Interpretations of New and Previous Lead Isotopic Data

Bulletin 139 2019

INTERPRETATIONS OF NEW AND PREVIOUS LEAD ISOTOPIC DATA FOR LATE CRETACEOUS TO EOCENE SATELLITE INTRUSIONS, BUTTE GRANITE, VOLCANIC ROCKS, AND BASE METAL VEINS OF THE BOULDER BATHOLITH, SOUTHWESTERN MONTANA

Stanley L. Korzeb

Montana Bureau of Mines and Geology, Butte, Montana Cover photo: Kirk Waren, MBMG, Rampart Mountain. Bulletin 139 2019

Stanley L. Korzeb

Montana Bureau of Mines and Geology, Butte, Montana

CONTENTS Abstract ...... 1 Introduction ...... 2 Geologic Setting ...... 3 Previous Investigations ...... 5 Analytical Procedures ...... 6 Results ...... 7 Discussion ...... 9 Base Metal Veins ...... 13 Satellite Plutons and Intrusions ...... 14 Lowland Creek Volcanic Field ...... 14 Elkhorn Mountains Volcanic Field ...... 17 Stream Sediments ...... 17 Big Belt Mountains ...... 18 Belt Supergroup Formations and Sulﬁ de Veins ...... 19 Magma Source ...... 23 Ore Deposit Genesis ...... 27 Conclusion ...... 29 Acknowledgments ...... 29 References ...... 30 Appendix A ...... 33 Appendix B ...... 35 Appendix C ...... 36 Appendix D ...... 37 Appendix E ...... 38 Appendix F ...... 40

FIGURES Figure 1. Geologic map of Boulder Batholith showing mining districts and approximate locations for base metal vein galena samples ...... 4 Figure 2. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for base metal vein and breccia pipe galena lead isotope ratios generated by this investigation ...... 7 Figure 3. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for Boulder Batholith intrusions published by Doe and others (1968) with added data generated by this investigation ...... 8 Figure 4. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for Lowland Creek volcanic rocks published by Dudás and others (2010) with added data generated by this investigation...... 8 Figure 5. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for Elkhorn Mountains volcanic rocks published by Doe and others (1968) with added data generated by this investigation ...... 9 Figure 6. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for Basin Mining district stream sediment samples published by Uhruh and others (2000) ...... 10 Figure 7. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for the Big Belt Mountains published by du Bray and others (2017) ...... 11 Figure 8. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for the Belt Supergroup formations and sulfi de veins published by Zartman (1992)...... 11 Figure 9. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for base metal vein and breccia pipe galena lead isotope data generated by this investigation...... 13 Figure 10. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for Boulder Batholith intrusions published by Doe and others (1968) with additional data generated by this investigation ...... 15 Figure 11. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for Lowland Creek volcanic rocks published by Dudás and others (2010) with additional data generated by this investigation ...... 16 Figure 12. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for Elkhorn Mountains volcanic rocks published by Doe and others (1968) with additional data generated by this investigation ... 17 Figure 13. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for stream sediment samples from the Basin mining district (Uhruh and others, 2000) ...... 18 Figure 14. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for the Big Belt Mountains, data published by du Bray and others (2017) ...... 19 Figure 15. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for the Belt Supergroup formations and hosted sufi de veins, data published by Zartman (1992)...... 20 Figure 16. Lead isotope ratios generated by this investigation for galena samples from base metal veins and galena from Montana Tunnels breccia pipe plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios ...... 21 Figure 17. Lead isotope ratios for Boulder Batholith intrusions from data published by Doe and others (1968) with additional data generated by this investigation (red points) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios...... 22 Figure 18. Lead isotope ratios for Lowland Creek volcanic rocks from data published by Dudás and others (2010) with additional data generated by this investigation (red points) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios ...... 23 Figure 19. Lead isotope ratios for Elkhorn Mountains volcanic rocks from data published by Doe and others (1968) with additional data generated by this investigation (red points) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios ...... 24 Figure 20. Lead isotope ratios for stream sediments from the Basin mining district from data published by Uhruh and others (2000) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 208Pb/204Pb vs. 206Pb/204Pb ratios ...... 25 Figure 21. Lead isotope ratios for the Big Belt Mountains from data published by du Bray and others (2017) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios ...... 26 Figure 22. Lead isotope ratios for Belt Supergroup formations and hosted sulfi de veins from data published by Zartman (1992) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios ...... 28

TABLE Table 1. Summary of lead isotope ranges for samples showing degrees of compatible isotope signatures ...... 14 Montana Bureau of Mines and Geology Bulletin 139 ABSTRACT New lead isotope data generated from this investigation were used in conjunction with data from previous investigations to identify a metal source for the Boulder Batholith base metal veins and a magmatic source for the Butte Granite, Boulder Batholith satellite plutons, and associated volcanic fi elds. To constrain the lead isotope data, covariant plots generated from 2.7 Ga initial lead isotope data for Stillwater and lead isotope data from the Big Belt Mountains were used. Lead isotope data for the base metal veins, satellite plutons, Butte Granite, Elkhorn Mountains, and Lowland Creek volcanic rocks overlap, indicating a common source. This data set plotted on covariant diagrams follows an 85 Ma mixing line with lead isotopes sourced from both a subcon- tinent lithospheric component and upper crust. Lead isotopes suggest the magmas that generated the Boulder Batholith intrusions and associated volcanic rocks originated from Archean–Proterozoic mafi c to intermediate basement that assimilated upper crust. The variations in 207Pb/204Pb and 208Pb/204Pb between diff erent intrusions and volcanic rocks indicate a heterogeneous isotopic source. Past investigations suggest assimilated upper crust contributed inherited zircons to the intrusions with Proterozoic cores, and Cretaceous age rims and base metal veins have sulfur isotopes with Belt Supergroup signatures. Proximity and Cretaceous ages of the Boulder Batholith and Big Belt Mountains imply both originated from the same geologic process of subduction- related magmatism, including mantle-derived components. Lead isotopes for base metal vein galena samples overlap the intrusions and volcanic rocks hosting the veins, are in isotopic equilibrium, and share a common isotopic source. Overlapping of Pb isotopes for the base metal veins, intrusions, and volcanic rocks imply the Pb isotopic source for the base metal veins are magmatic fl uids that exsolved from cooling intrusions that assimilated crustal rocks or Belt Supergroup formations. The assimilated crustal rocks may have contributed additional metals and sulfur to intrusions that are geneti- cally related to mineral resources. Galena from the Montana Tunnels breccia pipe is more radiogenic than the base metal vein galena. The increase in 207Pb/204Pb and 208Pb/204Pb from the Montana Tunnels breccia pipe suggests a diff erent lead isotope source than for the base metal veins or in situ radioactive decay of U and Th introduced into the breccia pipe during development.

1 Stanley L. Korzeb INTRODUCTION Zartman and Haines (1988) used the distinct U/ Pb and Th/U characteristics to suggest three ideal- Radiogenic lead isotopes are the decay products ized crustal reservoirs for U-Th-Pb. These reservoirs of uranium and thorium isotopes. The lead isotope are identifi ed to be mantle, lower crust, and upper 206Pb is from the decay of 238U, 207Pb is generated crust. The three reservoirs mix in the orogene where from 235U decay, and 208Pb is a product of 232Th decay geologic processes such as magmatism, crustal (Tosdal and others, 1999). The 204Pb isotope is almost deformation, metamorphism, and sedimentation take stable and does not decay to another isotope, nor place (Zartman and Haines, 1988). Zartman and Doe does it have a long-lived parent isotope. The abun- (1981) derived model curves for each of the major dance of radiogenic isotopes has grown since the Pb sources. Lead isotope studies of ore deposits and earth formed 4.57 billion years ago, building upon an magmatism relate Pb isotope data to these model initial concentration. The measured present day iso- reservoirs (Tosdal and others, 1999). Due to geologic tope composition is equal to the sum of the initial Pb events such as partial melting or deformation, Pb isotope composition plus radiogenic lead added over isotope data from ore deposits and rocks often devi- time (Tosdal and others, 1999). ate from the ideal growth curves (Tosdal and others, Variations in Pb isotopes are impacted by U/Pb 1999). The plumbotectonic model derived by Zartman and Th/U ratios in the geologic environment over time and Haines (1988) is a broad generalization that is (Faure, 1977). U and Th have tetravalent oxidation diffi cult to apply on a local scale. The model does not states and can substitute for each other and other discriminate between depleted and enriched mantel high-valence ions in compounds. During partial melt- reservoirs, in part because the role of an enriched ing and fractional crystallization, U and Th will con- reservoir within the mantle was not recognized at the centrate in the liquid phase over the residual phase time Zartman and Haines (1988) developed the model or crystallized parts of a magma (Tosdal and others, (Frances Dudás, written commun., 2018). 1999). Thorium has one oxidation state and U has a Variations in 207Pb/204Pb for a whole rock or ore second oxidation state (U6+), making U soluble and mineral sample is signifi cant for rocks or minerals mobile in aqueous fl uids. Under oxidizing conditions U younger than one billion years (Tosdal and others, forms uranyl ions and will fractionate from Th (Tosdal 1999). Elevated 207Pb/204Pb values indicate regions and others, 1999). of the crust where radiogenic Pb evolved in Ar- Lead is soluble at moderate to high temperatures chean eon rocks because 235U was more abundant found in hydrothermal, magmatic, and metamorphic in the Archean (Doe and Zartman, 1979). Elevated environments. At low temperatures, lead is generally 207Pb/204Pb further indicates the Pb was not reworked not soluble but will complex with organic matter (Tos- by geologic processes and/or diluted by material with dal and others, 1999). Lead has a larger ionic radius lower 207Pb/204Pb values (Tosdal and others, 1999). than U and Th, causing it to behave diff erently during In contrast, lower 207Pb/204Pb values indicate a lack of partial melting, metamorphism, and low-temperature old radiogenic Pb derived from 235U decay (Tosdal and alteration events (Tosdal and others, 1999). others, 1999). The lack of old radiogenic Pb indicates Stacey and Kramers (1975) derived a model the lead was sourced from an environment such as Pb isotope growth curve from worldwide Pb isotope the mantle that was isolated from old radiogenic crust values. The curve represents the Pb isotope evolution or derived during recent times from a mantle or oce- of continental crust based on Pb isotope composi- anic environment (Tosdal and others, 1999). tions of galena from ore deposits whose hydrothermal However, there is a possibility of analytical un- system’s Pb isotopes refl ected an average of large certainty in 207Pb/204Pb data showing a smaller range segments of crust (Tosdal and others, 1999). The than 206Pb/204Pb or 208Pb/204Pb data. The analytical average crustal growth curve derived by Stacey and uncertainty is caused by the anomalous behavior of Kramers (1975) is based on two stages of growth 207Pb during thermal ionization mass spectrometry characterized by diff erent U/Pb and Th/U ratios. The analysis. Amelin and others (2005) determine even growth curve provides a reference crustal value to mass isotope ratios follow standard mass fraction- compare Pb isotope compositions measured in rocks ation laws, but in contrast, odd to even isotope ratios and minerals (Tosdal and others, 1999). Deviations of follow mass-dependent fractionation laws. The diff er- measured Pb isotope values from the average crustal ences in fractionation during analysis between odd growth curves are caused by geologic events that and even isotopes causes anomalous fractionation of transport U-Th-Pb from reservoirs with typical crustal 207Pb relative to even mass isotopes (Amelin and oth- values (Tosdal and others, 1999).

