Andrew R. Campbell

Lateral subsurface flow pathways in a semiarid Ponderosa pine hillslope

The mechanisms controlling lateral subsurface flow in semiarid environments have received relatively little attention despite the fact that lateral subsurface flow can be an important runoff process in these environments. The objective of the current study is to better understand lateral subsurface flow process in semiarid environments. Natural chloride, dissolved organic carbon, and stable isotope (δD and δ18O) tracers were used to investigate the lateral subsurface flow process and the chemical changes that occur as a result of lateral subsurface flow. Observed differences in chemistry between soil matrix water and lateral subsurface flow were large (for example, chloride concentrations in matrix soil water samples were >200 mg/L, compared with only 2 mg/L in lateral subsurface flow samples obtained at the same time). This difference in chemistry is indicative of a two-domain flow system in which macropores conduct lateral subsurface flow that is not in chemical or hydrological equilibrium with the soil matrix. The size of precipitation events appeared to have a strong influence on the variations in old/new water percentages, and examples of both old and new water dominated events were observed. There were also large variations in the chemistry of lateral subsurface flow with time. For example, chloride and dissolved organic carbon concentrations were 10 and 70 times greater, respectively, under saturated conditions than under unsaturated conditions.

Rhenium behavior in molybdenite in hypogene and near-surface environments: Implications for Re-Os geochronometry

Abstract Rhenium is concentrated mainly in molybdenite (MoS2) and occurs as a major cation in only a few rare minerals. This affinity makes molybdenite an ideal mineral for Re-Os geochronometry, but research on the behavior of Re in molybdenite is limited. Infrared microscope, XRD, back-scattered electron (BSE), and microprobe techniques have been used in this study to document Re behavior in molybdenite affected by hypogene and near-surface processes. In the hypogene environment, both 3R and 2H molybdenite can experience Re loss during hydrothermal alteration, which also causes increased infrared transparency (IR). Alteration at temperatures as low as ~ 150°C can cause Re loss in the presence of advecting fluids, and will affect Re-Os dating if it occurs long after primary mineralization. Re loss in 3R and 2H molybdenite under supergene conditions does not increase IR transparency. Rhenium is not incorporated into supergene ferrimolybdite, but is enriched in K-Al-silicate intergrowths which may be illite(?). These minute intergrowths are present in several samples and would not be detected in a simple optical examination. In the supergene environment elemental Os is stable, whereas Re is not. Rhenium in molybdenite may be removed by supergene fluids after some has decayed to 187Os, causing erroneously old ages, or it may be adsorbed into the illite intergrowths, creating ages which are too young. In the weathering environment, Proterozoic molybdenites have altered to Re-enriched powellite, which can be detected using back-scattered electron imagery. Combined microprobe, XRD, BSE, and infrared microscopy can be successfully used to detect alteration in molybdenite prior to dating. These techniques are non-destructive and should be performed before any molybdenite is dated by the Re-Os system.

Water movement in desert soil traced by hydrogen and oxygen isotopes, chloride, and chlorine-36, southern Arizona

Abstract Understanding soil water movement is important for water resource management and for analyzing pollutant transport in the vadose zone. Seasonal variations in shallow soil profiles may have profound effects on deep soil water and solutes. We have investigated seasonal soil water movement in the top meter of undisturbed desert soil in the southern Arizona using 2 H, 18 O, Cl, and bomb- 36 C1. Six soil profiles were sampled from a young terrace (Holocene) during 1991 to 1992 on a seasonal basis. One soil profile was sampled from an older terrace (latest Pleistocene) in October 1992, and one from a yet older fan surface (late Pleistocene) in March 1992. The results indicate that repeated seasonal cyclic movement of soil water in the top 60–80 cm active zone tends to produce a consistent stable isotope composition for the soil water below the active zone in different seasons. The deep δ 18 O on the oldest surface is about 6‰. heavier than on the younger surfaces, probably resulting from greater evaporative loss due to the larger proportion of fines in the older soil. This deep isotopic composition is closely related to the composition of the local average annual precipitation. An annual average evaporation rate of 35 mm year −1 was estimated using a steady-state diffusion model, whereas the actual regional evapotranspiration rate is 175–250 mm year −1 . This suggests that about 80% of the soil water is lost through transpiration and first-stage evaporation before quasi steady-state is reached. The long-term average infiltration rate below the active zone is about 4 mm year −1 for the younger terrace, based on both chloride mass balance and bomb 36 Cl, and 0.02 mm year −1 for the older terrace and the fan surface, based on chloride mass balance. The correlation between the deep δ 18 O, the reduction of downward flux and the surface age suggests that increasing soil development has resulted in a significant decrease of soil water infiltration and hence an increase in evaporation and surface runoff.

