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Western Kentucky UniversityTopSCHOLAR®Honors College Capstone Experience/ThesisProjects Honors College at WKU
5-13-2015
Geothermobarometry and PetrographicInterpretations of Christensen Ranch Meta-Banded Iron Formation from the Ruby Range,MontanaJacob HughesWestern Kentucky University, [email protected]
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Recommended CitationHughes, Jacob, "Geothermobarometry and Petrographic Interpretations of Christensen Ranch Meta- Banded Iron Formation from theRuby Range, Montana" (2015). Honors College Capstone Experience/Thesis Projects. Paper 580.http://digitalcommons.wku.edu/stu_hon_theses/580
GEOTHERMOBAROMETRY AND PETROGRAPHIC INTERPRETATIONS OF
CHRISTENSEN RANCH META- BANDED IRON FORMATION FROM THE RUBY
RANGE, MONTANA
A Capstone Experience/Thesis Project
Presented in Partial Fulfillment of the Requirements for
the Bachelor of Science Degree with
Honors College Graduate Distinction at Western Kentucky University
By
Jacob Hughes
*****
Western Kentucky University
2015
CE/T Committee: Approved by
Professor Andrew Wulff Ph. D, Advisor
Professor Fredrick Siewers Ph. D ________________________
Professor Clay Motley Ph. D Advisor
Department of Geography &
Geology
Copyright by
Jacob Hughes
2015
ii
ABSTRACT
The Ruby Range in southwestern Montana was tectonically uplifted during the
mountain-building event of the Big Sky orogeny. Contained within the Christensen
Ranch Formation, made up of sedimentary units metamorphosed during the Big Sky
orogeny, are a small number of metamorphosed banded iron formations (BIFs). It is the
presence and composition of these BIFs which is the focus of this research, principally
their depositional origin, relationship to surrounding sedimentary packages, elemental
and mineral compositions, and lastly, peak metamorphic conditions. Samples were
collected, cut into thin sections, and powdered for textural and compositional analyses
employing polarized light microscopy, Raman microscopy, scanning electron
microscopy, X-ray diffraction and electron microprobe analyses. Mineral phase
assemblages and compositional data suggest differences in protoliths between Stone
Creek and Iron mine metamorphosed BIFs in addition to greater activity of fluid phases
during the metamorphism of Iron Mine formations.
Keywords: Geology, Montana, Ruby Range, Big Sky orogeny, Metamorphosed Banded
Iron Formation, Geothermobarometry
iii
ACKNOWLEDGMENTS
The author would like to thank the Keck Geology Consortium for the incredible
opportunity, Julie Baldwin (University of Montana) and Tekla Harms (Amherst College)
for their guidance and patience throughout the course of the project, the other members of
the “Ruby Rangers” for their help and camaraderie, Andrew Wulff (Western Kentucky
University) for his tremendous instruction and continued support, as well as John
Andersland (Western Kentucky University) and Dave Moecher (University of Kentucky)
for their expertise and kindness.
iv
VITA
2015…………………………………….. Bachelor of Science, Western Kentucky
University, Bowling Green, Kentucky
2014…………………………………….. Research Experience for Undergraduates, Keck
Geology Consortium
2010…………………………………….. Greenwood High School, Bowling Green,
Kentucky
PUBLICATIONS AND PRESENTATIONS
Hughes, J.M., Wulff, A.H., 2015 Geothermobarometry and Petrographic Interpretations
of Christensen ranch Meta- Banded Iron Formation from the Ruby Range,
Montana: Keck Geology Consortium Spring Symposium Journal.
