Thomas H. Heine, Manitoba Energy and Mines Regional Geologist, Flin Flon
The Earth comprises several discrete layers, roughly similar to the structure of an onion. The crust, the rigid, outer 20 to 40 kilometre thick layer of the Earth, is a very dynamic part of our planet. Plates of this thin, relatively rigid, surface veneer are slowly being moved across the surface of the Earth by convection systems in the plastic mantle that underlies the crust. A simple model for this process would be the foam on a pot of simmering jam slowly moving across the surface towards the cooler edges of the container. As these rigid plates move across the surface of the Earth, they can interact in a variety of ways. If thy collide head-on, huge mountain ranges can result. The Himalayas, currently the largest mountain range exposed on the planet, are the result of the continuing collision if the Indian Eurasian plates. Huge earthquakes are periodically reported from this region, a direct result of this continuing subduction zone, as under the west coast of South and Central America, or slides past each other along a series of transform faults as under the western margin of most of North America. The circum-Pacific Ring of Fire is seismically (earthquakes are common) and volcanically very active because it marks the interaction of the Pacific and marginal continental plates. Volcanoes such as Mount St. Helens and continuing seismic activity along the western margin of North America occur along transform faults associated with the collision of the Pacific and North American plate. Volcanoes also occur where plates are moving apart along spreading centres, as at the Mid-Atlantic Ridge. Iceland, a volcanic island, straddles this feature.
The City of Flin Flon is located on the remains of submarine volcanoes that erupted between 1920 and 1880 million years ago. Crustal plates were smaller then than they are now, and probably moved at a faster rate. The chemical characteristics of the volcanic rocks in the Flin Flon area indicate that they formed near a convergent plate margin, that is, an area where two plates collided to form a subduction zone. As the temperature, causing them to be recrystallized ( a process called metamorphism), faulted and folded, eventually forming a mountain range. Between 1850 and 1830 million years ago, volcanism ceased and the mountain range, now exposed above sea level, began to erode. Erosion and associated weathering are probably very rapid then as land plants had another 1400 million years to evolve before they could provide protection to the Earth's surface.
This tour will examine some of the primary volcanic features that are well preserved in rock exposures in Flin Flon, structural features associated with the folding and faulting of the volcanic sequence, and the features associated with the weathering and erosion of the mountainous terrane following cessation of volcanic activity in the region.
THE WALKING TOUR
Proceed to the corner of Main Street and Fourth Ave. at the south end of the downtown shopping area (stop 1). You will be at the Co-op store. Stay on the north side of Fourth Avenue and proceed east past Hapnot and Hill Street, over Bellevue Ave., veer south (right) past Terrace, then east (left) along Ross Street. At the three-way stop turn north (left) on Hudson Street and proceed to the rock outcrop behind the house at 32 Ross Street.
This is stop 2.
STOP 2. The volcanic rocks in Flin Flon are dominated by basalt that was erupted underwater. Basalt lava has a temperature of about 1200°C, and the water it is erupted into is much cooler, probably no more than 2°C. This caused the surface of the lava to freeze extremely rapidly. The rapid freezing combined with continued movement of the underlying still-liquid rock resulted in the breaking up (brecciation) of the frozen lava flow surface. As the lava advanced it was able to incorporate fragments of the frozen rock into the still liquid underlying flow forming a flow braccia. At this stop you can see interlayered massive, rather featureless rock that represents the main bassalt flow, and heterogeneous rock consisting of basalt fragments enclosed in a matrix of similar composition. This type of rock is a basaltic flow breccia. If you look closely at the basalt you will note millimetre-size rectangular white spots. These are feldspar crystals, a common mineral that formed when the magma started to cool. The geological name for these crystals is phenocrysts, and the basalt can be described as being feldspar-phyric. Individual flows that comprise the volcanic sequences in Flin Flon can be classified on the basis of their phenocryst content: some flows are aphyric (they have no phenocrysts), some are feldspar-phyric, while still others are feldspar-amphibole( a dark colored mineral) phyric. Return to Ross Street and continue down Sipple Hill to 50 Ross Street.
STOP 3. Across the street, along the east side, you can see a white quartz vein. This vein was deposited in a fault zone, in part represented by the valley that Ross Street follows to Third Avenue. The rock behind the fault (to the east) is a basalt flow that contains many gas cavities. We will have another chance to look at amygdules (gas cavities now filled with minerals) at a later stop.
Continue down Sipple Hill along the west side of Ross Street until you get to the first house on the east side (77 Ross Street). Walk across Ross Street (watch for cars!) to the bottom of the north-facing cliff.
STOP 4. If you look closely, the rocks in some of the low outcrops at the base of the hill have a much different appearance than the volcanic rocks that we examined at the first to stops: They are more granulose and occasionally you can see some rounded pebbles. Parts of this metamorphosed sandstone are layered (bedded), with some of the beds defined by thin black bads of a heavy, magnetic mineral, magnetite. This sandstone and associated coarser metamorphosed sedimentary rocks(metasediments), part of the Missi Formation, are the erosional products of the weathering and erosion of the surrounding terrane between 1850 and 1830 million years ago. You have just crossed over a time interval of some 20 million years. Studies of this sequence of metasedimentary rocks indicate that they were deposited by a series of braided streams located at the front of a mountain range. Periodically these streams ever-thickening sequence. In the Flin Flon area the Missi Formation is preserved to the east only as far as Perimeter Drive (Highway 10), the loop around the main residential section of the town.
