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Igneous Intrusions/Plutons origin of batholiths Geologists think that such large bodies of magma are associated with subduction of oceanic crust beneath continental crust. As the oceanic crust subducts, it begins to melt releasing great quantities of intermediate magma that slowly rise toward Earth''s surface, intruding into overlying continental crust. During this process, the magma begins to cool and differentiate (see discussion of Bowen''s Reaction Series on the Introduction to Igneous Rocks page), and incorporates continental crust elements as well. This forms felsic magma that eventually crystallizes into large bodies of granite in the form of batholiths. The Sierra Nevada Mountains of California are composed of a number of batholiths emplaced during a period of many millions of years, collectively called the Sierra Batholith. The same is true for much of southern California, where the California Batholith extends from the Mojave Desert southward to near the tip of Baja California! This massive series of intergrown batholiths is referred to as the California Batholith. The presence of both the Sierra and California batholiths tell geologists that subduction was occurring from what is now central California all the way down to the tip of what is now Baja California from roughly 150 to 80 million years ago - a grand tale of plate tectonics and igneous intrusive activity! Mesozoic Plutonism in the central Sierra Nevada Batholith A review of works on mineralogy and isotopes in relation to models for batholith formation Prepared by Tanya S. Unger The problems associated with the origin and emplacement of the Sierra Nevada batholith are anything but trivial. Questions that have remained difficult to answer since Lawsons 1937 seimic observations of the Sierra Nevada Range include and are not limited to the following: 1.) What was the nature of the western North American plate margin in Mesozoic time? 2.) What are the sources for magmas generated from Jurassic to Cretaceous time that make up the plutons of the Sierra Nevada batholith? 3.) How were these magmas emplaced? 4.) What is the compostion and age of the Sierra Nevada root? In answering these questions, it is necessary to integrate geochemistry, geophysics, tectonics, and geology. Mineralogical and isotopic data suggests that the batholith has both vertical and lateral trends oweing themselves to the nature of magma generation and emplacement. This webpage is an effort to summarize select studies of the granitiod rocks of the Sierra Nevada batholith and present current interpretations of mineralogical and isotopic data. Ague, J. J., and G.H. Brimhall (1988) Regional variations in bulk chemistry, mineralogy, and compositions of mafic and accessory minerals in the batholiths of California. Geological Society of America Bulletin, v.100, p. 891-911. Ague, J. J., and G.H. Brimhall (1988) Magmatic arc assymetry and distribution of anomalous plutonic bets in the batholiths of California: Effects of assimilation, crustal thickness, and depth of crystallization. Geological Society of America Bulletin, v.100, p. 912-927. Bateman, P.C. (1983) A summary of the critical relations in the central part of the Sierra Nevada Batholith, California, U.S.A. Geological Society of America Memoir No. 159, p. 241 252. Bateman, P.C. (1988) Constitution and Genesis of the Central Sierra Nevada Batholith, California. U.S. Geological Survey Open File Reports: 88-382, 284p. Bateman, P.C. and F.C.W. Dodge (1970) Variations of major chemical constituents across the Central Sierra Nevada Batholith. Geological Society of America Bulletin, vol. 81, p. 409-420. Bateman, P.C. and J. P. Eaton (1967) Sierra Nevada Batholith, California. Science, vol. 158, no. 3807, p. 1407-1417. Bateman, P.C., L.K. Clark, N.K. Huber, J.G. Moore and C.D. Rinehart (1963) The Sierra Nevada batholith A synthesis of recent work across the central part. U.S. Geological Survey Professional Paper 414-D. 46p. Chen, J. H., and G.R. Tilton (1982) Applications of lead and strontium isotopic relationships to the petrogenesis of granitoid rocks, central Sierra Nevada batholith, California. Geological Society of America Bulletin, v.103, p.439-447. Chen, J. H., and J. G. Moore (1991) Uranium-Lead ages from the Sierra Nevada batholith, California. Journal of Geophysical Research, v.87, no. B6, p. 4761-4784. Crough, S.T., and G.A. Thompson (1977) Upper mantle origin of the Sierra Nevada uplift. Geology, v. 5, p. 396-399. DeFant, M.J., and M.S. Drummond (1990) Derivation of some modern arc magmas by melting of young subducted lithosphere. Nature, v. 347, p. 662-665. DePaolo, D. J. (1980) Sources of continental crust: Neodymium isotope evidence from the Sierra Nevada and Peninsular Ranges. Science, v. 209, no. 8, p. 684-687. DePaolo, D. J. (1980) A neodymium and strontium isotopic study of the Mesozoic calc-alkaline granitic batholithis of the Sierra Nevada and Peninsular Ranges, California. Journal of Geophysical Research, v. 86, no.B11, p. 10470-10488. deSaint Blanquat, M., and B. Tikoff (1997) in Bouchez, J.L., at al., Granite: From segregation of melt to emplacement fabrics. Kluwer Acedemic Publishers, p. 231-252. Dodge, F.C.W., J.J. Papike, and R.E. Mays (1969) Hornblendes from granitic rocks of the Central Sierra Nevada Batholith, California. Journal of Petrology, v. 10, p. 378-409. Dodge, F.C.W., V.C. Smith, and R.E. Mays (1969) Biotites from granitic rocks of the Central Sierra Nevada Batholith, California. Journal of Petrology, v. 10, p. 250-271. Dodge, F.C.W., and R.E. Mays (1972) Rare-Earth Element Fractionation in Accessory Minerals, Central Sierra Nevada Batholith, California. U.S. Geological Survey Professional Paper 800-D, p. 165-168. Dodge, F.C.W. (1972) Trace-element