Stable isotopic signature of a palaeoaquifer, Mississippian Alamogordo Member limestones, Sacramento Mountains, New Mexico, USA

Nine stratigraphic sections, each ≈5 m thick, were sampled from the Alamogordo Member limestones of the Lake Valley Formation, Sacramento Mountains, New Mexico, USA. Four stratigraphic sections consist entirely of lime mudstone and wackestone, whereas the other five sections have a prominent layer of crinoidal packstone about 1 m thick at their base. Stable isotopic analyses reveal that the lime muds in the sections with basal packstone layers show a downward decrease in δ¹⁸O and constant δ¹³C values, whereas those in the sections solely composed of lime mudstone and wackestone have, in general, relatively uniform δ¹⁸O and δ¹³C values. The diagenesis of the Alamogordo Member limestones was previously believed to have been governed by the downward percolation of meteoric water from a regional pre-Pennsylvanian exposure surface ≈100 m above this unit. However, the uniform δ¹³C and downward decrease in δ¹⁸O values in the lime muds in the sections with basal packstones indicate that the meteoric water ascended within the Alamogordo Member, rather than descended from the overlying exposure surface. This indicates that the basal packstones were probably a conduit for meteoric water. This is further supported indirectly by the relatively uniform δ¹⁸O and δ¹³C values of the lime mud in the sections without basal packstones. The implications are that the oxygen isotopic gradients may be used to identify palaeoaquifers, flow directions within these aquifers and that meteoric diagenesis below an exposure surface could be governed by flow through a palaeoaquifer.     

The sources and budget for dissolved sulfate in a fractured carbonate aquifer,southern Sacramento Mountains,New Mexico,USA

Climate change in the SW USA is likely to involve drier conditions and higher surface temperatures. In order to better understand the evolution of water chemistry and the sources of aqueous SO₄ in these semi-arid settings, chemical and S isotope compositions were determined of springs, groundwater, and bedrock associated with a Permian fractured carbonate aquifer located in the southern Sacramento Mountains, New Mexico, USA. The results suggest that the evolution of water chemistry in the semi-arid carbonate aquifer is mainly controlled by dedolomitization of bedrock, which was magnified by increasing temperature and increasing dissolution of gypsum/anhydrite along the groundwater flow path. The δ³⁴S of dissolved SO₄ in spring and groundwater samples varied from +9.0‰ to +12.8‰, reflecting the mixing of SO₄ from the dissolution of Permian gypsum/anhydrite (+12.3‰ to +13.4‰) and oxidation of sulfide minerals (−24.5‰ to −4.2‰). According to S isotope mass balance constraints, the contribution of sulfide-derived SO₄ was considerable in the High Mountain recharge areas, accounting for up to ∼10% of the total SO₄ load. However, sulfide weathering decreased in importance in the lower reaches of the watershed. A smaller SO₄ input of ∼2–4% was contributed by atmospheric wet deposition. This study implies that the δ³⁴S variation of SO₄ in semi-arid environments can be complex, but that S isotopes can be used to distinguish among the different sources of weathering. Here it was found that H₂SO₄ dissolution due to sulfide oxidation contributes up to 5% of the total carbonate weathering budget, while most of the SO₄ is released from bedrock sources during dedolomitization.     

Chertification in the Mississippian Lake Valley Formation, Sacramento Mountains, New Mexico

Chert distribution in the Lake Valley rocks is selective to mud-supported facies; it is not related to proximity to unconformities. The facies selectivity of the chertification is believed to be a function of the depositional distribution of indigenous silica as sponge spicules, an interpretation that is supported by high positive qualitative correlation of chert with spiculitic rocks. Petrography indicates that the spicules were all originally siliceous, and that they all went through a moldic stage during which many molds were compactively destroyed and distorted. Remaining molds were subsequently cemented by calcite or chalcedony. Chert distribution and spicule petrography argue for an intraformational source for much of the silica. Chert micro-fabrics are dominated by microquartz, a replacement of grains and lime mud; length-fast chalcedony, a pore-filling cement; and megaquartz, a post-chalcedony pore-filling cement. Petrography of compaction features within chert masses indicates that chertification occurred after some burial. Based on stratigraphic reconstruction this burial depth was a maximum of about 215 m. and was most likely a few metres to a few tens of metres. Petrography of chert-calcite cement relationships indicates that chertification occurred before and during first generation, pre-Pennsylvanian non-ferroan calcite cementation, and was completed before late-stage, post-Mississippian ferroan calcite precipitation. Petrography of chert clasts in basal Rancheria (Meramecian) and basal Pennsylvanian conglomerates proves these clasts derived from the Lake Valley Formation and were chertified before redeposition. Thus, some cherts in the Lake Valley are pre-Meramecian in age, but all are pre-Pennsylvanian in age. Furthermore, association of the cherts with the non-ferroan cements indicates the cherts were probably precipitated in meteoric phreatic lens established beneath the pre-Meramecian and pre-Pennsylvanian subaerial unconformities.     

