Melones Fault Zone Research Articles

Geochemical Evolution of a High Arsenic, Alkaline Pit-Lake in the Mother Lode Gold District, California

The Harvard orebody at the Jamestown gold mine, located along the Melones fault zone in the southern Mother Lode gold district, California, was mined in an open-pit operation from 1987 to 1994. Dewatering during mining produced a hydrologic cone of depression; recovery toward the premining ground-water configuration produced a monomictic pit lake with alkaline Ca-Mg-HCO3-SO4–type pit water, concentrations of As up to 1,200 μ g/L, and total dissolved solids (TDS) up to 2,000 mg/L. In this study, pit-wall rocks were mapped and chemically analyzed to provide a context for evaluating observed variability in the composition of the pit-lake waters in relationship to seasonal weather patterns. An integrated hydrogeochemical model of pit-lake evolution based on observations of pit-lake volume, water composition (samples collected between 1998–2000, 2004), and processes occurring on pit walls was developed in three stages using the computer code PHREEQC. Stage 1 takes account of seasonally variable water fluxes from precipitation, evaporation, springs, and ground water, as well as lake stratification and mixing processes. Stage 2 adds CO2 fluxes and wall-rock interactions, and stage 3 assesses the predictive capability of the model. Two major geologic units in fault contact comprise the pit walls. The hanging wall is composed of interlayered slate, metavolcanic and metavolcaniclastic rocks, and schists; the footwall rocks are chlorite-actinolite and talc-tremolite schists generated by metasomatism of greenschist-facies mafic and ultramafic igneous rocks. Alteration in the ore zone provides evidence for mineralizing fluids that introduced CO2, S, and K2O, and redistributed SiO2. Arsenian pyrite associated with the alteration weathers to produce goethite and jarosite on pit walls and in joints, as well as copiapite and hexahydrite efflorescences that accumulate on wall-rock faces during dry California summers. All of these pyrite weathering products incorporate arsenic at concentrations from <100 up to 1,200 ppm. In the pit lake, pH and TDS reach seasonal highs in the summer epilimnion; pH is lowest in the summer hypolimnion. Arsenic and bicarbonate covary in the hypolimnion, rising as stratification proceeds and declining during winter rains. The computational model suggests that water fluxes alone do not account for this seasonal variability. Loss of CO2 to the atmosphere, interaction with pit walls including washoff of efflorescent salts during the first flush and seasonal rainfall, and arsenic sorption appear to contribute to the observed pit-lake characteristics.

Chronology of Gold Mineralization in the Sierra Nevada Foothills from 40Ar/39Ar Dating of Mariposite

Mariposite (Cr-rich muscovite) crystals formed the during metasomatic replacement of serpentinite yield 40Ar/39Ar ages demonstrating that gold-quartz-carbonate mineralization in the Sierra Nevada Foothills Metamorphic Belt occurred as early as 152 Ma and continued until at least 122 Ma. The earliest gold mineralization is found in the Grass Valley District; mineralization subsequently migrated southward along the Melones fault zone, forming the Mother Lode Gold Belt. These ages correspond to a lull in Sierra Nevada plutonism, and with periods of ductile deformation along the Melones and related fault zones. Our data suggest that gold-quartz-carbonate mineralization was not associated with Sierra Nevada plutonism, but instead was related to changes in plate dynamics, and generation and migration of CO2-rich ore-forming metamorphic fluids along the continental margin of North America. In this model, shallow gold mineralization was produced by heating of the lower crust within the structural mélange of the Sierra Nevada Foothills Metamorphic Belt as a response to an increase in dip of the subducting slab and oceanward migration of the saw-tooth thermal structure.

Arsenic speciation in pyrite and secondary weathering phases, Mother Lode Gold District, Tuolumne County, California

