Contrasting Styles of Alteration with Barren and Auriferous Quartz Veins from the Contact Lake Lode Gold Deposit 1

Contrasting Styles of Alteration with Barren and Auriferous Quartz Veins from the Contact Lake Lode Gold Deposit 1 M. Fayel<2, T.K. Kyse?, R. T. Kusmirskf, V. Sopuck3, and D. Chan 3 Fayek, M., Kyser, T.K., Kusmirski, R.T., Sopuck, V., and Chan, D. (1993): Contrasting styfes of alteration with barren and auriferous quartz veins from the Contact Lake lode gold deposit; in Summary of nvestigations 1993, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 93-4. dentification of alteration zones associated with barren and auriferous quartz veins is an important exploration tool used in the search for structurally-controlled gold deposits. The Contact Lake lode gold deposit is located within the La Ronge lithostructural domain approximately 45 km northeast of La Ronge, Saskatchewan. t is the largest high-grade gold deposit in Saskatchewan, with an estimated geological reserve of 1.6 million tonnes at a grade of 9.6 g/tonne. The Bakos shear zone, which hosts the Contact Lake gold deposit, crosscuts the central granite and granodiorite phases of the composite Little Deer Lake pluton (Figure 1 ). Barren and auriferous veins that occur within the shear zone provide an opportunity to study the interaction of barren and auriferous fluids with two lithologies within a single deposit. ~ Shear zone Mineralization 1. The Bakos Shear Zone The Bakos shear zone is poorly exposed on surface, however, diamond drilling has outlined a strike length of 1.4 km, 700 m of which lies beneath Contact Lake. The shear zone is typically from 20 to 30 m thick, trends from 060 to 080, and dips 55 to 65 to the southeast, and crosscuts the north-trending granite-granodiorite contact, which dips steeply to the west (Figure 1 ). The hanging-wall contact is typically abrupt, whereas the footwall contact is diffuse and appears as a thick zone of moderately foliated rocks (Chapman et al., 1990). Within the granodiorite phase of the pluton, the shear zone splays into several diffuse zones, whereas within the granitic phase, the shear zone is well defined (Chapman et al., 1990). Mineralization occurring in the granodiorite is referred to as the BK3 zone and mineralization occurring in the granitic phase is referred to as the Main zone mineralization (Figure 1). The extent of mineralization and the contact between the granite and granodiorite phases has been outlined from data obtained from diamond drill ing. n the vicinity of the main Bakos shear zone, there are many small (1 to 10 cm wide) north and northeast-trending structures which are locally mineralized. A A - - J\FEU0:1> ( '0'.'iT:\ ( r ~ Ml:'EkAJ.J J\TO~ A '....,. Granite, f J / : J :Colitatt Lake :t( HO SOrn Figure 1 - Schematic level plan and longitudinal section (insert) showing the extent of gold mineralization within the Bakos shear zone. Specimens from diamond drill core exhibit features typical of brittle ductile shear zones (Ramsey, 1980; Hodgson, 1989). The dominant shear fabric bends from about 45 in relation to the shear zone boundary at its margins, to sub-parallel to the boundary in the central portions of the shear zone (Hodg son, 1989). n the central portions of the shear zone, a second fabric is developed, which consists of spaced zones of mineral alignment and reduced grain size (mylonitization) paralleling the shear zone boundary (Figure 2b). These spaced zones are small slip sur- (1) This project is supported by a Univers ity-ndustry grant from NSERC and Cameco. (2) Department of Geological Sciences, University ol Saskatchewan, Saskatoon. Saskatchewan, S7N OWO. (3) Cameco Corporation, 2121 11th St. W., Saskatoon, Saskatchewan, S7M 1J3. Saskatchewan Geological Survey 153