Biogeographic implications of a packrat midden sequence from the Sacramento Mountains, south-central New Mexico

Thirteen packrat (Neotoma spp.) and two porcupine (Erethizon dorsatum) middens from 1555 to 1690 m elevation from the Sacramento Mountains, New Mexico, provide an 18,000-yr vegetation record in the northern Chiuahuan Desert. The vegetation sequence is a mesic, Wisconsin fullglacial (18,000–16,000 yr B.P.) pinyon-juniper-oak woodland; a xeric, early Holocene (ca. 11,000–8000 yr B.P.) juniper-oak woodland; a middle Holocene (ca. 8000-4000 yr B.P.) desert-grassland; and a late Holocene (ca. 4000 yr B.P. to present) Chihuahuan desertscrub. The frequency of spring freezes and summer droughts in the late Wisconsin probably set the northern limits of Pinus edulis and Juniperus monosperma at about 34°N, or 6° south of today's limit. Rising summer tempratures in the early Holocene eliminated pinyon and other mesic woodland plants from the desert lowlands and allowed the woodland to move upslope. At this time pinyon-juniper woodland and pine forest dominated by Pinus ponderosa probably began their spectacular Holocene expansions to the north. Continued warming in the middle Holocene led to very warm summers with strong monsoons, relatively dry, cold winters, and widespread desert-grasslands. Desertscrub communities in the northern Chihuahuan Desert did not develop until the late Holocene when the biseasonal rainfall shifted slightly back toward the winter, catastrophic winter freezes decreased, and droughts in all seasons increased. The creosote bush desertscrub corridor across the Continental Divide between the Chihuahuan and Sonoran deserts was probably connected for the first time since the last interglaciation.     

A new method for analyzing the effects of joints and stratigraphy on spring locations: a case study from the Sacramento Mountains, south central New Mexico, USA

A novel method for analyzing the effects that stratigraphic layers and joints have on spring locations is presented, with specific relevance for the southern Sacramento Mountains, south central New Mexico, USA. Bedrock in this heavily vegetated mountain block is mostly covered by alluvium and soil, making direct field observations difficult, and necessitating alternative methods. Elevations of a mapped geologic contact are used to produce a three-dimensional stratigraphic surface. A histogram of spring stratigraphic positions, calculated from this surface, indicates zones of preferential groundwater flow. Field measurements are combined with GIS analyses to quantify joint control on spring locations within stream valleys, and these results are compared to traditional lineament analysis. Stream valleys are parallel to joints observed in the field, and joint-parallel segments selected from digital elevation model (DEM)-mapped streams are up to 900 m in length. The abundance of springs throughout the study area decreases logarithmically with distance from these joint-parallel stream segments. Springs are more than twice as abundant within 25 m of joint-parallel stream segments compared with manually mapped lineaments from air-photos. Higher correlation with spring locations and significant time reduction by using GIS to select joint-parallel streams for lineament interpretation indicate that the method provides a good alternative to manual lineament analysis.     