contents of someplutonic rocks of the Sierra Nevada Batholith. U.S. Geological Survey Bulletin 1314-F, 13p. Dodge, F.C.W., H.T. Millard, Jr., and H.N.Elsheimer (1982) Compositional Variations and Abundances of selected elements in granitoid rocks and constituent minerals, Central Sierra Nevada Batholith, California. U.S. Geological Survey Professional Paper 1248, 24p. Dodge, F.C.W., and P.C. Bateman (1988) Nature and origin of the root of the Sierra Nevada. American Journal of Science, v. 288-A, p. 341-357. Domenick, M.A., R.W. Kistler, F.C.W. Dodge, and M. Tatsumoto (1983) Nd and Sr isotopic study of crustal and mantle inclusions from the Sierra Nevada and implications for batholith petrogenesis. Geological Society of America Bulletin, v. 94, p. 713-719. duBray, E. A., ahd D. A. Dellinger (1988) Potassium-Argon ages for plutons in the eastern and southern Sierra Nevada Batholith, California. U.S. Geological Survey Bulletin 1799, 10p. Ducea, M.N., and J.B. Saleeby (1998) The age and origin of a thick mafic-ultramafic keel from beneath the Sierra Nevada batholith. Contrib Mineral Petrol, v. 133, p. 169-185. Evernden, J.F., and R.W. Kistler (1970) Chronology and Emplacement of Mesozoic Batholithic complexes in California and Western Nevada. U.S. Geological Survey Professional Paper 623, 42p. Farmer, G.L., and D.J. DePaolo (1983) Origin of Mesozoic and Tertialry granite in the Western United States and implications fro pre-Mesozoic Crustal Structure 1. Nd and Sr isotopic studies in the geocline of the northern Great Basin. Journal of Geophysical Research, v. 88, no. B4, p. 3379- 3401. Hay, E.A. (1976) Cenozoic uplift of the Sierra Nevada in isostatic response to North American and Pacific plate interactions. Geology, v. 4, p. 763-766. Hietanen, A. (1973) Origin of andesitic and granitic magmas in the northern Sierra Nevada, California. Geological Society of America Bulletin, v. 84, p. 2111-2118. Hurley, P.M., P.C. Bateman, H.W. Fairbairn, and W.H. Pinson, Jr. (1965) Investigation of Initial 87Sr/86Sr Ratios in the Sierra Nevada Plutonic Province. Geological Society of America Bulletin, v. 76, p. 165-174. Kistler, R.W., P.C. Bateman, and W.W. Brannock (1965) Istotopic ages of minerals from granitic rocks of the central Sierra Nevada and Inyo Mountains, California. Geological Society of America Bulletin, v. 76, p. 155-164. Kistler, R.W., J.F. Evernden, and H.R. Shaw (1971) Sierra Nevada plutonic cycle: Part I, Origin of composite granitic batholiths. Geological Society of America Bulletin, v. 82, p. 853-868. Kistler, R.W., and Z.E. Peterman (1973) Variations in Sr, Rb, K, Na, and initial 87Sr/86Sr in Mesozoic granitic rocks and intruded wall rocks in central California. Geological Society of America Bulletin, v. 84, p. 3489-3512. Moore, J. G., and F. C.W. Dodge (1980) Late Cenozoic volcanic rocks of the southern Sierra Nevada, California: I. Geology and petrology: Summary. Geological Society of America Bulletin, Part I, v. 91, p. 515-518. Pickett, D.A., and J.B. Saleeby (1993) Thermobarometric contraints on the depth of exposure and conditions of plutonism and metamorphism at deep levels of the Sierra Nevada batholith, Tehachapi Mountains, California. Journal of Geophysical Research, v. 98, no. B1, p. 609-629. Pickett, D.A., and J.B. Saleeby (1994) Nd, Sr, and Pb isotopic characteristics of Cretaceous intrusive rocks from deep levels of the Sierra Nevada batholith, Tehachapi Mountains, California. Contrib Mineral Petrol, v. 118, p. 198-215. Ramo, O. T., and J. P. Calzia (1998) Nd isotopic composition of cratonic rocks in the southern Death Valley region; evidence for a substantial Archean source component in Mojavia. Geology, 26, (10), 891-894. Reed, J. C., Jr., T. T. Ball, G. L. Farmer, and W. B. Hamilton (1993) A broader view, in Precambrian: Conterminous U. S., The Geology of North America, vol. C-2, edited by J. C. Reed, Jr. and others, pp. 614-622, Geol. Soc. Amer., Boulder, Colo. Rudnick, R.L. (1995) Making Continental Crust. Nature, v. 378, p. 571-578. Stern, T.W., P.C. Bateman, B.A. Morgan, M.F. Newell, and D.L. Peck (1981) Isotopic U-Pb ages of zircon from the granitoids of the central Sierra Nevada, California. U.S. Geological Survey Professional Paper 1185, 17p. Stevens, C. H., P. Stone, G. C. Dunne, D. C. Greene, J. D. Walker, and B. J. Swanson, (1998) Paleozoic and Mesozoic evolution of East-central California, in Integrated Earth and Environmental Evolution of the Southwestern United States, edited by W.G. Ernst and C. A. Nelson, pp. 119-160, Bellweather Publ., Columbia Maryland. Tobisch, O.T., J.B. Saleeby, P.R. Renne, B. McNulty, and W. Tong (1995) Variations in deformation fields during development of a large volume magmatic arc, central Sierra Nevada, California. Geological Society of America Bulletin, v. 107, no. 2, p. 148-166. Van Kooten, G. K. (1980) Mineralogy, petrology, and geochemistry of an ultrapotassic basaltic suite, Central Sierra Nevada, California, U.S.A. Journal of Petrology, 21, (4), 651-684 Sierra Nevada Deformation Deformation during the late Mesozoic to early Cenozoic along the western margin of the United States has had multiple orientations (Figure 1). Some mention has been made of a dextral component causing substantial displacement of the oceanic plate north from the Baja to British Columbia. A plate margin normal component is also believed to have been present producing the Sevier and Laramide orogenies. These tectonic orientations may have occurred at different times or may have occurred concurrently through strain partitioning. To unravel the various possibilities, the timing, sense and magnitude of displacement must be determined. Timing is generally constrained by isotope dating of volcanic deposits or fossils present in the stratigraphy and the relationship of the timed