faces termed C planes (Hodgson, 1989). 2. Vein Paragenesis Four stages of veining have been identified on the basis of mineralogy and textural relationships, observed in hand specimen and thin-section. These are: stage, microcline-quartz-biotite and albite-quartz veins (Figure 2a); stage, quartz-biotite veins (Figure 2b); stage ll, quartz-muscovite-pyrite±gold veins (Figure 2c); and stage V, quartz-muscovite-gold-chalcopyrite-sphaleritegalena±albite±calcite±chlorite±pyrrhotite±pyrite± bismuthinite, and ±native bismuth veins (Figure 2d). The mineral paragenesis of the veins is schematically illustrated in Figure 3. a) Stage Stage veins are not restricted to the Bakos shear zone or related structures in that they occur along brittle fractures or as pods throughout the granitic and granodioritic phases of the pluton and are crosscut and deformed by the Bakos shear zone and related structures. These relations imply that stage veins were not precipitated as a result of fluid flow through the shear zone and predate shearing. Stage veins occurring within th e shear zone are brittly fractured with fractures infilled with stage, ll, and V vein material. Stage veins range in size from 0.2 m to 1 m in width and are characterized by microcline, albite, quartz, and biotite and are devoid of gold. Veins in the granodiorite phase of the pluton consist mainly of albite, quartz, and biotite; whereas, in the granitic phase of the pluton, microcline is the dominant feldspar. The earliest vein filling mineral is feldspar which comprises from 25 to 100 percent of the vein. n hand specimen, the microcline is brecciated with quartz and biotite filling the brittle fractures. Quartz is typically transparent to translucent. n thin section, quartz forms a mosaic of interlocking grains showing variable degrees of strain in the form of undulose extinction, serrated grain boundaries, and quartz subgrains. Adjacent to cross-cutting stage V veins, stage vein quartz is dynamically recrystallized forming a mosaic of 0.01 to 0.05 mm polygonal quartz grains. Biotite laths occur along brecciated microcline grain boundaries and fractures, and are variably deformed. n addition to pre-,,,.,.- A~,i.' : - -.,..... ).... -.... T- Figure 2 - a) Photograph of typical stage veins showing microcline-quartz biotite; b) s_tage, veins e'!'p_laced parallel to C plane; c) stage ll, quartz-muscovite-pyrite±.gold veins; and d) photomicrograp~ of stage V m1cro~emlet cons,stmg of _dyna_m,cally ~ecrystal fized quartz, muscovite, and gold (sample 89-135-43.5). Base of photo,s 2 mm. Abbrev1allons: Qtz:::quartz; M1c=m1croclme, B1o=b1 otite; Py==pyrite; Musc=muscovite; and Au=gold. 154 Summary of nvestigations 1993

K-frldspar Biotite _\lusrn" ilc Allli1c l'yrrh(ltilt' C'halc 1p_~ rltc Cjal ena G uld Sphalcrik u,~orile Hlsmuth Bi, mulhinile MA:"/ ZONE (~ranile hosted) BK., ZO~E (granodiorile hosted) cent of infilling material, and muscovite and pyrite the remaining up... Stag~ C rc,q Mallit.cdl Sla~,e ll V tl t~s '!... ~ Rcla. livc Timi.: Sta~, V ve1m.ate S1.1gc vein~ - [.:a1rly Sla~e 11 Stai,:c JJJ S t:1ge JV vt ns Rd al in Timl' '! ---! Figure 3 - Mineral paragenesis of veins from the main (granite-hosted) and BK3 (granodiorite-hosted) zones. dating shearing, the relatively high-temperature mineral assemblage of stage veins implies that they are the product of fluids separated from the latest phase of the pluton during cooling. b) Stage Stage veins are restricted to the Bakos shear zone and related structures and are emplaced parallel to, and at oblique angles to, the C plane of the shear zone (Figure 2b). Stage veins, therefore, precipitated from hydrothermal fluids infiltrating through the shear zone. These veins range from 0.2 to 1.0 m in width and consist of quartz and biotite, and are generally devoid of gold mineralization. Quartz is transparent to translucent and comprises between 90 and 100 percent of the