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USGS2005

Supergene destruction of a hydrothermal replacement alunite deposit at Big Rock Candy Mountain, Utah: Mineralogy, spectroscopic remote sensing, stable-isotope, and argon-age evidences

Big Rock Candy Mountain is a prominent center of variegated altered volcanic rocks in west-central Utah. It consists of the eroded remnants of a hypogene alunite deposit that, at ∼21 Ma, replaced intermediate-composition lava flows. The alunite formed in steam-heated conditions above the upwelling limb of a convection cell that was one of at least six spaced at 3- to 4-km intervals around the margin of a monzonite stock. Big Rock Candy Mountain is horizontally zoned outward from an alunite core to respective kaolinite, dickite, and propylite envelopes. The altered rocks are also vertically zoned from a lower pyrite–propylite assemblage upward through assemblages successively dominated by hypogene alunite, jarosite, and hematite, to a flooded silica cap. This hydrothermal assemblage is undergoing natural destruction in a steep canyon downcut by the Sevier River in Marysvale Canyon. Integrated geological, mineralogical, spectroscopic remote sensing using AVIRIS data, Ar radiometric, and stable isotopic studies trace the hypogene origin and supergene destruction of the deposit and permit distinction of primary (hydrothermal) and secondary (weathering) processes. This destruction has led to the formation of widespread supergene gypsum in cross-cutting fractures and as surficial crusts, and to natrojarosite, that gives the mountain its buff coloration along ridges facing the canyon. A small spring, Lemonade Spring, with a pH of 2.6 and containing Ca, Mg, Si, Al, Fe, Mn, Cl, and SO 4 , also occurs near the bottom of the canyon. The 40 Ar/ 39 Ar age (21.32±0.07 Ma) of the alunite is similar to that for other replacement alunites at Marysvale. However, the age spectrum contains evidence of a 6.6-Ma thermal event that can be related to the tectonic activity responsible for the uplift that led to the downcutting of Big Rock Candy Mountain by the Sevier River. This ∼6.6 Ma event also is present in the age spectrum of supergene natrojarosite forming today, and probably dates the beginning of supergene alteration at Big Rock Candy Mountain. The δ 34 S value (11.9‰) of alunite is similar to those for replacement alunite from other deposits in the Marysvale volcanic field. The δ 34 S values of natrojarosite (0.7‰ to −1.2‰) are similar to those for aqueous sulfate in Lemonade Spring, but are larger than those in pyrite (0.4‰ to −4.7‰). The δ 34 S and δ 18 O SO 4 values of gypsum show an excellent correlation, with values ranging from 15.2‰ to −5.2‰ and 7‰ to −8.2‰, respectively. The stable-isotope data indicate that the aqueous sulfate for gypsum is a mixture derived from the dissolution of hypogene gypsum and alunite, and from the supergene oxidation of pyrite. The aqueous sulfate for the natrojarosite, however, is derived largely from the supergene oxidation of pyrite, with a minor contribution from the dissolution of alunite and gypsum. The exceptional detailed spectral mapping capabilities of AVIRIS led to the recognition of a small amount of jarosite that is probably the top of the steam-heated system that produced the primary hypogene alteration at Big Rock Candy Mountain.

Utah↗

USGS1969

Geochemistry and hydrodynamics of the Paradox Basin region, Utah, Colorado and New Mexico