beds to structural features such as faults, mylonites, unconformities, etc. Orientations can be inferred from structural fabrics such as lineations on faults or within mylonites. Magnitude along the plate boundary may be extrapolated from measured amounts of slip along faults or strain accumulated within shear zones and integrated over the plate. A single place to study all of these aspects with respect to relative oceanic plate and North American plate movement is in the Sierra Nevada. This range has been present during this period and has several different types of measurable markers to indicate relative plate motion. This web site will concentrate on lineations within a NW-trending shear zone along the eastern Sierra Nevada known as the Sierra Crest shear zone. During the late Cretaceous to Paleocene (90-50 Ma), the south west margin of North America is believed by some to have been translated northward thousands of kilometers due to north-oblique convergence of the Kula Plate with North America [Housen and Beck, 1999]. Further to the east, the Sevier (120-50 Ma) [DeCelles and Mitra, 1995] and then Laramide orogenies (75-35 Ma) [Dickinson, 1988] were forming. The mean direction of foreland shortening between 75 and 50 Ma was 40o [Bird, 1998]. These concurrent tectonic regimes may indicate oblique convergence causing transpression within the continental interior of the North American plate. Shear zones within the Sierra Nevada should record this transpression. The Sierra Nevada batholith was emplaced in multiple contractional episodes from 210 to 80 Ma [Unger, 1999; Stevens et al., 1998]. The Eastern Sierran thrust system, consisting of a NW-trending belt of NE-vergent thrust and reverse faults and folds, developed during this time [Dunne et al., 1983]. Present, though less common, are shear zones (Figure 2) and conjugate strike-slip faults. There are three main shear zones within the eastern Sierra Nevada. They are generally connected and contemporaneous along a 200 km transect. The Cascade Lake shear zone is the northernmost [Davis et al., 1995]. It has lineations that are nearly horizontal (Figure 3) and strike N. Sense of shear indicators within the Cathedral Peak and Half Dome plutons[Tikoff, 1994] suggest dextral motion. Ar/Ar dating [Sharp et al., 1993] and U/Pb dating on zircon in the syntectonic Mono Creek and Cathedral Peak plutons [Stern et al., 1981] suggest that this shear zone became inactive at 85-80 Ma. In structural contact to the southeast lies the Gem Lake shear zone. This shear zone strikes more NW and has lineations that are nearly vertical. It is approximately 1 km wide and shear indicators suggest nearly 20 km of dextral displacement [Greene and Schweickert, 1995]. This shear zone deforms metavolcanic and metasedimentary rocks of the Ritter Range pendent as well as the Kuna Crest Granodiorite (91 Ma). To the southeast of the Gem Lake shear zone is the Rosy Finch shear zone. This shear zone strikes NS and has nearly horizontal lineations. It cuts the syntectonic Mono Creek granite (80-85 Ma). There is approximately 8 km of dextral displacement over the 3.5 km wide shear zone [Tikoff and Teyssier, 1994]. The Sierra Nevada Batholith: Close-up Shots A granitic body often displays areas of mafic rock, and mafic bodies often display felsic areas. Some areas appear to have been liquid when they were injected into the larger intrusive magma: they show rounded edges and can be bent. Other areas entered the large intrusive body as solid rock: they show angular edges and lithologies that may match rocks surrounding the granite. These are termed xenoliths (foreign rocks). Geologists examine the lithology and especially the chemistry of both types of enclaves in order to understand the processs of intrusive rock formation and the geology of areas deep beneath the surface. The Sierra Nevada Batholith The Sierra Nevada Batholith of eastern California forms the largest mountain range in the continental U.S. Although the prospect of walking across huge areas of one type of rock--granite--might seem dull, in fact the Sierras offer spectacular scenery and lots of interesting details about what went on in the magma chambers of a suite of subduction zone volcanoes that were active from roughly 150 to 80 million years ago. Intrusive rocks offer many clues as to the inner workings of the Earth! Cenozoic/Mesozoic Volcanism of the Eastern Sierra Nevada Author David R. Jessey Superimposition and timing of deformations in the Mount Morrison roof pendant and in the central Sierra Nevada, California GSA Bulletin; March 1977; v. 88; no. 3; p. 335-345; DOI: 10.1130/0016-7606(1977)88<335:SATODI>2.0.CO;2 1977 Geological Society of America STEPHEN RUSSELL1 and WARREN NOKLEBERG1 1 Department of Geology, California State University, Fresno, California 93740 The Mount Morrison roof pendant, the only roof pendant in the central Sierra Nevada containing Paleozoic fossils, is complexly deformed and contains three generations of structures, including folds, reverse faults, schistosities, and lineations. All three generations of structures occur in the Ordovician-Silurian(?) metasedimentary rocks, whereas only the younger two are recorded in the Pennsylvanian-Permian(?) metasedimentary rocks and the Permian(?)