Project 8832

Geology and Geochemistry of the Uranium-Gold Mineralization in the Nicholson Bay-Fish Hook Bay Area 1

E.P. W. Peiris2 and G.R. Pars/a..,.;

Peiris, E.P.W. and Parslow, G.R. (1988): Geology and geochemistry of the uranium-gold mineralization in the Nicholson Bay­Fish Hook Bay area; in Summary of Investigations 1988; Saskatchewan Geological Survey; Saskatchewan Energy and Mines, Miscellaneous Report 88-4.

A study of the uranium-gold mineralization in the Nichol­son Bay-Fish Hook Bay area was begun last year as part of an M.Sc. program by the senior author (EPWP) at the University of Regina and reported upon in the 1987 Summary of Investigations (Peiris and Parslow, 1987). This report describes the progress of the study since that time. Some of the preliminary interpretations which are made here may be changed with further re­search.

1. Field Work Completed During 1988

Field work by the senior author covered three weeks in June and July of this year. Cores of eleven diamond­drill holes from the Nicholson no. 2 zone were logged and sampled for petrographic study; two of the cores (MCD 88-2 and 88-14) were sampled for later geochemi­cal analysis. In addition, further surface mapping of the geology in and around the mineralized zones was car­ried out, resulting in the discovery of more outliers of Athabasca Group sandstones in the region. The presence of intense red staining and alteration in rocks adjacent to the unconformity precludes, in many instan­ces, the ready discrimination between the Athabasca Group and the underlying basement quartzites.

Drill core was checked for possible uranium mineraliza­tion with a scintillometer, but significant radioactive zones were not detected. According to company drillhole logs and assay results, the gold mineralization is within the red zone; however, no obvious features dis­tinguishing barren from mineralized sections of the drill cores were observed.

The two drillholes chosen for detailed geochemical sam­pling (MCD 88-2 and 88-14) are 38 m apart and plunge respectively at 55° and 50° on an azimuth of 115°. The red hematitic alteration zone extends from surface to 77 m depth in MCD 88-2 and from surface to 124 m depth in MCD 88-14 (Figures 1 and 2). Three mineral­ized sections are identified in MCD 88-2, with the highest gold assay (104,000 ppb or 3.33 oz./ton) en­countered between 37.2 and 37.7 m; the other two mineralized sections occur from 62.4 to 66.1 m and 70.3 to 76.9 m. The company drillhole logs describe the MCD 88-14 core as essentially unmineralized (the

highest gold assay is 320 ppb or 0.01 oz./ton at 119.1 to 120.1 m).

2. Petrography and Electron Probe Micro­Analyses

Petrography of selected surface and drill core samples from Nicholson Bay and Fish Hook Bay was performed during the year. The petrographic interpretations were checked and assisted by the electron probe micro­analyses carried out at the Natural History Museum in Vienna.

The probe data supported the petrographic identifica­tions except in two cases: some euhedral crystals of gersdorffite turned out to be an arsenic pyrite and some skutterudite was actually bravoite.

Diopside, with ubiquitous tremolite, is abundant in the marbles, calc-silicates, and quartzites which are the dominant host rocks to mineralization in the area (Peiris and Parslow, 1987). The probe work identified talcose material, approaching anthophyllite in composition, as one of the alteration products of diopside in addition to talc and serpentine. Since talc is formed above 430°C to 470°C at 1 to 3 kb of pressure (Hyndman, 1972, p. 109) and serpentine is formed below these temperatures, the presence of talc may indicate an earlier(?) hydrothermal alteration of the calc-silicates. The calc-silicates were selected for the study of alteration because pure quartzites and marbles are insensitive to the alteration processes. Typical compositions of the diopside and its alteration products are given in Table 1.

The alteration processes tend to remove calcium totally, leach iron and enrich magnesium (see analyses Ta and An in Table 1). Diopside is pseudomorphed by a dark green or reddish, soft talcose mass. The development of the red variety is probably due to oxidation of any fer­rous iron present, as the total iron contents of both the green and red "pseudomorphic diopside" are very similar. Within the hematitic zone, reddish-brown chert also pseudomorphs diopside. Additionally, chert and quartz replace carbonates, particularly in marbles.

Two different types of chlorites are found in fresh and al­tered rocks. Chlorites in the fresh marbles, interpreted

(1) Project funoed under the Saskatchewan component of the can ad a - Saskatchewan Sut>sldlary Agreement on Mineral Development 1984 -89 (2) Department of Geology, Unlverslly of Regina

70 Summary of lnwstigations 1988

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Saskatchewan Geological Survey 71

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Summary of Investigations 1988

as products of the retrogressive metamorphism in the area, have a lower iron content than the chlorites pen­ninite and pyrochlore associated with the mineralization within the red alteration zone.

