Alteration Geochemistry of the Rio Zone; Bootleg Lake Gold Mine, Creighton Area

Edward C. Appleyarc?

Appleyard, E.C. (1992): Alteration geochemistry of the Rio zone, Bootleg Lake Gold Mine, Creighton area; in Summary of Inves­tigations 1992, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 92-4.

Gold mineralization was discovered in 1936 in a zone of strongly carbonatized, altered basalt, volcanic heterolithic breccia, granodiorite, and granite confined to the Rio fault zone about 6 km southwest of Creighton, Saskatchewan. No development took place on the property until the early 1980s when Flin Flon Mines Ltd. developed a ramp to the ore body and built a 125-ton per day mill {Coombe Geoconsultants Ltd.,

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f

1984). Production began in 1984 but, by year end, the operation had shut down.

Between 1986 and 1989, Vista Mines Inc. undertook fur­ther exploration and development of the property and processed a 10,000 ton bulk sample. The grade of the sample averaged 0.12 oz/ton gold, which was too low to warrant production. The property has been variously known as the Reo Mine, the Rio Mine, and latterly as

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the Bootleg Lake Gold Mine. The property is currently registered to Vista Mines Inc.

1. General Geology

..•• ,,--· v ( The general geology around the deposit is illustrated in Figure 1.

[: :.:, :J SILICA- CARBONATE ALTERATION

~ RIO SHEAR ZONE

~ PHANTOM LAKE GRANITE

0 50

[.',\I BOOT LAKE PLUTON

1~1 BASALTIC DEBRIS FLOWS

' x

Iv'/ v I MASSIVE TO AMYGDALOIDAL BASALT

100 m

Figure 1 - Generalized geological map of the Rio alteration zone (modified from Pear­son, 1984b}. Analyzed sample locations are marked by crosses.

Bedrock supracrustals consist of basaltic flows and debris flows with minor intercalated layers of inter­mediate to acid fragmentals belong­ing to the Beaver Road As­semblage (basalts) and the Douglas Lake Assemblage (heterolithologic tuff breccias) of the Amisk Group volcanics {Thomas, 1989a, 1989b, 1990a, 1990b). These units are disposed in a westerly-facing homoclinal se­quence on the southwestern limb of a larger structure known as the Beaver Road Anticline {Byers et af., 1965). Amisk-age rocks have been radiometrically dated at ca. 1886 Ma {U-Pb zircon, Syme et af., 1987).

These rocks are intruded by mem­bers of the multiphase Boot Lake­Phantom Lake Intrusion, specifical­ly, at this site, by Phase 3, plagioclase-porphyritic granodiorite, and Phase 4, microcline-porphyritic granite; the latter unit is commonly called the Phantom Lake granite. Boot Lake granodiorite has been dated as

(1 ) Saskatchewan Pro1ect A. 143 of 1991-92 was funded under the Canada- Saskatchewan Partnership Agreement on Mineral Development 1990· 95.

(2) Department of Earth Sciences, University of Waterloo, Waterloo, Ontario, N2L 3G1.

138 Summa,y of Investigations 1992

1842 ± 13 Ma (Pb-Pb zircon) by Ansdell and Kyser (1991) and as 1841 ±3 Ma (U-Pb zircon) by Heaman et al. (1991). The Phantom Lake granite intrusion is dated as 1840 ±7 Ma (weighted average of 3 Pb-Pb zircon determinations) and a dyke of the intrusion as 1834 ± 13 Ma (minimum age of 4 Pb-Pb zircon determina­tions) by Ansdell and Kyser (1991).

Granodiorite occupies much of the ground on the southeast side of the Rio fault with the exception of an inlier of massive and pillowed basaltic flows (Figure 1). A dyke of Phantom Lake granite intrudes the footwall (southeast side) volcanics and is found underground in the ore zone (Ansdell and Kyser, 1990). Similar granitic rocks north and northwest of the mill are assigned by Thomas (1990b) to a different, but co-magmatic, dyke.

The Rio fault trends northeasterly across the property and dips steeply (75°) to the west (Byers et al., 1965; Pearson, 1984a). There is a marked change in strike of the fault zone within the zone of alteration from 040° in the southwest to 025° in the northeast. Over most of its length, the fault consists of a narrow 5 m wide deforma­tion zone showing little evidence of alteration. On the Rio property, however, the fault zone widens to a broad belt (90 m) of fracturing accompanied by strong car­bonatization and the development of a siliceous (cherty) facies.