Quantity and location of groundwater recharge in the Sacramento Mountains,south-central New Mexico (USA), and their relation to the adjacent Roswell Artesian Basin

The Sacramento Mountains and the adjacent Roswell Artesian Basin, in south-central New Mexico (USA), comprise a regional hydrologic system, wherein recharge in the mountains ultimately supplies water to the confined basin aquifer. Geologic, hydrologic, geochemical, and climatologic data were used to delineate the area of recharge in the southern Sacramento Mountains. The water-table fluctuation and chloride mass-balance methods were used to quantify recharge over a range of spatial and temporal scales. Extrapolation of the quantitative recharge estimates to the entire Sacramento Mountains region allowed comparison with previous recharge estimates for the northern Sacramento Mountains and the Roswell Artesian Basin. Recharge in the Sacramento Mountains is estimated to range from 159.86?×?10⁶ to 209.42?×?10⁶ m³/year. Both the location of recharge and range in estimates is consistent with previous work that suggests that ~75 % of the recharge to the confined aquifer in the Roswell Artesian Basin has moved downgradient through the Yeso Formation from distal recharge areas in the Sacramento Mountains. A smaller recharge component is derived from infiltration of streamflow beneath the major drainages that cross the Pecos Slope, but in the southern Sacramento Mountains much of this water is ultimately derived from spring discharge. Direct recharge across the Pecos Slope between the mountains and the confined basin aquifer is much smaller than either of the other two components.     

Petrogenesis of Pre-caldera Mafic Lavas, Jemez Mountains Volcanic Field (New Mexico, USA)

The Miocene–Quaternary Jemez Mountains volcanic field(JMVF), the site of the Valles caldera, lies at the intersectionof the Jemez lineament, a Proterozoic suture, and the CenozoicRio Grande rift. Parental magmas are of two types: K-depletedsilica-undersaturated, derived from the partial melting of lithosphericmantle with residual amphibole, and tholeiitic, derived fromeither asthenospheric or lithospheric mantle. Variability insilica-undersaturated basalts reflects contributions of meltsderived from lherzolitic and pyroxenitic mantle, representingheterogeneous lithosphere associated with the suture. The Kdepletion is inherited by fractionated, crustally contaminatedderivatives (hawaiites and mugearites), leading to distinctiveincompatible trace element signatures, with Th/(Nb,Ta) and La/(Nb,Ta)greater than, but K/(Nb,Ta) similar to, Bulk Silicate Earth.These compositions dominate the mafic and intermediate lavas,and the JMVF is therefore derived largely, and perhaps entirely,from melting of fertile continental Jemez lineament lithosphereduring rift-related extension. Significant variations in Pband Nd isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 17·20–18·93;¹⁴³Nd/¹⁴⁴Nd = 0·51244–0·51272) result fromcrustal contamination, whereas ⁸⁷Sr/⁸⁶Sr is low and relativelyuniform (0·7040–0·7048). We compare theeffects of contamination by low-⁸⁷Sr/⁸⁶Sr crust with assimilationof high-⁸⁷Sr/⁸⁶Sr granitoid by partial melting, with Sr retainedin a feldspathic residue. Both models satisfactorily reproducethe isotopic features of the rocks, but the lack of a measurableEu anomaly in most JMVF mafic lavas is difficult to reconcilewith a major role for residual plagioclase during petrogenesis. KEY WORDS: Jemez Mountains volcanic field; Rio Grande rift; lithospheric mantle; crustal contamination; trace elements; radiogenic isotopes     

新疆东天山玉海铜(钼)矿床流体包裹体和稳定同位素研究

玉海铜(钼)矿床成矿岩体为石英闪长(玢)岩,矿化呈细脉状、细脉-浸染状和稀疏浸染状。围岩蚀变主要为钾硅酸盐化、石英-绢云母化、青磐岩化和黏土化蚀变。矿床类型为斑岩型。铜(钼)矿化主要发育于钾硅酸盐化阶段、石英-绢云母化阶段和青磐岩化阶段。流体包裹体可划分为气液两相包裹体、含子晶三相包裹体和CO_2包裹体3种类型。钾硅酸盐化阶段的均一温度为307~423℃,盐度w(NaCleq)为4.18%~10.11%,密度0.62~0.77g/cm~3,属于高温、中-低盐度流体;石英-绢云母化阶段均一温度为172~336℃,盐度为w(NaCleq)为3.23%~8.55%,密度0.70~0.93 g/cm~3,属于中温、低盐度流体;晚期青磐岩化阶段均一温度155~296℃,盐度w(NaCleq)为3.71%~9.08%,密度0.80~0.96 g/cm~3,属于中低温、低盐度流体。从早阶段到晚阶段,成矿流体温度逐渐下降,各成矿阶段成矿流体盐度均小于11%,但钾硅酸盐化阶段成矿流体盐度稍高。石英-绢云母化阶段成矿流体δD=-91.6‰~-72.1‰,δ~(18)OH_2O=-1.8‰~6.3‰;青磐岩化阶段成矿流体δD=-97.1‰~-68.3‰,δ~(18)OH_2O=-6.3‰~2.2‰;成矿流体具有岩浆水和大气降水混合特征,但青磐岩化阶段大气降水含量更高。硫化物的δ~(34)S值为-3.5‰~2.8‰,硫来自石英闪长(玢)岩。     

