This chapter presents a synthesis of data pertaining to post-early Miocene slip on the San Andreas fault in central California and suggests a three-phase evolition of the San Andreas system. The cricial evidence that supports the three phases of evolution conies from the reversed positions of two exotic rock fragments in the vicinity of Parkfield, California. The three-phase evolution of the San Andreas is also supported by the correlation of other exotic fragments, the basement rocks on which they lie, overlying Tertiary stratigraphic sequences, and distinctive Miocene strata derived from these fragments during their transport along the fault. The 40-km-long section of the San Andreas fault near Parkfield is characterized by exotic blocks composed of Cretaceous hornblende quartz gabbro at Gold Hill and lower Miocene volcanic rocks in Lang Canyon. The gabbro is correlated petrographically with similar rocks near Eagle Rest Peak, 145 km to the southeast, and near Logan, 165 km to the northwest. The lower Miocene volcanic rocks, informally termed the volcanic rocks of Lang Canyon, are correlated with the Neenach Volcanics 220 km to the southeast and the Pinnacles Volcanics 95 km to the northwest. All three fragments of volcanic rocks are unconformably overlain by similar successions of Tertiary sedimentary rocks. The original positions of the bodies of gabbro and volcanic bodies and their overlying sedimentary cover may be reconstructed from these exotic fragments that now lie along the San Andreas fault between San Juan Bautista and the northwestern Mojave Desert. The original undeformed gabbroic body was composed of the hornblende quartz gabbro of Eagle Rest Peak, Gold Hill, and Logan. In its initial prefaulted position, the original gabbroic body lay about 55 km northwest of the early Miocene volcanic assemblage. The undeformed volcanic assemblage was composed of the Neenach Volcanics, Pinnacles Volcanics, and volcanic rocks of Lang Canyon. The original spatial relationship between the undeformed gabbro and volcanic assemblage and their sedimentary cover is preserved in the present position of the gabbro of Logan and the Pinnacles Volcanics. However, in the Parkfield segment of the San Andreas, the gabbro of Gold Hill lies east of the main trace of the San Andreas fault, and the volcanic rocks of Lang Canyon lie 2 km west of the fault. The reversed relative positions of the gabbro of Gold Hill and the volcanic rocks of Lang Canyon suggest a complex history of movement on the San Andreas fault. Consequently, plainspastic reconstruction of these bodies and their overlying sedimentary cover is constrained by the unusual distribution of exotic blocks near Parkfield. The resulting proposed history of movement is divided into three stages that begins with the eruption of the early Miocene volcanic rocks about 24 Ma. The Neenach-Pinnacles Volcanics, erupted after passage of the Mendocino triple junction, were soon cut by the growing San Andreas transform system. During the first phase of movement the Salinian block, which contains the Pinnacles and Logan godies, was detached from the Mojave and Sierran blocks. The Pinnacles and Logan bodies were transported about 95 km northwest from the Neenach Volcanics and the gabbro of Eagle Rest Peak. At the end of the first phase, the Logan and Pinnacles fragments lay adjacent to the west side of what is now the San Joaquin Valley. Concurrently, fan-deltas deposited debris that was derived from the Gabilan Range, the fan-deltas spread across the San Andreas fault into the middle Miocene sea in the San Joaquin trough. During the second phase of movement, the San Andreas—at least locally—stepped eastward and detached a second fragment from the Neenach Volcanics. This fragment consists of the volcanic rocks of Lang Canyon. Slip was transferred to the new trace of the San Andreas fault, and the older trace became completely or largely inactive. After transferral of slip to the new trace of the San Andreas fault, the volcanic rocks of Lang Canyon and the Pinnacles Volcanics remained about 95 km apart on the Salinian Block west of the San Andreas fault. During the third phase, the Gold Hill fragement was slivered off the Logan fragment and was tectonically emplaced on the east side of the San Andreas fault when the Logan fragment lay at the latitude of Gold Hill. The process of slivering off of the Gold Hill fragment was accomplished by deformation of the San Andreas in an eastward bend along what is now the Jack Ranch fault. Bending of the fault was stimulated by the presence of highly sheared Franciscan rocks that crop out near the San Andreas and extend to great depth. Eventually the San Andreas bent to such a degree that slip could not be conducted around the bend, and a new, stable, straight segment was formed. The straightening of the fault resulted in slivering of the Gold Hill fragment from the Logan fragment. After detachment of the Gold Hill fragment, the Salinian block containing the gabbro of Logan, the Pinnacles Volcanics, and the volcanic rocks of Lang Canyon was transported an additional 160 km northwest to its present position. This reconstruction honors the current positions of all the related exotic fragments of gabbro, volcanics, and sedimentary rocks. The timing of the sequence of movements required to reconstruct the original bodies suggests that the three phases of evolution of the San Andreas fault in central California are characterized by increasing slip rates. The rate for the first phase probably averaged about 10 mm/yr over a period of about 8 m.y. The rate for the second phase averaged about 8 mm/yr over a period of about 7 m.y. The rate rate for the third phase averaged about 33 mm/yr over a period of about 5 m.y.