Within this zone of alteration the ore is of two types: (1) very fine-grained gold-bearing albite-pyrite-quartz­ankerite rock with an overall banded aspect and (2) quartz-ankerite-pyrite-chlorite-muscovite veins (Ansdell and Kyser, 1990; Pearson, 1984a). The former ore type is most evident on the hanging wall (northwest) side of the alteration zone where the banding trends ca. 025°, i.e. parallel to the fault, while the latter ore type is more evident on the footwall (southeast) side and trends sub­parallel to a set of fractures at 350° to 355° (Pearson, 1984a; Middleton, 1985). Middleton (1985) identifies the 025° to 040° surfaces as 82 structures and the 350° to 355° surfaces as S3. The quartz veins cross-cut S2 and show much less evidence of deformation than their host rocks.

Most of the alteration zone occurs in basaltic rocks but, to the northeast, alteration extends through Boot Lake granodiorite. The Phantom Lake granite dyke has also been deformed and strongly altered. Significant move­ment and alteration must, therefore, have followed the emplacement of the Boot Lake-Phantom Lake pluton.

Pearson (1984a) and Middleton (1985) explain the localization of alteration to this section of the fault as a consequence of stress reorientation, either due to dif­ferent rheological properties of the footwall basalt and granodiorite respectively, or to the change-of-strike of the fault. The development of two sets of fractures (025° and 350° to 355° respectively) in this locality is sup­posed to have resulted in extreme dilatancy providing a channel for hydrothermal fluids.

Saskatchewan Geological SuNey

2. Previous Alteration Studies

The studies by Pearson (1984a) and Middleton (1985) contain field and petrographic descriptions of the al­tered rocks and their relationship to structural features within the alteration zone and host rocks.

Mellinger and Pearson (1987) undertook an extensive sampling program of both surface and underground ex­posures and analyzed the resultant lithogeochemical data using statistical correspondence analysis proce­dures. They concluded that Au in the Rio mine rocks is statistically associated with Fe, which they correlate with the development of pyrite in the rocks, and less strongly with Pb, W, and Ag. On the other hand, Au is not as­sociated with K nor with Cu, but the latter elements are mutually associated and thus represent mineralization which is distinct and separate from the Au event.

Galley and Franklin (1987) reported on a study of mineralization associated with the Boot Lake-Phantom Lake Intrusive Complex. Porphyry-type mineralization was found to be linked to the high-level emplacement of the Phantom Lake granite and was marked by the elemental association of Au-W-Cu-Mo. Three types of al­teration were recognized all of which have a carbonate component. Their type-3 alteration comprises deformed and planar quartz-ankerite-sulphide veins within, or close to, shear and fracture zones displaying variably developed carbonate alteration and associated dissemi­nated sulphides. They identify the Rio zone as an ex­ample of this type of mineralization. This hypothesis would derive the mineralizing fluids from the Phantom Lake granite in a high level crustal setting. An ancillary part of the model identifies the northeast- and northwest­trending fracture-shear systems as a strike-slip con­jugate system and places mineralization events where maximum dilation occurs.

Ansdell and Kyser (1990) undertook a fluid inclusion and O and H isotopic study of samples of the vein-type mineralization in the Rio zone and concluded that the fluids were of low salinity ( <4 wt.% NaCl equiv.) with highly variable H20-C02 ratios (15 to 100 vol.%). The stable isotope compositions of the fluids indicated that they were derived by regional metamorphic dehydration and decarbonation reactions and that mineralization oc­curred at 300° to 400°C and at depths of 6 to 10 km (2 to 3 kbar pressure). They proposed that Au was probab­ly leached from a variety of host rocks and then focussed into permeable structural zones, such as the Rio fault. The Rio zone is categorized as a mesothermal­type, similar to many structurally-hosted Archean gold deposits.

3. Objectives of Present Study

This study is part of several investigations of gold deposits in the La Range and Flin Flan domains designed to characterize and quantify elemental fluxes during alteration events associated with mineralization. The mandate includes the directive that criteria will be developed, where possible, that will enable ex­plorationists to recognize 'prospective' alteration effects.