Hughes, J.M., Wulff, A.H., 2015 Geothermobarometry and Petrographic Interpretations
of Christensen ranch Meta- Banded Iron Formation from the Ruby Range,
Montana: WKU Student Research Conference, Bowling Green, KY,
Hughes, J.M., Wulff, A.H., Garmon, W. T., Higgins, B., 2014, A Comparison of
Kentucky Mississippi Valley Type Deposits: Geological Society of America
Annual Meeting, Vancouver, BC,
Hughes, J.M., Wulff, A.H., Garmon, W. T., Higgins, B., 2014, A Comparison of
Kentucky Mississippi Valley Type Deposits: WKU Student Research Conference,
Bowling Green, KY,
FIELD OF STUDY
Major Field: Geology
v
TABLE OF CONTENTS
Page
Abstract…………………………………………………………………………………... ii
Acknowledgments………………………………………………………………………. iii
Vita………………………………………………………………………………………. iv
List of Figures…………………………………………………………………………… vi
List of Tables………………………………………………………………………….... vii
Chapters:
1. Introduction……………………………………………………………………….. 1
2. Literature Review…………………………………………………………………. 6
3. Methods………………………………………………………………………….. 15
4. Results…………………………………………………………………………… 20
5. Conclusion………………………………………………………………………. 32
References………………………………………………………………………………. 35
vi
LIST OF FIGURES
Figure Page
Figure 1. Map of the Ruby Range and Surrounding Ranges…………………………….. 2
Figure 2. MMT and BBMT within the Wyoming Craton……………………………….. 7
Figure 3. Tectonic Evolution Model for the MMT……………………………………... 10
Figure 4. General Stratigraphy and Structure of the Ruby Range……………………… 12
Figure 5. Reference map of sample 14-JH-14………………………………………….. 18
Figure 6. SC BIF field photo…………………………………………………………… 21
Figure 7. Actinolite exsolution/intergrowth lamellae…………………………………... 23
Figure 8. Grunerite exsolution/intergrowth lamellae…………………………………… 23
Figure 9. SEM traverse of amphibole exsolution from sample 14-JH-6……………….. 24
Figure 10. SEM back-scatter image of sample 14-JH-6………………………………... 24
Figure 11. IM BIF field photo #1……………………………………………………….. 27
Figure 12. IM BIF field photo #2……………………………………………………….. 28
Figure 13. Photomicrographs of representative mineral species……………………….. 30
Figure 14. EDS spectrum from 14-JH-6 magnetite…………………………………….. 31
Figure 15. EDS spectrum from 14-JH-13d pyroxene…………………………………... 31
Figure 16. EDS spectrum from 14-JH-18b garnet……………………………………… 31
vii
LIST OF TABLES
Table Page
Table 1. XRD results of select samples..……………………………………………….. 25
Table 2. Estimated modal percents from sample thin sections.………………………… 30
1
CHAPTER 1
INTRODUCTION
Iron formation is defined as: a chemical sediment, typically thin bedded or
laminated, whose principal chemical characteristic is an anomalously high content of
iron, commonly but not necessarily containing layers of chert (Klein, 2005; James, 1954;
Trendall, 1983). The metamorphosed Banded Iron Formations (BIFs) found in the Ruby
Range and the surrounding Tobacco Root and Highland Mountains account for a
volumetrically small portion of a metasupracrustal (metamorphosed rocks deposited
directly on basement rocks) package (herein referred to as the Christensen Ranch suite) in
the Ruby Range. The presence and composition of these BIFs is the focus of this
research, principally their depositional origin, relationship to surrounding sedimentary
packages, elemental and mineral compositions, and lastly, peak metamorphic conditions.
Located in southwestern Montana near the town of Dillon (Figure 1), the Ruby
Range was formed by a mountain-building event in the Proterozoic which has been
termed the Big Sky or Trans-Montana orogeny (Brady et al., 2004; Cheney et al., 2004;
Alcock and Muller, 2012; Harms, et al., 2004; Roberts et al., 2002). The Ruby Range and
surrounding ranges comprise the Montana Metasedimentary Terrane (MMT) in the
northwestern portion of the Wyoming Province (Mogk et al., 1992, Mueller et al., 1993)
and are associated with Proterozoic orogenic activity. Farther to the east is the Beartooth-
Bighorn Magmatic Terrane (BBMT) believed to be arc-granitoids intruded
2
Figure 1. Map of the Ruby Range and Surrounding Ranges. Taken from Immega and
Klein, 1976.
3
into Archean crust (Roberts et al., 2002; Wooden and Muller, 1988). Three
metasedimentary units have been identified within the MMT (James, 1990). The criteria
for this categorization is largely predicated upon the presence of marble and iron
formation, as exhibited by the younger Christensen Ranch suite, or the absence thereof,
identified in regional basement as the Pre-Cherry Creek suite (Alcock and Muller, 2012).
The third group found between the two is the Dillon Gneiss which, while poorly
characterized, is known to include abundant biotite gneiss, quartzofeldspathic gneiss,
metagranite, as well as localized marble and amphibolite (James, 1990). These general
unit definitions were established based on structural relationships in the Madison and
Gravelly ranges where they were first recorded (Alcock and Muller, 2012; Erslev, 1983;
Erslev and Sutter, 1990; Hadley, 1969), but have been adapted and correlated throughout
the region.
Metamorphosed Banded Iron Formations (meta-BIFs) in the region are found
interspersed throughout inferred metasupracrustal sequences containing amphibolite,
metapelite, calc-silicate, marble, and quartzite. This Christensen Ranch metasupracrustal
suite has been identified as a part of the Cherry Creek sequence (Dahl, 1979; Garihan,
1973; James, 1990; Karasevich et al., 1981), while the Pony Group of the Tobacco Root
Mountains is roughly correlative with the Pre-Cherry Creek Group in the Ruby Range
(Wilson and Hyndman, 1990). The exact paleo-environmental setting has been contested
within the literature for some time. Disagreement between an accretionary wedge, fore-
arc depositional model and a passive margin, back-arc or intra-arc basin model has fueled
debate on the subject (Wilson and Hyndman, 1990), though the depositional and tectonic
history of southwestern Montana is likely even more complex.
