Bulletin 141 2020

GENESIS AND EXPLORATION POTENTIAL FOR LATE CRETACEOUS VEINS OF THE BIG FOOT MINING DISTRICT, JEFFERSON COUNTY,

Stanley L. Korzeb

Montana Bureau of Mines and Geology, Butte, Montana Cover photo: The Big Four mine, the largest producer in the district, located on State Creek road. Bulletin 141 2020

GENESIS AND EXPLORATION POTENTIAL FOR LATE CRETACEOUS VEINS OF THE BIG FOOT MINING DISTRICT, JEFFERSON COUNTY, MONTANA

Stanley L. Korzeb

Montana Bureau of Mines and Geology, Butte, Montana

TABLE OF CONTENTS

Abstract ...... 1 Introduction ...... 3 Methods ...... 5 Previous Investigations...... 5 History ...... 5 Regional Geology ...... 5 District Geology ...... 7 Geophysics ...... 7 Infrared Spectrometry ...... 7 Results...... 9 Wall-Rock Alteration Types ...... 10 Propylitic ...... 10 ...... 11 Silicic– ...... 12 Argillic–Illite ...... 12 Alteration and Litho-Geochemistry ...... 13 Alteration Trace Elements ...... 19 Vein ...... 24 Host Rock Alteration ...... 26 Base Stage Minerals ...... 26 Supergene Stage Minerals ...... 26 Vein Trace Elements...... 26 Cathodoluminescence ...... 29 Fluid Inclusions ...... 34 Isotopes...... 39 Isotopes...... 41 Discussion ...... 42 Wall-Rock Alteration ...... 43 Pressure and Depth of Mineralization ...... 43 Vein Formation Temperature ...... 44 Trace Elements ...... 44 Sulfi de-Quartz Vein Genesis ...... 44 Geologic Model...... 47 Exploration Potential...... 48 Acknowledgments ...... 49 References ...... 49 Appendix A...... 55 Appendix B ...... 61 Appendix C ...... 65 Appendix D ...... 69 Appendix E ...... 73 Appendix F ...... 79 Appendix G ...... 83

FIGURES Figure 1. Generalized geologic map of the Boulder Batholith showing location of the Big Foot mining district...... 3 Figure 2. Geologic map of the Big Foot mining district showing sample and major mine locations and aeromagnetic low anomalies...... 4 Figure 3. Geologic map of a portion of the Ratio Mountain 7.5’ quadrangle EDMAP showing relationship of aplite and alaskite intrusions to area of aeromagnetic low anomaly...... 8 Figure 4. Histogram showing the distribution of illite spectral maturity (ISM) values indicating high temperature for silicic–illite alteration and low temperature for argillic–illite alteration...... 9 Figure 5. Histogram showing distribution of Al-OH shortwave infrared wavelengths for silicic–illite alteration...... 10 Figure 6. Histogram showing distribution for Al-OH shortwave infrared wavelengths for argillic–illite alteration...... 11 Figure 7. Propylitic alteration...... 11 Figure 8. Carbonate alteration, microphotograph of carbonate altered granite showing calcite fi lling open spaces...... 12 Figure 9. Silicic–illite alteration microphotographs showing microbrecciated granite with schorl fi lling fractures and open spaces, and replacement of granite with fi ne- to coarse-grained quartz...... 13 Figure 10. Argillic–illite alteration...... 14 Figure 11. Alteration box diagram identifying and comparing diff erent alteration types and corresponding alteration index (AI) and carbonate–chlorite– index (CCPI)...... 15 Figure 12. Isocon diagram of whole-rock oxides for propylitic, argillic–illite, silicic–illite, and carbonate alteration showing oxide losses and gains compared to the unaltered granite isocon reference line. ....17 Figure 13. Isocon diagram showing trace elements losses and gains for silicic–illite, argillic–illite, propylitic, and carbonate alteration compared to unaltered granite isocon reference line...... 21 Figure 14. Paragenesis of base metal and supergene mineralizing events for the Big Foot veins...... 25 Figure 15. SEM-BSE images of primary sulfi de minerals in quartz...... 27 Figure 16. wires in quartz vug with euhedral quartz crystals and galena inclusion from Attowa mine...... 27 Figure 17. SEM-BSE images of secondary minerals replacing primary sulfi de minerals...... 28 Figure 18. Isocon diagram showing trace element loss and gains for quartz veins compared to unaltered granite isocon reference line...... 30 Figure 19. Primary quartz grain in silicic–illite altered host granite showing extensive microbrecciation and fracturing...... 31 Figure 20. Multiple generations of quartz crystallization...... 32 Figure 21. Fractured and partially brecciated generation two and three euhedral quartz crystals ...... 33 Figure 22. Intense microbrecciation of second- and third-generation quartz crystals and grains...... 33 Figure 23. A fi fth quartz generation showing euhedral crystals with weak oscillatory growth that crystallized with sphalerite and pyrite in open space within brecciated second and third generations. .. 34 Montana Bureau of Mines and Geology Bulletin 141

Figure 24. Primary type I two-phase and liquid fl uid inclusions...... 35 Figure 25. Primary type II two-phase fl uid inclusion consisting of a vapor bubble and brine...... 35

Figure 26. Three-phase fl uid inclusion with liquid CO2, vapor, and brine...... 36 Figure 27. String of secondary type I and II fl uid inclusions in a healed ...... 36 Figure 28. Histogram plot showing distribution of homogenization temperatures...... 38 Figure 29. Histogram plot showing distribution of salinity in weight percent NaCl equivalent...... 39 Figure 30. Salinity (NaCl wt% equiv.) versus homogenization temperature diagram for type I and II, three-

phase CO2 , and three-phase halite daughter fl uid inclusions showing groups of low- and high- salinity fl uids...... 40 Figure 31. Histogram showing distribution of total sulfur isotopes for all phases...... 41

TABLES Table 1. Production history for the Big Foot mining district...... 6 Table 2. Whole-rock analysis for altered and unaltered granite and aplite ...... 14 Table 3. Alteration index and chlorite-carbonate pyrite index values with corresponding alteration types and sample numbers ...... 15 Table 4. Isocon analysis of whole-rock averages or propylitic alteration ...... 18 Table 5. Isocon analysis of whole-rock averages for argillic–illite alteration ...... 18 Table 6. Isocon analysis of whole-rock averages for silicic–illite alteration ...... 18 Table 7. Isocon analysis of whole-rock averages for carbonate alteration ...... 19 Table 8. Trace element averages for unaltered granite and corresponding argillic–illite, silicic–illite, propylitic, and carbonate alteration...... 20 Table 9. Isocon analysis of trace element averages for propylitic alteration ...... 22 Table 10. Isocon analysis of trace element averages for argillic–illite alteration ...... 22 Table 11. Isocon analysis of trace element averages for silicic–illite alteration ...... 23 Table 12. Isocon analysis of trace element averages for carbonate alteration ...... 23 Table 13. Summary of minerals identifi ed from the Big Foot veins: vein paragenesis ...... 24 Table 14. Analytical averages of elements with anomalous results for sulfi de-quartz veins from mines and prospects throughout the Big Foot mining district compared to unaltered granite ...... 29 Table 15. Isocon analysis of ICP-MS averages for quartz veins ...... 31 Table 16. Summary of fl uid inclusion data for type 1 and II, three phase with halite, and three phase

with CO2 ...... 37 Table 17. Summary of sulfur isotope analytical results...... 41 Table 18. Oxygen isotope compositions for six quartz vein samples from the Big Foot mining district ...... 42

iii Stanley L. Korzeb

iv Montana Bureau of Mines and Geology Bulletin 141 ABSTRACT The Big Foot veins are hosted by the Butte Granite and have characteristics of both epithermal and meso- thermal veins. Age dating indicates the veins developed 73.8 ± 0.12 to 75.3 ± 0.25 Ma, about the same time aplite intruded the Butte Granite at 74.5 ± 0.6 Ma. Vein age dates indicate vein emplacement was contempora- neous with a late-stage aplite intrusion of the 74.5 ± 0.9 to 76.28 ± 0.14 Ma Butte Granite. Four types of wall- rock alteration developed during quartz vein emplacement: propylitic, carbonate, silicic–illite, and argillic–illite. Silicic–illite and argillic–illite alteration occur throughout the district, whereas propylitic and carbonate alteration occur at two and one locations, respectively. The most common alteration minerals are K–illite, illite–smec- tite, , , , and schorl. The schorl occurs in the silicic–illite alteration as needles included in quartz and fracture fi llings. Anomalous trace elements from the altered wall rocks and quartz veins Ag, As, Pb, Sb, and Zn are characteristic of epithermal systems, and Au, Cd, Co, Cu, Mn, and Mo are charac- teristic of modern back arc geothermal fi elds. Cathodoluminescence revealed six quartz generations and two episodes of microbrecciation. The granite wall rock was microbrecciated during alteration. The fi rst three quartz generations show evidence of dissolution and overgrowths and were later microbrecciated. A fourth generation of quartz stringers with sulfi de minerals fi lled open spaces in the microbrecciated quartz. The fourth generation of quartz stringers partially replaced early quartz generations. A fi fth quartz generation and sulfi de mineralization crystallized simultaneously in open spaces generated by microbrecciation. A fi nal sixth quartz generation, which is barren of sulfi des, fi lled remain- ing open spaces. The veins formed at a lithostatic pressure of 2 kb, refl ecting the 7 to 8 km crystallization depth for the Butte Granite. Pressure corrected fl uid inclusion homogenization temperatures range from 310⁰ to 508⁰C and average 430⁰C. Some fl uid inclusions revealed liquid CO2, and others had halite daughter minerals; salinities for all 181 inclusions ranged from 0.2 to 38.1 wt% NaCl eq. There is no fl uid inclusion evidence that indicates boiling, but decompression of the hydrothermal fl uid took place as indicated by microbrecciation of the quartz veins and host wall rock. This caused CO2 loss, decreasing temperatures, and increasing salinities from fl uid loss. Sulfur and oxygen isotopes indicate the veins originated from a magmatic-derived hydrothermal fl uid. Sul- fur isotopes have a wide range, from -0.7‰ to 16.6‰, but all the samples with the exception of one range from -0.7‰ to 5.2‰, with a single outlier at a δ34S value of 16.6‰. Equilibrium temperatures calculated for sphaler- ite–galena δ34S pairs gave a temperature range of 304.2⁰ to 578.6⁰C, which overlaps the pressure-corrected fl uid inclusion temperatures. Oxygen isotopes for vein quartz range from 11.0‰ to 12.3‰, and calculated δ18O compositions for the hydrothermal fl uids range from 6.5‰ to 9.0‰. Calculated oxygen isotopes suggest the hydrothermal fl uids originated from an I type granitoid pluton. Vein quartz δ18O and sulfi de δ34S show the hydrothermal fl uids were infl uenced by crustal rocks assimilated into an I type granitoid pluton. Based on the depth of formation, lithostatic pressure, fl uid inclusion, and stable isotope data, the veins fi t the mesothermal classifi cation. The similarities in ages between the veins and aplite imply the veins formed from a hydrothermal system established during the time of aplite intrusion, although their relation to a porphyry system cannot be ruled out. Oxygen and sulfur isotopes suggest the hydrothermal fl uids originated from a cooling felsic intrusion that assimilated crustal rocks. The hydrothermal fl uids underwent periods of pressure changes between lithostatic and hydrostatic and decompression; this generated microbreccias in the vein quartz and granite host. Decompression caused temperature, salinity, and pH changes and loss of CO2 leading to quartz dissolution and overgrowths, as well as sulfi de mineralization. The veins crystallized from a high- to low-temperature, variable salinity hydrothermal fl uid at a lithostatic pressure of 2 kb, similar to conditions re- lated to deep porphyry systems.

1 Stanley L. Korzeb

2 Montana Bureau of Mines and Geology Bulletin 141 INTRODUCTION stockwork –molybdenum–tungsten deposits and disseminated/stockwork and silver deposits The Big Foot mining district is located 15 mi (24 at depth. Their assessment was based on data from km) north of Whitehall, and 9 mi (14.5 km) south of previous published and unpublished sources and Boulder in Jeff erson County (fi g. 1). The seven mines studies consisting of geologic mapping, geochemical in the district with past production are accessible from and geophysical surveys, remote sensing, and geo- State Creek Road and are located in the Deer Lodge chronologic studies. Past mining had limited produc- National Forest (fi g. 2). The district covers about 30 tion for gold, silver, copper, , and from narrow mi2 (48 km2) and is located in the Late Cretaceous vein deposits. Boulder Batholith, near its eastern contact with the co- The goal of the current investigation was to as- magmatic Elkhorn Mountains volcanic fi eld (Klepper sess the future exploration potential and update and and others, 1957). generate modern geologic data on the known mineral The district was chosen for this investigation resources. Data generated from the geologic map- because it was not studied in detail by past inves- ping done by Olson and others (2016) was expanded tigations, it could have potential for future minerals with new data sets for the known narrow vein mineral exploration or undiscovered resources for critical ele- resources. From the geologic data, a genetic model ments, and the genesis of mineral resources hosted of the mineral resources was determined, a geologic by the Butte Granite is not fully understood. Elliot and model constructed, the deposit classifi ed, and others (1993) assessed the district as having a high future exploration potential assessed. potential for the occurrence of undiscovered porphyry/

Figure 1. Generalized geologic map of the Boulder Batholith showing location of the Big Foot mining district.

3 Stanley L. Korzeb

Figure 2. Geologic map of the Big Foot mining district showing sample and major mine locations and aeromagnetic low anomalies. Box outlines Ratio Mountain 7.5’ quadrangle (Olson and others, 2016).