vein filling material. Biotite forms from 1 to 10 percent and occurs along vein selvages and rarely in the central portions of the vein. n thin section, interlocking grains of quartz exhibit undulose extinction, serrated grain boundaries and subgrain formation. Biotite exhibits undulose extinction and kinkbanding in proximity to crosscutting stage ll veins, and quartz is dynamically recrystallized in proximity to later cross-cutting stage V veins. c) Stage ll Stage ll veins are also restricted to the Bakos shear zone and related structures and are the products of infiltrating hydrothermal fluids focussed through the shear zone. They occur at low and high angles, and parallel to the C planes, locally truncate one another (Figure 2c), and overprint stage veins. Veins range from 2 mm to 1 m in width and consist of quartz, muscovite, and pyrite±gold. Quartz is translucent with minor transparent and milky masses. Pyrite and muscovite occur along vein selvages and only locally occur in the central portions of the veins along the edges of rafted wall-rock fragments. Quartz comprises between 80 and 100 per- - l..1tt to 20 percent of the vein, but typically about 5 percent. n thin section, quartz shows variable degrees of deformation in the form of undulose extinction, serrated grain boundaries, and pervasive subgrain formation. Dynamically recrystallized quartz, forming irregular mosaics of fine (0.01 to 0.5 mm) polygonal grains, occurs along contacts with stage V veins. Subgrain formation and dynamic recrystallization of quartz is more pronounced in stage ll veins emplaced parallel to C planes. Abundant healed microfractures, outlined by fluid inclusions, and brittly fractured quartz grains indicate episodes of brittle fracturing. The healed fractures crosscut deformed quartz grain boundaries, and in places dynamically recrystallize the quartz grains. Brittle fracturing of quartz, therefore, postdated ductile deformation, which was accompanied by recrystallization and recovery of quartz. Muscovite is present as large (-500 µm) deformed laths intergrown with pyrite, proximal to vein margins. Muscovite shows signs of both brittle and ductile deformation in the form of fracturing, undulose extinction, and kink banding. Pyrite is typically present along vein selvages as large anhedral to euhedral grains that were later fractured by brittle deformation. A few gold grains are found completely enclosed by pyrite. This stage of veining accounts for the majority of the pyrite and only a small fraction of the gold in the Contact Lake gold deposit. Stage ll veins are mainly restricted to the granitic portion of the deposit. d) Stage V Stage V veins occur along late brittle micro-fractures (Figure 2d) crosscutting stage,, and ll vein minerals. The veins are typically <1 mm wide making identification in hand specimen difficult. n thin section, dynamically recrystallized quartz grains surrounded by calcite, chlorite, and muscovite, and intergrown, undeformed laths of muscovite surrounding gold, indicate that stage V mineralization and dynamic recrystallization of quartz were virtually contemporaneous, with mineralization occurring slightly after recrystallization of quartz. Stage V vein muscovite is fine grained (>200 µm), in marked contrast to the relatively large grain size of muscovite in stage ll veins. t is also higher in FeO. Gold in stage V veins is generally located where the micro-fractures have encountered or crosscut previously deposited minerals such as feldspars, pyrite, epidote and biotite, and rafted wall-rock fragments or have propagated into the wall rock. Small anhedral to euhedral grains of sphalerite (typically exhibiting chalcopyrite Saskatchewan Geological Survey 155