-Jurassic(?) metavolcanic rocks. The average strike directions of axial planes of folds are north-south, N25W, and N60W in the first, second, and third generations, respectively. Generations of structures having similar styles, orientations, and relative age relations occur in other pendants of the central Sierra Nevada. The pendant is interpreted as a thin sequence with tight isoclinal folds instead of a thick homoclinal sequence. The first deformation occurred during Devonian or Mississippian time, perhaps during the Antler orogeny. Uplift, erosion, and volcanism occurred in Late Permian time between the first and second deformations, perhaps as an expression of the Sonoma orogeny. The second generation structures formed in several pulses between Early Triassic and Early Cretaceous time, as indicated by temporal relations between deformed wall rocks and younger, crosscutting granitic plutons. The third generation structures formed between Early and Late Cretaceous time, during which these structures were crosscut by granitic rocks. The wall rocks of the batholith may form an anticlinorium instead of a synclinorium. Other roof pendants in the axial portion of the batholith may be relatively old, because they contain the same three sets of structures as found in the Ordovician-Silurian(?) rocks of the Mount Morrison roof pendant. Locally, various age belts of granitic rocks have shielded roof pendants from subsequent deformation. Implications of Polonium Radiohalos in Nested Plutons of the Tuolumne Intrusive Suite, Yosemite, California Answers Research Journal 2 (2009): 53-78. www.answersingenesis.org/articles/arj/v2/n1/radiohalos-in-yosemite-granites by Dr. Andrew Snelling and Dallel Gates April 8, 2009 Keywords: Granite plutons, Yosemite National Park, Magma emplacement, Magma cooling, Polonium radiohalos, Hydrothermal fluids, Accelerated U decay, Nested plutons, Tuolumne Intrusive Suite, Sequential emplacement, Explosive volcanism Abstract The formation of granite plutons has conventionally been thought to be a slow process requiring millions of years from generation to cooling. Even though new mechanisms for rapid emplacement of plutons have now been proposed, radioisotope dating still dominates and dictates long timescales for pluton formation. However, a new challenge to those long timescales has arisen from radiohalos. Polonium radiohalos found in biotite flakes of granites in Yosemite National Park place severe time constraints on the formation and cooling of the granite plutons due to the short half-lives of the polonium isotopes. The biotite flakes must have formed and cooled below 150C before the polonium supply was exhausted and the radiohalos could be preserved, so the U decay had to be grossly accelerated and the formation of the plutons had to be within 610 days. Furthermore, rapid cooling of the plutons was facilitated by the hydrothermal fluid convection that rapidly generated the Po radiohalos, challenging conventional thinking that cooling is a slow process by conduction. It is evident that there were greater volumes of hydrothermal fluids in the later central intrusions of the nested plutons of the Tuolumne Intrusive Suite. So as expected, more Po radiohalos were generated in these plutons as they were sequentially intruded, confirming the hydrothermal fluid transport model for Po radiohalo formation. Thus granite pluton formation is consistent with the timescale of a young earth, and accelerated radioisotope decay renders the absolute ages for these granite plutons grossly in error. Granites constitute a major portion of the continental crust. They outcrop over many areas of the earths surface as discrete bodies called plutons, ranging in size from 10 km2 to thousands of km2. The granite magmas are believed to be sourced from great depths in the lower to mid levels of the continental crust, but the plutons crystallize in the upper crust, typically at depths of 15 km. Furthermore, granite plutons are often part of batholiths, which are regional areas comprised of hundreds of plutons. As far as can be ascertained, granite magmatism primarily occurs in the continental crust and involves four separate but potentially quantifiable stagesgeneration, segregation, ascent, and emplacementthat operate over length scales ranging from 10-5 to 106 meters (Petford et al. 2000). Once in place, the final stage is cooling. Explanations for the formation of even a single granite pluton have become somewhat controversial, even in the conventional geologic community, as once held conventions are being challenged. No longer is research focusing on just the mineralogy, geochemistry, and isotopes of granites as clues to their formation (which has hithertofore supposedly required long timescales of millions of years), but on the physical processes as well. The results of such research have drastically shortened the intrusion timescales of many plutons to just centuries and even months (Petford et al. 2000). Various evidences are now being cited that change the long-held, extended time frames for granite formation (Coleman, Gray, and Glazner 2004). Conventional thinking has been that plutons form from the slow rising of diapirs, large molten masses that intrude into the host rocks and then cool. However, the problem of how the host rocks provide the space for these intruding diapirs has been increasingly recognised. In contrast, there are field data that indicate persuasively that plutons have formed from small batches of magma that accumulated in succession by dike intrusions. This new thinking drastically reduces the timescales for magma intrusion to form granite plutons, but most geologists are still convinced, based on their unerring commitment to radioisotope dating, that plutons require long uniformitarian time frames to form and then to cool primarily by conduction. Thus, the formation and the cooling of granite plutons are still regarded as prima facie evidences against the year-long, catastrophic global Flood on a young earth (Young and Stearley 2008). A Brief Story of the Geology of Yosemite Valley (1943) by M. E. Beatty About the Author Matthew Edward "Ed" Beatty was born August 30, 1901. He was Associate Park Naturalist in Yosemite from 1932 to 1944. In 1944 he transferred to Glacier National Park in Montana, where he was Chief