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BRITISH GEOLOGICAL SURVEY TECHNICAL REPORT WC/93/ I5 International Geology Series Industrial Minerals Exploration Guide No. 1 BIOGENIC SEDIMENTARY ROCKS S J Mathers International Division Natural Environment Research council British Geological Survey Keyworth. Nottingham United Kingdom NG I2 5GG British Geological Survey Natural Environment Research Council Technical Report WC/93/15 International Geology Series Industrial Minerals Exploration Guide No. 1 BIOGENIC SEDIMENTARY ROCKS S J Mathers International Division Bibliogrophic reference Mathers, S J. 1993. Industrial Minerals Exploration Guide No.1. Biogenic sedimentary rocks. British Geologic01Survey Technic01 Report WC/93/15. 0 NERC copyright I993 British Geological Survey Keyworth Nottingham 1993 TECHNICAL REPORT WC/93/15 Industrial Minerals Exploration Guide No. 1 BIOGENIC SEDIMENTARY ROCKS Preface A vast range of industrial minerals is consumed by society as primary raw materials for the construction, chemical, fertilizer, metallurgical, ceramics, refractory and glass industries. Many others are used in manufacturing processes as abrasives, fillers, filter-aids and pigments. Clearly industrial minerals are literally and commercially the foundation stones of development. Self-sufficiency is the key to the economies of many of the world's Less Developed Countries (LDCs) due to trade inbalances and their limited reserves of foreign exchange. A few LDCs are major world exporters of important industrial mineral commodities, providing them with a substantial income of "hard" currency. To help realize development potential it is essential that these nations are aware of their industrial mineral resources and the uses to which they can be put. This requires comprehensive exploration for, and evaluation and laboratory testing of, industrial mineral raw materials. In recent years, funding from the Overseas Development Administration has enabled the British Geological Survey to provide much assistance and advice to LDCs in assessments of their industrial mineral potential. This has in particular been carried out by the Minerals for Development R & D project. Two parallel series of publications are currently being produced by the staff'of this project, which set out much of the knowledge and techniques developed. This series of Exploration Guides is intended to provide ideas and advice for those geoscientists involved in the identification and field evaluation of industrial minerals in the developing world. There are over 50 industrial mineral commodities and, unlike metallic mineral exploration, each requires a distinct approach. The series will comprise eight volumes each dealing with a specific genetically-related group of commodities. A complementary series of Laboratory Manuals is also being produced, each volume dealing with a single industrial mineral commodity. These manuals contain tests and procedures for identifying and evaluating commodities that may be of use in industry. Copies of both Exploration Guides and Laboratory Manuals may be ordered from the British Geological Survey, Keyworth, Nottingham NG12 5GG. S.J. Mathers Author D.J. Morgan Project Manager Published to-date are; Laboratory Manuals: Limestone; Flake Graphite; Diatomite and Kaolin. Exploration Guides: Biogenic Sedimentary Rocks. CONTENTS Page INTRODUCTION 1 CARBONATE ROCKS 4 DIATOMITE 33 PHOSPHATE ROCK 52 SULPHUR 66 Industrial Minerals Exploration Guide INDUSTRIAL MINERALS EXPLORATION GUIDE No 1: BIOGENIC SEDIMENTARY ROCKS INTRODUCTION This series of Industrial Mineral Exploration guides will comprise eight volumes each dealing with a genetically-related group of commodities (Table 1). As with all classifications of industrial minerals there are anomalies; for example with those commodities with diverse origins or styles of occurrence. However using a geologicallybased structure helps to minimise these difficulties and seems the most appropriate for the provision of advice and "handy hints" on exploration and evaluation to field geologists. This guide describes the geological occurrence, exploration and field evaluation of those industrial mineral commodities that accumulate predominantly, but not exclusively,by biogenic sedimentary processes (Carbonate Rocks, Diatomite, Phosphate Rock and Sulphur). The techniques described encompass traditional field geology, remote sensing, geophysical and geochemical prospecting, drilling and procedures for sampling. The basic principles behind the main techniques are outlined; key references are cited for those requiring more detailed information. British Geological Survey @ MRC. LIST OF FIGURES 1. Principal carbonate minerals. 2. Terminology applied to Carbonate Rocks. 3. Elements of karst geomorphology. 4. Carboniferous Limestone, Gordale Scar, northern England. 5. Field use of point-load testing equipment. 6. Location of a clay pocket in limestone using a Geonics EM 31 conductivity meter; after Penn & Tucker (1983). 7. Location of clay-filled depressions in the Chalk of southern England using a proton precession magnetometer; after McDowell (1975). 8. Bed of weathered tuff within Carboniferous Millers Dale Limestone, near Buxton, England. 9. Geological map and conductivity profiles across the Hog’s Back Monocline, southern England; after Zalasiewicz and others (1985). 10. The Mount Sopris down-the-hole logger which includes a gamma-ray recorder. 11. The Tororo Rock carbonatite plug, southeastern Uganda. 12. Geophysical and geochemical exploration of the Butiriku carbonatite, eastern Uganda; based on Reedman (1974). 13. Scanning electron micrograph showing the detailed structure of a lacustrine diatomite from Costa Rica. 14. Diatomite sample from Loma Camastro, Costa Rica. 15. Principal sedimentary processes of lake basins in active volcanic terrain; from Mathers (1989). 16. Outcrop of very pure lacustrine diatomite, near Burney, northern California. 17. Rhymically laminated impure lacustrine diatomite from Los Nubes, Costa Rica. 18. Graphic lithological log and bulk density of section in the Loma Camastro lacustrine diatomite of Costa Rica; from Mathers and others (1990). 19. The Eijkelkamp extendable auger system with cutting shoes for A, sand and gravel, B, clays, and C, soft and water-logged sediments. 20. Continental distribution, postulated ocean circulation patterns and major phosphorite deposits in the Late Tertiary; after Sheldon (1982). 21. Stratigraphical distribution of sedimentary phosphorites produced in 1978; after Notholt (1980). 22. Dark grey peloidal phosphate with interbeds of chalk and chert; A1 Hisa phosphorite, Cretaceous, Central Jordan. 23. Idealised mode of occurrence of elemental sulphur deposits in salt domes; after Gittinger (1975). 24. Section through the Mishraq stratiform sulphur deposit, northern Iraq; after Cortesini (1966). 25. Sulphur-rich crater lake, Poas volcano, Costa Rica. 26. Sublimation sulphur deposits formed around fumaroles at La Solfatara near Naples, Italy. LIST OF TABLES 1. Geological classification of industrial minerals used in this series of guides, based loosely on the classification employed in Geology of the Nonmetullics by Harben & Bates (1984). 2. Basic mineralogical and chemical data for the principal rock-forming carbonate minerals. 3. Classification of limestones (Dunham, 1962). 