4
Harms et al. (2004) present a cogent model for the region’s tectonic evolution on
the of basis exhaustive field and analytical work involving samples from the near-by
Tobacco Root Mountains. They describe the tectonic evolution as the product of partial
cratonic break-up followed by two successive mountain building events separated by
approximately 700 million years. The evidence put forward in support of this model is
some of the most compelling to date, encouraging the adoption of this early back-arc or
supra-arc deposition followed by a later arc collision model for the purposes of this
research (See Harms et al., 2004).
Previous work in the region (Immega and Klein, 1976) found iron formation
metamorphism ranging from 650-750°C and 4-6 kbar in the Tobacco Root Mountains,
temperatures from 500-600°C in the Carter Creek area of the Ruby Range and <400°C
and 2-4 kbar in the Ruby Creek area of the Gravelly Range. Additional work involving
samples taken predominately from the Kelly and Carter Creek iron formations of the
Ruby Range indicate that peak metamorphic conditions were 7.2 +/- 1.2 kbar, 745 +/-
50°C and 6.2 +/- 1.2 kbar, 675 +/- 45°C, respectively (Dahl, 1980).
This thesis is the product of fieldwork conducted during the summer of 2014 as
part of a Keck Geology Consortium Research Experience for Undergraduates. Samples
were collected from various meta-BIF locales within the Ruby Range for analysis using
polarized light microscopy (PLM), Raman microscopy, X-ray diffraction, (XRD),
scanning electron microscopy (SEM), and electron microprobe (EMP). Results from field
observations and various analytical methodologies named above have been collected with
the principal goals of textural and mineralogical characterization as well as constraining
peak metamorphic conditions experienced by meta-BIFs observed in both the previously
5
unstudied Stone Creek and well-documented Iron Mine (often referred to as “Carter
Creek”) localities within the Christensen Ranch. The data and conclusions presented in
this research aim to corroborate and augment existing work conducted on
metamorphosed iron formations in the region. The Ruby Range and the Big Sky orogeny
are of particular tectonic interest because the subsequent uplift of basement rocks in the
region offers a unique glimpse into the processes at work behind the amalgamation of
what went on to become the North American craton, the basement of the continent. The
area in and around the Ruby Range serves as an excellent natural laboratory for
modelling Archean and Proterozoic tectonism while further elucidating the conditions of
the Big Sky orogeny for the benefit of a greater scientific understanding of regional and
meso-scale metamorphism.
6
CHAPTER 2
LITERATURE REVIEW
Geologic Setting
The Ruby Range owes its name to localized occurrences of alluvial garnets
mistaken by early prospectors as rubies (Sisson, 2007). The Ruby Range and surrounding
Tobacco Root, Gravelly, Highland, Madison, and Greenhorn Mountains of southwestern
Montana lie in a region known as the Montana Metasedimentary Terrane (MMT). The
MMT is located on the northwestern flank of the Wyoming Province, an Archean craton
stabilized approximately 2.6-2.7 Ga (Harms et al., 2004, Houston et al., 1993; Frost and
Frost, 1993; Frost, 1993; Dutch and Nielson, 1990; Hoffman, 1988). The Beartooth-
Bighorn Magmatic Terrane (BBMT), named after the tonalite–trondhjemite–granodiorite
assemblages exposed in the Beartooth and Bighorn Mountains, lies to the east of the
MMT toward the center of the of the craton (Roberts et al., 2002) and contains zircons
dated to 2.9 Ga and greater (Foster et al., 2006). These spatial relations are shown in
Figure 2. The Wyoming Province underlies parts of Wyoming and Montana and is bound
by the Great Falls tectonic zone to the northwest (O’Neill and Lopez, 1985), the
Medicine Hat block to the north (Roberts et al., 2002; Harms et al., 2004), and the
Archean Wyoming Greenstone province (Hoffman, 1988; Sisson, 2007) to the south. The
western boundary is poorly defined due to Laramide and Tertiary events (Sisson, 2007).
7
Figure 2. MMT and BBMT within the Wyoming Craton. Taken from Harms, et al., 2004.
8
Giletti (1966) collected samples throughout the region for whole rock, biotite,
muscovite and K-feldspar K-Ar age-dating (Roberts et al., 2002) and arrived at the
conclusion that there were two distinct domains present within the MMT: one in which
no age greater than 1.8 Ga was identified and the other wherein ages were found to be
consistent with established Archean (approximately 2.6 Ga) ages for the area. Giletti’s
work shed new light on the uplift of the MMT which was previously thought to have
been uplifted solely by an identified 2.45 Ga orogenic event known as the Tendoy or
Beaverhead orogeny (Cheney et al., 2004; Baldwin et al., 2014; Jones, 2008). The two
domains are separated by a northeast-trending line now referred to as “Giletti’s Line”
(Harms et al., 2004). This geochronological discrepancy was interpreted to be the result
of a previously unrecognized tectono-thermal event characteristic of an orogeny which
reset K-Ar “clocks” on one side of the line, but left a hypothesized foreland to the
southeast unaffected (Erslev and Sutter, 1990; O’Neill, 1998; Harms et al., 2004). The
MMT is now interpreted to represent two separate orogenic events: the older
Tendoy/Beaverhead orogeny and the younger Big Sky or Trans-Montana orogeny (Brady
et al., 2004; Cheney et al., 2004; Alcock and Muller, 2012; Roberts et al., 2002) separated
by approximately 600 to 700 million years.