Tectonic evolution of Proterozoic rocks in the Cimarron Mountains, northern New Mexico, USA

Abstract Portions of three Proterozoic tectonostratigraphic sequences are exposed in the Cimarron Mountains of New Mexico. The Cimarron River tectonic unit has affinities to a convergent margin plutonic/volcanic complex. Igneous hornblende from a quartz diorite stock records an emplacement pressure of 2–2.6 kbar. Rocks within this unit were subsequently deformed during a greenschist facies regional metamorphism at 4–5 kbar and 330 ± 50° C. The Tolby Meadow tectonic unit consists of quartzite and schist. Mineral assemblages are indicative of regional metamorphism at pressures near 4 kbar and temperatures of 520 ± 20° C. A low-angle ductile shear zone separates this succession from gneisses of the structurally underlying Eagle Nest tectonic unit. Gneissic granite yields hornblende pressures of 6–8 kbar. Pelitic gneiss records regional metamorphic conditions of 6–7 kbar and 705 ± 15° C, overprinted by retrogression at 4 kbar and 530 ± 10° C. Comparison of metamorphic and retrograde conditions indicates a P–T path dominated by decompression and cooling. The low-angle ductile shear zone represents an extensional structure which was active during metamorphism. This extension juxtaposed the Tolby Meadow and Eagle Nest units at 4 kbar and 520° C. Both units were later overprinted by folding and low-grade metamorphism, and then were emplaced against the Cimarron River tectonic unit by right-slip movement along the steeply dipping Fowler Pass shear zone. An argon isotope-correlation age obtained from igneous hornblende dates plutonism in the Cimarron River unit at 1678 Ma. Muscovite associated with the greenschist facies metamorphic overprint yields a ⁴⁰ Ar/³⁹ Ar plateau age of 1350 Ma. By contrast, rocks within the Tolby Meadow and Eagle Nest units yield significantly younger argon cooling ages. Hornblende isotope-correlation ages of 1394–1398 Ma are interpreted to date cooling during middle Proterozoic extension. Muscovite plateau ages of 1267–1257 Ma appear to date cooling from the low-grade metamorphic overprint. The latest ductile movement along the Fowler Pass shear zone post-dated these cooling ages. Argon released from muscovites of the Eagle Nest/Tolby Meadow composite unit, at low experimental temperatures, yields apparent ages of c. 1100 Ma. Similar ages are not obtained north-east of the Fowler Pass shear zone, suggesting movement more recently than 1100 Ma.     

Evaluation of groundwater residence time in a high mountain aquifer system (Sacramento Mountains,USA): insights gained from use of multiple environmental tracers

The New Mexico Bureau of Geology and Mineral Resources (USA) has conducted a regional investigation of groundwater residence time within the southern Sacramento Mountains aquifer system using multiple environmental tracers. Results of the tracer surveys indicate that groundwater in the southern Sacramento Mountains ranges in age from less than 1 year to greater than 50 years, although the calculated ages contain uncertainties and vary significantly depending on which tracer is used. A distinctive feature of the results is discordance among the methods used to date groundwater in the study area. This apparent ambiguity results from the effects of a thick unsaturated zone, which produces non-conservative behavior among the dissolved gas tracers, and the heterogeneous character and semi-karstic nature of the aquifer system, which may yield water from matrix porosity, fractures, solution-enlarged conduits, or a combination of the three. The data also indicate mixing of groundwater from two or more sources, including recent recharge originating from precipitation at high elevations, old groundwater stored in the matrix, and pre-modern groundwater upwelling along fault zones. The tracer data have also been influenced by surface-water/groundwater exchange via losing streams and lower elevation springs (groundwater recycling). This study highlights the importance of using multiple tracers when conducting large-scale investigations of a heterogeneous aquifer system, and sheds light on characteristics of groundwater flow systems that can produce discrepancies in calculations of groundwater age.     