4. Classification of limestones (Wright, 1992). 5. Geophysical location of solutional features in carbonates; from Geological Society Engineering Group Working Party (1988). 6. Basic mineralogical and chemical data for iron sulphide minerals. 2 Industrial Minerals E-cploration Guide Table 1. Geological/genetic classification of industrial mineral commodities based loosely on Harben & Bates (1984). Biogenic Sedimentary Rocks - Carbonate Rocks - Diatomite - Phosphate Rock - Sulphur Clastic Sedimentary Rocks - Sand and Gravel - Silica Sands - Clays (Common, kaolins, bentonites) - Mineral Sands Chemical Sedimentary Rocks - Barite - Salt - Sodium salts - Gypsum and Anhydrite - Potassium minerals - Borates - Celestite - Nitrates - Other halides Surfkial Diagenetic Deposits - Vermiculite - Manganese minerals - Bauxite - Iron oxides (Ochres, Umbers) - Zeolites Intrusive Igneous Rocks - Olivine - Chromite - Nepheline Syenite - Granite Continued overleaf British Geological Survey NERC. 3 Industrial Minerals Ecploration Guide Pegmatitic & Hydrothermal Minerals - Feldspar - Mica - Quartz crystal - Lithium minerals - Beryllium minerals - Fluorspar Extrusive Igneous Rocks - Basalt - Pumice and Scoria - Perlite - Obsidian Metamorphic Rocks & Minerals - Slate - Asbestos - Talc and Pyrophyllite - Graphite - Sillimanite Group minerals - Corundum and Garnet - Wollastonite - Jadeite British Geological Survey @ NERC. 4 Carbonate Rocks CARBONATE ROCKS Introduction A wide range of industrial mineral commodities is encompassed by the term carbonate rocks, the most important being limestone, dolomite, magnesite and marble. These commodities have a myriad of different applications utilizing the specific physical or chemical properties of the carbonate minerals they contain. The principal uses of limestone for example are as an aggregate, in cement and lime manufacture and as a filler (for a more complete review see Hankon, 1992). Dolomite and magnesite are commonly utilized for their refractory properties and in steel making. Marble is chiefly used as a dimension and ornamental stone and as a high-quality filler. Geological occurrence Geologically, carbonate rocks have diverse origins; they include sediments, igneous intrusions and the products of metamorphism. Sedimentary and diagenetic carbonates The main carbonates are limestone and dolomite which together comprise over 15% of the world's sedimentary rocks and range in age from Precambrian to Recent. By definition limestones contain more than 50% calcite or aragonite (both are forms of CaCO,). With substitution of half the calcium by magnesium, dolomite CaMg (CO,), is produced (Figs 1, 2; Table 2). Complete substitution yields British Geological Survey @ hERC. 5 Figure 1. Principal carbonate minerals. Other minerals Aragonite Figure 2. Terminology applied to Carbonate Rocks. British Geological Survey @ NERC. 6 Carbonate Rocks magnesite (MgCO,) another useful industrial mineral commodity. Carbonates of very high purity are rare and particularly valuable industrial minerals. Table 2. Basic mineralogical and chemical data for the principal rock-forming carbonate minerals. Hardness Crystal System Solubility (cold HC1) -2.7 3.0 Trigonal Good CaCO, -2.95 3.5-4.0 Orthorhombic Good Dolomite CaMg(CO,), -2.85 3.5-4.0 Trigonal Poor Magnesite MgC0, -3.0 3.5-4.5 Trigonal Slight Mineral Formula Calcite CaCO, Aragonite S.G. Most limestones form in warm shallow marine environments and many are biogenic, e.g. reef limestones; others are bioclastic - composed mainly of detrital carbonate grains. Chemically precipitated limestones are also widespread and form in both marine and lacustrine environments. Limestones (s.1.) can here be taken to include consolidated rocks and also unconsolidated beach sands or shell banks (e.g. coquina) composed essentially of calcium carbonate. Other well known variants are chalk, a fine grained pure limestone formed of coccolith skeletons, and travertine, a calcareous tufa commonly deposited around springs, particularly in volcanic areas. Dolomites are predominantly produced by the early-stage diagenesis of limestone through interaction with magnesium-rich fluids including seawater; they form extensively in the intertidal zones of arid coastlines. Crystalline magnesite is produced by complete substitution British Geological Survey @ NERC. 7 Carbonate Rocks of calcium by magnesium, often as a result of hydrothermal alteration of dolomites. Cryptocrystalline magnesite on the other hand occurs as veins produced by reaction of host olivine-rich ultrabasic/ultramafic intrusives and serpentinites with carbonate groundwater. Magnesite, in finely disseminated form, is also found in fine-grained sediments derived from these parent lithologies Carbonatites Carbonatites are intrusive igneous rocks, composed mainly of calcite; they tend to occur either as individual plugs, or in nested ring complexes associated with alkaline and ultrabasic lithologies. Carbonatites are volcanic "roots" and tend to occur in belts located along major structural lineaments in the basement terrain of stable cratonic blocks. As carbonates they tend to be relatively impure but are extracted commercially in areas where they represent the only available source of carbonate. They are chiefly used for lime and cement manufacture. Some carbonatites, and in particular the residual soils developed on them, contain economic concentrations of fluorapatite (cf Phosphate Rock, this volume) and rare-earth bearing minerals such as pyrochlore. Metamorphic carbonates Isochemical metamorphism of a relatively pure carbonates produces marble. In regional metamorphic terrains marbles tend to occur as beds which usually trace out the considerable structural complexity and dislocation characteristic of such environments. Contact metamorphic British Geological Survey @ NERC. 8 Carbonate Rocks marbles are more irregular in form and may grade with increasing distance from the intrusive contact into unaltered limestone. Near contacts with acidic plutons, occurrences of wollastonite and other less common calcium-rich skarn minerals are sometimes closely associated with marble. The transformation to marble produces a tough, compact, massive lithology with few fractures. When pure white or with attractive hues (produced by minor impurities or textural variations), marble is a valuable commodity for use as a dimension or ornamental stone. Exploration and field evaluation Sedimentary, diagenetic and metamorphic carbonates Carbonate rocks are by no means rare, and whilst not ubiquitous they do occur in one form or another in most areas. In most regions the outline geology is sufficiently well-known, and carbonate deposits sufficiently common, for their occurrence and broad distribution to be known. Therefore primary exploration for most carbonates is not needed; usually what is required is a more detailed evaluation of a poorly-known occurrence to determine its characteristics, namely type, volume, grade, variability and suitability for a particular end-use. An initial appreciation of the lateral extent of any carbonate deposit would normally be obtained by a combination of remote sensing and reconnaissance field traverses. British Geological Survey NERC. 9 Carbonate Rocks Remote sensing Modern remote sensing techniques such as satellite images derived from the Thematic Mapper (TM) are