Wilson and Hyndman (1990) favor a fore-arc accretionary wedge model to
explain the tectonic setting of the Montana Archean assemblage. They point to the
Mesozoic Franciscan Formation of California as an analog for the assemblages observed
within southwestern Montana. The Franciscan Formation, characterized as a large marine
accretionary wedge, is comprised of sediments which are compositionally and
volumetrically comparable to similar rocks exposed within the MMT. Wilson and
9
Hyndman (1990) detail the lithologies of the Franciscan Formation as dominantly
greywackes with various pillow basalts, cherty limestones, and black shale. They contend
that the relative uniformity of lithologic and structural features, as well as the notable
absence of ophiolitic sequences of the MMT basement cannot be the product of a back-
arc setting. They posit that thick deposits of terrestrial clastics, pelagic sediments and
marine turbidites characteristic of a fore-arc setting conform well to the observations
made concerning the MMT. The discrepancy between the typical blue-schist grade
metamorphism common to an accretionary wedge setting and the upper-amphibolite
grade metamorphism of southwestern Montana in this model is attributed to higher
thermal gradients during the Archean. While Wilson and Hyndman’s proposal offers
unique insight into disentangling vexing paleo-environmental questions, it falls short in
explaining the multiple tectonic episodes experienced by the region and the events
leading up to them.
In an attempt to reconcile the complexities of the paleo-environmental setting and
tectonics of the northwestern Wyoming Province, Harms and colleagues (2004) have
proposed a new model for the tectonic history of the region (Figure 3). An initial paleo-
environmental setting of inferred Pre-Cherry Creek units is postulated as a back-arc or
supra-arc basin within the Wyoming Province from which arc protoliths are generated
from 3.55 to 3.2 Ga and deposited between 3.13 and 2.85 Ga. After this phase the
Tendoy/Beaverhead orogeny occurs during a collisional event at approximately 2.54 Ga.
Rifting to the northwest at around 2.06 Ga causes the subsequent injection of
metamorphosed mafic dikes and sills (MMDS). The resultant ocean basin was later
10
Figure 3. Tectonic Evolution Model for the MMT. Model proposed based on
observations and data from the Tobacco Root Mountains taken from Harms, et
al., 2004.
11
closed between 2.0 to 1.8 Ga. Arc sediments from a terrane exotic to the Wyoming
Province were then accreted to the craton between 1.78 and 1.72 Ga. Harms et al. (2004)
substantiate their claims by citing extensive zircon and monazite age dating (Cheney et
al., 2004), to accurately constrain the timing of events, REE profiles of MMDS samples
consistent with modern continental rifting (Brady et al., 2004), and thorough examination
of unit contacts and structural features.
Although structural complexity, discontinuous and poor exposure, and locally
defined lithologies (Wilson and Hyndman, 1990) have hampered stratigraphic correlation
within the ranges of the MMT, three general subdivisions have been widely accepted
within the literature although some contentions have been raised about established
naming conventions (see Wilson and Hyndman, 1990; James, 1990). These are, moving
up stratigraphic sequence, the Pre-Cherry Creek Group, the Dillon Gneiss, and the
Christensen Ranch suite (James, 1990) all of which comprise three northeast trending
belts (Figure 4) within the Ruby Range (Sisson, 2007). The Pre-Cherry Creek suite
consists largely of various gneisses and schists interlayered with minor quartzite (James,
1990) and a lack of marble (Alcock and Muller, 2012). Dillon Gneiss is comprised of
biotite and quartzofeldspathic gneiss, metagranite, and localized occurrences of
amphibolite and marble (James, 1990). James (1990) also notes that characterization of
the Dillon Gneiss is lacking to refine a unit definition. James also asserts that a
sedimentary or volcanic origin is, as of yet, unclear. Lastly, the Christensen Ranch suite
contains interbedded metapelites, quartzites, amphibolites, meta-BIFs, marbles, calc-
silicates and small ultramafic pods.
12
Figure 4. General Stratigraphy and Structure of the Ruby Range. Taken from Harms
(Proc. Keck Symp., 2015).
