Tuffaceous Sediments Research Articles

Eurypterids from the Viséan of East Kirkton, West Lothian, Scotland

ABSTRACTThe hibbertopteroid eurypterid genera Hibbertopterus, Dunsopterus, and Cyrtoctenus occur in the East Kirkton Limestone of Bathgate, West Lothian. Most specimens are fragmentary and appear to have been washed into the depositional area with plant debris and tuffaceous sediment. Nearly all of the identifiable material can be attributed to Hibbertopterus scouleri (Hibbert), but two isolated Cyrtoctenus combs, and a femur comparable with Dunsopterus have also been recognised.New material of Hibbertopterus scouleri (Hibbert) reveals that the posterior legs and telson resemble those of the cyrtoctenids more closely than was previously thought. It is possible that Hibbertopterus, Dunsopterus, and Cyrtoctenus are indistinguishable except by relatively minor characteristics, and that all of the material from East Kirkton Quarry might be derived from a single eurypterid species. However, associations of the diagnostic sclerites which would demonstrate that these taxa are either cogeneric or distinct have not yet been found.

Resolving volcanic activity of 20MA ago with relative accuracy of 1 yr from tree rings of petrified woods

Tree rings of petrified woods were analyzed to resolve a volcanic activity of 20 Ma ago with a relative accuracy of 1 yr. Samples are those in the volcanic breccia of the Early Miocene formation of volcaniclastic rocks and tuffaceous lake sediments in central Japan. Field observations indicate that these petrified woods are the fossils of the trees that were carried away from their growth sites with volcanic breccia ejected upon explosive eruption and included in it. Using a dendrochronological procedure we measured the relative years of the last rings of the petrified woods. The relative years of the last rings are concentrated in 17 yrs, where the ring‐width patterns show an anomalous behavior similar to one observed in firs growing in the area devastated by Mt. St. Helens. We suggest that the last‐formed tree ring of a petrified wood is a record of volcanic eruption and that the activity of volcano continued over a period of 17 yrs, in which eruption occurred at least five times. High resolution regarding the activity period demonstrates uniqueness of tree‐ring study to investigate volcanism in the geologic past.

K/Ar dates of hydrothermal clays from core hole VC-2B, Valles caldera, New Mexico and their relation to alteration in a large hydrothermal system

Abstract Seventeen K/Ar dates were obtained on illitic clays within Valles caldera (1.13 Ma) to investigate the impact of hydrothermal alteration on Quaternary to Precambrian intracaldera and pre-caldera rocks in a large, long-lived hydrothermal system (∼ 1.0 Ma to present). Clay samples came from scientific core hole VC-2B (295°C at 1762 m) which was spudded in the Sulphur Springs thermal area and drilled into the boundary between the central resurgent dome and the western ring-fracture zone. Six illitic clays within Quaternary caldera-fill debris flow, tuffaceous sediment, and ash-flow tuff (48 to 587 m depth) yield ages from 0.35 to 1.09 Ma. Illite from Miocene pre-caldera sandstone (765 m) gives an age of 6.74 Ma. Two dates on illite from sandstones in Permian red beds (1008 and 1187 m) are 4.33 and 4.07 Ma, respectively. Surprisingly, three dates on illites from altered andesite pebbles within the red beds (1010–1014 m) are 0.95 to 1.06 Ma. Four illite dates on variably altered Precambrian quartz monzonite (1615–1762 m) range from 2.90 to 276 Ma. Post-Valles age illite is not correlated with alteration style (argillic to propylitic). Rather, post-Valles ages are uniformly obtained from illites in highly fractured, intensely altered, caldera-fill rocks and the Permian volcanic clasts. Generally, finer clay fractions from identical samples yield younger ages. Plots of 40 Ar/ 36 Ar versus 40 K/ 36 Ar and 40 Ar∗ versus 40 K for the illites in caldera-fill rocks lie close to a 1-Ma isochron. Most illite dates older than Valles caldera are difficult to interpret because they correspond to the ages of pre-Valles volcanic and hydrothermal episodes in the Jemez volcanic field (⩽ 13 Ma). In addition, older dates may be caused by co-mingling of different illites during sample preparation, or by inherited argon or lost argon in illites from rocks with potentially complex hydrothermal histories. However, the range of ages obtained from illites in Permian sands and pebbles and from Precambrian crystalline rocks indicates that Valles hydrothermal activity is overwhelming illite produced by earlier geologic events.