disease), chalcopyrite, and galena are always associated with gold. Pyrrhotite occurs with gold along deformed quartz grain boundaries, and as overgrowths around and in fractures within pyrite deposited during stage ll veining. Pyrite deposited during stage V veining occurs as skeletal overgrowths around stage ll pyrite, and as small anhedral blebs with chalcopyrite and gold. Native bismuth and bismuthinite are minor phases in pyrrhotite and were only observed with the scanning electron microprobe. The mineral assemblage associated with gold is dependent on the environment in which the auriferous stage V veins were emplaced. n the granodioritic phase of the deposit, interaction between the mineralizing fluid and biotite, epidote, and pyrite resulted in the precipitation of quartz, muscovite, albite, calcite, chlorite, pyrrhotite, sphalerite, chalcopyrite, galena, and gold (Figure 3). n contrast, in the granitic portion of the deposit, the mineralizing fluid interacted with potassium feldspar and pyrite, precipitating quartz, muscovite, chalcopyrite, sphalerite, galena, pyrite, and gold, but no chlorite, calcite or pyrrhotite (Figure 3). 3. Alteration A petrographic study of the central granitic and granodioritic phases of the pluton and the Bakos shear zone revealed two styles of alteration. The first is pervasive and affects the entire Little Deer lake pluton, whereas the second is restricted to the shear zone (Figure 4). Pervasive alteration is marked by sericitization of potassium feldspar, epidote formation in the cores of plagioclase, and the breakdown of hornblende to green biotite and epidote. Primary igneous textures are generally preserved. The alteration is interpreted to be deuteric in origin. n the shear zone, alteration has resulted in distinct mineral assemblages that can be related to the two macroscopic veining events (Stages and ll) (Figure 4). An alteration envelope consisting of biotite and epidote (and excluding hornblende) is associated with stage veins. Biotite is present as large green to brown laths, forming rosettes up to 1 mm in diameter, accompanied by euhedral epidote. n proximity to stage ltl and V veins, this biotite exhibits undulose extinction, kink banding, and is chloritized, and the epidote is highly fractured and corroded. Closer to Stage ll veins, alteration is marked by sericitization and pyritization and replacement of biotite and epidote. Ribbons of sericite occur along C planes paralleling vein selvages. Pyrite is in textural equilibrium with sericite occurring as small cubes in wall rock. Low gold grades ranging from 0.7 to 1.7 g/tonne (Chapman et al., 1990) characterize this alteration. n the upper 175 m of the deposit, late pervasive hematization occurs in weakly to moderately fractured zones where there is an abundance of pyrite. The hematite overprints sericite alteration associated with stage ll and stage V veins and is interpreted to represent the infiltration of meteoric fluids along macro and micro-frac- Figure 4 - Schematic plan view of the Bakos shear zone showing the various stages of veining and the alteration associated with each vein type. tures, oxidizing pyrite and occasionally oxidizing chalcopyrite to form bornite. 4. Stable sotope Geochemistry a) Oxygen and Hydrogen Quartz samples from the barren stage and veins have similar 01ao values ranging from 10.3%0 to 10.8%0, whereas stage ll and stage V vein quartz samples tend to be more 1BQ-rich, with o1bq values ranging from 11.4%0 to 13.5%0 (Table 1 ). Stage ll veins, that consist mainly of dynamically recrystallized quartz (i.e. quartz related to the auriferous stage V veins), have the highest 151ao values (11.8%0 to 13.5%0), whereas stage ll veins, that consist mainly of large deformed quartz grains and are devoid of dynamically recrystallized quartz, have lower 151ao values (11.4%0 to 12.0%0). The isotopic composition of the quartz from the hydrothermal veins suggests that there are three distinct fluids, one high-temperature fluid that precipitated barren quartz-biotite veins (stage veins) with o1bq quartz values of 1 0.5%o, a second lower temperature fluid that precipitated low-grade quartz-muscovite±gold veins (stage ll veins) with quartz 81BQ values of -12.0%0, and a third, low-temperature fluid that accompanied the main stage of gold deposition (i.e. stage V veins), that 156 Summa,y of nvestigations 1993