2 Montana Bureau of Mines and Geology Bulletin 139 ers, 2005). The anomalous fractionation of 207Pb could Tunnels breccia pipe, and identify a magmatic source lead to analytical uncertainties in some data sets. Due for the plutons and volcanic rocks. 207 204 to analytical uncertainties, Pb/ Pb data need to be The study region included the mining districts lo- cautiously interpreted. cated in the northern half of the Boulder Batholith and The model lead isotope growth curves derived by east of the Elkhorn Mountains (fi g. 1). These districts Stacey and Kramers (1975), Zartman and Doe (1981), were some of the most productive in the Batholith and Zartman (1974) are too general to yield much outside the Butte mining district. Previous lead isotope useful information, but they do provide model ages for investigations by Dudás and others (2010), Zartman potential lead isotope sources. The Pb isotope study (1992), Zartman and Stacey (1971), Doe and others of the Stillwater complex by McCallum and others (1968), Unhruh and others (2000), and du Bray and (1999) provides a specifi c data set for Archean mag- others (2017) covered regions in and near the Boul- matism in Montana and can be used to constrain data der Batholith. The previous investigations generated sets from other locations in the State. McCallum and lead isotope data from intrusions, volcanic rocks, and others (1999) derived their covariant diagrams from a stream sediments throughout the Boulder Batholith model crust at 2.7 Ga based on the crustal evolution and intrusions and mineralized rocks from the High- model of Stacey and Kramers (1975) and the mantle land Mountains, Little Belt Mountains, and Big Belt value of 2.7 Ga from the Plumbotectonics IV mantle Mountains. evolution model of Zartman and Doe (1981). In this GEOLOGIC SETTING study, the covariation diagrams derived by McCal- lum and others (1999) are used to interpret the lead The Late Cretaceous Boulder Batholith (fi g. 1) isotope data along with the model lead isotope growth covers an area of 2,200 mi2 and intruded Precam- curves by Stacey and Kramers (1975). Due to the brian to Late Cretaceous rocks of diverse lithology proximity of the Big Belt Mountains to the east of the (Doe and others, 1968). The Batholith is in the Great Boulder Batholith and shared Late Cretaceous ages, Falls Tectonic Zone within the fold-thrust belt of Pb isotope data from du Bray and others (2017) were southwestern Montana. The collision of Archean and used to further constrain the Boulder Batholith Pb Paleoproterozoic terrane about 1,800 mega-annum isotope data. (Ma) created the Great Falls Tectonic Zone. A north- The objectives of this study were to determine the west-trending depositional trough known as the Belt lead isotopic relationships between mineral resources Basin formed approximately 1,500 Ma and crosses and magmatic events in the Boulder Batholith, identify the Great Falls Tectonic Zone. Proterozoic northwest- a magma source for the magmatic events, and iden- striking faults forming the Lewis and Clark Line are tify a metal source for the mineral resources. With the exposed in the west-central and southeast part of the aid of new lead isotope data from 15 mining districts, Belt Basin (Berger and others, 2011). the Elkhorn Mountains volcanic fi eld, the Lowland Magmatic and tectonic activity took place in Creek volcanic fi eld, intrusions, and lead isotope data western Montana during the Laramide orogeny (Late from previous investigations, an upper mantle to lower Cretaceous to Eocene; Berger and others, 2011). crust source was identifi ed for igneous rocks. Lead Magma moved into the crust during the Laramide isotope data suggest the mineral resources have a orogeny between about 85 and 65 Ma and Mississip- genetic relationship with the Boulder Batholith intru- pian–Cretaceous sedimentary rocks were regionally sions that may have assimilated upper crust rocks. shortened and deformed, developing the fold-thrust Seventeen galena samples from base metal veins belt (Robinson and others, 1968; Kalakay and others, and a breccia pipe, and fi ve whole rock samples, were 2001; Lageson and others, 2001; Lund and others, analyzed for 206Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb 2007). Concurrent left-lateral transpression along the isotope ratios. This data set includes samples from Lewis and Clark Line cuts the Cordilleran fold-thrust 15 mining districts in the northern half of the Boulder belt. Middle Proterozoic Belt Supergroup formations Batholith, three samples from the Elkhorn Mountains were juxtaposed by fold-thrust belt tectonism (Sears volcanic fi eld, one sample from the Lowland Creek and others, 2010). volcanic fi eld, and one sample from the Golden Sun- The Elkhorn Mountains volcanic fi eld (EMVF) and light mine representing a post mineralization diorite Boulder Batholith formed concurrently during crustal dike. This data set was used in conjunction with pub- shortening near the end of Mesozoic Cordillera arc lished Pb-isotope data to interpret the metal source magmatism between about 85 and 76 Ma (Mahoney and genesis of Pb in base metal veins and Montana and others, 2015; Rutland and others, 1989). The

3 Stanley L. Korzeb

Figure 1. Geologic map of Boulder Batholith showing mining districts and approximate locations for base metal vein galena samples.

EMVF is preserved along the west and east fl anks of Magmatic events that developed the Boulder the Boulder Batholith (fi g. 1) and was the fi rst mag- Batholith started at the same time as the EMVF erup- matic event during continental arc magmatism in the tion. Field relationships indicate the Batholith was em- region. Intruded into the EMVF and Mesoprotero- placed into the EMVF between 85 and 76 Ma (Tilling zoic to Mesozoic sedimentary rocks are the Boulder and others, 1968; Mahoney and others, 2015). A 950- Batholith or Butte Granite and a group of associated m section of EMVF ignimbrites south of Boulder, Mon- smaller plutons (du Bray and others, 2012). tana has an 40Ar/39Ar age ranging from 83.7 ± 0.3 to

