Boulder Batholith Research Articles

Mineralogy and sulfur isotope geochemistry of polymetallic, porphyry-epithermal mineralization peripheral to the Golden Sunlight gold mine, Montana

The Golden Sunlight mine, located 50 km east of the famous Butte porphyry/lode deposit, is the largest gold mine in Montana, and has produced over 3.6 M oz of gold in its 36-year history. Most of this gold has come from the Mineral Hill breccia pipe (MHBP), a west plunging, cylindrical body of brecciated latite porphyry and siliciclastic sedimentary rocks of the Mesoproterozoic Belt Supergroup. This paper focuses on four areas of mineralization outside the MHBP termed the Apex, Bonanza, North Pit, and 102/Sunlight zones. The Apex and 102/Sunlight areas consist of auriferous, pyrite-rich veins cutting Precambrian sediments and the Cambrian Flathead Quartzite, whereas the Bonanza and North Pit areas are polymetallic Mo-Cu-Ag-Au prospects hosted in latite porphyry and Belt sediments. Recent dating (40Ar/39Ar of igneous biotite) shows the porphyry at Bonanza was emplaced at 78 ± 1 Ma, which is several m.y. younger than the porphyry associated with the MHBP (81.9 ± 1.9 Ma, zircon U/Pb), and several m.y. older than the main phase of the Boulder Batholith that hosts the Butte deposit.Overall, the ore and gangue mineral assemblages in the peripheral deposits are similar to those described previously for the MHBP deposit. Pyrite, locally As-rich, is abundant in all settings, with varying amounts of galena, sphalerite, chalcopyrite, bornite, molybdenite (at Bonanza), tetrahedrite-tennantite, and traces of pearceite, tetradymite, aikinite, calaverite, petzite, goldfieldite, buckhornite, gold and electrum. Gangue minerals include quartz, carbonates (Fe-dolomite, siderite, magnesite), barite, anhydrite, adularia, and alumino-phosphate sulfate (APS) minerals. Spectral analysis indicates that some of the adularia is ammonium rich, i.e., buddingtonite.Values of δ34S for sulfide minerals from the peripheral deposits overlap with values obtained by previous workers from the MHBP. The bulk of the analyses (including all samples from the porphyry-hosted Bonanza deposit) have δ34S values between −12 and −4‰, with a tail to heavier values >+5‰. It is postulated that most of the lighter S was introduced into the district by late Cretaceous magmatic/hydrothermal fluids, whereas the heavier S is attributed to sedimentary pyrite in the Precambrian Greyson and LaHood formations. A weak negative correlation exists between gold grade and the δ34S of associated pyrite: all samples in this study that contain >0.1 oz/ton Au have δ34S < −5.9‰. Two possible explanations for the isotopically light S at Golden Sunlight are explored: 1) assimilation of biogenic pyrite from underlying sedimentary units; and 2) fractionation of S isotopes between SO4 and H2S in a system where oxidized S > reduced S.

Geochemical relationships in Cretaceous bentonites as inferred from linear discriminant analysis

Bentonites serve as an important stratigraphic tool for facies correlation due to their instantaneous deposition and preservation across multiple depositional environments. Correlating one bed to another can be a complex issue limited by the 3-dimensional availability of outcrops and core. This process is reliant on the assumption that the geochemical signatures of ash beds do not vary enough to statistically alter the correlation coefficients that tie one ash-fall to another. This assumption may be faulty, however, and the problem compounds as bentonite bed thickness increases because the chemical composition varies through an eruption due to stratification of the magma chamber and through physical transport. Linear discriminant analysis (LDA) can significantly aid with this issue, allowing for the utilization of a multivariate dataset to statistically separate chemically-similar groups. Here we present a statistical approach to assessing the primary discriminatory variables on a suite of 87 Cretaceous bentonites deposited in the Bighorn Basin, Wyoming, USA, that span five geologic stages and seven formations, from the Cloverly Formation (110 Ma) to the Meeteetse Formation (70 Ma), and includes nine named ash beds. Using LDA, bentonites were sorted into groups based on geologic stage, formation, and unique beds using key elemental concentrations and radiogenic isotopes (e.g., 87Sr/86Sr, TiO2, Sc, Na, and Th). Additionally, distinct similarities in the discrimination pattern between formations reveal two periods of magmatism, the development of the Idaho batholith and the migration and development of the Boulder batholith and surrounding related plutons. With well-preserved, temporally-constrained ash, LDA can correlate ancient tephra deposits while also shedding light on the cyclical nature of geochemical trends in a migrating subduction zone.