beginning to enable some lithological discrimination under favourable conditions. The older LANDSAT images however are of limited use in this regard. Carbonate minerals cannot be distinguished one from another, since most have their principal absorptions around 2.34pm. This wavelength lies on the edge of TM band 7. Studies of sedimentary sequences on the Sinai Peninsula, Egypt (Marsh & O'Connor, 1992), have met with some success in discriminating carbonates from adjacent clays and sulphate-bearing rocks. This was achieved because the carbonate lithologies encountered tended to develop a weathered surface which produced a relatively weak Band 7 response, contrasting with a much stronger signal for the other lithologies. Aerial photographs are of considerable importance in regional assessments. In general carbonates are relatively tough rocks and, especially when thickly or massively bedded, they tend to resist mechanical erosion. Outcrops of relatively pure carbonates tend to produce a light tone although impure variants may be considerably darker. In wet climates, or areas that have experienced wet conditions in the geologically recent past, limestones and marbles have a distinct morphological expression. This develops as a result of their relatively high solubility in rainwater (the magnesium-bearing carbonates are much less soluble). Internal (subsurface) drainage systems readily British Geological Survey @ hERC. 10 Carbonate Rocks develop; these are marked by features such as sinkholes, solutional collapse structures and other karstic landforms, and springlines at the base (Fig. 3). Surface outcrops usually show preferrential solution along high angle fractures and faults; sinkholes are commonly located at their intersections. By contrast, in arid climates limestones and marbles tend to form resistant beds and may be difficult to distinguish from sandstones using aerial photographs. Limestones do tend to display more jagged forms with prominent screes, and usually have longer dip-slopes and smoother bedding planes. Pure limestones and marbles being almost entirely soluble tend to develop little soil cover and vegetation; impure limestones and carbonatites can produce thick residual soils such as "terra rosa", and are commonly thickly vegetated. In addition these insoluble residual soils often contain economic concentrations of chemically resistant minerals such as barite and, in the case of carbonatites, apatite, magnetite and rare earth-bearing minerals. In humid tropical climates and volcanically active areas travertine deposits are commonly developed around springs. Provisional interpretations based on remotely sensed data are usually followed up by reconnaissance field traverses to confirm lithological identity and detail. British Geological Survey @ NERC. 11 Limestone platform Clay plain Sandstone hills (with surface drainage) & Limestone gorge formed by the collapse ofthe roof of an underground cave River re-appea rs at the foot of the Limestone cliff ,Dry va I ley River disappears down swallow hole bnderground caverns Figure 3. Elements of karst geomorphology. British Geological Survey @ NERC. 12 Carbonate Rocks Field geology At a more detailed level of investigation, and in areas where remote sensing data are not available or helpful, geological surveying is the prime tool of investigation. Detailed topographic maps (published or if necessary surveyed specially) and/or aerial photographs are required for locating and recording observations. Mapping out the distribution of a carbonate bed involves the careful and systematic recording of its geomorphological expression, available sections and distribution of diagnostic soil types. Despite being soluble in acidic water, many carbonates form prominent scarp and dip features due to their resistance to mechanical erosion. This is particularly marked in gently dipping stata where the carbonate is interbedded with "soft" argillaceous sequences. In flat-lying strata extensive karstic plateaux develop controlled by major bedding planes. Spring lines are particularly useful in mapping out the base of limestones where they rest on impermeable beds; conversely swallow holes are likely to occur close to the top of a unit. (Fig. 3). Soils developed on carbonates are also particularly diagnostic. Soluble (calcium) carbonates tend to produce soils composed of their insoluble residues; many have a characteristic orange-red colouration due to iron (e.g. rezina soils). Other common constituents include barite, pyrite, base metal sulphide minerals, clay minerals and silica; in carbonatites residual soils are usually dominated by fluorapatite and magnetite. Relatively impure carbonates tend to form thicker residual soils which are able to support more extensive vegetation. Also diagnostic of carbonate outcrops, and sometimes a useful additional clue to their British Geological Survey @ NERC. 13 Carbonate Rocks presence, are the very distinct floral assemblages of calcium-loving species that develop. Exposures of stratiform sedimentary and metasedimentary carbonates should be recorded carefully by not only gathering structural information (strike, dip etc.), but also by the preparation of a carefully annotated graphic log showing the thickness and lithological character of the beds. These logs should ideally be constructed to scale on graph paper and incorporate detailed observations such as colour, lithology, fauna, degree of fracturing, mineralization and alteration. These graphic logs are invaluable for lithological correlation of sequences and provide key information on the internal variability of the deposit and thus its suitability for many end-uses. Much useful information can be derived from the examination of surface exposures (Fig. 4), although in isolation this style of assessment suffers from several disadvantages: a) Exposures rarely display thick or complete stratigraphic sequences thus posing uncertainties for lithostratigraphical correlation. b) Bias easily creeps into the recording and sampling of exposures; care must be taken to be representative not selective. c) Some sections tend to occur as vertical rockfaces making access for examination and sampling difficult. d) Care should be taken to avoid sampling weathered material; in humid climates carbonate rocks tend to be leached producing a British Geological Survey @ NERC. 14 Carbonate Rocks ~ Figure 4. Carboniferous Limestone, Gordale Scar, northern England. Figure 5. Field use of point-load testing equipment. British Geological Survey @ NERC. 15 Carbonate Rocks relatively impure carbonate layer whereas in arid climates the surficial crust may be enriched in carbonate. By being aware of these factors considerable amounts of useful data can be obtained from the logging and sampling of outcrops. If funds permit the preferred method of assessment of a carbonate resource involves drilling. In this case information gathered from exposures should be used as a supplement. It is recognized however that in many developing countries either budgets for drilling may be unobtainable or there may be operational difficulties which make drilling impractical. Construction of graphic logs of sections (or cores) involves careful examination of hand specimens. Using the naked eye and/or a hand lens the principal textures and constituents of a specimen can be