Abstract The Leadville district has yielded gold, silver, and base metals for more than 125 years, principally from dolostone-hosted massive sulfide replacement bodies (mantos). The carbonate host rocks were deposited on a Paleozoic shallow marine shelf overlying a Proterozoic (1.4 Ga) granitic basement, and subsequent diagenetic processes converted the shelf rocks to do-lostones. Multiple stages of dissolution with infilling generated paleokarst units within the uppermost dolostone, the Leadville Dolomite. The ore host rocks lie in the Mosquito Range on the eastern flank of the Laramide Sawatch uplift. Southwest-directed, east-dipping, low-augle thrust faults offset by high-angle reverse faults characterized the compressional environment in the Leadville district during the Laramide orogeny. The first Phanerozoic igneous eveut in the area, emplacement of the Pando Porphyry, occurred contemporaneously with low-angle thrusting. The Pando Porphyry was emplaced along thrust planes and along an unconformity at the top of the Leadville Dolomite. Six igneous events resulting in sills, dikes, and a small stock followed during Tertiary time, culminating with the emplacement of the Breece Hill stock in the eastern part of the Leadville district. All of the preore igneous rocks are latite to quartz monzonite in composition: Four genetic types of ore deposits are present within the Leadville district: (1) contact-related magnetite-serpeutine bodies, (2) dolostone-hosted barite-silver-minor base metal replacement and open-space filliugs termed “Sherman-type ore,” (3) dolostone-hosted zinc-lead-silver-gold massive sulfide replacement bodies with associated veins, referred tg as “Leadville-type ore,” and (4) placer gold. The contact-related magnetite-serpentine bodies replace dolostone aud formed during emplacement of the Breece Hill stock as well as locally around the margin of au older stock southeast of Breece Hill. These ores were used sporadically as smelting flux. The main Leadville district ore deposits are principally massive sulfide replacement bodies in dolostone, but there are related quartz-pyrite-gold veins, disseminated quartz-pyrite-gold in porphyry, and quartz-base metal veins. These are zoned spatially about the Breece Hill stock with central-most quartz-pyrite-gold veins and local disseminations surrounded by quartz-base metal veins and mautos. Mantos closer to Breece Hill have higher gold coutent (Ag/Au = 20), as do those localized along an east-dipping low-angle thrust fault west of Breece Hill; with increasing distance from Breece Hill silver/gold ratios of ores within mantos increase to>75. Manganese in gangue and alteration products increases away from the stock as well. Leadville orebodies in dolostone are dominated by pyrite with lesser marmatite, galena, chalcopyrite, tetrahedrite, pyrrhotite, marcasite, eleetrum, and late-stage veinlets with elevated silver, gold, bismuth, and tellurium contents. Late golden barite, dolomite, and rare fluorite are found in veinlets or vugs within ore. The sulfide coutent of ores in dolostone is>65 vol perceut. The ores are massive and commonly exhibit banding due to textural, size, or min-eralogical differences between bands. The bands parallel sedimentary layering or fracture ‘ planes cutting the dolostones. Diffusion-controlled replacement resulted in ores that locally exhibit concentric banding about unreplaced dolostone bodies, reflecting the stratigraphic and structural controls on ore fluid movement. Local sulfide replacement of paleokarst results in complete preservation of finely laminated cave fill with soft sediment impact features due to rock falls in the paleocaves; however, Leadville ores replaced principally massive dolostone with paleokarst localizing only miuor amouuts of ore. Hydrothermal alteration of the Breece Hill stock is extensive with both sericitic and argillic products common. Adjacent to veins and mantos more intense sericitic alteration is friuged by argillic and propylitic assemblages, extending as much as 100 m from ore. All assemblages contain abundant siderite or manganosiderite. The carbonate host rocks exhibit limited visible alteration exteuding uo more than 5 m from ore. Thermal effects on dolostone insoluble residue phyllosilicates is more extensive. Potassium-argon dates on sericite in Johnson Gulch Porphyry yield an average age on the Leadville ore event of 39.6 ± 1.7 Ma or early Oligocene. Fission-track dates on apatite and zircon from altered preore Johnson Gulch Porphyry indicate that cooling of the hydrothermal system occurred over a 6-m.y. period. The Leadville orebodies are cut by rhyolite porphyry dikes and plugs; these, in turn, have been broken by fragmental porphyry, a fluidized breccia system of tabular to irregular bodies around the perimeter of the Breece Hill stock. Late-stage quartz-base metal sulfide veins cut the fragmental porphyry and yield similar fluid inclusion data as found in the main Leadville-type orebodies. The rhyolite and fragmental porphyries were apparently derived from an intrusive system centered beneath the Breece Hill stock. As the fragmental porphyry bodies were emplaced, the downdropped block, a large triangular-shaped graben in the southeastern part of the district, subsided. Postmineral normal faulting (east side up) associated with development of tbe Rio Grande-Upper Arkansas rift exposed the ores in several blocks up the western flank of the Mosquito Range, leading to oxidation and supergene enrichment of the Leadville-type orebodies and the development of placer gold in California Gnlch, the site of initial discovery.