Succession of Laramide Magmatic and Magmatic-Hydrothermal Events in the Patagonia Mountains, Santa Cruz County, Arizona

This investigation of the space-time progression of magmatism and hydrothermal activity in the Patagonia Mountains of southern Arizona is based on field and paragenetic relationships, and on U-Pb and 40 Ar/ 39 Ar geochronology of igneous and hydrothermal minerals. The Patagonia Mountains consist of Precambrian, Paleozoic, and Mesozoic sedimentary, granitic, and volcanic rocks, Laramide volcanic rocks, and a core of Laramide intrusions that comprise the Patagonia Mountains batholith. Laramide igneous rocks and adjacent Paleozoic and Mesozoic rocks contain significant porphyry Cu-Mo deposits, Mo-Cu breccia pipes, Ag replacement deposits, and numerous other Cu-Pb-Zn-Ag replacement and vein deposits. Ages of igneous and hydrothermal minerals from 20 U-Pb and 52 40 Ar/ 39 Ar determinations define four magmatic and magmatic-hydrothermal events that formed the batholith and altered parts of it and adjacent rocks; cumulatively the events span at least 16 m.y., from ~74 to 58 Ma. The oldest event of this succession includes the 74 Ma Washington Camp stock and spatially associated Cu-Pb-Zn-Ag replacement deposits in Paleozoic carbonate rocks of the Washington Camp-Duquesne district. Eruption of 73 to 68 Ma volcanic rocks in the northern part of the range was the next youngest event, which coincides temporally with replacement and vein deposits in Paleozoic carbonate rocks at the Flux mine (~71 Ma). An event at 65 to 62 Ma is marked by emplacement of small-volume quartz monzonite, granodiorite, and diorite intrusions, formation of the Ventura breccia deposit in Jurassic granite at 65 to 64 Ma, and formation of other Pb-Zn-Ag-Cu replacement and vein deposits (~62 Ma; Blue Nose and Morning Glory). The Red Mountain porphyry Cu-Mo system is hosted by ~62 Ma granodiorite and Laramide volcanic rocks (73–68 Ma) at the northern end of the batholith. It includes a deep, chalcopyrite-bornite resource (~60 Ma) that is associated with potassic and sericitic alteration and a near-surface chalcocite-enargite resource (60 Ma) that is associated with advanced, supergene-enriched argillic alteration. The youngest event includes the Sunnyside porphyry Cu-Mo system and a Cu-Mo breccia deposit at Red Hill (Four Metals mine), both of which formed in large-volume quartz monzonite, granodiorite, quartz monzonite porphyry, and quartz feldspar porphyry (~61–59 Ma). Similar to the Red Mountain system, the Sunnyside system consists of a deep chalcopyrite resource that occurs in ~60 to 59 Ma quartz feldspar porphyry, and a near-surface, slightly younger (~59–58 Ma) enargite-chalcocite-tennantite resource that occurs in quartz feldspar porphyry, quartz monzonite porphyry, and Mesozoic rocks. The Red Hill Cu-Mo breccia deposit is hosted by large-volume quartz monzonite, granodiorite, and quartz monzonite porphyry (~61–59 Ma). Discrepancies between field and paragenetic relationships and some analytic ages at Sunnyside and Red Hill preclude precise dating of mineralization stages, and may reflect disturbance of isotope systems by multiple, co-spatial to juxtaposed intrusive and hydrothermal events, and/or by unrecognized intrusions. Numerous vein and replacement deposits at the northern end of the batholith, including the Hardshell Ag resource and the Three R supergene chalcocite resource, are distal deposits of the Sunnyside and Red Mountain systems. Small, ~61 to 59 Ma Cu-Mo deposits in large-volume intrusions in the southern part of the batholith consist of hydrothermal quartz, biotite, K-feldspar, muscovite, chalcopyrite, and molybdenite. The age span of magmatic and magmatic-hydrothermal events in the Patagonia Mountains, minimally 16 m.y., is comparable to that of certain other magmatic-hydrothermal successions that contain porphyry Cu-Mo systems. Magmatic-hydrothermal events of the Wasatch-Oquirrh igneous trend, Utah, and the Boulder batholith, Montana, both span ~17 m.y. and include the Bingham and Butte porphyry Cu-Mo, vein and replacement deposits, respectively. Plutons and mineral deposits in the Pima district, Arizona, which includes the porphyry Cu-Mo deposits at Sierrita-Esperanza, Mission-Pima-San Xavier North, and Twin Buttes, formed over an interval of ~14 m.y. The diversity of igneous and hydrothermal products likely reflects evolutionary processes occurring at multiple sites in the lithosphere and at different time scales from >10 m.y. to less than the geochronologic precision currently achievable.

