A detailed mineral paragenesis for the different alteration and vein types is shown in Figure 3.7. The main sulphide minerals are pyrite and chalcopyrite with minor pyrrhotite, sphalerite, galena and molybdenite, whereas the main oxide mineral is hydrothermal magnetite with minor ilmenite. A general zonal pattern occurs with a pyrite ± chalcopyrite ± magnetite ±
113 pyrrhotite-bearing hydrothermal biotite breccia and pyrite ± chalcopyrite ± pyrrhotite-bearing veins and disseminations outward from the breccia unit.
In the biotite alteration assemblage the sulphide paragenesis is complex, but importantly it is consistent through all alteration styles (i.e., disseminations, veins, breccias). Three
generations of pyrite can be distinguished petrographically and are summarized as follows: (1) fine-grained, euhedral to subhedral, inclusion-bearing pyrite which may contain chalcopyrite, magnetite and/or pyrrhotite inclusions (pyrite 1; Fig. 3.20A-C); (2) fine- to coarse-grained, anhedral to subhedral, inclusion-rich pyrite that may contain sphalerite and/or galena inclusions (pyrite 2; Fig. 3.20A, D); and (3) fine-grained, euhedral, inclusion-free pyrite (pyrite 3; Fig. 3.20A,C, D). Rarely, vein pyrite has a skeletal texture. Two stages of chalcopyrite have been documented based its on relative timing with pyrite: (1) early chalcopyrite, which may be intergrown with magnetite, that is documented as pre- to syn-pyrite 1 (Fig. 3.20C); and (2) late chalcopyrite, which may be associated with pyrrhotite, that postdates pyrite (Fig. 3.20E). The sulphide paragenesis demonstrates that gold is closely associated with both the early chalcopyrite ± magnetite stage (Fig. 3.20B) and the later chalcopyrite ± pyrrhotite stage (Fig. 3.20F, G). Gold has not been documented to be associated with pyrite. In addition, gold is syn- to post-
molybdenite, as it occurs along cleavage planes in the molybdenite (Fig. 3.20G).
The paragenesis described above is also found in chlorite and often in muscovite altered samples, which suggests the sulphides were predominantly deposited during biotite alteration. Muscovite and chlorite alteration may, therefore, inherit the sulphide mineralization of the earlier biotite alteration. In addition, the sulphide paragenesis in the deposit area is documented to be the same as with regional east-west trending, sheeted veins outside the deposit.
114 With regards to gold occurrence, both ore microscopy and SEM studies of gold-rich samples from breccia-, vein- or disseminated-type samples rarely provided visible gold or electrum. Thus, the presence of sub-microscopic gold is suggested, either in sulphides or as disseminations. This conclusion is commensurate with the results of laser ablation elemental mapping of similar samples by Gao et al. (2015) that indicated the presence of disseminated, submicron size gold that correlates with biotite and chlorite zones. Importantly this study also revealed that for the samples analyzed, the pyrite was not enriched in gold.
3.9 U-Pb Geochronology
Seven carefully selected samples were used for U-Pb dating to constrain the time of magmatic and hydrothermal events. Three samples (A-C) were analyzed with the high-precision ID-TIMS (Table 3.6) and SHRIMP (Table 3.7) method and a further four samples (D-G) with the LA ICP-MS method (Table 3.8). Photos of the samples used in this study are presented in Figure 3.21 and the concordia diagrams are found in Figures 3.22 and 3.23. A paragenesis with the new ages showing the timing of hydrothermal events with respect to magmatic events is presented in Figure 3.24.
A) Albite altered diorite (Sample E12-360-LK13-117.8; z11115): This sample is an
intensely albite altered diorite cut by a vein of quartz-carbonate-titanite. Titanite occurs as fine- to medium grained disseminated crystals and as fine- to very coarse-grained, subhedral to euhedral vein titanite that are partially pseudomorphed and replaced by yellowish rutile (Fig. 3.21A), both of which were analyzed in this study. The disseminated titanite occurs in the margin of the vein and is interpreted to be secondary in origin. The sample contains abundant light
115 brown to brown, anhedral titanite grains with minor fractures and inclusions (Fig. 3.22D). Four multigrain titanite fractions were analyzed using ID-TIMS techniques (Fig. 3.22A, Table 3.6). A linear regression of all four titanite analyses has an upper intercept of 2737.5 +2.2/-1.8 Ma (MSWD = 0.19, probability of fit (pof) = 0.82), which is interpreted to be the age of the titanite in the highly altered diorite.
