Conclusiones parciales

Fig. 4.22 shows chondrite-normalised PGE data for each of the three principal East Bull Lake Suite intrusions. Fig. 4.22 shows that in general, the East Bull Lake suite intrusions are enriched in the Palladium Group PGE (PPGE) relative to the Iridium Group PGE (IPGE) with the East Bull Lake, Agnew and River Valley intrusions recording average Ir/Pt(N) of 0.20, 0.08 and 0.04 respectively. Rocks from the unmineralised zones of the three intrusions commonly contain low abundances of PGE and record a range of ΣPGE of between 0.003-0.03 × chondritic values.

Specifically, unmineralised rocks of the East Bull Lake intrusion contain ΣPGE

3

117 abundances of between 11-18 ppb, while unmineralised rocks of the Agnew and River Valley intrusions contain ΣPGE abundances of between 21-61 ppb and 46-101 ppb respectively. The samples with the highest PGE contents (RV014 and RV015) are sampled from the mineralised gabbronorite matrix of the inclusion-bearing zone of the River Valley intrusion and contain 2892 ppb and 980 ppb total PGE respectively. Sample AG007 comes from the equivalent zone in the Agnew intrusion but contains 61 ppb ΣPGE, which is within the range recorded by unmineralised East Bull Lake Suite rocks. These results may demonstrate the heterogeneity of the

‘blebby’ mineralisation observed in the inclusion-bearing zone (Peck et al. 2001;

James et al. 2002).

Fig. 4.22. Chondrite-normalised PGE diagrams for individual East Bull Lake Suite intrusions.

Normalising values from Naldrett and Duke (1980).

When the East Bull Lake Suite samples are divided into rock type as calculated using the CIPW norm (Fig. 4.13), rather than intrusion, it would appear that gabbronoritic rocks are the most prospective (Fig. 4.23). However, the data for the gabbronoritic rocks of the East Bull Lake suite are skewed by the two mineralised samples which have gabbronoritic compositions. When these samples are removed, the remaining gabbronoritic rocks contain similar PGE abundance to other unmineralised East Bull

0.0001

118 Lake Suite lithologies, which indicates that lithology is not a totally useful indicator of degree of mineralisation in the East Bull Lake Suite.

Fig. 4.23. Chondrite-normalised PGE diagrams for average values for each of the lithologies which make up the East Bull Lake Suite intrusions (See Fig.

4.13). Gabbronorite (A) = Average Gabbronorite including mineralised samples and (B) = Average Gabbronorite not including mineralised samples. Normalising values from Naldrett and Duke (1980)

4.5. Thessalon Formation

4.5.1. Alteration and Element Mobility

Due to constraints of time and expense, only a few samples were collected from the Thessalon Formation volcanics of the Huronian Supergroup and made into thin-sections. Instead, this study uses the data of Kirsty Tomlinson who collected 84 samples related from the Thessalon Formation of the Huronian Supergroup as part of her Ph.D. thesis on early Precambrian Greenstone Belts (Tomlinson 1996). The data presented here includes 78 samples of Thessalon Formation basalts and basaltic andesites from Tomlinson (1996) which can be found in full in Appendix C.

The LOI for the Thessalon Formation volcanics range from 1.0 to 6.7 wt.%. Some of these LOI values are relatively high and may suggest that the Thessalon Formation rocks have experienced little alteration. However, previous studies (Jolly 1987a;

Tomlinson 1996) have documented alteration assemblages indicative of greenschist facies metamorphism in all but the coarsest flows of the Thessalon Formation. A subset of element vs. Zr graphs is shown in Fig. 4.2. Examination of these graphs shows that the Thessalon Formation volcanics have a quite complex geochemistry and that, based on trace and major element relationships, the volcanic rocks can be divided into at least four separate groups. The distinction between Group 1 and Group 2 volcanics is not immediately obvious on plots of major elements vs. Zr, but does become apparent with plots of the MREE and HREE vs. Zr. On these plots,

119 volcanics do. Group 3 volcanics have much higher concentrations of MgO, Ni and Cr and also have much lower concentrations of Al2O3 relative to the other Thessalon samples which have similar Zr contents. Group 4 volcanics have much higher Th and U contents than other Thessalon samples with similar Zr contents.