Structural Geologic Evolution of the Butte District, Montana

The Butte district contains two porphyry Cu-Mo deposits and the younger zoned Main stage base metal lode deposits that formed in the period from 66 to 62 Ma, synchronous with Laramide compressional thrust faulting and folding in southwest Montana. New geologic mapping of the Butte district reveals three distinct episodes of deformation: 1. Gentle (20° ± 10°) NE tilting produced during Laramide compressional deformation, uplift, and exhumation during and after ore formation in the Late Cretaceous to Paleocene (~66–~53 Ma); 2. Moderate (10°–50°) NW tilting of Eocene age produced by NE-striking normal faults that are limited to the northwest part of the district and synchronous with the eruption of the Lowland Creek Volcanics; and 3. Miocene to present gentle (15°) ESE tilting in the hanging walls of the Continental, East Ridge, and Rocker normal faults. Stratigraphic relationships, isotopic ages, hornblende barometry, and fluid inclusion data indicate that exposures of the Butte Granite, which hosts mineralization, crystallized at ~6- to 9-km depth and that early porphyry Cu-Mo deposits formed at similar depths of 5 to 9 km. A period of rapid uplift and erosion removed about 7 km of rock from above the ore deposit between 66 to 64 Ma and 53 Ma, but was mostly completed by 59 Ma. Estimated exhumation rates range from 0.5 to 2 mm/yr, and uplift was likely most rapid between ~63.5 and 62 Ma during Main stage mineralization, which occurred at depths of 2 to 6 km. The Butte Granite and Boulder batholith were emplaced at 79 to 76 Ma into thrust fault ramps, which later became inactive near Butte as Laramide crustal shortening moved eastward in the Helena salient. The accelerated period of uplift and unroofing is attributed to renewed deformation along the Montana disturbed belt south of Butte that uplifted the south end of the Boulder batholith and produced ~20° of gentle NE tilting (episode I) in the district. The exhumation history of the deposit is therefore broadly contemporaneous with ore formation, and is consistent with the general transition from lithostatic to hydrostatic conditions that is evident from the telescoping of early porphyry Cu-Mo ores cut by the Main stage lode veins. Within a 5-m.y. period, the Laramide compressional regime near Butte evolved into an extensional environment characterized by normal faulting. The normal faults were synchronous with eruption of the 53 to 48 Ma Lowland Creek Volcanics, which directly overlie the Main stage veins in the northwest part of the district. Structural reconstruction of the district at the time of mineralization produces orientations of quartz porphyry dikes, porphyry Cu-Mo veinlets, Main stage veins, and alteration zonal patterns that are steeply dipping and symmetrical, similar to observations from other upright or restored porphyry copper deposits. Early porphyry Cu-Mo veins and dikes are oriented E-W and likely originated as tensile hydrofractures above a ductile magma carapace during Laramide shortening. Additionally, during the Laramide compressional phase, two conjugate fault systems occupied by Main stage veins are normal oblique-slip faults that record minor E-W and vertical shortening and N-S extension. These normal faults with minor offset likely resulted from brittle failure caused by doming above a volatile-rich and buoyant granitic magma chamber and cupola at depth. The structural evolution of the Butte district therefore reflects the interaction of a far-field compressive stress regime with local buoyant magmatic forces.

Extension of the Anaconda metamorphic core complex: 40Ar/39Ar thermochronology and implications for Eocene tectonics of the northern Rocky Mountains and the Boulder batholith

Thermochronologic data define the extension and exhumation history of the Anaconda metamorphic core complex and have implications for the Eocene tectonic setting of the northern Rocky Mountains. The 40 Ar/ 39 Ar data indicate that relatively rapid extension on the Anaconda detachment started at ca. 53 Ma and continued through ca. 39 Ma. Apatite fission-track data reveal that faulting and exhumation of the footwall continued until ca. 27 Ma. The average displacement rate on the Anaconda detachment was on the order of 1 mm/yr between ca. 50 and 39 Ma based on the lateral gradient in mica 40 Ar/ 39 Ar ages in the direction of fault slip. The total displacement along the Anaconda detachment in Eocene and Oligocene times is estimated to be ≥25–28 km based on reconstruction of the Cretaceous Storm Lake Stock with its detached roof, which is now exposed in the Deer Lodge Valley. Extension exhumed crust from ∼12 km depth and exposed middle-greenschist-facies mylonites in the easternmost part of the Anaconda complex footwall. On a regional scale, the Anaconda detachment dips east beneath the Cretaceous Boulder batholith, indicating that the batholith and the Butte mineralization were transported east in the hanging wall. The Anaconda metamorphic core complex formed at the transition between the Cordilleran hinterland and the foreland at the same time as extension occurred in the Bitterroot and Priest River metamorphic core complexes but exhumed a shallower part of the Eocene crustal section than the contemporaneous complexes to the west.

Rise and emplacement of magma during horizontal shortening of the brittle crust: Insights from experimental modeling

Magmatic activity tends to concentrate at plate margins. At divergent margins, extensional tectonics provide steep conduits for magma to reach the surface. At rapidly convergent margins, such as the Andes, one might imagine that horizontal compression prevents the rise of magma. Nevertheless, volcanoes are also common. In order to study the mechanisms by which magma rises in a compressional context, we resorted to laboratory experiments, in which a brittle crust was shortened, while magma was intruding. Our model materials were (1) cohesive fine‐grained silica powder, representing brittle crust, and (2) molten low‐viscosity vegetable oil, representing magma. In general, horizontal shortening and injection were coeval but independent processes. Thrust faults accommodated the shortening, while overpressured oil formed hydraulic fractures. In those experiments where there was no shortening, injection resulted in a saucer‐shaped intrusive body. In the other experiments, where there was shortening, oil formed a basal sill, before rising along thrust faults. Once in place, the sill lubricated the base of the model, so that arcuate thrusts formed at the leading edge of a plateau. Uplift of the plateau promoted further intrusion of oil at depth. In general, the pattern of deformation and intrusion depended on the kinematic ratio R between rates of shortening and injection. The lengths of the basal sill and plateau increased with decreasing R. On the basis of these results, we have reexamined two natural examples of magmatic complexes, which were emplaced in compressional tectonic settings, Tromen volcano in Argentina and the Boulder Batholith of Montana.

