Geological modelling of mineral deposits for prediction in mining

Accurate prediction of the shape, location, size and properties of the solid rock materials to be extracted during mining is essential for reliable technical and financial planning. This is achieved through geological modelling of the three-dimensional (3D) shape and properties of the materials present in mineral deposits, and the presentation of results in a form which is accessible to mine planning engineers. In recent years the application of interactive graphics software, offering 3D database handling, modelling and visualisation, has greatly enhanced the options available for predicting the subsurface limits and characteristics of mineral deposits. A review of conventional 3D geological interpretation methods, and the model struc- tures and modelling methods used in reserve estimation and mine planning software packages, illustrates the importance of such approaches in the modern mining industry. Despite the widespread introduction and acceptance of computer hardware and software in mining applications, in recent years, there has been little fundamental change in the way in which geology is used in orebody modelling for predictive purposes. Selected areas of current research, aimed at tackling issues such as the use of orientation data, quantification of morphological differences, incorporation of geological age relationships, multi-resolution models and the application of virtual reality hardware and software, are discussed.     

Control‐volume mixed finite element methods

A key ingredient in simulation of flow in porous media is accurate determination of the velocities that drive the flow. Large‐scale irregularities of the geology (faults, fractures, and layers) suggest the use of irregular grids in simulation. This paper presents a control‐volume mixed finite element method that provides a simple, systematic, easily implemented procedure for obtaining accurate velocity approximations on irregular (i.e., distorted logically rectangular) block‐centered quadrilateral grids. The control‐volume formulation of Darcy’s law can be viewed as a discretization into element‐sized “tanks” with imposed pressures at the ends, giving a local discrete Darcy law analogous to the block‐by‐block conservation in the usual mixed discretization of the mass‐conservation equation. Numerical results in two dimensions show second‐order convergence in the velocity, even with discontinuous anisotropic permeability on an irregular grid. The method extends readily to three dimensions.     

Relationship of larval-stage growth and mortality to recruitment of striped bass,Morone saxatilis, in Chesapeake Bay

Variable recruitments of striped bass were hypothesized to be caused by factors influencing growth and survival of larvae. Eggs and larvae were collected in the Potomac River from 1987 to 1989 and in the Upper Chesapeake Bay in 1988 and 1989 to estimate abundances, larval growth and survival rates, and environmental variability. Larval batch dates, ages, and growth and mortality rates were estimated from analysis of otolith daily increments. A retrospective analysis of Potomae River ichthyoplankton data from 1974–1977 and 1980–1982 provided additional estimates of larval abundances and vital rates for comparative purposes. Significant correlations betweens vital rates (growth and mortality) and abundances of striped bass larvae, and the Maryland juvenile recruitment index indicated that recruitment level may be fixed during the larval stage. The ratio of mean daily growth and mortality rates (G:Z) of larvae in the Potomac River for 1987–1989 was highest in 1987 when the juvenile index was relatively high, and was lower in 1988 and 1989 when juvenile indices were low. In the Upper Bay, mean larval growth rate, survival rate, and the G:Z ratio were highest in 1989 when the juvenile index also was high. In both tributaries, abundances of late-stage larvae (8 mm SL) were correlated with juvenile-stage recruitment indices. The retrospective analysis provided additional evidence that Potomac River larval abundances and G:Z ratios were positively correlated with juvenile recruitment indices in the 1974–1977 and 1980–1982 periods. Conditions favoring striped bass larval abundance and potential recruitment differed between the Potomac River and the Upper Bay. In the Potomac, late-stage larval abundances coincided with late-season water temperatures that were relatively warm, low river discharges and high, late-season densities of zooplankton prey, which favored larval growth. In the Upper Bay, the high abundance of late-stage larvae in 1989 relative to 1988 was attributed to a higher egg production that was coincident with high zooplankton abundances.     

