CHAPTER 7NORTHERN SLIDE MOUNTAIN TERRANE

CASSIAR GOLD CAMP

INTRODUCTION

The Cassiar camp, is situated in northern British Co-lumbia, a few kilometres east of the abandoned Cassiartownsite and asbestos mine. It includes mines on TableMountain (Erickson and Cusac) and at Quartz Rock Creek(Taurus), The camp encompasses a north-trending belt overa distance of about 12 kilometres of productive east to north-east-trending gold-silver-bearing quartz veins (Figure 7.1,Photo 7.1). During its decade-long production history from1979 to 1988, a combined total of 8 524 000 grams (274 055ounces) gold and 5 472 300 grams (175 938 ounces) silverwere produced from quartz lodes (Schroeter and Lane,1991). Boronwski (1988) reports that total production fromthe camp was roughly 490 000 tonnes grading approxi-mately 15.6 grams (0.5 ounce) per tonne gold and 11.3grams per tonne silver mostly from the Erickson mine. Sig-nificant amounts of gold have also been recovered fromplacers on McDame Creek and its tributaries. Holland(1950) reports that from the initial discovery of placer in1874 until 1950, reported recovery was 2 021 700 grams (65000 ounces). Since that time small-scale operations have re-covered gold during seasonal placer mining.

Investigations in the Cassiar camp in the course of thisproject were conducted for approximately one week in1989. In the following pages, we summarize thelithotectonic setting of mesothermal gold-quartz vein de-posits in the Cassiar camp. A conventional K-Ar isotopicage on mariposite from listwanite and an Ar-Ar age on seri-cite from quartz are presented and discussed. Matt Ball gra-ciously provided several days of his time to review both theunderground and surface geology of the mine, which con-tributed significantly to our understanding of the deposit ge-ology. Andre Panteleyev provided several unpublishedAr-Ar age dates of hydrothermal vein mica from the campwhich represents a significant contribution to the followingdiscussion.

PREVIOUS WORK

The geology of the area has been mapped at a variety ofscales. It was first investigated as part of a Geological Sur-vey of Canada regional 1:4 mile mapping program of theMcDame map sheet (Gabrielse, 1963). Gordey et al. (1982)mapped a west-trending belt across the Sylvesterallochthon. A larger area of the allochton has been subse-quently mapped at a 1:50 000 scale (Nelson and Bradford,1993). More localized mapping in the area of the Cassiarcamp has been conducted by Diakow and Panteleyev(1981), Panteleyev and Diakow (1982) and Harms (1989).

The detailed geology and characteristics of vein mor-phology, alteration and mineralization in the camp are well

documented (Mandy, 1936, 1938; Panteleyev, 1980, 1983,1992; Diakow and Panteleyev, 1981; Panteleyev andDiakow, 1982; Hopper, 1984; Dursell, 1986; Sketchley,1986; Sketchley et al., 1986; Gunning, 1988; Boronowski,1988; Anderson and Hodgson, 1989; Nelson and Bradford,1989; 1993; Panteleyev et al., 1997). Most of this work hasfocused on the central part of the camp, near the Ericksonmine, and provides a wealth of detailed information. Pro-duction at the mine coincided with the period whenmesothermal gold-quartz vein deposits in general were af-forded considerable research attention. More recent explo-ration activity concerned with the potential for bulk tonnageore has been reviewed by Panteleyev et al. (1997).

