Download Hydrothermal Alteration Associated with Gold Mineralization at the

Hydrothermal Alteration Associated with Gold Mineralization
at the Giant Yellowknife Mine, NWT
Frank Santaguida* (Carleton University)
present address: Ontario Geological Survey, 933 Ramsey Lake Rd., Sudbury ON, P3E 6B5
[email protected]
H. Falck (GNWT Resources, Wildlife and Economic Development)
K. Krey (Independent Researcher, Calgary, AB)
Systematic studies of hydrothermal alteration within the Yellowknife Greenstone Belt have been
initiated to characterize mesothermal gold mineralization in the Giant Mine area. Funding for this study
has been supported by the Yellowknife EXTECH III project.
The Giant Mine alone has produced over 7 million ounces of Au and the immediate area
contains several past-producing mines as well as several gold prospects. The focus of these studies is
to identify the chemical and mineralogical signatures of hydrothermal alteration imparted on the volcanic
rocks in the area which may be utilized in further exploration programs for gold mineralization
throughout the Yellowknife Supergroup.
Previous work at the Giant Mine has recognized that gold mineralization is associated with
deformation and intense hydrothermal alteration of the mafic volcanic rocks. General alteration mineral
assemblage transitions consist of: (1) a quartz-carbonate-sulphide core that hosts Au mineralization, (2)
a sericite-carbonate-quartz-chlorite schist and (3) a chlorite-carbonate-sericite wallrock mineral
assemblage. These transitions are typically gradational and are variable in size around individual
mineralized zones throughout the mine. The mineralogical transitions typify gold mineralization that
occurs as composite quartz-carbonate veins where alteration selvages to veins are discernible; however,
where gold mineralization coincides with quartz-carbonate-sericite schists and breccias, alteration is
diffuse and distinct mineral transitions are not as obvious. Other exceptions to the general mineralogic
transitions, such as the high grade gold veins at the Brock open pit which are associated with only minor
chlorite-carbonate alteration, further complicate the direct relationship between alteration mineralogy
and gold mineralization at the Giant Mine. Post-mineralization deformation in the Giant Mine area has
also affected alteration mineral textures, but has not been previously considered in evaluating the
chemical trends in the volcanic wallrocks due to alteration.
Outside of the intensely deformed and altered rocks that host gold mineralization, the mafic
volcanic rocks are composed of a chlorite-albite-quartz-actinolite-epidote-carbonate mineral
assemblage that represents a typical seafloor metamorphic assemblage common in subaqueous mafic
volcanic environments. The mafic volcanic rocks are well-preserved and locally contain
unmetamorphosed glass. The relationship between volcanogenic hydrothermal alteration and the gold
mineralization system is not presently understood, but will be a focus for future studies.
Preliminary petrologic and electron microprobe investigations of rocks mainly from mineralized
areas within the Giant Mine reveal complex paragenetic and geochemical relationships between the main
alteration minerals. A quartz-sericite-carbonate mineral assemblage is typically associated with gold
mineralization; however, an asymmetrical distribution of these minerals around some gold-bearing veins
throughout the mine may reflect multiple hydrothermal fluid generations. Textural varieties of sericite
range from fine grained schistose bands containing up to 10% sulphide minerals to distinct quartzsericite veins with only minor sulphides. Green-coloured sericite is obvious in schistose rocks containing
disseminated gold and as selvages to laminated quartz veins bearing free gold. Recrystallization textures
of metallic minerals, mainly pyrite and arsenopyrite, are obvious throughout the mine in both mineralized
and unmineralized areas and suggest remobilization of whole-rock chemical components may be
significant and further support a composite hydrothermal system. Sericite compositions from throughout
the mine demonstrate widespread variations in K-Na and Si-Al. Sporatic enrichments of Fe, Mg, V,
and Cr also occur. Chlorite compositions within the mafic volcanic wallrocks immediately adjacent to
mineralized zones are variable, but are constrained to the ripidolite compositional field. Outside of the
mineralized zones, chlorite is Fe-enriched. Consistent compositions of Si-rich chlorite occur outside of
the mine area in the undeformed mafic volcanic rocks.
Gold mineralization near Gold Lake occurs at the equivalent stratigraphic level as the Giant
orebodies, north of the mine, and is well exposed at surface in a series of trenches. The deformation
zone that hosts mineralization at Gold Lake is discrete in outcrop exposures and contrains the extent of
the alteration zone. Iron-rich carbonate is easily identified by the rust-staining of outcrops around the
trenches. Within the trenches, sulphide mineralization in quartz-carbonate veins is accompanied by
sericite and chlorite. The similarities at Gold Lake in geologic setting, alteration mineralogy, and gold
mineralization to the Giant Mine suggest the smaller hydrothermal systems throughout the Yellowknife
Greenstone Belt exhibit high potential for gold mineralization and warrant further investigation.
Biography
Frank Santaguida is currently a Geoscientist with the Ontario Geological Survey. He obtained his Ph.D.
from Carleton University in Ottawa in 1999. His research studies have concentrated on mapping and
petrochemical studies of volcanogenic massive sulphide deposits as well as lode gold deposits. He has
recently worked in the Notre Dame Bay area, central Newfoundland, the Noranda VMS District,
Abitibi Subprovince and in the Yellowknife Greenstone Belt, NWT.