Deep-sea avulsion and morphosedimentary evolution of the Rhône Fan Valley and Neofan during the Late Quaternary (north-western Mediterranean Sea)

The Petit-Rhône Fan Valley (north-western Mediterranean) is a broad, sinuous, filled valley that is deeply incised by a narrow, sinuous thalweg. The valley fill is differentiated into three seismic subunits on high-resolution seismic-reflection profiles. The lower chaotic subunit probably consists of channel lag deposits that seem to be in lateral continuity with high-amplitude reflections representing levee facies. The intermediate transparent subunit, which has an erosional base and clearly truncates levee deposits, is interpreted to be mass-flow deposits resulting from the disintegration of the fan-valley flanks. The upper bedded subunit shows an overall lens-shaped geometry and the seismic reflections onlap either onto the top of the underlying transparent subunit or onto the Rhône levees. Piston core data show that the upper few meters of this upper subunit consist of thin turbidites, probably deposited by overflow processes. The few available ¹⁴C ages suggest that the upper stratified subunit filled the Petit-Rhône Fan Valley between 21 and 11 kyr BP. The upper bedded subunit is deposited within the Petit-Rhône Fan Valley downslope of a major decrease in slope gradient. This upper subunit and the thalweg are genetically related and represent a small channel/levee system confined within the fan valley. Previous studies interpreted this thalweg to be an erosional feature resulting from a recent avulsion of the major channel course. Our interpretation implies that the thalweg is not a purely erosional feature but a depositional/erosional channel. This small channel/levee system is superimposed on a large muddy channel/levee system after the sediment supply changed from thick muddy flows during the main phase of aggradation of the Rhône Fan levees, to thin, mixed (sand and mud) flows at the end of Isotope Stage 2 (～16–18 ka BP). The pre-existing morphology of the Petit-Rhône Fan Valley played a determinant role in the sediment dispersal leading to the creation of this small and confined channel/levee system. These mixed flows have undergone flow stripping resulting from the changes in the slope gradient along the thalweg course. The finer sediment overflowed from the thalweg and were deposited in the Petit-Rhône Fan Valley. Coarser channelled sediment remaining in the thalweg were deposited as a ‘sandy’lobe (Neofan). As indicated by ¹⁴C dating, sedimentation on this lobe continued until very recently, suggesting a further evolution of the turbidity flows from small mixed flows to small sandy flows. the deposition of this study lobe and the sedimentary fill of the Petit-Rhône Fan Valley may be related to widespread shelf edge and canyon wall failures with a resulting downslope evolution of failed sediment into turbidity currents.     

Timing and Extent of Late Quaternary Paleolakes in the Trans-Pecos Closed Basin, West Texas and South-Central New Mexico

The Trans-Pecos Closed Basin is a hydrographically closed region covering 20,000 km²centered on Salt Basin, 160 km east of El Paso, Texas. Geomorphic and limnetic evidence have been used to identify four major highstands for Lake King during the last glacial maximum (LGM). Additional geomorphic features from a second, recently identified, paleolake, Lake Sacramento, have been found in the Beargrass subbasin, a nested subbasin approximately 75 km northwest of Salt Basin. Radiocarbon ages of the organic material in Lake King sediments date four abrupt climate changes and rapid lacustrine transgressions during the LGM with a quasi-periodicity of 2000 yr. Geomorphic evidence in the Beargrass subbasin identifies lake cycles contemporaneous with those in Lake King. The dates for these transgressions correlate with the dates of freshening events identified by researchers in paleolake basins elsewhere in New Mexico. The quasi-periodicity of the events approximates that of Dansgaard–Oeschger events identified from Greenland ice cores. The contemporaneity of the Trans-Pecos transgressions with transgressive events in other basins in the region suggests that paleolakes in the region were in phase with respect to abrupt climate changes during the latter stages of the LGM.     

Diatoms in sediments of perennially ice-covered Lake Hoare, and implications for interpreting lake history in the McMurdo Dry Valleys of Antarctica