If you look a short distance to the west of the Missi Formation sandstone you will see a rock showing irregular dark patches. A closer examination will reveal that this is a volcanic rock, part of the sequence that we examined earlier, but its texture is somewhat different form what we saw previously. This mottled rock is part of a regolith, an irregular layer of rock that formed when the volcanic sequence was uplifted above sea level and was subject to weathering and erosion. As the rock was exposed to the atmosphere, soil gradually formed and the basalt became deeply weathered. Because of the absence of plant cover (remember, land plants hand not evolved yet), loose parts of the soil are easily eroded and have not been preserved, but the lower parts of the weathered layer can still be seen in the rocks underlying the Missi Formation.
Continue down Ross Street to Third Avenue. Turn left (west) and go to the top of Ross Hill. As you walk up Ross Hill, notice the large outcrops on your left behind the white fuel tank at the water heating plant. These are all volcanic rocks that show the mottled dark green and brown patches typical of the bottom part of the regolith that we just looked at. Stop at the rock outcrops just past Terrace immediately east of and underlying the house at 1A Third Ave.
STOP 5. We again see a combination of massive basalt lava flows interlayered with flow breccia. Pieces of frozen lava were incorporated into still-molten parts of the cooling flow, reheated and stretched like taffy as the flow advanced.
If you look at the massive flow rocks you will see common oval features a few millimetres in diameter. These are commonly filled with minerals such as white quartz and calcite, or dark green chlorite, and represent gas bubbles that came out of the lava as it crystallized. We know from the presence of these amygdules that the depth of the water that covered these lava flows during their eruption was less than 2 000 metres, and was probably of the order of 500 m. If the depth was greater than this , the water pressure would have been too great to allow the gas to come out of the solution, and would have remained dissolved in the liquid rock. This is the same as releasing the pressure on a pop bottle when you open it: as long as pressure is maintained in the container, the carbon dioxide remains in solution. When the bottle is opened, the pressure is released and the drink fizzes.
Continue west along Third Avenue to the Hundred Stairs. Climb up to the landing approximately half way up the stairs.
STOP 6. The Hundred Stairs are constructed on a cliff that consists of gabbro, an intrusive rock made up of plagioclase ( a type of feldspar) and lesser dark-coloured ferromagnesian minerals. This intrusion is a synvolcanic sill, a body of magma (liquid rock that crystallizes below the Earth's surface, as opposed to lava, liquid rock that crystallizes at the Earth's surface) that was emplaced between layers of lava during volcanic activity. As magma moves through the crust within a volcanic system, it can sometimes be emplaced between the layers of the accumulating volcanic pile, forming sills similar to the one you are looking at. If the magma crosscuts the layers of the sequence, it forms a dyke. Third Avenue joins Callinan Lane, you can see the basalt that the gabbro sill was emplaced into, but the contact between the two rock types is not exposed.
We have now come to the end of this excursion through part of the volcanic and metasedimentary sequence in Flin Flon. We have seen a fault (there are many in the area), but we have not seen other features that reflect the collision of the two plates or deep burial and associated metamorphism of the volcanic and younger overlying metasedimentary rock sequences. Or have we? In a modern volcano, successive lava flows accumulate subhorizontally with the youngest flows at the top of the sequence. You have walked through a series of flows, eventually ending up at the Missi Formation metasedimentary rocks that overlie the volcanic rock. We have travelled upward through the rock sequence. Through geologic forces the rocks have been tilted, and now dip eastward approximately 45°. The geological picture is further complicated because the section we examined was once deeply buried. Compressive forces and elevated temperatures at depth metamorphosed the rocks and caused them to behave in a plastic fashion, recrystallizing them and developing complex fold structures.
Understanding the geology of the Flin Flon area has more than an academic interest. The reason for the existence of this community is the presence of seral zinc-copper ore deposits. Some of these also contain important quantities of gold, recovered as a by-product. It has only been within the last 25 years that a proper understanding of the way these deposits form has been achieved. All of the deposits currently being exploited in the Flin Flon area are hydrothermal (ie. deposited by hot fluids similar to those that occur in hot springs) accumulations of copper, zinc and iron sulphides, as the minerals chalcopyrite and sphalerite, and the iron sulphides pyrrhotite and pyrite. These minerals were deposited at the ocean floor, and can form volcanogenic massive sulphide deposits. Hydrothermal activity often follows volcanic eruptions, and metals can be leached from the surrounding rocks to be concentrated at the point of discharge of the hot spring. Hydrothermal solutions and dissolved metals may also originate from a cooling magma at depth. Economic metal accumulations do not occur in a random or haphazard fashion, but are limited to a single 'layer' representing a narrow time interval in the volcanic sequence.
continue to the top of the Hundred Stairs and proceed west back to Main Street to Stop 7, which marks the end of the tour.
STOP 7. If you are interested in seeing how copper and zinc are concentrated from the ore and the metals extracted from the concentrate, tours of the smelter complex are available from Hudson Bay Mining and Smelting during the summer months, starting about May 6. You must be at least 16 years of age to participate. Reservations are also necessary, so you should call HBM&S a day or so ahead of time at (204) 687-2050. Tours are conducted Mondays, Wednesdays, and Fridays, and depart the main gate promptly at 8:30 AM.
If you would like additional information about the geology of the Flin Flon-Snow Lake region, why not visit the Manitoba Department of Energy Mines office at Room 202 - 143 Main Street. It is open weekdays 8:30 AM until noon and 1:00 until 4:30 PM. Maps and publications are available for reference and sale. Inquiries can be made by calling (204) 687-4221.
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