房総半島南部江見海岸における江見層群の地質構造

The Early to Middle Miocene Emi Group is well exposed along the coast line in the Emi area, southern part of the Boso Peninsula. It is composed mainly of tuffaceous siltstone with tuffaceous and volcaniclastic sandstones. The strata are highly faulted, folded and disrupted. The minor structures were analyzed by precise mapping and careful observation of various styles of deformations under semi-lithified conditions.First, dish structures and other dewatering structures including web structures were formed. Next, bedding parallel or slighly oblique thrust faults occurred to make a possible duplex structures. Abundant thrust faults cut the previous structures to bring all the strata horizontally shortened. Assuming the thrusting occurred when the bedding was still nearly horizontal, NWSE horizontal compression was inferred. Strike slip faults of NE trend at last occurred.Such series of deformation indicate that the Izu forearc tuffaceous sediments were accreted along the proto-Sagami trough during Early to Middle Miocene subduction of the Philippine Sea plate to Honshu.

The stratigraphy of a sheet of supracrustal rocks within high-grade orthogneisses and its bearing on Late Archaean structure in southern West Greenland

An extensive sheet of presumed middle Archaean supracrustal rocks within high-grade orthogneisses in inner Godthåbsfjord includes a section 3 km thick with exceptional preservation of pillow structures in low K tholeiitic amphibolites unambiguously indicating way up. The amphibolites are associated with pillow-structured and pyroclastic, komatiitic ultramafic schists, dunitic-harzburgitic sills, a stratiform gabbro, pyritous metacherts and quartzo-feldspathic gneisses and schists with field and chemical characteristics suggesting they formed as acid-intermediate tuffaceous sediments. The sheet is tectonically overlain by older orthogneisses (Amîtsoq) with tracts and enclaves of the early Archaean Isua/Akilia supracrustal association. It is underlain by younger orthogneisses and granites with tracts of older, variably reworked Amîtsoq gneisses. Rocks of the higher level were emplaced on a tectonic slide which marks the upper boundary of the supracrustal sheet. The original lower boundary with Amîtsoq gneisses in the lower level may also have been a slide which has been obscured by emplacement of younger orthogneisses. Composite dome-in-dome structures with intervening synclinal cusps were superimposed on the sub-horizontal layer cake structure of the supracrustal sheet and the higher and lower levels of orthogneisses. Syntectonic granitic and amphibolite dykes (Kangilíngua dykes) were emplaced in parts of the synforms, whilst granite emplacement and feldspar blastesis took place in the cores of some domes. The dome and cusp system appears to be part of a regional series of late Archaean, steeply inclined to upright structures trending northeast in the Godthåbsfjord region. In inner Godthåbsfjord the system evolved in response to buoyancy forces generated by injection of basaltic magmas and partial melting of orthogneisses at depth within a regime of regional compression associated perhaps with transcurrent intraplate motions.

Zeolitic diagenesis of Oligocene pyroclastic rocks of the Metaxades area, Thrace, Greece

AbstractThe unstable glassy component of the Oligocene tuffaceous sediments of the Metaxades area, Thrace, Greece, has undergone extensive zeolitic diagenetic alteration. The authigenic minerals occur as crypto- to micro-crystalline aggregates making up most of the matrix in the altered tuffs and as precipitates in cavities produced by dissolved glass fragments. Glass-shard pseudomorphs are ubiquitous and most of them are partly filled by one or more of the minerals smectite, heulandite, mordenite and silica.The heulandite group zeolites are mostly high-silica calcium-rich heulandites showing intermediate thermal behaviour between most heulandites and clinoptilolites. Their Si/Al ratios are similar to clinoptilolite (4.74–5.19) but their divalent/monovalent cation ratios (1.5–3.24) are partly superposed onto the ratios of heulandite group 1 and 2 and differ considerably from the values of the relevant ratio in clinoptilolite. Rare K-rich clinoptilolite crystals have been identified in one sample only.Based on field observations, compositions and paragenetic relationships of coexisting authigenic minerals and the absence of critical phases such as laumontite, analcime or authigenic albite, it is suggested that the Metaxades pyroclastic rocks underwent burial diagenesis intermediate between Iijima's (1978) zones II and III commonly developed in silica saturated environments.