Brecciation, which assisted in the development of per­meability in the crystalline rocks, is evident and predates the introduction of the pitchblende veins.

In summary, the following general sequence of events can be inferred from the thin and polished sections:

1) Hydrothermal alteration of the host rocks charac­terized by pseudomorphic replacement of calc-sili­cate minerals.

2) Remobilization of pyrite, chalcopyrite and pyrrhotite into fractures, seen as veinlets within the "pseudo­diopside".

3) Brecciation. 4) Introduction of euhedral carbonate and specular

hematite as veins within the red alteration zone. 5) Formation of colloform pitchblende; veins commonly

contain pyrite and chalcopyrite. 6) Development of rammelsbergite in cracks and voids

in the pitchblende. 7) Formation of a second generation of pitchblende,

emplaced along with colloform silica. 8) Formation of a second generation of rammelsber­

gite, with later niccolite filling cracks within the ram­melsbergite.

9) Introduction of veins of gersdorffite, maucherite, skut­terudite, bravoite, linnaeite, polydimite and millerrte. The paragenetic sequence is complex, but is generally: gersdorffrte, maucherite, polydimrte and linnaeite. Bravoite and violarite are observed as al­teration products of millerrte. Euhedral grains of bravoitewith alternate pinkish-brown to brown zones were recognized by the probe analyses.

1 O) Finally, development of a second generation of green colloform silica very similar to that in stage 7.

Within the context of the above scheme, certain impor­tant problems of paragenesis remain unresolved:

Table 1 - Probe Analyses of Selected Minerals

Di Tr Ta An Pi

Si02 55.1 58.7 63.6 64.1 1.53 Al203 0.14 0.08 0.01 0.08 Ti02 0.01 0.0 0.02 0.0 0.0

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1) Timing of gold introduction: Gold has been indi­cated by the electron probe within a grain of ram­melsbergite and the senior author has tentatively identified tiny inclusions of gold within goethrte in the red carbonate from high-grade sections of drillhole MCD 88-2. Thus, the gold may be associated with one or more of stages 4, 6 and 8 above.

2) Timing of hematite introduction: At least two genera­tions of colloform hematite can be observed within the hematitic zone. Whether or not both generations of hematite belong to stage 4 is unknown at the present time.

3) Age of hydrocarbon "buttons": Hydrocarbon "but­tons· are quite common in the wall rocks of the prtchblende veins. Pitchblende and arsenides fill cracks in the "buttons", suggesting that the mineralization is later than the organic material, but just how early the "buttons· are is not known.

4) Position of nolanite in the paragenetic sequence: Nolanite was idenmied in samples from the Nichol­son no. 1 zone and in the Fish Hook Bay samples, forming pinkish to dark brown, hexagonal and strongly anisotropic crystals. Its position in the se­quence is not known.

5) Position of bornite and chalcocite in the paragenetic sequence: Bornrte and chalcocite are present in samples from the Nicholson no. 1 and no. 3 zones, and from Fish Hook Bay. They form up to 30 per­cent of the mineralization in the no. 3 zone. Their age is unknown but is presumably post stage 5.

Definition of possible chemical controls and age(s) of mineralization are fundamental factors in deposit modell­ing. Textures of the pitchblende, arsenides and sul­phides typically indicate replacement of pyrrte and euhedral carbonate. Goethite and hematrte pseudo­morphs of pyrite, and relict pyrite within these minerals, are common in the red marbles. Close by, chalcopyrrte is commonly altered to native copper and covellite. Clearly, therefore, the oxidation of pyrite (stage 4) could

have produced a reducing environment that deter­mined the preciprtation of later phases (stages 5 to 9).