4 Montana Bureau of Mines and Geology Bulletin 141 METHODS claims: Hoosier Boy, Big Four, Terror, Nickel Plate, Searchlight, and Ajax (fi g. 2). The Big Four group pro- Methods used for this investigation involved duced 1,577 tons of ore from 1920 to 1945. Produc- sampling dumps and ore stockpiles of the existing tion was from a 100-ft shaft on the Ajax and Hoosier mines, because underground workings were inacces- claims and from a 180-ft shaft on the Big Four claim, sible. Vein outcrops were mapped and sampled along which had three levels at 40, 60, and 100 ft depths. with associated altered host rocks. Samples were By 1959, the shafts were no longer accessible. The analyzed for alteration types, fl uid inclusions, stable State mine (fi g. 2) produced the most gold from the isotopes, mineralogy, trace element and whole-rock district. Between 1905 and 1940, a total of 544 tons of geochemistry, near infrared and shortwave infrared ore yielded 384 oz of gold with a calculated grade of spectroscopy, and cathodoluminescence imaging. 0.7 oz/t gold. Workings consisted of numerous adits, Minerals were identifi ed from thin sections, polished one above the other at about 100-ft intervals develop- sections, hand samples, and SEM-EDS analysis. The ing an east–west vein. analytical methods used are detailed in the results Lyden (2013) reports placer mining was attempted sections for each analysis type. along Big Foot Creek from 1934 to 1935. For the 2-yr PREVIOUS INVESTIGATIONS period of operation, 0.54 oz of gold was recovered. The sources are suggested to be gold-bearing veins The fi rst investigation of the Big Foot mining dis- located at the State mine and other prospects. trict was a preliminary geologic map of the southwest Production ended in 1956 (table 1; Roby and quarter of the Boulder 7.5´ quadrangle by Pinckney others, 1960). The Corporation of America and Becraft (1961). Their geologic map included leased the State mine in 1956 for a period of 10 yr. the Big Foot mining district veins. Olson and others There is no recorded production or exploration activi- (2016) mapped the geology of the Ratio Mountain ties reported for the lease period. The lease expired 7.5´ quadrangle covering most of the Big Foot district. in 1966 with control of the State mine returning to Olson and others (2016) mapped the veins, included the original owners. No new activity was reported for a brief discussion of the hydrothermal mineral depos- the State mine after 1966. After production ended, its, dated the veins at the State and Ajax mines using activity in the district consisted of exploration at the 40 39 Ar/ Ar methods, and determined the depth of crys- Mountain Queen and State mines (fi g. 2). At the tallization for the Butte Granite hosting the veins. Mountain Queen mine, from 1963 to 1984, activity HISTORY consisted of reopening and assaying samples from the underground workings, identifi cation of ore min- Mining history of the Big Foot district is summa- erals, and geologic surface mapping. Activity ended rized from records preserved in the MBMG archives with the death of the owner in 1984 and the property and from Roby and others (1960). The earliest re- has remained idle since. Exploration at the State mine corded activity took place in the 1880s at the Moun- consisted of trenching across known vein exposures tain Queen mine (fi g. 2), which was worked for a short and surface sampling and assaying vein outcrops. At time before being closed. The mine reopened and the time of this investigation, there was no exploration operated sporadically from 1939 to 1956, producing activity taking place within the district. Some patented 839 tons of ore. Records for the Summit mine show mining claims are being used for recreation and con- development in the 1880s when a 750-ft adit was struction of vacation homes or cabins. driven on an E–W-trending shear zone. There is no recorded production from the Summit mine, and it was REGIONAL GEOLOGY most likely closed shortly after the adit was driven. In- The mining district is located on the east side of terest in the property was renewed in 1951 when the the Boulder Batholith west of the Elkhorn Mountains Defense Minerals Exploration Administration issued a volcanic fi eld and hosted by the Butte Granite (fi g. 1). loan to the owner William Mulcahy for exploration and The Boulder Batholith and Elkhorn Mountains volca- reopening the adit. Exploration activities continued nic fi eld (EMVF) formed concurrently during crustal through 1967, but the property was not developed into shortening between 85 and 76 Ma near the end of an operating mine. Mesozoic Cordillera arc magmatism (Mahoney and Most production from the district (table 1) was others, 2015; Rutland and others, 1989). During con- generated from the Big Four group and the State tinental arc magmatism, the fi rst magmatic event was mine. The Big Four group consists of six patented the eruption of the EMVF, which is preserved along the

5 Stanley L. Korzeb

west and east fl anks of the Boulder Batholith (fi g. 1). The batholith is about 90 km long and 50 km wide Based on regional rock exposures, Smedes (1966) es- and elongated in a northeast–southwest direction. timated the original thickness of the EMVF sequence Plutonic rocks composing the batholith are subdivided to be 4.6 km. Diorite stocks, dikes, and sills fed the based on age dating into an early stage and younger EMVF eruptions, and some diorite bodies may repre- Butte Granite stage (Berger and others, 2011). The sent volcanic rocks recrystallized during emplacement Boulder Batholith is an amalgamation of plutons that of the Boulder Batholith (Klepper and others, 1957). began forming approximately 81 Ma and continued Activity in the EMVF started as fi ssure eruptions of until approximately 74 Ma (Lund and others, 2002). andesitic lava (lower member) that overfl owed the Houston and Dilles (2013) suggested the Boulder edges of a broad N35oE-trending regional syncline Batholith within the Butte mining district crystal- (Klepper and others, 1957; Smedes, 1966). A middle lized at a depth of 6 to 9 km based on aluminum-in- member consisting of two massive tuff and tuff breccia hornblende barometry studies. Based on additional units overlies the early andesite beds on the west fl ank aluminum-in-hornblende barometry studies, the Butte of the Boulder Batholith (Scarberry and others, 2019a; Granite within the Big Foot district is reported by Ol- Scarberry, 2016). The middle member on the east son and others (2016) to have crystallized at a pres- side of the batholith consists of three dacite to rhyolite sure of 2.1 to 2.4 kb with a depth of 7 to 8 km similar ignimbrite sheets capped by two thin pyroclastic units to the Butte mining district. The early stage plutonic (Scarberry and others, 2019b). Conglomerates con- rocks span an interval of about 81 to 76 Ma and the sisting of bedded and water-laid tuff and andesite, and Butte Granite stage emplaced about 75 to 74 Ma mudstone form the top or upper member. Lenticular (Lund and others, 2002). Early stage magmas were beds of fresh water limestone and andesite fl ows oc- emplaced concurrently with superjacent thrust faulting cur locally (Klepper and others, 1957). and related folding. The Butte Granite was likewise

6 Montana Bureau of Mines and Geology Bulletin 141 structurally controlled and follows pre-Butte Granite GEOPHYSICS thrust faulting (Berger and others, 2011). Intrusions of pegmatite, alaskite, and aplite dikes mark the close of The USGS conducted a gravity and aeromagnetic Butte Granite magmatism (Berger and others, 2011). survey of the Butte 1o x 2o quadrangle that covered Sulfi de-bearing quartz veins with east–west orienta- the Big Foot mining district (Hanna and others, 1994). tion developed in the northern half of the batholith The aeromagnetic survey showed a magnetic low after crystallization of the Butte Granite and dikes 6 mi long and 2 mi wide in the Butte Granite in the (Berger and others, 2011). northeast part of the district (fi gs. 2, 3). The magnetic low encompasses the Big Four, Nickel Plate, Ajax, A combination of Laramide shortening, Eocene and Attowa mines and a major northeast-trending crustal extension, and Basin and Range faulting vein that extends through the center of the anomaly. caused the EMVF and Boulder Batholith to be up- Another magnetic low identifi ed by Hanna and oth- lifted, eroded, and exhumed. Laramide crustal short- ers (1994) is west of the St Anthony mine. This ening occurred about 65 to 55 Ma followed by exten- magnetic low is 2 mi long and 1 mi wide. Clusters of sion forming a graben across the Boulder Batholith. aplite and alaskite dikes were mapped by Olson and Steepening of the subducting Farallon plate under the others (2016) within and adjacent to the magnetic North American plate caused back-arc volcanism to lows (fi g. 3). Hanna and others (1994) interpret the start during the early Eocene (Feely, 2003). Back-arc granite-hosted magnetic low anomalies as an underly- volcanism caused the eruption of the Lowland Creek ing pluton altering the Butte Granite, using the Butte volcanic fi eld into a northeast–southwest-trending half district as an example. During alteration of the host graben traversing the Boulder Batholith (Sillitoe and granite by a porphyry intrusion, such as in the Butte others, 1985; Dudás and others, 2010). Eruption of district, magnetite oxidizes to hematite. The oxidation the Lowland Creek volcanic fi eld took place between of magnetite to hematite changes the oxidation state 53 and 49 Ma (Dudás and others, 2010; Scarberry of from Fe2+ to Fe3+, giving a negative magnetic and others, 2015, 2019c, 2019d; Olson and others, response. 2017). DISTRICT GEOLOGY INFRARED SPECTROMETRY The entire district lies within the Butte Gran- Visible-near infrared (VNIR) and short-wave ite of the Boulder Batholith and is included in the infrared (SWIR) spectrometry were conducted on 1:24,000-scale Ratio Mountain geologic map gen- the altered host rock and sulfi de–quartz veins using erated by Olson and others (2016; fi gs. 2, 3). Past a PANalytical TerraSpec Halo mineral analyzer. The producing sulfi de-quartz veins <4 ft wide occupy PANalytical TerraSpec Halo mineral analyzer mea- east–west- to northeast-striking fractures. The Butte sures wavelengths from 350 to 2,500 nm and is used Granite pluton consists of medium- to coarse-grained to quantify rock alteration. The mineral analyzer iden- –hornblende granite (Olson and others, 2016). tifi es phyllosilicate minerals, hydroxylated silicates, Alaskite and aplite dikes and sills cutting the Butte sulfates, , ammonium-bearing minerals Granite mapped by Olson and others (2016) are the and minerals with hydroxyl (OH) bonds (Shankar, youngest intrusions in the district. Ages determined 2015; Madubuike and others, 2016), and infrared from U/Pb zircons for the Butte Granite range from scalers based on ratios of refl ectance at diagnostic 74.5 ± 0.9 Ma (Lund and others, 2002) to 76.28 ± wavelengths. Scalers used for this investigation are 0.14 Ma (Martin and others, 1999). Olson and oth- the illite spectral maturity (ISM), aluminum hydroxide ers (2016) determined a 40Ar/39Ar plateau age for the bonds (Al-OH), and iron hydroxide bonds (Fe-OH). vein at the State mine of 75.2 ± 0.25 Ma (95% confi - The ISM scaler is a qualitative refl ection of alteration dence) and a 40Ar/39Ar plateau age for the vein at the temperature. ISM numbers below 1.0 indicate high Ajax mine of 73.81 ± 0.12 Ma (95% confi dence). The temperature, and numbers greater than 1.0 indicate ages of the veins are about a million years younger low temperature (Madubuike and others, 2016). Min- than the oldest age for the host Butte Granite. Litho- erals with Al-OH bonds have compositional variations static pressure estimates for the intrusion of the Butte that show the chemistry of fl uids at the time of altera- Granite based on hornblende compositions was deter- tion. The compositional variations cause wavelength mined to be 2.1 to 2.4 (± 0.2) kilo bars (kb) based on shifts, indicating geochemical conditions at the time the calibration of Anderson and Smith (1995) sug- of alteration. Minerals with Fe-OH bonds have com- gesting emplacement depths of 7 to 8 km (Olson and positional variations that are refl ected in wavelength others, 2016). shifts. The wavelength position of the Fe-OH scaler is

7 Stanley L. Korzeb

Figure 3. Geologic map modifi ed from Olson and others (2016) of a portion of the Ratio Mountain 7.5’ quadrangle EDMAP showing relationship of aplite and alaskite intrusions to area of aeromagnetic low anomaly.

8 Montana Bureau of Mines and Geology Bulletin 141 a refl ection of the geochemical conditions at the time caused by substitution of Al by Mg and/or Fe (Yang of alteration. and others, 2001). Yang and others (2001) report shorter wavelengths, close to 2,190, are characteristic RESULTS of Na–illite. Quartz veins and silicic–illite alteration show a wide range of infrared wavelengths ranging Multiple infrared analysis was conducted on 42 from 2,189.2 to 2,215.2 nm (appendix B). The quartz altered granite and 28 quartz vein samples from veins and silicifi ed granite histogram (fi g. 5) shows throughout the district. Complete results are in ap- infrared wavelengths falling between 2,196 and 2,209 pendices A and B. ISM values are plotted on a his- nm, refl ecting an abundance of K–illite. The next most togram (fi g. 4) and show two alteration types related abundant mineral is phengite, which is refl ected in the to temperature. Development of the quartz veins and 2,209 to 2,215 nm infrared wavelength (fi g. 5). A few silicifi cation of the granite host develops silicic–illite al- samples had infrared wavelengths below 2,196 nm teration, with ISM scaler values varying from 0.162 to (fi g. 5), refl ecting a minor presence of Na–illite. Argil- 0.976 nm, indicating high-temperature alteration (ap- lic–illite altered granite has infrared wavelengths vary- pendix B). Argillic–illite altered host rocks have ISM ing from 2,201.6 to 2,214.3 nm (appendix A). K–illite scaler values ranging from 1.006 nm to 3.918 nm, is the most common alteration mineral in the argillic– indicating low-temperature alteration (appendix A). illite altered granite and is refl ected by the distribution K–illite is the most common mineral identifi ed in of infrared wavelengths of 2,200 to 2,209 nm (fi g. 6). the altered wall-rock and quartz veins. Wavelengths The argillic–illite histogram (fi g. 6) also shows a range from 2,196 to 2,208 nm characterize K–illite with octa- of 2,210 to 2,214 nm; this refl ects the presence of hedral Al contents close to muscovite (Yang and oth- phengite, the next most abundant alteration mineral. ers, 2001). Phengite has longer wavelengths (>2,210)

Figure 4. Histogram showing the distribution of illite spectral maturity (ISM) values indicating high temperature for silicic–il- lite alteration and low temperature for argillic–illite alteration.

9 Stanley L. Korzeb

Figure 5. Histogram showing distribution of Al-OH shortwave infrared wavelengths for silicic–illite alteration.

The Fe-OH scaler has an infrared wavelength WALL-ROCK ALTERATION TYPES spectrum that ranges from 2,240 to 2,270 nm and is reported for minerals with Fe-OH bonds. The scaler is Hydrothermal fl uids altered the Butte Granite commonly a refl ection of chlorite, , and phlogo- along vein contacts. Altered wall rock extends from pite (Yang and others, 2001). The minerals usually as- the vein contact into brecciated and fractured granite sociated with the Fe-OH scaler were rarely detected up to 5 ft distal from the veins. Field observations, in the altered granite and quartz veins. examination of hand specimens and thin sections, (identifi ed as schorl by SEM-EDS) was detected by and infrared spectral analysis identifi ed propylitic, car- infrared analysis in unaltered aplite, quartz veins, ar- bonate, silicic–illite, and argillic–illite alteration. Pro- gillic–illite, and silicic–illite alteration. When detected, pylitic and carbonate alteration are not common but tourmaline yielded Fe-OH infrared wavelengths vary- silicic–illite and argillic–illite alteration are widespread ing from 2,245.3 to 2,296.9 nm (appendices A, B). All throughout the district. the samples examined fall within the 2,240 to 2,270 Propylitic nm infrared wavelength range reported by Yang and others (2001), with the exception of one sample that Propylitic alteration is not common and was identi- ranges from 2,296.0 to 2,296.9 nm. fi ed at two locations: the Big Major (sample location

10 Montana Bureau of Mines and Geology Bulletin 141

Figure 7. Propylitic alteration. Microphotograph A shows replacement of plagioclase, orthoclase, and annite with calcite, K–illite, and other clay minerals. Microphotograph B is the same image, showing remains of replaced annite and feldspars and anhedral quartz grains. Photograph A is un- der crossed polarizers and B is under plane-polarized light. Figure 6. Histogram showing distribution for Al-OH short- wave infrared wavelengths for argillic–illite alteration. VNIR-SWIR are K–illite, muscovite, phengite (diocta- 33) and St. Anthony mines (sample locations 49, 51, hedral between muscovite– 56; fi g. 2). In thin section, propylitic alteration is char- and muscovite– join), ( acterized by chlorite replacing biotite, minor , polymorph), hydrobiotite (smectite group), and recto- and carbonate minerals, with quartz fi lling fractures rite (smectite group). and replacing feldspars (fi g. 7). Minerals identifi ed by VNIR-SWIR related to propylitic alteration are anker- Carbonate ite and dolomite. Other minerals identifi ed by VNIR- Carbonate alteration was only found at the Big SWIR are related to argillic–illite alteration overprint- Major mine (sample locations 34, 35; fi g. 2). In thin ing propylitic alteration. In thin section, illite replaces section, carbonate alteration is characterized by brec- plagioclase and fi lls fractures and open spaces with ciated and silicifi ed granite with secondary quartz and secondary quartz stringers and calcite. Schorl needles calcite fi lling open spaces (fi g. 8). Minerals identifi ed fi ll open spaces and are included in quartz stringers. by VNIR-SWIR are calcite, smithsonite, dolomite, The most common minerals related to argillic–illite K–illite, epidote, clinozoisite, muscovite, montmo- alteration overprinting propylitic alteration identifi ed by rillonite, kaoliniteWX (WX is well crystallized), ka-

11 Stanley L. Korzeb

Figure 8. Carbonate alteration, microphotograph of carbonate altered granite showing calcite fi lling open spaces. Photo under crossed polarizers. olinitePX (PX is poorly crystallized), and hematite. muscovite, halloysite, and . Secondary Weathering infl uenced the sample as indicated by the minerals are jarosite, goethite, , and fer- presence of hematite and poorly crystallized kaolinite. rihydrite. Argillic–illite alteration overprints carbonate altera- Argillic–Illite tion, indicated by K–illite replacing plagioclase and the presence of muscovite. Argillic–Illite alteration is widespread throughout the district and is adjacent to all quartz veins. Abun- Silicic–Illite dant K–illite and other clay minerals replacing plagio- Silicic–illite alteration is widespread throughout clase and fi lling fractures in the host granite character- the district, contacting and grading into all quartz ize argillic–illite alteration. In thin section, argillic–illite veins. Silicic–illite alteration is recognized by quartz alteration shows illite partially to completely replacing fi lling open spaces and fractures in brecciated granite feldspar crystals and annite along with clay miner- and replacing the granite host rock along vein mar- als (fi g. 10). Depending on the degree of alteration, gins, and complete replacement of granite breccia primary quartz of the original granite remains in a fragments trapped in quartz veins. In thin section, clay–illite ground mass. Where argillic–illite alteration silicic–illite alteration is characterized by quartz with overprints silicic–illite alteration, fractures are fi lled schorl needle inclusions, schorl fi lling open spaces with schorl and quartz stringers with minor sulfi de in microbreccia and fractures, illite, and minor sulfi de minerals. The most common minerals identifi ed by minerals fi lling fractures in brecciated granite (fi g. VNIR-SWIR, and found in all samples, include K–illite, 9). At locations where the granite is completely re- illite/smectite, and phengite. Less common minerals placed, fractured primary anhedral quartz grains of include beidellite, , , muscovite, the original granite occur as inclusions in hydrother- dickite, kaoliniteWX (WX is well crystallized), roscoe- mally deposited quartz veins. When granite is partially lite, rectorite, montmorillonite, magnesite, glauco- silicifi ed, plagioclase is replaced by illite. Minor illite phane, vermiculite, and chabazite. Secondary miner- and calcite fi lls fractures in quartz veins. als related to weathering include ferrihydrite, jarosite, minerals identifi ed by VNIR-SWIR are K–illite, illite– goethite, hematite, and kaolinitePX (PX is poorly smectite, chabazite, schorl, dickite, montmorillonite, crystallized).