Abstract Epithermal precious-metal and mercury deposits are present in the Sonoma and Clear Lake volcanic fields of central California and several hot springs in the Clear Lake volcanic field are presently depositing mercury and gold. The deposits and hot springs are associated with late Miocene to Holocene volcanic centers developed above a zone of thin crust and hot asthenosphere termed a slab window (Dickinson and Snyder, 1979, Benz and others, 1992) as the end of Pacific plate subduction was marked by the passage of the Mendocino triple junction along the California coast. Mercury deposition is actively occurring at the Sulphur Bank mercury mine, but no precious metals are present there because the geothermal system is vapor-dominated. In the water dominated geothermal systems at Wilbur Springs (Peters, 1990, Donnelly and others, 1993) and springs near the Cherry Hill gold deposit, both cinnabar and gold are being deposited (Pearcy and Petersen, 1990). Transport of mercury and gold is in a fluid which also contains high concentrations of petroleum and associated methane and CO2 derived from thermal degradation of organic matter in sedimentary rocks (Peabody, 1989). Chemical and isotopic analysis of oxygen and deuterium of the hot springs indicate that three types of fluid are present: moderate chloride, isotopically heavy, evolved formation fluid equilibrated with oceanic sedimentary rocks; evolved meteoric water; and isotopically light meteoric water (Peters, 1990,1991, Sherlock and Jowett, 1992, and Donnelly-Nolan and others, 1993). High concentrations of Hg, As, Sb, Au, and Ag occur in precipitates from hot springs composed dominantly of the isotopically heavy fluid, but not in the moderate-temperature, oxidized springs that are mixtures of these two fluid types (Peters, 1990, Donnelly-Nolan and others, 1993). The McLaughlin gold deposit (initial reserves of 2.9 million oz of gold) is economically the most important deposit in the Clear Lake and Sonoma volcanic fields. This precious metal-mercury hydrothermal system developed within and adjacent to andesitic vents and dikes emplaced along the Stony Creek fault zone (Lehrman, 1986). Gold occurs in opal, chalcedony, and quartz veins, and the highest gold values typically occur in amber to brown opal containing petroleum. Gold occurs in several sites within the petroleum-bearing opal: as a filling of 2050 micron- diameter oval voids representing large fluid inclusions; as 2-4 micron size crystals that coalesce to form dendrites of gold along primary vein banding; and in syneresis cracks which cut the vein banding. Oxide phases of Ga, In, Sn, and Ni are present within the petroleum-bearing opal. The isotopically heavy McLaughlin ore fluid plots in the field of andesitic magma volatiles (Hedenquist and Aoki, 1990, Giggenbach, 1987) and evolved formation waters (Sherlock and Jowett, 1992) suggesting that these two components are present Andesitic vents and dikes at the McLaughlin gold deposit suggest that a larger intrusion underlies the area and provided the heat source for the hydrothermal system. Andesitic vents along the Stony Creek fault provided a conduit for volatiles degassing from the intrusion to become entrained within the hydrothermal fluid composed of gas-oil-field water derived from the Great Valley sequence. The McLaughlin gold deposit reflects the complex interaction of three types of fluid each transporting a different elemental suite: evolved gas-oil field formation water transporting petroleum, Ga, In, Sn, Ni, and Hg; andesitic magmatic fluid transporting Au, Ag, Hg, Sb, and As; and near-surface meteoric water. Prospective areas for precious metal hot-spring deposits occur in the volcanic-structural environment above the thin crust and hot asthenosphere within the slab window in the Coast Ranges and parts of the Great Valley sequence where blind thrusts and associated faults are intruded by Pliocene to Holocene intrusive rocks. Mercury deposits with little or no gold content form along major structures from gas-oil field fluids with little or no magmatic component in the fluid and contain petroleum, Ni, Ga, In, Sn, and other transition elements. Epithermal gold deposits contain a significant magmatic component characterized by Au, Ag, As, Sb, and Hg as well as a gas-oil- field fluid component characterized by petroleum and transition metals. Both deposit types may occur along the same structures.