SHRIMP U-Pb and 40Ar/39Ar Age Constraints for Relating Plutonism and Mineralization in the Boulder Batholith Region, Montana

SHRIMP U-Pb and 40Ar/39Ar Age Constraints for Relating Plutonism and Mineralization in the Boulder Batholith Region, Montana

SHRIMP U-Pb and 40Ar/39Ar Age Constraints for Relating Plutonism and Mineralization in the Boulder Batholith Region, Montana

The composite Boulder batholith, Montana, hosts a variety of mineral deposit types, including important silver-rich polymetallic quartz vein districts in the northern part of the batholith and the giant Butte porphyry copper-molybdenum pre-Main Stage system and crosscutting copper-rich Main Stage vein system in the southern part of the batholith. Previous dating studies have identified ambiguous relationships among igneous and mineralizing events. Mineralizing hydrothermal fluids for these types of deposits and magma for quartz porphyry dikes at Butte have all been considered to be late-stage differentiates of the Boulder batholith. However, previous dating studies indicated that the Boulder batholith plutons cooled from about 78 to 72 Ma, whereas copper-rich Main Stage veins at Butte were dated at about 61 Ma. Recent efforts to date the porphyry copper-molybdenum pre-Main Stage deposits at Butte resulted in conflicting estimates of both 64 and 76 Ma for the mineralizing events. Silver-rich polymetallic quartz vein deposits elsewhere in the batholith have not been dated previously. To resolve this controversy, we used the U.S. Geological Survey, Stanford, SHRIMP RG ion microprobe to date single-age domains within zircons from plutonic rock samples and 40 Ar/ 39 Ar geochronology to date white mica, biotite, and K-feldspar from mineral deposits. U-Pb zircon ages are Rader Creek Granodiorite, 80.4 ± 1.2 Ma; Unionville Granodiorite, 78.2 ± 0.8 Ma; Pulpit Rock granite, 76.5 ± 0.8 Ma; Butte Granite, 74.5 ± 0.9 Ma; altered Steward-type quartz porphyry dike (I-15 roadcut), 66.5 ± 1.0 Ma; altered Steward-type quartz porphyry dike (Continental pit), 65.7 ± 0.9 Ma; and quartz monzodiorite of Boulder Baldy (Big Belt Mountains), 66.2 ± 0.9 Ma. Zircons from Rader Creek Granodiorite and quartz porphyry dike samples contain Archean inheritance. The 40 Ar/ 39 Ar ages are muscovite, silver-rich polymetallic quartz vein (Basin district), 74.4 ± 0.3 Ma; muscovite, silver-rich polymetallic quartz vein (Boulder district), 74.4 ± 1.2 Ma; muscovite, early dark micaceous vein (Continental pit), 63.6 ± 0.2 Ma; biotite, early dark micaceous vein (Continental pit), 63.6 ± 0.2 Ma; potassium feldspar, early dark micaceous vein (Continental pit), 63 to 59 Ma; and biotite, biotite breccia dike (Continental pit), 63.6 ± 0.2 Ma. Outlying silver-rich polymetallic quartz veins of the Basin and Boulder mining districts probably are directly related to the 74.5 Ma Butte Granite, whereas Steward-type east-west quartz porphyry dikes and Butte pre-Main Stage deposits are parts of a 66 to 64 Ma magmatic-mineralization system unrelated to emplacement of the Boulder batholith. The age of the crosscutting Main Stage veins may be about 61 Ma as originally reported but can only be bracketed as younger than the 64 Ma pre-Main Stage mineralization and older than the about 50 Ma Eocene Lowland Creek intrusions. The 66 Ma age for the quartz monzodiorite of Boulder Baldy and consideration of previous dating studies in the region indicate that small ca. 66 Ma plutonic systems may be common in the Boulder batholith region and especially to the east. The approximately 64 Ma porphyry copper systems at Butte and gold mineralization at Miller Mountain are indicative of regionally important mineralizing systems of this age in the Boulder batholith region. Resolution of the age and probable magmatic source of the Butte pre-Main Stage porphyry copper-molybdenum system and of the silver-rich polymetallic quartz vein systems in the northern part of the Boulder batholith documents that these deposits formed from two discrete periods of hydrothermal mineralization related to two discrete magmatic events.