Naturalist to 1955. He was Regional Chief of Interpretation in 1961. Ed Beatty wrote several articles and booklets for Yosemite Nature Notes, while he was in Yosemite, including this one. Other subjects he wrote about include birds, bears, firefall, and photographer C. E. Watkins. M. E. Beatty died October 22, 1989 at Polson, Montana (which is on the shore of Flathead Lake, south of Glacier National Park). Sierra Nevada Geologic History Oldest rocks Paleozoic metamorphosed marine sediments Appear to match unmetamorphosed rock exposed in Great Basin Ranges Deposited in stable marine environment west of North American coast Prominent as dark roof pendants exposed above Sierra Nevada granite Caps several peaks in Sierra crest Mount Tom North American plate starts moving west around 400 million years ago (myr) Series of fragments pasted onto west coast over period 400-200 million years ago (myr) Western metamorphic belt created in Sierra foothills as oceanic fragments collide with North America Separated by major fault zones Fault zones often marked by serpentenite from deep ocean and mantle West coast of North America advances into region Original gold veins formed by metamorphosis of marine sediments in foothill zone Mother lode closely follows Melones fault which separates westernmost (youngest) belt from older fragments to east Gold probably formed originally near volcanic vents on ocean floor Mobilized and concentrated in veins after sediments pasted onto continent by emplacement of batholith Water drawn to deep depths Molten plutonic rock heats water to high temperatures Dissolves gold and other minerals Circulates into cracks, deposits gold and quartz veins Emplacement of Sierra Nevada batholith starts around 210 myr Subduction zone present west of Sierra, roughly parallel to current coast Massive amounts of molten magma rise toward surface east of subduction zone Intrusive granitic rocks of batholith cool slowly below surface Probably roots of large volcanic chain similar to modern Andes Distinctive large-grained, light-colored rock More than 100 individual units with ages ranging 200-90 myr Generally uniform in appearance Solid, resistant rock Around 80-90 myr slope of subduction zone appears to flatted and magma generation shifts eastward Region of accretion along continental margin shifts west to coast ranges Granites and gold-bearing veins exposed by uplift and erosion of ancestral Nevadan Range 100-65 myr Formation of Tertiary river deposits 50-40 myr Extensive weathering and erosion due to humid conditions Sea level fluctuations produce alternating cutting and filling of large river beds Gold-bearing quartz gravels deposited in river channels Volcanic activity buries river deposits beneath layers of lava: 33-16 myr, again 9-5 myr Table Mountain formed from resistant lava flow deposit Remnant volcanic deposits exposed in spots along Sierra crest Geology and Geomorphology of the North Fork of the American River Geologic History of the North Fork The North Fork of the American River There is magic and mystery in this wildest of all canyons in the Northern Sierra The great canyon of the North Fork of the American River, often more than 2,000 feet deep, for miles of its length over 3,000 feet deep, and at Snow Mountain, the massive sentinel of the Royal Gorge, over 4,000 feet deep, has resisted roads and development down through the 20th century and now into the 21st century. It is one of the great refugia for wildlife left in the Northern Sierra, and is renowned for its wildness and beautiful scenery. It is this canyon and no other which, historically, was known as The American River Canyon. Tectonic character of the Melones Fault Zone Dissertation of Lee.R.Russell, 1977 The Mother Loge Composite Devonian island-arc batholith in the northern Sierra Nevada, California GSA Bulletin; March 1988; v. 100; no. 3; p. 446-457; DOI: 10.1130/0016-7606(1988)100<0446:CDIABI>2.3.CO;2 1988 Geological Society of America RICHARD E. HANSON1, JASON B. SALEEBY2 and RICHARD A. SCHWEICKERT3 1 Department of Geology and Mineralogy, Ohio State University, Columbus, Ohio 43210 2 Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125 3 Department of Geological Sciences and Mackay School of Mines, University of Nevada, Reno, Nevada 89557 The Bowman Lake batholith intrudes the pre-Upper Devonian Shoo Fly Complex in the northern Sierra Nevada. The eastern margin of the batholith extends to within 1.5 km of the base of a thick sequence of Paleozoic island-arc rocks that rests unconformably on the Shoo Fly Complex. Hypabyssal silicic intrusions associated with the batholith penetrate the Upper Devonian Sierra Buttes Formation, the lowest volcanic unit of the arc sequence, but do not extend to higher stratigraphic levels. The intrusions in the Sierra Buttes Formation possess marginal peperites and show other evidence of their injection into wet, unconsolidated sediments. On the basis of these field relations, the batholith is interpreted to represent a subvolcanic magma chamber emplaced concurrently with deposition of Sierra Buttes arc rocks. The batholith is composite and consists of trondhjemite, granodiorite, biotite granite, and hornblende tonalite, formed from discrete batches of magma injected into a common plutonic chamber. Mingling of tonalitic and trondhjemitic magmas produced abundant rounded inclusions of tonalite dispersed within trondhjemite and led to hybridization of the two magmas by fine-scale intermixing. Zircons from the trondhjemitic, granodioritic, and granitic phases of the batholith and an associated Sierra Buttes rhyolite sill show complex U-Pb isotopic systematics. A multistage history is suggested, involving the incorporation of earliest Paleozoic and/or Proterozoic zircons during magma generation or ascent in Devonian