Abstract Today's field trip examines late Eocene and Oligocene granitoid intrusions, cogenetic volcanic rocks (Keetley Volcanics), and associated hydrothermally altered and mineralized rocks in the central Wasatch Mountains. Because of late Cenozoic tilting related to Basin and Range extension, a continuum of mid-Tertiary paleodepths is exposed that ranges from about 11 km on the west side of the Little Cottonwood stock to the actual paleosurface on the east side of the range (Fig. 1; John, 1989a). Consequently, we will see a wide variety of textures and styles of emplacement in the intrusive rocks, and a correspondingly wide variety of hydrothermal alteration types and mineral deposits (Lawton et al., 1980; John, 1989a). Mid-Tertiary igneous rocks in the central Wasatch Mountains consist of three phaneritic stocks exposed in the western and central parts of the range (Little Cottonwood, Alta, and Clayton Peak stocks); six porphyry to fine-grained phaneritic stocks exposed in the middle and eastern parts of the range, primarily in the Park City mining district (Flagstaff, Glencoe, Mayflower, Ontario, Pine Creek, and Valeo stocks); a subvolcanic porphyry complex (Park Premier stock) exposed near the Park Premier mine which is now partly covered by water filling the Jordanelle Reservoir; and coeval volcanic rocks (Keetley Volcanics), subvolcanic intrusions, and a volcanic neck (Indian Hollow plug) are exposed on the east side of the range (Figs. 2 and 3). The intrusive rocks range from coarse-grained, coarsely porphyritic on the west to fine-grained, porphyroaphanitic on the east (John, 1989a). They form a high-K, calc-alkaline series (Vogel

Abstract 0 Road log begins at the Keystone Lodge in Keystone, Colorado. North of the hotel, Proterozoic granite and felsic gneiss are exposed on the upper elevations of the un-named mountain. The lower slopes of the mountain consist of Pleistocene till that overlies the Precambrian rocks. Drive west on U. S. Highway 6 to the intersection with Interstate 70 (exit 205). 0.6 North of the highway, the Williams Range thrust fault that placed Proterozoic rocks over Cretaceous shale and sandstone is exposed just north of the borrow pit (situated above the shale cliff). The trace of the fault south is covered by Quaternary rocks in the Snake River area, is then exposed along the lower slopes of Keystone Mountain ( Widmann et al., 2003 ). Shale and sandstone of Cretaceous age crop out adjacent the road to Dillon and form the northern slope of Swan Mountain (south of the Snake River Arm of the reservoir) and the peninsula to the west ( Kellogg et al., 2008 ). 1.4 Highway crosses the Snake River. 2.4 Snake River crossing by highway. Pierre Shale (Cretaceous) forms the dark gray outcrops just north of the highway. 4.0 Dillon Reservoir. The west portal of the 21-mile long Harold Roberts Tunnel is located on the end of the peninsula west of here. The tunnel transfers water beneath the continental divide into the South Platte River which flows through Denver. Eastward, the Williams Range thrust is concealed beneath Quaternary units at about the elevation of the power transmission line along the bottom slopes of Tenderfoot Mountain (elevation of 11,441 feet).

Abstract Historical production of metals in the western United States has left a legacy of acidic drainage and toxic metals in many mountain watersheds that are a potential threat to human and ecosystem health. Studies of the effects of historical mining on surface water chemistry and riparian habitat in the Animas River watershed have shown that cost-effective remediation of mine sites must be carefully planned. of the more than 5400 mine, mill, and prospect sites in the watershed, ∼80 sites account for more than 90% of the metal loads to the surface drainages. Much of the low pH water and some of the metal loads are the result of weathering of hydrothermally altered rock that has not been disturbed by historical mining. Some stream reaches in areas underlain by hydrothermally altered rock contained no aquatic life prior to mining. Scientific studies of the processes and metal-release pathways are necessary to develop effective remediation strategies, particularly in watersheds where there is little land available to build mine-waste repositories. Characterization of mine waste, development of runoff profiles, and evaluation of ground-water pathways all require rigorous study and are expensive upfront costs that land managers find difficult to justify. Tracer studies of water quality provide a detailed spatial analysis of processes affecting surface- and ground-water chemistry. Reactive transport models were used in conjunction with the best state-of-the-art engineering solutions to make informed and cost-effective remediation decisions. Remediation of 23% of the high-priority sites identified in the watershed has resulted in steady improvement in water quality. More than $12 million, most contributed by private entities, has been spent on remediation in the Animas River watershed. The recovery curve for aquatic life in the Animas River system will require further documentation and long-term monitoring to evaluate the effectiveness of remediation projects implemented.