Persistence of a Zeolite in Tuffaceous Soils of the Texas Trans-Pecos

Clinoptilolite, a highly siliceous zeolite, was observed in tuffaceous sediments of the Texas Trans-Pecos volcanic field. A toposequence of three soils (a Ustollic Calciorthid, a Lithic Torriorthent, and a Ustollic Haplargid) was selected to study the pedogenic fate of clinoptilolite formed in the tuff. X-ray diffraction and electron microscopy were used to determine the mineralogy of soil particlesize separates. Clinoptilolite was observed in the silt (0.002–0.05 mm) and coarse clay (0.0002–0.002 mm) of all horizons, including the tuff. Well-crystallized clinoptilolite was not detected in the fine clay (<0.0002 mm) of the near-surface horizons of the pedons on nonerosional positions, but was present in the fine clay of deeper horizons in these soils. Scanning electron microscopy revealed that most of the clinoptilolite occurred in the interiors of volcanic glass-shard pseudomorphs. The shard pseudomorphs were postulated to act as a shielding mechanism protecting clinoptilolite from intensive weathering. The fact that well-crystallized clinoptilolite was not found in the fine clay of the surficial horizons suggests that, when the shards are fractured or weathered, clinoptilolite weathers rather quickly, at least in the finer fractions. Clinoptilolite cannot, therefore, be considered a stable mineral in this environment, although, because of its mode of formation, it is remarkably persistent.

Palaeoceanography and tectonic setting of the Jurassic Coast Range ophiolite, central California: evidence from the extrusive rocks and the volcaniclastic sediment cover

The primary thesis of this paper is that the character of sedimentary rocks within or directly on top of an ophiolite complex can be used to constrain tectonic models of oceanic crustal genesis. Late Jurassic ( c. 169-161 Ma) Coast Range ophiolite remnants are exposed E and W of the San Andreas Fault in central California. Ophiolite volcanic rocks are mainly sub-alkaline tholeiitic basalts, but include subordinate intermediate and silicic flows and breccias. Available ‘immobile’ major and trace element data indicate that the Coast Range ophiolite formed above a subduction zone, possibly in a fore-arc setting, as suggested by the presence of high magnesian andesites and boninites. The ophiolite extrusives mainly comprise pillow lava and lava breccia formed on a rugged fault-controlled ocean floor. Interlava pelagic sediment is restricted to very small amounts of interstitial red chert and nannoplankton limestone. Widespread occurrence of vesicular lavas probably reflects high primary volatile contents rather than eruption in shallow water. Local interbeds of arc-derived silicic tephra in the uppermost extrusives suggest that the ophiolite and cover sediments formed together in a single tectonic setting. Depositionally overlying deep-water successions up to 600 m thick comprise radiolarian-rich tuffaceous and volcaniclastic sediments of ?Callovian-mid-Tithonian age (165-148 Ma). Depositional processes included airfall, turbidity currents, mass flow and current reworking. Small SiFeMn lenses along the lava-sediment contact formed by hydrothermal replacement of basal siliceous tuffaceous sediments or by direct chemical precipitation. At one locality (Llanada), the ophiolite is overlain by up to 1.8 km of younger (?Tithonian) basic-intermed, and acid lava flows, submarine lahars and mainly silicic pyroclastic sediments, of inferred volcanic arc origin. The volcaniclastic cover passes conformably upwards into late Tithonian and younger terrigenous deep-water clastic sediments of the Great Valley Group, deposited in a fore-arc setting. The Coast Range ophiolite and its cover are interpreted as an incipient intraoceanic volcanic arc, possibly developed above an eastward (continent-ward)-dipping subduction zone within the equatorial Pacific Ocean. In this scenario, short-lived (5–8 Ma) supra-subduction zone spreading to form the ophiolite was followed by ?Callovian-Tithonian arc volcanism, shedding tuffaceous debris onto the adjacent fore-arc. Subduction was active separately along the western American continental margin with the result that the Coast Range arc-ophiolite complex eventually collided with North America, triggering the Late Jurassic Nevadan orogeny and initiating Franciscan subduction-accretion. The Coast Range ophiolite remnants, at least those west of the San Andreas Fault, were apparently translated northwards to their present positions after Late Cretaceous.