REGIONAL SETTING

Gold-quartz vein deposits of the Cassiar camp arehosted by late Paleozoic (Gabrielse et al., 1993) oceaniccrustal volcanic, plutonic and mantle metamorphic rocks ofthe Sylvester ophiolitic assemblage (Harms, 1989). Theseoceanic rocks occur as individual thrust-bounded sliceswhich form part of an imbricated stack of fault-boundedlithotectonic units of predominantly oceanic crust-mantleand sedimentary rocks and subordinate arc-volcanic andcontinentally derived sedimentary rocks that comprise theSylvester allochthon (Gabrielse and Mansy, 1980; Gordeyet al., 1982; Harms, 1984, 1985, Harms et al., 1989; Nelsonand Bradford, 1989, 1993). This oceanic assemblage is avast, composite, klippe resting on the basal Sylvester fault(Harms, 1989) above displaced ancestral North Americancontinental margin rocks (Figure 7.2). The timing ofobduction of the Sylvester assemblage is poorly con-strained. Harms (1985) suggested an age between Late Tri-assic and Early Cretaceous (Figure 7.3) constrained by theupper age of the Table Mountain sediments and the oldestage of hydrothermal sericite within the imbricating struc-tures. AMiddle Jurassic emplacement age is, however, mostgenerally advocated (Struik et al., 1992; Nelson and Brad-ford, 1993). Whether or not the Middle Jurassiccompressional event is related to obduction of the SlideMountain terrane or is due to telescoping of a Late Triassicrifted fore-arc basin that was constructed on previouslyaccreted oceanic basement, remains to be resolved. Harms(1985) reported a tonalite intrusion cross-cutting aslice-bounding fault near the southern end of the SylvesterAllochthon in the Rapid river area yielded late Permian zir-cons which indicates pre-Permian tectonism within theophiolitic assemblage. This is a rare and possibly unique ob-servation that provides the only evidence for pre-Late Trias-sic tectonism in oceanic Slide Mountain terrane in theCanadian Cordillera, known to the author. An obduction age

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74 British Columbia Geological Survey

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Figure 7.1. Geology of the Cassiar gold camp, after Harms (1989) and Nelson and Bradford (1993). Generalized cross-section of theErickson mine area is from Harms (1989).

of Late Permian would explain why there are no youngerrocks in the Slide Mountain terrane and why it does notoverlie rocks anywhere that are younger than Late Permian.

POST-COLLISIONAL MAGMATISM

Several episodes of post-collisional felsic magmatismhave affected rocks of the Sylvester assemblage (Figure7.3). The earliest identified intrusive event is evidenced bythe north-trending Early Cretaceous Cassiar batholithwhich extends along, and in part cuts, the western margin ofthe Sylvester allochthon (Figure 7.2). It is a compositebatholith including quartz monzonite, granodiorite and gra-nitic phases (Gabrielse, 1963). Potassium-argon isotopicdata for biotite and amphibole give a range of mid Creta-ceous ages between 109 and 89 Ma (Baadsgard et al., 1961;Wanless et al., 1968, 1970, 1972; Panteleyev, 1985).

A small quartz monzonite body near Cassiar, theTroutline Creek stock, intrudes North American platformalsedimentary rocks between the Cassiar batholith and theSylvester allochthon and is considered to be part of a dis-crete magmatic suite independent of the Cassiar Batholith(Panteleyev, 1983). Mica and hornblende K-Ar dates indi-cate a Late Cretaceous age of intrusion which is coeval witha recently obtained U-Pb ziron age of 73±0.5 Ma (APanteleyev and R. Friedman, personal communication2000).

The small stocks at Mount Reed and Mount Haskin areproducts of Early Tertiary magmatism. They intrude NorthAmerican strata along the eastern periphery of the Sylvesterallochthon. Biotite K-Ar ages range from 51.4 to 48.8 Ma(Christopher et al., 1972).

DEPOSIT GEOLOGY

Gold-quartz veins in the Cassiar camp are hosted by agently dipping thrust zone, 300 to 400 metres wide, whichimmediately underlies Late Triassic Table Mountain sedi-mentary rocks (Photo 7.2). This thrust zone comprises a se-quence of narrow imbricated metabasaltic slices, roughly100 metres thick, separated by thinner, discontinuous tec-

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Photo 7.1. View to the south overlooking Needle Point Mountain.

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Figure 7.2. Location and regional geological setting of the Cassiarcamp.

tonic slivers of variably listwanite-altered ultramafic rocks(Figure 7.4).