Group 1 volcanics show good correlations between Zr and Nb and Hf only.

Moderately strong correlations are observed between Zr and Ce, Pr, Nd, Yb, Lu, Th and U while there are weak correlations between Zr and the remaining elements.

For Group 2 volcanics, the elements which show a poor correlation with Zr include the major, LIL, and compatible trace elements, Eu, Ta, Th and U. Elements which show a moderate correlation with Zr include Y, Er, Tm, Yb, Lu and Hf. The REE (La, Ce, Pr, Nd, Sm, Gd, Tb, Dy and Ho) are the only elements to show a good, linear positive correlation with Zr. Group 3 samples exhibit poor correlations between Zr and the major elements, Sc, Cr, Zn, V, Ba, and the MREE-HREE, moderately good correlations between Zr and the LREE, K2O, P2O5, Rb, Cs, Yb and Ta and good correlations between Zr and Ni, Nb, Th, U, Sr and Y. However, as only three Group 3 samples have trace element data, the R² values recorded above for the trace elements cannot be used with confidence as, with such a small population size, the product moment correlation coefficient becomes less robust as it can be affected by anomalous samples. Group 4 volcanics contain a very narrow range of Zr (5 ppm) relative to the other trace elements and all elements record a poor correlation with Zr.

The correlations described above are interesting for two reasons. Firstly, it is not uncommon for elements which are considered to be immobile under greenschist facies metamorphism (Pearce 1996; Hastie et al. 2007) to record poor correlations with Zr in the Thessalon Formation rocks, despite them having experienced relatively low degrees of metamorphism (Mossman and Harron 1983). Secondly and perhaps more surprisingly is the fact that the strength of the correlations is not uniform between the different groups despite the groups belonging to the same volcanic formation and thus sharing a common tectonic and metamorphic history.

Potential reasons behind the different geochemistry preserved by the Thessalon Formation volcanic rocks are explored later in the thesis.

120

Fig. 4.24. Bivariate diagrams of selected elements vs. Zr for the Thessalon Formation samples.

R² = 0.43

121 4.5.2. Classification

On the TAS diagram (Fig. 4.25), the majority of the Group 1 Thessalon volcanics plot as tholeiitic basalts-andesites, while approximately 30% of Group 1 samples plot within the alkaline series as trachybasalts-trachyandesites. The majority of the Group 2 volcanic rocks plot as tholeiitic basalts and fewer basaltic andesites and trachyandesites. The Group 3 samples form a fairly tight cluster which straddles the tholeiitic basalt-basaltic andesite transition. The Group 4 samples form a diffuse scatter in the tholeiitic basaltic andesite, andesite, basaltic trachyandesite and trachyandesite fields.

Fig. 4.25. Total alkali vs. SiO2 (TAS) diagram for the Thessalon Formation samples. Field boundaries and names as in Fig. 4.2.

The poor correlation of Na2O, K2O and SiO2 with Zr observed in the Thessalon volcanic groups suggests that these elements may have been remobilised. Therefore, the Zr/Ti vs. Nb/Y diagram may be of more use in classifying the Thessalon Formation rocks. On this diagram (Fig. 4.26), the majority of Group 2 samples plot as a fairly tight cluster in the subalkaline basalt and basaltic andesite fields. The Group 1 samples also lie within the subalkaline basalt and basaltic andesite fields but plot separately from Group 2 samples and typically record lower Nb/Y ratios. Three Group 3 samples form a very tight cluster in the subalkaline basalt field while Group 4 samples plot as a tight cluster on the transition between subalkaline basaltic andesite-andesite.

0 4 8 12 16

35 45 55 65 75

Na2O + K2O (wt.%)

SiO₂ (wt.%) Group 1

Group 2 Group 3 Group 4

122

Fig 4.26. Zr/Ti vs. Nb/Y diagram for the Thessalon Formation volcanic rock. Field boundaries defined by Pearce (1996).