139

4. Study Plan Large hand specimens were collected from the Rio al­teration zone and from the various rocks that host it beyond the area of visible alteration effects. Twenty-four samples were selected for analysis of 54 components, including Fe203 and FeO. Samples were collected along four sections (see Figure 1 ): section 1, B samples of basalt-hosted alteration along a northerly transect of the southwest end of the alteration zone; section 2, 7 samples of basalt-hosted alteration from the footwall to hanging wall on a northwesterly transect of the central part of the alteration zone; section 3, 6 samples of altera­tion of granodiorite on a westerly-trending transect across the northeast part of the alteration zone; and sec­tion 4, 3 samples of Phantom Lake granite dyke. Analyses were carried out by Activation Laboratories Ltd., Ancaster, Ontario, using a variety of analytical methods. Densities were measured on analyzed samples using a Beckman Model 930 Air Comparison Pycnometer.

5. Mass Balance Corrections Lithogeochemical data were corrected for mass chan­ges that occurred during alteration using the Gresens (1967) methodology and the procedures outlined by Ap­pleyard (1990). Reference samples showing no evidence of alteration and shearing were collected mar­ginal to the alteration zone along each of the 3 sample transects.

Mass change calculations presume that the immobile elements may be identified by mutual congruence of their Fv0 n factors (volume change factor for zero elemen­tal change, see Appleyard, 1990). The elements found to be immobile in the Rio zone were as follows (frequen­cy in parentheses): Ti (19) , Al (17), Yb (16), IFe (15), Lu (15), Sm (15), Ce (14), Sc (13), Hf (12), Zr (11), La (7) , and V (6) .

Data are expressed as net compositions, i.e. the ab­solute amount in grams of each element remainin~ in the sample after 100 cm3 (major elements) or 1 m (trace elements) was altered. The data are illustrated in two forms: 1) as mean compositions of least altered and altered facies of the basalts, granodiorite, and granite (Table 1), and 2) as variation diagrams versus distance for mass change systematics of 11 elements along sec­tions 1 and 2 of the altered basalt sequence (Figure 2).

Table 1 illustrates that density values tend to fall and the volume occupied by the sample to increase as a conse­quence of alteration. Mass changes (the product of den­sity and volume change ratios) also tended to increase. For basalts the mean mass change is 1.04 (range 0.586 to 1.48, n = 13), for granodiorite the mean is 1.09 (range 0.951 to 1.39, n - 5), and for granite, it is 1.22 (range 0.986 to 1.45, n=2).

The evidence for both a positive net mass change and a positive net volume change during the alteration and mineralization processes is thus consistent with and sup-

140

ports models that invoke strain dilation along the Rio shear zone.

6. Elemental Mobilities

a) Amisk Basalts: Sections 1 and 2

Patterns of changes in mass produced during alteration are somewhat erratic across the alteration zone (Figure 2), but in general conform to a region of large mass in­creases toward the footwall (southeast) side of the zone and a parallel region of mass loss on the hanging wall (northwest) side. Rocks mapped as unaltered host basalts at the hanging wall end of section 2 contain geochemical evidence of mass increases attributable primarily to the introduction of Si, Fe, and O (but not C nor S). Rocks from the hanging wall end of section 1, also collected beyond the mapped limit of alteration, are still within the region of mass loss, correlating here with loss of Si, Al, and 0 . Geochemical evidence of sig­nificant mass changes and elemental fluxes thus extend tens of metres beyond the limits of megascopic altera­tion.

Elements that most strongly correlate with calculated mass changes include Ca, (Mg), Na, C, S, As, (Sr), (Nb), and Au. The Ag values remain relatively low and do not commonly correlate strongly with the relatively er· ratic behaviour of Au.

It is noteworthy that Lfe does not correlate well with C, Au, or S; fluctuations of IFe appear to be the result of local migration within the shear zone. Pyrite, which is paragenetically closely related to Au precipitation (Pear­son, 1984a; Middleton, 1985), thus appears to be a product of sulphidation of Fe previously present in the host rocks with only minor local redistribution.

In partial agreement with the statistical conclusions of Mellinger and Pearson (1987) , there is an inverse relationship between Cu and Au with large magnitude Cu-losses where carbonatization and/or Au mineraliza­tion has been intense, but a positive correlation between Cu and K does not appear to be significant in these data.

Several of the elements said to characterize the Phan­tom Lake Granite (Galley and Franklin, 1985), i.e. Cu, W, and Mo, as well as K and F, show either a poor positive or a negative correlation with the elements positively cor­related with Au and the intensity of alteration. Tungsten values in the least altered rocks were below the limit of detection (1 ppm), but elevated values (up to 76 ppm) were returned from altered samples.

b) Boot Lake Granodiorite: Section 3

The granodiorite, cut off on its northwest side by the Rio fault, is in contact with Amisk volcanics. The most northwesterly samples were thus still within the visible al­teration zone; unaltered granodiorite was obtained at the southeast end of Section 3.