Table 1 Summary of stable isotope data. Sample 6 18 0 qtz 0 18 0 min. 60 min. Pluton 9.4 biotite 3.3-73 feldspar 9.4 whole rock 8.1 T equil. ( C) 470 ±25 Veins: Stage 1 10.5 10.8 biotite 6.5 515 ±25 Stage 2 10.3 10.8 biotite 103 to 117 505 ±25 4.3 to 6.3 muscovite 8.5 to 9.3 Stage 3 11.4 12.0-46 to SB 325 ±25 Stage 4 11.8 13.5 muscovite B.B to 9.0 66 to -88 290 ±25 exchanged oxygen with stage and ll quartz during the recrystallization process, resulting in dynamically recrystallized quartz with o1 BQ values of 11.8%0 to 13.5%0. C~arse grai~ed (-500 µm) muscovite from stage ll veins, associated muscovite produced as a result of wall-rock alteration, and fine-grained muscovite from stage V veins have similar 818Q values between 8.5%0 a_nd 9.0%0. The hydrogen isotopic composition of musco v1te from stage ll veins, however, is distinct in compari son to the hydrogen isotopic composition of muscovite from stage V veins. The 80 values of stage ll vein muscovite and associated alteration muscovite range from 58'roo to -46%0 whereas the od values of stage V muscovite range from -88%0 to -66%0. The oxygen and sulphur isotopic composition of co-existing mineral pairs can be used to determine the tem ~erature at which the minerals formed, and by implication, the temperature of the fluid from which they precipitated. Stage ll and stage V quartz-muscovite pairs give oxygen isotope equilibration temperatures of 290 to 325 C (Table 1 ). Sulphur isotope equilibration temperatures calculated using stage V sulphide mineral pairs associated with gold give similar temperatures (Table 2). Q~artz-biotite mineral pairs from barren stage ~nd veins have much higher oxygen isotope equilibration temperatures up to 515 C, which are similar to quartz-biotite temperatures from the pluton (Table 1). Table 2 : 6 34 S values of sulphides from the Contact Lake gold deposit. Sample 6348 (534$ 6345 6345 6348 Toqud (Py) (Ccp) (Sph) (Gal) (Po) (OC) Main zone: 89-69-247 6.4 5.9 6.2 4.0 285 89-91-166 6.2 5.6 89-119 6.1 BK3 zone: 90.150 3.0 5.2 330 92-167-180.5 5.3 4.6 4.4 92-160-246.5 4.2 4.4 92-172-153 4.4 4.1 Abbreviations: Py=pyrite, Ccp=chalcopyrlte, Sph=sphalerite, Gal=galena, and Po=pyrrhotite. Fractionation factors used are cited in Kyser (1987) and references therein. b) Sulphur Stage l~ vein pyrite and stage V vein chalcopyrite, sphalente, galena, and pyrrhotite were analyzed for their sulphur isotopic composition. The 834S values of all sulphides throughout the deposit vary from 3.0%0 to 6.4%0 (Table 2). The narrow range of S34S implies both a constant source and similar reducing conditions. However,. sulphides from the granodioritic phase of the pluton (1.e. BK3 zone), where pyrrhotite is the dominant sulphide phase, tend to be isotopically lighter in compari ~on to ~ulphides from the granitic phase of the pluton (1.e. main zone), where pyrite is the dominant sulphide ~hase. This difference in isotopic composition most likely resulted from slight variations in the redox conditions encountered during transport and deposition of the sulphides, which is also reflected in the mineralogy of the sulphides. 