4 Montana Bureau of Mines and Geology Bulletin 139

84.9 ± 2.6 Ma (Olson and others, 2016). The Boulder Compositions of the plutons making up the Boul- Batholith plutons and Butte Granite were emplaced der Batholith are similar to those of moderately dif- from 81.7 ± 1.4 Ma to 73.7 ± 0.6 Ma (du Bray and ferentiated subduction-related magmas (du Bray and others, 2012). The Butte Granite is the main pluton by others, 2012). Compositions vary from subalkaline, volume of the Boulder Batholith, and U/Pb zircon ages magnesian, calcic-alkalic to calcic and metaluminous are 74.5 ± 0.9 Ma (Lund and others, 2002) and 76.28 to peraluminous (du Bray and others, 2012). du Bray ± 0.14 Ma (Martin and others, 1999). The Butte Gran- and others (2012) report the major oxide abundances ite could be a composite formed from two episodes of of the Butte Granite intrusion are homogeneous. The magmatism (du Bray and others, 2012). satellite plutons are characterized by distinct trace Intrusion of pegmatite, alaskite, and aplite dikes element abundances, but these trace element abun- took place at the close of Butte Granite magmatism dance variations are minor. Even though the Boulder (Berger and others, 2011). The dikes have a north- Batholith hosts signifi cant mineral deposits, ore metal east orientation and are found in a northeast-trending, abundances in the Butte Granite and satellite plutons 4- to 6-km-wide zone extending across the Batholith. are not elevated and are comparable to global aver- After the Butte Granite and dikes crystallized, sulfi de- age abundances in igneous rocks (du Bray and oth- bearing quartz veins with an east–west orientation ers, 2012). developed in the northern half of the Batholith (Berger PREVIOUS INVESTIGATIONS and others, 2011). Olson and others (2016) determined 40Ar/39Ar plateau ages for two veins in the Big The fi rst lead isotope study on the Boulder Batho- Foot district. A vein at the State mine was determined lith including the Butte mining district was done by to be 75.2 ± 0.25 Ma (95% confi dence) and a 40Ar/39Ar Murthy and Patterson (1961). They suggested the plateau age for the vein at the Ajax mine to be 73.81 ore metals came from multiple sources that were ± 0.12 Ma (95% confi dence). The ages of the veins transported into the Batholith by late-stage magmatic are about a million years younger than the host Butte fl uids after the Batholith was intruded. They proposed Granite. ore mineral and host rock lead isotopes were mixed in and near the ore zone. The hypothesis presented Eocene crustal extension and Basin and Range by Murthy and Patterson (1961) contradicted the faulting caused the EMVF and Boulder Batholith to vein genesis hypothesis of Grunig and others (1961). be uplifted, eroded, and exhumed. Laramide crustal Grunig and others (1961) disputed Murthy and Pat- shortening about 65–55 Ma followed by extension terson’s (1961) hypothesis by providing evidence the formed grabens in the upper plate of the Anaconda Butte ores were derived from the diff erentiation and Metamorphic Core Complex detachment. The Low- concentration of a late-stage fl uid with a magmatic land Creek volcanic fi eld (LCVF) erupted between 53 origin and not from multiple outside sources. and 49 Ma during crustal extension, fi lling a northeast–southwest-trending half graben traversing the Doe and others (1968) published the results of Boulder Batholith (Sillitoe and others, 1985; Scarberry a lead and strontium isotope study on the Boulder and others, 2015; Dudás and others, 2010; fi g. 1). Batholith that included satellite plutons, volcanic The Lowland Creek volcanic rocks overlie the Elkhorn rocks, and Butte Granite. They determined a Pre- Mountains volcanic rocks and the Butte Granite. Un- cambrian age for the lead source and discussed derlying the LCVF near the western end of the graben three possible models for the origin of the plutons are Mid-Proterozoic, Permian, and Cretaceous sedi- and volcanic rocks. Doe and others (1968) concluded mentary rocks (Dudás and others, 2010). The LCVF the plutons and volcanic rocks originated from partial hosts the Montana Tunnels and Tuxedo mine breccia melting of lower crustal and/or mantle material and, pipes or diatremes (Sillitoe and others, 1985; Lawson in some plutons, Precambrian Belt and pre-Belt rocks and others, 1937). were assimilated into the melts. Extension-related core complex formation contin- Lead isotopes and mineralization ages for sulfi de ued during and after Lowland Creek volcanism until veins hosted by Belt Supergroup rocks in northwest- about 35 Ma (Lonn and Elliott, 2010). Near the close ern Montana and northern Idaho were investigated by of Lowland Creek volcanism, basin sedimentation Zartman and Stacey (1971). They used contrasting began (Portner and others, 2011). Overlying the LCVF lead isotope populations to distinguish between Pre- are lacustrine and fl uvial sedimentary rocks in the cambrian and Mesozoic or Cenozoic mineralization. Deer Lodge and Warm Springs Creek Valleys, which Zartman and Stracey (1971) concluded Precambrian are locally unconformable above the volcanic rocks veins have a uniform lead isotope composition that is (Dudás and others, 2010). 5 Stanley L. Korzeb the product of single-stage development. They identi- represented by andesite, was probably derived from a fi ed the possibility that vein deposits that formed at a lithospheric mantle source. later time incorporated lead remobilization from Belt Lead isotopic data and element concentrations rocks. Zartman and Stacey (1971) noted Mesozoic of streambed sediments from Basin Creek, Cata- and Cenozoic vein deposits have more radiogenic ract Creek, and High Ore Creek in the Basin mining lead and wider isotopic compositions than Precam- district were determined by Unruh and others (2000). brian veins, refl ecting two-stage development. The purpose of their study was to determine the pre- Zartman (1974) identifi ed three lead isotope prov- mining trace element concentrations in streambed inces in the Cordillera of the western United States sediments to be used as a baseline to determine the that includes western Montana. The provinces Zart- impact of mining on the Boulder River. Samples were man (1974) identifi ed refl ect the geology of the source taken upstream from historic mining activity, stream rocks from which the lead was acquired. The three terrace deposits located upstream and downstream provinces are designated as area I, area II, and area of the major tributaries of the Boulder River, and III. In area I, lead is sourced principally from Precam- sediment cores from over bank deposits, abandoned brian crystalline basement. Area II lead isotopes are stream channels, or beneath fl uvial mill tailings depos- sourced from sedimentary rocks eroded from Precam- its. They reported on lead isotope data for 6 stream- brian rocks, and area III contains lead derived from terrace samples and 13 sediment core samples. eugeosynclinal plutonic, volcanic, and sedimentary Isotopic data were generated by du Bray and oth- rocks forming a belt adjacent to the Pacifi c coast. ers (2017) for the Big Belt Mountains located east of Western Montana is located in lead isotopic province the Boulder Batholith. They used O, Nd, Sr, Rb, Sm, area I, suggesting lead sourced from Precambrian and Pb isotopes to identify the petrogenesis of the crystalline basement rocks. Mount Edith and Boulder Baldy intrusions. From the A lead isotope study on the Helena embayment isotopic study, a combination of compositionally het- by Zartman (1992) included Belt rocks exposed in the erogeneous sources was identifi ed as having variable Highland Mountains, Little Belt Mountains, Bridger isotopic contamination from crustal assimilation (du Range, and Belt rock hosted sulfi de veins. Zartman Bray and others, 2017). Altered and mineralized rocks (1992) found two lead isotope populations for galena related to the intrusions demonstrate two isotopically hosted by Middle Proterozoic Belt Supergroup rocks. distinct mineralizing events involving major inputs Most of the Belt Basin is characterized by uniform iso- from associated Late Cretaceous igneous rocks (du topic compositions with minor variability in isotope ra- Bray and others, 2017). The isotopic source for the tios. Zartman (1992) attributed the minor variability of mineralizing events identifi ed by du Bray and oth- more radiogenic isotope ratios to remobilization of the ers (2017) was fl uids that equilibrated with Newland original stratabound sediments and mixing between Formation quartzite. They found oxygen isotopes for marine and continental sediments. Zartman (1992) the Boulder Baldy intrusive to be similar to those of found a distinctive Archean lead isotope signature for subduction-related magmatism that included mantle- the Helena embayment, suggesting an Archean crys- derived components. talline sedimentary source for Belt rocks. However, ANALYTICAL PROCEDURES only the LaHood Formation had an isotopic signature related to an Archean provenance; lead isotopes from Isotopic compositions of Pb in whole-rock and the other formations had a marine provenance (Zart- galena samples were determined at the USGS man, 1992). Crustal Geophysics and Geochemistry Science The Lowland Creek volcanic fi eld was investigated Center, Radiogenic Isotope Laboratory in Denver, for lead isotopes and 40Ar/39Ar geochronology by Colorado. Seventeen galena samples representing Dudás and others (2010). The investigation included base metal veins from 15 mining districts and 5 whole an andesite sample from the Elkhorn Mountain volca- rock samples were submitted for analysis. Samples nic fi eld near the Emery mining district and a Boul- representing vein sections were collected from mine der Batholith related intrusion near Mill Creek. They dumps and whole rock samples were collected from identifi ed four diff erent 206Pb/204Pb components rep- outcrops. Slabs were cut from vein section samples resenting diff erent Lowland Creek volcanic eruptions to expose the sulfi de minerals and embedded galena and rock types. The fi rst three lead isotope compo- grains. Some slabs were coarsely crushed to aid in nents were probably derived from a lower to middle separating disseminated galena grains from unwanted crustal source. The fourth lead isotope component, sulfi des and quartz. Galena grains were handpicked

6 Montana Bureau of Mines and Geology Bulletin 139 from cut slabs and crushed samples under a bin- isotope data based on multiple runs of NIST standard ocular microscope. Whole rock samples were pow- SRM-981 is shown in appendix A. dered samples used for previous ICP-MS analysis RESULTS determined at the Washington State University, Peter Hooper Geo Analytical Laboratory, School of the Envi- Lead isotope compositions from base metal vein ronment, Pullman, Washington. galena, breccia pipe galena, and whole rock samples Lead was separated from whole rock samples are shown in appendix A. New analytical results (ap- for isotope analysis on AG1-X8 resin columns in HBr pendix A) and previous data are plotted on covariant medium. The Pb analyses was conducted by isotope- diagrams ﬁ gures 2–15. Analytical results are given dilution, thermal-ionization, mass spectrometry (ID in atomic percent ratios relative to 204Pb with analyti- TIMS). Splits of powdered whole rock were digested cal errors of ± 0.014 to ± 0.018 for 206Pb/204Pb, ± 0.02 over 3 days on a hot plate at 135°C using a mixture to ± 0.023 for 207Pb/204Pb, and ± 0.061 to ± 0.065 for 205 208Pb/204Pb. The lead isotope data for the base metal of HNO3 and HF with Pb spike solution. The total procedural Pb blank for whole rock analysis was ≈10 vein galena, volcanic, and intrusive rocks is relatively picograms and the whole rock data were corrected for consistent with little variation. The galena sample from the blank and spike contribution. No Pb blank correc- the Montana Tunnels breccia pipe was more radio- tion was necessary for the galena analyses, which genic than the galena samples from the base metal were conducted on un-spiked digested mineral with- veins, volcanic rocks, and intrusions. The results are out chemical separation of Pb. similar to those of past investigations with little variation (appendices B–F). The isotopic analysis was conducted using a multi-collector TIMS Triton. Isotope ratios were mea- The lead isotope data from the base metal veins sured in static mode using Faraday cups. Measured plotted on covariant diagrams overprints data from Pb-isotope ratios were corrected for mass fraction- previous intrusive rock investigations. The overprint- ation of 0.0007 ± 0.0003 per mass unit using raw data ing of the base metal veins and intrusion Pb isotopes for NIST Pb isotope standard SRM-981 measured constrains the Pb isotopes to a single source and con- at the same run conditions. Correction factors for Pb strains vein development to hydrothermal ﬂ uids with a

Figure 2. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for base metal vein and breccia pipe galena lead isotope ratios generated by this investigation. MT, Montana Tunnels data point.

7 Stanley L. Korzeb

Figure 3. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for Boulder Batholith intrusions published by Doe and others (1968) with added data generated by this investigation (red points).

Figure 4. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for Lowland Creek volcanic rocks published by Dudás and others (2010) with added data generated by this investigation (red point).

