Gypsum rosettes southern Manitoba pale

Document Sample
Gypsum rosettes southern Manitoba pale Powered By Docstoc
					Gypsum Rosettes in Southern Manitoba: Tools for Environmental Analysis and Public Awareness

J. Young, N. Chow, I. Ferguson, V. Maris, D. McDonald, D. Benson, N. Halden (University of Manitoba)
               Dept. of Geological Sciences, 125 Dysart Road, Winnipeg, MB R3T 2N2,
                        Gaywood Matile (Manitoba Industry, Trade and Mines)

          GeoCanada 2000 - The Millennium Geoscience Summit, Calgary, Alberta, May 2000

ABSTRACT: Gypsum crystals have been found in Quaternary Lake Agassiz clays at numerous
locations in southern Manitoba (Matile and Betcher, 1998). “Ore-grade” concentrations of gypsum
(selenite) rosettes along the Red River floodway have been commercially exploited by AMA Mines.
Several researchers and students from the Department of Geological Sciences at the University of
Manitoba are conducting an ongoing, multidisciplinary project to study the gypsum rosettes at several
localities in southern Manitoba (Fig. 1). In co-operation with Dr. Anne Walker, a teacher at Glenlawn
Collegiate in Winnipeg, chemistry and physics students in Grades 11 and 12 are actively participating in
the research as part of an outreach program in the Department of Geological Sciences.


                                               The proglacial Lake Agassiz succession in southern
                                               Manitoba has been subdivided into three units: Agassiz
                                               Units 1 and 2, unconformably overlain by Agassiz Unit 3
                                               (Teller, 1976). Units 1 and 2 cannot be differentiated at the
                                               investigation sites and are herein referred to as Agassiz
                                               Unit 1&2. Agassiz Unit 1&2 is characterized by dark grey
                                               and brown, massive clay. Silt and clay thin-beds are
                                               observed locally and show soft-sediment deformation.
                                               Sub-vertical fractures crosscut Agassiz Unit 1&2. The
                                               overlying Agassiz Unit 3 is characterized by grey to
                                               brown, thinly interbedded and interlaminated silt and clay,
                                               showing planar and ripple-cross lamination.

                                                  In Agassiz Unit 1&2, gypsum rosettes, up to 10 cm in
                                                diameter, have been found in the upper 10 m in localized
Figure 1. Location map showing Lake             concentrations or associated with fractures; the maximum
Agassiz basin (blue), known locations of        depth of occurrence of rosettes is unknown due to sampling
gypsum rosettes () and an iceberg-scour        limitations. There are also mm-size nodules of finely
location ().                                   crystalline white gypsum that occur near concentrations of
                                                rosettes or in thin beds. In Agassiz Unit 3, gypsum occurs
as fracture-filling or laminated alabaster, small rosettes and individual crystals.

Rosette types and petrography

In this study, three types of gypsum rosettes have been recognized on the basis of habit and spatial
arrangement of the crystals. Type T rosettes are characterized by colourless to pale-yellow, tabular
crystals. These rosettes occur in Agassiz Unit 1&2 and have only been observed to date at the Red River
floodway site where there have been deep excavations for “ore-grade” rosettes. They are most abundant
at ~1-2 m below the contact between Agassiz Unit 1&2 and Unit 3 but they have been collected from
depths up to 10 m. Type L rosettes are characterized by amber to pale-yellow lenticular crystals. These
rosettes have been observed at all sites studied to date (Birds Hill, Red River floodway and Letellier),
                                Gypsum rosettes in southern Manitoba
occurring in fractures near the contact between Agassiz Unit 1&2 and Unit 3. Type B rosettes are solitary
or aggregate balls of finer crystalline, amber to pale-yellow lenticular crystals. They occur in the upper
part of Agassiz Unit 1&2 and in Agassiz Unit 3 at all study sites.

Crystals comprising Type T and L rosettes show irregularly concentric zoning that is readily observable
using epi-fluorescence (epi-FL) microscopy (436 nm, blue-violet filter): an inner, dark-blue fluorescing
zone; and an outer, light-green fluorescing zone, which may show thin, concentric banding (Fig. 2).


