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Variation in clinopyroxene texture, composition, and crystallization depth of Late Cretaceous to Early Eocene lamprophyric rocks from alkaline calc-alkaline magmatic complexes of Montana, USA




McCane, Jacob, author
Ridley, John, advisor
Bareither, Christoper, committee member
Scarberry, Kaleb, committee member
Sutton, Sally, committee member

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Lamprophyres are spatially and temporally associated with many types of hydrothermal ore deposits and have been argued to be a marker in mineral exploration. A reconnaissance study of lamprophyric rocks in central Montana, namely the Central Montana Alkalic Province (CMAP) is presented that details the variations of petrographic textures and clinopyroxene (Cpx) mineral chemistry from different Late Cretaceous to Early Tertiary aged alkalic to calc-alkaline magmatic complexes. Using current International Union of Geological Sciences (IUGS) standards, lamprophyric rocks studied from the Late Cretaceous Bull Mountain/Golden Sunlight mining operations are found to be predominately minette > vogesite > kersantite varieties of lamprophyres. The Highwood Mountains lamprophyric rocks of Eocene age are classified as vogesites > sannaites > minnettes ≈ spessartites. Crazy Mountain (Crazies) lamprophyric rocks of Eocene age are monchiquites > vogesites > kersantites > spessartites. The sapphire-bearing Yogo dyke lamprophyre of Eocene age is classified as an ouachitite. Lamprophyric rocks occur as dykes and display porphyritic and panidiomorphic textures with dominant macrocrysts of Cpx, biotite, with lesser olivine and hornblende that range in size from a few centimeters in length to microcrystalline (<10 µm) matrix constituents. Leucocratic circular ocelli structures as well as bent and strained crystals of biotite are present within all lamprophyric rocks from the CMAP. Extreme heterogeneity exists within lamprophyres ranging from macroscale dyke swarm features to microscale Cpx macrocrysts assemblages captured in a single thin section. Cpx from every region of the CMAP display intricate optical zonations associated with complex compositional variations across core to rim analytical traverses and display disequilibrium features like spongy textured melt pockets. Within the CMAP the Cpx classify as diopside, Fe-rich diopside, Ca-rich diopside or augite and do not contain uniform textures or zonations. Cpx of the Crazies are strictly diopsides and the only region with augite is Bull Mountain/Golden Sunlight. Cpx macrocrysts from each region of this study are separated into Types based off of differing textural features from back scatter electron imaging and under the petrographic microscope. All Types across the CMAP are predominately normally zoned and are interpreted as antecrysts or xenocrysts that host compositions and textures that record disequilibrium with the host magma during crystallization. B Types of Bull Mountain/Golden Sunlight are enriched in Cr2O3 and FeO. H Types from the Highwoods are similar to B Types but record Step-zoning characteristics and contain the greatest proportion of both normal and reverse zoning characteristics. C Types of the Crazies record lower values of TiO2, Al2O3, CaO, and Fe2+ /Fe3+ ratios but have higher Na2O levels than all other Types of the CMAP. Y Types of the Yogo Dyke host the most ultramafic signatures with elevated Mg#, TiO2, Al2O3, Cr2O3, and CaO levels along with the lowest SiO2 and FeO composition of all Types of the CMAP. Barometric estimates of the pressure of crystallization for Cpx are presented from the CMAP, mean crystallization estimates are as follows: Bull Mountain/Golden Sunlight = 15.1 ± 1.5 kbar (56 km), Highwood Mountains = 11.4 ± 1.5 kbar (42 km), Crazy Mountains = 7.1 ± 1.5 kbar (26 km), Yogo Dyke = 14.3 ± 1.5 kbar (53 km). Cpx macrocrysts crystalized near the lower crust-mantle boundary and barometry from Bull Mountain/Golden Sunlight likely records a deeper magma reservoir and plumbing system than previously thought for the Boulder Batholith. Lamprophyric rock petrogenesis cannot be explained by simple fractionation processes because the antecrysts do not follow regular fractionation trendlines. Multiple complex open system magmatic processes are likely at play when controlling lamprophyric magma composition. Drastically different compositional variations of C Types compared to others of the CMAP confirm that the parent magma of lamprophyric rocks originating from the Crazies is significantly different than other areas of this study. Primary melts from the Crazies and the Highwoods likely had an enriched composition and heavy metasomatic influence. Evidence for multiple recharge and mixing events exists within all lamprophyric rocks of the CMAP. Hybridization of mafic and felsic magmas likely influenced the petrogenesis of lamprophyric rocks from Bull Mountain/Golden Sunlight, the Highwood Mountains, the Crazy Mountains, and the Yogo Dyke.


2022 Spring.
Includes bibliographical references.
Zip file contains data spreadsheet.

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clinopyroxene mineral chemistry
Highwood Mountains
Yogo Dyke
Crazy Mountains
Bull Mountain/Golden Sunlight


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