Abstract Alkaline igneous rock-related gold deposits, primarily of Mesozoic to Neogene age, are among the largest epithermal gold deposits in the world. These deposits are a subset of low-sulfidation epithermal deposits and are spatially and genetically linked to small stocks or clusters of intrusions possessing high alkali-element contents. Critical-, near-critical, or energy-critical elements associated with these deposits are F, platinum-group elements (PGEs), rare earth elements (REEs), Te, V, and W. Fluorine and tungsten have been locally recovered in the past, and some other elements could be considered as future by-products depending on trends in demand and supply. The Jamestown district in Boulder County, Colorado, historically produced F from large lenticular fluoritebearing breccia bodies and Au-Te veins in and adjacent to the Jamestown monzonite stock. Several hundred thousand metric tons (t) of fluorspar were produced. Some alkalic epithermal gold deposits contain tungstenbearing minerals, such as scheelite, ferberite, or wolframite. Small tungsten orebodies adjacent to and/or overlapping the belt of Au telluride epithermal deposits in Boulder County were mined historically, but it is unclear in all cases how the tungsten mineralization is related genetically to the Au-Te stage. Micron-sized gold within deposits in the Ortiz Mountains in New Mexico contain scheelite but no record of tungsten production from these deposits exists. The most common critical element in alkaline igneous-rock related gold deposits is tellurium, which is enriched (>0.5%) in many deposits and could be considered a future commodity as global demand increases and if developments are made in the processing of Au-Te ores. It occurs as precious metal telluride minerals, although native Te and tetradymite (Bi2Te2S) have been reported in a few localities. Assuming that the Dashigou and adjacent Majiagou deposits in Sichuan province, China, are correctly classified as alkalic-related epithermal gold deposits (exact origin remains unclear), they represent the only primary producers of Te (as tetradymite) from this deposit type. It is worth noting that some epithermal veins (and spatially or genetically related porphyry deposits) contain high contents of Pt or Pd, or both. The Mount Milligan deposit typically contains >100 ppb Pd, and some values exceed 1,000 ppb. However, owing to the presence of other large known PGE resources in deposits in which PGEs are the primary commodities, it is unlikely that alkaline-related epithermal gold deposits will become a major source of PGEs. Similarly, many epithermal gold deposits related to alkaline rocks have high vanadium contents, but are unlikely to be considered vanadium resources in the future. Roscoelite (V-rich mica) is a characteristic mineral of alkalic-related epithermal deposits and is particularly abundant in deposits in Fiji where it occurs with other V-rich minerals, such as karelianite, Ti-free nolanite, vanadium rutile, schreyerite, and an unnamed vanadium silicate. A few alkaline intrusive complexes that contain anomalous concentrations of gold or were prospected for gold in the past are also host to REE occurrences.The best examples are the Bear Lodge Mountains in Wyoming and Cu-REE-F (±Ag, Au) vein deposits in the Gallinas Mountains in New Mexico, which have REE contents ranging up to 5.6% in addition to anomalous Au.

Abstract The Gold King mine water release that occurred on 5 August 2015 near the historical mining community of Silverton, Colorado, highlights the environmental legacy that abandoned mines have on the environment. During reclamation efforts, a breach of collapsed workings at the Gold King mine sent 3 million gallons of acidic and metal-rich mine water into the upper Animas River, a tributary to the Colorado River basin. The Gold King mine is located in the scenic, western San Juan Mountains, a region renowned for its volcano-tectonic and gold-silver-base metal mineralization history. Prior to mining, acidic drainage from hydrothermally altered areas was a major source of metals and acidity to streams, and it continues to be so. In addition to abandoned hard rock metal mines, uranium mine waste poses a long-term storage and immobilization challenge in this area. Uranium resources are mined in the Colorado Plateau, which borders the San Juan Mountains on the west. Uranium processing and repository sites along the Animas River near Durango, Colorado, are a prime example of how the legacy of mining must be managed for the health and well-being of future generations. The San Juan Mountains are part of a geoenvironmental nexus where geology, mining, agriculture, recreation, and community issues converge. This trip will explore the geology, mining, and mine cleanup history in which a community-driven, watershed-based stakeholder process is an integral part. Research tools and historical data useful for understanding complex watersheds impacted by natural sources of metals and acidity overprinted by mining will also be discussed.