Summary of Investigations 1992

Mass change ratios rise from southeast to northwest along the section, reaching a peak of 1.39, then, as in the two basalt-hosted sections, fall to slightly negative values (-5%) in the hanging wall region of the alteration zone. Mass changes in granodiorite correlate much more closely with C values than with Si, although, the mass losses on the northwest side of the zone are at­tributable to Si-loss in the region of the shear zone where strain intensity appears to be greatest. Elements that correlate with carbonatization and Au mineralization include Mg, Ca, Na, F, S, Sr, Y, (Nb), (Eu), and (Pb). Al­though mean Cu values (Table 1) decrease with car­bonatization, individual values rise in parallel with C on the footwall side of the zone, but this increase is offset by an order-of-magnitude drop in Cu on the hanging wall side. Sodium has a strong positive gradient across the entire sampled alteration zone from southeast to northwest and correlates more strongly with Au than with C, which declines on the northwest side of the zone. Arsenic has a relatively weak correlation with Au, but in this host, F has a distinctly positive correlation with C.

The most intensely carbonatized sample is very anomalousix.-enriched in a large group of elements in­cluding Ti, 2:Fe, Mn, Mg, Ca, F, S, Sc, V, Cr, Co, Cu, Zn, Ga, Y, Nd, Sm, Eu, Yb, and Lu. Some of these ele­ments accompany carbonatization of the basalts, but many of the otherwise immobile elements are enriched to highly anomalous degrees in this sample. The same sample is depleted in Na, Mo, Ag, La, Th, and U and has Au values that are somewhat elevated but not ex­tremely so. Mobility of some of the high field strength elements, such as Ti, Y, and Zr, in highly carbonatized mafic lavas in southeast Quebec has been suggested by Hynes (1980). In the Rio zone, only one sample shows this anomalous distribution of elements, but Zr was not among the elements, thus this case cannot be used to incontrovertibly demonstrate Hynes' hypothesis.

A number of elements are distributed along strongly declinin9...gradients from southeast to northwest. These include 2:Fe, K, Zn, Rb, Sb, and Ba. For the alkali ele­ments, the gradients may reflect progressive breakdown of primary feldspars in the granodiorite along a gradient of increasing strain within the shear zone accompanied by strong albitization.

c) Phantom Lake Granite

A small number of analyses illustrate alteration effects in the dyke of Phantom Lake granite that cuts the footwall basalts. Two analyses from the centre of the main pluton, about 3 km to the southeast, and one analysis of the Rio dyke taken from the literature, are included with the data in Table 1 (see caption).

Strongly altered granite is highly strained and injected with quartz veinlets. A maximum mass change of + 45 percent was calculated and is associated with strong increases in Si, 0, S, B, As, Mo, Au, and (Pb). Alkali elements, viz. K, Rb, and Ba, as well as Cu, in­crease from the least carbonatized sample to the inter­mediate sample, but decrease in the most intensely al­tered sample probably due to dilution by quartz.

Saskatchewan Geological Survey

Curiously, in the sample with the highest C values, Mg, Ca, and Sr all decline from higher values in the inter­mediate sample. Altered samples are lower in F than the least altered dyke rock and Fe oxidation ratios also decrease during alteration as in the other host rocks.

d) Rare Earth Elements

There has been considerable interest in the behaviour of rare earth elements in hydrothermal systems ever since it was suggested that their assumed immobility would make them ideal indicators of the primary geochemistry and even tectonic setting prior to altera­tion. It has also been suggested that light and heavy rare earth elements might behave differently according to the primary anionic character of the fluid, specifically that Cr bearing fluids might be more effective in mobiliz­ing light and intermediate rare earth elements and F and CQ3-2 bearing fluids more effective in mobilizing heavy rare earth elements (Taylor et al., 1981 ), although Wendlandt and Harrison (1979) had concluded that C02 rich vapour is responsible for transporting and con­centrating all the rare earth elements and especially the light rare earths in carbonatites, alkaline-rich melts, and kimberlites.