REE composition of an aqueous magmatic fluid: A fluid inclusion study from the Capitan Pluton, New Mexico, U.S.A.

Abstract The REE content of aqueous magmatic-derived fluids trapped in fluid inclusions, has been determined by ICP-MS after crush-leach extraction of the fluids in 4 samples. The total REE concentration varies between 200 and 1300 ppm and is dominated by the LREE, especially La, Ce and Nd. Fluids were released at different times from a melt, which changed composition as it underwent fractional crystallisation, and this is reflected in the concentration of REE in solution. Early formed quartz-fluorite veins, hosted by granophyre, contain the highest concentration of REE, and appear to be in equilibrium with aplite melt from which the fluid was inferred to have been derived since calculated fluid/melt distribution coefficients are in broad agreement with experimentally derived values. Variation in the REE content of the fluids is independent of salinity which remains constant at ∼ 80 wt% total salts. Later veins, hosted by aplite, contain fluid derived from a porphyritic melt and have lower REE concentrations, reflecting the greater incorporation of REE into mineral phases crystallising from the melt: titanite and allanite occur in these later veins. REE mineral/fluid distribution coefficients have been calculated for these minerals and show there is a strong preference for REE to partition into the minerals.

Stable carbon and oxygen isotopes of pedogenic carbonates, Ajo Mountains, southern Arizona: implications for paleoenvironmental change

Abstract In this study we compare the stable-isotope composition of late Holocene pedogenic carbonate from a late Holocene soil developed on the pediment below the Ajo Mountains in southern Arizona (USA) with measurements of the δ 18 O of the soil water throughout the seasonal cycle and with δ 18 O and δ 13 C measurements on older, dated soils. The comparison with the soil-water composition shows that the pedogenic carbonate is forming in equilibrium with highly evaporated soil water at the highest summer temperatures. The δ 13 C of the carbonate apparently reflects equilibrium with CO 2 of an isotopic composition consistent with the present mixture of C3 desert shrubs and CAM cactus. Comparison with glacial-period soil carbonates shows that there has been little change in the δ 18 O, but that the δ 13 C was about 4.5‰ heavier. Independent evidence indicates that glacial-period summer temperatures were probably much cooler, and that the δ 18 O of precipitation was also lighter. The increase in the carbonate-water fractionation factor with decreasing temperature probably counteracted the decrease in the δ 18 O of precipitation, producing little net change in the δ 18 O of the soil carbonate. We attribute the decrease in the δ 13 C to replacement of a glacial-period C4 grassland by Holocene C3/CAM desert shrubs and succulents. This replacement was probably in response to an increase in temperature and reduction in summer precipitation at the end of the last glacial period. The δ 13 C of older soil carbonates indicates that landscape was dominated by grassland for most of the past 700 ka and that the present vegetation likely represents a relatively brief anomaly.

Microthermometry of enargite-hosted fluid inclusions from the Lepanto, Philippines, high-sulfidation CuAu deposit

