Review paper: Exploration geophysics for intrusion‐hosted rare metals

Igneous intrusions, notably carbonatitic–alkalic intrusions, peralkaline intrusions, and pegmatites, represent significant sources of rare‐earth metals. Geophysical exploration for and of such intrusions has met with considerable success. Examples of the application of the gravity, magnetic, and radiometric methods in the search for rare metals are presented and described. Ground gravity surveys defining small positive gravity anomalies helped outline the shape and depth of the Nechalacho (formerly Lake) deposit within the Blatchford Lake alkaline complex, Northwest Territories, and of spodumene‐rich mineralization associated with the Tanco deposit, Manitoba, within the hosting Tanco pegmatite. Based on density considerations, the bastnaesite‐bearing main ore body within the Mountain Pass carbonatite, California, should produce a gravity high similar in amplitude to those associated with the Nechalacho and Tanco deposits. Gravity also has utility in modelling hosting carbonatite intrusions, such as the Mount Weld intrusion, Western Australia, and Elk Creek intrusion, Nebraska. The magnetic method is probably the most successful geophysical technique for locating carbonatitic–alkalic host intrusions, which are typically characterized by intense positive, circular to sub‐circular, crescentic, or annular anomalies. Intrusions found by this technique include the Mount Weld carbonatite and the Misery Lake alkali complex, Quebec. Two potential carbonatitic–alkalic intrusions are proposed in the Grenville Province of Eastern Quebec, where application of an automatic technique to locate circular magnetic anomalies identified several examples. Two in particular displayed strong similarities in magnetic pattern to anomalies accompanying known carbonatitic or alkalic intrusions hosting rare‐metal mineralization and are proposed to have a similar origin. Discovery of carbonatitic–alkalic hosts of rare metals has also been achieved by the radiometric method. The Thor Lake group of rare‐earth metal deposits, which includes the Nechalacho deposit, were found by follow‐up investigations of strong equivalent thorium and uranium peaks defined by an airborne survey. Prominent linear radiometric anomalies associated with glacial till in the Canadian Shield have provided vectors based on ice flow directions to source intrusions. The Allan Lake carbonatite in the Grenville Province of Ontario is one such intrusion found by this method. Although not discovered by its radiometric characteristics, the Strange Lake alkali intrusion on the Quebec–Labrador border is associated with prominent linear thorium and uranium anomalies extending at least 50 km down ice from the intrusion. Radiometric exploration of rare metals hosted by pegmatites is evaluated through examination of radiometric signatures of peraluminous pegmatitic granites in the area of the Tanco pegmatite.     

The age of orogenesis in the Nambucca Slate Belt: A K‐Ar study of low‐grade regional metamorphic rocks

K‐Ar ages of biotite and hornblende from undeformed granodiorite plutons and of slaty and phyllitic rocks, ranging from prehnite‐pumpellyite metagreywacke to greenschist fades, have been determined in an attempt to define the age of orogenesis in the eastern part of the Nambucca Slate Belt. The plutons have K‐Ar ages of 226–227 m.y. (biotite) and 228–231 m.y. (hornblende) that provide a younger age limit for deformation. The lower grade metamorphic rocks yield a range of ages including some comparable with the depositional age of the rocks as indicated by fossils. Rocks of pumpellyite‐actinolite and greenschist facies give a more coherent group of ages which suggest orogenesis at about 250–255 m.y. Specimens of these latter rocks that have been affected by a later structural episode than that during which slaty cleavage formed, yield slightly older ages, which may result from the inclusion of minor amounts of environmental excess ⁴⁰Ar.

Support for the 250–255 m.y. age comes from previously determined radiometric ages from the western part of the Slate Belt, although the presence of granitic bodies perhaps as old as 289 m.y., some closely associated with high‐grade regional metamorphic rocks, may indicate the presence of additional earlier orogenic movements in this region.     