13
Extensive characterization and analysis of meta-BIF in the region has been carried
out by the likes of Immega and Klein (1976) and Dahl (1980). Immega and Klein (1976)
describe typical iron formation occurrences throughout the Tobacco Root, Gravelly, and
Ruby Ranges as thin, discontinuous, banded, lenticular bodies. Their work was derived
from samples collected during the summer of 1973 from which over 70 thin sections
were created for extensive petrographic characterization, as well as microprobe, and bulk
chemistry analyses. Samples were taken near Copper Mountain in the Tobacco Root
Mountains, from within the Ruby Creek area of the Gravelly Range and from the Carter
Creek area in the Ruby Range. Electron Microprobe data combined with computer
modelling of Fe2+
/Fe3+
in amphiboles, pyroxenes, and garnets, as well various
geothermobarometers (see Immega and Klein, 1976 for specific models used) were used
to arrive at peak pressure and temperature conditions. Through these methods they report
that likely metamorphic conditions were approximately 650-750°C and 4-6 kbar for
Tobacco Root BIFs, temperatures of about 600-650°C in the Carter Creek, Ruby Range
BIFs, and around 400°C and 3-4 kbar in the Ruby Creek area of the Gravelly Range.
Immega and Klein cite the potential effects of PH2O on calculated temperature differences
between the Tobacco and Ruby Ranges.
Dahl (1980) examined six metamorphic lithologies including meta-BIFs from two
locales within the Ruby Range: the Kelly region in the northeastern part of the range as
well as samples taken from the aforementioned Carter Creek locality. Dahl’s work was
centered on the distribution of iron and magnesium between garnet and pyroxene pairs.
Electron microprobe data from 13 mineral pairs were collected. This compositional data
was combined with known distribution coefficients of iron and magnesium into various
14
mineral structures, often expressed as an elemental KD, for calculations based on
established garnet-clinopyroxene geothermometers developed by Saxena (Saxena, 1976),
Ganguly (Ganguly, 1979) and others. Dahl reports that calculations predicated on these
geothermometers yield anomalously high metamorphic conditions for the study areas.
Refinement of existing work by Dahl’s incorporation of thermodynamic equations
designed to account for the effects of calcium and manganese substitution in garnets
yielded metamorphic conditions of 745 +/- 50°C, 7.2 +/- 1.2 kbar in the Kelly area and
675 +/- 45°C, 6.2 +/- 1.2 kbar for Carter Creek samples. Linear regression models served
to corroborate these new results and their use as a relative geothermometer that conforms
more closely to known KD temperature dependences. Dahl explains the importance of
larger data sets within the region and the need to account for the possible effects that
other elemental substitutions within pyroxenes may have on KD.
15
CHAPTER 3
METHODS
Sample Collection
Sample collection occurred during the summer of 2014 as part of a Keck Geology
Research Experience for Undergraduates (REU). The procedure for sampling involved
the division of the seven REU participants into working groups (typically 2 to 4 students)
supervised by one of two Keck project leaders. First, exploratory traverses were
conducted perpendicular to strike in order to locate units of interest as shown on local
quadrangle maps. Once target units were located, traverses were then conducted along
strike of units of interest and samples were collected from well-exposed areas and/or
areas characterized by textural or compositional differences. Sample positions were
recorded on a geologic map along with corresponding GPS coordinates for each sample.
Each sample was assigned a number and a sequential alphanumeric digit as necessary to
denote multiple samples collected from an individual locale. Detailed notes concerning
bed thickness in outcrop, textural observations, and adjacent stratigraphic units were
compiled for each sample. Oriented sampling was not conducted due to difficulties
induced by BIF magnetism. Sample collection was accomplished with chisels and
sledgehammers, which were used to field trim samples to remove heavily weathered
surfaces and moss. Edges were rounded before samples were wrapped with masking tape,
labelled, and placed into appropriately labelled Ziploc bags.
16
Sample Preparation
Upon returning to the University of Montana campus slabs were cut perpendicular to
foliation and perpendicular to any fold hinges. Select slabs were also cut parallel to
foliation. Once samples had been transported to Western Kentucky University slabs were
cut into billets and then ground using 400 grit followed by 600 grit to remove saw marks.
Slides were then frosted using 400 grit to better facilitate the epoxy hold between the
billet and the slide. A mix of three parts Buehler epoxy to one part Buehler epoxy
hardener was applied to affix the billet to the slide. The slab and slide were then left on a
hot plate overnight to cure. Once cured the slab was placed on a Microtek
Microsectioneer to cut the billet off of the slide leaving a thick section. Selected thin
sections were then made from samples of interest. Optical thin sections were prepared by
grinding down to a standardized 30 micron thickness, in addition to a number of “thick
sections” (thicknesses greater than 30 microns). These samples were then readied for
polarized light microscopy, scanning electron microscopy and electron microprobe
analyses as necessary. Thin sections were polished using Buehler .1 micron and .05
micron diamond paste on a lapidary wheel to produce an even polish.
Samples were readied for X-ray diffraction first by powdering sample using a ceramic
mortar and pestle and an acetone slurry. Once samples were sufficiently powdered the
sample powders were packed into sample disks for XRD analysis. High resolution scans
of each slide used for PLM, Raman, SEM, and EMP analyses were created using an
Epson scanner and Microsoft Silverlight photo editing software to produce useful thin
section reference maps. Zones or grains of interest identified using PLM or Raman
microscopy were noted accordingly on these reference maps to facilitate efficient
17
relocation for complimentary analyses. An example reference map produced from this
method is presented in Figure 5.
