Jamies Essay

8/13/2019 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.

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Jamies Essay