Arsenian pyrite, formed during Cretaceous gold mineralization, is the primary source of As along the Melones fault zone in the southern Mother Lode Gold District of California. Mine tailings and associated weathering products from partially submerged inactive gold mines at Don Pedro Reservoir, on the Tuolumne River, contain ∼20–1300 ppm As. The highest concentrations are in weathering crusts from the Clio mine and nearby outcrops which contain goethite or jarosite. As is concentrated up to 2150 ppm in the fine-grained (<63 μm) fraction of these Fe-rich weathering products. Individual pyrite grains in albite-chlorite schists of the Clio mine tailings contain an average of 1.2 wt.% As. Pyrite grains are coarsely zoned, with local As concentrations ranging from ∼0 to 5 wt.%. Electron microprobe, transmission electron microscope, and extended X-ray absorption fine-structure spectroscopy (EXAFS) analyses indicate that As substitutes for S in pyrite and is not present as inclusions of arsenopyrite or other As-bearing phases. Comparison with simulated EXAFS spectra demonstrates that As atoms are locally clustered in the pyrite lattice and that the unit cell of arsenian pyrite is expanded by ∼2.6% relative to pure pyrite. During weathering, clustered substitution of As into pyrite may be responsible for accelerating oxidation, hydrolysis, and dissolution of arsenian pyrite relative to pure pyrite in weathered tailings. Arsenic K-edge EXAFS analysis of the fine-grained Fe-rich weathering products are consistent with corner-sharing between As(V) tetrahedra and Fe(III)-octahedra. Determinations of nearest-neighbor distances and atomic identities, generated from least-squares fitting algorithms to spectral data, indicate that arsenate tetrahedra are sorbed on goethite mineral surfaces but substitute for SO 4 in jarosite. Erosional transport of As-bearing goethite and jarosite to Don Pedro Reservoir increases the potential for As mobility and bioavailability by desorption or dissolution. Both the substrate minerals and dissolved As species are expected to respond to seasonal changes in lake chemistry caused by thermal stratification and turnover within the monomictic Don Pedro Reservoir. Arsenic is predicted to be most bioavailable and toxic in the reservoir’s summer hypolimnion.

Sodium metasomatism along the Melones Fault Zone, Sierra Nevada Foothills, California, USA

AbstractAlbitite, locally aegirine- and riebeckite-bearing, formed as a result of sodium metasomatism of felsic dykes and argillites along the Melones Fault Zone near Jamestown, California. Pyrite, magnetite, hematite and titanite are common in small amounts in altered dykes. The dykes were originally plagioclase-hornblende porphyritic, and had major and trace element abundances typical of calc-alkaline rocks, whereas they now have Na2O contents as high as 11.40%. Associated fracture-filling veins are dominated by albite, but locally include aegirine, analcime, paragonite, calcite and sodic scapolite. Quartz is present in most albitic rocks, but is absent in riebeckite- and aegirine-bearing samples. Albitization predated CO2 metasomatism and formation of sericite-pyrite assemblages that are typical of gold deposits of the Mother Lode Belt.Alkaline fluids responsible for Na-metasomatism had elevated Na+/K+ and Na+/H+ relatively high fO2, and low aH4SiO4. The presence of titanite indicates fluid. The presence of titanite indicates fluid XCO2 was low, in contrast to fluids that formed later carbonate-bearing assemblages. Sodic scapolite suggests that, at least locally, the fluids attained very high salinities.Mass balance calculations indicate that alteration involved addition of large amounts of sodium, and the removal of SiO2 and K2O. Textural preservation, combined with volume factors calculated from specific gravity and whole rock analytical data, indicate that Na-metasomatism was essentially isovolumetric.Sodium-rich zones along the Melones Fault Zone are closely associated with fault-bounded bodies of ultramafic rock, typically altered to talc-carbonate or quartz-magnesite-Cr muscovite assemblages. Carbonatization and talc-forming reactions in the ultramafic rocks may lead to SiO2-undersaturated fluids. Expansion of the muscovite stability field in terms of Na+/K+-Na+/H+, as a result of incorporation of Cr (up to 7.7% Cr2O3) in muscovite, would result in H+- and K+-depletion as the fluid interacts with ultramafic rocks. This could lead to fluids with elevated Na+/K+ and high pH, as documented in this occurrence.

Strains and structures associated with a terrane bounding stretching fault: the Melones fault zone, central Sierra Nevada, California

