The geology, geochemistry, and gold metallogeny of the Star Lake area, Northern Saskatchewan
Date
1992
Authors
Journal Title
Journal ISSN
Volume Title
Publisher
ORCID
Type
Degree Level
Masters
Abstract
The Star Lake gold prospect encompasses rocks of the Central
Metavolcanic Belt of the La Ronge Lithostructural Domain in northen
Saskatchewan, Canada. The La Ronge Domain itself is a component of the
Trans-Hudson Orogen, which makes up much of the Proterozoic rocks of
the westen canadian Shield, and is believed to represent an 1850-1900
Ma island-arc sequence. The gold prospect harbours many small gold
occurrences as well as the first gold-producing mine to operate in the
province after a 40 year hiatus.
The David lake Metabasalts, possessing a blastoporphyritic
texture and a mineral assemblage of hornblende + plagioclase + epidote
+ calcite +/- salite, biotite, and quartz, is the dominant
metavolcanic rock in the area. These rocks typically contain, in
variable proportions, phenocrysts of plagioclase, and relict diopside
that are largely pseudomorphed by hornblende. Breccia fragments occur
at various localities throughout this rock. The Rush lake Metabasalt
is composed almost entirely of hornblende with local occurrences of
Mg-chlorite and magnetite, and could represent a metamorphosed
cumulate lava flow, or a fluid discharge zone. The mineral assemblage
in these lavas is typical of arrphibolite-grade metamorphism.
The geochemical analyses of the David lake Metabasalts
indicate the presence of a high-Mg metabasalt and a high-A1
metabasalt. The high-Mg metabasalt contains a high proportion (up to
20 modal per cent) of pseudomorphed clinopyroxene phenocrysts,
suggesting that it may be an ankaramitic or a cumulatic lava flow. It
is believed that the high-A1 metabasalt evolved from a more mafic
magma by clinopyroxene-dominated fractionation coupled with a lack of
plagioclase fractionation. this hypothesis is consistent with the
presence of the clinopyroxene-phyric high-Mg metabasalt. Major- and
trace-element geochemistry of the metabasalts point to an islarid-arc
tholeiite to calc-alkaline basalt affinity, thus supporting the
hypothesis that the La Ronge Domain represents an island arc.
The Mallard lake Volcanogenic Metasediments in the area are
composed of mafic metatuffs, felsic metatuffs, a shoshonitic metatuff,
and rocks that could be sediments derived from erosion of the
volcanic rocks. The mineral assemblage is typically hornblende +
plagioclase + biotite +/- quartz. These rocks differ from the
metabasalts not only in texture but also in their lack of calcite, and
higher modal abundance of biotite, quartz, and sulphide minerals. The
geochemistry of these rocks suggest that they could be equivalents of
the metabasalts or rocks petrogenetically evolved from the
metabasalts.
The compositionally zoned Star lake Pluton that intrudes the
metavolcanic rocks consists of a feldspar porphyry margin, a dioritic
outer zone, and a monzodioritic-monzonitic progression towards the
core. The diorite is an orthocumulate with plagioclase being the
cumulate mineral, and pyroxene, hornblende, biotite, and apatite
having crystallized from the intercumulus liquid. The geochemistry
suggests a calc-alkaline nature to the pluton with the diorite being
metaluminous and the monzonite being peraluminous. Calcic plagioclase
was the major crystallizing mineral but minor sodic plagioclase and K-
feldspar in later felsic magmas also crystallized. Biotite and
hornblende do not appear to have played a major role in the magma's
fractionation history. The geochemistcy also suggests that the pluton
was emplaced in an island-arc setting.
The dikes that intrude the area include hornblendite dikes,
felsite dikes, diorite dikes, and sheared mafic dikes. The
hornblendite dikes were probably originally emplaced as a pyroxene
crystal mush that has since altered to hornblende. The felsite dikes
are highly siliceous dikes made up of quartz + feldspar +/- biotite,
and spessartine, and may have formed by partial melting of basaltic
rocks deep in the arc pile. There are perhaps three generations of
felsite dikes in the area. The diorite and sheared mafic dikes are
most likely the same rock unit with the sheared mafic dikes being more
deformed and altered. Their mineral assemblage is hornblende +
plagioclase + epidote +/- biotite, K-feldspar, and quartz. 'Ihe
mineralogies of the dikes are consistent with the amphibolite grade of
metamorphism, and with that of the metabasalts except there is a
distinct lack of calcite in the dikes. The age relations of the dikes
are not fully understood
Stable isotope analyses of the metavolcanic rocks suggest
that the original isotopic compositions of these rocks have been reset
by progressive metamorphism, perhaps initially involving seawater. Low
SD values suggest exchange with deuterium-depleted water. For the
pluton, original oxygen and hydrogen isotopic compositions of wholerock
and mineral samples have been largely retained suggesting that
the pluton has not been greatly altered, after emplacement, or that
alteration took place at temperatures high enough to allow total
water-rock equilibrium. The dikes have also retained their original
oxygen isotopic compositions but hydrogen data show that they have
been affected by deuterium-depleted waters.
Gold mineralization in the Star lake area occurs in
pyritiferous quartz veins associated with sheared mafic dikes. The
shear zones locallized by the mafic dikes strike NE and dip steeply
NW, and most likely represent D2 structures formed by collisional
tectonism during the Trans-Hudson Orogeny. Alteration in the shear
zones is restricted to the sheared mafic dikes where biotite + epidote
assemblages are fonned at the expense of hornblende. K-feldspar
enrichment is locally present. Enriched levels of Nb, Zr, Y, Rb, and
Th within the surrounding plutonic country rocks are associated with a
few gold occurrences. This is attributed to volmne loss during
alteration, and is associated with tourmaline mineralization.
The gold deposits are apparently localized where NE-striking
D
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Citation
Degree
Master of Science (M.Sc.)
Department
Geological Sciences
Program
Geological Sciences