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Lithologging – Borehole Cores and Sampling techniques

Exploration Geology

Lithologging – Borehole Cores and Sampling techniques

Lithologging of drill core of boreholes is an important stage in mineral exploration and it is done in reconnoitery, exploratory and evaluation stages of drilling to characterise the lithology.

What is lithologing?

Lithologging is systematic, depth wise enumeration of details of Core/Sludge samples or rock exposure in cross sections.

Objective of lithologging:

The basic objective of lithologging of core is to provide a factual, accurate and concise record of the important geological characteristics of exploration significance. Standard drill core logging and formation identification will assist in understanding and interpretation of localised stratigraphy, structure, depositional environment and thickness of rock unit encountered in the boreholes. The basic observations recorded while carrying out lithologging include sediment or rock type, formation boundaries or lithological contacts, facies correlation, physical properties such as colour, texture, mineralogy, grain size, grain-angularity, angle of planar structures etc. Accurate sample description is basic geologic work – the foundation upon which the entire structure of subsurface investigation rests. The source, transporting medium, environment of deposition, and post-depositional history of the sediments all can be determined by sample examination. Most important aspect of lithologging is that it provides basic idea on the nature, type, grade and thickness of the mineralization within the host rock. The lithologging of sludge/chip samples recovered from DTH drilling (non core drilling) does not require very extensive work because there is lot of mixing and contamination in the samples received. Hence, only colour and rock type may be noted.

Required Equipment:

The geologist should ensure availability of certain field items while Lithologging like field diary, scintellometer (For radiometric logging), core logging sheets, hammer, steel tape (3m), penknife, streak plate, hand lens, magnet, protractor and acid.

Geological tools required for lithologing

Fig. 1- Geological tools required for lithologing

How to carry out lithologging? 

The procedures followed for core display, core arrangement in the core boxes and core marking, examination of drill core, descriptive lithologging etc. are discussed below in different sub-headings one by one:

 Core display: The borehole core is displayed, either spread on the corrugated GI sheets or channel ways dug in the ground or stacked in the core boxes. If the core is already kept in the core boxes the arrangement would normally be in “Book Fashion” where one can examine the core as reading a book. The other type of arrangement is “Serpentine fashion”.

Fig. 2- Different styles of core display in core boxes. Arrow is indicating the direction of progress of the borehole.

Core and Core boxes: The core boxes can be wooden or metal boxes 1.5 m long with 4 to 5 longitudinal compartments. The core boxes should invariably be marked with inscriptions like, Name of the area, Box Number, Borehole Number, Inclination and Depth on the top and front.

Core markings: Depth of the borehole is marked at the end of every run usually of 3.05m lengths. However these can be of shorter lengths. As a rule the driller must mark every run independent of its length. The other marks on the core are a ‘T’ indicating the top side of the hole and an arrow indicating the direction of progress of the borehole. These markings should be on every piece of core.

Examination of Core: Following parameters are examined in core and the details may be entered in core logging sheets/register. It may be mentioned here that the core logging should be done on the wet core as it reveals the features sharply.

Core recovery: Expressed in terms of percentage it is the length of core recovered against the meterage drilled in a run of the borehole. This parameter is very important because the part of the core that is lost is to be distributed within the run and measurement of any depth within the run is to be calculated taking core recovery into account. How the core recovery is distributed will be dealt in later section on core assay.

Broad Lithology: Enumerating the broad lithology based on local stratigraphy of the area is the first step in lithological examination of the core. The very purpose of lithologging is to broadly correlate the various lithological formations in the area. This also helps in understanding the inter-borehole correlation and tallying up the hypothetical sections of the boreholes with the factual details.

Descriptive lithologging: This column should be filled with maximum details. At first a broad lithology of the core is must and finer variation at different depths may be noted mentioning depth in parenthesis. These may also include features like mineralogy, grain size, textural variations, litho-contacts and their types (gradational, sharp or erosional), associated minor minerals, inclusions, veins and their relation with the country rocks etc. In the sedimentary terrains the Lithologging should be done at close intervals and may include variation in grain size, sphericity, roundness, sorting, colour (Shade cards may be used), facies variations, type and nature of matrix, degree of effervescence, sedimentary structures etc. Various alteration and secondary features like silicification, kaolinisation, ferruginisation, sericitisation, chloritisation etc, formation of vugs and cavities and infilling materials may also be noted depth wise.