8 Montana Bureau of Mines and Geology Bulletin 139

Figure 5. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for Elkhorn Mountains volcanic rocks published by Doe and others (1968) with added data generated by this investigation (red point). DISCUSSION magmatic source. These constraints restrict the timing of mineralization and vein development to intrusive Western Montana is dominated by Archean and and volcanic events. The more radiogenic Montana Proterozoic crust that may have been an initial source Tunnels galena sample might have been infl uenced for lead isotopes found in the veins and intrusions of by the process that formed the breccia pipe or rep- the Boulder Batholith. Zartman (1974) identifi ed three resent a diff erent lead isotope source than the base lead isotopic provinces based on lead isotope compo- metal veins. In situ radiogenic decay of uranium and sitions from volcanic and plutonic rocks in the western thorium transported into the breccia pipe from an out- United States. Crustal lead isotopes found in western side source during mineralization may have increased Montana are principally derived from the Precambrian the concentration of radiogenic lead. crystalline basement (Zartman, 1974). Average lead isotopic data from Zartman (1974) suggest western Lead isotope results from this investigation and Montana is underlain by 2.7 Ga basement rocks. past investigations are plotted on 206Pb/204Pb vs. 207Pb/204Pb covariant diagrams (fi gs. 2–15). On the Data from the Stillwater J-M reef sulfi des pub- diagrams, the line labeled 2.7 Ga source isochron lished by McCallum and others (1999) can provide a and 2.7 Ga reference isochron are from McCallum lead isotope baseline for the 2.7 Ga Archean rocks and others (1999). The line labeled 66 Ma Big Belt in Montana. Covariant plots for the 2.7 Ga initial lead Mountains reference isochron is based on the least isotope data for Stillwater was used to constrain radiogenic sample from the Boulder Baldy intrusion the lead isotope data from this investigation of the core (du Bray and others, 2017) and is constrained by Boulder Batholith along with data comparisons from the Stillwater initial Pb isotope value. The line labeled previous investigations. Lead isotope data determined 85 Ma mixing line Boulder Batholith is based on a by du Bray and others (2017) from the Big Belt Moun- point cluster average for all samples of the Boulder tains was used to further constrain the Boulder Batho- Batholith and is constrained by the Stillwater initial Pb lith data. Lead isotope data from past investigations isotope value. by Doe and others (1968), Zartman (1992), Unruh and others (2000), Dudàs and others (2010), and du Bray

9 Stanley L. Korzeb and others (2017) were used to compare and con- Lead isotope ratios plotted on the 206Pb/204Pb vs. strain data from this investigation and are summarized 207Pb/204Pb covariant diagram (fi gs. 2–7) shows a in appendices B–F. clustering of points below the 2.7 Ga source isochron Lead isotope data from this investigation and reference line. Lead isotope data from past investiga- past investigations are plotted on a 206Pb/204Pb vs. tions and this investigation are similar and overlap 207Pb/204Pb covariant Pb isotope diagrams (fi gs. 2–8) following similar trends on the covariant diagrams. based on the Stillwater investigation by McCallum These similarities between the diff erent data sets im- and others (1999). The line on the plot from McCal- ply they share a similar isotopic source. Plotting below lum and others (1999) labeled 2.7 Ga source isochron the 2.7 Ga reference isochron suggests the isotopic represents a range of source compositions based on compositions were infl uenced by mixing between 2.7 Stacey and Kramers (1975) parameters at 3.7 Ga and Ga crustal lead and less radiogenic sources. An ex- evolution to 2.7 Ga with a variable μ(238U/204Pb). This ception can be found for the Proterozoic Belt Super- line serves as a reference for possible lead isotope group formations and sulfi de veins (fi g. 8); these Pb compositions sourced from Archean rocks in west- isotopes plot above the 2.7 Ga reference line. Lead ern Montana. Lead isotopic ratios that plot along the isotopes plotting above the 2.7 Ga reference isochron source isochron line were not infl uenced by outside imply an Archean crustal infl uence for the Proterozoic geologic processes such as intrusive and metamor- Belt Supergroup formations and sulfi de veins. phic events causing additions and/or subtractions of A mixing line for the Boulder Batholith constrained Pb isotopes and were directly sourced from the crust. to pass through the Stillwater initial isotopic composi- The source isochron line constrained by the Stillwater tion was drawn through the point cluster for all the initial Pb ratios passes through the model crust value Boulder Batholith data and represents the addition of but not the model mantle value. The 2.7 Ga reference externally derived lead isotopes. The Boulder Batho- isochron line is constrained to pass through the Still- lith mixing line suggests lead isotopes sourced from water initial Pb isotopic composition (McCallum and Archean crust was infl uenced by geologic processes others, 1999). adding additional radiogenic isotopes to the magmas that generated the plutons and volcanic rocks. This

Figure 6. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for Basin Mining district stream sediment samples published by Uhruh and others (2000).

10 Montana Bureau of Mines and Geology Bulletin 139

Figure 7. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for the Big Belt Mountains published by du Bray and others (2017).

Figure 8. Covariant diagram of 206Pb/204Pb vs. 207Pb/204Pb plot for the Belt Supergroup formations and sulﬁ de veins published by Zartman (1992).

11 Stanley L. Korzeb mixing of Pb isotopes took place at the beginning of Callum and others (1999), was added to the covariant the Laramide orogeny about 85 Ma when subduction diagrams. Lead isotopes that plot on or below the 2.7 zone magmas were being generated and continued Ga reference line could originate from a crustal- or until the close of magmatic and mineralizing events. mantel-derived magma contaminated with additional The age (68 to 66 Ma) and proximity of the Big radiogenic crust. Belt Mountains to the Boulder Batholith imply they A 66 Ma Big Belt Mountains reference isochron were developed from the same geologic process dur- was plotted using the least radiogenic sample from ing the Laramide orogeny. Due to this geologic and the Boulder Baldy intrusion core and age of the age relationship, the Big Belt Mountains will be used intermediate zone (du Bray and others, 2017) and as a reference for constraining Pb isotope composi- constrained to pass through the Stillwater initial lead tions from the Boulder Batholith. In fi gures 2 to 8, the isotope value (fi gs. 9–15). Lead isotopes that plot on Big Belt Mountains reference isochron was drawn or near the Big Belt Mountains reference isochron using the least radiogenic sample from the core of contain a higher subcontinental lithospheric lead iso- the Boulder Baldy intrusion and constrained through tope component than lead isotopes that plot below the the Stillwater initial Pb isotope reference point. The line, which was infl uenced by crustal assimilation. isochron age is based on the 66.2 Ma U-Pb age for A mixing line was constructed for the Boulder zircon from the intermediate zone of the intrusion (du- Batholith passing through the cluster of points and Bray and others, 2017). Petrogenesis of the Boulder constrained by the Stillwater initial Pb isotope com- Baldy intrusion core involved Archean to Proterozoic position (fi gs. 9–15). Most lead isotope data for the mafi c to intermediate basement-derived magma (du Boulder Batholith is constrained between the 66 Ma Bray and others, 2017). Lead isotope data that plot Big Belt Mountains reference isochron and the 2.7 on or near the Big Belt Mountains reference line have Ga reference isochron (fi gs. 9–13) and cluster around a greater subcontinental lithospheric-derived compo- the 85 Ma mixing line. The constraint of the Boulder nent than those that plot above the line, which have a Batholith lead isotopes shown on fi gures 9–13 sug- greater crustal-derived contamination. Lead isotopic gest they are more radiogenic than the 2.7 Ga model data constrained between the 2.7 Ga reference iso- crust. This implies that the lead isotope compositions chron and the 66 Ma Big Belt Mountains isochron will were infl uenced by the addition of radiogenic lead have greater Archean to Proterozoic crustal-derived from an outside source or in situ radioactive decay. contamination with more radiogenic Pb isotopes than those plotting on or near the 66 Ma Big Belt Moun- Lead isotope mixing lines for plutons in the tains isochron. Boulder Batholith was proposed by Doe and others (1968). The mixing lines on 206Pb/204Pb vs. 207Pb/204Pb 208 204 206 204 The covariant Pb/ Pb vs. Pb/ Pb diagram and 206Pb/204Pb vs. 208Pb/204Pb covariant diagrams by based on the Stillwater investigation by McCallum and Doe and others (1968) are similar to the mixing lines others (1999) was used to constrain the lead isotope proposed for this investigation shown in fi gures 2–15. data from this investigation and past investigations Lead isotope mixing lines have likewise been identi- (fi gs. 9–15). The 2.7 Ga source isochron represents fi ed by du Bray and others (2017) for the Big Belt the range of possible model crust lead isotope values Mountains intrusions located on the northeast edge sourced from the Archean crust. McCallum and others of the Boulder Batholith. The mixing lines (fi gs. 2–6 (1999) constructed the line based on Stacey-Kramers and 9–13) for the veins, intrusions, and volcanic rocks model values using a Th/U value of 3.8 and a vari- of the Boulder Batholith suggest the initial magma able μ . The 2.7 Ga source isochron is constrained o source was contaminated with the addition of radio- to pass through the Stillwater initial lead isotope genic crust during intrusive events. The single mixing value (McCallum and others, 1999). Lead isotopes lines on fi gures 2–6 and 9–13 suggest the veins were that plot close or below the 2.7 Ga source isochron mineralized during single events related to diff erent are undisturbed, possibly originating from a mantle- magmatic intrusions that started with the eruption of derived magma (McCallum and others, 1999). Lead the Elkhorn Mountains volcanic fi eld and ended with isotopes plotting above the model crust values of the the eruption of the Lowland Creek volcanic fi eld. 2.7 Ga source isochron will most likely originate from crustal- or mantel-derived magmas contaminated with To further diff erentiate between lead isotope the addition of radiogenic crust. A 2.7 Ga reference sources, data were plotted on model lead growth isochron constrained to pass through the Stillwater curves from Stacey and Kramers (1975; fi gs. 16–22). initial lead value, calculated for Th/U = 3.8, from Mc- The ages on the model lead growth curves are model ages and do not refl ect the Boulder Batholith 85 Ma

