https://bereket-haileab.geology.sites.carleton.edu/files/original/40cf4f4d20bb0365794623892f6c2917.jpg d70c664e97f59e7510d6d20ebcad8a0c Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource Sedimentary Rocks Subject The topic of the resource This collection includes sandstones, limestones, breccias, conglomerates, dolostones, shales, oolites, and BIF. Creator An entity primarily responsible for making the resource Bereket Haileab Source A related resource from which the described resource is derived Housed in Anderson Hall, Carleton College. Format The file format, physical medium, or dimensions of the resource Thin sections, thin section scans, hand samples, hand sample images, photomicrographs. Hand sample Dublin Core The Dublin Core metadata element set is common to all Omeka records, including items, files, and collections. For more information see, http://dublincore.org/documents/dces/. Title A name given to the resource BH250-215 Subject The topic of the resource Hydrothermally Altered Feldspar-Rich Cataclasite / Cataclastic Breccia Coverage The spatial or temporal topic of the resource, the spatial applicability of the resource, or the jurisdiction under which the resource is relevant Location: Cooke City, Montana, USA GPS Coordinates: 45° 0'25.00"N 109°59'30.26"W Source A related resource from which the described resource is derived From the rock collection of Bereket Haileab. Sample 215. Housed at Carleton College in Minnesota. Type The nature or genre of the resource Hand sample Relation A related resource <iframe width="100%" height="600" style="border: 0;" src="https://carleton.maps.arcgis.com/apps/instant/media/index.html?appid=854b2ec06027408aac540447f8c771ad" frameborder="0" allowfullscreen="allowfullscreen">iFrames are not supported on this page.</iframe><br />View on ArcGIS Online <a href="https://carleton.maps.arcgis.com/home/item.html?id=854b2ec06027408aac540447f8c771ad">here</a><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1BAVzy406eyAw4rD4AjRooLlBgRWho5dV/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/139s3RiHL3MWkoFp5INRWRlwk2DfVowHc/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1akQ4rSxLU-qA7PQi46h_0te_cWpsynrk/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/11lrTEhFPtz7NaUJeQhgNWJZZbAkjp_7r/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1mZQe8GhScE3GsL263unvqEfUYMcFj3tx/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/16yd1QM27xA0_94HH57Iu97D1w2r0FYxA/preview" allow="autoplay"></iframe><br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1kaV0Oo8FRsptk7VS0a67REzQdicfiRr7/preview" allow="autoplay"></iframe> <br /><br /><iframe width="100%" height="480" src="https://drive.google.com/file/d/1Wa8zgtCen3Q6m6mpWLS0lJArCbhKtgRf/preview" allow="autoplay"></iframe> Description An account of the resource <p>BH250-215 — Hydrothermally Altered Feldspar-Rich Cataclasite / Cataclastic Breccia (Cooke City, Montana)</p> <p data-start="112" data-end="1094">BH250-215 is best interpreted as a<span>&nbsp;</span>hydrothermally altered feldspar-rich cataclasite or cataclastic breccia, collected as float near Cooke City, Montana from a geologically complex region shaped by repeated tectonic deformation, magmatism, and hydrothermal activity. In hand sample, the rock is visually striking because of its large, light-colored clasts set within a dark, fine-grained matrix. At first glance, the rounded to subrounded clasts resemble a conglomerate or volcaniclastic rock, but closer examination suggests that these lighter domains are more likely fractured feldspar-rich lithic fragments rather than transported sedimentary pebbles. Several clasts preserve irregular boundaries, internal fractures, and locally jigsaw-like relationships that indicate brittle fragmentation in place rather than sedimentary transport. The dark matrix surrounding these fragments likely represents finely crushed material generated during fault-related grain-size reduction.</p> <p data-start="1096" data-end="2040">Thin section observations further support a cataclastic origin. Many of the larger clasts appear to be plagioclase-rich and show evidence of internal fracturing, grain-size reduction, and extensive alteration. Several fragments appear to be surrounded by dark reaction or alteration rims, suggesting that hydrothermal fluids migrated through fractures after brecciation and chemically altered clast margins. These rims may contain secondary chlorite, epidote, iron oxides, sericite, or other alteration minerals commonly produced during fluid-rock interaction in brittle fault zones. In some areas, the relationship between clasts and matrix resembles classic fault breccia textures in which fragments were fractured, rotated, and later cemented or altered by mineralizing fluids. Together, these petrographic features strongly suggest that both brittle deformation and hydrothermal alteration played major roles in the evolution of the rock.</p> <p data-start="2042" data-end="2692">Bulk geochemical data are also consistent with a feldspar-rich protolith. The rock contains relatively low silica (SiO₂ = 46.9 wt.%) but elevated Al₂O₃ (23.14 wt.%) and CaO (11.06 wt.%), strongly indicating abundant plagioclase feldspar in the original rock. Moderate Fe₂O₃, MgO, Na₂O, and K₂O values suggest the presence of mafic minerals and secondary alteration products. This chemistry is inconsistent with quartz-rich sedimentary rocks such as quartzite and does not match typical granitic compositions. Instead, it is more compatible with an intermediate intrusive, volcanic, or volcaniclastic precursor that was later brecciated and altered.</p> <p data-start="2694" data-end="3696">Regional geology provides an important additional clue. The sample was collected near the Cooke City–Silver Gate–New World Mining District structural corridor, an area characterized by Precambrian crystalline basement, Paleozoic sedimentary rocks, Eocene intrusions, Absaroka volcanic rocks, hydrothermal breccia systems, and numerous fault zones. USGS studies have documented intrusive breccias, hydrothermal breccias, mineralized breccia pipes, and long-lived structural zones that were repeatedly reactivated during magmatism and mineralization in this region. The Cooke City structural zone likely served as a conduit for both magma emplacement and hydrothermal fluids, making it a plausible source for this rock. BH250-215 may therefore represent a brecciated intermediate intrusive rock that was subsequently crushed along a fault zone and altered by hydrothermal fluids, or it may represent a tectonically overprinted intrusive breccia associated with mineralization in the New World district.</p> <p data-start="3698" data-end="4155">Although additional petrographic work would help refine the classification further, the current evidence strongly supports identifying BH250-215 as a<span>&nbsp;</span>hydrothermally altered cataclasite or tectonic breccia derived from a feldspar-rich igneous protolith.<span>&nbsp;</span>The sample is an excellent example of how structural deformation, magmatism, and hydrothermal alteration can combine to produce unusually complex rocks that do not fit neatly into a single category.</p> <br /><iframe width="1800" height="100" src="https://docs.google.com/spreadsheets/d/e/2PACX-1vS4nyqCmZQGn3t6vX4g4nhDPwHMBaKrigt1LaY2pykWbBGXB1wNb6FYArx625I-JTtD7lyg4uz1OS1r/pubhtml?gid=1362744383&amp;single=true&amp;widget=true&amp;headers=false"></iframe> Creator An entity primarily responsible for making the resource Bereket Haileab Absaroka volcanic field altered igneous rock Brecciation brittle deformation Cataclasite cataclastic breccia Cooke City Montana fault breccia fault zone rocks Hydrothermal alteration hydrothermal fluids Montana geology New World Mining District Reaction rims tectonic breccia