Crystals from selected rosettes were analyzed using
inductively-coupled plasma atomic emission spectroscopy
(ICP-AES). The major element chemistry is similar for
both Type L and T rosettes but the concentration of Ca is
significantly less than that expected in stoichiometric
gypsum. Trace abundances of Sr and Na have also been
detected; Type L rosettes have higher Sr and Na
concentrations than those from Type T rosettes.
Furthermore, preliminary analyses using proton-induced
x-ray spectrometry (-PIXE) reveal that the inner, dark-       Figure 2. Epi-FL photomicrograph of
blue fluorescing zone of crystals from both Type T and         crystal from Type L rosette, showing
Type L rosettes have significantly higher Sr concentrations    concentric zoning with an inner (I) and
than the outer, light-green fluorescing zone.                  outer (O) zone.

Oxygen and sulfur isotope analyses of gypsum show variations between the different sites that have been
sampled. Type T and L rosettes from the Red River floodway and Birds Hill sites have 18O and 34S
values for gypsum sulfate that overlap (+2.2 to +8.5‰ SMOW and -14.7 to -12.8‰ CDT, respectively);
Type L rosettes have the highest values. In contrast, Type L and B rosettes from the Letellier site have
18O values that range from -2.5 to -3.8‰ SMOW, with 34S values that are slightly lower than those
from the previous two sites. Within each site, the different rosette types have different isotopic
composition. Nonetheless, based on current data, there appears to be no systematic correlation between
rosette type and location.

The stratigraphic distribution and preliminary geochemical analyses of the gypsum rosettes, in
conjunction with previous ground-water data for Agassiz clays (Day, 1977; Pach 1994), suggest that
gypsum formation was associated with lowering of the piezometric surface, followed by infiltration and
ponding of meteoric water. Isotopic data from the three study sites further suggest an organic sulfur or
sulfide origin for the sulfate, and in situ formation of the sulfate.


Geophysical surveys have been done at the Red River floodway and Letellier sites in order to provide
more information on the genesis of the gypsum rosettes and to assess the application of geophysics as an
exploration tool for new deposits.

The physical properties of the Agassiz units, as well as overlying and underlying units, have been
characterized in a number of geotechnical studies on the Winnipeg area (e.g. Baracos, 1977; Graham and
Shields, 1985). Seismic refraction surveys conducted during the construction of the Red River floodway
(Hobson et al., 1964) defined the seismic velocity response of the different geological units and revealed
considerable variation in the depth to bedrock along the floodway associated with karstification of the
bedrock. Near the floodway site, the bedrock is at 22 m depth but there is a sharp decrease to 12 m depth,
                                 Gypsum rosettes in southern Manitoba
several hundred metres to the north. Previous geoelectric studies in the Winnipeg area have indicated that
the conductivity of the upper several metres of soil is ~100 mS.m-1, and the conductivity of the Agassiz
units is 125-200 mS.m-1.

Surveys at the Red River floodway and Letellier sites. Geophysical surveys at the Red River floodway
site included electromagnetic/electrical (EM31, EM38, DC-resistivity, PROTEM47) and seismic
refraction methods. The seismic refraction results resolved the thickness of the soil layer, the combined
thickness of the clay units, and the thickness of the till, but not resolve the interface between Agassiz Unit
1&2 and Unit 3. Small-scale EM measurements in the excavation pit revealed the conductivity of the
Agassiz Unit 1&2 to be 200 mS.m-1 and that of Agassiz Unit 3 to be 120-150 mS.m-1. Surface DC
resistivity measurements identified three layers corresponding to more resistive soil and Agassiz Unit 3,
conductive Agassiz Unit 1&2, and more resistive till/bedrock.