determined. Primary carbonates are usually classified in hand specimen with reference to the system of Dunham (1962) shown in Table 3 or using subsequent schemes developed from it (e.g. Wright, 1992; shown in Table 4). For more detailed description and identification of textures, cements and carbonate grains, the reader should consult standard reference texts such as Bathurst (1975). The development of secondary dolomite usually results in an overprinting or obliteration of the primary texture by the characteristic pale brown sacharroidal appearance of the closely interlocking dolomite crystals. Marbles are usually coarse equigranular crystalline rocks in which traces of fracturing and bedding have largely disappeared due to annealing and recrystallization of the carbonate minerals. British Geological Survey @ NERC. 16 Carbonate Rocks - Table 3. Classification of limestones (Dunham, 1962). ROCK TYPE TEXTURAL FEATURES Grainstone Mud absent Grain-supported Packstone >10% grains Wackestone >10% grains Mudstone Carbonate mud present Mud supported BI0LOGICAL DEPOSITIONAL Matrlx-supported Graln-supported (clay and silt grade) <lO%grains>lO%grains Calcimudstone Wacke. stone withmatrix nomatrix DIAGENETIC Encrusting Organisms Rtgld Main Manygram Mostgrain binding acted to organisms component contacts as contacts are organisms baffle dominant iscement mtcrosrylol mcrosrylolites ites Packstone Grainstone Boundstone Bafflestone Framestone Cement- Clay-filled pipes and hollows Obliterative Non-obliterative In situ organisms stone Depth : diameter ratio less then 2 : 1 max. depth 30 m Condensed Fined grainstone gramstone EM traversing Magnetic Crystals >loym Sparstone Depth of investigation/coil separation Local magnetic gradient Presence of ferrous waste Sand-filled pipes and hollows Max. depth 5 m Radar Thickness of cover and conductivity Caves Depth : diameter ratio less than 2 : 1 max. depth 30 m EM traversing Depth of investigatiodcoil separation Nature of fill >30 m depth Cross-hole shooting Radar ER or E M traversing Gravity Cross-hole shooting Borehole spacing Ground conductivity Cavity infill Cavity infill, terrain Borehole spacing > 1.O at less Caverns than 10 m cover > 1.0 at 10 m cover + British Geological Survey @ NERC. 17 Carbonate Rocks Several simple chemical tests can be applied in the field to help distinguish the individual carbonate mineral species. One method is to etch a relatively flat surface of the rock with a weak acid solution (say 1/10 Normal HC1); this has the effect of preferentially dissolving calcium carbonate minerals and leaving dolomite, magnesite and other less soluble and insoluble impurities standing proud. The etching will also enhance the texture and fabric of the rock. A more sophisticated approach involves the use of staining techniques which enable more precise distinctions to be made between most of the common carbonate minerals. One of the most popular stains for carbonates - potassium ferricyanide - needs to be handled with caution and skin contact avoided. The other commonly used reagent, Alizarin Red S is not regarded as hazardous. Always follow the manufacturers guidelines when handling chemicals! Staining solutions can be prepared in dropper bottles and used in the field directly on rock surfaces. The most useful stains have been described in the corresponding ODA/BGS Industrial Mineral Laboratory Manual on Limestone (Harrison, 1992); these are summarized below: Alizarin Red S in 30% NaOH. Stains Mg-calcite and dolomite purple. Alizarin Red S in 1.5% HC1. Stains calcite and aragonite red; ferroan dolomite and ferroan calcite purple. British Geological Survey @ NERC. 18 Carbonate Rocks Potassium ferricyanide in dilute HC1. Stains iron-bearing carbonates dark blue (dolomite usually contains some iron whereas calcite does not) Potassium ferricyanide and Alizarin Red S in 1.5% HC1. Stains calcite pink or red and ferroan calcite purple. Dolomite is unaffected but ferroan dolomite turns turquoise. This technique allows all common phases to be identified in one stage (Dickson, 1965; 1966). All staining solutions should be freshly mixed and not stored for more than 24 hours before useage. Other useful summaries of carbonate staining techniques are given in Leeder (1982) and Miller (1988). Field determination of Specific Gravity (SG) using a simple balance can be useful in identifying magnesite (SG = -3.0) from dolomites (SG = 2.85) according to Wicken & Duncan (1983). Such determinations however are better performed in field laboratories. Simple determinations of the strength of samples or core can also be obtained in the field by using portable point-load testing equipment (Franklin and others, 1971; Broch & Franklin, 1972; Brook & Misra, 1970), as shown in Fig. 5. Alternatively, the Schmidt hammer test has been used (Deere & Miller, 1966; Harrison and others, 1982). However experience with such techniques has revealed problems of reproducibility in many limestone lithologies (Harrison, 1983) and these technique are probably best reserved for laboratory use on large sawn samples of carbonates with few bedding or fracture surface (e.g. marbles). A further drawback to the use of these tests is that they measure the strength of the rock rather than the strength in its final British Geological Survey @ NERC. 19 Carbonate Rocks form as a crushed rock aggregate. Geophysics Due to the tendency of carbonates to dissolve in weak acidic solutions and develop underground drainage, many carbonates and in particular limestones are cavernous and contain surficial solutional depressions which can become infilled with soil, clay and windblown sediments. Clearly the location of voids and clay-filled pockets within the deposit are important in terms of carbonate purity, available reserves, extraction plans, ground stability and hydrogeology . Although the presence of some clay-filled hollows may be deduced from soil type or geomorphology, geophysical techniques represent the best means of assessing the presence of caverns and infilled solution hollows. Table 5 shows the techniques most suited to this type of problem. Examples of the detection of shallow clay-rich pockets on limestone outcrops are given in Figures 6 and 7 . These demonstrate the effectiveness in this context of a portable non-contacting ground conductivity meter and magnetic profiling using a proton precession magnetometer. However it cannot be stressed too strongly that despite the capabilities of these techniques, some ground truth (i.e boreholes or augerholes) must be established in order to properly interpret (calibrate) the geophysical information. Where carbonates are interbedded with clay-rich beds of at least l m thick (Fig. 8) these can often be located at outcrop by traversing across British Geological Survey @ NERC. 20 CONDUCTIVITY PROFILE rnillirnhos/rn Vertical scale Horizontal scale ? 