The application of oxygen isotopes in silicate rocks and minerals is useful for locating zones of increased hydrothermal alteration associated with fluid mineralizing conduits. Oxygen isotopes in silicate minerals have been utilized in many other studies related to a variety of rock types, ore deposits and geological settings in order to identify areas of increased low-and high-temperature alteration associated with hydrothermal systems (e.g., Taylor, 1974; Muehlenbachs & Clayton, 1976; Huston et al., 1995). The purpose of this chapter is to report and discuss oxygen and carbon isotope results for the Young-Davidson gold deposit, in order to determine the fluid source(s) and conditions responsible for hydrothermal alteration.
Samples were collected for the purpose of characterizing the geochemical trends from least altered to mineralized rocks from different zones of the deposit along two main vertical N-S cross sections, looking west, through the Lucky Zone and Lower Boundary Zone (Y-Y’; Figure 1.6) and the Lower YD Zone (X-X’; Figure 1.6). Veins were avoided during sampling but nevertheless it is inevitable that some of the samples contain small amounts of vein material in them (Martin, 2012). A total of 74 samples from 8 drill holes taken from Lower Boundary Zone (LBZ) and Lucky Zone (LZ) have been analyzed. This consists of 68 samples of syenite, 14 samples of sediments and 2 samples of
lamprophyre. The whole-rock oxygen isotope (δ18OWR) results for LBZ and LZ are listed
investigated; all of these samples are syenite. The whole-rock oxygen isotope results for LYDZ are given in Appendix B.
The lithology, drill-hole traces and sample locations from each cross section are
presented in Figure 4.1. Cross sections were created using a 45m window on either side (90m total slice width) to allocate for sampling of drill core above and below the mineralized zone.
Figure 4.1: Geology of N-S vertical cross sections at (A) 23240mE and (B) 22790mE looking west. Drill holes on these cross sections intersect mineralization in the Lower YD Zone, Lucky Zone and the Lower Boundary Zone, as labeled. Drill hole surface traces (plan view) are shown above each cross section with the distances east and west of each section designated by the negative and positive distances, respectively. The trace of mineralization is shown in yellow and defined by 3D modeling by Young-Davidson geologists during resource modeling. Grid spacing is 100m. Ground surface is at
approximately 10300z; these cross sections do not intersect the surface (reproduced from Martin, 2012).
The whole-rock δ18O values (δ18OWR)of LZ and LBZ in the syenite range from
9.2 ± 0.2 to 11.3 ± 0.2 ‰ and in sedimentary rocks the δ18OWR values range from 9.8
± 0.2 to 11.1 ± 0.2 ‰ (Figure 4.2). The δ18OWR values in the LYDZ exhibits a range from
8.2 ± 0.2 to 10.7 ± 0.2 ‰, which are 1.0 ± 0.2 ‰ lower compared to the syenite in LZ
and LBZ (Figure 4.2). This difference might be due to a change in mineral assemblages as petrographic studies of 3 samples that display δ18
OWR =8.2 ± 0.2 ‰ (2 samples) and
δ18
OWR = 8.6 ± 0.2 ‰ (1 sample) contain tremolite alteration; all these 3 samples are
mineralized. In general most of the samples taken from the LZ and LBZ contain quartz,
which is likely responsible for the observed higher δ18OWR values.
Figure 4.2 displays δ18
O values, K2O concentration, sulfur concentration, chlorite
alteration and biotite alteration contours along two N-S cross sections. Only syenite data is contoured in all figures. The location of each cross section was selected so that each zone of mineralization was intersected for study. The 23240mE cross section intersects mineralization in the LZ and LBZ located in the central portion of the deposit whereas the cross section through 22790mE intersects mineralization in the LYDZ at the west end of the deposit (Martin, 2012). The data presented in Figure 4.3 includes the ore body as modeled by AuRico geologists (Edmunds pers. comm., 2008). Sample points have been plotted using Surfer ™ (Version 10) and projected to the 2D cross section surface, they were contoured using the Kriging interpolation method with search ellipse: anisotropism = 2 and slope = 75-80˚, after parameters given in Martin (2012). The specified search ellipse was arranged to interpolate data up-dip between drill holes following the syenite contact and the zone of mineralization in principal as indicated by the ore trace (pale yellow). As in Martin (2012), Kriging parameters were used to orient the long axis of the
search ellipse along the direction of the least data density, selecting between adjacent drill holes rather than along the drill holes. This process was used to generate contours that are prolonged up-dip, paralleling the zone of mineralization and preferentially searching from neighboring drill holes (Martin, 2012). Martin (2012) created the geology, K2O
wt% and sulfur wt% contours. Oxygen isotope data, chlorite alteration and biotite alteration are created in this study.
Figure 4.2 (displayed on page 59): Whole-rock δ18O values, chlorite alteration, biotite alteration contours, K2O concentration and sulfur concentration, along two N-S cross
sections at Young-Davidson. Geology, K2O wt% and sulfur wt% contours are from
Martin (2012), oxygen isotope data, chlorite alteration and biotite alteration contours are from this study. A) 23240mE intersecting LZ and LBZ mineralization. B) 22790mE intersecting LYDZ mineralization. Ore trace is shown in yellow on all figures and only syenite data is contoured in all figures.
There is a strong correlation between the presence and absence of chlorite
determined by XRD patterns and δ18OWR values in the LZ, LBZ and LYDZ (Figure 4.2).
There is a chlorite-boundary that corresponds to a δ18OWR value of 10.3 ± 0.2 ‰. Where
chlorite is absent the δ18OWR values are higher than 10.3 ± 0.2 ‰, and where chlorite is
present the δ18OWR values are lower than 10.3 ± 0.2 ‰. Of the 71 whole-rock samples
only 8 do not follow this trend. It should be noted that chlorite abundances of less than approximately 1% cannot be detected by XRD and so trace chlorite may be present in some samples that are described as chlorite absent. It should also be noted that at constant temperature and fluid composition constant, chlorite alteration would decrease the
δ18
OWR values (Taylor, 1978).
The LBZ in Figure 4.3 shows a correlation between low δ18O values and both elevated K2O concentration and high sulfur concentration. These regions spatially
overlap zones of gold mineralization, outlined in the yellow ore trace. Similarly, the LYDZ in Figure 4.3 demonstrates the relationship between δ18O lows, increased biotite alteration, high K2O content, and high sulfur concentration in the mineralization zone.
The Lower Boundary Zone and Lower YD Zone display δ18O values of 8.2 to 10.8 ‰ (Appendices A and B). These values and the oxidized nature of the alteration suggest that the fluids are either meteoric or magmatic waters. The combination of low δ18
O values (higher temperature) associated with increased potassic alteration, high sulfur concentrations (associated with elevated pyrite abundance) and also with areas of Au mineralization, suggests that these fluids have a magmatic component.
The δ18O whole-rock values in the Lucky Zone are higher, 10.5 to 11.3 ‰
(Appendix A). This most likely reflects lower temperatures and/or a greater influence of metamorphic fluids. Martin (2012) concluded that the Lucky Zone is vein-dominated and there is less potassic alteration. The Lower YD Zone displays a gradual change in whole- rock δ18
Ovalues in syenite and the δ18Ovalues in the Lower YD Zone are lower compared to the Lower Boundary Zone and Lucky Zone.