Raman microscopy was utilized to corroborate and augment PLM mineral
observations. This instrument employs a specialized optical microscope fitted with an
excitation laser and a number of filters designed to take advantage of the Raman effect
for mineral phase identification and quantification. The excitation of the molecules within
the sample produces characteristic frequency and pattern shifts in the scattered beam
which are unique to particular molecular species and the intensity is directly proportional
to the number of molecules in the path of the beam. The spectrum that is produced from
this process is compared to spectra in a user-generated RRUF database for identification
based on percent correlation. Inferences can be made from this data concerning not only
the identity of a given mineral phase but also its relative abundance.
Geothermobarometric assertions rely not only on the compositions of the mineral
phases in question, but also on its crystallographic structure and site occupancy.
Goldschmidt’s rules state that given ions of equal size and charge, there should be free
substitution in and out of appropriate structural sites in a given crystal lattice. In reality,
minute differences in atomic radii or charge will cause one ion to be more “preferred”
since its smaller size results in less distortion of the crystal lattice. Furthermore, different
mineral species possess a number of different structural sites, each with different criterion
for the acceptance of ionic species. The pyroxenes, for example, possess two dominant
cation accepting sites referred to as M1 and M2, as well as a tetrahedral (T) site into
which certain elements may substitute. Certain elements are known to only substitute into
particular sites, while others substitute between different sites as a consequence of site
18
Figure 5. Reference map of sample 14-JH-14.
19
occupancy, and/or pressure and temperature conditions, or because their atomic radius
allows them to satisfy radius ratio requirements for multiple structural sites. Therefore,
compositional data acquired from a microprobe is simply not enough to indicate likely
pressure and temperatures of formation. Elemental partitioning between structural sites
must be taken into account to produce more detailed chemical formulae depicting not
only elemental composition, but the locations of these elements within the crystal lattice.
20
CHAPTER 4
RESULTS
Stone Creek Iron Formations
Stone Creek (SC) meta-BIFs, while often marked on geologic maps of the area,
are in many cases only thin, laterally continuous beds typically less than one or two
meters in width. Where exposed, these iron-formations are commonly weathered to a dull
red-brown and accompanied by distinctive red soil with ant colonies in some places. In
outcrop SC meta-BIFs exhibit alternating magnetite/hematite and quartz bands ranging
from less than .5 cm to 2.5 cm in width. Banding is continuous, though quartz lenses are
not uncommon. Magnetite/hematite bands (containing additional minor silicate
mineralization) are dominant, constituting 55% to 80% of the rock by composition with
quartz largely responsible for the remainder. Figure 6 provides a field photo of SC BIF
for reference. Trendall and Blockley (1970) have suggested terminology for the scale of
banding in BIF. According to this scheme, banding in the SC meta-BIFs ranges from
mesobands (average thickness of less than one inch) to microbands (0.3-1.7 mm). Grain
size of the magnetite/hematite is typically sub-millimeter in scale. Non-quartz and non-
iron oxide mineralization is sparse (accounting for two percent or less of total
composition), and, where present, generally occurs between quartz and iron oxide bands.
21
Figure 6. SC BIF field photo. Rock hammer for scale. Photo taken during the summer of
2014.
22
Meta-BIF occurrences are interlayered with a number of different lithologies
including amphibolite, marble, and metapelite. Narrow (between .5 to 2 meter) sections
of SC meta-BIF are bounded on both sides by a single lithology (dominantly metapelite)
or formed a boundary between disparate lithologies (commonly between metapelite and
either marble or amphibolite). The complex folding and faulting of the region makes it
difficult to resolve with certainty whether the observed iron formations in the study area
are all stratigraphically distinct or if many are simply the expression of the same unit
(James, 1990).
Banding variations include highly folded, two-centimeter width monomineralic
bands, and sub-centimeter scale, planar bedding. SC meta-BIFs (samples 14-JH-1a to 14-
JH-10b as well as 14-JH-20) tend to exhibit more well-defined, thin bedded layers. These
samples exhibit generally equal modal percentages of both quartz as well as various
oxides such as magnetite and hematite (40% to 50%, respectively). PLM inspection also
showed 5% to 15% amphibole as well as up to 5% pyroxene within SC meta-BIF.
Pyroxenes identified employing Raman microscopy on SC samples 1c and 6 were found
to be diopside with minor augite and hedenbergite. Most amphiboles from SC sample
numbers 3B, 5, and 6, were identified as clinoamphiboles on the basis of polarized light
microscopy techniques, and vary from tremolite to actinolite. Some observed grains have
exsolution/intergrowth features (crystallization of two mineral phases) showing both
actinolite and grunerite (Figures 7 and 8). Figure 9 shows a SEM transect highlighting
this phenomenon and Figure 10 demonstrates its relative abundance. Sample 6 also
contains sodium-rich phases: katophorite and winchite. Table 1 presents various non-
quartz phases identified in XRD analyses of selected samples.