1 f Ground level 0U 2rn BOREHOLES f LI M ESTONE LI M ESTON E + Borehole terminated at 5.5rn Vertical scale x 2 horizontal 0 2rn Horizontal scale U Figure 6. Location of a clay pocket in limestone using a Geonics EM 31 conductivity meter; after Penn & Tucker (1983). ~~ ~ 47870 47860 47850 ~~ ! ~~ - - ~ ~- F(nT) MAGN ETlC TRAVERSE - n-a GEOLOGICAL SECTION SCALES: Horizontal f La/# ' - Figure 7. Location of clay-filled depressions in the Chalk of southern England using a proton precession magnetometer; after McDowell (1975). British Geological Survey @ NERC. 21 Carbonate Rocks the strike using portable ground conductivity meters. These meters have also been used to refine the outcrop pattern of carbonates in poorly exposed terrain (e.g. Zalasiewicz, Mathers & Cornwell, 1985: Fig. 9 herein). Down-the-hole geophysical logs of carbonate sequences are also useful in pinpointing clay-rich layers and units and in determining the overall stratigraphy and unit boundaries. In this situation the most useful information usually comes from the natural gamma ray logs produced by equipment such as the Mount Sopris logger shown in Fig. 10. On the gamma logs most argillaceous strata give strong positive anomalies due to a relatively high content of radioactive elements (commonly 40K). The very low frequency electromagnetic method (VLF-EM) technique has been applied in the basement terrain of Nigeria where mapping of a steeply-dipping bed of dolomitic marble (about 60m wide) within quartzites was refined (Aina & Emofurieta, 1991). The equipment used in this study was the Geonics EM 16 instrument. The rippability (ease of excavation) of carbonates can be gauged by assessing seismic refraction P-wave velocities. In simple terms, beds with P-wave velocities of less than 1200 m/sec can be considered relatively easy to excavate. With increasing seismic velocity (indicating for example less fracturing, stratification or weathering) excavation becomes progressively more difficult; rocks with velocities over about 2000 m/sec generally require blasting (Geological Society Engineering Group Working Party, 1988; Weaver, 1975.). British Geological Survey @ NERC. 22 Carbonate Rocks Figure 8. Bed of weathered tuff within Carboniferous Millers Dale Limestone, nea Buxton, England. a’ N 50 r , LC - S 1 200m , N LondonClay S WRB - Woolwich and Reading beds CK - Chalk UG - Upper Greensand G - Gault LG - Lower Greensand 200m , Figure 9. Geological map and conductivity profiles across the Hog’s Back Monocline, southern England; after Zalasiewicz and others (1985). British Geological Survey @ NERC. 23 Firmre 11. The Tororo Rock carbonatite ~ l u e southeastern . Uganda. British Geological Survey @ NERC. 24 Carbonate Rocks Carbonatites Carbonatite intrusions tend to form upstanding plugs and arcuate ridges (Fig. 11) since they are usually more resistant than associated intrusive and sub-volcanic lithologies. Because they differ markedly from other carbonates, carbonatites are investigated using a different range of techniques. Carbonatites are composed of coarsely crystalline calcite with accessory apatite and magnetite; they often resemble marbles. At outcrop they rarely show much internal structure; however the presence of any large xenoliths, magmatic segregations rich in noncarbonate minerals and cross-cutting sheet intrusions of other lithologies need to be carefully recorded since these constitute "waste" material that needs to be rejected during quarrying. Many carbonatite complexes are known; any new finds are most likely to result from the evaluation of airborne magnetic and radiometric surveys. This is due to the presence in most carbonatites of anomalously high concentrations of magnetite, uranium and rare earth elements. Airborne geophysical surveys planned specifically to search for and investigate carbonatites should be flown at low altitude with a closely-spaced grid of flightlines so as to avoid missing the often areally small anomalies produced by the bodies. In addition to their importance as a source of carbonate rock, many carbonatites contain economically important concentrations of fluorapatite (see Phosphate Rock chapter herein) and rare earth elements. British Geological Survey @ NERC. Carbonate Rocks Residual soil geochemistry has proved invaluable in the delineation of carbonatite bodies within the previously poorly-known Butiriku Complex of south-eastem Uganda (Reedman, 1974). The detailed structure of this body had evaded previous studies due to an extensive, thick blanket of residual soil. An area of 38 km2 of residual soils was sampled on a grid pattern leading to the detection of multi-element (P, Nb, REE, Zn, PB, MO and Be) anomalies which indicated the presence of buried masses of carbonatite. (Fig. 12a,c). When combined with geophysical studies, pitting and drilling, the geology of the whole complex was revealed. Ground gravimetric surveys are also useful in delineating the structure of carbonatite complexes. Again the study of Reedman (1974) illustrates how residual gravity profiles of five carbonatites from southeastern Uganda give rise to strong positive anomalies associated with masses of carbonatite (density 2.9 - 3.4) which were discriminated from lighter surrounding granitic basement (density 2.6- 2.7) and allied agglomerates (density 2.1-2.4) (Fig, 12b,c). Ground surveys using scintillometers and magnetometers are also useful in delineating the structure of carbonatite complexes and separating their constituent intrusive lithologies. Seismic techniques should also be effective in defining the interface between unconsolidated residual soil cover and the bedrock. British Geological Survey @ NERC. 26 c) Stages of exploration a t Butiriku a) Butiriku Phosphorus Anomdy c) vvvvv~" v v v v v v~vvvvvvvvv v v v v v v v~vvv~vvvvvvvvv v v v v v v v v v v v v v v v v v v v v v v ~vvvvvvvvvvv v v v v v v v v v v ...' 0 1. Original survey(after Davies) 2. Interpretation of the gravitysurvey 3 . Afterfollow-up drilling on Cu anomalies 4. Afterfollow-up workon multi-element anomalies b) Residual gravity profiles over carbonatite complexes of southeast Uganda OL V / / ' sw . - NE - TORORO SU KULU 40 - - 30 Carbonatite - 20 10 Vent agglomerate - . . Alkaline rocks O L SE NW BUKUSU Figure 12. Geopl~ysicaland geochemicd exploration of l h ~ ,E1 0 [O 7 SE NW BUT1R I KU Ventagglomerate density2.1-2.4 Carbonate, alkaline and ultrabasic rocks density2.9-3.4 Granite basement density2.6-2.7 0 0 5 kilometres British Geological Survey @ NERC. 10 ~- ~ Rut iri fcu _ carb 0~1lat ite, ~ eastern Uganda; based on Reedman (1974). ~ 27 Carbonate Rocks Drilling and sampling Once the surface distribution of a carbonate is relatively well known detailed assessment can commence to evaluate the grades and tonnages available. Individual studies vary from regional resource assessments of carbonates for all uses, to site-specific reserve calculation with a view to quarrying for a particular end-use. The type of drilling programme will depend mainly on: the level of detail required (i.e. resources or reserves), the exploration budget and the volume of core material needed for testing. Rotary diamond drilling is normally employed. Except in the most general of assessments drilling is usually carried out with regularly spaced boreholes; in more detailed work these are usually drilled on a grid pattern. In regional assessments of the Carboniferous Limestone resources of the Peak District in central England, boreholes could be correlated satisfactorily over distances up to 6 km apart (Cox, Bridge & Hull, 1977). Whereas at the other end of the scale quarry operators commonly employ drilling grids with 30 m centres to detect minor variations in properties, for example where Mg content is of concern in cement manufacture. The choice of core diameter is governed by cost, availability, or