As noted above, a number of the intermediate and heavy rare earth elements appear among the immobile elements. The light rare earths, however, appear to be mobile more frequently than they are immobile and Eu displays a unique behaviour. These relationships are il­lustrated in Figure 3 in which the ratio of the three ele­ments L~. Eu, anq ~u versus the ~mount in the parent rock (La , Eu , Lu ) 1s plotted against the net (corrected) amount of carbon in each rock as an index of alteration intensity. The slopes of the regression lines for the three elements are La = -0.025, Eu = 0.050, and Lu = -0.007 illustrating that the heavy element Lu is essentially im­mobile, the light element La is lost during alteration, and Eu is gained. The introduction of Eu correlates with a decrease in the Fe oxidation ratio and is thus consistent with Eu+2 being mobile in a reducing environment. Thus, in this occurrence it appears that fluids, which must have been C03-2 bearing, have preferentially mobi­lized the light and intermediate rare earth elements.

Table 1 - Mean compositions of least altered host rocks and progressively altered facies from the Rio shear zone. Values fisted are net compositions and the units are g/100 cmJ of the host for major elements (Si to S) and g/mJ for trace elements (B to U). The abundance of carbon (C) in the rocks is taken as an index of intensity of alteration. Samples of the Phantom Lake granite, from the pluton centre, and one slightly altered sample from the Rio dyke were obtained from Galley and Franklin (1987) and Ansde/1 and Kyser (1990). Major element values are arithmetic averages and trace element values are log normalized averages. Be, W, Hg, Tl, and Bi were analyzed but many values fell below the levels of detection. Density su­perscripts (*) indicate assumed densities in one or more samples. Superscripts on composition values indicate the num­ber of samples included in the mean where this is Jess than the number n.

14 1

Basalt Host Granodioritc Host Phantom Lake Granite

<0.3g 0.3-2.0g 2.0-10.0g >10.0g <0.3g 0.3-5.0g >5.0g Pluton Rlo Alt'd Rlo c c c c c c c Centre Dyke Dyke

n 2 5 5 3 1 3 2 2 2 2 S.G. 2.92 2.91 2.78 2.83 2.85 2.72 2.81 2.12· 2.12· 2.68 Fv 1.00 0.91 1.08 1.29 1.00 1.09 1.18 1.00 1.00 1.24

Si 69.2 61.0 70.1 65.2 87.2 84.9 72.6 86.1 82.0 106.5 Ti 1.06 1.10 1.06 1.06 0.769 0.740 2.05 0.563 1.04 1.00 Al 24.7 21.2 21.8 23.8 23.9 24.2 25.7 22.6 22.8 21.5 Fe+ 3 5.54 6.06 3.07 1.74 4.25 1.68 2.24 - 2.331 1.14 Fe+ 2 15.7 16.0 17.7 19.5 7.66 7.14 18.6 - 4.631 4.98 :EFe+2 21.2 22.0 20.8 21.2 11.9 8.81 20.8 4.63 5.63 6.72 Mn 0.55 0.44 0.33 0.52 0.22 0.16 0.47 0.08 0.10 0.10 Mg 10.3 6.68 9.18 11.0 3.70 2.81 6.77 1.80 2.45 2.15 Ca 16.8 13.8 12.5 27.7 9.82 7.39 15.5 2.51 5.61 5.29 Na 9.65 5.90 10.7 13.6 2.24 11.3 10.5 9.63 10.6 7.19 K 1.66 1.82 3.36 5.05 8.71 6.38 6.65 10.6 6.44 10.6 p 0.18 0.26 0.21 0.24 0.20 0.20 0.27 0.20 0.34 0.38 c+• 0.103 0.684 4.99 18.1 0.288 3.25 9.48 0.4451 0.1631 1.58 oc1dc 126. 117. 135. 175. 135. 140. 154. - 125.1 159. F 0.074 0.080 0.061 0.102 0.140 0.102 0.161 - 0.4081 0.169 5-2 0.048 0.092 0.265 0.639 0.041 0.279 0.274 - 0.0071 0.116

B 50.3 29.4 23.6 45.6 65.6 67.9 81.1 - 46.21 89.1 Sc 117 81.3 74.0 108 25.4 24.0 57.5 - 15.01 15.5 v 845 662 496 608 262 187 566 68.01 1961 202 Cr 518 41.4 45.7 123 28.5 43.0 62.6 48.9 45.l 114 Co 123 75.4 74.6 68.4 31.4 29.6 52.8 13.61 18.8 1 16.9 Ni 41.3 57.43 85.22 72.2 57.0 43.6 2 46.4 1391 54.41 107 Cu 227 218 61.8 11.3 43.9 38.6 27.6 19.01 62.01 190 Zn 368 235 208 167 190 87.1 124 1851 1291 106 Ga 4.67 11.9 20.04 8.022 4.28 5.032 6.00 - 13.61 23.61

