Banded Iron Formation from the Ruby Range, Montana Jacob Hughes Western Kentucky University, Jacob.Hughes163@Topper.Wku.Edu

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5-13-2015 Geothermobarometry and Petrographic Interpretations of Christensen Ranch Meta- Banded Iron Formation from the Ruby Range, Montana Jacob Hughes Western Kentucky University, [email protected]

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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

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

Jacob Hughes

2015

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 ii

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.

iii

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

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

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

LIST OF TABLES

Table Page

Table 1. XRD results of select samples..……………………………………………….. 25

Table 2. Estimated modal percents from sample thin sections.………………………… 30

vii

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

Figure 1. Map of the Ruby Range and Surrounding Ranges. Taken from Immega and

Klein, 1976.

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.

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

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.

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).

Figure 2. MMT and BBMT within the Wyoming Craton. Taken from Harms, et al., 2004.

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

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

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.

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.

Figure 4. General Stratigraphy and Structure of the Ruby Range. Taken from Harms (Proc. Keck Symp., 2015).

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

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.

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.

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

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

Figure 5. Reference map of sample 14-JH-14.

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.

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.

Figure 6. SC BIF field photo. Rock hammer for scale. Photo taken during the summer of 2014.

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.

Figure 7. Actinolite exsolution/intergrowth lamellae. Crystal denoted by red cross-hair.

Figure 8. Grunerite exsolution/intergrowth lamellae. Crystal denoted by red cross-hair.

Intensity (CPS)

Ca µm

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.

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

2+ cummingtonite (Fe Mg)7Si8O22(OH)2

tremolite Ca2Mg5Si8O22(OH)2

magnetite Fe(Fe.04Ti.67)O4

3+ ferripedrizite NaLi2(Fe 2Mg2Li)Si8O22(OH)2

3+ 14-JH-6 magnesioferrite MgFe 2O4

feriwinchite NaCaMg4Fe(Si8O22)(OH,F)2

2+ cummingtonite (Fe 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).

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

Figure 11. IM BIF field photo #1. Rock hammer for scale. Photo taken during the

summer of 2014.

Figure 12. IM BIF field photo #2. Rock hammer for scale. Highly contorted meta-BIF

banding. Photo taken during the summer of 2014.

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.

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.

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.

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

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

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.

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