4.5.3. Major Element Variation

Group 1 volcanics range in MgO, TiO₂ and Fe₂O₃ between 4.1-7.1, 0.6-1.5 and 10.1-17.0 wt.% respectively. These values correspond to a range in Mg# of 25-43. SiO₂ in Group 1 volcanics ranges from 45.2-56.6 wt.% while total alkalis range from 2.2-8.2 wt.%. Group 2 volcanics range in MgO, TiO₂ and Fe₂O₃ between 1.5-8.2, 1.1-2.0 and 10.2-16.7 wt.% respectively. These values correspond to a range in Mg# of 14-38.

SiO2 in Group 2 volcanics ranges from 47.8-61.7 wt.% while total alkalis range from 3.3-7.4 wt.%. The Group 3 volcanics range in MgO, TiO2 and Fe2O3 between 8.4-9.9, 1.1-1.3 and 12.5-14.6 wt.% respectively. These values correspond to a range in Mg# of 41.2-46.4. SiO2 in Group 3 volcanics ranges from 50.1-52.9 wt.% while total alkalis range from 2.8-3.9 wt.%. Group 4 volcanics range in MgO, TiO2 and Fe2O3

between 3.7-5.2, 0.7-0.7 and 9.7-13.3 wt.% respectively. These values correspond to a range in Mg# of 28-32. SiO2 in Group 4 volcanics ranges from 53.1-58.1 wt.%

while total alkalis range from 3.1-6.5 wt.%.

All of the major elements in Group 1 and 2 volcanics have weak correlations with MgO (Fig. 4.27), which may be further evidence that MgO (and the other major elements) has been remobilised in the Thessalon Formation. Although the correlations are weak, Group 1 and 2 volcanics show negative correlations between MgO and SiO₂, TiO₂, Na₂O, K₂O and P₂O₅ while also showing positive correlations between MgO, Fe₂O₃ and CaO. These correlations may suggest that the evolution of Group 1 and 2 Thessalon volcanics was controlled by the fractionation and removal

0.001 0.01 0.1 1

0.01 0.1 1 10 100

Zr/Ti

Nb/Y

Group 1 Group 2 Group 3 Group 4

123

Fig. 4.27. Bivariate diagrams of selected major elements vs. MgO for the Thessalon Formation.

of Mg, Fe and Ca bearing minerals in a deeper magma chamber. Group 3 and 4 volcanics typically show very weak correlations with MgO and trends are not immediately obvious for the majority of the major elements although Group 4 volcanics show significant negative linear correlations between MgO, SiO2 and Fe2O3, similar to those observed in Group 1 and 2 samples.

124

Fig. 4.28. Bivariate diagrams of trace elements vs. MgO for the Thessalon Formation samples.

4.5.4. Trace Element Variation

In the Thessalon Formation volcanic groups, the elements which are compatible during basaltic fractionation show variable negative correlations with MgO (Fig.

4.28). Group 2 volcanics contain the largest ranges in Cr and Ni of all of the groups (1-444 ppm Cr and 1-205 ppm Ni) and encompass the smaller ranges observed in the Group 1 and 4 volcanics. Group 3 samples contain much greater concentrations of Ni (320-357 ppm) and Cr (997-1080 ppm) than the other groups and also show strong

R² = 0.39

125 negative linear correlations between these elements and MgO. Weak linear negative correlations are observed between MgO and the incompatible trace elements in the Group 1 and 2 Thessalon volcanics. Interestingly, Group 3 and 4 samples record positive correlations between MgO and some of the incompatible trace elements, however, these correlations may be erroneous due to the relatively small population size of these two groups.

Fig. 4.29. Chondrite-normalised REE diagrams for (A) Group 1, (B) Group 2, (C) Group 3 and (D) Group 4 of the Thessalon Formation. Normalising values from McDonough and Sun (1995).

Chondrite-normalised REE diagrams for the Thessalon Formation (Fig. 4.29) show that the four groups are characterised by LREE enrichment relative to the HREE with variable (commonly negative) Eu anomalies. The degree of LREE enrichment varies between the groups with Group 2 volcanics being most enriched [(La/Yb)N = 13.07], much greater than that recorded by Group 1 volcanics (mean (La/Yb)N = 4.37).