23
Figure 7. Actinolite exsolution/intergrowth lamellae. Crystal denoted by red cross-hair.
Figure 8. Grunerite exsolution/intergrowth lamellae. Crystal denoted by red cross-hair.
24
Figure 9. SEM traverse of amphibole lamellae from sample 14-JH-6.
Figure 10. SEM back-scatter image of sample 14-JH-6. Amphibole
exsolution/intergrowth prolific and marked by alternating light and dark layers.
Ca
Ca
Fe
Al
Ca
Intensity (CPS)
µm
25
XRD Results
Sample Mineral Formula
14-JH-1b petedunnite CaZnSi2O6
spinel Cu.6Zn.4Al2O4
diopside CaMgSi2O6
14-JH-4 ferriwinchite NaCaMg4Fe(Si8O22)(OH,F)2
cummingtonite (Fe2+
Mg)7Si8O22(OH)2
tremolite Ca2Mg5Si8O22(OH)2
magnetite Fe(Fe.04Ti.67)O4
ferripedrizite NaLi2(Fe3+
2Mg2Li)Si8O22(OH)2
14-JH-6 magnesioferrite MgFe3+
2O4
feriwinchite NaCaMg4Fe(Si8O22)(OH,F)2
cummingtonite (Fe2+
Mg)7Si8O22(OH)2
14-JH-13d diopside CaMgSi2O6
sodic-amphibole Na(Na1.97Mg.03)Mg3(Mg1.03Cr.97)Si8O22F2
14-JH-17a magnetite Fe.89Co.19(Fe1.02Co.94)O4
fluoro-edenite NaCa2Mg5(Si7Al)O22F2
hematite Fe2O3
14-JH-18b hematite Fe1.98H.06O3
fluor-boron edenite NaCa2Mg5Si7BO22F2
magnetite Fe.88Co.19(Fe.02Co.94)O4
Table 1. XRD results of select samples. All samples contain quartz (SiO2).
26
Iron Mine Iron Formations
Unlike its Stone Creek counterparts the meta-BIF of the Iron Mine (IM) is well-exposed
in almost continuous outcrop. Located near the abandoned Christensen Ranch homestead
off of Sweetwater Road the IM meta-BIF forms a prominent ridge adjacent to recent
unconsolidated sediments. IM meta-BIFs are often highly-folded, with a thickness in
outcrop greater than six meters, and form the crest of a ridge spanning over 100 meters.
Folded IM meta-BIFs feature thickened, parallel folded hinges coupled with elliptical
folds. The intense folding suggests that IM BIFs may have undergone higher grade
metamorphism than that of SC BIFs. The texture of the iron formation varies from wavy
bands to fine, well-laminated bands (up to .25 cm), discontinuous quartz lenses, locally
folded, quartz-rich bands, pyroxene-rich and quartz-poor bands, well-layered bands of
equal proportions quartz and other phases up to 1.5 cm thick, and poorly developed
fabrics with coarse grain sizes of .25 cm. Figures 11 and 12 showcase some of the highly
folded sections of IM meta-BIFs. This variability in texture parallels a pronounced
variability in composition as respective portions of the outcrop contain green, pyroxene-
rich bands up to .5 cm, abundant specular hematite, and garnet-rich zones. Grain-size is
typically fine to microscopic. Pyroxene and garnet phases are largely concentrated at the
contact between magnetite/hematite and quartz layers. Pyroxenes from sample 13d range
from diopside to hedenbergite, but are dominated by augite. Raman spectra of garnets
found in sample 18b are strong matches to andradite. Amphiboles in IM samples 13d, and
14 vary from tremolite to actinolite. Modal percentages of major mineral phases are also
highly variable with quartz ranging between 30% and 70%, oxides from 20% to 65%,
pyroxene from
27
Figure 11. IM BIF field photo #1. Rock hammer for scale. Photo taken during the
summer of 2014.
28
Figure 12. IM BIF field photo #2. Rock hammer for scale. Highly contorted meta-BIF
banding. Photo taken during the summer of 2014.
29
0% to 30%, amphibole from 0% to 15%, and garnet up to 10%. In addition to this
mineralogical variability, many of the observed IM thin sections contain pyroxene and
amphibole phases with distinct exsolution lamellae. Table 2 provides a more complete
representation of major mineral phases observed in select thin sections and Figure 13
provides photomicrographs taken of both SC and IM samples. Additionally SEM back-
scatter images of energy dispersive spectroscopy (EDS) analyses included in Figures 14,
15, and 16 were conducted to obtain additional compositional data from a variety of
mineral phases.