5. Radiogenic sotope Systematics a) Rb-Sr sotope Systematics The crystallization age of the Little Deer Lake pluton de!ermined by the single-zircon Pb-evaporation technique, 1s 1837 ±5 Ma (Kyser et al., 1992). A Rb-Sr isochron made up of a granitic whole rock sample from the pluton, pluton biotite, and feldspar from the pluton gives an age of 1719 ±300 Ma, with a B7Sr/86Sr initial ratio of 0.7050. The large error can be attributed to differential deuteric alteration of the feldspar. A Rb-Sr isochron made up of whole rock-biotite and biotite-feldspar give ages of 1720 ±8 Ma (Table 3) and 1719 ±8 Ma, respectively, with 87Sr/B6Sr initial ratios of 0.7044 and 0.7059, respectively.!hese ages of ca. 1720 Ma presumably reflect the last time the temperatures were in excess of ca. 300 C, the closure temperature for Rb-Sr systematics in biotite (Giletti, 1991 ). Using the zircon age of the pluton a~d assuming the whole rock remained relatively closed with respect to Rb-Sr, an initial 87Sr/86Sr ratio of 0.7040 is calculated. There are no Rb-poor minerals associated with the stage ll quartz-muscovite-pyrite veins, therefore an initial ratio must be assumed. The high B7Rb/86Sr ratio of muscovite means that the choice of an initial 87Sr/B6Sr ratio does not significantly affect the calculated model age. As an example, model ages for stage ll vein muscovite, using initial 87Sr/86Sr ratios of 0.7020, 0.7040, and 0.7050, give model ages of 1774 ±9 Ma, 1770 ±9, and 1767 ±9 Ma, respectively (Table 3). That the muscovite in stage ll veins was not affected by the event that reset the biotite in the pluton can be attributed to much higher closure temperatures of ca. S00 C for Rb Sr isotope systematics in muscovite (Giletti, 1991 ). ~b-sr isotope systematics in sphalerite and chalcopynt~ from sta~e V veins in conjunction with stage V vein muscovite, all of which are cogenetic with gold, have a Rb-Sr isochron age of 1699 ±8 Ma with an initial B 7 Sr/86Sr ratio of 0.7097 (Table 3). Model ages for the muscovite alone using initial B7Sr/86Sr ratios of 0.7020, 0.7040, and 0.7050, are 1730 ±9 Ma. 1724 ±9 Ma, and 1718 ±9 Ma, respectively. Although the Rb-Sr systemat- Saskatchewan Geological Survey 157

Table 3 Rb-Sr isotope data for the Little Deer Lake pluton and for stage ll and V vein minerals. Sample No. Mineral Rb (ppm) Sr (ppm) 87 Rbf86Sr B 7 Srf86Sr nitial 87 Srf86Sr Age (Ma) Pluton separates: 89 89 5 feldspar 32.79 805.03 0.10182 0.70844 ±43 89-89 5 whole rock 48.80 601.8 0.22057 0.70986 ±23 0.7044* 1720 ±8* 89 89-5 biotite 543.1 17.49 106.38 3.3351 ±36 Stage ll vein minerals: 89 91-166 pyrite 4.647 1.426 9.6836 0.94478 ±222 89 91-166 muscovite 239.9 26.24 28.353 1.4252 ±14 0.7020 1774 ±9** 0.7035** 1770 ±9.. 0.7050** 1767 ±9** Stage V vein minerals: 89-69-247 sphalerite 0.09667 0.6588 0.54581 0.72321 ±43 0.7097*** 1699 ta 89-69-247 chalcopyrite 0.4872 6.012 0.24148 0.71551 ±29 89-69-247 muscovite 235.9 39.97 16.702 1.1175 ±44 0.7020.. 1730 ±9** 0.7035** 1724 ±9** 0.7050** 1718 ±9** 90 150-96 galena 1.812 3.263 1.6408 0.73323 ±220 89-91-166 sphalerite 0.009931 0.2045 0.26115 0.71575 ±36. Ages and corresponding initial ratios derived from mineral isochrons using biotite and whole rock sample only. Model ages for muscovite and their corresponding initial ratio. Ages and corresponding ini1ial ratios derived from mineral isochrons using sphalerite, chalcopyrite, and muscovite. ics in sulphide minerals may have been partially affected by later events, the Rb-Sr isochron ages of minerals paragenetically associated with gold are in general agreement with 40Arf39Ar plateau ages for muscovite (see below). b) 40 Art3 9 Ar sotope Systematics Stage V vein muscovite, stage ll vein muscovite, wallrock alteration muscovite related to stage ll veins, and pluton biotite were dated by the 4 0Arf39Ar technique. Stage V vein muscovite, stage ll vein muscovite, and wall-rock alteration muscovite related to stage ll veins give plateau ages of 1717 ±7 Ma, 1721 ±5 Ma, and 1717 ±3 Ma (Figure 5), respectively. Their spectra show that almost all the gas released gives the same age, within error, and therefore the muscovite has either been undisturbed