Abstract Gold deposits associated with alkaline rocks include high-grade, gold-rich epithermal deposits, porphyry-type Cu(Au) and Mo(Au) deposits, and several other deposit types more speculatively linked to alkaline magmatism. These deposits can be large and high grade; several contain >1,000 tonnes Au. Alkaline rocks associated with gold deposits range from mafic-ultramafic lamprophyres to fractionated alkaline rhyolites, they have widely variable K/Na ratios, and they are found in a variety of tectonic settings, most notably in arc environments and in areas of extensional tectonics. Alkaline Cu (Au) deposits tend to form in volcanic arcs with thin or mafic crust, whereas Mo(Au) deposits are typically found in areas of thickened continental crust. Alkaline rocks are found to be as old as the Archean, but alkaline-related gold deposits are usually associated with shallow-level Phanerozoic alkaline magmatism, particularly within Cenozoic orogenic zones. In many cases deposits are found in clusters or in regions with recurring episodes of alkaline magmatism. Key characteristics of alkaline rocks associated with gold deposits are their hydrous and oxidized nature, as well as their ability to produce hydrothermal systems with ideal chemistries for transporting gold. Magmatic endowments may be variable but are not likely to exceed tens of parts per billion. In addition to their association with a distinctive group of igneous rocks, gold deposits related to alkaline magmatism are characterized by telluride-rich mineralization, extensive carbonation, and voluminous K metasomatism. Hydrothermal quartz is much less prominent in many alkaline systems than in most sub-alkaline systems and is absent in some high-temperature alkaline deposits. Likewise, hydrolytic (acid) alteration tends to be poorly developed in many alkaline systems. Where sericitic alteration is observed, it is commonly accompanied by carbonate minerals and a significant gain of K 2 O. These features and other geochemical data (such as isotopes and fluid inclusions) reflect formation from hydrothermal fluids of predominantly magmatic origin. Alkaline porphyry-style systems in which SiO 2 < 60 wt percent tend to develop deposits rich in Cu and platinum-group elements (PGE), whereas more felsic systems were enriched in Mo, and ultimately, F, Be, Hg, W, and Sn in the most evolved systems. Alkaline gold deposits also exhibit distinctive metal ratios and zonations. Both Cu(Au) and Mo(Au) porphyry-type deposits may grade upward or outward into telluride-rich epithermal deposits. Epithermal parts typically are base metal poor and have Au > Ag, whereas porphyry-style parts contain significant gold but have Ag > Au in a base metal-rich core. Both epithermal and porphyry-type deposits typically have low total sulfides. Exploration for these deposits is encouraged by their large sizes and high grades and because they are environmentally favorable to mine (low total sulfides and high acid-buffering potential). The most productive deposits show evidence for voluminous metasomatism and multiple magmatic and hydrothermal events, in addition to structurally focused zones of high-grade mineralization. Geophysical and geochemical signatures of these deposits are variable, but their characteristic styles of mineralization and alteration can be recognized in almost all examples, providing an effective exploration tool.

Abstract The Barrick Golden Sunlight Mine (GSM) in Whitehall, Montana, is an industry leader in safe, responsible resource extraction. With more than 3 million ounces of gold poured since 1983, and current proven and probable reserves of 318,000 ounces of gold, GSM is the largest gold producer in Montana. The gold-silver deposit is localized in a hydrothermal breccia pipe related to Late Cretaceous latite porphyry magmatism hosted by the Mesoproterozoic Belt Supergroup, and is influenced by younger cross-cutting faults and fracture systems. The deposit has been mined by both underground and open pit methods, and the current open pit operation was recently permitted for expansion. The mill and tailings operations practice efficient and environmentally responsible resource recovery by processing ore from historical tailings and dumps from around the state in addition to ore from the Golden Sunlight property. This trip will explore the complex geologic and tectonic controls on mineralization and review how GSM has addressed the technical challenges of mining, milling, and reclamation.