Metabasalts are the dominant rock type throughout themap area and are typically grey-green, massive and aphani-tic. Where they contain gold-quartz veins the rock is con-verted to orange-brown weathering ankerite (Photo 7.3).Locally in outcrops and extensively in underground mineworkings, pillow structures and pillow breccias are wellpreserved. Major, trace and rare-earth element analysis ofthese mafic volcanic rocks suggest that they are mid-oceanridge basalts (MORBs; Ash, unpublished data). The ultra-mafic rocks are either intensely serpentinized or variablycarbonatized, making recognition of their protolith lithol-ogy difficult. Nelson and Bradford (1989) have suggestedthat they are attenuated tectonic slivers of the Zus Mountainultramafic thrust sheet which crops out at a similar

76 British Columbia Geological Survey

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Figure 7.3. Relative age and tectonostratigraphic relationships forlithologies in the Cassiar camp.m - muscovite K-Ar; hb - hornblende K-Ar; bi - biotite K-Ar; mp- mariposite/fuchsite K-ArData sources used to constrain age relationships for the elementsdepicted are;� Christopher et al., 1972; � Panteleyev, 1985; � Panteleyev,personal communication 1991; � Panteleyev and Diakow, 1982;Sketchley et al., 1986; � Harms, 1989; � Nelson and Bradford,1989, 1993; Gabrielse et al., 1993 � this study and � A.Panteleyev and R. Friedman, personal communication, 2000).

Photo 7.2. Flat-lying thrust contact between hanging wall graphiticblack argillite and footwall carbonate altered ultramafics in thearea of the Sky vein (photo courtesy of Andre Panteleyev).

Photo 7.3. Example of relatively unaltered massive basalticdiabase/volcanic in faulted contact with orange-brown weatheringcarbonate altered equivalent (photo courtesy of AndrePanteleyev).

tectonostratigraphic level to the north. Zus Mountain ultra-mafic rocks consist of plutonic and metamorphic lithologiesrepresentative of an ophiolitic crust-mantle transitionalsuite. It has been argued (Anderson and Hodgson, 1989;Nelson and Bradford, 1993) that the Cassiar vein deposits,although fitting the overall classification for mesothermalveins in terms of morphology, mineralogy and alterationcharacteristics, are unique because they lack evidence of anassociation with a deep crustal fault. We contend that thepresence of ophiolitic crustal and upper mantle lithologieswithin the zone of mineralization implies that it was part of adeeply rooted, through-going crustal fault, during and mostlikely subsequent to, obduction of the Sylvester allochthon.

Gold-bearing veins are commonly white crystallinequartz (Photo 7.4) with locally laminated, ribboned andbrecciated textures. They pinch and swell along strike andare typically 1 to 2 metres wide but locally can be up to sev-eral metres in width. Productive veins are commonly from100 to several hundred metres in strike length, with theVollaug and the Elan veins having persistent strike lengthsof 2600 and 1500 metres, respectively. Down-dip extent ofthe veins vary from several tens to several hundreds ofmetres. Most trend easterly with minor deviations. Differ-ences in both structural style and hostrock lithology have ledto a twofold subdivision of the vein system (Panteleyev andDiakow, 1982; Boronowski, 1988). Tensional veins (Type 1veins of Panteleyev and Diakow, or steeply dipping veins ofBoronowski) occupy steeply-dipping dilational fractures insheared metabasalt and are typically truncated by thelow-angle thrust faults. These are the most common andeconomically the most significant vein type. Fissure veins(Type 2 veins of Panteleyev and Diakow, or gently dippingveins of Boronowski) occur along the low-angle thrustfaults in association with listwanite-altered ultramafic rocksand hangingwall argillites in the upper part of the thrustzone.