Abstract The spatial relation between porphyry and high-sulfidation epithermal deposits is particularly well revealed in the Mankayan mineral district of northern Luzon, Philippines, where the Lepanto high-sulfidation Cu Au deposit lies over and adjacent to the Far Southeast (FSE) porphyry Cu Au deposit. Consequently, a study was undertaken to characterize the fluids responsible for epithermal mineralization in this environment. The ore stage at Lepanto consists of enargite-luzonite (Cu 3 AsS 4 ), pyrite, tennantite-tetrahedrite, and chalcopyrite. Infrared petrography of the enargite reveals variable transparency, with growth banding and twinning visible in euhedral specimens. Two phase (liquid > vapor) fluid inclusions occur as primary and secondary types ranging from T h ) of fluid inclusions in enargite were measured from within the lateral (3.0 km) and vertical (0.5 km) extent of the enargite mineralization. These values show a cooling trend toward the northwest, away from the area over the porphyry deposit, with average T h ranging from 285°C (proximal) to 166°C (distal). Ice melting temperatures ( T m ) were measured using a cycling technique, as ice was usually not visible in frozen inclusions. Apparent salinities range from 4.5 to 0.2 eq. wt% NaCl, with samples from the margins of the deposit showing a general decrease in apparent salinity with lower T h . Secondary fluid inclusions in quartz phenocrysts tend to have a higher average T h and lower apparent salinities compared to enargite-hosted inclusion fluids from the same locations. Several samples of pyrite are also transparent to IR radiation, and show internal features such as growth banding, and in one instance a two phase (liquid > vapor) fluid inclusion. This inclusion yielded a salinity of 1.2 eq. wt% NaCl. There is a large discrepancy in T h and apparent salinities between the enargite mineralization and the subjacent porphyry deposit despite contemporaneous formation, suggesting that the hot, (>500°C) hypersaline, magmatic brines associated with porphyry mineralization did not directly ascend to the elevation of the enargite deposit. However, absorbtion of magmatic vapors into overlying meteoric water may have created the mineralizing fluid of the Lepanto deposit. As this liquid moved to the northwest along the Lepanto fault, it was cooled and diluted by mixing with groundwater, resulting in enargite deposition.

Interstadial climatic cycles: A link between western North America and Greenland?

During the interval 33.6 to 26.1 ka, Searles Lake in southeastern California went through six major cycles of expansion and contraction. A comparison of U/Th ages for these events with the chronologies for quasi-cyclic interstadial episodes in the ice core from Summit, Greenland, suggests that the episodes of low water at Searles Lake are synchronous with the interstadial episodes at Summit. The two phenomena may be linked by variations in the strength of the global hydrological cycle, driven by oscillations in the Atlantic Ocean thermohaline circulation.

Geochemistry of southwestern New Mexico fluorite occurrences implications for precious metals exploration in fluorite-bearing systems

Abstract Fluorite-bearing Cu–Au, Ag–Au, and Ag–Pb–Zn deposits comprise a geologically and genetically varied group of mineral deposits in southwestern New Mexico. Additionally, several small to moderate tonnage former fluorite orebodies and numerous non-economic fluorite occurrences are distributed throughout much of New Mexico, western Texas, and northern Sonora, Mexico. We differentiate fluorite-bearing deposit types utilizing variations in fluorite geochemistry. Several types of data were collected by fluid inclusion microthermometry, δD and δ18O analysis of inclusion fluids, and trace element analysis of fluorite separates. Fluid inclusion microthermometry indicates that fluorite in most of the veins formed from low salinity (0–7 eq. wt.% NaCl) fluids at temperatures between 100° and 220°C. A small number of vein and replacement deposits hosted by limestone or granite, including the Hansonburg Mississippi Valley Type (MVT) deposit, and the Zuni Mountains deposits, formed at similar temperatures, but from moderately saline (10–20 eq. wt.% NaCl) fluids. Magmatic fluids (>600°C, 80 eq. wt.% NaCl), are trapped in fluorite veins in the Capitan pluton. δD and δ18O values of inclusion fluids range, respectively, from −108 to −27‰ and from −9.9 to 7.3‰ relative to SMOW. Horizontal to subhorizontal trends in δD versus δ18O space suggest that variably-exchanged meteoric waters were responsible for fluorite deposition in most deposits. Trace elements show a broad range in concentration from tenths to hundreds of parts per million. The sediment-hosted fluorites in this study tend to have low ΣREE concentrations relative to fluorite hosted by igneous rocks. Limestone-hosted fluorites associated with felsic intrusive rocks have high ΣREE concentrations relative to all other sediment-hosted fluorites. Signatures indicative of precious metals mineralization are present in low-salinity epithermal systems containing Au and/or Ag. These are characterized by small and/or isolated ranges in Sr concentrations, Sc/Eu ratios, (Tb/Yb)n ratios, and Eu anomalies. Two apparently barren fluorospar districts in southwestern New Mexico contain fluorite with signatures indicative of precious metals mineralization and, therefore, it is suggested that precious metals mineralization may also be present within these districts.

