked similarity of thestrstigraphy and petrography of the Eocene volcanicrocks of the Okanagan an83Roundary areas.This might also partly account for the resetting to Eocene of radiometric dates of crystalline basementrocks inthe region (Ross, 1974).
The hypotheticalrepositioning of Penticton Group rocksin the south Okanagan andBoundary areasto juxtaposition on a northeast-trending geothermal belt would require a southeast-northwesttranslation of ab,out 80 kilometres.Extending the hypothetical geothermal belt and lithologicalcorrelations to the Highwood area of central Wmtana (similarities noted by Church, 1973, p. 75), a translation :in the order ofseveral hundred kilometres muld be required. It is notedthat lateral translations of this magnitu*have been recorded in theNorthern Cordillera(Norris, 1981, G.A.c. abstract, p. 29).
REFERENCES
B.C. Ministry ofEnergy, Mines & Pet. Res.,Geology inBrit.ish Columbia, 1975, pp. G99-Gl18; GeologicalFieldwork, 1978, Paper 1979-1, pp. 7- 15; Prelim. Map 35, Geologyof thePenticton Tertiary Outlier (revised 1980); Prelim. Map 37, Geologyof theTerrace Mountain Tertiary Outlier(revised 1980); Prelim. Map 39, Geoloqy of the Kelowna TertiaryOutlier (West Half), 1980; Prelim. Map 41, Geology of the Rock Creek Tertiary Outlier, 1980; Prelim. Map 45, Geologyof the Kelowna Tertiary Outlier (East Half), 1981.
15 Church, B.N. (1973): Geology of the White Lake Basin, B.C. Ministry of Energy, Mines .s Pet. Res., 5ull. 61, 120 pp. Ewing, T.E. (1981): RegionalStratigraphy and StructuralSetting of the Kamloops Group, South-centralBritish Columbia, Cdn. Jour. Earth Sei., Vol. 19, NO. 9, pp. 1464-1477. GeologicalAssociation of Canada (1981): Symposium on 'The Last 100 Mil- lion Years of Geology and MineralDeposits in theCanadian Cordillera', in Abstracts, pp. 14 and 29. Grim, P.J. (1977): GeothermalEnergy Resources of theWestern United States,National Geophysical and Solar-Terrestrial Data Center. Monger, J.W.H. (1968): EarlyTertiary Stratified Rocks, Greenwood Map- Area (82E/2), British Columbia, Geol. Surv.,Canada, Paper 67-42, 39 PP . Pearson, R.C. andObradovich, J.O. (1977): Eocene Rocks in Northeast Washington - Radiometric Agesand Correlation, U.S.G.S., Bull. 1433, 41 PP. Ross, J.V. (1974): A Tertiary TnermalEvent in South-centralBritish Columbia, Cdn. Jour.Earth Sci., Vol. 11, No. 8, pp. 1116-1122.
16 THE RIDDLE CREEK URANIUM-THORIUM PROSPECT (82E/12W)
By B.N. Church
The Riddle Creek uranium-thorium prospect, 15 kilometres west: Of Summerland, was discovered in 1977 andacquired the same year by British Newfoundland ExplorationLtd. Work on theproperty to dateincludes line- cutting, mapping, soil geochemistry, and severalshort drill holes.
The presentreport is based on recentgeological and scintillometer surveysand a lithogeochemicalstudy sponsored by theMinistry.
GEOLOGICAL SETTING
A large radioactive anomaly coincideswith an Eocene volcanic: centre near theheadwaters of RiddleCreek (Figure 1). The principalradioactive rocksinclude trachytes andmafic phonolites of the Marron Formationand consanguineousigneous intrusions of theCoryell-type.
LOW-RADIOACTIVE COUNTRY ROCKS
At thebase of the Tertiary section and north of the zone of anomalous radioactivity,poorly exposed polymictic boulder conglomerate beds are tentativelyassigned to the SpringbrookFormation. These rocks appear to beunconformably underlain by graniticphases of the Okanagan Batholith (Jurassic-Cretaceous) and overlain byunnamed andesites. The andesiteis form a significantformation in the northeast part of the area where lava and breccia are 250 metres thick.Alkaline andesite dominates (No. 1, Table 1) and is characterized by scatteredmicrophenocrysts of plagioclase and hornblendeusually less than 1 mtllimetre indiameter.
Scintillometer readings on these rocks and other basal andbasement units range from 40 to 80 counts per second.
RADIOACTIVE ROCKS
Rocks that show anomalous radioactivity are principally mafic: phonolites andtrachyte lavas and breccias (Nos. 2 and 4, Table 1). Thetse overlie theandesites and onlap parts of the Okanagan Batholith. They are dated 52.7t1.8 Ma (K/Ar on biotite) and correlatewith the Yellow Lake Member of the Marron Formationnear Penticton.