The following description of the vein mineralization istaken primarily from references cited at the beginning ofthis chapter. Gold was the primary metal recovered in thecamp, with silver as a significant byproduct, although it wasthe dominant precious metal derived from tetrahedrite insome veins. A variety of mineralization styles are evident.Gold is most common as free gold in quartz or along gra-

phitic or micaceous fractures but a significant amount alsooccurs as either fracture fillings or disseminated blebswithin sulphide grains. Sulphides are generally dissemi-nated and comprise from 1 to 3% of vein material, locally in-creasing to 5 to 10%. Pyrite is the dominant sulphide and isusually associated with tetrahedrite, sphalerite, chalcopy-rite and lesser galena and arsenopyrite. In type 1 veins, gold

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Figure 7.4. Schematic cross-section of the Erickson mine compiled after Sketchley, 1986; Boronowski, 1988 and Anderson and Hodgson,1989).

Photo 7.4. Massive bull quartz of the Jenni vein (photo courtesy ofJoanne Nelson).

Photo 7.5 Native gold in quartz from the Eileen vein, Ericksonmine (photo courtesy of Joanne Nelson).

is most commonly found associated with pyrite and vuggyquartz whereas gold within ribboned quartz; is dominant intype 2 veins.

Wallrocks are pervasively carbonatized with variablesericite and pyrite alteration. Metabasaltic rocks hosting thetype 1 veins display relatively uniform alteration envelopes,from 1 to 15 metres wide depending on vein width.Sketchley (1986) identified ankerite, siderite, dolomite,quartz, sericite, kaolinite, pyrite, carbon and minor titaniumoxides from the alteration assemblage in the metabasalticrocks. Carbonatization of ultramafic rocks is most com-monly manifest as schistose talc-serpentinite, talc-carbon-ate and quartz-carbonate-mariposite alteration assemblages(listwanite suite, Appendix III). The dominant hydrother-mal carbonate is iron-rich-magnesite. Locally, throughoutt h e l i s t w a n i t e , a b u n d a n c e s o f s c h i s t o s emariposite-quartz-sulphide rock, with subordinate carbon-ate, produce visually striking, bright-green exposures.

Studies of primary fluid inclusions in quartz veins indi-cate that the mineralizing fluids were moderately saline andrich in carbon dioxide, with homogenization temperaturesranging from 250° to 300°C (Hopper, 1984; Dursell, 1986;Nelson, 1990). By using an arbitrary load pressure of 6300kilopascals, Dussell calculated an entrapment temperatureof 350°C. He proposed that gold was transported as a goldbisulphide complex Au(HS)2 with deposition triggered ei-ther by decreasing sulphur and oxygen or increasing iron ac-tivity or some combination of these mechanisms, all ofwhich relate to fluid-wallrock reactions and fluid degassing.

AGE OF GOLD MINERALIZATION

The age of mineralization for the Cassiar gold veins hasbeen traditionally interpreted at 130 Ma (Panteleyev, 1985;Sketchley, 1986), the mean apparent age of potassium-ar-gon isotopic analysis of hydrothermal sericites that rangebetween 115 and 137 Ma (Figure 7.3). A number of recentAr-Ar isotopic age determinations on similar hydrothermalsericites from throughout the camp (A. Panteleyev, personalcommunication) indicate that the age is slightly older thaninitially interpreted. Six samples of hydrothermal sericitewere dated (Layer and Drake, 1997). Four of the dated sam-ples have well defined plateaus with only slight amounts ofAr loss and identical ages between 133 and 135 Ma indicat-ing an age of mineralization at ca. 134 Ma. The two othersamples were slightly to significantly older than the 134 Maage, however those samples showed evidence of Ar losswith less well defined plateaus (Layer and Drake, 1997).

Additionally a quartz vein sample with sericite from theQuartzrock Creek area was provided by Andre Panteleyevand dated as part of this study (Appendix II). The age spec-trum for this sample (Figure 7.5) has a steep gradient; appar-ent ages increase rapidly from approximately 100 Ma toapproximately 132 to 136 Ma. A plateau age of 132 to 136for the last 8 steps is clearly consistent with a 134 Ma miner-alization age indicated above.

