From a report by Dr.Eva Schandl, consulting geologist research associate, Dep. of geology Uof T.
DISCUSSION AND CONCLUSIONS
(1)
The present fluid inclusion study demonstrates that the fractured and brecciated
quartz veins and qu~rtz-rich pods with pyrite aggregates at Cobalt Hill crystallized from
chloride-rich brines. The significance of chloride-rich brines is, that they mobilize and
carry metals as chloride complexes. Such scenario has been described at the Sudbury
Igneous Complex by Farrow and Watkinson (1992) and Jago et al. (1994); at the
Merensky Reef of the Bushveld Complex by Ballhaus and Stumpfl (1987), at the New
Rambler mine by Wyman et al. (1990) and at the Lac des lies Complex by Tellier et al.
(1991). Results of this study suggest that the fluids that crystallized the quartz veins and
pyrite at Drill hole DOH 92-1 and its vicinity, were similar to late-magmatic flurds
dpcumented at the Sudbury Igneous Complex and at other Ni-Cu-PGE deposits. The
ubiquitous occurrence of small inclusions of gersdorffite, bravoite, pentlandite,
chalcopyrite, pyrrhotite, Hg-telluride (coloradoite) and some gold in pyrite (Table
1) in the quartz veins at Cobalt Hill is 'evidence for the mobilization of metals by the
chloride-rich solutions. The predominance of Ni-sulfides and chalcopyrite inclusions in
pyrite at Cobalt Hill suggests that the source of these fluids must have been Ni and Cu-
rich, whereas the previously reported occurrence of fuchsite in the same quartz + pyrite
veins implies that the source of metals (and Cr in the fuchsite) was probably a mafic-
ultramafic intrusion at some (not too distant) depth. (Schandl, February, 2002).
(2)
Fluid inclusion microthermometry demonstrates the strong similarities between
late-magmatic fluids of the Deep Copper Zone of Strathcona mine and the fluids that
crystallized the Cobalt Hill quartz veins. Homogenization temperature, salinity and the
presence of halite daughter minerals in fluid inclusions in quartz veins at Cobalt Hill are
comparable to the temperature and salinity of fluid inclusions in quartz at the Deep
Copper Zone of the Strathcona mine (Farrow and Watkinson, 1992). In addition, the
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pressure-corrected crystallization temperature for the Cobalt Hill quartz veins is in the
range of 175°- 325°C, and the pressure-corrected crystallization temperature for quartz
in the Deep Copper Zone was estimated at 175° to 280°C by Farrow and Watkinson
(1992). The salinity of the Cobalt Hill inclusions are also comparable to the salinity of
fluid inclusions in quartz at Barnet and at the epidote zone of the Fraser mine (Farrow
and Watkinson, 1992). Table 2 summarizes and compares similarities in temperature
and salinity between the Cobalt Hill fluid inclusions and fluid inclusions from the North
Range deposits of the Sudbury Igneous Complex.
(3)
On basis of the present fluid inclusion and earlier mineralogical study, I consider
Cobalt Hill to have an excellent potential for Sudbury-type mineralization. The quartz +
pyrite veins originated at relatively shallow depths of maximum 1 kb (ca. 3km) (as
estimated from the CO2-H2O fluid inclusions) by chloride-rich brines that carried various
Ni-Cu sulfides and gold, all of which are included in pyrite within the quartz veins. This
depth is comparable to the (shallower) estimated depth of ore-formation for the North
Range (3-8 km: Farrow and Watkinson, 1992). Uplift and erosion must have since
reduced this depth considerably.
The present study demonstrates the existence of a hydrothermal system at
Cobalt Hill that is known to be characteristic of certain types of ore deposits. Metal-
carrying chloride-rich brines are commonly associated with porphyry-type deposits on
the global scale, as well as with Ni-Cu-PGE-type deposits. While a porphyry system
cannot be ruled out at this stage, the occurrelJce of fine-grained Ni & Cu sulfide
inclusions in pyrite would favor Sudbury-type mineralization for the vicinity of Cobalt Hill.
As sodium metasomatism in the area post-dated the quartz veins + pyrite, as well as
brecciation at Cobalt Hill (Schandl, February 2002 report), the quartz veins must be
older than the reported age of sodium metasomatism in the area at 1700 Ma (Schandl
et alo, 1992), but significantly younger than the Lorrain quartzites. |