FeO 3.6 5.3 2.48 1.83 3.2 0.36

The proximity of this basement-hosted mineraliza­tion to the sub-Athabasca Group Unconformity as­serts the role of the unconformity and suggests two probable ages for mineralization: prior to and subsequent to deposition of the Athabasca Group. If mineralization is pre-Athabasca, then a paleoregolith origin is favoured; if rt is post­Athabasca, an "unconformity-type" origin (as in the high-grade uranium deposits of the Athabasca Basin) is favoured The following evidence sup­ports the latter:

MnO 0.9 0.68 0.0 0.05 CaO 25.0 13.4 0.02 0.02 2.81 2.91 MgO 16.1 21.1 29.3 29.8 PbO 1.35 0.3 Na20 0.03 0.0 0.03 0.03 Th02 0.0 0.0 1<20 0.0 0.01 0.01 0.0 U02 80.98 70.0 Cr203 0.0 0.0 0.0 0.04 NiO o.o 0.02 0.03

Total 100.88 99.27 95.49 95.94 89.87 84.74

DI. dlopslde; Tr, tremorne: Ta, talc; An, "anlhophylllte•; Pl, pltcht>lende

Saskatchewan G60/ogica/ Surwy

1) Available discordant ages of pitchblendes from Nicholson indicate an oldest date of 1115 Ma (Koeppel, 1968), much younger than the deposi­tional age of the Athabasca Group. Addrtionally, some of the probe analyses of pitchblende indi

73

cate virtually no Pb, reflecting a much more recent origin than even 1 Ga (compare the two pitchblende analyses in Table 1).

2) Chertification and grmvth of euhedral quartz and car­bonate are observed in both the Athabasca sandstones and the basement alteration zones. The chertification is later than the authigenic syntaxial quartz overgrowths in the Athabasca sandstones; that is, post major diagenetic changes.

3) In the hematized sections in the basement (the 'red' zone), euhedral quartz and carbonate veins with specular hematite postdate the red alteration.

3. Geochemistry Thirty-two samples from the MCD 88-2 and 20 samples from the MCD 88-14 were analyzed for their multi-ele­ment composition by Elemental Research Inc. of Van­couver. The samples were prepared for analysis in the geochemical laboratories at the University of Regina. Another 20 samples (mainly surface samples collected by T.1.1. Sibbald}, comprising the different rock types found in the area, were also analyzed. The analyses were carried out using inductively coupled plasma mass spectrometry. Up to 63 elements were determined for each sample. Precision of the analytical method was checked by repeating every tenth sample. The following elements were generally below the detection limit of the method: As, Se, Rh, Cd, I, Te, Cs, Os, Ir, Re, Tl and Pt. Tungsten values are unreliable because of the possible contamination during the crushing process.

From the repeat analyses, it is clear that the technique defines most elements to a precision of ± 1 O percent, with the exception of Cr, Co, Mo, and Zr which often run to ± 10 percent, and particularly B and Hg which ex-ceed ±50 percent. In the time available, it proved impossible to obtain good standards covering the 63 elements; consequently, the accuracy of the ICPMS method is essentially unknown. On the other hand, the reasonable levels of precision allow the authors to draw the fol­lowing conclusions:

1) As expected, the ultrabasic rocks and pelitic rocks have much higher concentrations of many trace elements than the quartzite, carbonate, calc­silicate and Athabasca sandstone samples.

Al < 1 oo > 80

hematized' (brick red in colour), approximating the trend: host rock to altered and in part mineralized zone. A comparison of elemental values, in fresh and altered rocks, indicates a definite increase in Co, Cu, Zn, Rb, Sr, Y, Nb, Pd, Ag, Cd, Ba, Ta, Au, Sb, Se and Ga with increasing hematization. It ap­pears that Ni and Br are enhanced only in slightly hematized samples and that Sn is enhanced only in highly hematized samples. Conspicuous by its ab­sence in this correlation is the element U.

3) Irrespective of these general trends, it is important to note that rarely does a given sample exhibit an en­hancement of more than half the elements listed above. Thus, the elemental distributions 'controlled' by the increasing hematization are very erratic, as can be seen in the values of selected elements for samples from MCD 88-2 and MCD 88·14 (Figures 1 and 2). Strong and perfectly matching positive cor­relations of Au with other elements do not occur, only general correlations. Uranium tends towards a negative correlation with Au.

4) Although hematization of the altered rocks is visually impressive, total iron content remains essentially the same, suggesting that the 'red' zone was produced by a change in the oxidation state of the iron, rather than an introduction of iron. These data are shown in Figure 3 along with two samples of paleoreg­olith(?) which contain much more iron.

5) Although U does not correlate well with other ele­ments in the hematized rocks (except perhaps nega. tively), two samples of pitchblende veins show strong enrichments of all the elements mentioned in (2), plus other elements such as As, Sc, Mo, etc.

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grouped into 'fresh', 'slightly hematized' (light red in colour) and 'strongly Figure 3 - Plot of Mg·AI-Fe ternary diagram for fresh marbles (left) and hematized

marbles (right). Two points at 40 percent Fe on right are samples of paleoregolith(?).