A sample of carbonatized ultramafic rock with abun-dant mariposite collected from a trench exposure oflistwanite marginal to the Eileen vein was analyzed by con-ventional K-Ar methods at The University of British Co-

lumbia. The mariposite yielded an apparent K-Ar age of52.6 ±2.1 Ma. This age is significantly different fron the ca.134 Ma mineralization ages defined by hydrothermalsericites, and so probably reflects resetting by a later ther-mal event perhaps related to intrusion of the Mount Haskinand Mount Reed stocks which are coeval with the K-Armariposite age (Figure 7.3). This suggests that fault zoneswithin the Sylvester allochthon were reactivated during theEarly Tertiary and channeled fluids that either depositednew micas, or reset pre-existing vein micas.

RELATIONSHIP TO MAGMATISM

In contrast to gold-quartz vein mineralization de-scribed previously, there is no defined magmatic episodecoeval with vein mineralization in the Cassiar camp. Nelson(1990) presented two indirect lines of evidence to suggestthat a pluton might be present at depth. An actinolite-epidote metamorphic assemblage within metabasalticrocks, that defines a broad zone consistent with the distribu-tion of the vein system, is interpreted by Nelson to resultfrom contact metamorphism related to an underlying intru-sion. Its existence was indirectly supported by the presenceof granodioritic inclusions in a lamprophyre dike. An Ar-Arage determination on hornblende from these clasts pro-duced a well defined plateau at 76.4 Ma, suggesting that theinclusions may be related to the Troutline Creek Stock. Asimilar lamprophyre dike elsewhere which cuts theophiolitic assemblage rocks (A. Panteleyev, personal com-munication, 1991) returned a biotite K-Ar date of 110±8.0Ma suggesting that there are two distinct episodes oflamprophyre diking.

It is possible that there are earlier, cryptic phases of theCassiar Batholith coeval with vein mineralization that arecurrently unrecognized. The Sierra Nevada Batholithwhich is associated with the Mother Lode belt in Californiaprovides a possible analogy. Both these gold camps are inaccreted oceanic terranes spatially associated with, and lo-cally intruded by, large composite batholiths. Although thebulk of the Sierra Nevada Batholith is late Cretaceous in ageit also includes Early Jurassic phases that were only detect-

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Figure 7.5. Ar-Ar age spectra plot for sericite in quartz vein fromthe Quartz rock Creek area.

able by U-Pb dating of zircon. All K-Ar systematics of olderphases of the intrusion were effectively reset by the latestand dominant magmatic event. More detailed evaluation ofthe various phases of the pluton by U-Pb dating may con-strain the possibility of a similar relationship in the CassiarBatholith.

SUMMARY

· Gold-quartz vein mineralization in the Cassiar camp ishosted almost exclusively by ophiolitic crustal volcanic,hypabyssal and mantle metamorphic rocks of theSylvester ophiolitic assemblage along a terranecollisional suture zone. This suture is inferred by the pres-ence of pervasively listwanite altered ultramafic rockswithin the imbricated zone of vein mineralization thoughtto be derived from upper mantle lithosphere.

· The best developed veins in terms of thickness and conti-nuity are hosted by competent mafic metabasaltic volca-nic and hypabyssal rocks. Vein thickness and continuity isgenerally poorly developed in the ultramafic rocks. How-

ever the highest gold grades are found in quartz veins im-mediately adjacent to the listwanite-altered ultramaficrocks.

· Economic concentrations of gold are found only in thequartz veins or in peripheral fractures and highest valuesoccur in the deformed parts of the veins associated withconcentrations of sulphide minerals.

· The limited thickness of suitable ophiolitic gold quartzvein hostrocks within this regionally flat-lying imbricatedsequence restricts the vertical extent of productive goldveins.

· Unlike other gold quartz vein camps described previouslyin this work there is no defined magmatic episode coevalwith the gold-quartz mineralization. In addition, there hasbeen no described spatial association between felsic dikerocks and gold quartz veins in any of the mined vein sys-tems in the Cassiar camp.

· An early Cretaceous age of ca. 134 Ma is suggested byisotopic data of hydrothermal vein micas for the age ofgold vein mineralization in the Cassiar camp.

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80 British Columbia Geological Survey