The Paradox Basin region is approximately bounded by the south flank of the Uinta Basin to the north, the Uncompahgre uplift and San Juan Mountains to the east, the Four Corners structural platform to the southeast, the north rim of the Black Mesa Basin and the Grand Canyon to the south and southwest, and the Wasatch Plateau and Hurricane fault system to the west. Some of these geologic features are areas of ground-water recharge or discharge whereas others such as the Four Corners platform do not directly influence fluid movement. The aquifer systems studied were: (1) Mississippian rocks; (2) Pinkerton Trail Limestone of Wengerd and Strickland, 1954; (3) Paradox Member of the Hermosa Formation; (4) Honaker Trail Formation of Wengerd and Matheny, 1958; (5) Permian rocks. Recharge in the Paradox Basin occurs on the west flank of the San Juan Mountains and along the west side of the Uncompahgre uplift. The direction of ground-water movement in each analyzed unit is principally southwest-ward toward the topographically low outcrop areas along the Colorado River in Arizona. However, at any point in the basin, flow may be in some other direction owing to the influence of intrabasin recharge areas or local obstructions to flow, such as faults or dikes. A series of potentiometric surface maps was prepared for the five systems studied. Material used in construction of the maps included outcrop altitudes of springs and streams, drill-stem tests, water-well records, and an electric analog model of the entire basin. Many structurally and topographically high areas within the basin are above the regional potentiometric surface; recharge in these areas will drain rapidly off the high areas and adjust to the regional water level. With a few exceptions, most wells in formations above the Pennsylvanian contain fresh (< 1,000 mg/l T.D.S. 2 ) to moderately saline (< 10,000 mg/l T.D.S.) water. In only a few cases are true brines (> 35,000 mg/l T.D.S.) reported. Most water samples from strata below the Permian are brines of the sodium chloride type but with large amounts of calcium sulfate or calcium chloride type water commonly occurring. Because evaporite facies occur in the Paradox Member, this unit has brines with as much as 400,000 mg/l dissolved solids content. Previous analysis of the San Juan Basin has indicated the presence of an osmotic membrane system. The highly permeable Jurassic formations were postulated to be the outflow side of the membrane. It is also possible that the Upper Paleozoic units with known brines and with an otherwise inexplicably high potentiometric surface in the Four Corners area of New Mexico could be the outflow receptors of the San Juan membrane system.

Colorado, New Mexico, Utah↗

USGS2016

Three whole-wood isotopic reference materials, USGS54, USGS55, and USGS56, for δ2H, δ13C, δ15N, and δ18O measurements

Comparative measurements of stable hydrogen and oxygen isotopes in wood are hampered by the lack of proper reference materials (RMs). The U.S. Geological Survey (USGS) has prepared three powdered, whole-wood RMs, USGS54 ( Pinus contorta , Canadian lodgepole pine), USGS55 ( Cordia cf. dodecandra , Mexican ziricote), and USGS56 ( Berchemia cf. zeyheri , South African red ivorywood). The stable isotopes of hydrogen, oxygen, carbon, and nitrogen in these RMs span ranges as δ 2 H VSMOW from –150.4 to –28.2 mUr or ‰, as δ 18 O VSMOW from + 17.79 to + 27.23 mUr, as δ 13 C VPDB from –27.13 to –24.34 mUr, and as δ 15 N AIR-N2 from –2.42 to + 1.8 mUr. These RMs will enable users to normalize measurements of wood samples to isotope–delta scales, and they are intended primarily for the normalization of δ 2 H and δ 18 O measurements of unknown wood samples. However, they also are suitable for normalization of stable isotope measurements of carbon and nitrogen in wood samples. In addition, these RMs are suitable for inter-laboratory calibration for the dual-water suilibration procedure for the measurements of δ 2 H VSMOW values of non-exchangeable hydrogen. The isotopic compositions with 1-σ uncertainties, mass fractions of each element, and fractions of exchangeable hydrogen of these materials are: USGS54 ( Pinus contorta , Canadian Lodgepole pine) δ 2 H VSMOW = –150.4 ± 1.1 mUr (n = 29), hydrogen mass fraction = 6.00 ± 0.04 % (n = 10) Fraction of exchangeable hydrogen = 5.4 ± 0.6 % (n = 29) δ 18 O VSMOW = + 17.79 ± 0.15 mUr (n = 18), oxygen mass fraction = 40.4 ± 0.2 % (n = 6) δ 13 C VPDB = –24.43 ± 0.02 mUr (n = 18), carbon mass fraction = 48.3 ± 0.4 % (n = 12) δ 15 N AIR- N2 = –2.42 ± 0.32 mUr (n = 17), nitrogen mass fraction = 0.05 % (n = 4) USGS55 ( Cordia cf. dodecandra , Mexican ziricote) δ 2 H VSMOW = –28.2 ± 1.7 mUr (n = 30), hydrogen mass fraction = 5.65 ± 0.06 % (n = 10) Fraction of exchangeable hydrogen = 4.1 ± 0.5 % (n = 30) δ 18 O VSMOW = + 19.12 ± 0.07 mUr (n = 18), oxygen mass fraction = 35.3 ± 0.2 % (n = 6) δ 13 C VPDB = –27.13 ± 0.02 mUr (n = 18), carbon mass fraction = 53.3 ± 0.6 % (n = 12) δ 15 N AIR-N2 = –0.3 ± 0.4 mUr (n = 16), nitrogen mass fraction = 0.25 % (n = 4) USGS56 ( Berchemia cf. zeyheri , South African red ivorywood) δ 2 H VSMOW = –44.0 ± 1.8 mUr (n = 30), hydrogen mass fraction = 5.65 ± 0.05 % (n = 10) Fraction of exchangeable hydrogen = 6.6 ± 0.3 % (n = 30) δ 18 O VSMOW = + 27.23 ± 0.03 mUr (n = 12), oxygen mass fraction = 41.1 ± 0.2 % (n = 6) δ 13 C VPDB = –24.34 ± 0.01 mUr (n = 12), carbon mass fraction = 47.3 ± 0.2 % (n = 12) δ 15 N AIR-N2 = + 1.8 ± 0.4 mUr (n = 15), nitrogen mass fraction = 0.27 % (n = 4)