12 Montana Bureau of Mines and Geology Bulletin 139

Figure 9. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for base metal vein and breccia pipe galena lead isotope data generated by this investigation. Base Metal Veins to 45 Ma ages for the veins or intrusions. The model The base metal veins plotted on 206Pb/204Pb vs. ages on the lead growth curves are related to the lead 207Pb/204Pb covariant diagram (fi g. 2) show little de- isotope source ages. Lead isotopes from the base viation from the mixing line. On the 206Pb/204Pb vs. metal veins, intrusions, and volcanic rocks have simi- 208Pb/204Pb diagram (fi g. 9), the Pb isotope data show lar lead isotope compositions with overlapping ranges a wide deviation from the mixing line. The inconsisten- (table 1). Plotted on covariant diagrams and model cies between the two covariant diagrams (fi gs. 2 and lead growth curves (fi gs. 2–6, 9–13, and 16–21), lead 9) suggest a heterogeneous isotope source for the isotopes from the intrusions, stream sediments, base base metal veins. On the model lead growth curves metal veins, and volcanic rocks overlap along mixing (fi g. 16), model ages vary from 800 Ma to 200 Ma, lines and model lead growth curves. This suggests suggesting a variety of isotope sources during vein a common origin between vein mineralization and development. Vein mineralization took place from intrusive events. about 74 Ma to 45 Ma, representing diff erent mineralizing events. For each mineralizing event, lead isotopes may have originated from diff erent sources refl ected on the model lead growth curve. Lead iso-

13 Stanley L. Korzeb

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topes from Montana Tunnels plots beyond the lead model ages suggest the plutons and intrusions origi- isotope growth curve representing a future date (fi g. nated from the same isotopic source with little varia- 16) caused by an excess of radiogenic lead, most tion in lead isotope compositions between intrusive likely generated from in situ radioactive decay of ura- events. nium and thorium incorporated into the breccia pipe when it formed. Lowland Creek Volcanic Field The LCVF shows a wide range of lead iso- Satellite Plutons and Intrusions tope compositions (table 1). On the 206Pb/204Pb vs. Lead isotopes plotted on covariant diagrams for 207Pb/204Pb covariant diagram (fi g. 4), LCVF Pb the Boulder Batholith satellite plutons and intrusions isotopes plot along the 85 Ma mixing line and form (fi gs. 3 and 10) partially overlap the base metal veins, three point clusters. On the 206Pb/204Pb vs. 208Pb/204Pb forming a tight cluster of points above and on the 85 covariant diagram (fi g. 11), LCVF Pb isotopes cluster Ma mixing line. These plots suggest some of the intru- on the 85 Ma mixing line. Other points are scattered sions originated from a diff erent isotopic source than above and on the 66 Ma Big Belt Mountains reference the base metal veins. Other intrusions that plot on the isochron, showing a wide deviation from the mixing 85 Ma mixing line and overlap the base metal veins line. The two covariant plots (fi gs. 4 and 11) suggest may share the same lead isotope origin. Plutons and the LCVF lead isotopes originated from heteroge- intrusions plotted on the model lead growth curves neous diverse sources of crustal- and mantel-derived (fi g. 17) have model ages varying from 600 Ma to 200 magmas. The diversity of lead isotope sources is Ma, with most points clustering at 400 Ma. The narrow refl ected on the model lead growth curves (fi g. 18).

14 Montana Bureau of Mines and Geology Bulletin 139

Figure 10. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for Boulder Batholith intrusions published by Doe and others (1968) with additional data generated by this investigation (red points).

15 Stanley L. Korzeb

Figure 11. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for Lowland Creek volcanic rocks published by Dudás and others (2010) with additional data generated by this investigation (red point).

16 Montana Bureau of Mines and Geology Bulletin 139

Figure 12. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for Elkhorn Mountains volcanic rocks published by Doe and others (1968) with additional data generated by this investigation (red point).

Model ages vary from 1,200 Ma to 0 Ma and three for the EMVF plot with a model age of 400 Ma. This point clusters similar to the covariant diagrams occur model age is the same for the satellite plutons, LCVF, along the model lead growth curves. Based on their and base metal veins (fi gs. 16–18), indicating similar lead isotope study, Dudás and others (2010) sug- lead isotope sources. gested two magmatic sources for the LCVF eruptions, Stream Sediments consisting of a lower to middle crustal source and a lithospheric source. Stream sediment Pb isotopes from the Basin mining district plotted on covariant diagrams are Elkhorn Mountains Volcanic Field tightly clustered on the 85 Ma mixing line (fi gs. 6 and Lead isotopes from the EMVF plotted on covariant 13). The stream sediment plots are similar to the diagrams fall near the 85 Ma mixing line (fi gs. 5 and base metal veins, showing the stream sediment Pb 12) and overlap the satellite pluton point clusters, one isotopes originated from the base metal veins. The of the point clusters for the LCVF, and the base metal stream sediment Pb isotopes represent the Basin veins (fi gs. 2–4 and 9–11). These diagrams suggest mining district veins and the tight clustering of points the EMVF isotopes originated from similar sources suggest the Basin mining district veins originated from as the satellite plutons, LCVF, and base metal veins. a single homogenous source during one mineralizing Model lead growth curves (fi g. 19) show lead isotopes event. Stream sediment lead isotopes plotted on mod-

17 Stanley L. Korzeb

Figure 13. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for stream sediment samples from the Basin mining district (Uhruh and others, 2000). el lead growth curves (fi g. 20) show a clustering of 85 Ma mixing line and below the 2.7 Ga reference points around a model age of 400 Ma. This model age isochron. The lead isotope plots in fi gures 7 and 14 in- is shared by the base metal veins, satellite plutons, dicate an isotope source contaminated by assimilation EMVF, and LCVF (fi gs. 2–5 and 9–12), demonstrating of isotopically evolved crustal components including they have a common isotopic source. a portion of lithospheric mantle and subcontinental- derived material as suggested by du Bray and oth- Big Belt Mountains ers (2017). Lead isotopes for the Boulder Batholith The Big Belt Mountains intrusions and mineralized and Big Belt Mountains intrusions are constrained rocks partially overprint the lead isotope composi- between the 2.7 Ga reference isochron and 66 Ma tions from the Boulder Batholith (table 1). On covari- Big Belt Mountains reference isochron (fi gs. 2–7 and ant diagrams these primarily plot between the 85 Ma 9–14), indicating they underwent similar geologic pro- mixing line and the 66 Ma Big Belt Mountains refer- cesses during intrusion. ence isochron (fi gs. 7 and 14). Lead isotopes plotting Big Belt Mountains lead isotopes plotted on lead below the mixing line in fi gure 7 indicate they are less growth curves show a wide range of model lead ages, radiogenic than the Boulder Batholith isotopes shown varying from 1,200 Ma to 0 Ma (fi g. 21). This wide 206 204 208 204 in fi gures 2–6. On the Pb/ Pb vs. Pb/ Pb covariation on model lead ages implies a heterogeneous variant diagram (fi g. 14), Pb isotopes plot below the

18 Montana Bureau of Mines and Geology Bulletin 139

A B

C D

Figure 14. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for the Big Belt Mountains, data published by du Bray and others (2017). lead isotope source for the Big Belt Mountains Intru- Belt Supergroup Formations and Sulfi de Veins sions, as argued for by du Bray and others (2017). Most lead isotopes for the Belt Supergroup forma- The lead growth curves show a clustering of points tions and associated sulfi de veins plotted on covari- consistent with model ages of 1,200 Ma, 600 Ma, and ant diagrams are constrained between the 2.7 Ga from 400 Ma to 0 Ma (fi g. 21). Two points plot with reference isochron and 2.7 Ga source isochron (fi gs. future model ages, which could be attributed to in situ 8 and 15), suggesting a crustal isotopic source. The radioactive decay. Lowland Creek volcanic fi eld lead covariant diagrams show the lead isotope source for isotopes show a similar trend, with a clustering of the Belt Supergroup formations and associated sulfi de points at model ages of 1,200 Ma, between 800 Ma veins are diff erent from those of the Boulder Batho- and 600 Ma, and between 400 Ma and 0 Ma (fi g. 18). lith and may not be directly related. Belt Supergroup These similarities in model ages imply both the 68 to formations and associated sulfi de vein lead isotopes 66 Ma Big Belt Mountains intrusions and the 53 to 49 plotted on lead growth curves show model ages Ma LCVF developed from similar geologic processes between 1,200 Ma and 1,000 Ma, which is older than and lead isotope sources. Proximity of the Boulder Boulder Batholith model ages (fi g. 22). The exception Batholith to the Big Belt Mountains and similar late is a cluster of LCVF lead isotope points with a model Cretaceous ages suggest they have similar isotopic age of 1,200 Ma (fi g. 18). Even though one LCVF sources and petrogenetic histories.

19 Stanley L. Korzeb

Figure 15. Covariant diagram for 208Pb/204Pb vs. 206Pb/204Pb plot for the Belt Supergroup formations and hosted suﬁ de veins, data published by Zartman (1992).

20 Montana Bureau of Mines and Geology Bulletin 139

Figure 16. Lead isotope ratios generated by this investigation for galena samples from base metal veins and galena from Montana Tunnels breccia pipe plotted on Stacey and Kramers (1975) model lead isotope growth B curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios.

21 Stanley L. Korzeb

Figure 17. Lead isotope ratios for Boulder Batholith intrusions from data published by Doe and others (1968) with additional data generated by this investigation (red points) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios.