An examination of the structures, total strains, and rock types across the Melones fault zone indicates that in the central Sierra Nevada this fault separates and locally imbricates two distinct terranes. East of the fault, a widespread early Jurassic cleavage displays west-northwest to northwest strikes and is associated with moderate to high strain intensities. These structures form in Jura-Triassic metasedimentary rocks in part derived from a continental source. A weak northwest striking cleavage and low strain intensities occur west of the fault in lower Jurassic volcanic rocks and upper Jurassic slates and sandstones. The fault zone itself consists of steep east-dipping ductile-brittle shear zones or stretching faults along which the early Jurassic structures are transposed parallel to the Jura-Cretaceous and fault-related structures. Large flattening type strains are common in the fault zone and may be the result of heterogeneous strains developed when the shear zones bend around resistant blocks or by volume losses. Total strains in the eastern hanging wall reflect the superposition of fault-related on older strains; therefore, stretching lineations in this hanging wall do not reflect directions of fault motion. Stretching lineations in the footwall and fault zone, kinematic indicators in the fault zone, and a small eastward jump in metamorphic grade suggest an east-over-west or reverse sense of movement; no evidence for significant strike-slip or oblique-slip motion was found. Removal of footwall strains indicates that the Jura-Cretaceous shortening across the fault was responsible for both the transposition of older structures east of the fault and the rotation of the fault to its present steep dips. Thus, the Melones fault is a large ductile-brittle shear zone along which the eastern hanging wall (Merced River Terrane) moved to the southwest over the western footwall (eastern Foothills Terrane).

Curved slickenfibers: a new brittle shear sense indicator with application to a sheared serpentinite

Brittle fault zones are commonly characterized by penetrative fracturing of the rock to form an aggregate of blocks, so that the rock becomes, in essence, like a granular material composed of rigid ‘grains’. The surfaces of the blocks, or ‘grains’, have a wide distribution of orientations, and shearing of the blocks past one another on these planes accommodates the large-scale motion, i.e. the macromotion, in the fault zone. During a macromotion that is a non-coaxial deformation, the rigid blocks and their surfaces may undergo a progressive rigid rotation that is distinct from the macromotion and is described by the microspin. If a macroscopic non-coaxial deformation has monoclinic symmetry, this symmetry should be reflected in the sense of rigid rotation of the blocks and their surfaces. On the surfaces of the blocks, which are local shear planes, the history of displacement may be recorded by mineral fibers that grow progessively with displacement. Because of the rigid rotation of the local shear planes, slickenfiber lineations commonly are curved. The sense of curvature from the youngest to the oldest part of the fiber, looking down the normal to the local shear plane, is different for those planes whose normals are on opposite sides of the unique monoclinic symmetry plane for the macroscopic shearing. The intersection of the symmetry plane with the plane of the fault zone defines the macroscopic slip direction, and the sense of rigid rotation of the local shear planes determined from the lineations defines the shear sense. Application of this technique to the disrupted and sheared margins of the Feather River Peridotite in the northern Sierra Nevada of California indicates that late deformation involved dextral-normal oblique slip along the Melones fault zone.

Extent of the Nevadan orogeny, central Sierra Nevada, California

The Late Jurassic Nevadan orogeny has long been considered responsible for much of the deformation that occurred in the central Sierra Nevada. On the basis of our recent work and that of other workers, we suggest that the intensity of Nevadan deformation in the central Sierra Nevada decreases eastward within the foothills belt, is restricted to domainal deformation in rocks between the Melones fault zone and the batholith, and is virtually absent in the continental volcanic arc along the east side of the range. These data and interpretations suggest that in the central Sierra Nevada, Nevadan deformation does not represent as intense and widespread an orogeny as originally supposed.

Role of the Melones Fault Zone in the Structural Chronology of the North Yuba River Area, Western Sierra Nevada, California