Chemical Geology↗

USGS2005

Paleoproterozoic high-sulfidation mineralization in the Tapajós gold province, Amazonian Craton, Brazil: geology, mineralogy, alunite argon age, and stable-isotope constraints

The Brazilian Tapajós gold province contains the first evidence of high-sulfidation gold mineralization in the Amazonian Craton. The mineralization appears to be in large nested calderas. The Tapajós–Parima (or Ventuari–Tapajós) geological province consists of a metamorphic, igneous, and sedimentary sequence formed during a 2.10 to 1.87 Ga ocean−continent orogeny. The high-sulfidation mineralization with magmatic-hydrothermal alunite is related to hydrothermal breccias hosted in a rhyolitic volcanic ring complex that contains granitic stocks ranging in age from 1.89 to 1.87 Ga. Cone-shaped hydrothermal breccias, which flare upward, contain vuggy silica and have an overlying brecciated cap of massive silica; the deposits are located in the uppermost part of a ring-structure volcanic cone. Drill cores of one of the hydrothermal breccias contain alunite, natroalunite, pyrophyllite, andalusite, quartz, rutile, diaspore, woodhouseite–svanbergite, kaolinite, and pyrite along with inclusions of enargite–luzonite, chalcopyrite, bornite, and covellite. The siliceous core of this alteration center is surrounded by advanced argillic and argillic alteration zones that grade outward into large areas of propylitically altered rocks with sericitic alteration assemblages at depth. Several occurrences and generations of alunite are observed. Alunite is disseminated in the advanced argillic haloes that envelop massive and vuggy silica or that underlie the brecciated silica cap. Coarse-grained alunite also occurs in branching veins and locally is partly replaced by a later generation of fine-grained alunite. Silicified hydrothermal breccias associated with the alunite contain an estimated reserve of 30 tonnes of gold in rock that grades up to 4.5 g t −1 Au. Seven alunite samples gave 40 Ar/ 39 Ar ages of 1.869 to 1.846 Ga, with various degrees of apparent minor Ar loss. Stable isotopic data require a magmatic-hydrothermal origin for the alunite, typical for high-sulfidation mineralization. The δ 34 S values of most samples of alunite range from 14.0&permil; to 36.9&permil;. Sulfur isotopic alunite–pyrite and oxygen isotopic alunite SO 4 −OH temperatures range from 130 to 420 °C. The δD H 2 O and δ 18 O H 2 O values for alunite-forming hydrothermal fluids suggest a predominance of magmatic water, with a small meteoric contribution. A rare sample of supergene alunite has a δ 34 S value of 4.1&permil; and an 40 Ar/ 39 Ar age of 51.3±0.1 Ma. Other than local foliation in the volcanic rocks and recrystallization of alunite near faults, the mineralization and associated alteration appears to have been remarkably undisturbed by later metamorphism and by supergene alteration. The Au mineralization was preserved because of burial by sediments and tuffs in taphrogenic basins that probably developed shortly after mineralization and were probably first exhumed at about 60 Ma. Because high-sulfidation mineralization forms at relatively shallow crustal levels, the discoveries in Tapajós province provide new perspectives for mineral exploration for the Amazonian and perhaps for other Precambrian cratons.