Abstract The discovery of the McLaughlin hot-spring type gold deposit in the old Knoxville mercury district sparked considerable interest and research into the origin and relation of mercury and epithermal gold mineralization in the California Coast Ranges Province. Silica-carbonate mercury mineralization occurs throughout the Coast Ranges, from Santa Barbara County in the south to Lake and Colusa counties in the north (Fig. 1). Since their discovery in the mid 1800’s California’s Coast Ranges mercury deposits have produced over 50% of the total production for the United States, and two of the mines, the New Almaden mine in Santa Clara County and the New Idria mine in San Benito County, rank as the fifth and sixth largest producing deposits in the world respectively (Bailey and others 1973). Prior to the discovery of the McLaughlin gold deposit in 1978, lode gold production from the Coast Ranges was very minor with an estimated total of about $750,000, with two thirds of this production from the Palisade and Silverado mines in the Calistoga silver-gold district (Clark, 1976). Notable quantities of lode gold have been produced from only two other areas; the Los Burros district in south western Monterey County where mesothermal gold-bearing quartz veins occur in Franciscan Complex rocks; and, from the Knoxville and Sulfur Creek mercury mining districts where gold was produced as a byproduct. The McLaughlin and Cherry Hill epithermal gold deposits (Fig. 1) were subsequently discovered in the Knoxville and Sulfur Creek mining districts respectively. Current reserves at the McLaughlin mine are

ABSTRACT This field trip explores the geology and mining history of the Bodie Hills with a focus on the Bodie and Aurora mining districts. The field trip starts and ends in Bridgeport, California. Our first geologic stop is at Travertine Hot Springs, which provides an analogy for the style of mineralization observed in the Bodie and Aurora districts. We then proceed south on Highway 395 to the Bodie exit at Highway 270. At this location on Highway 395 is the placer camp of Dogtown with outcrops of till from the Sherwin glaciation. A side trip up Cinnabar Canyon exposes rocks with veinlets of cinnabar and silicic alteration. From here, we travel through several different components of different stratovolcanoes and arrive at the preserved ghost town of Bodie. Bodie Bluff and Standard Hill contain most of the mine workings for the Bodie district. At Bodie we discuss different interpretations of volcanic history and stratigraphy and learn about the U.S. Bureau of Land Management’s 1997 economic appraisal of the mineralization there: 69.8 million tons of ore averaging 0.4 oz/ton Au. Leaving Bodie, we pass through a placer field where William Bodie made his discovery of the mine and the district that bears his name. And then we move through the Bodie volcanics to those volcanic rocks associated with Aurora. On the way to Aurora, we will see the stage stop site of Del Monte. At Aurora, we view the active open pit mining operation and compare this district with Bodie and others in the Bodie Hills. Both Bodie and Aurora were classified as principal gold deposits of the United States.

The Eagle Rest Peak igneous complex in the San Emigdio Mountains, and Logan gabbro, near San Juan Bautista, California, crop out on opposite sides of the San Andreas fault. They have identical U-Pb zircon ages of 161 Ma and similar Sr initial isotopic ratios. These data support previous correlations of these rocks (Ross, 1970) and require 305 km of post-Jurassic slip on the northern San Andreas fault, a figure equal to the total slip on the southern segment. The Eagle Rest Peak complex has previously been proposed as the source for gabbro clasts in the Upper Cretaceous Gualala Formation near Point Arena (Ross, 1970; Ross and others, 1973). This implies >440 km of post-Late Cretaceous slip on the San Andreas fault and the existence of a proto-San Andreas fault. Also, granitic to quartz dioritic clasts in the Gualala Formation have been interpreted as detritus derived from the Salinian block in the Cretaceous. However, new U-Pb zircon data from Gualala Formation gabbroic clasts indicate minimum ages of 163 and 165 Ma, slightly older than the 161-Ma age of the Eagle Rest Peak complex and Logan gabbro. Published K-Ar ages also suggest the Gualala cobbles are older than the Eagle Rest Peak complex. These data and the presence of alternate sources for the Gualala cobbles indicate that the Gualala-Eagle Rest Peak tie is not suitable for determining slip on the San Andreas fault. Despite the differences between specific areas, the Eagle Rest Peak complex, Gold Hill and Logan gabbros, and the Gualala gabbro clasts are similar in age and lithology to mafic-ultramafic complexes that form a widespread part of the Jurassic Sierran-Klamath arc. Although Gualala gabbro clasts cannot be uniquely matched to the Eagle Rest Peak complex and Logan rocks, they probably are derived from similar rocks cropping out in the Sierra foothills or buried in the Great Valley. U-Pb-age, Sr, and Pb isotopic data from a single Gualala Formation granodiorite clast do not support a Salinian provenance. The clast is older than 154 Ma, older than the 80- to 120-Ma Salinian granites. The clast also has less radiogenic Pb and Sr isotopic ratios than plutonic rocks from the Salinian block. A reevaluation of paleocurrent data and clast types from the Gualala Formation also suggests a non-Salinian source. The Gualala area is apparently not part of the Salinian block.