Influence of Late Cretaceous magmatism on the Sevier orogenic wedge, western Montana

Late Cretaceous large-volume pluton emplacement and accompanying volcanism within the evolving western Montana thrust wedge may have played important roles in determining the geometric and kinematic development of the thrust wedge, thereby influencing patterns of sediment dispersal and subsidence in the adjacent foreland basin. Intrusion of the Upper Cretaceous Boulder batholith and coeval eruption of the Elkhorn Mountains Volcanics were focused mainly at the trailing margin of the Lombard-Eldorado allochthon. The resultant thick (16–17 km) igneous culmination evolved over a relatively short interval (80–70 Ma), thickening the orogenic wedge to the point of supercritical taper and facilitating continued motion along the Lombard-Eldorado thrust system, thrust imbrication at the wedge toe, and forelandward translation of the Helena salient. Delivery of eroded volcaniclastic detritus from the thickened thrust wedge and accelerated basin subsidence due to thrust loading resulted in accumulation of a thick (4–5 km) sequence of Campanian-Maastrichtian volcaniclastic strata in the foreland basin (Livingston Group). The structural and sedimentological effects of this structural-magmatic culmination are similar to those of basement-cored culminations elsewhere in the Sevier orogen and to the Neogene central Andean orogenic wedge.

Taphonomy of a Petrified Forest in the Two Medicine Formation (Campanian), Northwest Montana: Implications for Palinspastic Restoration of the Boulder Batholith and Elkhorn Mountains Volcanics

Taphonomy of a Petrified Forest in the Two Medicine Formation (Campanian), Northwest Montana: Implications for Palinspastic Restoration of the Boulder Batholith and Elkhorn Mountains Volcanics

Taphonomy of a Petrified Forest in the Two Medicine Formation (Campanian), Northwest Montana: Implications for Palinspastic Restoration of the Boulder Batholith and Elkhorn Mountains Volcanics

Abstract The taphonomy of a newly discovered petrified forest in the Two Medicine Formation, south of Choteau, Montana, provides new constraints on the pre-thrusting position of the Boulder batholith and Elkhorn Mountains Volcanics. The petrified forest consists of more than 200 charcoalified, subaligned trees encased in an ash flow tuff and an immediately overlying bentonite. The ash flow tuff has been dated at 80.002± 0.114Ma (39Ar/40Ar on plagioclase). The ash flow and bentonite are interpreted to record a single catastrophic eruptive event that toppled and entombed the forest. The mean azimuth of the prostrate trunks, which are interpreted as the direction of travel of the ash flow, is 037° (2σ = 35°). Based upon age, mineralogy, and the absence of other contemporaneous volcanic centers, it is concluded that the Late Cretaceous Elkhorn Mountains Volcanics were the most likely source of the tuff and bentonite that encase the trees. The mean azimuth of the tree trunks is consistent with palinspastic res...

Seismic reflection profiling in the Boulder batholith, Montana

Seismic reflection profiling combined with gravity data allows more exact determination of the geometry of the controversial Boulder batholith of Montana, reveals laminated structure of the lower crust beneath the batholith, and identifies the Moho at a depth of 38 km. The batholith has inward-dipping contacts, the dip being about 50° on the west side, on the basis of seismic data; and the depth to the batholith floor is constrained between 12 and 18 km, indicating a great volume for the batholith. The Boulder batholith was emplaced between 80 and 70 Ma during an eastward thrusting in the fold-and-thrust belt. A presumed basal decollement of the thrust system might coincide with the batholith floor and may correspond to the top of the lower-crustal layering at a depth of 18 km.

Fractal patterns of fractures in granites

Fractal measurements using the Cantor's dust method in a linear one-dimensional analysis mode were made on the fracture patterns revealed on two-dimensional, planar surfaces in four granites. This method allows one to conclude that: (1)|The fracture systems seen on two-dimensional surfaces in granites are consistent with the part of fractal theory that predicts a repetition of patterns on different scales of observation, self similarity. Fractal analysis gives essentially the same values of D on the scale of kilometres, metres and centimetres (five orders of magnitude) using mapped, surface fracture patterns in a Sierra Nevada granite batholith (Mt. Abbot quadrangle, Calif.). (2)|Fractures show the same fractal values at different depths in a given batholith. Mapped fractures (main stage ore veins) at three mining levels (over a 700 m depth interval) of the Boulder batholith, Butte, Mont. show the same fractal values although the fracture disposition appears to be different at different levels. (3)|Different sets of fracture planes in a granite batholith, Central France, and in experimental deformation can have different fractal values. In these examples shear and tension modes have the same fractal values while compressional fractures follow a different fractal mode of failure. The composite fracture patterns are also fractal but with a different, median, fractal value compared to the individual values for the fracture plane sets. These observations indicate that the fractal method can possibly be used to distinguish fractures of different origins in a complex system. It is concluded that granites fracture in a fractal manner which can be followed at many scales. It appears that fracture planes of different origins can be characterized using linear fractal analysis.

