jamies essay
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Introduction
Flin Flon, located at the Saskatchewan-Manitoba border, was established in 1927
by the Hudson Bay Mining Company. A Cu-Zn mine was constructed at this time to
exploit massive sulphide deposits discovered by Tom Creighton in 1915. The massive
sulphide deposits were formed by ancient volcanogenic island arcs, which underwent
extreme deformation and metamorphism during the Hudsonian orogeny around 1800 Ma.
Flin Flon field school 408.3 is designed to introduce one to the geological evolution of
the Trans-Hudson Orogen in this area, which produced these Cu-Zn deposits. The
objective of this field school is to become familiar with field mapping techniques with
hard rocks, identification and interpretation of volcanic, plutonic and sedimentary rocks,
identification and analysis of fabrics resulting from deformation and metamorphism, as
well as map production and thin section analysis. These skills are essential for students
to acquire before pursuing a career in geology. This paper outlines the regional geology
found in the Flin Flon domain, as well as the local geology of map area 6.
Regional Geology
During Paleoproterozoic time, an ancient seaway existed in the Churchill
Province of the Canadian Shield referred to as the Manikewan Ocean (Stauffer, 1984).
This ocean separated the Western craton to the northwest, and the Superior province to
the southeast. The Manikewan belt can be divided into two zones: The Cree Lake zone
and the Reindeer Lake zone. The Reindeer Lake zone is broken down into four domains:
the Rottenstone domain, La Ronge domain, Kisseynew domain, and the Flin Flon domain
(Stauffer, 1984). The western section of the Flin Flon domain is the best-studied area,
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and includes 6 major lithologic types: mafic volcanics (Amisk group), pre-tectonic
granitic plutons, pre-tectonic volcanic mafic plutons, coarse clastic sedimentary rocks
(Missi group), syn-tectonic granitic-granodioritic plutons to batholiths, and post-tectonic
granodiorite stocks and dikes (Stauffer, 1984).
The Amisk Group was deposited in or near an island arc in the Paleoproterozoic
Manikewan Ocean. The Amisk formation, which is at least 9,000 m thick, is mostly
composed of andesitic lava flows and pyroclastic deposits (Stauffer, 1974). Dacites,
rhyodacites, basalts and rhyolites are rare but are also found, as well as greywacke
turbidite beds near the top of the formation (Stauffer, 1974). The Amisk formation
demonstrates a continuous transition from submarine, to intermediate, to subaerial
volcanism during Paleoproterozoic time. This evolution is seen in most convergent
oceanic plates of Cenozoic island-arcs (Stauffer, 1984).
Intraoceanic collisions led to crustal thickening and subsequent uplift, causing
erosion of these arc crusts and deposition of fluvial sedimentary rocks of the Missi group
unconformably on top of the Amisk group. The Missi formation ranges up to 2700 m in
thickness and is divided into two upward fining and maturing sequences: the lower
Beaver Dam member and the upper Cliff member (Stauffer, 1990). The composition of
each member ranges from polymictic pebble-to-boulder conglomerates to medium-to-
coarse grained arkosic sandstones, with thin to massive cross bedding present (Stauffer,
1974). The source of 75% of the clasts in the Missi group is originally from the Amisk
volcanics. The Missi group is interpreted as a transition from alluvial fan to braided
stream deposits formed around 1.845 Ga (Stauffer, 1984).
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Afterwards, multiple tectonic collisions of the Flin Flon belt with the Archaen
Sask, Superior and Hearn cratons closed the Manikewan Ocean by 1800 Ma, forming the
Hudsonian orogeny (Stauffer, 1984). This led to three different deformation events,
regional metamorphism (beginning during the 2nd
deformation event), and plutonism
(large granodiorite stocks and small felsic to dioritic dykes and stocks) (Stauffer, 1974).
Local Geology: Mapping Area 6
In mapping area six, the geology trends from non-porphyritic andesitic basalts of
the Amisk in the West, to porphyritic andesitic basalts of the Amisk in the central region,
to Missi sandstone conglomerates in the Eastern section of our mapping area. Also,
numerous coarse grained gabbro boundary dyke intrusions as well as fine grained Flin
Flon dyke intrusions can be found in our mapping area.
In the western region of our mapping area, flow breccia, massive, and pillowed
non-porphyritic andesitic basalts of the Amisk occur. Greenschist metamorphism is
widespread over our entire mapping area, suggesting the original plagioclase, hornblende
and pyroxene composition of the basalts would not have survived, spare a few relict
grains. Therefore, although aphanitic and difficult to determine, the mineral composition
is assumed to be chlorite, actinolite, epidote, albite, and other greenschist minerals.