Group 3 and Group 4 volcanics show similar levels of LREE enrichment, with average (La/Yb)N values of 8.00 and 7.83 respectively. Group 2 volcanics also record the steepest HREE slopes (mean (Gd/Yb)N = 2.88), almost twice as steep as those of

1

126

Fig. 4.30. Primitive Mantle-normalised multi-element diagrams for the Thessalon Formation. (A) Group 1, (B) Group 2, (C) Group 3 and (D) Group 4. Normalising values from McDonough and Sun (1995).

1 10 100

Th Nb Ta La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

Rock / Primitive Mantle

1 10 100

Th Nb Ta La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

Rock / Primitive Mantle

1 10 100

Th Nb Ta La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

Rock / Primitive Mantle

1 10 100

Th Nb Ta La Ce Pr Nd Zr Hf Sm Eu Ti Gd Tb Dy Y Ho Er Tm Yb Lu

Rock / Primitive Mantle

127 Groups 1 and 4 which have (Gd/Yb)_N of 1.47 and 1.51 respectively. The Group 3 volcanics have an average (Gd/Yb)N of 2.25, intermediate between the other groups.

All of Group 3 and 4 samples show Eu anomalies of between Eu/Eu* = 0.50-0.89.

Group 1 and 2 samples show variable ranges in Eu/Eu* of 0.52-1.13 and 0.67-1.02 respectively. These ranges in Eu/Eu* may indicate that the Group 3 and 4 volcanics experienced plagioclase fractionation while the Group 1 and 2 lavas may have undergone plagioclase fractionation and removal/accumulation.

On Primitive Mantle-normalised multi-element plots (Fig. 4.30), the Thessalon Formation rocks all have similar patterns of incompatible element enrichment and negative anomalies in Nb-Ta, Ti, Y and Zr-Hf. The largest Nb-Ta (Nb/Nb* = 0.2) and Ti (Ti/Ti* = 0.4) anomalies are observed in Group 4 volcanics. The smallest Nb-Ta anomalies are recorded by Group 3 volcanics (Nb/Nb* = 0.7) and it is this group which is the only one that has (Th/La)N ratios less than 1. All samples from Groups 2, 3 and 4 have negative Zr-Hf anomalies between (Zr/Zr*) = 0.6-1.0. All but one of Group 1 samples show similarly large, negative Zr-Hf anomalies while, the one sample which does not, shows a positive Zr-Hf anomaly of (Zr/Zr*) = 1.3.

Fig. 4.31. Nb/Y vs. Zr/Y diagram for the Thessalon Formation samples. Field boundaries and end-member compositions from Condie (2005). Abbreviations as in Fig. 4.8.

On the Nb/Y vs. Zr/Y diagram (Fig. 4.31), the Thessalon Formation samples plot on an array of increasing Nb/Y and Zr/Y within the non-plume sources area of the diagram. Group 1 samples plot on a trend which crosses the NMORB and volcanic arc basalt fields. Group 2 samples produce a fairly diffuse scatter almost entirely

128 within the volcanic arc basalt field. The Group 3 samples define a trend in the overlapping portions of the oceanic plateau and volcanic arc basalts while Group 4 samples plot in a tight cluster in the space between the oceanic plateau, ocean island and volcanic arc basalt fields. On the Zr/Nb vs. Nb/Th diagram (Fig. 4.32), the Group 1 Thessalon volcanics form a moderately tight cluster within the volcanic arc basalt field, very close to the enriched component end-member. The majority of Group 2 samples plot within the overlapping portions of the volcanic arc and oceanic plateau basalt fields. The Group 3 samples fall in the space between the volcanic arc, oceanic plateau and ocean island basalt fields while the Group 4 samples fall just to the left of the volcanic arc basalt field and very close to the composition of the upper continental crust.

Fig. 4.32. Zr/Nb vs. Nb/Th diagram for the Thessalon Formation samples. Field boundaries and end-member compositions from Condie (2005). Abbreviations as in Fig. 4.8.