Clark Fork Lineament, West-Central Montana: A Model for Wrench Anticlines

The Clark Fork lineament is a 150-km-long left-lateral wrench fault zone developed between Missoula and Helena, Montana. It is part of the extensive Montana Laramide wrench system that includes classic wrench faults like Cat Creek, Lake Basin, and Nye-Bowler. The Clark Fork is remarkable in that it consists of a dazzling array of en echelon wrench anticlines. To better define it as a model for wrench anticlines, field mapping, gravity, and compilation were undertaken. Fourteen wrench anticlines occur between Missoula and Helena. Generally, they are symmetrical, south plunging, about 5 km wide, and 13-35 km long. They are arranged en echelon along the Clark Fork lineament forming a series of wrench anticline umbrellas over a basement fault. The anticlines are age-constrained by cross-cutting plutons of the Garnet stocks, the Elkhorn Mountain volcanics, and the Boulder batholith, all dating about 80 to 78 Ma. Following folding, a south-facing monocline formed along the entire length of the Clark Fork lineament, increasing from zero throw to about 5 km westward. The approximately 70 Ma Sapphire block, a 3,000 km{sup 3} thrust sheet from the southwest, was emplaced against the lineament and tectonically loaded the south side of the Clark Fork wrench zone, addingmore » to (or causing) down-to-the-south, tilt-to-the-west movement. Wrench anticlines adjacent to the Sapphire block margin were squeezed toward the northeast by the block and structurally modified.« less

Syncompressional Emplacement of the Boulder and Tobacco Root Batholiths (Montana‐USA) by Pull‐apart along Old Fault Zones

AbstractThe 15 major plutons comprising the 6000 km2 Late Cretaceous Boulder batholith were emplaced at mid‐ to upper‐crustal levels (&lt;20 km) during active thrusting in the Montana thrust belt. The Tobacco Root batholith, the largest satellite pluton of the Boulder batholith (&lt; 300 km2) was emplaced into uplifted Archean basement rocks immediately southeast of the frontal margin of the thrust belt.Emplacement of the Boulder batholith and its satellite plutons was controlled principally by three long‐lived and deeply penetrating regional fault sets. Two of these sets (a NE‐trending set and an E‐trending set) appear to have controlled the emplacement of the main mass of the batholith (Butte pluton). A third (NW‐trending) fault set controlled the emplacement of the Tobacco Root batholith and many of the smaller plutons of the Boulder batholith. The intersecting fault sets give the Butte pluton a distinctly rhomboid shape, elongate in a northeasterly direction. Geophysical data suggest that the pluton may have a flat floor at the level of the basal décollement of the thrust belt (∼17km).One mode of pluton emplacement consistent with a rhomboid pattern and with a compressional tectonic setting is the filling of the pull‐apart regions of shear zones. We propose that the main mass of the Boulder batholith was emplaced along a pull‐apart within a segment of a thin‐skinned thrust sheet during ENE translation in the thrust belt. Sinistral slip on the E‐trending faults at the north and south margins allowed the rhomboid‐shaped cavity to form within the thrust sheet above the basal décollement and to progressively fill with magma below a roof of cogenetic volcanic rocks.Late Cretaceous movement on the NW‐trending fault set was oblique with roughly equal components of sinistral slip and reverse slip. The main mass of the Tobacco Root batholith is elongate in a northwesterly direction between two of these faults and has a crudely rhomboid map pattern. We suggest that it, too, was emplaced by filling a sinistral pull‐apart between these faults.

The geochemistry and structural significance of a set of Middle Precambrian diabase dikes from the Highland Range, southwestern Montana

The Middle Precambrian diabase dikes of the Highland Range in southwestern Montana are moderate- to high-TiO2 continental tholeiites and are related along a differentiation trend involving strong iron enrichment. Postmagmatic metamorphism and K, Rb, and Sr metasomatism have altered the chemical composition of the igneous rocks of some of the samples. The metamorphic assemblage in the diabase dikes belongs to the low-pressure calcic plagioclase – actinolite hornfels facies, and we suggest that thermal effects associated with the intrusion of the Boulder Batholith are responsible for the metamorphic overprints in these rocks.Combined chemical data from the diabase dikes in the Highland Range, the Ruby Range, and the Tobacco Root Mountains produce smooth differentiation trends for most major oxides and trace elements, and we conclude that one magma was responsible for the dikes in the three ranges. Discrepancies in Rb–Sr age dates obtained for the dikes in the Tobacco Root Mountains can be explained if a Rb, Sr, and K metasomatic event like the one observed in the Highland Range had occurred in the Tobacco Root Mountains as well.Structurally, the diabase dikes in the Highland Range intruded into both east–west- and northwest-trending fractures at the same time. All dikes dip steeply to the north or northeast and are believed to have intruded into tensionally opened fractures related to the opening of the Belt Basin.