Angle of planar structures: Dip of the foliations, fractures and other planar structures should be noted with respect to core axis. Angles with respect to horizontal can be deduced by taking deviation data into account. Directions cannot be measured because the cores are un-oriented.

Structural features: These include recording of various deformational features namely joints, folds, faults, fractures etc. Details of weak zones, micro-faults, zones of brecciation, pulverization etc may also be examined, recorded and tallied with the core loss zones.

Mineralisation: Visible mineralisation of significance usually sulphides, magnetite, apatite etc may be included here. Visual estimates to quantify the minerals may also be made in terms of barren, low, medium or high grades. Such details may later on help in generating data on by-products. This also helps in determining the sampling intervals while assaying.

Geotechnical features: A very preliminary study of geotechnical character can be made by examination of core. It is called rock quality designation (RQD). It is the ratio expressed in terms of percentage of recovered with intact length of more than 10cm length to the total run by the borehole.

On the basis of the percentage RQD the rocks can be classified over a rock quality scale with excellent RQD (90%) at one end and very poor RQD (<25%) at the other, which would indicate that the rock is very friable at poor end or self-supporting at the excellent end. These details are required at the time of mine planning. Another feature, which may be recorded, is joints per meter. It may be mentioned here that geotechnical studies of these nature should be done on the unbroken core hence it is advisable to perform Lithologging before the core is packed in the core boxes.

It is very essential to do sampling for further studies before discarding the core/sludge. To get the best results and interpretation proper sampling is required. So in second part of article I will discuss about sampling techniques.

Sampling Techniques:

Definition of drilling says that it is a method of sample collection from depth and the data obtained is by far more reliable than the surface data. The sampling by drilling can be regulated by planning the boreholes at various spacing. The location of the samples to be obtained by drilling is pre-decided in the space, hence depth of intercept of ore zone is more or less known before drilling. Drilling provides core and sludge samples. Drilling method that recovers core samples is best in mineral exploration. Sampling in mineral exploration represents a very important phase, because it directly relates to the representative of the rocks or ore being sampled and hence the confidence level at different stage of an exploration programme.

What is sampling?

Systematic collection of representative fraction of any material from its bulk is called sampling. As defined by Boxter and Parks, “Sampling is the process of taking small portion of an article such that the consistency of the portion shall be representative of the whole.” It may also be considered as a process of removing a part convenient in size for testing, from a whole which is of much greater bulk in such a way that the portion and distribution of the quality to be tested (metal content in the case of mineral exploration) are the same in both, the whole and that part removed.

None of the sample can be representative to the whole in a true sense, but to access the whole there must be a process that reasonably relates to the consistency with fair degree of confidence. For this, it is necessary that an explorer does not depend on a single sample but look for a reasonable greater number of samples. Variation in uranium ore body normally follows a rule, rather than an exception. Difference in ore grades within one body can be established by taking more number of samples at different intervals.

Nomenclature of samples has no regular basis. Classification can be made on the basis of nature of sample like rock, water, soil, plant sample etc., but more frequently used classification is based on sampling methods which includes:

(a) Point sampling: Random, grab or specimen.
(b) Linear sampling: Chip sample, channel samples, core samples and sludge sample.
(c) Volumetric sampling: Bulk sample.

a) Channel sampling: Channel sampling is the most important method of sampling in the detailed examination of ore body that out crops in the earlier stages. The method involves cutting channels across the face of exposed ore and collecting the resulting chips. Channel sampling involves the following steps.

(i) Before the channel is cut it is required to clean the exposure of dust, slime, soluble salts and un-wanted vegetation. It is better to chip off the outer part of rock along the band within which the sample is to be taken. After the surface is cleaned, location of channel is inscribed, by two parallel lines on the rock a few inches apart using chalk or paint. The channel should be at right angle (normal) to the plane of layering. This will give the shortest length to cut and the variation across the thickness of the ore body that is exposed. Sometimes cutting of channels at a convenient angle, which crosses all the layers at same angle, is also preferred.

(ii) Cutting of channel is done with the help of hammer and a chisel. It is necessary that the cuttings are equal from each part of the channel, for this width and depth of the channel is kept uniform. All the cuttings, chips and dust should be collected on a canvas sheet or an iron pan. Maximum care should be taken to collect all the pieces without losing them. Different parts of a channel are sampled individually at a regular interval (about 30 cm in the particular direction).