Application and limitation of fission-track geochronology to quarternary tephras

Zircon and glass are the principal materials used for fission-track dating of Quaternary tephras. This is because zircon is a common accessory mineral in rhyolitic tephras, and it usually contains sufficient uranium to produce detectable quantities of spontaneous fission tracks over the time span of the Quaternary. Volcanic glass, though uranium-poor, is typically the dominant component of rhyolitic tephras, comprising up to 99% of some distal tephra-fall deposits. Low uranium contents, however, result in low spontaneous track densities that make the age measurement difficult and laborious, a condition exacerbated by very young glasses. In addition, fission tracks in glass shards are unstable and can partially anneal at ambient temperatures. Therefore, unless the glass age is corrected by plateau or isothermal heating experiments, partial annealing causes a reduction in the measured age compared to the true age of the sample. Fission-track dating has a large subjective component that can contribute systematic errors to the analysis; microlite inclusions and some structural defects resemble fission tracks, and if misidentified and included in the count of spontaneous tracks the calculated age would be too old. The most prevalent source of systematic error is contamination in the natural environment, for many tephras in the stratigraphic record are reworked tuffaceous sediments. These deposits may contain detrital components that could mask the primary age of the tephra. However, by using the external detector fission-track method it is possible to calculate ages of single mineral grains, and to distinguish contaminants from primary phases on the basis of their age distribution. This ‘grain-discrete’ capability is the major advantage of the fission-track method over ‘bulk’ geochronological systems.

Stable isotopes of Holocene calcareous tufa in southern Poland as paleoclimatic indicators

The isotopic composition of oxygen in freshwater calcareous tufa seems to be a sensitive indicator of past climatic changes. Results of measurements of δ c 18O and δ c 13C in tufa samples dated with the 14C method are used to reconstruct Holocene climatic changes in southern Poland. Values of δ c 18O obtained on tufa samples from four sites (Raclawka, Rzerzuŝnia, Trzebienice, Sieradowice) representing different hydrodynamic conditions of tufa sedimentation seem to form a selfconsistent series. These values, with some simplifying assumptions concerning the course of tufa sedimentation, were therefore used to estimate mean annual temperatures in the interval ca. 9500-2000 yr B.P. When the resulting curve of thermal changes in southern Poland is compared with changes of deposition and erosion of tuffaceous sediments and with results of other methods of reconstruction of paleoclimate in central Europe, a reasonable agreement among the different specific methods is seen.

Late Miocene Fanglomerates in Lower Member of Starlight Formation, Northern Portneuf Range, Idaho: ABSTRACT

The lower member of the late Miocene Starlight Formation in the northern Portneuf Range, southeastern Idaho, represents sedimentation influenced by both Basin-and Range faulting and eastern Snake River plain (SRP) volcanism. The lower Starlight contains at least 610 m of fanglomerates and thin, interbedded basalt flows. The formations lower half is interpreted as medial alluvial fan facies and consists of coarse tuffaceous heterolithologic conglomerate lenses and sheets, and interbedded immature tuffaceous pebbly lithic arenites. The fine-grained upper half of the member is interpreted as distal-fan facies. The sandy deposits of the distal facies are cut by numerous gravel-filled channels. On the basis of clast assemblages and preservation of fragile rhyolite tuff grains that could not survive long transport distances, these deposits were likely derived from nearby sources. In addition, paleocurrent data indicate a general westerly transport direction. Individual drainage systems apparently controlled sedimentation, as indicated by different clast assemblages at various locations. The dominant clast type in The Cove, southwest of the Blackfoot River, is Eocene andesitic volcanics. In outcrops 20 km south, Paleozoic and lower Mesozoic sedimentary clasts predominate. Chert pebble conglomerate clasts, found only in southern exposures, may have been derived from distant eastern outcrops of Cretaceous conglomerates.more » Alluvial fan deposits in the lower member of the Starlight Formation developed as a response to early Basin-and-Range uplift. However, they were also influenced by eastern SRP volcanism, as indicated by the presence of tuffaceous sediments and basaltic lava flows. The fan deposits likely originated from a nearby eastern source that is now covered by Tertiary volcanics of the Blackfoot laval field.« less

Geochronology of Type Santacrucian (Middle Tertiary) Land Mammal Age, Patagonia, Argentina

Mammal-bearing lacustrine and tuffaceous sediments from three localities of the Santa Cruz Formation, type fauna of the Santacrucian Land Mammal Age, in Patagonia, southern Argentina, are calibrated by radioisotope dating with the aid of magnetostratigraphy. The strata range from about 17.6 Ma to perhaps 16.0 Ma, and are thus of late-early Miocene age. The Santacrucian Land Mammal Age ranges from about 18.0 Ma to about 15.0 Ma.

The Tuskar Group of southeastern Ireland: its geochemistry and depositional provenance

The Tuskar Group is a newly recognized succession of cleaved, steeply dipping, north younging basic volcanics, tuffs, tuffaceous sediments, black shales and medium to fine grained greywackes and turbidites which occur largely in the sea area around the Tuskar Rock, Co. Wexford and in xenoliths in the Carnsore Granite on the mainland. Foliated granodiorites occur along a probable shear zone at the northern contact with the Rosslare Complex. This succession is lithologically similar to parts of the Gwna Group on Anglesey which lies to the northeast along strike. The chemical affinity of metabasites within the Tuskar Group is dissimilar from that of the basic volcanic rocks of the Gwna Group, however, and the overall sense of younging is opposed to that seen on Anglesey.