Comparison of fluid inclusions in coexisting (cogenetic?) wolframite, cassiterite, and quartz from St. Michael's Mount and Cligga Head, Cornwall, England☆

Abstract The St. Michaels Mount occurrence and Cligga Head deposit are typical examples of W-Sn mineralization in the Cornwall district in S.W. England. Mineralization consists of quartz, wolframite, cassiterite, and various amounts of base metal sulfides in sheeted veins. Associated alteration includes greisenization and argillization. In both areas textural evidence for contemporaneous deposition of the ore minerals, wolframite and cassiterite, with quartz is generally ambiguous. Previous studies state that the quartz and wolframite are intergrown, but do not address the genetic relation between them. In this study microthermometry was performed on fluid inclusions in wolframite, cassiterite, and quartz. The inclusions in wolframite were observed using an infrared microscope and those in cassiterite and quartz were observed with visible light. At St. Michaels Mount Th values for primary inclusions in quartz average 311°C and in wolframite average 369°C. Salinities average 7.3 eq. wt% NaCl in quartz and 4.2 eq. wt% NaCl in wolframite. At Cligga Head, the average Th values for primary inclusions in the various minerals are: cassiterite, 352°C; wolframite, 324°C; and quartz, 295°C. Fluid inclusion salinities average 5.3 eq. wt% NaCl in cassiterite, 3.9 eq. wt% in wolframite, and 6.0 eq. wt% NaCl in quartz. The magnitude of the changes in Th measurements between minerals cannot be accounted for by post trapping changes and therefore must be due to actual differences in the temperatures of deposition. These data suggest that the ore minerals, wolframite and cassiterite, were deposited earlier than the associated quartz, even though conclusive textural evidence is lacking. Wolframite and cassiterite were deposited at about the same temperature at St. Michaels Mount but at Cligga Head cassiterite was deposited at higher temperatures than wolframite. Certainly in these areas the gangue minerals do not provide reasonable estimates of the depositional conditions of the ore minerals even though they are intimately intergrown.

The effects of K-metasomatism on the mineralogy and geochemistry of silicic ignimbrites near Socorro, New Mexico

Abstract K-metasomatism of the upper Lemitar and Hells Mesa silicic ignimbrites near Socorro, New Mexico is thought to be the result of downward percolation of alkaline, saline brines in a hydrologically closed basin [Chapin, C.E., Lindley, J.I., 1986. Potassium metasomatism of igneous and sedimentary rocks in detachment terranes and other sedimentary basins: economic implications. Arizona Geological Society Digest, XVI, 118–126.]. During the chemical changes associated with metasomatism, Na-rich phases, primarily plagioclase, are replaced by secondary mineral phases. Adularia, a low-temperature K-feldspar, is the dominant mineral formed during K-metasomatic alteration, but mixed-layer I/S, discrete smectite, and kaolinite can also be present in the assemblage. The formation of discrete smectite within the assemblage during K-metasomatism may have occurred during periods of low cation/H + ratios in the alkaline, saline brine. Upon an increase in the cation/H + ratio, such as could occur during evaporation of the alkaline lake, the solution may have become sufficiently concentrated to cause illitization of smectite resulting in the formation of mixed-layer I/S within the assemblage. Distribution of phases in the alteration assemblage strongly suggests a dissolution–precipitation reaction for K-metasomatism in the Socorro area as indicated by the presence of dissolution embayments in plagioclase crystals, the presence of euhedral adularia, and the common occurrence of authigenic clay minerals in the assemblage. K-metasomatism causes significant chemical modification of the silicic ignimbrite, particularly increases in K 2 O and Rb and depletion of Na 2 O and Sr with increasing adularia abundance. The correlation between Rb and K 2 O suggests that Rb is enriched during alteration due to substitution for K in adularia. The effect of hydrothermal activity, either prior to, or following metasomatism, is also observed in some samples, as shown by high concentrations of elements such as Ba, As, Sb, Pb and Cs. Enrichment of middle and HREE in the upper Lemitar Tuff samples and depletion of middle and HREE in Hells Mesa Tuff samples suggests attempted re-equilibration between the secondary alteration assemblage and the metasomatizing fluid. Preliminary data indicate clay minerals within the secondary assemblage may have played an important role in the incorporation of REE during redistribution.