22 Montana Bureau of Mines and Geology Bulletin 139

Figure 18. Lead isotope ratios for Lowland Creek volcanic rocks from data published by Dudás and others (2010) with additional data generated by this investigation (red points) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios. point cluster shares a model age with Belt Supergroup Magma Source formations, they do not share the same lead isotope Based on Sri values, du Bray and others (2017) source. This is supported by covariant diagrams, in report the intrusive rocks of the Big Belt Mountains which LCVF lead isotopes plot on the 85 Ma mixing have a less evolved isotopic source including contri- line (fi g. 4) and Belt Supergroup formations are con- butions from the asthenospheric mantle and are less strained between the 2.7 Ga reference isochron and contaminated by crustal rock assimilation than the 2.7 Ga source isochron (fi gs. 8 and 15). When covari- Boulder Batholith intrusions. Variations of 207Pb/204Pb ant diagrams and lead growth curves of the Boulder and 208Pb/204Pb refl ect radiogenic Pb compositions Batholith are compared with the Belt Supergroup for- of the magmatic source or the extent of geochemi- mations and associated sulfi de veins (fi gs. 2–6, 9–13, cal evolution and the Pb isotopic composition of and 16–20), it appears the lead isotopes for the Belt crustal-derived contaminants. Relative variations of Supergroup formations had a diff erent source than the 206Pb/204Pb are a function of age of the associated ig- lead isotopes for the Boulder Batholith.

23 Stanley L. Korzeb

Figure 19. Lead isotope ratios for Elkhorn Mountains volcanic rocks from data published by Doe and others (1968) with additional data generated by this investigation (red points) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios.

24 Montana Bureau of Mines and Geology Bulletin 139

Figure 20. Lead isotope ratios for stream sediments from the Basin mining district from data published by Uhruh and others (2000) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 208Pb/204Pb vs. 206Pb/204Pb ratios. neous rocks and their source. In contrast to the they are more contaminated with assimilated crustal Boulder Batholith intrusions, the Big Belt Mountains components and have a lesser proportion of subconti- intrusions 207Pb/204Pb and 208Pb/204Pb are less radio- nental- and lithospheric mantle-derived material. genic, indicating the intrusions are less contaminated The partial melting and mixing of crustal and by assimilation of isotopically evolved crustal compo- mantle magmas causes the transfer of U-Th-Pb from nents and include a greater proportion of lithospheric the mantle and crust into magmas generated in a mantle and subcontinental-derived material (du Bray subduction zone (Tosdal and others, 1999). Because and others, 2017). Boulder Batholith intrusions, on of U-Th-Pb mixing, lead isotope ratios generated in 207 204 the other hand, have more radiogenic Pb/ Pb and a subduction-related magma can vary, depending on 208 204 Pb/ Pb (table 1) and plot on an 85 Ma mixing line the degree of mantle, and upper and lower crust melt- (ﬁ gs. 2–6, and 9–13) independent of the 66 Ma Big ing (Faure, 1977). The volcanic rocks and intrusions Belt Mountains reference isochron. This indicates of the Boulder Batholith were formed in a volcanic

25 Stanley L. Korzeb

Figure 21. Lead isotope ratios for the Big Belt Mountains from data published by du Bray and others (2017) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios. back arc environment and originated from subduction- 208Pb/204Pb 38.98), showing a balance of magmas related magmatism (du Bray and others, 2012). The or heterogeneous isotopic sources for crustal con- volcanic rocks, intrusions, and base metal vein galena taminates that were assimilated into the magmas. In 207Pb/204Pb and 208Pb/204Pb populations are scattered addition, Lowland Creek volcanic rocks show a wide along 85 Ma mixing lines (ﬁ gs. 2–6 and 9–13), dem- range of 207Pb/204Pb ratios (table 1), further suggesting onstrating some isotopic diversity in crustal contami- a variable isotopic source for crustal contaminants in nants that assimilated into the magmas by partial magmas that generated these volcanic rocks. melting. Compared to most intrusions in the Boulder Intrusions in the Big Belt Mountains originated Batholith, the diorite intrusion from the Golden Sun- from subduction-related magmatism (du Bray and 207 204 light mine is the least radiogenic intrusion ( Pb/ Pb others, 2017). The proximity of the Big Belt Mountains 208 204 15.48, Pb/ Pb 38.00) and Lowland Creek volca- to the east of the Boulder Batholith and similar ages nic rocks are the most radiogenic (207Pb/204Pb 15.70,

26 Montana Bureau of Mines and Geology Bulletin 139 imply they originated from the same subduction event. igneous rocks refl ecting the magmatic lead isotope Compared to the Boulder Batholith, the intrusions in source. the Big Belt Mountains are less radiogenic (du Bray Lead isotopes for the Boulder Batholith veins and others, 2017; table 1). Similar to the Boulder overlap the intrusions and volcanic rocks, implying the Batholith, Pb isotopes for the Big Belt Mountains metals that developed the veins were sourced from intrusions have variable ratios, implying source region intrusion-generated magmatic fl uids. There is Pb, Sr, isotopic heterogeneity and variable contamination and Nd isotopic evidence the magmas that developed from diverse crustal assimilants (du Bray and others, the Boulder Batholith and Big Belt Mountains intru- 2017). The less radiogenic fl ows in the Lowland Creek sions and volcanic rocks assimilated crustal rocks volcanic rocks could have a higher proportion of (du Bray and others, 2017). Physical evidence for the derived lithospheric mantle, argued for by Dudás and assimilation of Proterozoic crustal rocks into the mag- others (2010), and subcontinental material than the mas when they were generated includes inherited zir- other Boulder Batholith volcanic rocks and intrusions. cons with core Proterozoic ages and rim Cretaceous In addition, the wide range of isotopic diversity in the ages for the Boulder Batholith intrusions (Lund and Lowland Creek volcanic rocks suggests a heteroge- others, 2002). Intrusions in the Butte, Emery, and Oro neous isotopic composition for crustal contaminants Fino districts also include Proterozoic age inherited assimilated into the Lowland Creek source magmas. zircons (Field and others, 2005; Korzeb and others, Ore Deposit Genesis 2018; Korzeb and Scarberry, in press). The base metal veins in the Boulder Batholith Cooling intrusions exsolving metal-enriched hy- have the characteristics of low to intermediate sul- persaline fl uids that become diluted with circulating fi dation epithermal systems. These systems develop meteoric waters or low to moderate density magmatic- from circulating hydrothermal fl uids associated with derived fl uids will rise to shallow depths, leading to magmas and could develop several kilometers away the development of epithermal veins (Rottier and from the intrusion site (Cooke and Simmons, 2000). others, 2018). There is indirect evidence that metals Metals transported by circulating hydrothermal fl uids and sulfur that formed the Boulder Batholith epither- that form epithermal veins are sourced from intrusive mal veins could have been derived from intrusions bodies, host rocks, and underlying formations (Hayba that assimilated Proterozoic crustal rocks. Field and and others, 1985; Hedenquist and Lowenstern, 1994). others (2005), using sulfur isotope data, argued that Hendenquist and Lowenstern (1994) suggest a cool- the sulfur found in the Butte district veins was sourced ing magma will discharge vapor and hypersaline- from magmas contaminated with evaporates from as- acidic magmatic fl uids that can leach metals from similated middle Proterozoic Belt Supergroup rocks. host formations; the leached metals and associated The cooling magma contaminated with assimilated isotopes become concentrated in magmatic-derived Proterozoic crustal rocks exsolved metal- and sulfur- hydrothermal fl uids. The metal-enriched, magmatic- enriched magmatic fl uids, leading to the development derived hydrothermal fl uids are transported, mixed of epithermal veins. In addition to the Butte mining with heated circulating meteoric fl uids, and vented district, veins in the Emery and Oro Fino mining dis- through fractures developing epithermal veins. tricts have sulfur isotope compositions that suggest they developed from a magmatic source that assimi- Galena from two types of ore deposits, base lated Proterozoic crustal rocks (Korzeb and others, metal veins, and a breccia pipe were sampled for lead 2018; Zimmerman, 2016; Korzeb and Scarberry, in isotope analysis. Lead isotope compositions for the press). galena samples overlap isotope populations from the intrusive and volcanic rocks (table 1), demonstrating Proterozoic crustal rocks assimilated into magmas the intrusions and volcanic rocks hosting the base during intrusion may have enriched the magmas in metal veins are in isotopic equilibrium. This rela- metals and sulfur. During the Proterozoic, Belt Su- tionship implies base metal vein lead has the same pergroup sediments became locally metal-enriched isotopic source as the igneous rocks and could have from hydrothermal fl uids venting on the sea fl oor been derived from magmatic fl uids exsolved from ig- and concentrating in the ocean sediments (Box and neous intrusions. Based on Pb isotopic compositions others, 2012). Lead isotope compositions indicate between galena from veins and feldspar from plutons, the magmas that generated the Boulder Batholith Doe and others (1968) concluded a genetic rela- intrusions assimilated Archean to Proterozoic crust. tionship between the plutons and base metal veins, Lead isotope compositions further demonstrate the suggesting the ore fl uids could have originated from epithermal veins are directly related to intrusions,

27 Stanley L. Korzeb

Figure 22. Lead isotope ratios for Belt Supergroup formations and hosted sulfi de veins from data published by Zartman (1992) plotted on Stacey and Kramers (1975) model lead isotope growth curves for 206Pb/204Pb vs. 207Pb/204Pb (below) and 206Pb/204Pb vs. 208Pb/204Pb ratios. and hydrothermal fl uids were sourced from magmatic Ages for base metal veins representing one of the fl uids exsolving from cooling intrusions. Proterozoic mineralizing events in the Boulder Batholith took place crustal rocks assimilated into Boulder Batholith mag- from 75.12 Ma to 73.1 Ma (Olson and others, 2017). mas contributed Proterozoic age inherited zircons, These ages overlap an aplite intrusion age of 74.5 sulfur isotopes with Belt Supergroup signatures, and, Ma (Korzeb and Scarberry, in press). The overlapping in some cases, metal enrichment. Intrusions related ages suggest a geochronological link between the ap- to epithermal vein development probably assimilated lite intrusive event and epithermal vein development. metal-enriched Belt Supergroup rocks that contributed Lead isotope ratios from base metal vein galena and metals and sulfur to the melt. When these intrusions aplite intrusions (table 1; fi gs. 2, 3, 9, and 10) overlap, cooled, they exsolved metal-enriched hypersaline indicating isotopic equilibrium between the base metal fl uids (Rottier and others, 2018) that led to the devel- veins and aplite. The overlapping of lead isotopes and opment of the Boulder Batholith epithermal veins. ages between the base metal veins and aplite intru-

