THE CANADIAN MINERALOGIST
INTRODUCTION
In the present study, I combine fluid-inclusion systematics
with mineralogy to investigate the nature of
hydrothermal fluids that gave rise to pyrite-rich quartz
veins containing small inclusions of millerite, pentlandite,
pyrrhotite, chalcopyrite, chalcocite, coloradoite and
gold at the Cobalt Hill base-metal – gold prospect, ca.
20 km northeast of the Sudbury Igneous Complex (SIC),
Ontario. Because numerous base-metal and preciousmetal
prospects are known in the area, a better understanding
of the source and origin of metal-bearing fluids
at Cobalt Hill could provide important information for
exploration east of the SIC.
Huronian sedimentary rocks are locally enriched in
base and precious metals east of the SIC and Lake
Wanapitei. The origin of these metals is enigmatic and
has been a topic of debate over the last few decades (cf.
Innes & Colvine 1979, 1984, Dressler 1982, Rowell &
Edgar 1986, Gates 1991). Owing to the complex tectonic
and metamorphic evolution of the area, the source
of the metals and the mechanisms responsible for their
mobilization and concentration are difficult to identify.
As high concentrations of metals are commonly associated
with shear zones and faults, and most mineralized
zones are located within brecciated and hydrothermally
altered sediments or sheared gabbroic rocks (cf. Gates
1991, Rowell & Edgar 1986), metamorphism is considered
by these authors to have played an important part
in their mobilization, redistribution and concentration.
My objectives here are: 1) to characterize the fluids
instrumental in the mobilization of base metals and gold
at Cobalt Hill, which represents one of several basemetal
and gold prospects east of the SIC, 2) to compare
the chemical composition and temperature of fluids that
precipitated the quartz–pyrite veins at Cobalt Hill with
fluids reported from the ore zone of the SIC, 3) to suggest
a possible source for the metals, and 4) to identify
the metamorphic event that may have been instrumental
in the mobilization and concentration of metals.
GEOLOGICAL SETTING
Cobalt Hill is located on the northeastern margin of
Lake Wanapitei in Mackelcan Township, Ontario, ca.
20 km northeast of the SIC (Fig. 1). The area straddles
the western margin of the Wanapitei magnetic and gravity
high anomaly. Situated approximately 200 m north
of Jones Lake and 200 m east of Jess Lake, Cobalt Hill
is contained within an intensely microbrecciated and
hydrothermally altered zone, ca. 100 m in diameter. This
small area is part of a northwest-trending, intermittently
FIG. 1. Regional map of Cobalt Hill and the Sudbury Igneous Complex (from Peck et al. 2001).
THE ROLE OF SALINE FLUIDS, COBALT HILL PROSPECT, ONTARIO 1543
albitized breccia zone 450 m long and 600 m wide that
also contains the Jess Lake gold prospect. Gold values
up to 7.5 grams per tonne have been reported from grab
samples at Cobalt Hill, and up to 22.7 grams per tonne
(in a 1.5-m-long drill-core section) from the Jess Lake
gold prospect 60 m west of Cobalt Hill (Flag Resources
Ltd., company reports). Gold mineralization at Cobalt
Hill is coupled with anomalous Ni (0.34%), Co (0.55%)
and Cu (0.09%) values (Goad 1991).
The property contains exposures of Huronian sedimentary
rocks of the Cobalt Group, which represents
the uppermost sedimentary cycle of the Huronian Supergroup.
The sedimentary units consist predominantly
of quartz arenites mixed with minor arkosic quartzites
of the Lorrain Formation. The age of the Huronian Supergroup
is bracketed between that of the Copper Cliff
rhyolites at the base of the Supergroup and the Murray
Granite, at 2.45 and 2.47 Ga, respectively (Krogh et al.
1984, 1996), and the Nipissing gabbro that was
emplaced in the Huronian sedimentary rocks at 2.22 Ga
(Noble & Lightfoot 1992).
The Lorrain quartzites at Cobalt Hill are extensively
brecciated and are cut by numerous quartz veins (up to
3 m wide) that contain and are converted to assemblages
with albite, mica and chlorite. Metamorphic grade in the
area is in the lower greenschist facies. Partial alteration
of the sedimentary rocks to albite is widespread on a
regional scale, extending for hundreds of kilometers; the
western limit has been traced to the Bruce mines, and
the eastern limit to Lake Temagami (Gates 1991). The
age of albitization has been determined from the U–Pb
age of Th-poor hydrothermal monazite in the nearby
Scadding and MacLennan townships at 1.7 Ga (Schandl
et al. 1992, 1994), indicating that this regional hydrothermal
episode postdated the Sudbury Event of 1.85
Ga (Krogh et al. 1984). The 1.7 Ga monazite age corresponds
to a period of granitic plutonism in the Southern
Province, the time of collisional orogeny and the development
of the Killarney Magmatic Belt (Easton 2000).