Biogeochemical Zones Within a Macrotidal,Dry-Tropical Fluvial-Marine Transition Area: A Dry-Season Perspective

The Fitzroy River delivers large amounts of nutrients and fine sediments to Keppel Bay (contiguous with the Great Barrier Reef Lagoon) during intermittent flow events. This study explores sources, forms and transformations of nutrients in Keppel Bay, and develops a functional process zonation that integrates seabed geochemistry and water column nutrient characteristics which are controlled by suspended sediment. The water column and seabed properties were investigated over two dry seasons, with supplementary core incubations taken to measure carbon decomposition rates and nutrient fluxes. Keppel Bay can be divided into three zones, the: zone of maximum resuspension (ZMR); coastal transitional zone (CTZ); and blue water zone (BWZ). Mineralisation of predominantly terrestrial organic matter occurs in the ZMR where nutrient uptake by phytoplankton is light limited. The CTZ and BWZ had higher light penetration and phytoplankton growth was likely limited by N and P, respectively. The identified zones conform to the bathymetry and hydrodynamic characteristics of the bay, allowing for the development of an integrated conceptual model accounting for the benthic and pelagic biogeochemical processes. Recognition of these different zones shows that considerable variation in benthic and water column properties is possible within a small system with the bathymetric and hydrodynamic characteristics of the fluidized bed reactor.     

Evaluation and comparison of spatial interpolators II

The performance of several variations on ordinary kriging and inverse distance estimators is evaluated. Mean squared errors (MSE) were calculated for estimates made on multiple resamplings from five exhaustive data bases representing two distinctly different types of estimation problem. Ordinary kriging, when performed with variograms estimated from the sample data, was more robust than inverse-distance methods to the type of estimation problem, and to the choice of estimation parameters such as number of neighbors.Notice: Although the research described in this article has been funded in part by the United States Environmental Protection Agency through Cooperative Agreement CR818526 to the Harry Reid Center for Environmental Studies, University of Nevada-Las Vegas, it has not been subjected to Agency review. Therefore it does not necessarily reflect the views of the Agency. Mention of trade names or commercial products does not constitute endorsement or recommendation for use.     

A high-resolution TEM-AEM, pH titration, and modeling study of Zn coprecipitation with ferrihydrite

Experiments of Zn²⁺ and Fe³⁺ coprecipitation as a function of pH were conducted in the laboratory at ambient temperature and pressure. X-ray diffraction patterns of the coprecipitates show two broad peaks at 0.149 and 0.258 nm, which is consistent with published patterns for pure 2-line ferrihydrite. Zn²⁺ uptake occurred at pH ≥5 while Fe³⁺ precipitation occurred between pH 3 and 4, although both Zn²⁺ and Fe³⁺ were present in the same solution during the entire range of pH titration. High-resolution transmission electron microscopy shows that the coprecipitates are 2 to 6 nm sized single crystalline particles but aggregated to 50 to 400 nm sized clusters. Analytical electron microscopy indicated that the 5% atomic Zn with respect to Fe was homogeneously distributed. No segregated phases were found in the clusters or at single crystal edges, which is consistent with published extended X-ray absorption fine structure (EXAFS) results at similar Zn/(Zn + Fe) ratios. Hence, occlusion and surface precipitation may be excluded as possible coprecipitation mechanisms. The bulk solution Zn²⁺ sorption edge was fitted to both solid solution and generalized diffuse layer surface complexation models. However, a solid solution model is inconsistent with published EXAFS results that show tetrahedral polydentate Zn²⁺ complexes sharing apices with Fe³⁺octahedra.     

Web-cam’s potential for collaborative activities in the Earth sciences

We report here our experiences from using easily acquired web-cam components for collaborative ventures in the Earth sciences. We have used a variety of hardware and different software. We demonstrate from various locations in the U.S.A. the feasibility of using web-cam in multitudinous activities, relevant for conducting research and knowledge dissemination. We summarize the quality of the connections from the various combinations of communicating parties. Today, web-cams can be utilized as an economical and viable means of point-to-point communication in the Earth science community. Greater bandwidth is sorely needed for activities such as multi-party conferencing on the present internet network. We propose that web-cam can be readily deployed as a web-service for facilitating collaborative research over the GRID infrastructure, using the middleware Narada-Brokering. Web-cam will play an important role in the emerging field of geoinformatics.     