time and one or more stages of disturbance in Mesozoic-Cenozoic(?) time. Consideration of these different discordance mechanisms results in a model U-Pb igneous age for the batholith of 364 to 385 Ma (Middle to Late Devonian); on the basis of the field relations, a Late Devonian age for the batholith is adopted. Zircon characteristics and isotopic systematics suggest that petrogenesis of the granite involved mixing of trondhjemitic and/or granodioritic magmas with partial melt from a separate, large-ion-lithophile-enriched source. The Mineral Calaverite Calaverite is an uncommon and much sought after mineral by mineral collectors and those seeking fortunes. Calaverite is one of the few minerals that is an ore of gold, besides native gold itself. It is the most common gold bearing mineral besides native gold. The element gold is typically either found as native gold (in its elemental state), as an alloy with other metals such as silver and copper and as trace amounts in a few minerals. To be an actual significant part of a non-alloyed mineral is really quite uncommon for gold and this makes calaverite a unique mineral indeed. For some reason gold has an affinity for the element tellurium, which is sometimes found naturally as native tellurium. Tellurium is a semi-metallic element which means that it has some properties of metals but not all or as strongly. This helps provide an explanation for gold''s, and other metals such as silver''s, attraction to tellurium. Other gold tellurides include sylvanite, (Silver Gold Telluride); kostovite, (Copper Gold Telluride); krennerite, (Silver Gold Telluride); nagyagite, (Gold Lead Antimony Iron Telluride Sulfide) and petzite (Silver Gold Telluride). Calaverite is closely related to sylvanite and differs only in silver content and slightly in hardness, cleavage, color and density. At times the two minerals are only distinguishable with chemical tests. Crystals of calaverite are unique and of interest to collectors. Typically found as striated prisms that can be twinned causing sharp bends, reticulated individuals and skeletal or arborescent formations. These clusters remind many collectors of writing. Associated Minerals include gold, quartz, celestite, fluorite, pyrite, nagyagite, sylvanite, krennerite and other rare telluride minerals Calaverite: Gold Telluride Calaverite is one of the names for gold telluride, the natural combination of gold and tellurium. Although Calaverite is named for Calaveras County in California, where it was first found, this sample is from the famous Cripple Creek district, in Colorado...presented in a 1.25 inch perky. Formation of Lode Gold Deposits by Brian Heike Ores bearing native gold consist of grains or microscopic particles of metallic gold embedded in rock, often in association with veins of quartz or sulfide minerals like pyrite. These are called "lode" deposits. Native gold is also found in the form of free flakes, grains or larger nuggets that have been eroded from rocks and end up in alluvial deposits (called placer deposits). Such free gold is always richer at the surface of gold-bearing veins owing to the oxidation of accompanying minerals followed by weathering, and washing of the dust into streams and rivers, where it collects and can be welded by water action to form nuggets. Gold occurs mainly in pyrite- and polymetallic sulfidequartz vein/veinlet stockworks. Fluid inclusions in the deposit are divided into three main types, namely CO2H2O, H2OCO2 CH4 and aqueous ones. A popular misconception is that small veins of gold or silver ore in a mining district are necessarily branches of a single rich and massive mother lode deep in the ground. This idea is contrary to modern theories of ore deposits. The term is also used metaphorically to refer to the origin of something valuable or in great abundance. The clustered distribution of low displacement faults and associated mesothermal Au deposits within fault systems can be governed by static stress changes (Dsf) associated with large rupture events on high displacement components of the fault systems. The low displacement faults, which have localized gold deposition and attendant fluid flow, are interpreted as being formed and repeatedly reactivated during aftershock sequences following numerous, high displacement rupture events. The distribution of aftershock activity and associated permeability enhancement is particularly influenced by the location of rupture arrest on main shock faults. The formation of lode gold deposits requires a history of repeated fault slip and fluid flow events. Accordingly, an important factor controlling the distribution of aftershock fault arrays is the development of long-lived structures which repeatedly inhibit rupture propagation and accumulation of fault slip on high displacement faults. Using two case studies, we demonstrate that both dilatant and contractional jogs on high displacement faults can be effective barriers to rupture propagation. Modeling of Dsf associated with large slip events demonstrates that the distribution of Au deposits on low displacement faults in both the Mount Pleasant and St Ives goldfields in the Archaean Yilgarn Craton (Western Australia) is well-matched by the domains of positive Dsf and enhanced aftershock probability. In geology a lode is the metalliferous ore that fills a fissure in a rock or a vein of ore deposited between layers of rock. Mother lode is a principal vein or zone of veins of gold ore. In the United States, Mother lode is most famously the name given to the long alignment of hard rock gold deposits stretching northwest to southeast in the Sierra Nevada of California. The zone contains hundreds of mines and prospects, including some of the best-known historic mines of the gold-rush era. Individual gold