the amount of material required for testing. Core diameters between about 40 mm and 60 mm are generally used for general assessment purposes. Drilling is usually rotary diamond drilling preferably with wireline equipment and using either air-, watedfoam- or water-flushing, the latter being more effective in cherty lithologies. Smaller portable drill British Geological Survey NERC. 28 Carbonate Rocks rigs (e.g. Minuteman or Winkie) can be used in inaccessible locations and in an ancillary capacity to obtain narrow diameter (up to 22 mm) core from the first few metres of ground where coring with larger capacity drilling rigs is usually difficult (Cox, Bridge & Hull, 1977). Where large volumes of core material are needed for physical and mechanical testing (for example for roadstones) much larger diameter boreholes are drilled with diameters ranging up to about 300 mm (Carr & Rooney, 1983). It is desirable to log uncased boreholes with a portable gamma ray logger to serve as a useful indicator of clay content and guide for correlation. Core should be logged provisionally on-site, and include an assessment of the fracture index (number of discontinuities divided by unit thickness). References Aina, A. & Emofurieta, W.O. 1991. The use of very low frequency electromagnetic method for non-conductive resource evaluation and geological mapping. Journal of African Earth Sciences, 12, 609-616. Bathurst, R.G.C. 1975. Carbonate Sediments and their Diagenesis. 2nd Edition. Developments in Sedimentology 12. Elsevier, Amsterdam. Broch, E. & Franklin, J.A. 1972. The point load strength test. Institute Journal of Rock Mechanics and Mineral Science 9, 669-697. British Geological Survey @ NERC. 29 Carbonate Rocks Brook, N. & Misra, B. 1970. A critical analysis of the stamp mill method of determining Protodyakonov Rock Strength and the development of a method of determining a Rock Impact Hardness Number. Proceedings of the 12th Symposium of Rock Mechanics, Missouri, 157-165. Can, D.D. & Rooney, L.F. 1983. Limestone and Dolomite. In Industrial Minerals and Rocks, 5th Edition, 2 Vols. Lefond, S.J. (Ed) 833-868. American Institute of Mining, Metallurgical and Petroleum Engineers, New York. Cox, F.C., Bridge, D. McC. & Hull, J.H. 1977. Procedure for the assessment of limestone resources. Mineral Assessment Report No. 30, Institute of Geological Sciences, London, 14p. Deere, D.U. & Miller, R.P. 1966. Engineering classification and index properties for intact rock. Technical Report Am-ZX-65-116, Air Force Weapons Ltd. Kirkland Air Force Base, New Mexico. Dickson, J.A.D. 1965. A modified staining technique for carbonates in thin section. Nature, 205, 587. Dickson, J.A.D. 1966. Carbonate identification and genesis as revealed by staining. Journal of Sedimentary Petrology, 36, 491-505. Dunham, R. J. 1962. Classification of carbonate rocks according to their depositional texture. In Classijkation of Carbonate Rocks, Vol 1. Ham, W.E. (Ed) 108-121. American Association of Petroleum Geologists, Tulsa. British Geological Survey @ NERC. 30 Carbonate Rocks Franklin, J.A., Broch, E. & Wilton, G. 1971. Logging the mechanical character of rock. Transactions of the Institute of Mining and Metallurgy, 80, A, 1-9. Geological Society Engineering Group Working Party, 1988. Engineering Geophysics. Quarterly Journal of Engineering Geology, 21, 207-271. Harrison, D.J. 1983. Geological problems associtaed with the assessment of resources of limestone and sandstone. In Prospecting and Evaluation of Non-metallic Rocks and Minerals. Institution of Geologists, London, 127-137. Harrison, D.J. 1992. Limestone. Industrial Minerals Laboratory Manual, British Geological Survey Technical Report WG/92/29,45pp. Harrison, D.J., Wild, J.B.L. & Adlam, K. McL. 1982. South Wales Hard Rock Feasibility Study - A Preliminary Report Mineral Assessment Internal Report Series No. 82/2. Institute of Geological Sciences. M e r , M.R. 1982. Sedimentology, Allen & Unwin. 344pp. McDowell, P.W. 1975. Detection of clay filled sink-holes in the chalk by geophysical methods. Quarterly Journal of Engineering Geology, 8, 303-310. Marsh, S.H. & O’Connor, E.A. 1992. Exploration for building materials and industrial minerals in arid regions using satellite remote British Geological Survey @ NERC. 31 Carbonate Rocks sensing: progress report for 1991/92. British Geological Survey, Technical Report WC/92/15. 17p. Miller, J. 1988. Microscopical techniques: Slices, slides, stains and peels. In Techniques in Sedimentology, Tucker, M. (Ed) Blackwell, Oxford. 86-107. Penn, S. & Tucker, D. 1983. The application of geophysics to quarry overburden problems. In Prospecting and Evaluation of Non-metallic Rocks and Minerals. Institution of Geologists, London, 149-156. Reedman, J.H. 1974. Residual soil geochemistry in the discovery and evaluation of the Butiriku carbonatite, southeast Uganda. Transactions of the Institute of Mining and Metallurgy, London. Section B, 83, B112. Weaver, J.M. 1975. Geological factors significant in the assessment of rippability. Civil Engineer of South Africa, 17, 313-316. Wicken, O.M. & Duncan L.R. 1983. Magnesite and related minerals. In Industrial Minerals and Rocks, 5th Edition, 2 Vols. Lefond, S.J. (Ed) 881 - 896. American Institute of Mining, Metallurgical and Petroleum Engineers, New York. Wright, V.P. 1992. A revised classification of limestones. Sedimentary Geology, 76, 177-185. Zalasiewicz, J.A., Mathers, S.J. & Cornwell, J.D. 1985. The application of ground conductivity measurements to geological British Geological Survey @ NERC. 32 Carbonate Rocks mapping. Quarterly Journal of Engineering Geology 18, 139-148. British Geological Survey @ NERC. 33 Diatornite DIATOMITE Introduction Diatomite is a soft , pale-coloured sedimentary rock composed of the skeletal remains of diatoms - minute unicellular aquatic algae commonly 20 - 200 pm in diameter (Figs 13, 14). Diatoms precipitate a perforate cell-wall of opaline silica to protect their soft organic interior. After death the organic material decays leaving the siliceous skeletons to sink to the bottom of the watermass. Here they accumulate in vast numbers to form diatomaceous oozes which are subsequently compacted to form diatomite. Processed diatomite powders have a unique physical structure and are moderately refractory and chemically inert. They are used in a wide variety of industrial applications as filter-aids, fillers and a mild abrasives. Geological occurrence Diatoms live in a wide variety of environments from the open ocean to nearshore; and in freshwater rivers, lakes and marshes. Based on their diagnostic floral assemblages and sedimentary associations, ancient diatomites are readily classified into marine and freshwater deposits. Many freshwater and marine deposits are closely associated with volcanism (Fig. 15) which provides a ready supply of dissolved silica which the diatoms need to construct their skeletons (Taliaferro, 1933; Mulryan, 1942; Williamson, 1966; Kadey, 1983). A rapid expansion in diatom population following the supply of pyroclastic material from British Geological Survey @ NERC. 34 Diatomite Figure 13. Scanning electron micrograph showing the detailed structure of a lacustrine diatomite from Costa Rica. Figure 14. Diatomite sample from Loma Camastro, Costa Rica. British Geological Survey @ NERC. 35 Diatornite __ British Geological Survey @ NERC. _ _ 36 Diatomite an eruption has been recorded in the modern environment (e.g. Kurenkov, 1966). stratigraphic record Similar responses have been recorded in the (e.g. Abella, 1988). Under favourable environmental conditions diatomite ooze can accumulate very rapidly, perhaps at rates around 1 mm per year (Soper & Obson, 1922; Gams, 1927). Marine diatomites are commonly characterized by circular (centric), immobile planktonic forms. Diatomites may be grade into, or be interbedded with, a wide variety of sedimentary lithologies; the commonest are mudrocks, carbonates and waterlain pyroclastics. The most famous deposits are those of the Neogene Monterey and Sisquoc formations of southern California which are worked extensively in the Lompoc area (Dibblee, 1950; Woodring & Bramlette, 1950). Here shallow marine diatomite beds attain an aggregate thickness of several hundred metres and are associated with arkoses, rhyolitic tuffs, black and phosphatic mudstones and basaltic lavas. Individual beds 1 - 10 m thick are mined. Freshwater fluvial, lacustrine and paludal diatomaceous deposits are commonly rich in elliptical (pennate), mobile diatoms, which usually occur interbedded with waterlain pyroclastics, fine sands, silts, clays and peat. Economic diatomites of this type are sometimes closely associated with bentonite, zeolites and pumice. Diatomite-bearing sequences can be several hundreds of metres thick; however individual diatomite beds are commonly less than 2 m thick and are finely laminated, reflecting seasonal or episodic sediment supply. British Geological Survey @ NERC. 37 Diatomite Examples of freshwater diatomites associated with volcanism include the Neogene-Quaternary examples of the western United States (Williamson, 1966) and Costa Rica (Mathers, 1989). Recent lacustrine diatomites have been dredged from the floor of Lake Myvatn in Iceland. The impure diatomaceous deposits at Kentmere, England (Mitchell, 1934) represent Post-Glacial - Recent deposits that accumulated in a glaciated valley blocked by a terminal morraine. The close association with glaciated terrain probably reflects an abundant supply of silica derived from finely ground silica in glacial deposits (Conger, 1939). As relatively simple forms of life diatoms probably have a long geological history; fossil evidence seems to indicate that they flourished markedly in the Cretaceous and have been abundant in Tertiary and Quaternary times. However, older diatom-rich deposits may be difficult to recognize in the stratigraphic record due to their diagenetic conversion to chert. All commercially important deposits are late Tertiary or Quaternary in age. Diatomites are of variable purity, although the most commonly used commercial grades contain at least 85% SiO, and are used in the manufacture of fillers and filter-aids. The most common impurities are detrital sand and silt, clay, volcanic ash, carbonate and organic material. Impure clayey diatomite containing about 75% SiO, is used to make lightweight refractory bricks; such deposits are referred to in the industry as "moler". British Geological Survey @ NERC. 38 Diatomite Exploration and field evaluation Any exploration for diatomite must be based on an understanding of the geological environments in which it can form, and the age of strata which can be expected to contain economic deposits (see above). To be useful commercially, diatomite deposits must be only moderately compacted and free from the effects of diagenesis, chemical alteration or metamorphism. Therefore economic deposits are only found in strata of late Tertiary or Quaternary age. Conger (1942) suggests that uplift and exposure under good drainage conditions are important factors for the removal of organic material and formation of pure deposits. Literature searches and examination of regional geological maps will help pin-point potential areas, but preference must always be given to locations closest to the perceived market for the commodity or to coastal port facilities. Potentially economic deposits are most likely to be found in shallow marine and lacustrine sediments which are closely associated with penecontemporaneous volcanism. Other sites of potential interest include Quaternary - Recent lacustrine deposits in glaciated terrains and bog deposits. Recent deposits commonly lie beneath present-day valley floors and are invariably beneath the watertable; these "wet" deposits require a distinct investigative approach which is discussed separately at the end of this section. An example of a regional appraisal of diatomite occurrence and British Geological Survey @ NERC. 39 Diatomite potential of the western United States is given by Williamson (1966, figs 1 - 3). Most commercial deposits comprise uplifted and eroded marine and freshwater sediments. Reconnaissance starts with the examination of available and accessible sections. Most deposits are first located by "shows" in stream, road sections or as "float" in soils. Because of its often characteristic white colour and light weight, the location of many diatomites has been known for centuries by the local communities. In Costa Rica, for example, several rivers that flow accross diatomite deposits are called Rio "Tizatl" - the old preColumbian indigenous word for chalk - which it closely resembles (Mathers, 1989). The name thus serves as a clue to the presence of diatomite. Air photographs are likely to be useful for delineating areas of interest and locating sections at the reconnaissance stage of exploration especially in poorly known terrain where adequate topographic base maps are not available. A further possible approach is mentioned by Carter (1971), referring to developmental work then being carried out by G.W. Greene of the USGS. Greene had reportedly found that diatomite, because of its distinct thermal characteristics, could be distinguished using airborne narrow pass-band thermal infrared imagery (3 - 4 and 4.5 - 5.5 pm). The present author has tried unsuccessfully to trace subsequent details of this work. Once it is confirmed that an area contains beds of potentially-economic diatomite, more detailed exploration can be planned. Detailed geological surveying of the area, perhaps at 1:25 000 or larger scale, coupled with an air photograph interpretation, are essential to trace out the distribution of packets of strata containing diatomite. Logging of available sections should include the bed by bed recording British Geological Survey @ hERC. 40 Diatomite and sampling of the strata, involving the production of an annotated graphic lithological log together with measurements of the dip, faulting etc. In the rare event of relatively homogenous, massive beds, channel samples representing a maximum of 1.5 m of strata are recommended (Kadey, 1983), but most deposits are more thinly stratified and can be sampled on a bed-by-bed basis. Particular attention should be paid to recording the wet and dry colour, grit content, density, consolidation, alteration and any staining of the deposit. Basic observations such as these immediately enable beds of good and poor quality diatomite to be distinguished. Diatomite deposits vary in colour from snow white in relatively pure dry deposits, (Fig. 16) to greenish and olive hues in water-saturated deposits containing organic material. Colour is not however an infallible