Research Article| March 01, 1979 Role of the Melones Fault Zone in the Structural Chronology of the North Yuba River Area, Western Sierra Nevada, California S. E. Cebull; S. E. Cebull 1Department of Geosciences, Texas Tech University, Lubbock, Texas 79409 Search for other works by this author on: GSW Google Scholar L. R. Russell L. R. Russell 2Shell Development Company, Bellaire Research Center, P.O. Box 481, Houston, Texas 77001 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1979) 90 (3_Part_II): 528–544. https://doi.org/10.1130/GSAB-P2-90-528 Article history received: 07 May 1978 rev-recd: 02 Nov 1978 accepted: 14 Nov 1978 first online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation S. E. Cebull, L. R. Russell; Role of the Melones Fault Zone in the Structural Chronology of the North Yuba River Area, Western Sierra Nevada, California. GSA Bulletin 1979;; 90 (3_Part_II): 528–544. doi: https://doi.org/10.1130/GSAB-P2-90-528 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract INTRODUCTIONThe Melones fault zone is the easternmosi major element of the Foothills Fault system (Clark, 1960, 1964) of the Western Sierra Nevada metamorphic belt, California. In the southern part of the belt, it commonly separates. Mesozoic rocks on the west from Paleozoic rocks on the east, but to the north, the fault is bounded on both sides largely by Paleozoic rocks. Accordingly, E. M. Moores considers the northern segment to be significantly older than that farther south (Schweickert and Cowan, 1975). Sense of displacement along the zone is uncertain, as is the role of the fault in the tectonic evolution of the belt. For example, the zone has been interpreted as being both of possible dip-slip origin (Knopf, 1929; Russell and Cebull, 1914) and strike-slip origin (Chandra, 1953; Clark, 1960; Baird 1962). Probably, movement was complex and includes both components of displacement (Cebull, 1972), which may be related either to a single oblique-slip motion or to changes of sense with time. Recently, the Melones fault has been interpreted in terms of regional plate-tectobics models. Schweickert and Cowan (1974, 1975) depict the Melones, with some of the other Foothills faults, as sutures between collided arcs. At one place along the Melones fault zone, along the north fork of the Yuba River west of Downieville (Fig. 1), Schweickert (1976) reports lawsonite blueschist and cites such rocks as supporting the proposal that the zone is one of suturing “between diverse terranes juxtaposed by subduction” (p. 409). According to Schweickert and Cowan (1975), the rock succession along the North Yuba River, from Downieville westward for about 30 km, is a melange. If this is correct, the Melones fault zone there represents only the easternmost portion of the melange succession (see Figure 1 of Schweickert and Cowan, (1975) This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Analysis of recent crustal movement in the central and northern Sierra Nevada, California, using repeated geodetic leveling data

A comparative analysis of repeated geodetic leveling data was made along nine subparallel, E—NE-trending leveling lines located in the central to northern Sierra Nevada and the eastern Central Valley. The analysis was made to identify relative changes of elevation and evaluate these changes with respect to the regional geology and tectonics. The analysis used National Geodetic Survey first- and second-order, unadjusted, observed elevations. The relative changes in elevation indicate that crustal deformation is continuing to occur in the Sierra Nevada along pre-existing zones of crustal weakness and that this deformation is localized along some strands of Late Cenozoic faulting within the Mesozoic Foothills fault system. This deformation is characterized by variable and nonunifor westward tilt of the Sierran block west of the Melones fault zone, and relatively consistent eastward tilt of the Sierran block east of the Melones fault zone. Variable elevation changes occur within the Foothills fault system and are often associated with prominent geological or structural contacts. In addition, subsidence in the Central Valley appears to be of small magnitude and localized in extent, indicating nontectonic changes in elevation problably due to compaction of unconsolidated sediments.

Role of the Melones fault zone in the structural chronology of the North Yuba River area, western Sierra Nevada, California: Summary

INTRODUCTION The Melones fault zone is the easternmosi major element of the Foothills Fault system (Clark, 1960, 1964) of the Western Sierra Nevada metamorphic belt, California. In the southern part of the belt, it commonly separates. Mesozoic rocks on the west from Paleozoic rocks on the east, but to the north, the fault is bounded on both sides largely by Paleozoic rocks. Accordingly, E. M. Moores considers the northern segment to be significantly older than that farther south (Schweickert and Cowan, 1975). Sense of displacement along the zone is uncertain, as is the role of the fault in the tectonic evolution of the belt. For example, the zone has been interpreted as being both of possible dip-slip origin (Knopf, 1929; Russell and Cebull, 1914) and strike-slip origin (Chandra, 1953; Clark, 1960; Baird 1962). Probably, movement was complex and includes both components of displacement (Cebull, 1972), which may be related either to a single oblique-slip motion or to changes of sense with time. Recently, the Melones fault has been interpreted in terms of regional plate-tectobics models. Schweickert and Cowan (1974, 1975) depict the Melones, with some of the other Foothills faults, as sutures between collided arcs. At one place along the Melones fault zone, along the north fork of the Yuba River west of Downieville (Fig. 1), Schweickert (1976) reports lawsonite blueschist and cites such rocks as supporting the proposal that the zone is one of suturing “between diverse terranes juxtaposed by subduction” (p. 409). According to Schweickert and Cowan (1975), the rock succession along the North Yuba River, from Downieville westward for about 30 km, is a melange. If this is correct, the Melones fault zone there represents only the easternmost portion of the melange succession (see Figure 1 of Schweickert and Cowan, (1975)