Chemical Geology↗

USGS2005

Evolution of the magmatic-hydrothermal acid-sulfate system at Summitville, Colorado: Integration of geological, stable-isotope, and fluid-inclusion evidence

The Summitville Au-Ag-Cu deposit is a classic volcanic dome-hosted high-sulfidation deposit. It occurs in the Quartz Latite of South Mountain, a composite volcanic dome that was emplaced along the coincident margins of the Platoro and Summitville calderas at 22.5??0.5 Ma, penecontemporaneous with alteration and mineralization. A penecontemporaneous quartz monzonite porphyry intrusion underlies the district and is cut and overlain by pyrite-quartz stockwork veins with traces of chalcopyrite and molybdenite. Alteration and mineralization proceeded through three hypogene stages and a supergene stage, punctuated by at least three periods of hydrothermal brecciation. Intense acid leaching along fractures in the quartz latite produced irregular pipes and lenticular pods of vuggy silica enclosed sequentially by alteration zones of quartz-alunite, quartz-kaolinite, and clay. The acid-sulfate-altered rocks host subsequent covellite+enargite/luzonite+chalcopyrite mineralization accompanied by kaolinite, and later barite-base-metal veins, some containing high Au values and kaolinite. The presence of both liquid- and vapor-rich fluid inclusions indicates the episodic presence of a low-density fluid at all levels of the system. In the mineralized zone, liquid-rich fluid inclusions in healed fractures in quartz phenocrysts and in quartz associated with mineralization homogenize to temperatures between 160 and 390 ??C (90% between 190 and 310 ??C), consistent with the range (200-250 ??C) estimated from the fractionation of sulfur isotopes between coexisting alunite and pyrite. A deep alunite-pyrite pair yielded a sulfur-isotope temperature of 390 ??C, marking a transition from hydrostatic to lithostatic pressure at a depth of about 1.5 km. Two salinity populations dominate the liquid-rich fluid inclusions. One has salinities between 0 and 5 wt.% NaCl equivalent; the other has salinities of up to 43 wt.% NaCl equivalent. The occurrence of high-salinity fluid inclusions in vein quartz associated with mineralization, as well as in the deep stockwork veins, suggests that brines originating deep in the system transported the metals. The ??34S values of sulfides in magnetite (-2.3???) and of sulfate in apatite (5.4???) in unaltered quartz latite indicate that ??34S???S was near 0???. The ??34S values of coexisting alteration alunite and pyrite are 18.2??? to 24.5??? and -8.1??? to -2.2???, respectively. Deep in the system, most of the change in ??34S values occurs in the sulfates, indicating that the fluids were initially H2S-dominant, their redox state buffered at depth by equilibration with igneous rocks. However, in the main alteration zone, most of the change in ??34S values occurs in pyrite, indicating that the fluids moved off the rock buffer and became SO42- -dominant as pyrite precipitated and SO2 disproportionation produced the sulfuric acid requisite for acid leaching. The ??34S values of the late-stage barite and sulfides indicate that the system returned to high H2S/SO42- ratios typical of the original rock-buffered fluid. The ??DH2O of alunite parent fluids was near -45??? and their ??18O ranged from 7??? to -1???, depending on the degree of exchange in the alteration zone at low water-rock ratio, or mixing with unexchanged meteoric water. The low ??D values of some alunite samples are interpreted to result from postdepositional exchange with later ore fluids. Fluid exsolved fr om the magma at depth had ??DH2O and ??18OH2O values near -70??? and 10???, respectively. During and following migration to the top of the magma chamber, the fluid underwent isotopic exchange with the partially crystallized magma and its solid and cooler, but still plastic, carapace just below the transition from a lithostatic to hydrostatic pressure regime. These evolved magmatic fluids had ??DH2O and ??18OH2O values close to -40??? and 5???, respectively, prior to release into the superjacent hydrostatically pressured fracture zone, wherein the fluids separat

Chemical Geology↗

USGS2007

Regional uplift associated with continental large igneous provinces: The roles of mantle plumes and the lithosphere

The timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes. Here we review geological indicators of surface uplift associated with five continent-based magmatic provinces: Emeishan Traps (260??million years ago: Ma), Siberian Traps (251??Ma), Deccan Traps (65??Ma), North Atlantic (Phase 1, 61??Ma and Phase 2, 55??Ma), and Yellowstone (16??Ma to recent). All five magmatic provinces were associated with surface uplift. Surface uplift can be measured directly from sedimentary indicators of sea-level in the North Atlantic and from geomorpholocial indicators of relative uplift and tilting in Yellowstone. In the other provinces, surface uplift is inferred from the record of erosion. In the Deccan, North Atlantic and Emeishan provinces, transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma. Transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation. Observations of the spatial and temporal relationships between surface uplift, rifting and magmatism are also important in constraining models of LIP formation. Onset of surface uplift preceded magmatism in all five of the provinces. Biostratigraphic constraints on timing of uplift and erosion are best for the North Atlantic and Emeishan Provinces, where the time interval between significant uplift and first magmatism is less than 1??million years and 2.5??million years respectively. Rifting post-dates the earliest magmatism in the case of the North Atlantic Phase 1 and possibly in the case of Siberia. The relative age of onset of offshore rifting is not well constrained for the Deccan and the importance of rifting in controlling magmatism is disputed in the Emeishan and Yellowstone Provinces. In these examples, rifting is not a requirement for onset of LIP magmatism but melting rates are significantly increased when rifting occurs. Models that attempt to explain emplacement of these five LIPs without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift, rifting and magmatism. For example, small-scale convection related to craton or rift boundaries (edge-driven convection) cannot easily explain widespread (1000??km scale) transient surface uplift (Emeishan, Deccan, North Atlantic), and upper mantle convection initiated by differential incubation beneath cratons (the hotcell model) is at odds with rapid onset of surface uplift (Emeishan, North Atlantic). The start-up plume concept is still the most parsimonious way of explaining the observations presented here. However, observations of surface uplift cannot directly constrain the depth of origin of the hot mantle in a plume head. The short time interval between onset of transient surface uplift and magmatism in the North Atlantic and Emeishan means that the associated starting plume heads were probably not large (??? 1000??km diameter) roughly spherical diapirs and are likely to have formed narrow (??? 100??km radius) upwelling jets, with hot mantle then spreading rapidly outward within the asthenosphere. In cases where rifting post-dates magmatism (N Atlantic Phase 1) or where the degree of lithospheric extension may not have been great (Siberia), a secondary mechanism of lithospheric thinning, such as gravitational instability or delamination of the lower lithosphere, may be required to allow hot mantle to decompress sufficiently to explain the observed volume of magma with a shallow melting geochemical signature. Any such additional thinning mechanisms are probably a direct consequence of plume head emplacement. ?? 2007 Elsevier B.V. All rights reserved.

Chemical Geology↗

USGS2005

Evaluation of argon ages and integrity of fluid-inclusion compositions: Stepwise noble gas heating experiments on 1.87 Ga alunite from Tapajós Province, Brazil