In-situ SHRIMP analysis of titanite from this sample were also undertaken (Fig. 3.22B, Table 3.7). A weighted average of the 207Pb/206Pb ages is calculated to be 2747.4 ± 8.9 Ma (MSWD = 0.76, pof = 0.71, n = 15) using only the analyses that are <5% discordance. This date is in agreement with the more precise results from the ID-TIMS analyses described above.
B) Albite altered diorite (Sample E11-107-4.5; z11004): This sample represents
pervasively albite altered diorite which contains abundant secondary yellow-brown rutile crystals that replaced igneous ilmenite (Fig. 3.21B). The rutile grains range from altered opaque to clear, orange-brown anhedral grains with some fractures and inclusions (Fig. 3.22D). The highest quality rutile grains were selected for in-situ SHRIMP and ID-TIMS analysis, but high amounts of common Pb and did not produce reliable SHRIMP results. A number of multigrain rutile fractions were analyzed with ID-TIMS, however, the majority of these analyses are not reliable data as a result of very low uranium contents and high amounts of common Pb incorporated in the grains upon their formation. One multigrain rutile analysis (fraction R1) did, however, yield a reliable age of ca. 2667 Ma (Fig. 3.22C, Table 3.6).
C) Albite and muscovite altered diorite (Sample 205716; z11125): This sample is a
pervasively albite altered diorite containing bright-green fuchsitic mica and abundant secondary rutile crystals after primary titanite. The sample contained slightly cloudy, orange-brown
116 anhedral rutile fragments with some fractures and inclusions (Fig. 3.22D). In-situ U-Pb SHRIMP analyses of the rutile indicate high amounts of common Pb, hence reproducible results were not obtained. Two ID-TIMS analyses of multigrain fractions of rutile did, however, yield an age of ca. 2667 (fraction RA) and a very imprecise age of ca. 2741 Ma (fraction RB) (Fig. 3.22C, Table 3.6).
D) Quartz diorite (Sample E11-95-254B): This is a light- to dark-green, coarse- to very-
coarse-grained, inequigranular quartz diorite with minor apatite and fine- to coarse-grained, euhedral titanite, interpreted to be primary in origin (Fig. 3.21C). A total of nineteen titanite grains from the sample yielded a cluster of concordant to discordant analyses. The weighted mean of the 207Pb/206Pb ages analyses after doing a 207Pb-based correction for common Pb is calculated to be 2744 ± 18 Ma (MSWD = 0.62, pof = 0.89; Fig. 3.23A, Table 3.8). The common Pb composition used for the correction was determined from the Pb evolution model of Kramers and Tolstikhin (1997) at 2744 Ma. This date is interpreted to be the crystallization age of the diorite.
E) Amphibole-titanite-apatite-magnetite vein (Sample E11-116-LK13-172.7): This cm-
sized vein rich in amphibole with minor apatite and magnetite contains fine- to medium-grained, subhedral to euhedral titanite that appears to be part of the primary assemblage (Fig. 3.21D, E). Twenty-six of twenty-eight titanite grains yield a cluster of concordant to discordant analyses. The observed discordance is apparently due to sub-recent weathering, as the data are pulled from the concordia back towards the origin of a standard concordia diagram. The weighted mean of the 207Pb/206Pb ages with 2σ outlier rejection (N = 24 of 26) is calculated to be 2745 ± 3 Ma (MSWD = 0.72, pof = 0.85; Fig. 3.23B, Table 3.8), which is interpreted to be the crystallization age of titanite in the vein.
117
F) Titanite vein (Sample E10-19-154): This sample contains a <1 mm wide titanite
stringer vein that cuts a sheeted quartz-carbonate-pyrite vein in tonalite with a muscovite alteration halo (Fig. 3.21F). Twenty-five of thirty titanite grains yield a good cluster but with very slightly discordant analyses. A concordia age (Ludwig, 1998) for the cluster was calculated as 2736 ± 7 Ma (MSWD = 0.98, pof = 0.32; Fig. 3.23C, Table 3.8), which is interpreted as the age of the titanite vein.
G) Quartz-carbonate-ilmenite-sulphide vein (Sample E12-178-LK13-340.8): This quartz-
carbonate-ilmenite-titanite-rutile-pyrite-chalcopyrite vein, which has a cm-sized albite alteration halo, contains coarse- to very coarse-grained, anhedral to euhedral titanite that partially replaces ilmenite (Fig. 3.21G, H). The titanite is in turn partially replaced by rutile. A total of fourteen titanite grains yield a cluster of concordant analyses with a calculated concordia age of 2745 ± 9 Ma (MSWD = 0.54, pof = 0.46; Fig. 3.23D, Table 3.8) which is interpreted to be the age of titanite crystallization and vein formation.