International Geology Series Industrial Minerals Exploration

BRITISH GEOLOGICAL SURVEY

TECHNICAL REPORT WC/93/ I5

International Geology Series

Industrial Minerals Exploration Guide No. 1

BIOGENIC SEDIMENTARY ROCKS

S J Mathers

Natural Environment Research council

International Division

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. Exploration Guide No. 1. Biogenic sedimentary rocks. British Geologic01 Survey Technic01 Report WC/93/15.

Industrial Minerals

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

INTRODUCTION

CARBONATE ROCKS

DIATOMITE

PHOSPHATE ROCK

SULPHUR

Page

1

4

33

52

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 geologically-

based 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

- 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

- Quartz crystal

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.

Mineral Formula S.G. Hardness Crystal System

Calcite CaCO, -2.7 3.0 Trigonal

Aragonite CaCO, -2.95 3.5-4.0 Orthorhombic

Dolomite CaMg(CO,), -2.85 3.5-4.0 Trigonal

Magnesite MgC0, -3.0 3.5-4.5 Trigonal

Solubility (cold HC1)

Good

Good

Poor

Slight

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


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


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.


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


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.


11

Sandstone hills Limestone platform (with surface drainage) Clay plain

&

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.


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


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


14

Carbonate Rocks

~

Figure 4. Carboniferous Limestone, Gordale Scar, northern England.

Figure 5. Field use of point-load testing equipment.


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.


16

TEXTURAL FEATURES

Mud absent

Grain-supported

>10% grains Carbonate mud present

Mud supported

>10% grains

Carbonate Rocks -

Table 3. Classification of limestones (Dunham, 1962).

ROCK TYPE

Grainstone

Packstone

Wackestone

Mudstone

DEPOSITIONAL BI 0 LOGICAL DIAGENETIC

Obliter- ative Matrlx-supported Graln-supported In situ organisms Non- obliterative

(clay and silt grade)

<lO%grains>lO%grains withmatrix nomatrix Encrusting Organisms Rtgld Main Manygram Mostgrain Crystals binding acted to organisms component contacts as contacts are >loym organisms baffle dominant iscement mtcrosrylol mcrosrylol-

ites ites

Calci- Wacke. Packstone Grainstone Boundstone Bafflestone Framestone Cement- Condensed Fined Sparstone mudstone stone stone grainstone gramstone

Clay-filled pipes and Depth : diameter ratio less EM traversing Depth of investigation/coil hollows then 2 : 1 max. depth 30 m separation

Magnetic

Sand-filled pipes and Max. depth 5 m Radar hollows

Caves

Caverns

Depth : diameter ratio less than 2 : 1 max. depth 30 m

EM traversing

>30 m depth Cross-hole shooting > 1 .O at less than

10 m cover > 1.0 at 10 m + cover

Radar ER or E M traversing Gravity Cross-hole shooting

Local magnetic gradient Presence of ferrous waste

Thickness of cover and conductivity

Depth of investigatiodcoil separation

Nature of fill

Borehole spacing Ground conductivity Cavity infill Cavity infill, terrain Borehole spacing


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.


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


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


20

CONDUCTIVITY PROFILE rnillirnhos/rn

0 2rn Vertical scale

Horizontal scale U

Ground f ? 1 BOREHOLES level

f LI M ESTO N E LI M ESTO N E

+ Borehole terminated at 5.5rn

Vertical scale x 2 horizontal

Horizontal scale U 0 2rn

Figure 6. Location of a clay pocket in limestone using a Geonics EM 31 conductivity

meter; after Penn & Tucker (1983). ~- ~~ - - ~ ~~ ~~ ~

47870 F(nT)

MAGN ETlC 47860 TRAVERSE

f - n-a ! La/# 47850

GEOLOGICAL ' SECTION

SCALES: Horizontal -

Figure 7. Location of clay-filled depressions in the Chalk of southern England using a

proton precession magnetometer; after McDowell (1975).


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.).


22

Carbonate Rocks

Figure 8. Bed of weathered tuff within

Buxton, England.

LC - LondonClay

WRB - Woolwich and Reading beds

CK - Chalk

UG - Upper Greensand

G - Gault

LG - Lower Greensand

Carboniferous Millers Dale Limestone, nea

a’ N 50 r

S 1

, 200m ,

N S

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 non-

carbonate 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.


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 south-

eastern 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.


26

40

30

20

1 0

-

O L

a) Butiriku Phosphorus Anomdy

- - -

-

c) Stages of exploration a t Butiriku

0

b) Residual gravity profiles over carbonatite complexes of southeast Uganda

NE O L

sw V//' . - -

SU KU LU TORORO

NW SE

v v v v v ~ "

v ~ v v v v v v v v v

v ~ v v v ~ v v v v v v v v v

~ v v v v v v v v v v v

v v v v v

v v v v v v

v v v v v v v v v v v v v v v

v v v v v v v

v v v v v c) v v v v v v ...'

1. Original survey(after Davies)

3 . Afterfollow-up drilling on Cu anomalies

Carbonatite

Vent agglomerate

Alkaline rocks . .

2. Interpretation of the gravitysurvey

4. Afterfollow-up workon multi-element anomalies

BUKUSU

Figure 12. Geopl~ysical and geochemicd exploration of l h ~ ~ ~-

, 1 0 [ 7 E O Rut iri _ - ~ ~ fcu carb 0 1 la t ite, eastern ~ Uganda; based on Reedman (1974).

NW SE

BUT1 R I KU

Ventagglomerate density2.1-2.4

Carbonate, alkaline density2.9-3.4 and ultrabasic rocks 0 Granite basement density2.6-2.7

0 5 10

kilometres


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


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.


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.


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


31

Carbonate Rocks

sensing: progress report for 199 1/92. 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, B1-

12.

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


32

Carbonate Rocks

mapping. Quarterly Journal of Engineering Geology 18, 139- 148.


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


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 __ _ _


36

Diatomite

an eruption has been recorded in the modern environment (e.g.

Kurenkov, 1966). Similar responses have been recorded in the

stratigraphic record (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.


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".


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


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 pre-

Columbian 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


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.


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).


43

24

23-

22 -

21 -

20 -

1 g -

1 8 -

1 7 -

16--*

15-

1 4 -

13-1-

12 -

1 1 -

10--

9-

8 -

7-

6 -

5

4 -

3-

2

‘

6, . .

D

.::

...

D

- -

D -

... -

D -

.

-

. -

-

D -

-

-= a . . .

Grain size

- River level 0

& & .P

Thin ash beds(<l cm)

Thin ash beds (< 1 cm)

‘Thin? bentonite ,seams

Ashlayer

Ashlayer

-Leaf remains

T C h e r t n o d u l e s Abundant organic

7 material

Diatomite-rich sequence economic more siltyat base

Sub-economic si I ty diatom i t e/ diatomaceous silt

Uneconomic sand-silt sequence 0.8

DENSITYOF DRIED GROUNOPOWDER g / ~ r n - ~

0.4 0.3 0.2 0.1 - I I I I I I I

f

I \

I I

I I

0

0

0

I /

/ 0

\ 0’

\ \

\ \ I I

I I I I I I I I I I I

I I

1

/ /

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


45

Diatomite

also requires a blend of excavation and drilling to establish the three-

dimensional 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, bulldozer-

or 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


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.


Documents

International Geology Series Industrial Minerals Exploration