28 Montana Bureau of Mines and Geology Bulletin 139 sions suggest the hydrothermal fl uids that developed Lead isotopes for base metal vein galena samples the veins were magmatic fl uids sourced from the overlap the intrusions and volcanic rocks hosting the same magma that intruded the aplite dikes and sills. veins, suggesting isotopic equilibrium between the Galena from the Montana Tunnels breccia pipe veins, volcanic rocks, and intrusions. The intrusions, has elevated radiogenic lead compared to the base veins, and volcanic rocks could have had similar iso- metal veins galena (table 1). The elevated radiogenic topic sources. This overlapping of Pb isotopes implies lead suggest the breccia pipe developed from a diff er- the hydrothermal fl uids that generated the veins was ent lead isotope source compared to the base metal sourced from cooling magmas that assimilated Belt veins and could be a refl ection of the process that Supergroup rocks. The intrusions that contributed formed the breccia pipe. The breccia pipe is reported magmatic fl uids to vein development may have been to have the characteristics of a maar volcano (Sillitoe enriched in metals by assimilating Belt Supergroup and others, 1985) and originated during the eruption formations. Belt Supergroup formations are known of the Lowland Creek volcanic fi eld. Sillitoe and others to be locally enriched in sulfi des that originated from (1985) concluded the breccia pipe developed from hydrothermal activity during the Proterozoic (Box and phreatomagmatic explosive activity caused by the others, 2012). Assimilation of these formations into a interaction of ground water with an ascending quartz magma might enhance the metal content of the melt latite magma. As the magma was being intruded and contribute sulfur isotopes with a Belt Supergroup and the maar volcano was erupting, magmatic fl uids signature. Sulfur isotopes refl ecting a Belt Supergroup mixed with an infl ux of meteoric water. The underly- origin are known to occur in epithermal base metal ing Butte granite could be a source for U and Th that veins from the Butte, Emery, and Oro Fino mining was leached from the granite by meteoric waters districts (Field and others, 2005; Zimmerman, 2016; (Dodd, 1981). The infl ux of meteoric water into the Korzeb and others, 2018; Korzeb and Scarberry, in breccia pipe during mineralization may have intro- press). duced uranium and thorium into the primary magmatic Galena from the Montana Tunnels breccia pipe fl uids. In situ radioactive decay of the added U and has more radiogenic Pb than galena from the base Th may have generated an enrichment of radiogenic metal veins. The increase in radiogenic Pb com- Pb isotopes that occur in the Montana Tunnels galena pared to the base metal veins, volcanic rocks, and sample. plutons indicates a diff erent isotopic lead source or CONCLUSION might be caused by the geologic process that formed the breccia pipe. The breccia pipe developed from Lead isotope compositions for base metal veins, phreatomagmatic explosive activity caused by the breccia pipe, plutons, and volcanic rocks indicate interaction of groundwater with an ascending quartz they originated from a common source. The magmas latite magma (Sillitoe and others, 1985). Groundwater that generated the Boulder Batholith intrusions were enriched in uranium and thorium could be leached derived from partial melting of lithospheric mantle- from the underlying Butte granite (Dodd, 1981) and and subcontinental-derived material in a subduction mixed with magmatic fl uid exsolving from the quartz zone similar to the nearby Belt Mountains, as sug- latite magma adding U and Th to the breccia pipe. In gested by du Bray and others (2017). Mixing of Pb situ radioactive decay of U and Th introduced into the isotopes imply the magmas generated from partial breccia pipe from meteoric water may have increased melting of mantle and subcontinental crust assimi- the radiogenic lead concentration of the breccia pipe lated Proterozoic crust into the melts. Variations in Pb galena. isotope ranges between plutons and volcanic rocks ACKNOWLEDGMENTS indicate heterogeneous or variable isotopic sources. The assimilation of Proterozoic crust into the magmas This research was funded by Montana Techno- contributed inherited zircons to the melts. The inher- logical University of the University of Montana seed ited zircons have Proterozoic cores with Cretaceous grant program. I wish to thank Kaleb Scarberry, age rims, and sulfi de minerals have sulfur isotopes Montana Bureau of Mines and Geology, who provided with Belt Supergroup compositions (Lund and oth- samples for this research and valuable suggestions ers, 2002; Field and others, 2005; Korzeb and oth- as this study was progressing. I also would like to ers, 2018; Korzeb and Scarberry, in press), implying thank Susan Smith, who drafted the fi gures used in Proterozoic rocks were assimilated into the generating this publication, Susan Barth, who did the fi nal edits, magmas. and Phyllis Hargrave, Montana Bureau of Mines and Geology, who reviewed the manuscript. I extend a

29 Stanley L. Korzeb special thank you to Francis Dudás, Massachusetts Survey Data Series 1040, 12 p. Institute of Technology, and John Ridley, Department Dudás, F.Ö., Ispolatov, V.O., Harlan, S.S., and Snee, of Geosciences, Colorado State University, Fort Col- L.W., 2010, 40Ar/39Ar geochronology and lins, Colorado for their review. Their comments and geochemical reconnaissance of the Eocene Low- suggestions greatly improved the fi nal manuscript and land Creek volcanic fi eld, west-central Montana: data interpretation. Journal of Geology, v. 118, p. 295 ̶ 304. REFERENCES Faure, Gunter, 1977, Principles of isotope geochemistry, 2nd edition: New York, John Wiley and Sons, Amelin, Y., Davis, D.W., and Davis, W.J., 2005, 464 p. Decoupled fractionation of even-and odd-mass isotopes of Pb in TIMS: Goldschmidt Conference Field, C.W., Zhang, L., Dilles, J.H., Rye, R.O., and Abstracts 2005, Non-Traditional Stable Isotopes, Reed, M.H., 2005, Sulfur and oxygen isotopic re- p. A215. cord in sulfate and sulfi de minerals of early, deep, pre-main stage porphyry Cu-Mo and late main Berger, B.R., Hildenbrand, T.G., and O’Neill, J.M., stage base-metal mineral deposits, Butte district, 2011, Control of Precambrian basement defor- Montana: Chemical Geology, v. 215, p. 61–93. mation zones on emplacement of the Laramide Boulder Batholith and Butte mining district, Grunig, J.K., Guilbert, J.M., and Zeihen, L.G., 1961, Montana, United States: U.S. Geological Survey Discussions, lead isotopes in ores and rocks of Scientifi c Investigations Report 2011 ̶ 5016, 29 p. Butte, Montana: Economic Geology, v. 56, p. 215–216. Box, S.E., Bookstrom, A.A., and Anderson, R.G., 2012, Origins of mineral deposits, Belt-Purcell Hayba, D.O., Bethke, P.M., Heald, P., and Foley, N.K., Basin, United States and Canada: An introduc- 1985, Geologic, mineralogic, and geochemical tion: Economic Geology, v. 107, p. 1081 ̶ 1088. characteristics of volcanic-hosted epithermal pre- cious-metal deposits, in Berger, B.R., and Betke, Cooke, D.R., and Simmons, S.F., 2000, Characteris- P.M., eds., Geology and geochemistry of epither- tics and genesis of epithermal gold deposits, in mal systems: Reviews in Economic Geology, v. 2, Hagemann, S.G., and Brown, P.E., eds.: Gold in Society of Economic Geologists, p. 129–167. 2000, Reviews in economic geology, Society of Economic Geologists, v. 13, p. 221 ̶ 244. Hedenquist, J.W., and Lowenstern, J.B., 1994, The role of magmas in the formation of hydrothermal Dodd, S. P., 1981, The mineralogy, petrology and ore deposits: Nature, v. 370, p. 519 ̶ 527. geochemistry of the uranium-bearing vein deposits near Boulder, Motana, and their relationship to Kalakay, T.J., John, B.E., and Lageson, D.R., 2001, faulting and hot spring activity: Western Washing- Fault-controlled pluton emplacement in the Se- ton University, M.S. thesis, 71 p. vier fold-and-thrust belt of southwest Montana, USA: Journal of Structural Geology, v. 23, p. Doe, B.R., Tilling, R.I., Hedge, C.E., and Klepper, 1151–1165. M.R., 1968, Lead and strontium isotope studies of the Boulder Batholith, southwestern Montana: Korzeb, S.L., Scarberry, K.C., and Zimmerman, J.L., Economic Geology, v. 63 p. 884 ̶ 906. 2018, Interpretations and genesis of Cretaceous age veins and exploration potential for the Emery Doe, B.R., and Zartman, R.E., 1979, Plumbotectonics mining district, Powell County, Montana: Montana I, The Phanerozoic, in Barnes, H.L., ed., Geo- Bureau of Mines and Geology Bulletin 137, 49 p. chemistry of hydrothermal ore deposits, 2nd ed.: New York, Wiley Interscience, p. 22 ̶ 70. Korzeb, S.L., and Scarberry, K.C., in press, Genesis and exploration potential for Eocene age veins of du Bray, E.A., Aleinikoff , N.J., and Lund, K., 2012, the Oro Fino mining district, Deer Lodge County, Synthesis of petrographic, geochemical, and Montana: Montana Bureau of Mines and Geology isotopic data for the Boulder Batholith, southwest Bulletin. Montana: U.S. Geological Survey Professional Paper 1793, 39 p. Lageson, D.R., Schmitt, J.G., Horton, B.K., Kalakay, T.J., and Burton, B.R., 2001, Infl uence of Late du Bray, E.A., Unruh, D.M., and Hofstra, A.H., 2017, Cretaceous magmatism on the Sevier orogenic Isotopic data for Late Cretaceous intrusions and wedge, western Montana: Geology, v. 29, p. 723– associated altered and mineralized rocks in the 726. Big Belt Mountains, Montana: U.S. Geological