A map of the area that contains Cobalt Hill is shown
in Figure 2. Samples collected for the fluid inclusion
and mineralogical study include drill-core sections taken
at 640 m depth from the deepest hole (759 m) drilled on
Cobalt Hill (CH92–1), from outcrops adjacent to the
drill hole and from the main waste pile. DDH92–1 represents
one of the twelve drill holes put down at Cobalt
Hill. A schematic cross-section of the drill-hole log for
DDH92–1 is shown in Figure 3.
PREVIOUS WORK
Highly saline fluids have been reported in fluid inclusions
from ore zones along the North and South
ranges of the SIC by Farrow & Watkinson (1992), Farrow
et al. (1994), Li & Naldrett (1993), and Molnar et
al. (1997, 1999, 2001). In their detailed work on fluid
inclusions, these authors characterized the chemical
properties and temperature of the hydrothermal fluids
involved in the mobilization and deposition of some
metals in deposits of the North and South ranges. They
suggested a magmatic origin for the ore-bearing saline
fluids. Temperatures of homogenization (Th halite) of
primary fluid inclusions from the Strathcona Deep Copper
Zone, from Barnet, and from the Fraser Epidote zone
define a range of 180°–250°C (Farrow & Watkinson
1992). In sulfide and precious metal-rich veins at the
Little Stobie deposits, the primary fluid inclusions have
a total Th range of 180°–270°C (orebody 1) and 280°–
350°C (orebody 2) (Molnar et al. 1999). Most primary
fluid inclusions contain halite and a variety of other
daughter minerals, and the salinity of the fluid ranges
from 30 to 50 equiv. wt% NaCl or NaCl–CaCl2 (Farrow
& Watkinson 1992, Farrow et al. 1994, Li &
Naldrett 1993, Molnar et al. 1997, 1999). Molnar et al.
(2001) demonstrated the involvement of high-temperature
fluids (Th (total) 400–500°C) released from a granophyre
in the genesis of the vein-type Cu–Ni–PGE ores
in the North Range. Molnar et al. (1999, 2001) distinguished
between saline magmatic fluids and heated
Canadian Shield brines in the Sudbury ore zones on the
basis of their metal content, and suggested that the presence
of saline fluid inclusions with a high content of
metals could be used an exploration guide for vein-type
Cu–Ni–PGE ores within the footwall of the Sudbury
Structure.
The presence of high-salinity fluid inclusions at the
SIC is not surprising, as in earlier studies of basement
fluids, Fritz & Frape (1982) reported the presence of
saline brines in the Sudbury area. In fact, the occurrence
of saline waters and brines has been known for several
decades within the Canadian Shield, and their
geochemical and isotopic characteristics have been
documented in detail (Fritz & Frape 1982, Frape & Fritz
1987). The brines were encountered at depths exceeding
1 km, and generally occur in shear zones or in pockets,
under high pressure (Fritz & Frape 1982). Although
most saline brines described by these authors were collected
from operating mines, saline brines also have
been identified in granitic plutons in the area at 1 km
depth (Leech & Pearson 1981). The origin of the Shield
brines is controversial, and hypotheses suggested for
their origin include 1) modified seawater or basinal
brines (Kelly et al. 1986, Guha & Kanwar 1987), 2)
removal of saline fluids from fluid inclusions in crystalline
rocks during episodes of high water–rock interaction
(cf. Frape & Fritz 1987, Kamineni 1987), and 3)
equilibrium reactions between low-temperature fluids
and aluminosilicates (Kyser & Kerrich 1990). Kyser &
Kerrich demonstrated with activity diagrams that the
composition of the Shield fluids are controlled by kaolinite,
muscovite and, to a lesser degree, feldspars, and
thus, are distinct from seawater and brines. The isotopic
composition of the Shield brines in the Sudbury area is
unusual, as they have high D values and plot to the left
of the standard meteoric water line (SMWL) (Fritz &
Frape 1982). Kyser & Kerrich (1990) suggested that the
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