Accretionary lapilli-filled clastic dykes: a comparison of compaction strain estimates from dyke folding and lapilli shape factors

Within pyroclastic and tuffaceous sediments of the late Archaean Fortescue Group, Western Australia, small-scale clastic dykes have been infilled by accretionary lapilli. This unusual occurrence allows quantification of compaction by two independent means using the folded nature of the dykes and the shape factors of the accretionary lapilli. The dyke-fold method consistently gives a much lower estimate of the compaction strain than the accretionary lapilli. Two dykes giving shortening of 20 and 2% by the dyke method both contain accretionary lapilli recording 40% compaction.

Mesozoic Sedimentary Rocks and Depositional Facies, Vizcaino-Cedros Area, Baja California, Mexico: ABSTRACT

Mesozoic sedimentary rocks in the Vizcaino-Cedros area constitute a 135-m.y. history of arc-related marine sedimentation with a cumulative thickness of 14 km. The Upper Triassic San Hipolito Formation is the oldest sedimentary unit in the region, and is recognized only on the Vizcaino Peninsula. The formation depositionally overlies an ophiolite sequence and consists of 2.4 km of tuffaceous sediment including a limestone megabreccia. The upper two-thirds of the sequence in the type section is now believed to be of Early Jurassic age. On Cedros Island, the basal sedimentary rocks are 1.2 km of the Lower Jurassic Gran Canon Formation. This richly tuffaceous unit depositionally overlies both ophiolite and arc volcanics. Conformably overlying the Gran Canon on Cedros Island is the Coloradito Formation, a spectacular sedimentary olistostrome up to 0.4 km thick, containing Triassic and late Paleozoic metasedimentary blocks. This formation record the first evidence of continental detritus in the region. Conformably overlying the Coloradito and, in part, stratigraphically equivalent to it on Cedros is 0.4 km of volcanogenic-metasedimentary conglomerate designated the Eugenia Formation. The Eugenia is much thicker (to 2.7 km) and widespread on Vizcaino where it unconformably overlies the San Hipolito Formation. On Vizcaino, the lower Eugenia (Upper Jurassic)more » includes spectacular volcanic debris flows interbedded with sandstone, but it is transitional upward into finer facies with increasing plutonic detritus. Time equivalent to the upper Eugenia and locally deposited on tonalite and volcanics is the Lower Cretaceous Asunction Formation (0.8 km thick). This sequence includes calcareous coarse breccias of serpentinized gabbros and tonalite.« less

Geologic setting of the Teutonic Bore massive sulfide deposit, Archean Yilgarn Block, Western Australia

The Teutonic Bore massive sulfide deposit occurs in a sequence of Archean mafic and felsic volcanic rocks in the northeastern Yilgarn Block, Western Australia. Felsic volcanic rocks form part of a regionally continuous linear belt consisting of locally emergent volcanic centers separated by interbedded aquagene tuffs and water-lain epiclastic sedimentary rocks. Mafic and felsic rocks in the area have been extensively intruded by gabbro and subsequently by high-level granitoids believed to be cogenetic with the rhyolitic rocks.Regionally the deposit lies in a linear zone of complex structure and lithology characterized by repeated periods of extensional tectonism. Rhyolite and cogenetic granitoids are related to earlier phases of extension, whereas subsequent activity led to the formation of a suite of saturated to undersaturated syenites and fault-bounded units of polymictic conglomerate derived largely from the rhyolite and high-level granitoids.The Teutonic Bore deposit consists of one relatively small, steeply dipping lens of predominantly banded massive sulfides (mainly pyrite, sphalerite, chalcopyrite, and galena) underlain by concordant and discordant stringer mineralization. The mineralization occurs within a sequence of pillowed basalt and tuffaceous sediment 0 to 150 m stratigraphically above rhyolitic volcanic and volcaniclastic rocks. Primary textures within the massive sulfides and underlying chert indicate depositional instability. Active growth faulting controlled the deposition of felsic epiclastic sedimentary rocks, subsequent basaltic rocks, and the development of the third-order basin which provided the site for deposition of massive sulfides. The faults probably provided the conduits for hydrothermal solutions; these fluids spread laterally and vertically prior to exhaling at the rock-seawater interface during a volcanic hiatus. Features within the local environment are compatible with regional extension. Regional extension resulted in bimodal magmatism, and the associated rifting formed basins suitable for the deposition of extensive epiclastic sedimentary rocks.The bimodal volcano-plutonic host at Teutonic is similar to many examples from the Archean of Canada, but the style of felsic volcanism, the carbonate-rich alteration, and the lead- and silver-enriched ore are most similar to deposits in the Sturgeon Lake area.

