Douglas G. Brookins

Rhenium as analog for fissiogenic technetium: Eh-pH diagram (25°C, 1 bar) constraints

Abstract Fissiogenic Tc, found in radioactive waste from nuclear reactions, presents potential risk to public health due to its high radioactivity, long half life, and high solubility in surface waters. In an attempt to more thoroughly predict behavior of Tc in natural media, Eh-pH diagrams for both Tc and its chemical analog, Re, are constructed and discussed. Although Re is a rare element, its behavior in rocks is fairly well known. Eh-pH diagrams constructed for Tc and Re show that both elements are readily soluble under the conditions which occur in surface and near-surface environments but these elements will be insoluble with reduced Eh values characteristic of subsurface conditions. In reducing conditions, with sulfur absent, Re and Tc will be present primarily as oxides. With sulfur present these elements will occur as sulfide minerals. The Eh-pH diagrams indicate that Tc and Re mobility will be minimal under such low Eh conditions.

Geochemical behavior of antimony, arsenic, cadmium and thallium: EhpH diagrams for 25°C, 1-bar pressure

New Eh-pH diagrams have been derived for Sb, As, Cd and Tl. These diagrams are useful in discussing the common association of several or all of these elements around epithermal Au deposits. Slight differences in pH, Eh or both pH-Eh are sufficient to account for partitioning of these elements from deposit to deposit. In addition, chloride concentration-pH diagrams for Au, Hg and Tl are useful to further discuss the role of chloride (and possibly chloride complexes) in the migration of some of these elements under low T,P conditions.

Sr isotopic disequilibrium in lherzolites from the Puerco necks, New Mexico

Abstract Sr isotopic ratios in coexisting minerals in a lherzolite inclusion from the Puerco necks, 60 miles west of Albuquerque, New Mexico, are significantly different. Petrographic and chemical data indicate that these inclusions are typical of other localities except that the composition of each mineral in the ultramafic inclusion appears to be independent of the mode and is homogeneous among dunites, lherzolites and websterites. The lherzolites have 87 Sr/ 86 Sr ratios higher than their host Pliocene basalts. It seems likely that the lherzolites are fragments of a part of the mantle which was not involved in any significant thermal event which could have restored isotopic equilibrium during Mesozoic time. Carters partial fusion-partial crystallization model for the origin of ultramafic inclusions is unacceptable for the Puerco inclusions.

Rare-earth element geochemistry of fluorite-carbonate deposits in western Montana, U.S.A.

Abstract The fluorite-carbonate-quartz deposits of Mineral, Missoula and Ravalli Counties in western Montana, U.S.A., at one time described as carbonatitic, have recently been shown to be hydrothermal in origin based on chemical, O- and Sr-isotope, and field studies. We report the results for rare-earth elements (REE) in the fluorite and carbonate minerals and discuss the implications for their origin. Fluorite from Lime Gulch, White Cloud, Swallow, Spar, Wilson Gulch and Snowbird deposits exhibits REE/chondrite patterns with a pronounced enrichment in the middle REE (MREE), highly depleted light REE (LREE) and slightly depleted Lu, often accompanied by a similar small depletion in Yb. La is depleted relative to Sm by an order of magnitude. Calcite is also MREE-enriched, although La is not as depleted relative to Sm as in the associated fluorite; dolomite does not show La depletion relative to Sm. Calcite and dolomite have negative Eu anomalies whereas some of the fluorite does not. REE patterns for the fluorite and carbonates are typical of those from other hydrothermal deposits reported in literature. Chemical zonation caused by complexation of MREE and HREE with carbonate and fluoride ions and a high degree of REE fractionation apparently produced the REE distribution patterns observed for these deposits.

“Pyroxene”—Ilmenite xenoliths from the stockdale pipe, kansas: Chemistry, crystallography, and origin

Abstract Comparison of serpentine-ilmenite xenoliths from the Stockdale kimberlite pipe, Kansas, with pyroxene-ilmenite xenoliths from African kimberlite pipes indicates both physical and chemical similarity. The intergrown phases are largely single crystals with the ilmenite plates parallel to the crystallographic basal plane of ilmenite. Orthopyroxene and clinopyroxene are oriented such that the b crystallographic axes are parallel to h0h0 of ilmenite. The results of textural and crystallographic studies do not definitively answer whether these intergrowths have formed by eutectic crystallization, discontinuous precipitation or eutectoid transformation.

Potassium-argon dating of polyhalite in southeastern New Mexico

Abstract Polyhalite, K 2 Ca 2 Mg (SO 4 ) 4 -2H 2 O, is an important mineral in many evaporites. Although its use for K-Ar dating has never been investigated, our results indicate that it is a very useful mineral for dating events ranging from the time of potash mineralization to any younger events which may have affected the evaporite. Five K-Ar dates on pure polyhalite, including two from included material and from beds distorted by the formation of a rubble chimney, yield dates between 198 and 216 Myp, in good agreement with Rb-Sr dates and the diagenetic age of the potash deposits from the same rocks. Two polyhalites mixed with sylvite gave lower dates (154 and 174 Myr) which is to be expected because of radiogenic 40 Ar loss from the sylvite phase. One polyhalite, formed after the intrusion of a 31 Myr lamprophyre dike, yielded 21 Myr. Collectively our results indicate that pure polyhalite is satisfactory for K-Ar dating and may provide critical age information in studies of the geologic history of the evaporite sequences.