12 Montana Bureau of Mines and Geology Bulletin 141

Figure 9. Silicic–illite alteration microphotographs showing microbrecciated granite with schorl fi lling fractures and open spaces, and replacement of granite with fi ne- to coarse-grained quartz. Microphotograph A shows schorl fi lling open spaces in brecciated and silicifi ed granite. Microphotograph B shows schorl fi lling fractures in silicifi ed granite. Micropho- tograph C shows replacement of host granite with fi ne- to coarse-grained quartz and illite fi lling fractures. Microphoto- graph D is the same image showing schorl fi lling open spaces in brecciated granite, coarse-grained quartz fi lling open spaces, and illite fi lling fractures. Photographs A, B, and D are under plane-polarized light and C is under crossed polar- izers. ALTERATION AND To delineate litho-geochemistry for unaltered and LITHO-GEOCHEMISTRY altered rocks, the Ishikawa alteration index (AI) and chlorite–carbonate–pyrite index (CCPI) was deter- Samples representing altered and unaltered gran- mined. Argillic–illite and chlorite alteration is defi ned ite were analyzed for trace elements and whole-rock by an AI between 50 and 100, with 100 representing oxides by ALS Minerals, Inc. Twenty samples were complete replacement by sericite, illite, and/or chlo- analyzed for 33 trace elements by four acid digestion rite (Large and others, 2001). The degree of chlorite ICP-AES and for whole-rock oxides by fusion/ICP- alteration replacing albite, K-feldspar, and sericite is AES methods. ALS Minerals' procedure for whole- defi ned by the CCPI (Large and others, 2001). Large rock analysis consists of a lithium borate fusion prior and others (2001) determined that pyrite, magnetite, to acid digestion followed by ICP-AES analysis. Re- or hematite enrichment, and Mg-Fe carbonate al- sults are given in weight percent oxide, shown in table teration have a positive eff ect on the CCPI. Primary 2. For trace element analysis, the sample is subject to composition variations of rock types and magmatic a four acid digestion followed by an ICP-AES fi nish for fractionation have a strong eff ect on the CCPI, result- 33 elements, with results reported in parts per million ing in diff erent values for altered granite and aplite (ppm). (Large and others, 2001). 13 Stanley L. Korzeb

A B

C D Figure 10. Argillic–illite alteration. Microphoto- graph A shows complete replacement of plagio- clase, orthoclase, and annite with illite and other clay minerals. Microphotograph B shows partial replacement of orthoclase with illite. Both photo- graphs are under crossed polarizers.

14 Montana Bureau of Mines and Geology Bulletin 141

Values for the AI and CCPI and corresponding alteration types are shown in table 3 and are plotted on an alteration box diagram (fi g. 11). By plotting the CCPI against the AI, altered and unaltered gran- ite cluster in separate fi elds. Silicic–illite alteration plots in two separate fi elds depending on degree of silicifi cation. Argillic–illite alteration overlaps silicic–il- lite alteration and plots in a fi eld on the right side of the diagram (fi g. 11) near the sericite–illite edge and K-feldspar corner. The AI for silicic–illite and argillic– illite alteration varies from 67.7 to 98.0 (table 3), in- dicating near complete replacement of the feldspars by K–illite and phengite. Disseminated pyrite and magnetite occurs in the silicic–illite and argillic–illite altered granite and is refl ected in the CCPI varying from 9.4 to 91.9 (table 3) depending on the degree of pyrite and magnetite enrichment. Large and others (2001) noted that unaltered rocks will plot in the center of the box diagram at the location of unaltered granite (fi g. 11). Aplite, having a diff erent composition than granite, will have diff erent CCPI values and plot in a separate fi eld. Propylitic

Figure 11. Alteration box diagram identifying and comparing diff erent alteration types and corresponding alteration index (AI) and carbonate–chlorite–pyrite index (CCPI).

15 Stanley L. Korzeb alteration plots adjacent to the granite fi eld and has an bility (table 4). The oxide with the greatest loss com-

AI ranging from 54.1 to 74.5 and CCPI ranging from pared to unaltered granite was Na2O, with a ΔC/CUA 55.9 to 66.1. Propylitic alteration was weak, indicated value equal to -0.92 and ΔC value equal to -2.4 (table by the propylitic fi eld plotting near the center of the 4). On the isocon diagram (fi g. 12), oxides with posi- diagram and adjacent to the granite fi eld (fi g. 11). The tive values plot above the isocon reference line and wide range of AI values or propylitic alteration refl ects those with negative numbers plot below the isocon. overprinting of argillic–illite alteration. For propylitic alteration, most oxides plot near or on the isocon reference line, indicating minor oxide en- Two silicic–illite altered samples, having SiO2 >90 wt% and showing near complete replacement of the richments or depletions compared to the other altera- granite host by quartz, plot in a separate fi eld near tion types. The minor oxide gains and losses indicate the top of the diagram. Due to the strong silicifi cation, minor mobilization of these oxides when the granite these samples lack K–illite and sericite, causing them was propyliticly altered. The oxide with the most loss is Na O, which plots well below the isocon and has to plot in a separate fi eld between carbonate and 2 the same ΔC/C value as argillic–illite alteration. chlorite–pyrite alteration. The CCPI refl ects abundant UA disseminated pyrite that is often included within silici- Argillic–illite alteration ΔC/CUA and ΔC values for

fi ed host rocks. Carbonate alteration plots near the SiO2, Al2O3, and K2O are positive numbers (table 5) epidote–calcite corner, refl ecting the abundant car- and plot above the isocon reference line (fi g. 12). The bonate minerals within this alteration type. ΔC/CUA value for Al2O3 (0.01) approaches 0, indicating Gains or losses of whole-rock oxide concentra- a minor gain compared to unaltered granite and minor tions (table 2) caused by alteration of the host granite mobility, and was used to further defi ne the isocon are plotted on an isocon diagram (fi g. 12). The refer- reference line on fi gure 12. The rest of the oxides all have negative ΔC/CUA and ΔC values and plot below ence isocon was defi ned using TiO2, SiO2, and P2O5, given in tables 4–7 and plotted on an isocon diagram the isocon (fi g. 12), indicating a high degree of loss (fi g. 12), which have little gains or losses between compared to unaltered granite. Compared to propylitic unaltered granite and propylitic alteration. Aluminum alteration, argillic–illite alteration whole-rock oxides oxide has minor gains or losses between unaltered have higher gains and losses, indicated by higher negative and positive ΔC/C and ΔC values (tables 4, granite (UA), propylitic (PR), and agillic–illite (AI) UA 5). The positive ΔC/C value for K O could be related alteration (tables 4, 5). The oxides defi ning the isocon UA 2 reference line are nearly immobile, indicated by the to the presence of illite and phengite in the argillic–il- lite altered granite. The positive ΔC/C value for SiO C /C and C /C (slope of the isocon) being close UA 2 PR UA AI UA could be attributed to minor silicifi cation of the argil- to or equal to 1 and the ΔC/CUA (gain or loss of ox- ide) having values equal to or approaching 0 (Grant, lic–illite altered granite. The rest of the oxides with negative ΔC/C values (table 5) indicate they were 2005). Kuwatani and others (2020) determined that UA an isocon slope equal to one and ΔC equal to zero lost during alteration and overprinting of propylitic defi ne an immobile element and a reference isocon. alteration.

Whole-rock oxides for silicic–illite and carbonate Silicic–illite alteration ΔC/CUA and ΔC values for alteration have wide variations in gains and losses SiO2 make it the only whole-rock oxide with a posi- compared to unaltered granite and cannot be used to tive number (table 6) and that plots above the isocon defi ne a reference isocon for immobile oxides (tables (fi g. 12). The high positive ΔC value could indicate

6, 7). Isocon analysis methods from Grant (2005) and SiO2 (table 6) was mobilized from the hydrothermal Kuwatani and others (2020) were used to determine fl uids into the silicic–illite altered granite. The rest of the isocon reference line on fi gure 12 and determine the whole-rock oxides have negative ΔC/CUA and ΔC the gains and losses of whole-rock oxides. Analytical values (table 6) and plot below the isocon (fi g. 12), values too small to reasonably fi t the isocon diagram indicating a loss for these oxides compared to the were scaled up (tables 4–7). host granite. Compared to propylitic and argillic–illite The ΔC/C and ΔC values for propylitic altera- alteration, silcic–illite alteration has the highest losses UA in Al O , K O, and CaO, indicated by high negative tion oxides have both positive and negative numbers, 2 3 2 ΔC/C and ΔC values (table 6), refl ecting a high indicating both losses and gains (table 4). The oxides UA degree of mobilization. Other whole-rock oxides TiO , used to defi ne the reference isocon TiO and P O 2 2 2 5 P O , Fe O , MgO, MnO, and Na O have losses (table have ΔC/C and ΔC values equal to 0.0, indicating 2 5 2 3 2 UA 6) compared to unaltered host granite. they were immobile, and SiO2 has a ΔC/CUA value equal to -0.01, indicating a minor loss and minor mo-

16 Montana Bureau of Mines and Geology Bulletin 141

Figure 12. Isocon diagram of whole-rock oxides for propylitic, argillic–illite, silicic–illite, and carbonate alteration showing oxide losses (plotting below reference line) and gains (plotting above reference line) compared to the unaltered granite isocon reference line.

17 Stanley L. Korzeb

A

18 Montana Bureau of Mines and Geology Bulletin 141

A B

Carbonate alteration has two whole-rock oxides, tems, either directly or indirectly related to porphyry

CaO and MnO, with positive ΔC/CUA and ΔC values type mineralization, share the same characteristic (table 7) and that plot above the isocon reference trace elements (Halley and others, 2015). The tech- line (fi g. 12). The positive ΔC/C and ΔC values for niques used for evaluating trace elements for epither- C UA D CaO and MnO are a refl ection of the abundant calcite mal veins can therefore be applied to porphyry type that characterizes carbonate alteration. The rest of mineralization. the whole-rock oxides have negative ΔC/C and ΔC UA The degree of gain or loss of trace elements for values (table 7) and plot below the isocon reference the four alteration types compared to unaltered gran- line (fi g. 12). Compared to all the alteration types, ite are shown in an isocon plot (fi g. 13) and in tables carbonate alteration has the highest losses for K O, 2 9–12. Isocon analysis methods defi ned by Grant MgO, Na O, and SiO , indicated by high negative ΔC/ 2 2 (2005) and Kuwatani and others (2020) determined C and ΔC values. Carbonate alteration caused the UA the isocon reference line in fi gure 13. The isocon ref- greatest mobilization of K O, MgO, Na O, and SiO 2 2 2 erence line was defi ned using Sc and Cr. These have compared to unaltered granite and the other alteration C /C (slope of the isocon) values of 1.02 and 1.04, types. The whole-rock oxides TiO , P O , Fe O , and PR UA 2 2 5 2 3 respectively, which approaches 1, indicating a minor Al O have greater losses than argillic–illite alteration 2 3 gain between unaltered granite and propylitic altera- and lower losses than silicic–illite alteration (fi g. 12), tion. The ΔC/CUA (gain or loss) approaches 0 (table 9), indicating a high degree of oxide mobilization during indicating minor Sc and Cr mobility between propylitic carbonate alteration. alteration and unaltered granite. Analytical values too Alteration Trace Elements large to reasonably fi t on the isocon diagram were scaled down. Epithermal vein systems and modern hot springs have unique trace element assemblages and geo- Trace elements for propylitic alteration have both chemical signatures (Mosier and others, 1987; Sil- gains and losses compared to unaltered granite (table berman and Berger, 1985; Simmons and Browne, 9). Elements with gains plot above the isocon refer- 2000). Thirty-three trace elements were determined ence line and those with losses plot below the line by ICP-AES methods by ALS Minerals for 20 samples (fi g. 13). The trace elements Cd, Cu, Mn, Pb, Sb, and Zn have the highest positive ΔC/C and ΔC values representing four alteration types and unaltered gran- UA ite host rock (table 8; complete data set is shown in compared to the other elements (table 9) and plot well above the reference isocon (fi g. 13). The high ΔC/C appendix C). Trace elements known to be character- UA istic of epithermal vein systems, modern hot springs, and ΔC values indicate these elements had a high and other elements consistently above instrument degree of enrichment when propylitic alteration de- veloped. The other elements with positive ΔC/C and detection limits were selected from the total set and UA are shown in table 8. Unaltered granite compared ΔC values Ag and As, were also gained during pro- to altered rock analysis determined gains or losses pylitic alteration and plot above the isocon reference of trace elements. Epithermal and deeper vein sys- line (fi g. 13). Trace elements were also lost from the

19 Stanley L. Korzeb

unaltered granite during propylitic alteration and have and were the most concentrated during silicic–illite negative ΔC/CUA and ΔC values and plot below the alteration. Other elements Ag, Cd, Mo, and Sb have isocon reference line (fi g. 13). The elements Ba and positive ΔC/CUA and ΔC values, indicating concentra- Sr had the greatest losses with the highest negative tion gains compared to unaltered granite and a high