As 11.7 14.2 90.83 96.3 25.7 30.3 6.56 - 2.721 4.60 Rb 41.2 28.7 38.74 1022 200 118 145 173 157 146 Sr 716 282 477 1890 513 744 956 3750 3780 1650 y 72.5 81.1 83.9 60.7 45.6 70.3 98.2 - 49.01 47.8 Zr 102 133 190 135 302 314 298 366 349 361 Nb 5.8 8.14 10• 8.72 11 13 15 351 221 24 Mo 2.56 2.26 1.89 1.64 3.02 3.55 1.07 - 8.321 11.9 Ag 0.330 0.188 0.180 0.272 0.114 0.155 0.066 - - -Cd 0.585 0.5304 0.3203 0.827 - - - - - -

Sb 1.52 1.58 1.07 1.232 3.71 1.55 1.14 - 1.09 1 1.26 Cs - - - - 4.56 2.841 - - 1.36 1 1.071

Ba 559 402 259 473 2770 1260 1600 4287 3840 4280 La 11.4 15.5 16.3 22.5 30.8 34.9 31.5 89.81 99.6 1 90.1 Ce 28.5 42.9 38.4 59.0 68.4 79.3 72.9 - 2121 214 Nd 16.0 22.5 20.1 34.7 28.5 37.3 40.7 - 1061 113 Sm 4.95 6.36 5.54 7.65 6.84 6.96 8.19 - 16.01 16.3 Eu 1.74 2.06 1.73 2.71 1.65 1.65 2.67 - 4.221 3.94 Yb 6.84 8.23 6.71 4.30 5.99 5.77 7.73 2.721 2.12 1 2.08 Lu 1.01 1.23 1.03 1.02 1.00 0.92 1.17 - 0.301 0.10 Hf 2.44 3.41 3.32 4.30 8.55 9.45 8.03 - 10.6 1 10.8 Au 0.0057 0.022 0.035 0.334 0.0029 0.0078 0.0051 - 0.0033 0.0102 Pb 11.7 5.40 4.52 8.56 5.73 6.69 5.68 - 7.101 7.81 Th 1.43 2.07 2.47 3.40 7.41 10.7 6.99 - 11.41 11.3

u 3.19 1.684 2.744 2.87 6.27 6.59 3.93 - 7.621 8.10

142 Summary of Investigations 1992

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Figure 2 - Geochemical variation diagrams for selected elements from two sections across the Rio alteration zone. Solid circles/long dash represent section 1 across the southwest end of the alteration zone (see Figure 1) and open squares/ short dash represent s11ction 2 across the centre of the zone. All values are net compositions. Composition scales are linear for mass change ratios and major el11ments (Si to F) and logarithmic for trace elements (S to Au). Distance scales are fixed with point zero at the peak values of carbon (C) and minus values extending toward the footwa/1.

Saskatchewan Geological Survey 143

4 .0

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ter type of alteration is often focused on fracture zones as it is at the Rio deposit, and it is always charac­terized by reductive chan­ges in Fe oxidation ratios and mobility of Eu .

0 .0 5.0 10.0 15.0 20.0 5.0 10.0 15.0 20.0 5 .0 10.0 15.0 20.0

An implication of this sug· gestion is that the fluids may already have reacted with a large volume of rock by the time they reached the present level in the Rio shear zone. Inasmuch as Au can be deposited from less evolved fluids than these, exploration to greater depths is recommended.

C (ne t composition) Figure 3 • Plots of n:ass·balance corrected rapos of La, Eu, and Lu in all Rio zone altered rocks versus the amount ,n the unaltered rocks (La , etc.) plotted against net composition carbon.

7. Conclusions A study of this type cannot contribute any information on the specific nature of the mineralizing fluids except through the nature of the elemental fluxes that occurred when alteration occurred. Gains and losses are not ex­clusively controlled by the fluid composition, but are also influenced by the availability of elements within the host rock and by the thermodynamic controls on mineral stability and instability. The corrections under· taken with the data should have ensured that the net composition values reflect the actual elemental fluxes and are not artifacts of dilution by other added materials or of residual concentration as a consequence of leach· ing.