Mafic phonolites, whichform the base of the Yellow LakeMenher, are exposed on theridges north and northeast of RiddleCreek where interlayeredlava flows and lahardeposits attain a thickness of about 75 metres. Petrographicexamination shows conspicuous rhomb-shaped anorthoclasephenocrysts to 2 centimetres in length and smaller subhedral
17
Tableof Chemical Analyses
1 2 3 4 !j 6 Oxidesrecalculated to 100 si02 57.89 57.37 63.87 61.39 57 -58 66.51 Ti02 0.96 0.93 0.64 0.84 0.87 0.49 A1203 16.38 19.16 17.92 17.05 16 -21 17.01 Fe2& 4.43 3.01 3.14 3.43 3.83 2.58 FeO 1.03 1.77 0.73 1.07 2.48 0.24 M nO 0.08 0.11 0.09 0.09 0.12 0.14 MgO 4.331.10 2.40 1.99 3.73 0.28 C a0 6.90 4.05 0.64 3.34 5 -22 0.84 Na20 4.12 5.47 4.71 6.05 K%O 3.88 5.39 7.14 5.33 5.25 5.86 lorn mmrr;Crcr mrr;Crcr mm mm Oxidesand elements as determined 0.56 2.02 0.90 +HzO 2.02 0.56 0.20 1.14 0.70 0.58 0.35 0.75 -H;O __ 0.35 0.58 0.18 0 -41 0.70 co2 0.25 0.25 c 0.11 0.25 0.25 0.25 s 0.02.. " <0.01~.~- (0.005~.." 0.G 0.02 0.01 pa os 0.82 0.46 0.25 0.32 0.68 <0.15 BaO 0.30 0.21 0.10 0.20 0.23 0.013 SrO 0.30 0.07 0.42 0.21 0.31 0.02 Molecular Norm Quartz 2.0 1.3- 5.7 6.6 Orthoclase 22.7 31.1 41.8 31.0 30.6 34 .O A1 bi te 36.7 38.8 41.8 48.3 41 ,.8 53.0 Nephel ine - 7.3 - - - Anorthite 14.6 10 .o 3.1 6.1 7,.5 1.9 Woll astoni te 7.7 3.9 - 4.1 7.2 0.9 Enstatite 11.8 5.4- 3.0 0,.6 0.8 Ferrosili te - - - - - Forsteri te - 4.9 - - 7,.2 - Fayal ite - - - - - Ilmenite 1.3 1.3 0.9 1.2 1,.2 0.7 Magnetite 0.4 2.1 0.4 0.7 3.9 - Hematite 2.8 1.90.6 1.9 2.1 Corundum - - 1.4 - -
Key to Analyses 1. Alkalineandesite, basal volcanic assemblage 2. Maficphonolite lava (rhombporphyry), Yellow Lake Member, MarronFormation 3. Trachyte ash flow,Yellow Lake Member, MarronFormation (Skahil Creek area) 4. Trachytelava (rectangular porphyry), Yellow Lake Member, MarronFormation 5. Coryell-typemonzodiorite intrusion 6. Trachyte dyke, on northfork of Riddle Creek and euhedralphenocrysts of green diopsidicaugite, biotite, apatite, and magnetite set in a devitrifiedglassy or fine-grainedfeldspathic matrix. Scintillometer readingsare in the range 140 to 180 countsper second.
The most radioactiverocks are thick trachyte lava Elows thatcomprise the upper part of theYellow LakeWmber in thisarea. This; unit is
19 estimated to be between 150 to 200 metres in thickness. It underlies the ridges and slopes immediately northeast and south of the confluence of the forks of Riddle Creek. The trachyte contains large rectangularor platy mixed feldspar phenocrysts of anorthoclase, sanidine, and plagioclase: otherwise it is petrographically similar to the mafic phonolite (rhomb porphyry) suite. Scintillometer measurements are in the range 300 to 420 counts per second.
Coryell plutonic rocks crop outon the hillsides north and south of the westerly source of Riddle Creek. These are high level miarolitic syenomonzonite and monzodiorite phases (No. 5, Table1) that are mineralogically akin, and feeders to, the overlying Yellow Lake volcanic pile into which the Coryell pluton has evidently stoped. The rockis composed of about80 per cent alkali feldspar, mostly orthoclase with rhomb-shaped anorthoclase cores, and20 per cent smaller phenocrysts and interstitial grains of amphibole and pyroxene with poikilitic inclusions of biotite, magnetite, apatite, and sphene. The average scintillometer reading is 250 counts per second.