ΔC/CUA and ΔC values, followed by V, Co, Ga, and degree of mobility from the hydrothermal fl uids during Mo; all of these elements were depleted compared to silicic–illite alteration. Compared to the other alteration unaltered granite when propylitic alteration took place. types, silicic–illite alteration has the highest gains for Trace elements for argillic–illite alteration have Ag, As, Cd, Cu, Mo, Pb, Sb, and Zn; these plot well both gains and losses compared to unaltered granite above the isocon reference line (fi g. 13). Trace ele- (table 10). The trace elements with the highest posi- ments lost during silicic–illite alteration are Ba, Co, Cr, Ga, Mn, Sc, Sr, and V, indicated by negative ΔC/CUA tive ΔC/CUA and ΔC values, As, Cd, Pb, and Zn (table 10), indicate the greatest gains compared to unaltered and ΔC values (table 11), and are depleted compared granite and the other elements. The other elements to unaltered host granite. with positive ΔC/CUA and ΔC values, Ag, Ba, Cu, and Compared to unaltered granite, carbonate altera- Sb, have major gains compared to unaltered granite. tion has gains of As, Ga, Mn, Sb, and Sr, indicated by

All the trace elements with positive ΔC/CUA and ΔC positive ΔC/CUA and ΔC values (table 12). Of all the values mobilized from hydrothermal fl uids into the ar- trace elements, Mn had the greatest gain with a ΔC gillic–illite altered granite. Trace elements with nega- value of 1,184.5 ppm, refl ecting the precipitation of tive ΔC/CUA and ΔC values, Co, Cr, Ga, Mn, Mo, Sc, carbonate minerals. The trace elements with positive Sr, and V, were lost and depleted compared to unal- gains plot above the isocon reference line (fi g. 13) tered granite during argillic–illite alteration. Compared and were mobilized from the hydrothermal fl uids into to propylitic alteration, these elements have a greater the carbonate-altered granite. The rest of the trace loss except for Ga, which has a slight gain over propy- elements have losses compared to unaltered granite, litic alteration (fi g. 13). indicated by negative ΔC/CUA and ΔC values (table Silicic–illite alteration has the greatest gains and 12); these plot below the isocon reference line (fi g. losses for trace elements compared to the other 13), indicating depletion from the carbonate-altered alteration types and unaltered granite (table 11, fi g. granite. 13). The trace elements with highest positive ΔC/

CUA and ΔC values are As, Cu, Pb, and Zn (table 11). This indicates these elements had the highest gains

20 Montana Bureau of Mines and Geology Bulletin 141

A

B

Figure 13. Isocon diagram showing trace elements losses and gains for silicic–illite, argillic–illite, propylitic, and carbonate alteration compared to unaltered granite isocon reference line. ppm, parts per million. Elements plotting below the reference line are lost and those plotting above show gains. 21 Stanley L. Korzeb

22 Montana Bureau of Mines and Geology Bulletin 141

23 Stanley L. Korzeb VEIN MINERALOGY Mineral identifi cation techniques consisted of refl ected and transmitted light microscopy and scan- ning electron microscope-energy dispersive spec- troscopy (SEM-EDS) using 15 polished sections, 33 polished thin sections, and 17 thin sections. Mineral identifi cation was done at the Center for Advanced Mineral and Metallurgical Processing at Montana Tech using a Tescan Mira 3 scanning electron microscope equipped with a Tescan EDX detector for the SEM- EDS analysis and backscatter electron (BSE) images. Additional mineral identifi cations using SEM-EDS analysis utilized grains and crystals extracted from cut slabs and hand specimens. Results showed the identifi cation of 35 minerals from eight groups (table 13). The most important min- eral group is the primary and secondary sulfi de and sufosalt minerals. These minerals occur as dissemi- nations and fracture fi llings in the quartz veins. Most of the produced from the district came from the sulfi de and sulfosalt mineral group. Native silver was a minor occurrence found only at the Attowa mine in a microvug with quartz crystals. Native sulfur is a secondary mineral fi lling fractures in altered sulfi de minerals. The oxide minerals are both secondary and pri- mary, occurring as disseminations in the quartz veins and altered host granite. Rutile is the only primary occurring as disseminated grains in vein quartz. The secondary oxide minerals replace the sulfi de minerals along fractures and on mineral surfaces. Iron oxide minerals occur as disseminations and fracture fi llings in the altered host granite. Carbonate minerals are secondary, occurring in the altered host granite and quartz veins as dis- seminations, fracture fi llings, and replacements. The carbonate minerals , cerrusite, and malachite replace primary sulfi de minerals on surfaces and along fractures. Calcite and dolomite fi ll fractures and open spaces in carbonate altered host granite. Two primary silicate minerals, quartz and schorl, occur in the veins. The veins are composed of mas- sive fi ne- to coarse-grained white to clear quartz fi lling faults and open spaces between brecciated and silicifi ed granite. Quartz commonly occurs as massive compacted intergrown anhedral to subhedral grains completely fi lling open spaces. When open space fi lling is incomplete, quartz forms euhedral crystals in vugs. Schorl occurs as needles disseminated in quartz near the vein margins and in silicifi ed host granite. is a secondary copper silicate

24 Montana Bureau of Mines and Geology Bulletin 141 mineral fi lling fractures in quartz and host granite. tion of polished sections, thin sections, and polished The phosphate minerals are both primary and sec- thin sections, and examination of cut slabs and hand ondary. Apatite-(CaF) and monazite-(Ce) are primary specimens. Mineralizing textures, including mineral minerals that occur as disseminations in the quartz overgrowths, replacements on grain boundaries, open veins and host granite. Pyromorphite is a secondary space and fracture fi llings, and mineral disseminations mineral that replaces the primary sulfi de minerals revealed a mineralizing sequence (fi g. 14). This min- and occurs as fracture and open space fi llings in the eralization sequence is a refl ection of the physical and quartz veins and altered host granite. chemical changes of the hydrothermal fl uids during vein development. Physical events such as wall-rock The sulfate and arsenate minerals are second- reactions, fl uctuating temperatures and pressures, ary and replace the primary sulfi de minerals along and infl ux of meteoric water can infl uence the mineral- fractures and on grain surfaces. Secondary minerals ization sequence. The introduction of diff erent metals also occur in fractures and open spaces in vein quartz during vein development will generate diff erent miner- and altered host granite. Secondary minerals formed als, refl ected in the vein paragenesis shown in fi gure microcrystals in open fractures and spaces. 14. Two mineralizing events and host rock alteration Paragenesis defi nes the sequence of mineraliza- developed the Big Foot veins. tion within the vein systems. Characterization and identifi cation of vein paragenesis was determined from SEM-backscatter images, microscopic examina-

Figure 14. Paragenesis of base metal and supergene mineralizing events for the Big Foot veins.

25 Stanley L. Korzeb

Host Rock Alteration Minerals formed minerals (fi gs. 15A, 15D). At the close of the Short-wave infrared spectrometry and illite spec- base metal stage, carbonate minerals crystallized in tral maturity scaler identifi ed two alteration types, a fractures. high-temperature silicic–illite alteration and a low- Supergene Stage Minerals temperature argillic–illite alteration (fi g. 4). Silicic–illite alteration that took place during vein development is Supergene mineralization was the fi nal stage that characterized by K–illite, phengite, schorl, illite–smec- led to the development of oxide, arsenate, phosphate, tite, disseminated pyrite, and intense silicifi cation of sulfate, and secondary sulfi de minerals (fi g. 14). the host granite. Silicic–illite alteration grades into Some of the supergene minerals identifi ed by SEM- the quartz veins and becomes part of the veins along EDS are identifi ed on BSE images in fi gure 17. The the granite contact. Quartz veinlets with inclusions secondary minerals developed from meteoric fl uids of schorl needles and schorl veinlets occur as frac- reacting with primary sulfi de minerals and vein weath- ture and breccia fi llings in the silicifi ed granite. Low- ering. Supergene alteration was minor at most mines temperature argillic–illite alteration characterized by but strongly developed at the Mountain Queen mine. minerals generated from weathering and meteoric wa- Leaching of the primary sulfi de minerals and gran- ter alteration overprints high-temperature silicic–illite ite host rock supplied the elements that developed the alteration. K–illite and a variety of clay minerals are supergene minerals. Covellite and digenite replaced the most common minerals related to low-temperature chalcopyrite along grain margins (fi g. 17A). Gree- alteration. Other common minerals related to weath- nockite fi lls fractures and coats galena (fi g. 17E), and ering and meteoric water alteration include goethite, anglesite replaces galena along grain margins and ferrihydrite, jarosite, hematite, and poorly crystallized fractures (fi g. 17B). Native sulfur fi lls fractures in chal- kaolinite. copyrite (fi g. 17D). The sulfate minerals jarosite, plum- bojarosite, and linarite fi ll open spaces and fractures Base Metal Stage Minerals in quartz and may have developed from a weathering Minerals representing base metal stage mineral- process. Bindheimite, scorodite, and mimetite devel- ization are identifi ed on SEM-BSE images in fi gure oped from the dissolution of arsenopyrite, galena, and 15. The paragenetic sequence (fi g. 14) represents (fi g. 17C). Pyromorphite and mimetite fi ll changes in fl uid chemistry and temperatures during fractures and open spaces in quartz veins; malachite vein development. Schorl and quartz were the fi rst and chrysocolla likewise fi ll and coat fractures in vein minerals to crystallize as veinlets in the brecciated si- quartz. The iron oxide minerals deposited in fractures licic–illite altered granite wall rock and altered granite and replacing pyrite are most likely weathering prod- breccia fragments assimilated into the quartz veins. ucts. Rutile, monzanite-(Ce), and apatite-(Ca,F) crystal- lized along with schorl during early vein development. VEIN TRACE ELEMENTS Pyrite and arsenopyrite followed, representing the fi rst ALS Minerals, Inc. analyzed 18 vein samples sulfi de minerals to precipitate, and pyrite crystallized for 48 trace elements by four acid digestion ICP- continuously throughout the base metal stage. Later MS methods. Gold analyzed by fi re assay was by pyrite partially replaced arsenopyrite (fi g. 15A). ICP-AES or gravimetric fi nish. Analysis of ore-grade With the introduction of zinc, copper, , elements was by four acid digestion and ICP-AES and silver, shalerite started crystallizing and overprint- methods. Results reported in appendix D for 19 ele- ed arsenopyrite and early pyrite. Following sphalerite, ments show anomalous concentrations compared a variety of copper minerals developed, including to unaltered and altered granite. Table 14 lists aver- chalcopyrite, tennantite, , and . age analytical results for vein samples collected from Chalcopyrite was the fi rst copper mineral to crystal- mine dumps and ore stockpiles. Samples represent lize and replace sphalerite along planes. mined quartz veins with disseminated sulfi de minerals Tennantite and tetrahedrite precipitated simultane- that show multiple stages of quartz crystallization as ously followed by bournonite with the introduction open space fi llings in brecciated and silicifi ed granite. of lead (fi g. 15B). Tetrahedrite is silver bearing and Sample size ranged from 0.63 to 1.27 kg, averaging crystallized along with tennantite (fi g. 0.8 kg. 15C). Following pyrargyrite, native silver crystallized Losses or gains of trace element concentrations as wires (fi g. 16) in open spaces in quartz. Galena for quartz veins (Cv) compared to unaltered granite crystallized last, enclosing and fi lling fractures in early (CUA) are presented on an isocon diagram and table

26 Montana Bureau of Mines and Geology Bulletin 141

AB

CD

Figure 15. SEM-BSE images of primary sulfi de minerals in quartz. (A) Euhedral to anhedral arsenopyrite grains partially replaced by pyrite with open spaces and fractures fi lled with galena. (B) Bournonite fi lling open space and partially replac- ing tennantite and tetrahedrite. (C) Pyrargyrite partially replacing tennantite fi lling open space in arsenopyrite. Tennantite partially replaced by mimetite along fractures. (D) Rounded sphalerite, chalcopyrite, and tetrahedrite grains included in late stage galena.

Figure 16. Silver wires in quartz vug with euhedral quartz crystals and galena inclusion from Attowa mine.

27 Stanley L. Korzeb

AB

CD

E

Figure 17. SEM-BSE images of secondary minerals replac- ing primary sulfi de minerals. (A) Covellite partially replacing chalcopyrite with galena fi lling fractures. (B) Anglesite par- tially replacing galena. (C) Arsenopyrite, galena, and ten- nantite being replaced by bindhemite, mimetite, scorodite, and anglesite along fractures. (D) Fractures in chalcopyrite fi lled with sulfur. (E) Greenockite and anglesite partially replacing galena.

28 Montana Bureau of Mines and Geology Bulletin 141

(fi g. 18, table 15). Isocon analysis results show there Sc, Sr, and V, as indicated by negative ΔC/CUA and are no conserved elements with equal concentrations ΔC values (table 15), and these plot below the isocon between unaltered granite and quartz veins, indicated reference line (fi g. 18). Trace elements depleted dur- by isocon slopes CV/CUA being well below or above ing quartz vein development have negative ΔC/CUA 1, and cannot be used to defi ne an isocon reference and ΔC values and are not anomalous compared to line. The isocon reference line in fi gure 18 is taken unaltered granite. from fi gure 13 and uses trace elements Sc and Cr be- tween unaltered granite and propylitic alteration. Loss- QUARTZ CATHODOLUMINESCENCE es and gains of individual elements were determined Quartz is the most common mineral and occurs using isocon analysis methods from Grant (2005) and in all the veins throughout the district. Multiple quartz Kuwatani and others (2020). Analytical values too generations fi ll open spaces, and fractures in brecci- large to reasonably fi t on the isocon diagram were ated silicifi ed granite, and replace granite breccia frag- scaled down (table 15). ments assimilated into the veins. Quartz veins consist The isocon analysis in table 15 indicates gains of fi ne- to coarse-grained white to clear anhedral and losses by positive or negative values for ΔC/ compacted quartz grains with disseminated sulfi des

CUA and ΔC. The trace elements As, Cu, Pb, and Zn and sulfi de minerals fi lling open spaces included with have the highest positive ΔC/CUA and ΔC values (table euhedral clear to milky quartz crystals. Clear to white 15) compared to the other elements. These plot well euhedral quartz crystals line open vugs. The parage- above the isocon reference line (fi g. 18), indicating a netic sequence of these quartz generations reveals high degree of concentration during quartz vein de- physical changes that took place during vein develop- velopment. The other trace elements Ag, Cd, Co, Mo, ment. and Sb have positive ΔC/C and ΔC values (table UA Cathodoluminescence (CL) revealed the relation- 15) and plot above the isocon reference line (fi g. 18), ship between quartz crystallization and sulfi de min- indicating anomalous concentrations gained during eralization. A Tescan Mira3 scanning electron micro- quartz vein development. Trace elements that were scope equipped with a Tescan Pan Chromatic CL lost compared to unaltered granite are Ba, Ga, Mn, detector located at Montana Technological University

29 Stanley L. Korzeb

Figure 18. Isocon diagram showing trace element loss (plotting below reference line) and gains (plotting above reference line) for quartz veins compared to unaltered granite isocon reference line. ppm, parts per million.

30 Montana Bureau of Mines and Geology Bulletin 141

Center for Advanced Metallurgical Processing was used to examine 14 polished thin sections representing veins from six mine locations for CL response. The instrument was set up with an electron beam acceleration of 15 and 20 kV, and beam intensity of 13 and 15 kV, with a working distance ranging from 12.5 to 14.97 mm. CL response varied from no response, resulting in a black image, to a strong response, yielding a bright white im- age. Some quartz grains showed a mottled image varying from bright to gray and others revealed euhedral forms with oscillatory growth bands. CL imaging revealed six quartz gen- erations and two microbrecciation stages. Brecciated primary quartz grains from the granite host gave a strong to moderate CL response with a mottled pattern (fi g. 19). Quartz stringers Figure 19. Primary quartz grain in silicic–illite altered host granite showing ex- fi lling fractures in the brecciated tensive microbrecciation and fracturing. Fractures fi lled with secondary quartz primary quartz grains did not give a and schorl. strong CL response and appear as black to dark gray veinlets. The non- responsive black background (fi g. 19) is quartz and illite that partially replaces brecciated feldspar, and schorl and quartz fi lled fractures.