Several different structures and textures of the Amisk can be found in the western and
central region of mapping area six. These structures and textures all indicate rapid
cooling in a mostly subsea, occasionally subaerial, oceanic island arc environment.
Specifically in the west, hyaloclastics can be found. They are formed when subsea
eruptions of hot lava cause seawater to boil, rapidly releasing vapour and exploding or, in
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some cases, imploding pillows. Due to specific pressure conditions for seawater to boil,
hyaloclastics are evidence for shallower underwater conditions as well as way up
indicators.
In the central region of our mapping area, a shift from non-porphyritic to
porphyritic andesitic basalts is seen. Repeating massive, pillowed, as well as flow
breccia structures indicate trangressive/regressive cycles. Once again, several different
structures and textures found in the central region suggest rapid cooling in a subsea
oceanic island arc environment. The most common structures seen are porphyritic pillows
(see photo below), which are formed in a shallow oceanic arc environment during
eruption of lava flows.
The pillows have very fine grained crystals forming an exterior pillow case texture,
with a coarser grained interior. These chilled margins range up to two centimetres thick.
This is evidence for rapid cooling and shrinking underwater. Sediments and very fine
tuffaceous material infill the interstial crevasses between pillows, but are not immediately
precipitated relative to the pillows. The mineralogical composition of these basalts is
similar to the first mentioned non-porphyritic basalts to the West. The only differences
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are the presence of feldspar or mafic phenocrysts. In this region, feldspar phenocrysts
dominate over the mafic ones. Therefore, due to the presence of phenocrysts, when
comparing these basalts to those in the West it is evident they are from different flows at
different times. The phenocrysts suggest the original composition of the magma was
different, as well as a two stage cooling event to allow the phenocrysts to grow before
erupting as lava. Also, when comparing mafic phenocrysts to feldspar phenocrysts of the
porphyritic basalts, it is evident these are also from two different flows. Another feature
present in these basalts, as well as the basalts to the west, is amygdaloidal vesicles.
These vesicles are caused by gasses being released during cooling of the liquid hot lava,
which later infill with quartz or calcite. Due to gasses rising, large concentrations of
these vesicles occur at the top of flows, indicating a way up direction. Thinly foliated
tuffs can be found to the north and south end of Louis Lake in the central region, which
indicates subaerial exposure at the top of a flow. In the south of the central region fast
cooling is also evident from the presence of columnar basalts. Basalts to the west of
Louis Lake contain mafic phenocrysts. A lava tube and volcanoclastics are also found to
the west of Louis Lake. Felsic clasts within the volcanoclastics range up to ten
centimetres in size.
A shift to Missi sandstone occurs to the east of mapping area six. At the site of
the unconformity between the Amisk and the overlying Missi, spheroidal weathering is
present (see photo below).
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The majority of the Missi formation found within mapping area six is cross bedded
coarse grained sandstone, with small patches of conglomerate layers less than fifteen
centimetres thick. This formation was deposited in a fluvial environment, possibly a
braided stream.
Flin Flon diabase dykes are found in numerous locations throughout our entire
mapping area. These dykes are composed of fine-grained basalts, which are difficult to
separate from the Amisk basalts. The presence of chilled margins aids in distinguishing
between the two.
Within mapping area six, boundary Intrusions are also found intruding the earlier
deposited Amisk formation (see photo below), as well as the Flin Flon dykes. These
intrusions are composed of coarse grained diorite to gabbro with high amounts of
feldspar. The hand sample and thin section description was chosen from a boundary
intrusion at the North end of Louis Lake.
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A reverse fault was found near the Missi conglomerate unconformity between the
central and eastern region of our map area (see map). This fault brought the Amisk unit
up relative to the Missi conglomerate. A number of shear zones with dextral and sinistral
slip were also discovered around Louis Lake. These shear zones were quartz rich and
highly chloritized due to hydrothermal fluids reacting with the ferromagnesian minerals
of the Amisk volcanics (Deer, et al., 1992) (see photos below).
Diorite Boundary Intrusion
Amisk volcanics
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A number of structural and metamorphic measurements were made throughout
our mapping area to help determine bedding, younging direction, cleavage, lineations,
and shear sense:pillow basalts found in the central region gave a younging direction
trending 130and plunging 55, a thinly foliated tuff layer in the southern portion also
gave a bedding direction of 360- 55E, an amygdaloidal rich layer within a three
dimensional outcrop at the north end of Louis Lake gave a bedding direction of 015- 60
E, near the same area cleavage was determined from a chlorite rich pillowed unit to be
02080E with a plunge and trend of 50/150S. Numerous other measurements were
made, and can be found plotted on the map and within our field notebooks.