deposits within the Mother Lode are gold-bearing quartz veins up to 50 feet thick and a few thousand feet long. The California Mother Lode was one of the most productive gold-producing districts in the United States, but is now given over to tourism. The Carlin mine near Carlin, Nev., USA, is producing gold from a large low-grade deposit that was opened in 1965 after intensive scientific and technical work had been completed. Similar investigations have led to the more recent discovery of a Carlin-type gold deposit in Jerritt Canyon, Nev. Many placer districts in California have been mined on a large scale as recently as the mid-1950''s. Streams draining the rich Mother Lode region--the Feather, Mokelumne, American, Cosumnes, Calaveras, and Yuba Rivers--and the Trinity River in northern California have concentrated considerable quantities of gold in gravels. California Gold Region 6 was second only to the Mother Lode in California gold production. California Gold Region 6 includes Susanville, Greenville, Westwood, Shasta, Weed, Red Bluff, Redding, Enterprise, Yreka, Weaverville, French Gulch, Alturas, Happy Camp, Orleans and Crescent City. Both lode and placer mining have been done in this region, which is adjacent to and south of the Oregon/California state line. California Gold Region 6 has gold deposit sites ranging eastward from Crescent City on the Pacific Ocean to Modoc National Forest northeast of Alturas. The gold sites range southward from the Oregon state line to latitude 40 degrees, north, which is five miles north of Quincy. The Klamath Mountains region in northwestern California is the second-most gold-productive province in California. The principal gold districts are in Shasta, Siskiyou, and Trinity Counties. Although there are several important lode-gold districts, the placer deposits have been the largest sources of gold. In Australia, the Mosquito Creek belt has been the largest source of metasediment-hosted lode Au in the southeast Archean Pilbara Craton. The city of Johannesburg located in South Africa was founded as a result of the Witwatersrand Gold Rush which resulted in the discovery of some of the largest gold deposits the world has ever seen. Gold fields located within the basin in the Free State and Gauteng provinces are extensive in strike and dip requiring some of the world''s deepest mines, with the Savuka and TauTona mines being currently the world''s deepest gold mine at 3,777 m. Other major producers are United States, Australia, China, Russia and Peru. Mines in South Dakota and Nevada supply two-thirds of gold used in the United States. In South America, the controversial project Pascua Lama aims at exploitation of rich fields in the high mountains of Atacama Desert, at the border between Chile and Argentina. Today about one-quarter of the world gold output is estimated to originate from artisanal or small scale mining. The Rushan gold deposit in the Jiaodong Peninsula is currently the largest lode gold in China. California Gold Quartz Veins Veins of gold bearing quartz are well known in California, and historically they have produced millions of dollars worth of gold. Their formation and the gold they contain are of interest. Quartz mines are found and worked in a great many counties in California, from Siskiyou on the north to San Diego on the south. Most of these are in the mountain, foot-hill and desert regions, there being none of note in the valley counties and very few in the Coast Ranges. Most are mesothernal in origin, and they are often described as segregated veins, emplaced in schist, slates or metamorphosed igneous rocks (greenstones), and more or less parallel with the schistosity of these rocks. Less commonly the walls are massive, igneous rocks. The quartz contains auriferous pyrite, free gold, arsenopyrite, chalcopyrite, tetrahedrite, galena, and sphalerite, but pyrite is far the most abundant. Tellurides have been occasionally detected in small amounts and locally other rare minerals such as scheelite, native bismuth, pitchblende, tetrahedrite, stibnite, molybdenite and fluorite are also occasionally found. Productive quartz is grayish or bluish in many instances because of the presence of fine-grained sulfides. The veins approximate at times a lenticular shape, which is less marked in California than in some other regions, and which shows analogies of shape with pyrites lenses and magnetite lenses In such cases the fissure-vein character is somewhat obscure. In California the veins occupy previously existing fissures in the slates. The largest and best known is the so called Mother Lode, which is a lineal succession of innumerable larger and smaller quartz veins that run parallel with the strike, and rarely cut the steep dip of the slates at an angle of 10 degrees. It was doubtless formed by faulting along the Melones fault zone in steeply dipping strata. The veins are persistent along regional high-angle faults, joint sets. The best deposits overall form in in areas where greenstone / greenschist rocks (metavolcanics) are present. High-grade ore shoots form locally at or near metasediment-serpentine contacts. The wall rocks of the California veins embrace many types of igneous rocks, as well as sedimentary slates, for all these enter into the western slopes of the Sierras. The dominant type however, are meta-sedimentary rocks of one type or another. The frequent serpentine is a metamorphosed igneous rock, while the diabase and diorite form great dikes. Considerable calcite, dolomite, and ankerite occur with the quartz, and very often it is penetrated by seams of a green, chloritic silicate, which was provisionally called mariposite, but which has been shown by Turner to be a potassium mica, colored green by chromium. Altered wall rocks frequently show the presence