31 Stanley L. Korzeb

Figure 20. Multiple generations of quartz crystallization. The fi rst generation (1) was fractured and replaced by second-generation quartz (2), both generations form the cores of euhedral crystals. A third quartz generation (3) showing oscillating growth zones was overgrown on generation one and two euhedral crystals. A fourth quartz generation (4) fi lls open spaces and fractures in microbrecciated euhedral crystals. A sixth non-brecci- ated generation (6) following sulfi de crystallization fi lls open spaces adjacent to euhedral crystals.

Three quartz generations fi ll fractures and open After the fi rst three generations developed, they spaces in the silicifi ed and brecciated host granite. underwent extensive fracturing and microbrecciation. Microbrecciation followed the three quartz generations This was followed by a fourth quartz generation fi lling and a fourth quartz generation precipitated (fi g. 20). fractures and partially replacing early quartz genera- The fi rst generation (1, fi g. 20) is not common and tions and cementing breccia fragments (4, fi g. 20). occurs as fragments showing oscillatory growth with a The fourth quartz generation was nonresponsive to dark gray to black CL response located in the cores of slightly responsive to CL activation, resulting in black euhedral crystals and overgrown by second and third to dark gray stringers and patches. The degree of quartz generations. The second quartz generation fracturing and microbrecciation varied from moderate (2, fi g. 20) gives an even light gray CL response and to extensive (fi gs. 21, 22). Moderate fracturing of the forms the cores of euhedral crystals overgrowing and early quartz generations in fi gure 21 display numer- replacing fragments of the fi rst generation. Following ous quartz stringers fi lling fractures in euhedral quartz the second generation, a third quartz generation (3, crystals. Strongly brecciated early quartz generations fi g. 20) formed overgrowths on the fi rst and second shown in fi gure 22 consist of angular fragments of generations and defi nes the outer margins of euhe- generation 2 and 3 quartz cemented with generation dral crystals. The third quartz generation overgrowths 4 quartz. Sulfi de minerals represented by euhedral to gave a medium gray CL response and show oscillat- anhedral pyrite grains that fi ll open spaces in the mi- ing growth bands. crobreccia and that are scattered in the breccia matrix precipitated with the fourth quartz generation (fi g. 22).

32 Montana Bureau of Mines and Geology Bulletin 141

Figure 21. Fractured and partially brecciated generation two and three euhedral quartz crystals. Fractures fi lled with generation 4 quartz stringers and open space fi lling.

Figure 22. Intense microbrecciation of second- and third-generation quartz crystals (2 and 3) and grains. Open spaces between breccia fragments fi lled with a fourth quartz generation and pyrite (black grains).

33 Stanley L. Korzeb A fi fth quartz generation crystallized at the same FLUID INCLUSIONS time sulfi de minerals precipitated in intensely brecciat- ed early quartz stages. The fi fth quartz generation (5, Microthermometric measurements of fl uid inclu- fi g. 23) shows a strong Cl response and weak oscilla- sions were made to gain an understanding of the tory growth zones. Pyrite grains (black CL response) temporal evolution, salinity, and temperatures for the are included in the base of the euhedral quartz crys- hydrothermal fl uids that developed the veins. The fi fth tals, suggesting simultaneous growth. Shortly after quartz generation identifi ed by CL yielded the best the quartz crystals formed, sphalerite precipitation fl uid inclusions for study. Fluid inclusions in the other followed, fi lling in the remaining open space. The quartz stages were too small for reliable measure- fi fth quartz generation did not undergo brecciation or ments or were lost due to fracturing and brecciation. fracturing and neither did the sulfi de minerals; this Fluid inclusions were selected from euhedral quartz indicates sulfi de precipitation followed at the close of crystals (fi fth quartz generation) and were analyzed microbrecciation. The fi fth quartz generation, repre- on a Fluid, Inc. adapted USGS-type gas-fl ow heating sented by euhedral quartz crystals enclosed by sulfi de freezing stage mounted on an Olympus BH-2 micro- minerals and lining open vugs, yielded the best fl uid scope located at the Geological Engineering Depart- inclusions for study. ment, Montana Tech. Following sulfi de mineral precipitation, a sixth Six identifi ed fl uid inclusion types were type I, type quartz generation crystallized in the remaining open II, liquid, vapor, and two types of three-phase inclu- spaces (6, fi g. 20). The sixth generation gave a dark to sions (fi gs. 24–27). Type I inclusions are the most medium gray Cl response and has a mottled appear- common and consist of liquid and <50 vol% vapor ance. This quartz stage along with the fi fth stage did bubble (fi g. 24). Type II inclusions are vapor domi- not show evidence of microbrecciation or fracturing. nated, consisting of liquid and >50 vol% vapor bubble (fi g. 25). Single-phase inclusions consisting of all liquid or vapor were not common, and some of these may have developed from necking of larger two- phase inclusions. Three-phase inclusions with a halite

Figure 23. A fi fth quartz generation (5) showing euhedral crystals with weak oscillatory growth that crystallized with sphal- erite and pyrite in open space within brecciated second and third generations.

34 Montana Bureau of Mines and Geology Bulletin 141

Figure 24. Primary type I two-phase and liquid fl uid inclusions.

Figure 25. Primary type II two-phase fl uid inclusion consisting of a vapor bubble and brine.

35 Stanley L. Korzeb

Figure 26. Three-phase fl uid inclusion with liquid CO2, vapor, and brine.

Figure 27. String of secondary type I and II fl uid inclusions in a healed fracture.

36 Montana Bureau of Mines and Geology Bulletin 141 daughter mineral, brine, and vapor were also pres- ature (Tm) when the last ice crystal melted. Salinity ent but are rare. These were found at nine locations. calculated from the Tm used the methods of Roedder

Another type of three-phase inclusion with liquid CO2, (1984). a vapor bubble, and brine was identifi ed in samples Three-phase inclusions with liquid, vapor, and a from six locations (fi g. 26). halite daughter mineral homogenized in steps yielding The designation between primary and second- homogenization and halite dissolution temperatures ary inclusions used the criteria outlined by Roedder (table 16). Complete homogenization temperature (1984). Primary inclusions occur as groups trapped in was reached when both the vapor phase and halite quartz crystal growth zones and yielded data refl ect- phase assimilated into the liquid phase, yielding a ho- ing the time of entrapment. Primary inclusions are mogenized liquid-fi lled inclusion. Salinities were calcu- type I and II, liquid, and vapor or three phase. Sec- lated from the halite dissolution temperature method ondary inclusions are found in healed fractures and of Sterner and others (1988). can be type I and II, liquid, or vapor (fi g. 27). Second- Three-phase inclusions with liquid CO developed ary inclusions give information on late-stage fl uids 2 clathrate upon freezing, indicating dissolved CO2 in trapped in healed fractures during the close of crystal- the fl uids at the time of entrapment. Homogeniza- lization. Inclusions show evidence of necking, which tion temperatures were reached when the CO2 liquid will generate all liquid and all vapor inclusions. Neck- and vapor bubble assimilated into the brine phase ing may have taken place during fracture healing and and generated a homogenized liquid-fi lled inclusion. recrystallization of primary quartz crystals. Salinities were determined from the clathrate melt A total of 181 quartz-hosted fl uid inclusions were temperature using the methods of Darling (1991). analyzed, including 161 two-phase type I and II, 9 Homogenization temperatures illustrated on a three-phase inclusions with halite, and 11 three-phase histogram plot (fi g. 28) show two distinct populations. inclusions with CO liquid, vapor, and brine. Homog- 2 Most inclusions were trapped between 210⁰C and enization temperatures and wt% NaCl equivalent (eq) 350⁰C, indicating the veins developed from one hydro- are summarized in table 16, and the complete data thermal event. Near the close of hydrothermal activity, set is shown in appendix E. All two-phase inclusions a population of lower temperature inclusions rang- yielded both homogenization temperature (Th) and ing from 140⁰C to 200⁰C were trapped. These lower wt% NaCl eq data, with the exception of one inclusion temperature inclusions could represent the close of that only gave Th. Primary and secondary type I and hydrothermal activity. II inclusions homogenized into a liquid phase. Ho- mogenization temperatures for type I and II inclusions Salinities illustrated on a histogram plot (fi g. 29) were the same for those in assemblages of 3D clus- show two distinct populations between 0 to 8 and 32 ters. Fluid salinities for type I and II inclusions were to 38 wt% NaCl eq. Most fl uid inclusions fall in the 0 determined by freezing with liquid nitrogen followed to 9 wt% NaCl eq range, and nine three-phase inclu- by melting the ice and recording the melting temper- sions with halite daughter minerals fall in the 32 to 39 wt% NaCl eq range. Vein quartz from the State mine

37 Stanley L. Korzeb

Figure 28. Histogram plot showing distribution of homogenization temperatures. has type I and II fl uid inclusions with salinities ranging district. Further south, the Mountain Queen (BF-19) from 6.0 to 8.4 wt% NaCl eq. The largest vein in the and St. Anthony veins (BF-54) each have one three- district developed by the Big Four, Nickel Plate, Ajax, phase inclusion. Five three-phase inclusions were in and Attowa mines, has type I and II fl uid inclusions a quartz vein developed by prospect pits (BF-63) near with salinities ranging from 0.4 to 6.9 wt% NaCl eq. Whitetail Park (fi g. 2). Further south in the district, veins developed by the On a salinity versus homogenization temperature Mountain Queen and St. Anthony mines, and pros- plot, two distinct fi elds are displayed (fi g. 30). Near pects near Whitetail Park, have salinities ranging from the top of the diagram, a group of nine inclusions with 0.2 to 3.2 wt% NaCl eq for type I and II inclusions. halite daughter minerals with salinities ranging from Nine three-phase inclusions with halite daughter min- 32.6 to 38.1 NaCl wt% eq form one fi eld. The rest of erals have salinities ranging from 32 to 38 wt% NaCl the fl uid inclusions plot near the bottom of the dia- eq. These occur in four veins throughout the district. gram and form a fi eld represented by low salinity (0.1 One three-phase inclusion was found at the Big Four to 8.4 NaCl wt% eq) with a wide temperature range site (BF-4) and one at the Attowa site (BF-9); both of (148.9⁰ to 347.4⁰C). The temperature corresponding these sites are on the same vein cutting across the

38 Montana Bureau of Mines and Geology Bulletin 141

Figure 29. Histogram plot showing distribution of salinity in weight percent NaCl equivalent. to both the high- and low-salinity fi elds overlap, and Samples were extracted under a microscope from cut this shows salinities changed during vein evolution but slabs using a Dremel tool and diamond bit or hand temperature remained relatively constant. The dia- extracted under a microscope from specimens. All gram further shows lower (<200⁰C) or higher (>300⁰C) measured sulfur isotope values are reported in stan- temperatures did not have an impact on salinity. dard δ34S‰ notation (in per mil, ‰) relative to Vienna Canyon Diablo Troilite (VCDT) standard. SULFUR ISOTOPES Sulfur isotopic data for 16 pyrite, 16 galena, 11 Sulfur isotopes for 48 sulfi de mineral samples sphalerite, and 5 chalcopyrite samples are illustrated were analyzed at the stable isotope laboratory of on a histogram plot (fi g. 31) and summarized in table the U.S. Geological Survey Crustal Geophysics 17. Appendix F presents the completed data set. Geochemistry Science Center in Denver, Colorado. Although the sulfur isotope data shows a wide varia- Analysis of sulfur isotopes was done by vacuum-line tion, from -1 to 16‰, all but one sample vary within -1 to 5‰. The anomalous value is for a galena sample conversion to SO2 followed by an elemental analyzer using a Thermo Delta Plus XP mass spectrometer. above 16‰.

39 Stanley L. Korzeb

Figure 30. Salinity (NaCl wt% equiv.) versus homogenization temperature diagram for type I and II, three-

phase CO2 , and three-phase halite daughter fl uid inclusions showing groups of low- and high-salinity fl uids.

40 Montana Bureau of Mines and Geology Bulletin 141

Figure 31. Histogram showing distribution of total sulfur isotopes for all phases.

Geothermometery was determined for isotope OXYGEN ISOTOPES pairs from four locations using the methods of Ding and others (2003). Spahlerite–galena pairs, show- Oxygen isotopes were determined for six quartz ing co-genetic relationships and physical evidence of samples from fi ve locations (table 18) within the most being in equilibrium, were selected for geothermom- extensive and mineralized veins in the district (fi g. 2). etry calculations. Isotopes from 11 diff erent sphaler- Selected samples represent clear to milky euhedral to ite–galena pairs used in the calculations represent anhedal quartz crystals and grains from sulfi de–quartz the St. Anthony vein and the vein crossing the district veins of disseminated sulfi des and silicifi ed host sampled at the Big Four, Nickel Plate, and Attowa rocks. Coarsely crushing slabs and hand specimens mines (summarized in appendix G). Results showed freed and separated the quartz crystals and grains, equilibrium temperatures ranging from 304.2⁰C to which were subsequently hand selected under a bin- 578.6⁰C and averaging 429.8⁰C. The two lower equi- ocular microscope. The hand-picked 100 g of coarsely librium temperatures (304.2⁰C and 332.1⁰C) overlap crushed quartz grains was powdered in a pulverizer the higher fl uid inclusion temperatures, which range as instructed by Geochron Laboratories. from 306.9⁰C to 347.5⁰C. The powdered samples were sent to Geochron Laboratories, Chelmsford, Massachusetts for isotope analysis. The laboratory procedure for extracting oxygen isotopes from the quartz powders used the

CIF3 method of Borthwick and Harmon (1982), quan-

titatively converting O2 to CO2 over hot graphite. The oxygen isotope measurements were performed using an Optima dual-inlet stable isotope mass spectrom- eter. The oxygen isotope results (table 18) are in the normal delta-notation relative to Vienna standard