guide to purity: a snow-white diatomite deposit examined recently by the author from northern Uganda turned out to contain almost 40 % kaolinite! If relatively white deposits are encountered, moistening the sample often reveals the presence of fine lamination, showing up any variation in the sediment and in particular in the silt or clay content (Fig. 17). Fine rhythmic lamination on a mm-cm scale is common in many marine and freshwater diatomites. In contrast more coloured beds suspected of having a substantial diatomite content can be dried (often in the sun) to reveal their dry colour and so give a guide to their purity. A portable blow torch can also be used on the sample which will burn off any organic material British Geological Survey @ MRC. 41 Diatomite Figure 16. Outcrop of very pure lacustrine diatomite, near Burney, northern California. Figure 17. Rhymically laminated impure lacustrine diatomite from Los Nubes, Costa Rica. British Geological Survey @ hERC. 42 Diatomite and dry the sample to reveal its true fired colour. Samples taken to determine the in situ moisture content must be carefully sealed to avoid drying-out. A qualitative estimate of the grit content can be obtained by either chewing a small amount of the sediment or by spreading it onto a glass plate with a knife. Blocks of sediment can often be cut directly from sections in order to determine the bulk density. In a simple field laboratory grinding of dried samples with a pestle and mortar, weighing, and a standard compaction in a measuring cylinder (say tapping on a bench 20 times) will enable a crude idea of relative bulk density to be obtained. Such procedures can reveal useful information on the relative purity of individual diatomite beds or even cryptic changes in diatomite content (Mathers and others, 1990, see Fig. 18 herein). In general, samples with either a high content of solid crystalline rock and mineral grains (ie impurities) or a coarse grain size will have a higher bulk densities; (for a fuller discussion of this approach see Inglethorpe & Bloodworth, 1989). The SG is a more reliable guide to quality and can be readily determined in a field laboratory using the methodology described in the ODA/BGS Industrial Minerals Laboratory Manual on diatomite by Inglethorpe (1992). Any carbonate impurities present can be easily detected by dropping acid on the sample and checking for effervescence. A portable microscope has been used by experienced exploration geologists working for some of the major diatomite producers as a rapid means of screening samples by estimating their purity. In trained hands an initial appreciation of the diatomite assemblage present can also be obtained (Kadey, 1983). British Geological Survey @ NERC. 43 - Grain size DENSITYOF DRIED GROUNOPOWDER g / ~ r n - ~ &.P & ‘ 24 23- 0.4 0.3 0.2 0.1 Thin ash beds(<l cm) 6, .. 22 - I 21 20 I D - I I I 0 1g Thin ash beds (<1 cm) - :.... I I 18- D f 0 I 17- \ 16--* - ‘Thin? 15- - I bentonite ,seams I I Diatomite-rich sequence economic more siltyat base I 0 I 14- D / - / 0 13-1- 0’ ... - 12 - \ Ashlayer \ \ \ \ I D 11- - I 1 I I I 10-- . 9- I I I I I 8- - I 7- . I - I 6- I - 5 / Ashlayer 4- I D -Leaf remains - / Sub-economic si Ity diatom it e/ diatomaceous silt 3 -2 -= a . .. TChertnodules 7 - River level Abundant organic material Uneconomic sand-silt sequence 0.8 0 Figure 18. Graphic lithological log and bulk density of section in the Loma Camastro lacustrine diatomite of Costa Rica; from Mathers and others (1990). British Geological Survey NERC. 44 Diatomite Samples are disaggregated in water and spread onto a slide, excess water is removed by drying or pressing on a cover glass. A magnification of at least x250 is required; the refractive index of the diatoms is close to that of water making them difficult to see easily, although the quantity and nature of mineral impurities are readily apparent. This difficulty is easily overcome by the addition of a mounting medium such as Canada Balsam with a markedly different refractive index. Fuller details and discussion of this technique are given by Eardley-Wilmot (1928). In the detailed assessment of diatomites several geophysical techniques can be deployed. Seismic refraction is particularly useful for determining the overall form of poorly-consolidated lacustrine deposits where these occupy basins floored by lavas or more consolidated rocks (see pers. comm. in Kadey, 1983, page 689.). Given the marked density contrast between relatively dry diatomite and much denser intervening layers such as mudstone or limestone, it is perhaps surprising that gravity surveys have not been utilized in delineating deposits. The precise nature of modern microgravity techniques should be suitable for detecting relatively thin beds of diatomite. A further technique likely to be successful in mapping out diatomite beds within packets of diverse sediments is the use of portable ground conductivity meters (Zalasiewicz and others, 1985), which can exploit the different electromagnetic response of relatively resistive high-purity diatomite beds from much more conductive clay-rich lithologies. The principal advantage of these meters is that they can be used to make rapid surveys in accessible terrain. Detailed delineation of diatomite deposits and quantification of reserves British Geological Survey @ NERC. 45 Diatomite also requires a blend of excavation and drilling to establish the threedimensional form of the resource-body . In dry sediments hand-dug shafts using cheap local labour can be very effective in investigating poorly-consolidated sediments (see for example Stearns, 1931). The effective depth of vertical shafts is about 15 m, the sequence can be logged from a wire-cage lowered down the shaft. Relatively soft sediments can also be investigated using extendable auger systems such as those manufactured by Eijkelkamp (see Mathers & Zalasiewicz, 1985; Fig. 19). The limit of operation is normally 5 - 6 m although coarse deposits such as gravels and surficial pebbly layers considerably reduce the success rate. Where strata are slightly inclined or in areas with some topographic relief, bulldozeror hand-dug trenches provide effective means of examining and sampling the sequence. Drilling of diatomite-bearing sequences is difficult due to the relatively poorly-consolidated nature of many deposits. Furthermore the heavily consolidated variants which might be expected to core well are unlikely to be of economic interest. Experienced drill crews are required. Kadey (1983) states that 'little if any, good quality diatomite will produce satisfactory core with a diameter less than 10 - 15 cm'. Where diatomite occurs at relatively shallow depths (less than 30m) in wholly soft or poorly consolidated sequences the use of a percussion (shell and auger) drilling rig is recommended. Equipped with a 'down the hole sliding hammer' they can be used recover undisturbed 'U100' (formerly U4) samples in preparatory steel tubes preferably fitted with split plastic sample liners to facilitate easy removal and sample logging. If immediately wrapped in polythene and sealed these samples British Geological Survey @ NERC. 46 Diatomite B Figure 19. The Eijkelkamp extendable auger system with cutting shoes for A, sand and gravel, B, clays, and C, soft and water-logged sediments. British Geological Survey @ NERC.