Quantitative analyses are reported for active (N 2 , CH 4 , CO, CO 2 , H 2 , O 2 , HF, HCl, H 2 S, SO 2 ) and noble (He, Ar, Ne) gases released by crushing and step heating of magmatic-hydrothermal alunite from the Tapajós gold province in Brazil. This is the oldest known alunite ( 40 Ar/ 39 Ar age of 1.87 Ga), and because it has undergone minimal postdepositional thermal or tectonic strain, it is excellent material to test the retention of gas species in fluid inclusions and within the crystal structure over geological time. The gas compositions of a single sample, in combination with Ar age-spectrum data derived from stepwise heating of 10 related samples, have been used to constrain the limits of modification of primary gas compositions in fluid inclusions and the possible extent of the loss of radiogenic Ar. The observed variations in the isotopic compositions of He, Ne, and Ar released by stepwise heating have been used to identify the residence sites and determine the diffusion coefficients of the gases in the mineral. The data suggest that the only modification to primary gas compositions after entrapment in fluid inclusions and formation of the mineral is due to radiogenic and nucleogenic processes which affect the noble gas isotopic compositions. Three gas retention sites are recognized in alunite: (1) primary fluid inclusions, (2) crystal structure OH sites, and (3) crystal structure sulfate sites. Alunite undergoes OH loss at <500°C, and K-SO 4 structural decomposition occurs at >600°C. Fluid inclusions generally are ≤1 μm in diameter and have variable but high vapor/liquid ratios. The gases in inclusion fluids are quantitatively released in vacuo by heating at 200°C for &sim;1 h. In the inclusion fluids, H 2 O is 32 mol% of total gas, H 2 S/SO 2 ranges from approximately 4 to 2, and N 2 /Ar from 0.3 to 96.3. The presence of large amounts of H 2 and CO indicates disequilibrium among the gas species in the fluids. Helium abundance is 214 ppm. Helium from fluid inclusions ( R /Ra=19.5) makes up about 4% of the total helium, whereas He ( R /Ra=0.2–2.0) from the crystal structure makes up about 65% of the total. Argon from fluid inclusions has 40 Ar/ 36 Ar=584–629 and that from crystal structure sites is >9.6×10 4 . Most gases are released from fluid inclusions at 200 °C, whereas most Ar (≥95%) is released between 525 and 725°C. Argon released from fluid inclusions at 200°C has 38 Ar/ 36 Ar=0.0–0.064. In contrast, Ar released from the matrix of the mineral at high temperature has 38 Ar/ 36 Ar=3.6–14.7. This difference suggests that, since the formation of the alunite at 1.87 Ga, traces of Cl in the mineral structure have undergone Cl ( n , γ ) and 41 K ( n , α ) in situ reactions with neutrons derived from U–Th. The amount of 36 Ar production from Cl nucleogenic reactions used in correcting for atmospheric 40 Ar typically increases the calculated age by 1–5 m.y., which is generally an insignificant component of the determined Ar age. Decay of U–Th over this time contributes 4 He ( α ) buildup in the crystal structure K-SO 4 sites. Atmospheric corrected excess 21 Ne/ 22 Ne=0.028–0.409 indicates that nucleogenic Ne was also produced via ( α , n ) reactions in matrix sites. Diffusion coefficients and activation energies for the diffusion of Ar and He, as determined using Arrhenius plots, indicate two distinct groups definable by their differences in activation energies. Argon log D o =2.45 and 15.33, with activation energies of 225 and 465 kJ mol −1 , respectively; the diffusion of He in alunite is quantified with log D o =−4.33 and E =106.8 kJ mol −1 . Model calculations of simplistic 1/ e -folding times and diffusion distance–time curves indicate that He should remain in alunite for millions of years at ≤100°C, whereas at <200–220°C, the alunite will retain Ar almost indefinitely. The data demonstrate why alunite is suitable for Ar geochronological applications and also show that, unless the alunite is subjected to metamorphic deformation, the inclusion fluids should retain their primary compositions.

Tapajós Province↗

USGS1966

Observed variations in the strontium concentration of sea water

On samples obtained from five widely separated areas of the North Atlantic Ocean 91 determinations of strontium were made. These include surface and deep-water samples from both the cold North Atlantic area and the warm subtropical Caribbean area. The results differ from many of the previously reported values for strontium in sea water. It appears that the more correct value for soluble strontium in sea water is less than the commonly accepted figure of 8.0–8.2 mg/l and most likely lies between 7.2 and 7.8 mg/l. The use of an “average” figure for strontium in sea water should be discouraged as it is clear that the soluble strontium content in sea water does vary considerably; consequently the assumption of a “constant” strontium/chlorinity ratio in the ocean is in error.

Chemical Geology↗

USGS1969

Fractionation of gold in a differentiated tholeiitic dolerite

Gold content was determined, by neutron-activation analysis, in samples from a drill core through the Great Lake sheet, Tasmania, a differentiated tholeiitic dolerite. The gold content of parts of the core seems to be related to the mafic index. The variation of gold content with depth and mafic index is similar to that of copper, indicating that gold and copper may have been concomitantly crystallized from the magma.