41 Stanley L. Korzeb mean ocean water (VSMOW). DISCUSSION Temperatures acquired from fl uid inclusion data All the veins in the Big Foot district hosted by and sulfur isotope pairs from each sample site were the Butte Granite (fi g. 2) developed after the granite used to calculate δ18O for the hydrothermal fl uids solidifi ed and fractured by tectonic forces. Age dat- using quartz–water fractionation equations from ing suggests the veins could be contemporaneous Campbell and Larson (1998), Matthews and Beck- with a late-stage aplite intrusion that followed the insale (1979), and Clayton and others (1972). Aver- emplacement of the Butte Granite. The granite host- age temperatures used to calculate the hydrothermal ing the veins crystallized at a depth of 7 to 8 km, as fl uid δ18O composition for quartz veins from the Ajax, determined by hornblende barometric methods (Olsen Nickel Plate, and State mines are from pressure-cor- and others, 2016). Age dating of the veins by 40Ar/39Ar rected fl uid inclusion homogenization temperatures. methods yielded ages varying from 73.8 ± 0.12 to Galena–sphalerite sulfur isotope pairs were used for 75.3 ± 0.25 Ma (Olson and others, 2016), similar to the Big Four and Attowa mines temperatures. the aplite dike zircon age of 74.5 ± 0.6 Ma (Korzeb The δ18O (‰) was calculated using the quartz– and Scarberry, 2017). The similarities in ages sug- H2O water fractionation equations from Campbell and Lar- gest the veins developed from a hydrothermal system son (1998) for measured fractionation between quartz coeval with aplite intrusion and may have evolved and water. Fractionation equations from Matthews from magmatic fl uids originating from a cooling plu- and Beckinsale (1979) were applied for temperatures ton at depth. The presence of schorl further suggests ranging from 265⁰ to 465⁰C, and Clayton and others the early mineralizing fl uids were saturated in boron, (1972) fractionation equations were used for tempera- silica, and aluminum and may have had a magmatic tures ranging from 500⁰ to 750⁰C. source. Oxygen isotope results for the quartz samples Similarities between ages of the veins and ap- showed little variation between sampling sites along lite intrusions indicate the initial magmatic fl uids that the strike of the vein represented by sample sites at developed the veins and deposited schorl could have the Big Four, Ajax, Nickel Plate, and Attowa mines originated from the same felsic pluton. Aplite intru- (table 18). The vein mined at the State mine has sions hosted by the Boulder Batholith are known to be the same oxygen isotope compositions as the vein boron enriched, developing pegmatites with abundant represented by the other four mines. Calculating the schorl. Boron-enriched magmas could originate from oxygen isotope composition for the hydrothermal fl uid the anataxis of boron-rich protoliths such as marine using quartz–water fractionation equations showed sediments (London and others, 2002). The original a wide variation in isotope compositions (table 18). boron source for the aplite intrusions may have been These compositions ranged from 6.8 to 9.0‰ for the marine sediments of the Proterozoic Belt Super- main vein represented by Big Four, Ajax, Nickel Plate, group formations, which at some locations contain and Attowa mines. The vein at the State mine also tourmalinites (Beaty and others, 1988; Slack, 1993) showed a similar wide variation ranging from 6.5 to and underlie the Boulder Batholith (Harrison, 1972). 7.5‰. Assimilation of Proterozoic Belt Supergroup forma- tions into a magma could generate a boron-enriched felsic magma that was intruded as aplite and alaskite; this may be the mineralizing fl uid source for the Big

42 Montana Bureau of Mines and Geology Bulletin 141

Foot district. Boron, being an incompatible element, tion progressed from schorl deposition and quartz will exsolve along with water from the cooling magma to K–illite, illite–smectite, dickite, montmorillonite, and (London and others, 2002), generating a boron- muscovite alteration, the hydrothermal fl uid began enriched magmatic-hydrothermal fl uid. A magmatic- to neutralize, becoming less acidic (Simmons and derived, boron- and silica-enriched hydrothermal fl uid others, 2005; Fournier, 1985). The presence of schorl originating from a cooling magma could have invaded and dickite is indicative of acidic pH conditions while fractures currently occupied by the veins, depositing K–illite and illite–smectite indicate a neutral pH envi- schorl and quartz along the vein margins, and silici- ronment (Hedenquist and others, 2000; Reyes, 1990). fi ed the host granite, which would generate silicic–illite Alteration of the host granite continued after alteration. silicic–illite alteration, generating a low-temperature Wall-Rock Alteration argillic–illite alteration distal from the veins. The low- Four types of alteration occur in the granite host temperature hydrothermal fl uid was less acidic than rock adjacent to the veins. These include propylitic, the high-temperature hydrothermal fl uids and generat- carbonate, silicic–illite, and argillic–illite identifi ed by ed K–illite, illite–smectite, phengite, and kaolinite. Ka- VNIR-SWIR spectroscopy, whole-rock analysis, and olinite is both well crystallized and poorly crystallized, from thin sections and hand specimens. Propylitic suggesting a transition from late-stage hydrothermal alteration is not common and was identifi ed at two conditions (well-crystallized kaolinite) to supergene locations, at the Big Major mine and St. Anthony mine. weathering (poorly crystallized kaolinite). Supergene Propylitic alteration was overprinted by silicic–illite weathering generated ferrihydrite, jarosite, goethite, alteration throughout the district, which could account hematite, and poorly crystallized kaolinite. for its rarity. Carbonate alteration was found at one The overprinting of propylitic alteration by argil- location, the Big Major mine. Silicic–illite alteration lic–illite alteration may be an indication of a cooling and argillic–illite alteration are the most prolifi c altera- system. The high-temperature silicic–illite alteration tion types within the wall rock adjacent to all the veins overprinting propylitic alteration followed by cooler throughout the district. argillic–illite alteration indicates a drop in temperature Two alteration types, identifi ed as high tempera- as alteration was taking place. Two fl uid inclusion ho- ture and low temperature by VNIR-SWIR spectros- mogenization temperature populations ranging from copy, took place during vein development. High- 140⁰ to 200⁰ C and 210⁰ to 350⁰ C further suggest a temperature alteration, represented by silicic–illite cooling system during vein and alteration develop- alteration, developed by hydrothermal fl uids silicifying ment. and replacing the host granite with quartz. This took Pressure and Depth of Mineralization place during quartz vein mineralization. Following The surface exposures of the Butte Granite in the the silicic–illite alteration, low-temperature altera- Big Foot district are estimated to have crystallized at tion developed argillic–illite alteration. This alteration a depth of 7 to 8 km (Olson and others, 2016). Us- overprinted the silicic–illite alteration and formed ing a geobaric gradient of 3.5 km per kb, the litho- distal from the veins. The transition from silicic–illite static pressure would be approximately 2 to 2.3 kb. alteration to argillic–illite alteration and overprinting This lithostatic pressure is similar to the 2.1 to 2.4 kb of propylitic alteration implies a cooling hydrothermal pressure estimated for the crystallization of the Butte system. Schorl, along with a variety of clay minerals Granite from hornblende compositions (Anderson and present in the silicic–illite and argillic–illite alteration, Smith, 1995; Olson and others, 2016). Quartz vein indicate mineralization from a hydrothermal fl uid with lithostatic pressure was estimated from aqueous two- a variable pH. phase fl uid inclusions and an average temperature Schorl precipitates from fl uids with a pH below from sulfur isotope geothermometry of sphalerite–ga- 6.5 and is unstable in alkali fl uids with a pH above 7.0 lena pairs. By applying the average fl uid inclusion (London, 2011). The presence of schorl and extensive homogenization temperature (270⁰C) and average 2.6 silicifi cation suggest the initial hydrothermal fl uid was wt% NaCl eq to a diagram in Roedder (1984), a fl uid enriched in boron, aluminum, and silica with an acidic density of 0.75 g/cm3 was estimated. The estimated pH. The acidic boron–silica-enriched hydrothermal fl uid density (0.75 g/cm3) and average sulfur isotope fl uids altered the host granite. This released Al, Na, geothermometry temperature (430⁰C) were applied to and K and led to schorl and quartz precipitation in a pressure temperature diagram from Fisher (1976). open fractures, as well as the replacement of feldspar That yielded a lithostatic pressure of 2 kb for quartz minerals with quartz and clay minerals. As altera- vein mineralization. This pressure is close to the pres-

43 Stanley L. Korzeb sure determined by Olson and others (2016) from the Zn, are the most anomalous compared to unaltered Butte Granite hornblende compositions and calculated granite. These elements could have been sourced from the crystallization depth. from either the host granite by circulating meteoric water or a hydrothermal-magmatic-derived fl uid. Vein Formation Temperature Based on the depth of vein formation (7 to 8 km), The average two-phase fl uid inclusion tempera- trace element content, fl uid inclusions and stable ture (270⁰C) is 160⁰C lower than the average sulfur isotopes, the hydrothermal fl uid was derived from a isotope geothermometry temperature (430⁰C). There magmatic source and not from circulating meteoric is little evidence for boiling and all inclusions homog- water. The trace elements B, As, Cu, Pb, Sb, and enized to liquid, and this suggests trapping above the Zn are enriched in hydrothermal fl uids sourced from liquid–vapor curve in the one-phase fi eld. Diff erences volcanic arc subduction-derived magmas (Noll and in the fl uid inclusion homogenization temperatures others, 1996). The same trace element suite from and sulfur isotope geothermometry temperatures the Big Foot veins also occurs in subduction-derived could be attributed to the lack of boiling during sulfi de magmas, and this implies a magmatic-hydrothermal mineralization. fl uid source for the Big Foot trace element suite. Trace element content, fl uid inclusions, and stable isotopes An isobaric correction needs to be applied to the from the epithermal polymetallic deposit of Cerro homogenization temperatures to obtain the true fl uid de Pasco, Peru, indicates a magmatic origin for this inclusion trapping temperatures. The upper pressure deposit (Rottier and others, 2018). Compared to the is constrained by the depth of crystallization of the polymetallic Cerro de Pasco, Peru deposit, the Big Butte Granite at 2.3 kb. A fl uid density of 0.75 g/cm3 Foot veins have similar anomalous trace elements, was determined from average salinity and Th for two- fl uid inclusions, and stable isotopes. This further phase fl uid inclusions. By applying the fl uid density implies a magmatic trace element source for the Big and lithostatic pressure to a pressure density dia- Foot veins. gram from Fisher (1976), a temperature correction of 160⁰C was determined. Applying the 160⁰C correction Sulfi de-Quartz Vein Genesis yielded an inferred fl uid trapping temperature ranging Fluid inclusion data, sulfur isotope equilibrium from 310⁰C to 508⁰C with an average of 430⁰C. This temperatures, and lithostatic pressure evidence sug- pressure-corrected fl uid inclusion temperature range gest the veins exposed on the surface in the Big Foot overlaps the sulfur isotope, equilibrium temperatures, district represent the deep roots of an inferred epith- which range from 304⁰C to 579⁰C. ermal system. Fluid inclusion temperatures for two- phase fl uid inclusions, corrected for pressure, range Trace Elements from 310⁰C to 508⁰C with an average of 430⁰C, and Anomalous concentrations of trace elements from salinities range from 0.2 to 8.4 with an average of 2.6 the veins and altered rocks consist of Au, Ag, As, Cd, wt% NaCl eq. Three-phase fl uid inclusions with a ha- Co, Cu, Pb, Sb, Mn, Mo, and Zn. The trace elements lite daughter mineral have pressure-corrected homog- Ag, As, Pb, Sb, and Zn are characteristic of epither- enization temperatures averaging 436⁰C, with a range mal systems and active geothermal vents (Weissberg, of 377⁰C to 474⁰C, an average salinity of 33.6 wt% 1969; Silberman and Berger, 1985; Ewers and Keays, NaCl eq, with a range of 32.6 to 38.1 wt% NaCl eq. 1977; Simmons and Browne, 2000; Simmons and Three-phase CO2 inclusions that generated clathrate others, 2016). The other trace elements, including have an average pressure-corrected homogenization Au, Cd, Co, Cu, Mn, and Mo, are not characteristic of temperature of 452⁰C with a range of 407⁰C to 472⁰C. epithermal vein systems but are reported from active These have an average salinity of 1.4 wt% NaCl geothermal fi elds, precipitates, and sulfi de minerals equivalent with a range of 1.2 to 1.6 wt% NaCl eq. in modern volcanic back arc environments (Fouquet Lithostatic pressure and fl uid inclusion data suggest and others, 1993; Berkenbosch and others, 2012; the vein quartz started crystallizing from a high-tem- Simmons and others, 2016). In addition to epither- perature, hydrothermal fl uid with low to high salinities mal systems, the Big Foot trace element suite is also similar to those measured in deep porphyry systems characteristic of porphyry systems (Halley and others, (Rusk and others, 2008; Reed and others, 2013). 2015). The trace elements and boron represented Single-phase magmatic-hydrothermal fl uids rise in by schorl from the Big Foot veins could result from a pulses as over-pressure waves from cooling plutons hydrothermal fl uid derived from a magmatic source. (Monecke and others, 2018). Pressures intermittently The trace elements, As, Ag, Cd, Cu, Pb, Sb, and exceeding lithostatic pressures within the upward-

44 Montana Bureau of Mines and Geology Bulletin 141 moving pulses cause microfracturing of the host rock could cause dissolution of an early quartz phase back with the passing of each over-pressure wave, in- into the magmatic-hydrothermal fl uids (Monecke and creasing host rock permeability. Monecke and others others, 2018). When pressures return to lithostatic (2018) suggest with the passing of each over-pres- conditions or lower, quartz will reprecipitate over sure wave, early formed quartz will go into dissolution, the partially dissolved fi rst quartz generation. These and when pressure drops to lithostatic conditions, periodic changes between lithostatic pressures may quartz will precipitate and recrystallize overprinting have taken place when the Big Foot veins crystallized the early quartz phases. The cathodoluminescence and are recorded by quartz textures revealed by CL (CL) patterns and textures found in the cores of early responses. quartz crystals suggest the Big Foot veins formed A third quartz generation with oscillating growth during fl uctuating pressure events. The passage of bands revealed by CL response occurs as an over- multiple pressure waves will create a stockwork of growth on early stage one and two quartz crystals. early quartz veins in the host rock and reopen earlier- The quartz overgrowths could have precipitated from formed veins (Monecke and others, 2018). a rising and cooling single-phase, magmatic-hydro- Textures revealed by Cl response of primary thermal fl uid at lithostatic conditions. Monecke and quartz grains remaining within the altered host granite others (2018) suggest that during cooling of a hydro- show extensive microfracturing and brecciation. The thermal fl uid, pressure conditions may fl uctuate and microfractures are fi lled with quartz that give a black decompression to hydrostatic or lower pressure condi- to dark gray CL response. This microfracturing may tions may occur. This decompression and cooling have been caused by pulses of over-pressured fl uids of the hydrothermal fl uid would cause further quartz exceeding lithostatic pressure followed by pressure precipitation. During decompression to hydrostatic drops to lithostatic conditions and quartz precipita- conditions, brittle fracturing of the early formed quartz tion. Along with quartz, schorl also fi lls microfractures, veins could take place (Monecke and others, 2018). which does not give a CL response. Two microbrecciation events took place during Cathodoluminescence of the quartz veins shows vein development. One event brecciated the host six quartz generations and two episodes of microbrec- granite and another brecciated the quartz veins. The cia generation and microfracturing of primary quartz host granite underwent microbrecciation, which gener- grains in the host granite. Each quartz generation ates open spaces and fractures. This allowed quartz represents pressure and temperature changes that veinlets to develop, feldspar replacement by quartz took place during vein development. The fi rst two and clay minerals, and schorl to crystallize from quartz generations took place at high temperatures invading hydrothermal fl uids. After the third quartz (≥500⁰C) and fl uctuating lithostatic pressures. The generation precipitated, brittle fracturing generated cores of these crystals show CL patterns consist- microbreccias in the early quartz veins. A fourth quartz ing of bright to black responding grains and grains generation giving a black to dark gray CL response with dark oscillatory growth bands showing evidence fi lled fractures and open spaces generated by micro- of dissolution along the grain boundaries. The early brecciation of the early stage quartz crystals. A fi fth quartz generations formed euhedral crystals in open quartz generation gave a low to white CL response, spaces and crystallized at the same time as schorl. displaying oscillating growth bands, crystallized with When schorl is present, quartz is included with schorl sulfi de minerals in open spaces generated by micro- crystals resembling needles. Early quartz with schorl brecciation. Equilibrium temperatures determined inclusions suggest the quartz precipitated at similar from sulfur isotope pairs indicate sulfi de mineraliza- acidic pH, high temperatures, and lithostatic pressure tion and microbrecciation took place over a wide required for schorl crystallization. temperature range (304⁰ to 579⁰C). Monecke and The fi rst quartz generation might have precipitated others (2018) noted pressure changes from lithostatic from cooling magmatic-hydrothermal fl uids ascending to hydrostatic take place at wide temperature ranges from a cooling pluton. Pressure fl uctuations as well (≈350⁰C to ≈500⁰C) and pressures up to 2 kb for a as temperature infl uenced the precipitation of the fi rst deep porphyry system that is similar to the Big Foot quartz generations. Dissolution and recrystallization vein temperatures and pressures. When lower litho- textures revealed by Cl response indicate the fi rst static pressure conditions are reached, temperatures quartz generation went into dissolution after crystal- are typically below ≈350⁰C (Monecke and others, lization and was later replaced by a second genera- 2018), and this is refl ected in lower fl uid inclusion ho- tion. Fluid pressures exceeding lithostatic pressure mogenization and sulfur isotope equilibrium tempera-