Influence of Provenance on Detrital and Diagenetic Mineralogy of Small Tertiary Fans in Southwestern Montana: ABSTRACT

In the North Boulder River basin in southwestern Montana, alluvial fans of the Renova (Oligocene-Miocene) and the Sixmile Creek (Miocene) Formations were deposited on the flanks of north-south-trending uplifts that also supplied the detritus. The Elkhorn Mountain volcanics (78 m.y.) overlying the Boulder batholith make up the western highlands, a small patch of Precambrian Belt Group rocks occur in the southwest and Paleozoic siliciclastic and carbonate rocks forming the eastern margin. The fan sediments thus allow adequate control for studying the influence of source rocks on detrital and diagenetic mineralogy. Modal analysis of 6228 grains in 31 thin sections shows a decrease of VRF away from the igneous sources (37% to 1% in a north-south transect; 37% to 7% in a west-east transect) along with an increase in quartz (8% to 24% and 3% to 13%) and plagioclase (2% to 16% and 2% to 10%); orthoclase abundance is low except in the southwest. Volcanic ash and glass shards are found in the younger sediments in the northern part of the basin. Their data show a positive correlation between the abundance of orthoclase and kaolinite (north-south transect); between SRF and carbonate cement (west-east transect) and between glass shards and smectite (bothmore » north-south and west-east transects). They infer that the diagenetic mineralogy of these sands was controlled essentially by the detrital particles, which were strongly controlled by source rocks in this area.« less

Tectonic origin of fractures for fissure vein emplacement in the Boulder Batholith and adjacent rocks, Montana

In the Boulder batholith and adjacent rocks, the dominant and most productive metalliferous fissure veins trend approximately east and dip steeply. Other steep veins, generally narrower, trend northwest, northeast, and rarely north. Fissure veins also occur locally in thrust faults in volcanic rocks along the west side of the batholith. Fractures along which these veins were emplaced were probably initiated by a regional compressional stress field during Laramide (Late Cretaceous-early Tertiary) time. The batholith was emplaced during the Late Cretaceous and is synorogenic with respect to the surrounding Laramide thrust and fold belt, which is characterized by east-west compression. Postorogenic rocks (Eocene and younger) do not host major fissure vein deposits.Fractures utilized by east-trending metalliferous veins are parallel to the axis of maximum compressional stress and thus appear to be extensional; this may be the reason these veins are widest and longest. Northeast- and northwest-trending veins were probably emplaced along conjugate shears; these veins are narrower than east-trending veins. Veins that trend north are less common and probably formed along release fractures perpendicular to the principal axis of compressional stress. These veins are narrower than many of those that trend east.Mechanical behavior of various rock units during deformation had an important effect on ore deposition. The longest, widest, and most productive fissure veins occur in competent, brittle rocks (Precambrian crystalline basement, batholith intrusions, and Elkhorn Mountains Volcanics). Ductile rocks (Mesozoic shales and Belt Supergroup argillites) do not commonly contain major fissure veins, except where these rocks have been contact metamorphosed and have become more brittle. Paleozoic strata, mainly carbonates, are of intermediate ductility and are hosts to fissure veins in only a few districts adjacent to the batholith.

Sedimentation in a tectonically partitioned, nonmarine foreland basin: The Lower Cretaceous Kootenai Formation, southwestern Montana