SCINTILLOMETER SURVEY
In the course of routine geological investigation of the Riddle Creek area, rock outcrops weretested in a manner outlined by McDermott (1977) using a portable gam ray scintillometer (GeoMetrics/Exploranium Model GRS-101). Guantitative control was obtained for uranium from neutron activation of 24 samples, courtesy ofD.R. Boyle of the Geological Survey of Canada, and €or thorium from spectrometer analysis performedby the Analytical Division of the Ministry. The relationship between counts per second and uranium/thorium composition canbe reduced to two equations:
U = C.P.S. (0.072) - 0.538 Th = c.p.6. (0.231) + 6.913
Accordingly, the following averages are calculated for uranium and thorium levels for the main rock types, basedon C.P.S. values at 93 stations:
Rock Unil U Th PP Ppn Trachyte (Ye1 lo* Lake Vember) 27 94 Maficphonolite (Yellow Lake Member) 11 45 Coryel I Intrusions 18 66 A ndesite Unit Andesite (unnamed) 5 23 Springbrd Formation 4 22 Okanagan Batho I i th 4 22
scintillometer results (Figure1) were contoured using the method outlined in Geological Fieldwork, 1980, Paper 1981-1, page 27. A bull’s- eye arrangement of contours lies immediately south of the main courseof
20 Riddle Creek in an areaunderlain by trachytelavas and a volcanic centre.Thoroughly altered rocks are exposed below thetrachyte on Riddle Creek and mre distally on theslopes to the west. Pervasive hydrothermalalteration of thetrachyte and vent (7) brecciahas produced cream and whitekaolinized rocks of variableradioactive response.
THE PROSPECT
A (diamond-drillprogram consisting of approximately 270 metres in seven holes was completed in 1979. Sixholes were sitedsouth OF Riddle Creek nearthe westboundary of thetrachyte andone holesited north of the creek. The purpose of thedrilling was totest bedrocknear geochemical soil anomalies and projectedstructural traps For uranium-bearing solutions.
Northof Riddle Creek, drill hole No. 7 was directedat a northeast- dippingsection oE strata +30 metresthick OE coarsz nlasticsedimentary rocksthat is overlain by partly welded ashflow brecciaat t:he base 0.E thetrachyte unit. (This stratigraphic-structuraltarget is strikingly similarto the occurrence of radioactivetrachyte ash and breccia in clasticsedimentary beds in thevicinity of Farleigh Lake and &aha Creek, 15 to 20 kilometresto the southeast (No. 3, Table 1 -' unit lb on Preliminary Map 35). Although no significant uranium was Eound, the drilling proved goal porosity of the bedsbelow theash Elar and thus potential €or Eurther exploration.
Most of the drilling in thearea of soil anomaliessouth of XiddleCre,?k intersectedCoryell intrusive rocks. However, hole No. 1 nearthe west boundary oE thetrachyte cut altered rocks showing vestiges of conglomerate and brecciasimilar to the rocks at site No. 7.
A few prominentradioactive high spots were not tested by the drilling. The mst important of these is an easterlytrending Eelsic dyke about 4 metres wide exposed 450 metresnorth oE theconfluence oE the Eorks oE Riddle Creek. This is thoughtto be aEeeder tothe trachyte lavas oE the Marron Forrnation (No. 6, Table 1). Scinttllocneterreadings here averaging 1500 C.P.S. correspondto rock analyses showing 121 ppm U ant3 342 ppn Th. Radiographs of slabbedsamples show thatradioactive elementsare concentrated on lmnganese pitch and dendritic growths on nutnerous small cracks. A similar dyke with scintillometerreadings in therange of 600 to 900 C.P.S. wasEound atthe contact between Coryell plutonicrocks and mafic phonolitelavas in thenorthwest part of the 'nap area &tween Isintok Creek and the westernheadwaters of RiddleCreek.
DISCUSSION
The Riddle Creek Tertiaryoutlier lies near the western extremity of a helt of mcenealkaline volcanic rocks that are characterized by anomalous uranium and thoriumcontents (Church and Johnson, '1978). It is suggestedthat these rocks are the source oE therelatively highuranium
21 levels in streams of the Okanagan-Boundary area found by the 1976 URP survey. The possibilities of secondaryuranium deposition and enrichment inthis setting are numerous, includingdykes, permeable sedimentary rocks and alteration zones associatedwith volcanic vents (Culbert and Leighton, 1978).
High radioactiveresponse near the headwaters of RiddleCreek coincides with what appearsto be a trachytevolcanic centre (Figure 1). In 1978, a program of shortdiamond-drill holes was conductedperipheral to this centre and yielded low uraniumvalues. For future study the volcanic centre and associatedash flow deposits offer the best target.
REFERENCES
AssessmentReports Nos. 7362 and 6750. B.C. Ministry ofEnergy, Mines & Pet. Bs., GeologicalFieldwork, Paper 1981-1, p. 27; Prelim. Map 35 (Revised 1380). Boyle, D.R. andBallantyne, S.B. (1980): Geochemical Studies of Uranium Dispersionin South-central British Columbia, C.I.M., Bull.,Vol. 73, NO. 820, pp. 89-108. Church, B.N. andJohnson, W.M. (1978): Uranium andTnoriun inlkrtiary AlkalineVolcanic Rocks inSouth-central British Columbia,Western Miner,Vol. 51, No. 5, pp. 33-34. Culbert, R.R. and Leighton, D.G. (1978): Uranium in Alkaline Waters, Okanagan area,British Columbia, C.I.M., Bull.', Vol. 71, No. 783, pp. 103-110. McDermott, M. (1977): FieldSurveys Using a Portable Gamma Ray Scintillometer, Western Miner,Vol. 50, No. 8, pp. 16-20.
22