45 Stanley L. Korzeb tures for the Big Foot quartz veins. ous rocks not infl uenced by crustal sources are ex- 34 Quartz precipitation during and after breccia- pected to have a δ S composition of 0 ± 3‰ (Ohmoto 34 tion most likely took place during a transition from and Rye, 1979). The range of δ S found in the Big lithostatic to stable lower pressure conditions over Foot veins extends to 16‰, suggesting a magmatic- a wide temperature range. Microbrecciation of the hydrothermal fl uid source infl uenced by crustal rocks. early quartz phases allowed a rising magmatic-hydro- Proterozoic and Phanerozoic marine evaporates, 34 thermal fl uid to circulate throughout the vein system. which have a δ S range of 10‰ to 35‰, could be a Cooling of a circulating magmatic-hydrothermal fl uid crustal source for heavy sulfur (Claypool and others, could lead to quartz and sulfi de precipitation (Mo- 1980). Isotopically heavy marine sulfate assimilated 34 necke and others, 2018). As the hydrothermal fl uids into a magma could cause δ S enrichment of the cooled, quartz solubility decreased. This resulted in magmatic-hydrothermal fl uids exsolved from a source the precipitation of a fourth quartz phase with sulfi de pluton (Field and others, 2005). minerals fi lling fractures and open spaces. The regional extent of the Belt Supergroup shows Mineralization takes place as low-salinity hydro- a spatial relationship, suggesting it underlies the thermal fl uids of the lithostatic environment transition Boulder Batholith (Harrison, 1972) and that it is the into the lower pressure hydrostatic realm during brittle most likely source for heavy sulfur assimilated into the fracturing in deep porphyry systems (Monecke and Boulder Batholith intrusions. The Newland Formation 34 others, 2018). Sulfur isotope equilibrium and pres- has diagenetic pyrite with δ S values ranging from sure-corrected fl uid inclusion temperatures indicate -8.7‰ to 36.7‰ (Lyons and others, 2000). Strauss 34 that Big Foot vein sulfi de mineralization started under and Schieber (1990) inferred that baryte with δ S val- lithostatic conditions at high temperatures. Sulfi de and ues of 13.6‰, 14.4‰, and 18.3‰ replaced gypsum. vein quartz mineralization continued as decompres- Field and others (2005) suggest the Belt Supergroup sion and cooling of the magmatic-hydrothermal fl uid could serve as a potential source for heavy sulfur in occurred during microbrecciation of the early quartz the Butte district veins. The heavy sulfur was derived generations. Decompression of the hydrothermal fl uid from the quartz porphyry magma that assimilated Belt may have taken place during exhumation of the Butte Supergroup rocks. When Belt Supergroup heavy sul- Granite by erosion of the overlying Elkhorn Volcanic fur is combined and equilibrated with magmatic sulfur, the δ34S is raised from a magmatic value of 0‰ to sequence. ΣS 10‰ (Field and others, 2005). The δ34S from the Big At the close of vein development, a sixth quartz Foot veins could have originated from magmatic-hy- generation fi lling remaining open spaces precipitated, drothermal fl uids exsolved from a pluton that assimi- giving a low, even CL response. Textures revealed lated underlying Belt Supergroup rocks. This pluton by CL response indicate this fi nal quartz generation intruded the Butte Granite at depth, similar to the did not undergo fracturing or brecciation and was not pluton that formed the Butte district porphyry deposit. subject to fl uid decompression. This quartz generation precipitated after sulfi de mineralization and crystal- Oxygen isotopes for the quartz veins range from lized at low temperatures below 300⁰C at low-pres- 11.0‰ to 12.3‰. Calculated oxygen isotopes for the sure or hydrostatic conditions. The sixth quartz gener- hydrothermal fl uids range from 6.5‰ to 9.0‰ (table ation could have crystallized at a shallower pressure, 11). These fi ndings suggest a granitoid magmatic representing hydrostatic conditions caused by exhu- origin for the hydrothermal fl uids that precipitated the mation of the Butte Granite during vein development. vein quartz (Ohmoto, 1986). For I-type granitoids, δ18O values fall between 6‰ and 8‰ (Ohmoto, 1986), Sulfur isotopes from the quartz veins range similar to the calculated hydrothermal fl uid δ18O from -0.7‰ to 16.6‰. All the samples fall within the range. The vein quartz δ18O values fall between the S- -0.7‰ to 5.2‰ range with the exception of one ga- type granitoid range of 8‰ and 12‰ (Ohmoto, 1986). 34 lena sample, with a δ S value of 16.6‰. The sulfur An implication of the δ18O values is that the magmatic isotope range suggests the veins developed from a fl uids originated from an I-type granitoid pluton that magmatic-hydrothermal fl uid enriched in heavy sulfur assimilated crustal rocks. This imparted an S-type sourced from crustal rocks assimilated into a felsic δ18O signature to the vein quartz. pluton. Granitic magmas acquire most of their sulfur during emplacement from crustal sources (Ohmoto, Oxygen isotopes further imply the veins crystal- 1986). Granitoid magmas become enriched in crustal lized from a magmatic-hydrothermal fl uid. This fl uid sulfur by bulk assimilation of country rocks and selec- exsolved from a pluton that assimilated underlying 18 tive assimilation of sulfur (Ohmoto, 1986). Acidic igne- Belt Supergroup rocks. The average δ O value for

46 Montana Bureau of Mines and Geology Bulletin 141 the Ravalli Group is 12‰ (Rye and others, 1984), with sure conditions, and loss of CO2 that shifted the pH to values of 11.3‰ to 13.0‰ determined for the Revett basic. These changes resulted in the precipitation of Formation of the Belt Supergroup (Lange and Sherry, sulfi de minerals. 18 1986). The Belt Supergroup rock δ O values refl ect After vein development, minor to intense super- 18 the δ O measurements for marine sediments (10‰ to gene alteration generated a variety of secondary 22‰) reported by Campbell and Larson (1998). The minerals. In contrast to the other veins in the district, partial melting of marine sediments assimilated into the vein at the Mountain Queen mine experienced a pluton could generate an exsolved magmatic fl uid intense supergene alteration, with few primary sulfi de 18 with an S-type δ O signature similar to the Big Foot minerals remaining intact. The breakdown and oxida- quartz veins. tion of primary minerals by groundwater generated Rusk and others (2008) propose that low-salinity, abundant secondary minerals found at the Mountain

CO2-bearing fl uids derived from a magmatic source Queen mine. These minerals consist of iron–lead–an- provided the mineralizing fl uids for the Butte porphyry timony oxides, lead and copper carbonates, a copper copper deposit. Low-salinity, CO2-bearing fl uid inclu- silicate, a lead phosphate, copper–iron–lead sulfates, sions present in the Big Foot veins could likewise and lead and iron arsenates. The sulfur and , be an indication the veins were mineralized from a probably leached from the primary sulfi de minerals by magmatic-derived, low-salinity, CO2-bearing hydro- acidic ground water, was transported to locations with thermal fl uid similar to that in the Butte district. The conditions that favored mineral precipitation. Pyro- initial magmatic-hydrothermal fl uid that formed the morphite, a lead phosphate, is the most abundant veins and altered the host granite was probably CO2 secondary mineral at the Mountain Queen mine. To enriched and acidic. Pressure-corrected fl uid inclusion generate this mineral, lead was most likely leached temperatures and sulfur isotope geothermometry sug- from the vein while phosphorus was sourced from the gest fl uid temperatures varied from 310⁰C to 579⁰C altered host granite and transported by groundwater and calculated lithostatic pressure ranged up to 2 kb. to favorable precipitation sites. During vein development, the magmatic-hy- GEOLOGIC MODEL drothermal fl uid underwent physical and chemical changes caused by fl uctuating pressures and tem- A geologic model for the Big Foot mining district peratures and wall-rock reactions. These physical is based on data generated by this investigation, past and chemical changes led to quartz precipitation and geophysical data, and geologic mapping. The veins sulfi de mineralization. There is little evidence of boil- are hosted by the Butte Granite, which crystallized at ing during vein development, but separation from a a depth of 7 to 8 km (Olson and others, 2016). The low- to high-salinity fl uid occurred as noted at fi ve vein Butte Granite is 74.5 ± 0.9 to 76.28 ± 0.14 Ma as locations. The initial magmatic-hydrothermal fl uid may dated by Lund and others (2002) and Martin and oth- have changed from a high-temperature (≥500⁰C), low- ers (1999). The veins were dated at 73.81 ± 0.12 to salinity (1.2 to 1.6 wt% NaCl eq), CO2-bearing fl uid to 75.2 ± 0.25 Ma by Olson and others (2016). Following an intermediate-temperature (≤500⁰C), moderate to emplacement of the Butte Granite, aplite dikes were low-salinity (0.2 to 8.4 wt% NaCl eq), CO2-defi cient intruded into the granite at 74.5 ± 0.6 Ma (Korzeb and fl uid by decompression from lithostatic to lower pres- Scarberry, 2017). Age dating suggests the veins were sure conditions. Rapid decompression of a magmatic- emplaced at the same time as aplite intrusive events. hydrothermal fl uid from lithostatic to hydrostatic condi- Age dating of the veins by Olson and others tions will lead to a decrease in temperature (Monecke (2016) show they formed about 1 million years after and others, 2018) and could result in the loss of CO 2 the host granite crystallized, implying the veins were and increase in salinity, similar to a boiling event. The emplaced at the same depth as the granite host. The loss of CO from a hydrothermal fl uid can change the 2 veins now exposed at the surface formed at a litho- pH from acidic to alkali (Reed and Palandri, 2006). static pressure of 2 kb, refl ecting the 7- to 8-km depth A shift in pH to alkali and dropping temperatures will of the host granite, with temperatures varying from cause sulfi de precipitation to take place (Reed and 310⁰C to 579⁰C. The lithostatic pressure and tempera- Palandri, 2006). Microbreccias hosting sulfi de miner- ture suggest the veins represent the exhumed deep als developed in the Big Foot veins, implying rapid roots of an epithermal system. Based on the depth of decompression of the magmatic-hydrothermal fl uids formation, lithostatic pressure, and temperature, the after the fi rst three quartz generations crystallized. veins fi t the mesothermal classifi cation based on the Rapid decompression of the hydrothermal fl uids model from Ridge (1972). resulted in a temperature drop, a change to low-pres- 47 Stanley L. Korzeb

Age dating, trace element, isotopic, and fl uid tions. These fl uctuating pressure changes could have inclusion evidence suggest a magmatic fl uid source taken place during vein exhumation as the overlying for the veins. Timing of vein emplacement implies that Elkhorn volcanic sequence was eroded. Monecke magmatic fl uids were sourced from a magma with a and others (2018) suggest fl uid sourced from a pluton felsic composition. Schorl is known to have crystal- rises in pulses that intermittently exceed lithostatic lized from boron-enriched aplites within the Boulder conditions. This causes microfracturing in the host Batholith. Magmatic-hydrothermal fl uids generated rock and increases permeability. This microfracturing from a boron-enriched felsic magma will likewise be and brecciation is found in the quartz veins and host enriched in boron and could crystallize schorl during granite throughout the Big Foot district. These fl uctu- alteration of the host granite along the vein contacts. ating pressure conditions imply a plutonic source for Anomalous trace elements found in the veins and the hydrothermal fl uids that precipitated the Big Foot altered host rock consisting of As, Cu, Pb, Sb, Zn, and veins. B implied schorl crystallization; these are known to be EXPLORATION POTENTIAL enriched in volcanic arc subduction-derived magmas (Noll and others, 1996). This suggests the fl uids that The northern and southern part of the Big Foot developed the Big Foot veins may have a subduction- mining district may have exploration potential for a derived magmatic origin. Pressure-corrected fl uid deep-seated porphyry or stockwork vein system. inclusions indicate the veins started to crystallize at Based on the results from this study, the veins classi- temperatures above 500⁰C, from CO2-enriched min- fi ed as mesothermal might be related to a magmatic eralizing fl uids with a variable salinity from 0.2 to 38.1 source. Magmatic-hydrothermal fl uids sourced from wt% NaCl eq. The presence of three-phase CO2-en- a pluton intruded into the Butte Granite, or a cupola riched inclusions, and pressure-corrected homogeni- formed during cooling of the Butte Granite pluton, zation temperatures exceeding 500⁰C, further suggest may have supplied the metals that developed the the initial mineralizing fl uids had a magmatic origin. veins. Although speculative, there could be a deep- Likewise, sulfur isotope equilibrium temperatures ex- seated porphyry or stockwork vein system underlying ceeding 500⁰C also imply crystallization from a high- the district. Elliott and others (1993) ranked the district temperature magmatic-hydrothermal fl uid. as high potential for the occurrence of porphyry and/ The largest sulfi de–quartz vein and most aplite or stockwork deposits of copper, molybdenum, and dikes correlate with a magnetic low located in the tungsten or stockwork and disseminated gold and northeast area of the district (fi gs. 2, 3). A smaller silver deposits. magnetic low correlates with the vein at the St. Antho- Hanna and others (1994) identifi ed a magnetic low ny mine. These magnetic lows interpreted by Hanna covering 60 km2 in the Butte Granite in the northeast and others (1994) are related to a pluton that altered part of the district. The anomaly encompasses the Big the Butte Granite, and they are similar to the magnetic Four, Nickel Plate, Ajax, and Attowa mines and a ma- lows in the Butte district. The correlation of the aplite jor vein extending to the center of the anomaly (fi gs. dikes and sulfi de quartz vein with the magnetic anom- 2, 3). A smaller magnetic low in the southwest part of aly implies the veins and aplite dikes share a common the district identifi ed by Hanna and others (1994) lies felsic pluton source. northwest of the St Anthony mine. It extends 4 km Quartz textures revealed by CL response pro- northwest of the mine and covers 4 km2. Hanna and vide additional evidence that the Big Foot veins may others (1994) interpreted these magnetic lows in the have originated from mineralizing fl uids derived from Butte Granite as caused by altered intrusions in the a cooling pluton intruded into the Butte Granite. As granite, similar to those found in the Butte mining dis- the magmatic-hydrothermal fl uids ascended through trict. However, the two magnetic lows in the Big Foot fractures in the Butte Granite, they began altering district are smaller than the one in the Butte district. the granite and initiated quartz crystallization on the The Butte district anomaly covers 120 km and the fracture margins. From quartz textures, it appears Big Foot district anomalies cover a total of 64 km2. the veins underwent pressure changes of varying The Big Foot district anomalies are associated with a lithostatic conditions. Quartz that precipitated during clustering of aplite and alaskite dikes and intrusions initial vein development shows evidence of dissolu- mapped by Olson and others (2016; fi g. 3). These tion, reprecipitation, and microfracturing caused by clusters of dikes and intrusions might be sourced fl uctuating pressure changes from above lithostatic from an underlying pluton related to the magnetic low conditions and returning to lower lithostatic condi- anomaly or during cooling of the Butte Granite.

