The Boulder Batholith is a relatively small batholith in southwestern

Montana Montana () is a state in the Mountain West division of the Western United States. It is bordered by Idaho to the west, North Dakota and South Dakota to the east, Wyoming to the south, and the Canadian provinces of Alberta, British Columb ...

, United States, exposed at the surface as

granite Granite () is a coarse-grained ( phaneritic) intrusive igneous rock composed mostly of quartz, alkali feldspar, and plagioclase. It forms from magma with a high content of silica and alkali metal oxides that slowly cools and solidifies under ...

(more specifically

quartz monzonite Quartz monzonite is an intrusive, felsic, igneous rock that has an approximately equal proportion of orthoclase and plagioclase feldspars. It is typically a light colored phaneritic (coarse-grained) to porphyritic granitic rock. The plagiocl ...

) and serving as the host rock for rich mineralized deposits at

Butte __NOTOC__ In geomorphology, a butte () is an isolated hill with steep, often vertical sides and a small, relatively flat top; buttes are smaller landforms than mesas, plateaus, and tablelands. The word ''butte'' comes from a French word me ...

and other locations. The batholith lies roughly between Butte and Helena, and between the Deer Lodge (Upper Clark Fork) Valley and the Broadwater (Upper Missouri) Valley. The volcanic

Elkhorn Mountains The Elkhorn Mountains are a mountain range in southwestern Montana, part of the Rocky Mountains and are roughly 300,000 acres (1200 km²) in size. It is an inactive volcanic mountain range with the highest point being Crow Peak at , right n ...

are a large mass of forested lava associated with the batholith. The batholith is composed of at least seven, and possibly as many as 14, discrete rock masses called

plutons In geology, an igneous intrusion (or intrusive body or simply intrusion) is a body of intrusive igneous rock that forms by crystallization of magma slowly cooling below the surface of the Earth. Intrusions have a wide variety of forms and com ...

, which had formed beneath the Earth's surface during a period of

magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...

intrusion about 73 to 78 million years ago (

Late Cretaceous The Late Cretaceous (100.5–66 Ma) is the younger of two epochs into which the Cretaceous Period is divided in the geologic time scale. Rock strata from this epoch form the Upper Cretaceous Series. The Cretaceous is named after ''creta'', ...

time). The rising buoyant plutons resulted from subduction along what was then the west coast of North America, which was near today's border between

and

Idaho Idaho ( ) is a state in the Pacific Northwest region of the Western United States. To the north, it shares a small portion of the Canada–United States border with the province of British Columbia. It borders the states of Montana and Wyomi ...

. Regional uplift brought the deep-seated granite to the surface, where erosion exposed the rocks and the extremely rich mineral veins they contained. Hundreds of millions of dollars of copper, silver, gold, zinc, lead, and other metals have been mined from the batholith, both using underground mining and pit mining. The Butte Granite makes up the bulk of the batholith (about 70%), with additional named intrusions along the periphery consisting of

granodiorite Granodiorite () is a coarse-grained ( phaneritic) intrusive igneous rock similar to granite, but containing more plagioclase feldspar than orthoclase feldspar. The term banatite is sometimes used informally for various rocks ranging from gr ...

monzogranite Monzogranites are biotite granite rocks that are considered to be the final fractionation product of magma. Monzogranites are characteristically felsic (SiO2 > 73%, and FeO + MgO + TiO2 < 2.4), weakly syenogranite. The batholith intruded into

Mesoproterozoic The Mesoproterozoic Era is a geologic era that occurred from . The Mesoproterozoic was the first era of Earth's history for which a fairly definitive geological record survives. Continents existed during the preceding era (the Paleoproterozoic), ...

and Mesozoic

sedimentary rock Sedimentary rocks are types of rock that are formed by the accumulation or deposition of mineral or organic particles at Earth's surface, followed by cementation. Sedimentation is the collective name for processes that cause these particles ...

s, including the Elkhorn Mountains

volcanics Volcanic rock (often shortened to volcanics in scientific contexts) is a rock formed from lava erupted from a volcano. In other words, it differs from other igneous rock by being of volcanic origin. Like all rock types, the concept of volcanic ...

. The origin of these igneous rocks is related to subduction and continental magmatic arc processes associated with the Late Cretaceous western margin of North America. Later

hydrothermal vein In geology, a vein is a distinct sheetlike body of crystallized minerals within a rock. Veins form when mineral constituents carried by an aqueous solution within the rock mass are deposited through precipitation. The hydraulic flow involved ...

activity along fault and shear zones within the Butte quartz monzonite and its contact with adjacent rocks, resulted in major ore deposits of lead, zinc, and silver. The Boulder Batholith was named for the prominent rounded boulders that typify its landscape, the result of

spheroidal weathering Spheroidal weathering is a form of chemical weathering that affects jointed bedrock and results in the formation of concentric or spherical layers of highly decayed rock within weathered bedrock that is known as ''saprolite.'' When saprolite ...

of fractured granite. It measures approximately 75 miles (121 km) north-south by about 25 miles (40 km) east-west, rather small in comparison to most batholiths.

References

Batholiths of North America Cretaceous magmatism Igneous petrology of Montana {{US-geology-stub