Chemical Geology↗

USGS1969

Ion association in natural brines

Natural brines, both surface and subsurface, are highly associated aqueous solutions. Ion complexes in brines may be ion pairs in which the cation remains fully hydrated and the bond between the ions is essentially electrostatic, or coordination complexes in which one or more of the hydration water molecules are replaced by covalent bonds to the anion. Except for Cl − , the major simple ions in natural brines form ion pairs; trace and minor metals in brines form mainly coordination complexes. Limitations of the Debye-Hückel relations for activity coefficients and lack of data on definition and stability of all associated species in concentrated solutions tend to produce underestimates of the degree of ion association, except where the brines contain a very high proportion of Cl − . Data and calculations on closed basin brines of highly varied composition have been coupled with electrode measurements of single-ion activities in an attempt to quantify the degree of ion association. Such data emphasize the role of magnesium complexes. Trace metal contents of closed basin brines are related to complexes formed with major anions. Alkaline sulfo- or chlorocarbonate brines (western Great Basin) carry significant trace metal contents apparently as hydroxides or hydroxy polyions. Neutral high chloride brines (Bonneville Basin) are generally deficient in trace metals. With a knowledge of the thermodynamic properties of a natural water, many possible reactions with other phases (solids, gases, other liquids) may be predicted. A knowledge of these reactions is particularly important in the study of natural brines which may be saturated with many solid phases (silicates, carbonates, sulfates, etc.), which may have a high pH and bring about dissolution of other phases (silica, amphoteric hydroxides, CO 2 , etc.), and which because of their high density may form relatively stable interfaces with dilute waters.

Chemical Geology↗

USGS1969

Geochemistry of pore waters from Shell Oil Company drill holes on the continental slope of the northern Gulf of Mexico

Pore waters were analyzed from 6 holes drilled from M.V. “Eureka” as a part of the Shell Oil Co. deeper offshore study. The holes were drilled in water depths of 600–3,000 ft. (approximately 180–550 m) and penetrated up to 1,000 ft. (300 m) of Pliocene-Recent clayey sediments. Salt and anhydrite caprock was encountered in one diapiric structure on the continental slope. Samples from holes drilled near diapiric structures showed systematic increases of pore-water salinity with depth, suggestive of salt diffusion from underlying salt plugs. Anomalous concentrations of K and Br indicate that at least one plug contains late-stage evaporite minerals. Salinities approaching halite saturation were observed. Samples from holes away from diapiric structures showed little change in pore-water chemistry, except for loss of SO 4 and other variations attributable to early-stage diagenetic reactions with enclosing sediments. Thus, increased salt concentrations in even shallow sediments from this part of the Gulf appear to provide an indicator of salt masses at depth.

Chemical Geology↗

USGS1969

Interstitial brines in playa sediments

Study of several closed drainages in the Great Basin has shown that the interstitial solutions of shallow, fine-grained playa deposits store a large quantity of dissolved solids and are often more concentrated than associated lakes and ponds, except in peripheral zones of stream or ground-water inflow. These interstitial fluids, when compared with local runoff, impoundments, or spring waters, commonly have a distinctive ionic composition which sometimes cannot be explained by either simple mixing of surface and subsurface inflow or by evaporative concentration. At Abert Lake, Oregon, the interstitial solute concentrations increased with depth to values as much as five times greater than the lake, except where springs indicate significant ground-water input. Where Na + , Cl, and CO 2 species constitute more than 90% of the solutes, Na + Cl − "> Na+Cl− ratios in the lake water are lower than in interstitial solutions of bottom cores and higher than in playa fluids. At the same time, Na + K + "> Na+K+ ratios are highest in the fluids of lake bottom muds and lowest in playa interstitials. In deeper playa profiles, interstitial Na + Cl − "> Na+Cl− tended to decrease with depth (5 ft. maximum). In the Abert Lake area, as in other parts of the western Great Basin, Na + Cl − "> Na+Cl− ratios are indicative of total CO 2 in solution and the effects of organic decay in surficial sediments. These ratios, coupled with data on silica and bulk density, show that higher P CO2 accompanying decay promotes silicate dissolution and hydrogen ion exchange, stripping alkalis from sediment which had preferentially adsorbed K + when entering the lake. On subsequent loss of pore fluid in the playa regime, silica initially released to solution in the lake environment is readsorbed on dissolution products.

Oregon↗

USGS1969

Computer-produced tables, maps, and diagrams as tools in the interpretation of brine data from southeastern Kansas (U.S.A.)