48 Montana Bureau of Mines and Geology Bulletin 141

High-sulfi dation epithermal veins, and mesother- study would not have been possible without their mal veins in deep systems, are known to be related to cooperation and help. A special thank you is due porphyry systems (Chang and others, 2011; Cook and Gnanou Hamadou, who conducted the fl uid inclu- Bloom, 1990; Sillitoe, 2010). A relationship between sion analysis when he was a graduate student at the the Butte district porphyry system and associated Department of Geological Engineering, Montana Tech polymetallic veins is well documented (Dilles and oth- of the University of Montana. I would like to thank ers, 1999; Rusk and others, 2008; Reed and others, Kaleb Scarberry of the MBMG, whose comments and 2013; Houston and Dilles, 2013). Likewise, the Big suggestions improved the fi nal manuscript. I thank Foot district veins could be related to a deep porphyry George Brimhall, Clementine Exploration LLC, and system or to a stockwork vein system in the vicinity of Robert Houston, Oregon Department of Geology and the two magnetic lows identifi ed by Hanna and others Mineral Industries, whose reviews and comments (1994). The Big Foot district veins have some similari- greatly improved the manuscript. Cartography and ties to the early Ag-Au-polymetallic veins that occur graphic work by Susan Smith, MBMG; editing and in the Butte district. Lund and others (2018) dated the layout by Susan Barth, MBMG. Ag-Au-polymetallic veins to be 73 to 70 Ma, making them the oldest veins in the Butte district. Lund and REFERENCES others (2018) suggest the genesis of these early veins Anderson, J., and Smith, D., 1995, The eff ect of tem- are hydrothermal fl uids related to the emplacement perature and ƒ O on the Al-hornblende barom- of plutons in the Boulder Batholith. Veins with similar 2 eter: American Mineralogist, v. 80, p. 549–559. ages to the oldest veins in the Butte district are found in the Basin and Boulder districts, and these may Beaty, D.W., Hahn, G.A., and Threlkeld, W.E., 1988, have a genetic relationship with plutons in the Boulder Field, isotopic, and chemical studies of tourma- Batholith (Lund and others, 2018). The ages of the line-bearing rocks in the Belt-Purcell Supergroup: veins in the Big Foot district are also similar to those Genetic constraints and exploration signifi cance in the Basin and Boulder districts and early veins in for Sullivan type ore deposits: Canadian Journal the Butte district; the Big Foot veins may likewise be of Earth Science, v. 25, p. 392–402. related to a pluton emplaced in the Boulder Batholith. Berger, B.R., Hildenbrand, T.G., and O’Neill, J.M., There is supporting geologic, geophysical, trace 2011, Control of Precambrian basement defor- element, isotope, quartz textures, and fl uid inclusion mation zones on emplacement of the Laramide evidence suggesting the Big Foot district veins formed Boulder Batholith and Butte mining district, from a magmatic-hydrothermal fl uid, which originated Montana, United States: U.S. Geological Survey from an underlying pluton or cupola in the Butte Investigations Report 2011-5016, 29 p. Granite. Evidence further suggests the magmatic- Berkenbosch, H.A., de Ronde, C.E.J., Gemmell, J.B., hydrothermal fl uid source could also be related to a McNeill, A.W., and Goemann, K., 2012, Mineral- porphyry or stockwork vein system, and this could be ogy and formation of black smoker chimneys determined by deep drilling and detailed geophysical from Brothers submarine volcano, Kermadec arc: surveys. Exploration for a porphyry system in the Big Economic Geology, v. 107, p. 1613–1633. Foot district will require targeting the two magnetic Borthwick, J., and Harmon, S.R., 1982, A note re- lows identifi ed by Hanna and others (1994) with de- garding ClF3 as an alternative to BrF5 for oxygen tailed geophysical surveys followed by a deep drilling isotope analysis: Geochimica et Cosmochimica program. The veins themselves may not be viable ex- Acta, v. 46, p. 1665–1668. ploration targets for polymetallic lodes because they Campbell, A.R., and Larson, P.B., 1998, Introduction appear to represent the roots of an epithermal sys- to stable isotope applications in hydrothermal tem with one mineralizing event, and economic lode systems, in Richards, J.P., and Larson, P.B., deposits may not extend beyond the currently mined eds., Techniques in hydrothermal ore deposits depths. Deeper mineralization of a Cu-Mo porphyry geology: Reviews in Economic Geology, Society system may extend below the mesothermal veins. of Economic Geologists, v. 10, p. 173–193. ACKNOWLEDGMENTS Chang, Z., Hedenquist, J.W., White, N.C., Cooke, D.R., Roach, M., Deyell, C.L., Garcia, J. Jr., I extend a special thanks to the patented claim Gemmell, J.B., Staff ord, M., and Cuison, L. 2011, owners who granted permission to enter and sample Exploration tools for linked porphyry and epither- the mines and prospects on their properties. This mal deposits: Example from Mankayan intrusion-

49 Stanley L. Korzeb

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and Mineral Symposium, October 11–October Madubuike, C., Brikowski, T., and Moulding, A., 2016, 14, 2017: Montana Bureau of Mines and Geology Using infrared spectrometry to deduce fl uid his- Open-File Report 699, p. 39–44. tory from an exploration core, Emigrant Peak Kuwatani, T., Yoshida, K., Ueki, K., Oyanagi, R., Uno, geothermal prospect, northern Fish Lake Valley, M., and Akaho, S., 2020, Sparse isocon analysis: Nevada, USA: Geothermal Resources Council A data-driven approach for material transfer esti- Transactions, v. 40, p. 445–454. mation: Chemical Geology, v. 532, article 119345, Mahoney, J.B., Pignotta, G.S., Ihinger, P.D., Wittkop, in progress. C., Balgord, E.A., Potter, J.J., and Leistikow, A., Lange, I.M., and Sherry, R.A., 1986, Nonmassive 2015, Geologic relationships in the northern sulfi de deposits in the late Precambrian Belt Su- Helena salient, Montana: Geology of the Elliston pergroup of , in Roberts, S.M., region: Northwest Geology, v. 44, p. 109–135. Belt Supergroup: A guide to Proterozoic rocks of Martin, M., Dilles, J., and Proff ett, J.M., 1999, U-Pb western Montana and adjacent areas: Montana geochronologic constraints for the Butte porphyry Bureau of Mines and Geology Special Publication system [Abs.]: Geological Society of America 94, p. 269–278. Abstracts with Programs, v. 31, no. 7, p. A380. Large, R.R., Gemmell, J.B., and Paulick, H., 2001, Matthews, A., and Beckinsale, R.D., 1979, Oxygen The alteration box plot: A simple approach to isotope equilibration systematics between quartz understanding the relationship between altera- and water: American Mineralogists, v. 64, p. tion mineralogy and lithogeochemistry associated 232–240. with volcanic-hosted massive sulfi de deposits: Monecke, T., Monecke, J., Reynolds, T.J., Tsuruoka, Economic Geology, v. 96, p. 957–971. S., Bennett, M.N., Skewes, W.B., and Palin,

London, David, 2011, Experimental synthesis and sta- R.M., 2018, Quartz solubility in the H2O-NaCl bility of tourmaline: A historic overview: Canadian system: A framework for understanding vein Mineralogists, v. 49, p. 117–136. formation in porphyry copper deposits: Economic London, D., Morgan VI, G.B., and Wolf, M.B., 2002, Geology, v. 113, p. 1007–1046. Boron in granitic rocks and their contact aureoles, Mosier, D.L., Sato, T., Page, N.J., Singer, D.A., and in Grew, E.S., and Anovitz, L.M., eds., Boron min- Berger, B.R., 1987, Descriptive model of Creede eralogy, petrology and geochemistry: Reviews in epithermal veins, in Cox, D.P. and Singer, D.A., Mineralogy, Mineralogical Society of America, v. eds., Mineral deposit models: U.S. Geological 33, p. 299–330. Survey Bulletin 1693, p. 145–149. Lund, K., Aleinikoff , J.N., Kunk, M.J., Unruh, D.M., Noll, P.D. Jr., Newsom, H.E., Leeman, W.P., and Zeihen, G.D., Hodges, W.C., Du Bray, E.A., and Ryan, J.G., 1996, The role of hydrothermal fl uids O’Neill, J.M., 2002, SHRIMP U-Pb and 40Ar/39Ar in the production of subduction zone magmas: age constraints for relating plutonism and min- Evidence from sidophile and chalcophile trace el- eralization in the Boulder Batholith region, Mon- ements and boron: Geochimica et Cosmochimica tana: Economic Geology, v. 97, p. 241–267. Acta, v. 60, p. 587–611. Lund, K., McAleer, R.J., Aleinikoff , J.N., Cosca, M.A., Ohmoto, Hiroshi, 1986, Stable isotope geochemistry and Kunk, M.J. 2018, Two-event lode-ore deposi- of ore deposits, in Valley, J.W., Taylor, H.P. Jr., tion at Butte, USA: 40Ar/39Ar and U-Pb documen- and O’Neil, J.R., eds., Stable isotopes in high tation of Ag-Au-polymetallic lodes overprinted by temperature geologic processes: Mineralogical younger stockwork Cu-Mo and penecontem- Society of America Reviews in Mineralogy, v. 16, poraneous Cu lodes: Ore Geology Reviews, v. p. 491–559. 102, p. 666–700. Ohmoto, H., and Rye, R.O., 1979, Isotopes of sulfur Lyden, C.J., 2013, Gold placers of Montana: Montana and carbon, in Barnes, H.L., ed.: Geochemistry of Bureau of Mines and Geology Reprint 6, p. 34. hydrothermal ore deposits: Hoboken, N.J., Wiley Lyons, T.W., Luepke, J.J., Madeline, E.S., and Zieg, and Sons, 2nd ed., p. 509–567. G.A., 2000, Sulfur geochemical constraints on Olson, N.H., Dilles, J.H., Kallio, I.M., Horton, T.R., and Mesoproterozoic restricted marine deposition: Scarberry, K.C., 2016, Geologic map of the Ratio Lower Belt Supergroup, northwestern United Mountain 7.5’ quadrangle, southwest Montana: States: Geochimica et Cosmochimica Acta, v. 64, Montana Bureau of Mines and Geology EDMAP p. 427–437. 10, scale 1:24,000.

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Olson, N.H., Sepp, M.D., Mankins, N.E., Blessing, nics and the Boulder Batholith, southwestern J.M., Dilles, J.H., and Scarberry, K.C., 2017, Montana, in Henshaw, P., ed., Volcanism and Geologic map of the Mount Thompson 7.5’ quad- plutonism of western North America: Volume 2, rangle, southwest Montana: Montana Bureau of Cordilleran volcanism, plutonism, and magma Mines and Geology EDMAP 11, 13 p., 11 sheets, generation at various crustal levels, Montana and scale 1:24,000. Idaho, Field trips for the 28th International Geo- Pinckney, D.M., and Becraft, G.E., 1961, Preliminary logical Congress: American Geophysical Union, geologic map of the southwest quarter of the Monograph, p. 16–31. Boulder quadrangle, Montana: U.S. Geological Rye, R.O., Whelan J.E., Harrison, J.E., and Hayes, Survey Mineral Investigations Field Studies Map T.S., 1984, The origin of copper-silver mineraliza- MF-187, scale 1:24000. tion in the Ravalli group as indicated by prelimi- Reed, M.H., and Palandri, J., 2006, Sulfi de min- nary stable isotope studies, in Hobbs, S.W., The eral precipitation from hydrothermal fl uids, in belt abstracts with summaries, Belt Symposium Vaughan, D.J., ed., Sulfi de mineralogy and II, 1983: Montana Bureau of Mines and Geology geochemistry: Mineralogical Society of America Special Publication 90, p. 104–107. Reviews in Mineralogy and Geochemistry, v. 61, Scarberry, K.C., 2016, Geologic map of the Sugarloaf p. 609–631. Mountain 7.5’ quadrangle, Deer Lodge, Jeff erson, Reed, M., Rusk, B., and Palandri, J., 2013, The Butte and Powell Counties, Montana: Montana Bureau magmatic-hydrothermal system: One fl uid yields of Mines and Geology Open-File Report 674, all alteration and veins: Economic Geology, v. scale 1:24,000. 108, p. 1379–1396. Scarberry, K.C., Korzeb, S.L., and Smith, M.G., 2015, Reyes, A.G., 1990, Petrology of Philippine geothermal Origin of Eocene volcanic rocks at the south end systems and the application of alteration mineral- of the Deer Lodge Valley, Montana, in Mosolf, J., ogy to their assessment: Journal of Volcanology and McDonald, C., eds.: 40th annual fi eld con- and Geothermal Research, v. 43 p. 279–309. ference Geology of the Elliston area, Montana and other papers: Northwest Geology, v. 44, p. Ridge, J.D., 1972, Classifi cation of ore deposits, in 201–212. Ridge, J.D., ed., Annotated bibliographies of min- eral deposits in the western hemisphere: Geolog- Scarberry, K.C., Coppage, E.L., and English, A.R., ical Society of America Memoir 131, p. 673–678. 2019a, Field guide to the geology and metal- lic mineral deposits along the western contact Roby, R.N., Ackerman, W.C., Fulkerson, F.B., and between the Boulder Batholith and the lower and Crowley, F.A., 1960, Mines and mineral deposits middle members of the Elkhorn Mountains Volca- (except fuels), Jeff erson County, Montana: Mon- nic fi eld: Northwest Geology, v. 48, p. 61–70. tana Bureau of Mines and Geology Bulletin 16, p. 37–38. Scarberry, K.C., Gammons, C.H., and Kallio, I.M., 2019b, Field guide to the geology and metallic Roedder, Edwin, 1984, Fluid inclusions: Mineralogical mineral deposits along the eastern contact be- Society of America Reviews in Mineralogy, v. 12, tween the Boulder Batholith, the Elkhorn Moun- 646 p. tains Volcanic fi eld, and Cretaceous-Paleozoic Rottier, B., Kouzmanov, K., Casanova, V., Wälle, M., sedimentary rocks: Northwest Geology, v. 48, p. and Fontboté, L., 2018, Cyclic dilution of mag- 97–108. matic metal-rich hypersaline fl uids by magmatic Scarberry, K.C., Elliott, C.G., and Yakovlev, P.V., low salinity fl uid: Generating the giant epithermal 2019c, Geologic summary of the Butte North 30’ polymetallic deposit of Cerro de Pasco, Peru: x 60’ quadrangle, southwestern Montana: North- Economic Geology, v. 113, p. 825–856. west Geology, v. 48, p. 1–14. Rusk, B.G., Reed, M.H., and Dilles, J.H., 2008, Fluid Scarberry, K.C., Elliott, C.G., and Yakovlev, P.V., inclusion evidence for magmatic-hydrothermal 2019d, Geology of the Butte North 30’ x 60’ quad- fl uid evolution in the porphyry copper-molybde- rangle, southwest Montana: Montana Bureau of num deposit at Butte, Montana: Economic Geol- Mines and Geology Open-File Report 715, 30 p., ogy, v. 103, p. 307–334. scale 1:100,000. Rutland, C., Smedes, H., Tilling, R., and Greenwood, Shankar, Vikram, 2015, Field characterization by near W., 1989, Volcanism and plutonism at shallow infrared (NIR) mineral identifi ers—A new pros- crustal levels: The Elkhorn Mountains Volca- 52 Montana Bureau of Mines and Geology Bulletin 141

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the system NaCl-KCl-H2O under vapor-saturated conditions: Geochimica et Cosmochimica Acta v. 52, p. 989–1005. Strauss, H., and Schieber, J., 1990, A sulfur isotope study of pyrite genesis: The mid-Proterozoic

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APPENDIX A

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APPENDIX B

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APPENDIX C

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APPENDIX D

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APPENDIX E

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APPENDIX F

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APPENDIX G

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