of Quartz, siderite, and (or) ankerite and albite in veins with halo of carbonate alteration further out. Green Chrome bearing mica with dolomite and talc as well as siderite are found in areas where ultramafic rocks are present. The quartz veins vary somewhat in appearance, being at times milk white and massive (locally called "hungry," from its general barrenness), at times greasy and darker, and again manifesting other differences, which are difficult to describe, although more or less evident in specimens. The richer quartz in many mines is somewhat banded, and is called ribbon quartz. The quartz has been studied in thin sections, especially in rich specimens, by W. M. Courtis, who shows that fluid or gaseous inclusions of what is probably carbonic acid are abundant. In rich specimens the cavities tend to be more numerous than in poor, but more data are needed to form the basis of any reliable deductions. Some quartz shows evidence of dynamic disturbances. The walls of the veins are themselves at times impregnated with the precious metal and the attendant sulphides. The rich portions of the veins occur in chutes which run diagonally down on the dip. The so-called "auriferous slate belt" of California, of which the Mother Lode is only one part, begins south of Mariposa County and extends north-westerly along the western flank of the Sierra Nevada to the north line of Plumas county, and doubtless farther, but beyond that point it is covered with vast sheets of barren lava. As described, it is about 250 miles long and twenty to seventy miles wide, and forms about one-third of the precious mineral-bearing area of the State, but it is so far the most productive part. The general strike of the strata is north-westerly, which is parallel with the axis of the Sierra Nevada and they dip very steeply to the north-east. Within this area are a countless number of gold-bearing quartz veins, commonly following the strike and dip of the strata, occasionally crossing them. Rich Gold Bearing Quartz from the Mother Lode of California The great Mother Lode vein system is the largest group of veins in California. It extends 112 miles in a general northwest direction. Beginning in Mariposa County, in the south, it crosses Tuolumne, Calaveras, Amador, and El Dorado counties in succession. It is not strictly continuous nor is it one single lode, but rather a succession of related ones, which branch, pinch out, run off in stringers, and are thus complex in their general grouping. It parallels the axis of the range and the general trend of the formations. Many veins of this group strike about N. 25 W and dip about 60 E. Single deposits are developed for more than a mile along the strike. Over 500 patented locations have been made on it. The mother lode counties furnished a large percentage of the milling ores of the State. The average recovery per ton is much less than in other counties where the veins are smaller and richer. The average recovery from all the counties in the Mother Lode district is less than $4 per ton while in Nevada County the amount exceeds $10 in gold and silver per ton of ore mined. One characteristic of the Mother Lode is the permanancy of the ore with increasing depth. In Amador County the mines are over 5000 ft. deep on the dip and the ore is as good as that found at the surface. The ores occur in fissure veins in steeply dipping slates and altered volcanics of Carboniferous and Jurassic age. The ores are found at so great a distance from the granitic rocks of the Sierra Nevada range that they are supposed to bear no genetic relation to them. The veins occur both in the slates and at their contact with diabase dikes. The veins show a remarkable extent and uniformity. The veins are often left in strong relief by the erosion of the wall rock, and thus are called ledges, or reefs. In the tilted layers of the slate there lay planes of weakness which the mineral-bearing solutions followed. The chief gangue mineral is quartz, and the ore is native gold and auriferous pyrite. The California veins are younger than the igneous rocks with which they are associated. Granite and grano-diorite are especially frequent, but diorite, gabbro, diabase, porphyrite and serpentine, presumably derived from some basic intrusion, are also met. Although Von Richthofen stated that the veins seldom occur far from granite, this has been shown by Lindgren to be unjustified. The greater number are in slates, and the richest in a particular series of slates, but they also cut all manner of igneous rocks and have no constancy of direction. The California gold quartz type is in characteristic examples sufficiently pronounced to justify its special treatment. In fact, they are considered as a archetype of the low sulfide gold-quartz vein model. Other famous examples of this same type are found in the Ballarat Goldfields of Victoria, Australia and the Goldfields of Nova Scotia Intrusive, Uplift and Erosional History of the Northern Sierra Nevada batholith Van Buer, Miller, Dumitru, Grove, Wright NSF Tectonics 0809226 The Mesozoic Sierra Nevada Batholith preserves an extensive record of continental-margin arc magmatism which serves as a classic, worldwide model for such tectonic environments. However, about one-third of the batholith actually extends northward from the Sierra Nevada proper into the Basin and Range Province of northwest Nevada, where it has received little study. Extensive recent research on late Cenozoic Basin and Range extension in that region has yielded considerable preliminary information on that portion of the batholith, highlighting three key, related issues that form the focus of this project: 1.Magmatic arc history 2.Erosion and exhumation 3.Sedimentation and basin formation Eastern Sierra Geology Basic Plate Tectonics |