74 Summary of Investigations 1988

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Figure 4 - Chondrff&.normaliUJd REE plots (averages of three analyses) for 'fresh' (A), 'slightly hematized' (B), and 'strongly hematized' (C) marbles. Trend D is an average of two pitchblende analyses.

Thus, a similar ore fluid to that controlling the Au mineralization is envisaged for the pitchblende vein formation.

6) Interpretation of the REE data is presently incom­plete; however, some conclusions can be drawn from Figure 4, where the data for the 'fresh', 'slightly hematized' and 'strongly hematized' carbonates and pitchblende veins are compared. As hematization in­creases (A, B, C), the REE content increases and depletion of the lightest REE occurs; within the pitchblende veins, the total content of REE increases dramatically but the REE pattern is preserved.

7) The REE pattern for phosphorites or 'marine' apatites is essentially flat at an enrichment level of 2,

Saskatchewan Geological Survey

relative to the average for North American shales. A similar plot of REE data for a sample of Athabasca sandstone containing 10+ percent apatite (not ii· lustrated) shows a very strong depletion of the light REE. Thus, the apatites of the Athabasca Group are probably not of 'marine origin' and may represent an introduction of material as part of the mineraliza­tion process(?).

4. Conclusions and Implications for Ex­ploration

1) Visual indicators of Au mineralization, in either sur­face or drill core samples, are not apparent other than an association with hematized rocks. Since large areas of the Beaverlodge region are 'reddened', this association is of limited use in detailed exploration studies (i.e., Nicholson Bay). Conversely, this observation suggests that the whole Beaverlodge area is worthy of further study for Au­PGE mineralization.

2) The fact that many veins of gangue minerals as­sociated with the mineralization, as well as chert re­placement features, are found in both the basement and the small outliers of Athabasca sandstones indi· cates a post-Athabasca age for the mineralization and a possible relationship with the unconformity.

3) The elemental associations of the Nicholson Bay mineralization are almost identical to those en­countered in the 'unconformity-type' deposits of the Athabasca Basin, where U tends to be the dominant element in the association, although at Key Lake, U is subordinate to Ni in some parts of the ore. The authors conclude that an 'unconformity-type' model best explains the available data. The lack of clay al­teration haloes is readily explained by the lack of feldspars in the country rocks.

4) Since graphitic pelites are not prevalent in the Nichol­son Bay area, some other reductant process is re­quired to form the observed mineralization. The com­mon occurrence of pyrite and magnetite in the area and their destruction within the hematized and typi· cally mineralized zones is a possible oxidation/ reduction model that can be applied.

5) The generally negative correlation between U and Au in the hematitic alteration zone suggests either two periods of mineralization or, at least, a clear separation of these two elements within a single paragenetic sequence. It has been shown by Jaireth (1988) that Au and Pt can be transported with U in an oxidized saline fluid and thereby might be ex­pecte~ to be associated with U in unconformity-type deposits, at least from a thermodynamic viewpoint. However, at the present stage of research, the exact timing of Au introduction remains obscure.

6) Acceptance of the unconformity-related nature of the Nicholson Bay mineralization raises speculations about the exploration potential of other unconfor-

75

mities in the area such as those below the Martin Group and even the Thluicho Group.

5. Acknowledgements The authors are most grateful to Saskatchewan Energy and Mines for their continued financial support of this project. We wish to offer sincere thanks to the Kasner Group for allowing access, not only to its properties, but also to drill core, drill logs and assay information, and for the provision of accommodation and transport in the field.

Finally, the senior author (EPWP) wishes to acknow­ledge the fellowship granted by the International Atomic Energy Agency, which has allowed him to pursue this re­search as part of his M.Sc. program in the Department of Geology at the University of Regina.

76

6. References Hyndman, D.W. (1972): Petrology of Igneous and Metamor­

phic Rocks; McGraw-Hill, 533p.

Jaireth, S. (1988): Hydrothermal transport of platinum and gold in unconformity-related uranium deposits: a prelimi­nary thermodynamic investigation; Record, Bur. Miner. Resour., Australia.

Koeppel, V. (1968): Age and history of the uranium mineralization of the Beaverlodge area, Saskatchewan; Geol. Surv. Can., Pap. 67-31, 111p.

Peiris, E.P.W. and Parslow, G.R. (1987): Metallogenic studies, Nicholson - Fish Hook Bays area; in Summary of lnvestiga, tions 1987, Sask. Geol. Surv., Misc. Rep. 87-4, p58-59.

Summary of Investigations 1988