The Lower Cretaceous Kootenai Formation in southwestern Montana was deposited in the nonmarine, Cordilleran foreland basin in the United States during a period of intensified uplift in the westward adjacent, but increasingly impingent, Sevier fold-thrust belt. Concurrently, the foreland basin was partitioned by uplift of intra-foreland structural elements and incipient plutonism. Kootenai fluvial and fluviolacustrine depositional systems developed and evolved in response to changing basin architecture. The Kootenai thus provides a case study of nonmarine sedimentary responses to tectonic partitioning in a foreland basin. The following criteria for recognition of tectonic partitioning of a nonmarine foreland basin are demonstrated by the Kootenai: (1) complex, internalized drainage patterns which locally diverge from the over-all regional paleoslope direction and a typical longitudinal pattern; (2) intraformational and interformational unconformities, both proximal to the fold-thrust belt and within the foreland basin; (3) evidence for cannibalization and redistribution of foreland-basin sediments; (4) abrupt deflections of paleodrainage axes in response to initiation of intraforeland uplifts; and (5) dramatic base-level alterations and consequent changes of fluvial geomorphology as a result of tectonic partitioning and segmentation of the foreland basin. Kootenai sedimentation can be divided into six phases. 1. A tectonic pulse in the Sevier fold-thrust belt produced a thin but widespread sheet of gravel alluvium, referred to as the “Basal Conglomerate Member,” which was deposited by a shallow, perhaps ephemeral, gravel-bed, braided-stream system. Drainage was predominantly eastward, across the axis of the foreland basin. 2. Subsequently, initial activation of intra-foreland structures over the present Tobacco Root, Madison, Gravelly, and Beartooth Archean blocks and the Boulder batholith caused reworking of the Basal Conglomerate in all but the westernmost foreland basin. Although the topographic manifestations of these uplifts were subtle, the foreland basin was effectively partitioned into at least four distinct drainage basins. The resulting deposit, commonly called the “Pryor Member,” but here referred to as the “First Sandstone Member,” was deposited by sand- and gravel-dominant, braided-stream systems in the west and east, respectively. Paleodrainage directions in the western foreland basin were southward, but over-all drainage was northward. 3. A period of mud-dominant sedimentation followed deposition of the First Sandstone, probably resulting from a combination of decreased tectonism, influx of siliceous volcanic ash, and change in source lithology. 4. Renewed tectonic activity in the fold-thrust belt generated a second episode of coarse-grained alluvial sedimentation, producing the Kootenai Second Sandstone Member. Basin partitioning was accentuated, but drainage directions remained essentially unchanged. Initial activation of the Blacktail-Snowcrest uplift dammed the southern portion of the Second Sandstone fluvial system. Base level was elevated, and a major anastomosed-stream system developed upstream from the structural dam. Downstream from the axis of the uplift, a transport-efficient, probably entrenched, meandering system developed. 5. The remainder of Kootenai siliciclastic deposition was dominated by muddy fluvial and fluviolacustrine systems. The foreland-basin configuration remained essentially unchanged as drainage became ponded in response to further elevation of base level as basin partitioning continued. An extensive carbonate lacustrine interval punctuated this period of sedimentation. 6. Gradual southward transgression of the Cretaceous sea left a transitional fluviolacustrine to marine imprint on the sedimentary record of the uppermost Kootenai and overlying Blackleaf Formation. Rejuvenation of orogenesis in the fold-thrust belt, augmented by cratonic and major intraforeland volcanic and sedimentary lithic sources, renewed the supply of coarse-grained detritus to the foreland basin during deposition of the Blackleaf.

A diatreme-hosted gold deposit at Montana Tunnels, Montana

A bulk-minable, gold-silver deposit at Montana Tunnels, southwestern Montana, is located in the central part of a diatreme. This diatreme was emplaced along the faulted contact between andesitic volcaniclastic rocks, assigned to the Late Cretaceous Elkhorn Mountains Volcanics, and a sequence of quartz latitic ignimbrites, which are part of the Lowland Creek Volcanics of middle Eocene age. A north-northeast-striking swarm of quartz latite porphyry dikes was emplaced during the waning stages of Lowland Creek volcanism. The two largest dikes, dated at 45 to 50 m.y. (middle Eocene), were intruded into the diatreme prior to the cessation of brecciation and mineralization.The diatreme underlies a 1-km 2 area and is known from drilling to extend steeply downward for at least 310 m. The principal rock type in the diatreme is a matrix-rich breccia. It is characterized by a sand-size tuffaceous matrix of quartz latitic composition containing fragments of contiguous volcanic wall rocks and intrusive rocks derived from the Late Cretaceous Boulder batholith. The batholith is inferred to underlie the Montana Tunnels area. Foundered blocks of volcanic wall rocks lie at various attitudes against the walls of the diatreme and within it. Those composed of Elkhorn Mountains Volcanics underwent hydraulic brecciation after they sank into the diatreme. Blocks interpreted to be composed of pyroclastic base surge deposits from a subaerial tuff ring and a piece of carbonized wood subsided into the diatreme from the palcosurface. The diatreme is separated into two unequal parts by a west-northwest-striking oblique slip fault.Sulfide minerals in the diatreme occur as disseminations in the breccia matrix and, in sub-ordinate amounts, as widely spaced, multidirectional veinlets and as the matrix to brecciated blocks of Elkhorn Mountains Volcanics. All forms of mineralization consist of pyrite, sphalerite, galena, minor chalcopyrite, and rare electrum accompanied by a gangue of manganocalcite, siderite, and minor quartz. Clastic grains and fragments of the sulfides and gangue minerals are common and attest to multiple alternating episodes of brecciation and mineralization. In the ore zone the introduction of abundant manganocalcite and siderite was accompanied by pervasive sericitic alteration but only weak kaolinization and silicification. Sericitization grades outward beyond the ore zone to chlorite-montmorillonite-carbonate alteration, an assemblage that also characterizes the interiors of the late mineral dikes in the diatreme. Gold occurs in electrum, with a fineness of about 550, as inclusions of less than 200 mu m in pyrite and sphalerite. Inclusions of electrum and galena were precipitated together. Silver is present mainly in solid solution in galena.The Montana Tunnels gold-silver deposit is characterized geochemically by anomalously high concentrations of zinc, lead, and manganese and, in comparison to many volcanic-hosted precious metal deposits, is low in arsenic, antimony, and mercury. This metal association and the low fineness of the gold are features shared by the Wau gold deposit in Papua New Guinea. It is suggested that the mineralized diatreme observable at Montana Tunnels was emplaced several hundred meters beneath the palcosurface, upon which a gold-bearing maar volcano, like that partly preserved at Wau, was constructed.