(iii) For better understanding of the ore body and understanding the variation related to different lithological or other visible variation in the rocks, it is required to sub divide each channel according to the variations. The samples of each sub divisions are collected separately. This method has the advantage of greater accuracy as samples of harder and softer rocks are not mixed so the inaccuracy caused by cutting more bulk of softer rock in comparison to harder one is avoided, as well as the data in respect to variation within the thickness of ore body and its hang wall and foot wall rocks is also available.

b) Chip sampling: Certain drilling methods like rotary and hammer drilling produce rock chips. These are sampled at every 1 to 3 meter interval as the case may be. The sample should be collected in a small bucket or dish from unrestricted natural fallout from hole. The receptacle should be positioned so that:

(i) It gets filled gradually over the entire sampling interval.
(ii) It contains a fair proportion of fine and coarse material.

The samples are assayed individually and by making a composite sample to avoid analyzing large number of samples in view of the high cost of analysis as in case of gold etc.

c) Sludge Sampling: Sludge samples are collected in case of non-core drilling. In this case there is no water circulation and the dust, fine particles and sludge brought up by the compressed air are collected in polythene sample bags. Here the disadvantage is that the propped depth of the sample is not known and chance of contamination is very high.However the sludge is the integral part of the sample and its maximum recovery when drilling through mineralized zones is an important part. To obtain true sludge sample certain precautions are necessary.

(i) It must be ascertained that all the water pumped down the hole is returned. If not, it means that part of the sludge is being lost.
(ii) The drill hole must be free from tendency to cave by abrasion of drill string; otherwise the sample would be contaminated by foreign material.
(iii) Sufficient velocity of water must be maintained to raise the heaviest particles.
(iv) The tendency of sulphides to adhere to the drill rods and casing must be overcome by keeping drill rods as free from oil/grease as far as possible.
(v) The hole should be of uniform size.

Various methods for collecting the sludge are in common use like tub method, sludge box or Thompson sludge cutter. In non-coring boreholes the samples are generally collected at 1m intervals. The sludge then allowed to dry and sent for various assay.

d) Core sampling: Core sampling requires more accuracy because the overall financial scrutiny of a deposit depends on it. Depending upon the nature of mineralization such as vein type, lensoid or tabular, the sampling density is selected. More the variation in the ore body, the closer should be the sample interval. Drilling being costly affair needs to be planned in such a way that the numbers of boreholes are restricted without sacrificing the objective and clarity of purpose.

Preparation of samples:

Preparation of the samples is the next step because the samples collected are required to be sent to various laboratories for various types of analysis hence the quantity and size of samples differ with the objective.

a) Samples from reconnaissance survey are packed in sample bags with number, location, rock type, formation details etc marked on it.
b) Core samples in exploration require more detailed preparation. Core of ore zone is kept in core boxes. After the physical assay of the core of the ore zone, it is split into equal halves longitudinally with the help of core splitter. One half of the split core is preserved in the core boxes wrapped in a polythene bag with details like Sample No., B.H. No., depth and grade etc, for further reference. This half should not be disturbed in any circumstances.

 

Fig. 3- Hydraulic core splitter for preparation of core samples.

Preservation of core samples:

Core skeletonization: Non-mineralized core is required to be preserved for future reference and study. But the length of this core will involve huge number of core boxes and their storage may become an unnecessary burden. The non-mineralized core is therefore subjected to skeletonization, which is a process of keeping representative core after discarding the unwanted core out of the entire non-mineralized core from below and above the ore zones. Normally a core piece of about 15 to 20cm length at every 10m or at the change of the lithology is kept and the rest of the core is discarded. Care should be taken to preserve full section, which exhibits any special features such as sulphide mineralisation, structural elements or any other exhibits worth keeping. The core length, which is preserved, is termed as “Salvaged Core”.

Conclusion:

It is obvious that sampling and logging of the core are the main aspects in mineral exploration programme. Different sampling techniques are used for various determinations in different stages of exploration. It is the geologist’s duty to take maximum care in sampling. Optimization of sampling is equally important for cost effective exploration programme without compromising with the confidence level in the estimates.

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Exploration geologist specialising in field geology, geological mapping, clastic sedimentology and uranium geology.

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