The applicability of computer-oriented techniques to assist in the interpretation of brine data is demonstrated by the use of six programs on data from two example areas. These programs include a data tabling routine; routines for producing Stiff, Piper, and Ropes diagrams; programs for calculating ionic ratios; and programs for plotting maps of chemical data. The examples illustrate: (1) the range in water quality and contamination of potable water by brines in Stafford, Pratt, and Barber Counties, and (2) the change in water quality in the Arbuckle Group from southeast to south-central Kansas.

Kansas↗

USGS1972

Graphical methods for representing form and stability of aqueous metal ions

The equilibrium distributions of solute species of aluminum at 25°C and one atmosphere pressure are shown graphically in systems containing fluoride, as functions of the total dissolved aluminum and fluoride. The predominant form of complex and degree of complexing are also shown graphically as functions of pH and fluoride activity. The graphs are based on the simultaneous solution of the equations representing nine complexing equilibria and three stoichiometric summarizations, using a fixed value of ionic strength equal to 0.1. Solubility relationships for aluminum hydroxide and cryolite also are shown graphically, using the same coordinates and additional equilibria. By overlaying an appropriate species distribution graph with a solubility graph a relatively complete summary of chemical relationships in an aqueous aluminum system can be obtained. Although this type of model has important limitations, it can accommodate enough variables simultaneously to have practical value and similar procedures could be used for other elements and systems of interest in low-temperature aqueous geochemistry.

Chemical Geology↗

USGS1973

The selenium content of U.S.G.S. standard rocks

Selenium was determined in duplicate portions from three bottles of six U.S.G.S. standard rocks by a spect rofluorimetric procedure. The following averages, as p.p.m. Se, were obtained: PCC-1, 0.031; GSP-1, 0.088; BCR-1, 0.12; SCo-1, 0.91; MAG-1, 1.3; and SGR-1, 3.7. One-way analysis of variance of the several sets of data showed no significant differences in the selenium content among bottles of any specific rock; these samples may be accepted as homogeneous for their selenium contents by this analytical method.

Chemical Geology↗

USGS1973

Point of zero charge of weathered forsterite

The point of zero charge of forsterite shifts from an estimated 8.9 for the unaltered mineral to 8.4 and 8.0, respectively, after being weathered artificially for 1 h and 4 h. These results corroborate the presence of magnesium-deficient surfaces on the weathered minerals, also indicated by chemical analyses of the solutions. H + and OH − are shown to be potential-determining ions in this system

Chemical Geology↗

USGS1974

Comparison of elemental accumulation rates between ferromanganese deposits and sediments in the South Pacific Ocean

Rates of accumulation of Fe and Mn, as well as Cu, Ni, Co, Pb, Zn, Hg, U and Th have been determined for five ferromanganese deposits from four localities in the South Pacific Ocean. Manganese is accumulating in nodules and crusts at a rate roughly equivalent to that found to be accumulating in sediments in the same area. Iron shows a deficiency in accumulation in nodules and crusts with respect to sediments, especially near the continents, but also in the central and south-central Pacific. Copper is accumulating in nodules and crusts at a rate one order of magnitude less than the surrounding sediments. This is interpreted as meaning that most of the Mn is supplied as an authigenic phase to both sediments and nodules while Fe is supplied mostly by ferromanganese micro-nodules and by detrital and adsorbed components of sediments; and Cu is enriched in sediments relative to nodules and crusts most probably through biological activity.

Chemical Geology↗

USGS1974

Iridium in some calcic and calc-alkalic batholithic rocks of the western United States

Neutron-activation determinations of Ir in rocks of the Southern California and Boulder (Montana) batholiths show that the Ir content is low and variable in both batholiths, extending over 1 to 2 orders of magnitude and having a combined median of 0.006 ppb Ir. The data suggest that the calc-alkalic Boulder batholith (0.002–0.5 ppb Ir, median 0.007) may be slightly enriched in Ir relative to the calcic Southern California batholith (< 0.001–0.12 ppb Ir, median 0.005). No conspicuous relationship between Ir content and differentiation parameters is evident; however, the Boulder batholith rocks do show a slight tendency to concentrate Ir in the more mafic rocks. Our results in conjunction with previous analyses of mafic rocks leads to a crustal-abundance estimate of 0.03 ppb Ir

Chemical Geology↗