Diatom assemblages in surficial sediments, sediment cores, sediment traps, and inflowing streams of perennially ice-covered Lake Hoare, South Victorialand, Antarctica were examined to determine the distribution of diatom taxa, and to ascertain if diatom species composition has changed over time. Lake Hoare is a closed-basin lake with an area of 1.8 km², maximum depth of 34 m, and mean depth of 14 m, although lake level has been rising at a rate of 0.09 m yr^-1 in recent decades. The lake has an unusual regime of sediment deposition: coarse grained sediments accumulate on the ice surface and are deposited episodically on the lake bottom. Benthic microbial mats are covered in situ by the coarse episodic deposits, and the new surfaces are recolonized. Ice cover prevents wind-induced mixing, creating the unique depositional environment in which sediment cores record the history of a particular site, rather than a lake-wide integration. Shallow-water (<1 m) diatom assemblages (Stauroneis anceps, Navicula molesta, Diadesmis contenta var. parallela, Navicula peraustralis) were distinct from mid-depth (4–16 m) assemblages (Diadesmis contenta, Luticola muticopsis fo. reducta, Stauroneis anceps, Diadesmis contenta var. parallela, Luticola murrayi) and deep-water (26–31 m) assemblages (Luticola murrayi, Luticola muticopsis fo. reducta, Navicula molesta). Analysis of a sediment core (30 cm long, from 11 m water depth) from Lake Hoare revealed two abrupt changes in diatom assemblages. The upper section of the sediment core contained the greatest biomass of benthic microbial mat, as well as the greatest total abundance and diversity of diatoms. Relative abundances of diatoms in this section are similar to the surficial samples from mid-depths. An intermediate zone contained less organic material and lower densities of diatoms. The bottom section of core contained the least amount of microbial mat and organic material, and the lowest density of diatoms. The dominant process influencing species composition and abundance of diatom assemblages in the benthic microbial mats is episodic deposition of coarse sediment from the ice surface.     

Hydrologic and climatic implications of a multidisciplinary study of late Holocene sediment from Kenosee Lake, southeastern Saskatchewan, Canada

Sediment lithology and mineralogy, as well as ostracode, plant macrofossil and stable isotope stratigraphies of lake sediment cores, are used to reconstruct late Holocene hydrologic changes at Kenosee Lake, a relatively large, hyposaline lake in southeastern Saskatchewan. Chronological control is provided by AMS radiocarbon ages of upland and shoreline plant macrofossils. All indicators outline an early, low-water, saline phase of lake history (4100–3000 BP), when the basin was occupied by a series of small, interconnected, sulfate-rich brine pools, as opposed to the single, topographically-closed lake that exists today. A rapid rise in lake-level (3000–2300 BP) led to the establishment of carbonate-rich, hyposaline lake conditions like those today. Lithostratigraphic data and ostracode assemblages indicate peak salinities were attained early in this period of lake infilling, suggesting that the lake-level rise was initially driven by an influx of saline groundwater. Lake-level and water chemistry have remained relatively stable over the last 2000 years, compared to earlier events. Because of a lack of datable organic material in sediments deposited during the last 2000 years, the chronology of recent events is not well resolved. Plant macrofossil, lithostratigraphic and ostracode evidence suggests that lake draw-down, accompanied by slightly higher than present salinites, occurred sometime prior to 600 BP, followed by peak lake-level and freshwater conditions. This most recent high lake stand, indicative of a high water table on the surrounding upland, may also have led to the establishment of an extensive cover of Betula in the watershed, possibly in response to paludification. Ostracode assemblages indicate that peak freshwater conditions occurred within the last 100 years. Since historically documented lake-level fluctuations correlate with decadal scale climatic fluctuations in the meteorological record, and late-Holocene hydrologic dynamics correspond to well documented climatic excursions of the Neoglacial and Little Ice Age, Kenosee Lake dynamics offer insight into the susceptibility of the region's water resources to climate change.     

A record of accelerated erosion in the recent sediments of Blelham Tarn in the English Lake district

Two frozen cores from Blelham Tarn were subsampled and measured using mineral magnetic, loss-on-ignition (LOI), radiometric, granulometric and diatom analyses. A detailed chronology was established using varves, radioisotopes and diatoms. This has enabled an accurately dated reconstruction of sedimentation over the past forty years. Despite a large increase in lake productivity, evidence suggests that the observed exponential increase in sedimentation rates can be attributed to erosion within the catchment. The predominant sediment source has been identified as surface soil. A comparison between the trend of accelerated sedimentation and the record of increased sheep stocking density for the area within which the most of the catchment lies, as well as observations of contemporary surface processes within the catchment, both suggest that much of the recent erosion is a direct response to increased pressure from sheep grazing.