30
Table 2. Estimated modal percents from sample thin sections. Chart created from PLM
and Raman microscope observations
Figure 13. Photomicrographs of representative mineral species. A)
Exsolution/intergrowth lamellae showing alternating grunerite and actinolite in
sample 14-JH-6. B) Clinopyroxene layer cut parallel to banding in sample 14-
JH-13d. C) Discontinuous bands of andradite garnet in sample 14-JH-18b. D)
Cross-section of thickened fold hinge with disseminated alkali-amphiboles in
sample 14-JH-14. All images in plane polarized light.
31
Figure 14. EDS spectrum from 14-JH-6 magnetite.
Figure 15. EDS spectrum from 14-JH-13d pyroxene.
Figure 16. EDS spectrum from 14-JH-18b garnet.
32
CHAPTER 5
CONCLUSIONS
In the nearby Tobacco Root and Gravelly Mountains two-pyroxene and garnet-
pyroxene assemblages indicated metamorphic conditions of 650-700°C, 4-6 kbar and
<400°C , 2-4 kbar respectively (Immega and Klein, 1976). Peak pressure and temperature
constraints derived from Christensen Ranch metapelites by Hamelin (Proc. Keck Symp.,
2015) give temperatures of 700-800 °C and 7-9 kbar pressures. This agrees with other
temperatures calculated from other lithologies within the Christensen Ranch. Bland
(Proc. Keck Symp., 2015) found temperatures of 711-735 °C from Christensen Ranch
marble while Berg (Proc. Keck Symp., 2015) obtained temperatures and pressures of
680-710°C and 7.5-8.5 kbar from Christensen Ranch amphibolite. It seems likely,
therefore, that Christensen Ranch meta-BIFs were subjected to equivalent pressures and
temperatures. Forthcoming electron microprobe analyses and two-pyroxene
geothermobarometry techniques outlined by Saxena (1976) as well as Lindsley (1983)
should serve to constrain the temperatures of BIF metamorphism in the Ruby Range and
provide an important comparison.
X-ray diffraction and Raman microscope results suggest that the SC and IM meta-
BIFs represent two separate occurrences of iron formation within the Christensen Ranch
suite. This has been inferred on the basis of a number of relationships
33
including compositional and textural differences between SC and IM BIF as well as
stratigraphic relationships as noted in the field. Mineral assemblages within SC and IM
meta-BIF point to differences in protolith compositions. SC BIFs are well-layered and are
characterized by narrower bands than the irregular, folded bands present in the Iron Mine.
While this may be partially explained by layer thickening during deformation and
metamorphism, this same relationship may be observed in areas of IM BIFs that appear
to have experienced little to no folding suggesting that layer thicknesses of IM BIFs are a
primary feature. SC and IM BIFs are generally observed in contact with lithologies that
differ between the two locations. SC BIFs are commonly found adjacent to metapelites
and marbles whereas IM BIFs are generally in contact with metapelites and amphibolites.
This may also imply slightly different lithofacies.
The presence of specular hematite, various alkali and sodic-amphibole
mineralization, as well as andradite garnet within IM BIFs seems to argue for the
presence of a significant quantity of metamorphic water and/or decarbonation. This
agrees well with the postulate of relatively high PH2O within the Ruby Range put forward
by Immega and Klein (1976). Dewatering of hydrous phases from amphibolites and
pelites (amphiboles and micas) adjacent to IM BIFs or some other source of metamorphic
fluid may have been sufficient to mobilize sodium and calcium ions from nearby marble
lenses. This would greatly enhance ion diffusion between units during metamorphism and
permit the growth of andradite garnet and explain sodic and alkali amphibole
mineralization. An influx of metamorphic fluid during metamorphism of the IM region
may also partially explain the high degree of textural deformation (seemingly indicative
of higher-grade metamorphism). Introduction of volatiles such as metamorphic fluids and
34
CO2 can act to lower melting temperatures and provide a higher degree of deformation
and metamorphism. It is quite likely that SC BIFs were also exposed to metamorphic
fluids as well, given prolific sodic amphibole mineralization observed in samples
throughout the area, though the degree of saturation and/or the conditions of
metamorphism in the area can be assumed to be less.
FUTURE WORK
In conducting future analyses, careful attention must be paid to mineral
assemblages due to reactions occurring under set Eh-pH conditions as outlined by Klein
(Klein, 2005). Examples of such reactions include the formation of amphibole by the
reaction between iron-rich carbonates such as siderite and quartz during metamorphism
(Klein, 2005). Future work could also strive to assess the oxidation states of iron-rich
phases as a proxy for oxygen fugacity and the availability of metamorphic fluids to
provide empirical evidence to substantiate hypotheses made regarding the availability of
fluid during metamorphism of IM BIFs. Whole-rock geochemistry implementing X-ray
fluorescence can provide greater composition resolution and serve to better constrain
diffusion of trace elements, particularly Large Ion Lithophile Elements, during
metamorphism. This could then be used to see if the assumed differences in metamorphic
conditions between SC and IM BIFs can be substantiated on the grounds of differential
fluid saturation, pressures and temperatures, or both.
35
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