北秦岭南台钼多金属矿床成矿流体和稳定同位素研究

南台钼多金属矿床产于北秦岭构造带宽坪群内,矿体主要呈似层状、透镜体状赋存于花岗斑岩内及其与宽坪群大理岩的接触带。矿化主要呈浸染状、团块状和细脉浸染状。围岩蚀变主要为硅化-钾硅酸盐化、矽卡岩化和碳酸盐岩化。矿床的形成经历了高温蚀变-矽卡岩期和石英-硫化物期,钼多金属矿化主要形成于石英-硫化物期。斑晶石英和辉钼矿-石英脉中主要发育4种流体包裹体：L型富液相包裹体、V型富气相包裹体、C型含CO₂包裹体和S型含子晶多相包裹体。早阶段斑晶石英中发育140～200℃、220～280℃、340～400℃ 3个均一温度区间,主成矿期辉钼矿-石英脉中发育120～180℃、200～240℃和280～380℃ 3个均一温度区间,晚阶段无矿石英脉中仅发育120～240℃一个低温区间。早阶段斑晶石英中的包裹体盐度显示57.90%～>73.96%、30.06%～38.01%、3.39%～18.55% 3个不连续的区间,主成矿期的辉钼矿石英脉和晚期无矿石英脉中的盐度范围分别为0.43%～12.85%、1.91%～10.73%。在成矿早阶段的斑晶石英和主成矿期石英辉钼矿脉的3个温度区间,均出现S型、C型、L型、V型等两种或两种以上包裹体共存且均一温度相近,流体沸腾作用明显,表明流体的多次沸腾是南台钼多金属矿床矿质沉淀的重要机制,这一机制与北秦岭秋树湾铜(钼)矿床的成矿机制相似。硫化物的δ³⁴S值集中于-0.3‰～7.2‰,平均3.1‰,表明硫来自深部岩浆。含硫化物石英的δD_V-SMOW值介于-103‰～-76‰之间,δ¹⁸O_H2O的值为4.01‰～5.55‰,表明主成矿阶段的成矿流体主要为岩浆水,混合有大气降水。     

Development of a Continental Volcanic Field: Petrogenesis of Pre-caldera Intermediate and Silicic Rocks and Origin of the Bandelier Magmas, Jemez Mountains (New Mexico, USA)

The Miocene–Quaternary Jemez Mountains volcanic field(JMVF) is the site of the Valles caldera and associated BandelierTuff. Caldera formation was preceded by > 10 Myr of volcanismdominated by intermediate composition rocks (57–70% SiO₂)that contain components derived from the lithospheric mantleand Precambrian crust. Simple mixing between crust-dominatedsilicic melts and mantle-dominated mafic magmas, fractionalcrystallization, and assimilation accompanied by fractionalcrystallization are the principal mechanisms involved in theproduction of these intermediate lavas. A variety of isotopicallydistinct crustal sources were involved in magmatism between13 and 6 Ma, but only one type (or two very similar types) ofcrust between 6 and 2 Ma. This long history constitutes a recordof accommodation of mantle-derived magma in the crust by meltingof country rock. The post-2 Ma Bandelier Tuff and associatedrhyolites were, in contrast, generated by melting of hybridizedcrust in the form of buried, warm intrusive rocks associatedwith pre-6 Ma activity. Major shifts in the location, styleand geochemical character of magmatism in the JMVF occur withina few million years after volcanic maxima and may correspondto pooling of magma at a new location in the crust followingsolidification of earlier magma chambers that acted as trapsfor basaltic replenishment. KEY WORDS: crustal anatexis; fractional crystallization; Jemez Mountain Volcanic Field; Valles Caldera; radiogenic isotopes; trace elements     

Relative humidity recorded in tree rings: A study along a precipitation gradient in the Olympic Mountains, Washington, USA

Humidity is one of the fundamental variables controlling the energy balance of the climate, and thus its reconstruction represents a significant aspect of paleoclimate studies. We here report a study of oxygen and hydrogen isotopic compositions of tree-ring cellulose along a precipitation gradient in the Olympic Mountains of Washington, USA, to examine whether and how the relative humidity is recorded in the cellulose. According to the physically-based model described in this paper, the relationship between δD and δ¹⁸O of cellulose should have the same slope as that in the relationship between δD and δ¹⁸O in the source water (∼8.8 for the local meteoric water) if there is no systematic variation in the mean relative humidity among study sites and no physiologic differences among trees. However, our isotopic analyses of cellulose yielded a slope of 17.4, significantly greater than the slope of the Local Meteoric Water Line. We show that to produce such a slope, a positive covariation between the relative humidity and the δD and δ¹⁸O in the source water is required across the precipitation gradient. This work suggests that the δD vs. δ¹⁸O relationship in tree rings can be a useful tool for paleohumidity reconstruction.     

