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The Jefferson City quadrangle, in Jefferson and Lewis and Clark Counties, Mont., is near the northeastern margin of a mountainous area of moderate relief called the Boulder Mountains. Altitudes range from 4,880 to 8,150 feet. The quadrangle was mapped as part of a general study of the geology and mineral deposits in and around the Boulder batholith. 'l'he oldest r~cks in the quadrangle are the middle and upper units of the Elkhorn Mountains volcanics of Late Cretaceous age. The middle unit is characterized by quartz latitic welded tuffs and the upper unit by well-bedded sedimentary tuffs. Both units are cut by dikes of rhyodacite porphyry. During Late Cretaceous or early Paleocene time, the Elkhorn Mountains volcanics were intruded by the Boulder batholith ; they are now exposed in a large roof remnant near the center of the quadrangle and in many smaller remnants. Most of the quadrangle is underlain by batholithic rocks that constitute the Butte quartz monzonite. These rocks, which nre mainly quartz monzonite but include some granodiorite, fall within a narrow range in chemical and mineralogical composition and are believed to belong to a single large pluton. Slight but mappnble differences in grain size, texture, and mineralogical composition are attributed to intermittent movement and mixing between fractions of tpe magma that bad reached different degrees of crystallinity near the contacts with overlying rocks. Batholithic rocks that are slightly younger than the Butte quartz monzonite include alaskite, alaskite porphyry, aplite, pegmatite, granite porphyry, and a few fine-grained, porphyritic dil\:e rocks of diverse composition. Alaskite is the most abundant; it forms dikes and a few large, irregular bodies in Butte quartz monzonite and in the prebatholithic rocks near the margins of the batholith. The youngest consolidated rocks in the quadrangle are quartz latite of Oligocene age and rhyolite of Oligocene ( ?) or Miocene ( '!) age. 'l'hese rocks were emplaced after a long period of erosion during which most of the Elkhorn Mountains volcnnics were removed and a mature erosion surface was developed on the underlying rocks of the batholith. The quartz latite occurs as tuff, di1{es, breccia bodies, and probably flows. Small bodies of rhyolite are mostly flow remnants; a large body of rhyolite on Red Mountain is probably mostly intrusive. During the Pleistocene epoch, most of the stream valleys in the western rmrt of the quadrangle were occupied by glaciers. Glnc'inl erosion modified the topography only slightly, but glacial deposits nre widespread. Ground moraine is extensive, and the beds of all major eastward-flowing streams contain thick nccumulations of outwash gravel. After the ice retreated, three landslides occurred near Rimini on valley walls steepened by glacial erosion. Other Recent deposits include talus below outcrops of volcanic rocks and alluvial deposits along most streams. The major structural features in the quadrangle are a broad southeast-plunging faulted syncline in the prebatholithic Elkhorn Mountains volcanics; many joint sets in the batholithic rocks; large, steeply dipping alaskite dikes that trend dominantly northeastward; shear zones that trend generally eastward and contain quartz veins ;-chalcedony veins an:d vein zones that trend northeastward; quartz latite dikes that trend north to northeast; nonmineralized faults of different trends; and probable faults indicated by topographic lineaments. The persistent northeastward trends of many structural features are probably the r{!sult of recurring regional forces active since· emplacement of the batholith and possibly even before. Ore deposits in the quadrangle have been valuable chiefly for silver and lead, and, to a lesser extent, for gold, zinc, and copper. Mining was most active before 1900. Six mining districts have produced ore valued at more than $80 million ; they are served by a lead smelter at East Helena and are crossed by good roads, a railroad, and power lines: The Alta and Comet mines have been the most productive mines in the Boulder batholith outside of Butte. A few small uranium deposits have been discovered since 1949, and B: little uranium ore bas been shipped. The mineral deposits are almost entirely in veins along sheared or brecciated zones. Two distinctly different types of veins are distinguishable, and, in general, they occur in different parts of the quadrangle. Coarsely crystalline quartz is prevalent in one type; chalcedony and microcrystalline quartz are prevalent in the other. Most of the base and precious metals occur as sulfide ore bodies in quartz veins, either intermixed with the quartz or in the adjacent intensely altered rock. Many of the ore bodies are oxidized near the surface. In a few places tourmaline is a major constituent of the quartz veins, and most veins contain late primary carbonate minerals. The quartz veins were formed largely by replacement of rock adjacent to fractures in ·shear zones, and the structure of a vein therefore reflects the original structure of the fracture system, ranging from a single joint to a wide, complex shear zone. Distinct mineral assemblages can be recognized in the quartz veins. The chalcedony veins, which commonly have prominent reeflike outcrops, are almost barren of sulfide minerals, but they contain most of the small uranium deposits that have been found. In a few veins, coarsely crystalline quartz and chalcedony are about equally abundant. Some of these quartz~ chalcedony veins contain small deposits of base and precious

Geology and mineral deposits of the Jefferson City quadrangle, Jefferson and Lewis and Clark Counties, Montana