The elements consistently introduced to both basalts and granodiorite are Ca, (Mg), Na, C, 0 , S, As, Sr, Nb, and Au whereas Cu, and possibly K, Rb, and Ba are either lost or fluctuate independently. The behaviour of the elements said to characterize the Phantom Lake granite, viz. Cu, Mo, W, Au, K, and F, are not consistent­ly correlated with either carbonatization or Au-mineraliza. tion and in several cases show an inverse relationship. An exception would be the small-scale introduction of F into the altered granodiorite. It can be concluded then, that the fluids do not appear to be derived principally from the Phantom Lake granite dyke.

The evidence of Ansdell and Kyser (1990), that the fluids have characteristics indicative of having been derived by dehydration and decarbonation reactions at depth, is regarded as provisionally acceptable.

Introduced elements are typical of fluids sourced in se­quences of Archean mafic volcanics and volcanogenic and/ or poorly d ifferentiated sediments (Kerrich, 1983). Typical Archean lode Au deposits are characterized by major additions of Si02 and C02 along with K, Rb, Li, and Cs, coupled with a 'near quantitative stripping' of Na (Kerrich, 1983). Conversely, in the Rio deposit it is Na a_n_d Nb that are strongly introduced and the large alkali ions, K. Rb, Ba, and Cs, are either not involved in the fluxes or have been expelled during the process. This is in accord with fluids that have evolved from high K/ Na to high Na/K fluids accompanied by a drop in temperature and/or a rise in pH (Kerrich, 1983). The lat·

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Mineralization and alteration is strongly associated with indications of mass changes in the host rocks; the banded albite-pyrite-quartz-ankerite alteration is as­sociated with areas of the alteration zone that tend to return mass-loss indications, while the quartz-ankerite­pyrite-chlorite-muscovite vein mineralization tends to be found in areas indicating substantial mass increases. It is recommended that FM values (mass change ratios) be utilized in identifying prospective areas in altered shear zones and in characterizing their internal zonation. Mean FM and/ or mean Fv (volume change ratio) values from shear zones sampled at a statistically significant in· tensity should indicate net increases, which in this model are ascribed to initial dilation on the shear. Other approaches, that are consistent with this recommenda· tion, are to look for areas of low Fe oxidation ratios and tor areas with positive Eu anomalies; both should corre­late with areas of high fluid/rock ratios. Structural studies of shear zones should also be focused on iden· tifying regions where dilation may have been maxi· mized.

Although many of the altered rocks have been described as silicified or 'cherty' , the geochemical evidence for systematic introduction of Si is not convinc­ing. Rather the irregular distribution of Si gains and los­ses suggests substantial local migration within the sheared system. The same conclusions can be ad­vanced for IFe. In the absence of evidence for sys­tematic introduction of Si the term silicified should be avoided. The association of IFe with Au found by Mel· linger and Pearson (1987) has not been confirmed by this study and may be a residual enrichment phenomenon. On the other hand, their data set was greater and more extensive than that used in this study so the question can not be said to have been answered.

8. Acknowledgments

I would like to thank Dave Thomas for providing logisti· cal assistance and expert knowledge in guiding me to the gold occurrences of the Flin Flon area and to him, Tom Sibbald, and Pam Schwann for stimulating discus-

Summary of Investigations 1992

sions of the geological problems associated with these deposits and occurrences.

9. References Ansdell, K.M. and Kyser, T.K. (1990): Epigenetic gold

mineralization in the Flin Flan Domain: Fluid charac­teristics; in Beck, LS. and Harper, C.T. (eds.), Modern Ex· ploration Techniques, Sask. Geol. Soc., Spec. Puhl. 10, p219-234.

-~~--c- (1991): Plutonism, deformation, and metamor­phism in the Proterozoic Flin Ron greenstone belt, Canada: Limits on timing provided by the single zircon Pb-evaporation technique; Geel., v19, p518-521.

Appleyard, E.C. (1990): Mass balance corrections applied to lithogeochemical data in mineral exploration; in Beck, LS. and Harper, C.T. (eds.), Modern Exploration Techniques, Sask. Geol. Soc., Spec. Publ. 10, p27-40.

Byers, A.R., Kirkland, S.J.T., and Pearson, W.J. {1965): Geol­ogy and mineral deposits of the Flin Flan area, Sas­katchewan; Sask. Dept. Miner. Resour., Rep. 62, 95p.