30 Montana Bureau of Mines and Geology Bulletin 139

Lawson, R.W., Tarrant, B.R., and Gallagher, A.L., sponse to orogenic exhumation in the northern 1937, Geology of the Tuxedo mining district: Rocky Mountain Basin and Range province, Flint Butte, Montana School of Mines B.S. thesis, 35 p. Creek basin, west-central Montana: Canadian Lonn, J., and Elliott, C., 2010, A walking tour of the Journal of Earth Sciences, v. 48, p. 1131 ̶ 1153. Eocene Anaconda detachment fault on Stucky Robinson, G.D., Klepper, M.R., and Obradovich, J.D., Ridge near Anaconda, Montana: Northwest Geol- 1968, Overlapping plutonism, volcanism, and tec- ogy, v. 39, p. 81 ̶ 90. tonism in the Boulder Batholith region, western Lund, K., Aleinikoff , J.N., Kunk, M.J., and Unruh, Montana: Geological Society of America Memoir D.M., 2007, SHRIMP U-Pb and 40Ar/39Ar age 116, p. 557 ̶ 576. constraints for relating plutonism and mineraliza- Rottier, B., Kouzmanov, K., Casanova, V., Wälle M., tion in the Boulder Batholith region, Montana, in and Fontboté, L., 2018, Cyclic dilution of mag- Lund, K., ed., Earth science studies in support of matic metal-rich hypersaline fl uids by magmatic public policy development and land stewardship; low-salinity fl uid: A major process generating the Headwaters Province, Idaho and Montana: U.S. giant epithermal polymetallic deposit of Cerro Geological Survey Circular C 1350, p. 39 ̶ 42. de Pasco, Peru: Economic Geology, v. 113, p. Lund, K., Aleinikoff , J.N., Kunk, M.J., Unruh, D.M., 825–856. Zeihen, G.D., Hodges, W.C., du Bray, E.A., and Rutland, C., Smedes, H., Tilling, R., and Greenwood, O’Neill, J.M., 2002, SHRIMP U-Pb and 40Ar/39Ar W., 1989, Volcanism and plutonism at shallow age constraints for relating plutonism and min- crustal levels: The Elkhorn Mountains Volcanics eralization in the Boulder Batholith region, Mon- and the Boulder Batholith, southwestern Mon- tana: Economic Geology, v. 97, p. 241 ̶ 267. tana, in Henshaw, P., ed., Volcanism and pluto- Mahoney, J.B., Pignotta, G.S., Ihinger, P.D., Wittkop, nism of western North America; v. 2, Cordilleran C., Balgord, E.A., Potter, J.J., and Leistikow, volcanism, plutonism, and magma generation at A., 2015, Geologic relationships in the northern various crustal levels, Montana and Idaho, Field Helena Salient, Montana: Geology of the Elliston trips for the 28th International Geological Con- Region: Northwest Geology, v. 44, p. 109 ̶ 135. gress: American Geophysical Union, Monograph, p. 16 ̶ 31. Martin, M., Dilles, J., and Proff ett, J.M., 1999, U-Pb geochronologic constraints for the Butte porphyry Scarberry, K.C., Korzeb, S.L., and Smith, M.G., 2015, system: Geological Society of America Abstracts Origin of Eocene volcanic rocks at the south end with Programs, v. 31, no. 7, p. A380. of the Deer Lodge Valley, Montana, in Mosolf, J., and McDonald, C., eds., 40th annual fi eld confer- McCallum, I.S., Thurber, M.W., O’Brien, H.E., and ence, Geology of the Elliston area, Montana and Nelson, B.K., 1999, Lead isotopes in sulfi des other papers: Northwest Geology, v. 44, p. 201 ̶ from the Stillwater complex, Montana: Evidence 212. for subsolidus remobilization: Contributions in Mineralogy and Petrology, v. 137, p. 206–219. Sears, J.W., McDonald, C., and Lonn, J., 2010, Lewis and Clark Line, Montana: Tectonic evolution of a Murthy, V.R., and Patterson, C., 1961, Lead isotopes crustal-scale fl ower structure in the Rocky Moun- in ores and rocks of Butte, Montana: Economic tains, in Morgan, L.A., and Quane, S.L., eds., Geology v. 56, p. 59 ̶ 67. Through the generations: Geological and anthro- Olson, N.H., Dilles, J.H., Kallio, I.M., Horton, T.R., and pogenic fi eld excursions in the Rocky Mountains Scarberry, K.C., 2016, Geologic map of the Ratio from modern to ancient: Geological Society of Mountain 7.5 quadrangle, southwest Montana: America Field Guide 18, p. 1 ̶ 20. Montana Bureau of Mines and Geology ED- Sillitoe, R.H., Grauberger, G.L., and Elliott, J.E., 1985, MAP-10, scale 1:24,000. A diatreme-hosted gold deposit at Montana Olson, N.H., Sepp, M.D., Dilles, J.H., Mankins, N.E., Tunnels, Montana: Economic Geology v. 80, p. Blessing, J.M., and Scarberry, K.C., 2017, Geo- 1707–1721. logic map of the Mount Thompson 7.5 quad- Stacey, J.S., and Kramers, J.D., 1975, Approximation rangle, southwest, Montana: Montana Bureau of of terrestrial lead isotope evolution by a two- Mines and Geology EDMAP-11, scale 1:24,000. stage model: Earth and Planetary Science Let- Portner, R.A., Hendrix, M.S., Stalker, J.C., Miggins, ters, v. 26, p. 207–221. D.P., and Sheriff , S.D., 2011, Sedimentary re-

31 Stanley L. Korzeb

Tilling, R., Klepper, M., and Obradovich, J., 1968, K-Ar ages and time span of emplacement of the Boulder Batholith, Montana: American Journal of Science, v. 266, p. 671 ̶ 689. Tosdal, R.M., Wooden, J.L., and Bouse, R.M., 1999, Pb isotopes, ore deposits, and metallogenic terranes, in Lambert, D.D., and Rutz, J., eds., Application of radiogenic isotopes to ore deposits research and exploration: Reviews in Economic Geology, v. 12, p. 1 ̶ 28. Unruh, D.M., Fey, D.L., and Church, S.E., 2000, Chemical data and lead isotopic compositions of geochemical baseline samples from streambed sediments and smelter slag, lead isotopic compositions in ﬂ uvial tailings, and dendrochronology results from the Boulder River watershed, Jef- ferson County, Montana: U.S. Geological Survey Open-File Report 00-038, 75 p. Zartman, R.E., 1974, Lead isotopic provinces in the Cordillera of the western United States and their geologic signiﬁ cance: Economic Geology, v. 69, p. 792 ̶ 805. Zartman, R.E., 1992, Archean crustal lead in the Hel- ena embayment of the Belt Basin, Montana, in Bartholomew, M.J., Hyndman, D.W., Mogk, D.W., and Mason, R., eds., International conference on basement tectonics, Proceedings of the eighth international conference on basement tectonics, Butte, Montana, USA: August, 1988, p. 699 ̶ 710. Zartman, R.E., and Doe, B.R., 1981, Plumbotecton- ics—The model: Tectonophysics, v. 75, p. 135 ̶ 162. Zartman, R.E., and Haines, S.M., 1988, The plumbotectonics model for Pb isotopic systematics among major terrestrial reservoirs—A case for bi-directional transport: Geochimica et Cosmochi- mica, v. 52, p. 1327 ̶ 1339. Zartman, R.E., and Stacey, J.S., 1971, Lead isotopes and mineralization ages in Belt Supergroup rocks, northwestern Montana and northern Idaho: Economic Geology, v. 66, p. 849 ̶ 860. Zimmerman, J.L., 2016, Re-examination of ore-forming processes in the Emery mining district, Powell County, Montana: Butte, Montana Tech, M.S. thesis, 127 p.

32 Montana Bureau of Mines and Geology Bulletin 139

APPENDIX A

Summary of lead isotope data for intrusions, volcanic rocks and base metal vein galena assumed near zero U/Pb for galena samples from this investigation.

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33 Stanley L. Korzeb

Factors used for correcting lead isotope data based on multiple analysis of NIST standard SEM-981. Ac- cepted NIST standard SEM-981 values for 206Pb/204Pb = 16.9405, 207Pb/204Pb = 15.4963, and 208Pb/204Pb = 36.7219.

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34 Montana Bureau of Mines and Geology Bulletin 139

APPENDIX B Lead isotope data corrected to initial lead for the Boulder batholith, satellite plutons, Lowland Creek and Elkhorn Mountains volcanic rocks from Doe and others (1968).

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35 Stanley L. Korzeb

APPENDIX C

Lead isotope data from Zartman (1992) for sulﬁ de veins hosted by the LaHood and Newland Formations, Belt Supergroup, and whole-rock lead isotope analysis from the LaHood, Newland, and Greyson Formations corrected for 1,400 million years of in situ radioactive decay.

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Uncorrected present-day lead isotope data from Zartman (1992) for LaHood, Newland, and Greyson For- mations.

36 Montana Bureau of Mines and Geology Bulletin 139

APPENDIX D

Lead isotopes for Lowland Creek Volcanics from Dudás and others (2010).

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37 Stanley L. Korzeb

APPENDIX E

Lead isotope data corrected to initial Pb isotope ratios using an age of 66 Ma for associated altered and mineralized rocks and intrusions for the Big Belt Mountains from du Bray and others (2017).

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38 Montana Bureau of Mines and Geology Bulletin 139

Uncorrected present day lead isotope data for altered, mineralized rocks, and intrusions for the Big Belt Mountains from duBray and others (2017).

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39 Stanley L. Korzeb

APPENDIX F

Lead isotope data for streambed sediments, contaminated fl uvial mill tails, and uncontaminated overbank fl uvial mill tailings from the Basin mining district, Jeff erson County, Montana. Data summarized from Unruh and others (2000).

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