Eh—pH diagrams for elements from Z = 40 to Z = 52: Application to the Oklo natural reactor, Gabon

Abstract Eh—pH diagrams for elements from Z = 40 (Zr) to Z = 52 (Te) have been constructed in order to comment on migration/retention of these elements at Oklo. Although data for fissiogenic amounts of some of these elements are lacking, where such data are available to agreement between predicted migration/retention based on the Eh—pH diagrams and actual measurement is excellent. Based on Eh—pH diagrams, migration (to what degree is uncertain) of Mo and Cd is predicted whereas retention of Y, Zr, Tc, Ru, Rh, Pd, Ag and Te is also predicted. An earlier report of Frejacques et al. of Tc migration is in disagreement with Eh—pH prediction, and recent (unpublished) data argue for Tc retention. In view of the agreement between prediction and observation, the possible migration of Sb and retention of In and Sn is proposed. These data again demonstrate the usefulness of Eh—pH diagrams for the Oklo fossil nuclear reactor but, more important, allow constraints to be placed on repositories for nuclear waste now under consideration.

Platinoid element Eh-pH diagrams (25 °C, 1 bar) in the systems MOHS with geochemical applications

Abstract Theoretical Eh-pH diagrams for the platinoids (Ru, Rh, Pd, Os, Ir, Pt) have been constructed for 25 °C, 1 bar conditions. Thermodynamic data from recent sources are adequate for these diagrams. With caution these diagrams may be applied to higher P , T conditions. These diagrams help explain the following geochemical observations: 1. (1) Ru and Os form M (IV) -sulfides but not M (II,III) -sulfides, hence Ru and Os are segregated somewhat from Ir (III,IV), Pd (II,IV) and Pt (II,IV) during magmatic sulfide formation. 2. (2) Rh occurs as native Rh and as Rh (III) -oxide, but forms no sulfides, thus it is segregated from the other platinoids during sulfide mineral formation. 3. (3) Deep-sea nodules are enriched in Pt over Pd because Pt (IV) forms a stable oxide whereas Pd does not. 4. (4) Pt is enriched over Os in magmatic sulfides [see (1) above], but much less so in sedimentary rocks due in part of existence of aqueous Os (VI, VIII) oxyions. 5. (5) Os is enriched in sedimentary rocks for the same reason over Ir. 6. (6) PtIr segregation is complex; under magmatic sulfide conditions PtS forms earlier than Ir 2 S 3 with Pt enriched over Ir, but under alkaline oxidizing conditions Ir as IrO − 4 may be mobile thus accounting for the extreme Pt/Ir variation in sedimentary rocks. 7. (7) In the iridium-anomaly clay zones from the KT boundary, the Os/Ir, Pt/Ir and Os/Pt ratios are near one as are these ratios in meteorites. These same ratios in sedimentary rocks vary widely, and only rarely approach 1. The Eh-pH diagrams suggest that no simple terrestrial mechanism(s) can result in these ratios being near one. 8. (8) The Eh-pH diagrams calculated are useful in addressing platinoid element behavior in rocks.

Geochemical constraints on underground disposal of uranium mill tailings

Abstract A three phase investigation has been conducted on groundwater quality impacts of the underground disposal of tailings from acid-leach milling of uranium ores. These phases included field collection and analysis of samples obtained during backfilling of mill tailings in empty underground mine stopes, collection of soil samples from mill tailings piles and previously backfilled stopes, and evaluation of thermodynamic constraints on possible geochemical transformations. Contaminants of principal concern include As, Mo, Se, U, V and Ra-226. The investigation has shown that short-term degradation of groundwater due to backfill disposal of the sand fraction of uranium tailings is negligible. Long-term effects, defined as those occurring after mining operations cease and the mine fills with water, are predicted to also be very small. This is attributed to immobilization of pollutants through chemical reduction and precipitation, as well as adsorption onto aquifer materials. This conclusion is substantiated, in part, by observation of high concentrations of most of the contaminants on the silt and clay fraction of the soil samples collected, in contrast to the concentrations found on the sand fraction.

Natural analogues for radwaste disposal: Elemental migration in igneous contact zones

Abstract Chemical studies of various igneous rocks intrusive into many different rocks have been examined to see if elemental transfer has taken place. Systems examined include: granite intrusive into shale and siltstone; basalt intrusive into granodiorite, diorite, rhyolite and dolomite; diabase intrusive into rhyolite and extrusive over arkose; rhyolite extrusive over basalt; lamprophyre intrusive into evaporites; monzonite intrusive into tuff and andesite; and quartz monzonite intrusive into metasedimentary rocks. The mechanism for elemental transport in these systems is due in part to diffusion and in part to chemical reaction between intrusive and host rocks. Evidence for transport of elements from intrusive into intruded rocks is noted for several elements. In some cases, the distribution of some elements is mineralogically controlled, and variations in Co, Cr, REEs, Ni, Cu, Zr, Mn, Ti, Rb, Sr, U and Th can be attributed to more-or-less specific reactions. In a few contact zones, there has been limited migration of Cs, but usually only within 0.5-1.0 m of the igneous rock contact. Thermal effects of the igneous intrusions are to reset some mineral isotopic systematics (loss of 40Ar, redistribution of ★87Sr, 18O), but otherwise the whole-rock chemistry usually remains unchanged. Collectively, the data for elemental behavior in the vicinity of the igneous contacts studied show that elemental migration of redistribution is often restricted to within 0.5-1.0 m of the contact, thus suggesting that for hypothetically stored radioactive waste canisters, emplaced at much lower temperatures, little if any elemental transport would take place in the event of breaching of the waste package.