Coombe Geoconsultants Ltd. {1984): Gold in Saskatchewan; Sask. Energy Mines, Open File Rep. 84-1, 110p.

Galley, A.G. and Franklin, J.M. (1985): Gold-tungsten mineralization associated with the Phantom Lake granite, Creighton; in Summary of Investigations 1985, Sas­katchewan Geological Survey; Sask. Energy Mines, Misc., Rep. 85·4, p99-100.

____ (1987): Geological setting of gold, copper, tungsten, and molybdenum occurrences in the Phantom Lake region; in Summary of Investigations 1987, Sas­katchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 87-4, p115-124.

Gresens, R.L (1967): Composition-volume relationships of metasornatism; Chem. Geol., v2, p47-65.

Heaman, L._M., Kamo, S.L, Delaney, G.D., Harper, C.T., Reilly, B.A., Sl1mmon, W.L, and Thomas, O.J. (1991): U-Pb geochronological investigations in the Trans-Hudson Orogen, Saskatchewan: Preliminary results by the ROM laboratory in 1990-91; in Summary of Investigations 1991, Saskatchewan Geological Survey; Sask. Energy Mines, Misc. Rep. 91-4, p74-75.

Hynes, A. (1980): Carbonatization and mobility of Ti, Y, and Zr in Ascot Formation metabasalts, SE Quebec; Contrib. Mineral. Petrol., v75, p79-87.

Kerrich, R. (1983): Geochemistry of gold deposits in the Abitibi greenstone belt; Can. Inst. Min. Metall., Spec. Vol. 27, 75p.

Saskatchewan Geological Survey

Mellinger, M. and Pearson, J.G. (1987): Lithogeochemistry and metallogenesis of the Rio Gold Mine, and compara­tive geochemistry from the Flin Aon and Amisk Lake areas; in Summary of Investigations 1987, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 87-4, p154-159.

Middleton, T.A. (1985): The relationship between the structural setting and the development of the alteration zone at the Rio Gold Mine, Saskatchewan; Unpubl. B.Sc. thesis, Univ. Waterloo, 54p.

Pearson, J.G. (1984a): Gold metatrogenic studies, the Rio deposit; in Summary of Investigations 1984, Sas­katchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 84-4, p123·126.

--~~ (1984b): Geology of the Rio deposit (Part of NTS 63K-12); 1:1200 scale prelim. map with Summary of Investigations 1984, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 84-4.

Syme, E.G., Bailes, A.H., Gordon, T.M., and Hunt, P.A. (1987): U-Pb zircon geochronology in the Flin Flan belt: Age of Amisk volcanism; in Manitoba Energy and Mines, Report of Field Activities 1987, p105-107.

Taylor, R.P., Strong, D.F., and Fryer, B.J. (1981): Volatile con­trol of contrasting trace element distributions in peralkaline granitic and volcanic rocks; Contrib. Mineral. Petrol., v77, p267·271.

Thomas, D.J. (1989a): Geology of the Douglas Lake- Phan­tom Lake area (Part of NTS 63K-12 and • 13); in Summary of Investigations 1989, Saskatchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 89-4, p44-54.

-,__,.....~- (1989b); Bedrock geology, Douglas Lake-Phan­tom Lake area (part of NTS 63K-12 and -13) (1989); 1: 12 500 scale prelim. map with Summary of Investiga­tions 1989, Saskatchewan Geological Survey, Sask. Ener­gy Mines, Misc. Rep. 89-4.

--~~ (1990a): New perspectives on the Amisk Group and regional metallogeny, Douglas Lake-Phantom Lake area, northern Saskatchewan; in Summary of Investiga­tions 1990, Saskatchewan Geological Survey, Sask. Ener­gy Mines, Misc. Rep. 90-4, p13-20.

--c---- (1990b); Bedrock geology, Douglas-Phantom Lakes area (parts of NTS 63K-12, -13); 1:12 500 scale prelim. map with Summary of Investigations 1990, Sas­katchewan Geological Survey, Sask. Energy Mines, Misc. Rep. 90-4.

Wendlandt, R.F. and Harrison, W.J. (1979): Rare earth par­titioning between immiscible carbonate and silicate Ii· quids and C02 vapor: Results and implications for the for­mation of light rare earth-enriched rocks; Contrib. Mineral. Petrol., v69, p409-419.

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