or completely reset with respect to the 40Arf39Ar system. Biotite from the Little Deer Lake pluton yields a disturbed spectrum with low ages at the low and mid temperatures of gas release (Figure 5). This suggests that the biotite consists of two Ar reservoirs and partial Ar loss occurred from both reservoirs (York and Lopez-Martinez, 1986; Hanes, 1991). The 4DAr/39Ar age of the biotite is interpreted to be 1749 ±4 Ma. c) The Timing of the Gold Mineralization at the Contact Lake Deposit Biotite closure temperatures for Rb-Sr and Ar-Ar are 350 ±40 C (Giletti, 1991) and 300 ±50 C (Harrison et al., 1985), respectively. A Rb-Sr isochron using biotite and a granitic sample from the pluton gives an age of 1720 ±8 Ma, with an initial 87Srf86Sr ratio of 0.7044, which is younger than the Ar-Ar age of 1749 ±4 Ma for biotite from the pluton (Figure 6). The closure temperature of muscovite with respect to Rb-Sr is about 500 C (Giletti, 1991) whereas muscovite closure temperatures for Ar-Ar are only 350 ±50 C (Hanes, 1991 ). Rb-Sr model ages for stage ll vein muscovite range from 1774 to 1767 Ma and are older than the Ar-Ar plateau age of 1717 ±3 Ma (Figure 6). However, Rb-Sr model ages and Ar-Ar plateau ages for stage V vein muscovite, which is cogenetic with gold, are in agreement at ca. 1720 Ma, indicating that the main phase of gold deposition occurred at about 1720 Ma (Figure 6). Thermal perturbations associated with the introduction of the auriferous stage V vein fluids at 1720 Ma may have partially reset the Rb-Sr isotope systematics of minerals in the pluton, and the Ar-Ar systematics of biotite in the pluton. The Ar-Ar systematics of muscovite associated with stage ll veins were also reset to near 1720 Ma, but their Rb-Sr systematics were not reset because the auriferous stage V vein fluids had temperatures of 300 ±50 C, which are near the closure temperatures for Ar Ar but well below the Rb-Sr closure temperature for muscovite. 6. Summary The Contact Lake gold deposit has had a complex fluid history. Four stages of veining have been identified on the basis of mineralogy, textural, and cross-cutting relationships. High-grade gold mineralization is associated with the latest stage of veins (stage V veins). These veins are microveinlets consisting of dynamically recrystallized quartz, muscovite, chlorite, calcite, chalcopyrite, sphalerite, galena, ±pyrrhotite, ±pyrite, gold, ±bismuthinite, and ±native bismuth. Gold is always associated with one or more of these minerals; however, the mineral assemblage associated with gold is strongly dependent on the host rock in which the stage V veins are emplaced. n the granodioritic phase of the deposit, interaction between the mineralizing fluid and biotite, epidote, and pyrite resulted in the precipitation of 158 Summary of nvestigations 1993

a) h) 1900 1900 Little Deer Lake Pluton Biotite,.-.._,.-.._ ~ ~ :; 1700 :; '-' '-' 1700 -~ ~ Q,j Q,j - Alteration Muscovite Related to stage H veins ell ell < < - 1749!4 Ma 171~3 Ma 1500 1500 c) d) Muscovite from stage veins 2000 1900-1900,.-.._,.-.._ ~ ~ - :; 1800 :; '-' '-' Q,j Q,j - bi) bi) < 1700-1700 Muscovite from stage V veins < - 1600 1721!5 Ma 1717:7 Ma 1500 1500 Figure 5-40 Ar/ 39 Ar plateau spectra of pluton and vein minerals. a) biotite from the pluton, b) alteration muscovite related to stage ll veins, c) large stage ll vein muscovite, and d) stage V vein muscovite. quartz, muscovite, calcite, chlorite, pyrrhotite, sphalerite, chalcopyrite, galena, and gold. n the granitic portion of the deposit, the mineralizing fluid interacted with potassium feldspar and pyrite, precipitating quartz, musco vite, albite, chalcopyrite, sphalerite, galena, pyrite, and gold, but no chlorite, calcite or pyrrhotite. Dynamically recrystallized quartz associated with stage V veins has the highest 61BQ values (11.8%0 to 