Groundwater flow,nutrient, and stable isotope dynamics in the parafluvial-hyporheic zone of the regulated Lower Colorado River (Texas,USA) over the course of a small flood

Periodic releases from an upstream dam cause rapid stage fluctuations in the Lower Colorado River near Austin, Texas, USA. These daily pulses modulate fluid exchange and residence times in the hyporheic zone where biogeochemical reactions are typically pronounced. The effects of a small flood pulse under low-flow conditions on surface-water/groundwater exchange and biogeochemical processes were studied by monitoring and sampling from two dense transects of wells perpendicular to the river. The first transect recorded water levels and the second transect was used for water sample collection at three depths. Samples were collected from 12 wells every 2 h over a 24-h period which had a 16-cm flood pulse. Analyses included nutrients, carbon, major ions, and stable isotopes of water. The relatively small flood pulse did not cause significant mixing in the parafluvial zone. Under these conditions, the river and groundwater were decoupled, showed potentially minimal mixing at the interface, and did not exhibit any discernible denitrification of river-borne nitrate. The chemical patterns observed in the parafluvial zone can be explained by evaporation of groundwater with little mixing with river water. Thus, large pulses may be necessary in order for substantial hyporheic mixing and exchange to occur. The large regulated river under a low-flow and small flood pulse regime functioned mainly as a gaining river with little hydrologic connectivity beyond a narrow hyporheic zone.     

Quantification and characterization of the dynamics of spring and stream water systems in the Berchtesgaden Alps with a long-term stable isotope dataset

The understanding of alpine groundwater dynamics and the interactions with surface stream water is crucial for water resources research and management in mountain regions. In order to characterize local spring and stream water systems, samples at 8 springs, 5 stream gauges and bulk samples of precipitation at 4 sites were regularly collected between January 2012 and January 2016 in the Berchtesgaden Alps for stable water isotope analysis. The sampled hydro-systems are characterized by very different dynamics of the stable isotope signatures. To quantify those differences, we analyzed the stable isotope time series and calculated mean transit times (MTT) and young water fractions (YWF) of the sampled systems. Based on the data analysis, two groups of spring systems could be identified: one group with relatively short MTT (and high YWF) and another group with long MTT (and low YWF). The MTT and the YWF of the sampled streams were intermediate, respectively. The reaction of the sampled spring and stream systems to precipitation input was studied by lag time analysis. The average lag times revealed the influence of snow and ice melt for the hydrology in the study region. It was not possible to determine the recharge elevation of the spring and stream systems due to a lack of altitude effect in the precipitation data. For two catchments, the influence of the spring water stable isotopic composition on the streamflow was shown, highlighting the importance of the spring water for the river network in the study area.     

Evolution of a Porphyry-Cu Mineralized Magma System at Santa Rita, New Mexico (USA)

The magmatic processes leading to porphyry-Cu mineralizationat Santa Rita are reconstructed on the basis of petrographicstudies, thermobarometry, and laser-ablation inductively-coupled-plasmamass-spectrometry analyses of silicate melt and sulfide inclusionsfrom dikes ranging from basaltic andesite to rhyodacite. Combinedresults suggest that magma evolution at Santa Rita is similarto that of sulfur-rich volcanoes situated above subduction zones,being characterized by repeated injection of hot, mafic magmainto an anhydrite-bearing magma chamber of rhyodacitic composition.The most mafic end-member identified at Santa Rita is a shoshoniticbasaltic andesite that crystallized at 1000–1050°C,1–3 kbar and log f_O2 = NNO + 0·7 to NNO + 1·0,whereas the rhyodacite crystallized at 730–760°C andlog f_O2 = NNO + 1·3 to NNO + 1·9. Mixing betweenthe two magmas caused precipitation of 0·1–0·2wt % magmatic sulfides and an associated decrease in the Cucontent of the silicate melt from 300–500 ppm to lessthan 20 ppm. Quantitative modeling suggests that temporal storageof ore-metals in magmatic sulfides does not significantly enhancethe amount of copper ultimately available to ore-forming hydrothermalfluids. Magmatic sulfides are therefore not vital to the formationof porphyry-Cu deposits, unless a mechanism is required thatholds back ore-forming metals until late in the evolution ofthe volcanic–plutonic system. KEY WORDS: porphyry-Cu; sulfur; sulfides; magma mixing; LA-ICP-MS     