Structural-metamorphic chronology in a roof pendant near Oakhurst, California: Implications for the tectonics of the western Sierra Nevada

Foliation, broken bedding, and two “shear zones” in pre-Cretaceous clastic metasedimentary rocks in a roof pendant west of Oakhurst, California, are on trend with the western Sierra Nevada metamorphic belt to the northwest. In the pendant, foliations define a westward-verging, upward-diverging fan, and lineations have orientations that plot along a great circle, or a small circle of large diameter, with a maximum at the intersection of the circle and the synoptic foliation. Rock microtextures are characterized by porphyroblast clasts, especially of hornblende, in a matrix of aligned cataclastic fragments and some superimposed static mineral growth. Three major events are interpreted. The first, M 1 , is characterized by probable synkinematic metamorphism of the epidote-amphibolite facies. The second, here correlated with the classical Nevadan orogeny, involved the cataclastic development of the prominent, penetrative, northwest-striking foliation surface, S 1 , and the incomplete transposition of M 1 hornblende lineations ( a kinematic axis: N50°E, 64°NE). During this event, bodies of ultramafic and granodioritic rocks were emplaced and deformed along the “shear zones.” Finally, contact metamorphism, M 2 , chiefly of the albite-epidote-hornfels facies, resulted from the intrusion of batholithic rocks between approximately 98 and 136 m.y. ago. Regional implications include the following: (1) the shear zones may be southerly remnants of the Foothills fault system, probably the Melones fault zone; (2) M 1 could represent a moderately deep expression of the Sonoma or Antler orogenies; and (3) the kinematics of S 1 favor an interpretation of primarily vertical displacement for the Nevadan deformation and, hence, lend possible support to most subduction models.

Feather River Ultramafic Body, Northern Sierra Nevada, California

The Feather River ultramafic body is an elongate mass of metamorphosed peridotite and dunite with subordinate lenses of mafic schist. Classification of the body as an alpine-type peridotite is indicated by its mineral chemistry (olivine, Fo 88–92 ; enstatite, En 89–92 ; chromium-rich spinel) and the presence of “flow-layering” and kink bands in olivine and enstatite. The body is located along the northernmost part of the Melones fault zone and is faulted against surrounding slate units derived from thick mudstone-chert and volcaniclastic-carbonate sequences of Paleozoic and Triassic age. The ultramafic rock underwent an early episode of high-temperature flowage, probably as a crystal mush, and later was metamorphosed in the greenschist facies, at least in part contemporaneously with emplacement. Mafic schist lenses within the ultramafic body were metamorphosed under amphibolite- and then greenschist-facies conditions. The western contact of the northern part of the body is a zone of intense shearing and is bordered by a layer 100 to 600 m thick of complexly deformed schist which is intermediate in metamorphic grade between amphibolite and greenschist facies. Evidence is inconclusive as to whether the ultramafic body was intruded into the Sierran environment at high temperatures or was tectonically emplaced as a cool slab of oceanic upper mantle. The latter origin, however, is supported by the tabular form of the body and the discordance of high-temperature flow features with the northwest trend of the body.

Sense of Displacement along Foothills Fault System: New Evidence from the Melones Fault Zone, Western Sierra Nevada, California

Sense of displacement along the Foothills fault system, western Sierra Nevada, California, has been interpreted in terms of reverse, thrust, and strike-slip faulting. Within the Melones fault zone (part of the Foothills fault system) near Downieville, synoptic fault-plane intersections, in cataclastically deformed metasedimentary rocks and serpentinite, plunge steeply northwest, as do axes of minor flexure folds in metasedimentary rocks. Fold axes in serpentinite plunge steeply to the northeast and southeast; slickenside orientations are varied, but plunge chiefly southward at steep to moderate angles. Cleavage in slate on either side of the fault zone strikes approximately north and dips steeply eastward. These relationships imply a complicated chronology of events characterized by possible early emplacement of serpentinite and a late phase of strike-slip displacement. I conclude that present fault dips near Downieville probably approximate initial dips, that the pattern of faulting suggests a conjugate relationship best explained in terms of strike-slip displacement, that the latter interpretation is supported by attitudes of minor-fold axes, but not by those of slickensides, and that such an interpretation does not preclude earlier oblique-slip or dip-slip displacements.