The most surprising results of the geophysical surveys were the lateral variations in the electrical
response. Surveys using an EM31 in horizontal dipole mode (penetration 3 m) and vertical dipole mode
(5 m), DC resistivity profiling (5-10 m), and time-domain electromagnetics (5-20 m) all revealed a broad
resistive zone containg a more narrow conductive zone coincident with the excavation pit (Fig. 3). These
observations suggest that the location of the deposit may be related to structures within the overlying
Agassiz Unit 3.
                                                                                              Horizontal Vertical
Initial geophysical surveys at the Letellier site have revealed a                          260
conductive anomaly coincident with the gypsum deposit. EM31                                250
readings increase from 100 mS.m-1 in background areas to 250 mS.m-1                        240
in the anomaly. Conductivity readings correlate inversely with, and                        230
may be partially controlled by, a corresponding 75 cm variation in                         210
surface topography. Readings in the dugout, where gypsum rosettes                          200
were collected, are in the range of 190-220 mS.m-1, similar to those                       190
from the excavation pit at the Red River floodway, but less conductive                     180                         180
than the overlying units. At both the floodway and Letellier sites, there                  170                         170

                                                                                                                             Apparent Conductivity (mS/m)
                                                                                           160                         160
is an indication that the location of the gypsum deposits is related to
                                                                                               Excavation Pit
                                                                            Northing (m)

                                                                                           150                         150
linear conductive features in the overlying units.                                         140                         140
                                                                                           130                         130
Interpretation and discussion. Full interpretation of the geophysical                      120                         120
results requires a detailed understanding of the electrical conductivity                   110                         110
                                                                                           100                         100
of the Agassiz units. At the smallest-scale, factors controlling                                                       90
conductivity include porosity, saturation, groundwater salinity, and                        80                         80
clay mineralogy. An initial sampling program at the Red River                               70                         70
floodway site has indicated grain-size and mineralogical variations                         60
across the anomaly. Sampling studies are planned for the Letellier site.                    50
One significant control on lateral variations in the Agassiz clays is                       20
iceberg scouring of Agassiz Unit 1&2. The scouring produced low-                            10
angle faults below the scour trough, which extended at least 3 m below                       0
                                                                                              0 10 20 30 0 10 20 30
the scour, and high-angle normal faults outside the scour (Woodworth-                        Easting (m) Easting (m)
Lynas & Guigné, 1990). The scours can be recognized on the present-                          Figure 3. EM31
day ground surface as linear features that are on the order of 100 m wide                    horizontal and vertical
and have topographic relief of several decimetres. Shallow troughs at the                    dipole mode contours for
margins of the scours have been deepened locally by farmers to control                       the east berm of the Red
surface runoff. The scours appear to influence the near-surface hydrology,                   River floodway site.
and the associated deeper structures, particularly the normal faults, likely
                                Gypsum rosettes in southern Manitoba
influence the deeper hydrological conditions. In the absence of vertical structures, such as faults, the
Agassiz clays form an aquitard, with low hydraulic permeability across the laminae (Baracos et al., 1980).

The geophysical results suggest a relationship between gypsum rosette accumulations and linear
anomalies in near-surface units; iceberg scours can produce such linear anomalies. Hence, we have
started to investigate the geophysical response of iceberg scours in the Agassiz clays. To date,
geophysical surveys have been completed over two scours in the Lorette area that were investigated by
Woodworth-Lynas & Guigné (1990). Electromagnetic survey results show conductivity variations of a
magnitude and form similar to those observed at the gypsum sites.


High-school science education in Manitoba largely neglects the geosciences. To broaden the scope of
high-school science curricula, we have teamed up with interested chemistry and physics students from
Glenlawn Collegiate in Winnipeg to study the gypsum rosettes. The students work with us in an active
research environment. At the same time, Glenlawn Collegiate has developed a program that links their
students with industry, government and academia.

Important issues in running this outreach program are: (1) student participation at an appropriate level –
students are given the broad background of the project and assigned specific, but challenging, research
objectives; (2) “hands-on” use by students of state-of-the-art field and lab equipment and techniques; and
(3) scheduling research sessions that consider the timetables of both the researchers and students.

Over the past year, Glenlawn students have participated in: (1) field work including gypsum sampling and
electromagnetic and seismic surveys; (2) sample preparation for petrography; (3) characterization of the
textural relationships between gypsum and host clays using a binocular microscope and scanning electron
microscope; and (4) identification of clay minerals using powder X-ray diffraction methods.

Shared By:
Description: Gypsum rosettes southern Manitoba pale