The potential impact of herbivores on the susceptibility of the marsh plantSagittaria lancifolia to saltwater intrusion in coastal wetlands

The objective of this study was to experimentally evaluate the effects of simulated herbivory on the ability of a freshwater marsh plant to recover from temporary saltwater intrusion such as can be caused by tropical storms. Sods containingSagittaria lancifolia, a dominant plant in interior coastal marshes, were manipulated in the field so as to subject plants to a pulse of 15‰ salt water for a duration of 1 wk. In addition to the exposure to salt water, some plants were also subjected to both short-term and long-term flooding treatments of 20 cm, and to simulated herbivory (clipping). Following exposure to salt water, plants were allowed to recover over the winter and were harvested the next June. Neither simulated herbivory, nor salinity, nor flooding caused any long-term effect either singly or in pairwise combinations. However, when plants were subjected to herbivory, salt water, and flooding simultaneously, reduced growth and plant death occurred. These results suggest that high levels of grazing by herbivores may increase the susceptibility of coastal marsh plants to damage from saltwater intrusion. *** DIRECT SUPPORT *** A01BY073 00002     

Are we adapting to climate change?

Human systems will have to adapt to climate change. Understanding of the magnitude of the adaptation challenge at a global scale, however, is incomplete, constrained by a limited understanding of if and how adaptation is taking place. Here we develop and apply a methodology to track and characterize adaptation action; we apply these methods to the peer-reviewed, English-language literature. Our results challenge a number of common assumptions about adaptation while supporting others: (1) Considerable research on adaptation has been conducted yet the majority of studies report on vulnerability assessments and natural systems (or intentions to act), not adaptation actions. (2) Climate change is rarely the sole or primary motivator for adaptation action. (3) Extreme events are important adaptation stimuli across regions. (4) Proactive adaptation is the most commonly reported adaptive response, particularly in developed nations. (5) Adaptation action is more frequently reported in developed nations, with middle income countries underrepresented and low-income regions dominated by reports from a small number of countries. (6) There is limited reporting on adaptations being developed to take advantage of climate change or focusing on women, elderly, or children.     

Adaptation to climate change in glaciated mountain regions

Understanding of the human dimensions of climate change (HDCC) in glaciated mountain regions is limited by a deficit in systematically collated information on where, to what stressors, by whom, at what scale, and with what effect adaptation is occurring. This paper presents a systematic literature review of the recent English language peer-reviewed scholarship on adaptation in glaciated mountain regions. 4050 potentially relevant articles were examined, with 36 included for full review. Results indicate that scholarly investigation into adaptation in glaciated mountains is presently limited to only 40 % of countries with alpine glaciation. Seventy-four discrete adaptation initiatives were identified, with most occurring in Peru (28 %), Nepal (22 %) and India (17 %). Many documented adaptations were initiated in response to intersecting stressors related to cryospheric change and socio-economic development; were autonomous and initiated in reaction to experienced climatic stimuli; and were carried out at the individual, family, or community scale. The study contributes to an emerging literature tracking on-the-ground adaptation processes and outcomes, and identifies a need to raise the profile of human adaptation in glaciated mountain regions within the HDCC scholarship. A research agenda for addressing key knowledge gaps and questions is developed, providing a framework for future investigation.     

A comparison of two methods of estimating transpiration rates from a Sitka spruce plantation

Estimates of hourly transpiration from a 16–17 yr old Sitka spruce forest were calculated from the Penman-Monteith combination equation and compared with estimates from an eddy correlation/energy balance method.Canopy conductances were estimated from stomatal conductances measured using null balance diffusion porometers and took account of canopy variations of stomatal conductance and needle area index.Vertical heat fluxes were measured by the eddy correlation method; transpiration fluxes were then estimated from an energy balance of the forest.There was not a 1:1 relationship between the estimates of transpiration from the two methods. The major sources of error were concluded to be (i) difficulties of estimating the variation in stomatal conductance and leaf area through the canopy, (ii) errors in the value of total leaf area index, and (iii) errors in stomatal conductance measurements.The eddy correlation method was suggested as the more useful for future studies of the variation of forest transpiration in time or space, because the Penman-Monteith equation requires extensive biological measurements.