Synthesis and Characterization of Zeolites in the System Na2O-K2O-Al2O3-SiO2-H2O

AbstractZeolites having the structures of phillipsite, merlinoite, and gobbinsite were synthesized from clear solutions at 80°C in the system Na2O-K2O-Al2O3-SiO2-H2O and their morphologies, cell parameters, and compositions determined. At 3.5 M silica concentration, the formation of merlinoite (synthetic zeolite W) is favored over the formation of phillipsite (synthetic zeolite ZK-19) by solution conditions of high pH (&gt; 13.6) and low Na/(Na + K) ratios (&lt;0.5).Using the information obtained from the synthesis experiments, the presence of merlinoite was predicted in sediments from Searles Lake, a saline, alkaline lake in California with ideal physiochemical conditions for its formation. Merlinoite was subsequently discovered to occur in tuffaceous sediments as part of an authigenic silicate zonation pattern from phillipsite → phillipsite + merlinoite → merlinoite → K-feldspar with increasing depth. Because of the close similarities in the physical properties of phillipsite and merlinoite, merlinoite may be much more common as an authigenic mineral than is currently realized.

Late Cenozoic vertebrates from Honduras and El Salvador

ABSTRACT The subhumid Pacific slopes of El Salvador and Honduras contain a valuable record of late Cenozoic terrestrial vertebrates. Vast ignimbrite flows, mainly of mid-Miocene age, and subsequent block-faulting and basin-filling provide a substantial sequence of late Cenozoic tuffaceous sediments. This paper discusses the geological setting and faunal content of six local faunas ranging from the late Miocene through the late Pleistocene. Many other more limited late Pleistocene sites are known throughout Honduras and El Salvador. The late Miocene fauna includes no families not already known from North America at temperate latitudes, but does indicate an endemic turtle genus, Rhinoclemmys, and endemic mammal species Pliohippus hondurensis and Rhynchotherium blicki, and a much earlier record of brevirostrine proboscideans than in temperate North America. Although no Pliocene faunas are known, substantial numbers of previously South American taxa appear in the early Pleistocene. By the late Pleistocene, in...

Deep drilling on the Faeroe Islands

A 2178 m deep well was drilled in 1981 at Lopra on Suduroy into the oldest and non-exposed parts of the Lower Tertiary Faeroe Island basalt plateau. The drilled sequence consists of thick subaerial lavas and thin tuffaceous sediments and is very similar to the lower part of the exposed sequence. Secondary minerals indicate a fossil temperature of about 200°C at the bottom of the well. The present gradient is about 25°0 km. Migrated hydrocarbons suggest the presence of non-volcanogenic sediments at deeper levels.

Geological, geochemical, stable isotope, and fluid inclusion characteristics of epithermal gold mineralization, Velvet District, Nevada

Abstract Gold mineralization in the Velvet District occurs in an eastward dipping sequence of late Tertiary rhyolitic ash-flow tuffs, flows, and tuffaceous sediments in northwestern Nevada. Minor gold and silver concentrations are associated with irregular zones of brecciation, argillic alteration, and quartz veining along north-northeast trending normal faults. Reaction of mineralizing fluids with wallrock produced an argillic alteration assemblage of illite, mixed-layer clays, smectite, and kaolinite. Illite alteration and highest gold concentrations appear to be associated with zones of high water/rock ratios. Kaolinite, smectite, alunite, and opal are postulated to have formed during a steam-dominated episode of alteration. Fluid inclusion studies indicate that the quartz veins were deposited in the temperature range 230 to 280°C from fluids which had salinities equivalent to 0.2–0.8 weight percent NaCl. δ 18O of quartz veins varies from −2.5 to +6.7 ‰ and indicates that the ore fluid must have been Tertiary meteroric water. Stable isotope data appear to define a zone of concentrated fluid flow and potential subsurface mineralization in the southeastern part of the district. Fluid inclusion and isotope studies can be used in combination with more standard geochemical, geophysical, and geological information to provide site-specific targets for epithermal metal concentrations.