The sequence of events in the Group 6 map area is as follows.
-Non-porphyritic Amisk volcanics deposited
-Porphyritic Amisk volcanics deposited
Shear zone
Quartz + Chlorite
rich veining in
shear zone
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-Diabase Flin Flon Dykes intruded
-Crustal thickening and uplift causing erosion of volcanics
-Missi sediments deposited unconformably on top of the Amisk
-Boundary Intrusions
-Numerous ductile and brittle deformation events
-Regional greenschist grade metamorphism
Hand Sample
The hand sample was chosen from a boundary dyke intrusion at the north end of
Louis Lake. The intrusion is cross cutting Amisk volcanics and a tuff layer. The sample
taken is a porphyritic coarse grained gabbro, with large actinolite and biotite phenocrysts
up to 2 mm in size present. The original composition of the gabbro would have been
mostly plagioclase, clinopyroxenes, amphiboles (such as hornblende), and biotite. After
regional greenschist grade metamorphism occurred, the composition altered to
greenschist minerals with some relict grains still present. The newly formed greenschist
minerals include actinolite (from pyroxene and hornblende), chlorite (from hornblende
and biotite), albite (from plagioclase), and possibly epidote. The sample is dark green in
color on fresh surfaces, and light beige to light green on weathered surfaces. It is a
suitable representative of the overall boundary intrusion composition.
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Thin Section
Mineral: Percentage: Size Range:
Actinolite 40% 0.251.4 mm
Chlorite 20% 0.050.2 mm
Biotite 15% 0.21 mm
Albite 10% 0.030.5 mm
Cpx 5% 1.0 mm
Zoisite 4% 0.10.5 mm
Opaques 3% 0.10.3 mm
Carbonates 2% 0.10.5 mm
Hornblende 1% 0.51.0 mm
Textures:
-Roesette textureseen in actinolite
-Uralitic texturein clinopyroxenes altering to uralite (actinolite), also creating a
reactionrimwhich is also seen in hornblendes and other original minerals altering due to
metamorphism.
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-Mesh texture seen in almost all minerals, due to deformation and metamorphism.
-Poikiloblastic textureseen in large phenocrysts of clinopyroxene, hornblende, actinolite,
and biotite, with smaller grained infilling of chlorite, albite, and/or actinolite shards.
-Relict grainsare present, mostly clinopyroxene and some hornblende, possibly some
plagioclase which hasnt altered to albite yet?
PPL (Low Power 4x)
Biotite
Chlorite
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XPL (Low Power 4x)
Crystallization History
The original protolith was an intrusive, coarse-grained gabbro boundary intrusion. The
cooling intrusion would have crystallized the opaque sulphides first, followed by the
clinopyroxene phenocrysts, hornblende, and then biotite. At the same time plagioclase
phenocrysts may have formed, and at a later cooling stage, finer-grained plagioclase
filled the groundmass. Afterwards, the rock underwent regional greenschist
metamorphism. This would have altered the clinopyroxene into actinolite, biotite into
chlorite, hornblende into chlorite and actinolite, as well as plagioclase to albite. The
calcium released by plagioclase altering to albite, as well as the aluminum from
pyroxenes and amphiboles would have combined to form the zoisite present. The name
for this rock is an actinolite-chlorite greenstone (lacking schistosity). The protolith for
this rock was a pyroxene-hornblende gabbro boundary intrusion.
Actinolite
Albite
Calcite
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References
Stauffer, Mel. 1974. Geology of the Flin Flon area: A new look at the sunless city.
Department of Geological Sciences, University of Saskatchewan, p. 30-35.
Stauffer, Mel. 1984. Manikewan: An early Proterozoic ocean in central Canada, Its
igneous history and orogenic closure. Precambrian Research, 25 p. 257-281.
Stauffer, Mel. 1990. The Missi Formation: an Aphebian molasse deposit in the Reindeer
Lake zone of the Trans-Hudson orogen, Canada. Geological Association of Canada,
special paper 37, p. 121-141.
Stauffer, Mukherjee , A ., 1971. Superimposed deformation in the Missi meta-
sedimentary rocks near Flin Flon, Manitoba: Can. J of Earth Sciences. Vol. 8, P. 217-242
Stauffer, Mukherjee, A., 1975. The Amisk group: An Aphebian(?) island arc deposit.
Can. J. Earth SCI. Vol. 12, 1975. P. 2021-2035.
Deer, et al., 1992. An introduction to the rock-forming minerals. Pearson Education Ltd.