13.5'Yoo) and is interpreted to have exchanged oxygen with fluids responsible for gold deposition. Mildly auriferous stage ll veins have 61BQ quartz values of 11.4 to 12.0'Yoo whereas barren stage and vein quartz has o1bq values of 10.3 to 10.8%0. Stable isotope equilibration temperatures derived from minerals paragenetically related to the gold indicate that stage V veins were deposited at temperatures of 290 C, whereas the barren stage and veins were deposited at temperatures up to 515 C. Stage V vein muscovite and stage ll vein muscovite give 40Arf39Ar plateau ages of 1717 ±7 Ma and 1721 ±5 Ma, respectively, whereas the Rb-Sr model age for stage ll vein muscovite is 1770 ±9 Ma and the model age for stage V vein muscovite is 1724 ±9 Ma. The event responsible for the production of the auriferous stage V veins has reset the 40Arf39Ar systematics of stage ll vein muscovite. The Rb-Sr systematics of stage ll vein muscovite, however, were undisturbed because the Rb Sr closure temperature of muscovite was not exceeded. 7. Acknowledgments We would like to thank Frank Hrdy for his in help in sample collecting and his input during the initial stages of this project. 8. References Bickford, M.E. and Van Schrnus, W.R. (1985): Preliminary U Pb age data for the Trans-Hudson Orogen in northern Saskatchewan: New and revised results; in Summary of nvestigation 1985, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 85-4, p63-66. Chapman, R., Curry, G., and Sopuck, V. (1990): The Bakos Deposit discovery-a case history; in Beck, L.S. and Harper, C.T. (eds.), Modern Exploration Techniques, Sask. Geol. Soc., Spec. Publ. 10, p195 212. Giletti, B.J. (1991): Rb-Sr diffusion in alkali feldspars, with implications for cooling histories of rocks; Geochim. Cosmochim. Acta, v55, p1331-1 343. Saskatchewan Geological Survey 159

1650 1600 ittle Deer on evap) vite from stage veins (Ar-Ar) 1850 1800 1750 1700 1650 1600 Time (Ma) Figure 6 - Summary of the geochronology from the Contact Lake area. Regional geochronological data are from Bickford and Van Schmus (1985) and Kyser et al. (1992). Shaded areas are three proposed thermal events. The earliest event is perceived as aperiod of plutonism and is followed by regional metamorphism and two later, less intense thermal events, one resulting in the precipitation of stage ll veins, and the latest event being the high-grade gold producing event (stage JV vein emplacement). Abbreviations: (Rb-Sr)=Rb-Sr model age; (Ar-Ar)=Ar-Ar plateau age; (Rb-Sr-bio-wr)=Rb-Sr biotite-whole rock lsochron age; (U-Pb zircons)=u-pb dating of zircons by conventional means; and (Pb-Pb zircon evap)=single-zircon Pb-evaporation dating. Hanes. T.A. (1991): K Ar and 40 Ari3 9 Ar geochronology: Methods and applications; in Heaman, L.H. and Ludden, J.N. (eds.), Short course handbook on applications of radiogenic isotope systems to problems in geology, Miner. Assoc. Can., v19, p27-57. Harrison, T.M., Duncan,., and McDougall,. (1985): Diffusion of 40 Ar in biotite: Temperature, pressure and compressional effects; Geochim. Cosmochim. Acta., v49, p2461 2468. Hodgson, C.J. (1989): The structure of shear-related, vein-type gold deposits: A review; Ore Geology Reviews, v4, p231 273. Kyser, T.K. (1987) : Equilibrium fractionation factors for stable isotopes; in Kyser, T.K. (ed.), Short course in stable isotope geochemistry of low temperature fluids, Miner. Assoc. Can., v13, p1-84. Kyser, T.K., Fayek, M., and Sibbald, T.1.1. (1992): Geochronologic studies in the La Ronge and Glennie Domains; in Summary of nvestigations 1992, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 92-4., p130-134. Ramsey, J.G., (1980): Shear zone geometry: A review; J. Struc. Geol., v2, p83-99. York, D., and Lopez-Martinez, H. (1986): The two-faced mica; Geophys. Research Lett., v1243, p973-975. 160 Summary of nvestigations 1993