Petrography and stable isotope geochemistry of the cretaceous El Abra Limestones (Actopan), Mexico: Implication on diagenesis

Petrography and stable isotopes (carbon and oxygen) geochemistry of limestones from the El Abra Formation, Actopan, were studied to identify their digenetic environments. The major petrographic types identified are mudstone, wackestone, grainstone, and boundstone. Most of the studied samples show positive δ¹³C values, except two samples (2 and 28), which are slightly negative values (−0.27‰ and −0.02‰). The organic remains identified in foraminiferal wackestone type can be responsible for the negative δ¹³C values. The δ¹⁸O values range from −12.41‰ to −4.02‰ and indicate meteoric diagenesis.     

Stable isotopes of cherts and carbonate cements in the Lake Valley Formation (Mississippian), Sacramento Mts, New Mexico

Oxygen isotopic compositions of chert and calcite cements in the Lake Valley Formation indicate that these diagenetic features cannot be equilibrium co-precipitates in spite of their coexistence in the same interstices. Petrography of megaquartz and non-ferroan calcite cements indicates that both are original precipitates that formed during pre-Pennsylvanian time at shallow burial depths (< 215m) implying precipitation temperatures less than 30°C. Under these constraints the δ¹⁸Os of megaquartz (mean =+27.0^0/00 SMOW; range =+ 24.8 to + 28.9^0/00) and calcite (mean =+ 28.0^0/00 SMOW; range =+ 27.3 to + 28.4^0/00) are best interpreted as unaltered since precipitation; thus, they must reflect the oxygen isotopic composition of pre-Pennsylvanian pore waters. Microquartz and chalcedony are interpreted to have formed from recrystallization of pre-Pennsylvanian opal-CT precursors, and therefore probably re-equilibrated during recrystallization in late or post-Mississippian time. We propose a model integrating the isotopic data with regional petrographic and sedimentological data that explains the greater consistency and generally greater δ¹⁸Os values of the calcites compared to those of the cherts. This model is one of chertification and calcite cementation in a regional meteoric phreatic ground-water system, the seaward terminus of which moved southward during lowering of pre-Pennsylvanian sea level. The calcite cements and some of the opal-CT precursor to microquartz and chalcedony are interpreted to have formed in the more seaward portions of the groundwater system. The megaquartz precipitated in the more inland parts of the phreatic groundwater system where rainfall was isotopically lighter and more variable. As such, the δ¹⁸Os of the megaquartz reflect the isotopic composition of groundwaters in areas undersaturated with respect to calcite.     

Preliminary research on hydrogen and oxygen stable isotope characteristics of different water bodies in the Qilian Mountains,northwestern Tibetan Plateau

Hydrogen and oxygen stable isotope in water bodies is a widely used tracer in hydrological process studies. In order to provide a basis for stable isotopic characteristics in different water bodies at the high mountainous area of northwestern Tibetan Plateau, samples for river water, groundwater, soil water, and plant water were collected from 10 sites in the Qilian Mountains during July and August 2015, and then analyzed for δ¹⁸O and δD, respectively. Results indicated that the stable isotope values of soil water were mostly plotted below the global meteoric water line (GMWL), which suggested that evaporation made heavy isotope in soil water enriched. The stable isotope values of soil water were quite different in the top soil layer, but tended to be uniform in the deep soil layer. Furthermore, the stable isotope difference of plant water is related to climatic conditions, water isotopes utilized by plant, plant species, growing season, and so on. Additionally, the variation of δ¹⁸O values for river water and groundwater relatively coincided with each other, and this showed the recharge sources of above two water bodies may be consistent. The stable isotope values of river water and groundwater were mainly plotted on the upper left of GMWL, and the lower level of isotopic fractionation due to weak evaporation may accountable for this.