GB2572569A

GB2572569A - Nugget effect grade assessment

Info

Publication number: GB2572569A
Application number: GB1805456.9A
Authority: GB
Inventors: Mikhailov Alexandr; Yasilyev Igor; Thorne Rowan
Original assignee: Mineral Explor Network Finland Ltd
Current assignee: Mineral Explor Network Finland Ltd
Priority date: 2018-04-03
Filing date: 2018-04-03
Publication date: 2019-10-09
Anticipated expiration: 2038-04-03
Also published as: GB2572569C; GB2572569A8; GB201805456D0; GB2572569B

Abstract

A method and system for analysing the precious metal content of a geological sample. The method comprises using a spectrometer device 15 to direct primary electromagnetic (EM) radiation at a plurality of positions on the sample 10 to excite atoms and to detect secondary radiation emitted from the sample at each position, and processing the detected secondary radiation to assess the metal composition of the sample. A system includes a receptacle for receiving the sample and the spectrometer device which are movable relative to one another in order to scan the sample. The spectrometer may be an XRF device or a laser emission (laser induced breakdown) device operating in the IR, visible or UV regions.

Description

The present invention relates to methods and apparatus for assessing the precious metal content of geological samples. In particular, the invention provides techniques for exploiting the ‘nugget effect’ to make an assessment regarding whether a rock sample is likely to be a low, medium or high grade precious metal sample.

Background

Gold, silver and platinum are commonly known as precious metals. This is because the metals are relatively rare and in high demand, making them valuable commodities. The term 'precious metals' can also refer to other materials, such as ruthenium, rhodium, palladium, osmium and iridium.

Precious metals are generally found in their metallic form in nature, such as in nuggets, veins, or discrete particles, rather than as oxides or other compounds. This is due to the metals' relative chemical inertness. In addition, precious metals have a high malleability, density, and ductility, meaning that precious metals do not naturally disperse homogeneously through a portion of rock. As a result, it is relatively difficult to accurately, or reliably determine the precious metal content of a given geological material (e.g. rock formation). This is known as the nugget effect.

The nugget effect is defined as the inability to take a representative sample from a portion of precious metal bearing geological material. For example, a first sample of a given portion of material may contain no nuggets or particles of precious metal, suggesting that no precious metal is present in the material, but a second sample of the same size and same portion of material may contain multiple nuggets of precious metal. As such, the nugget effect is seen as a significant barrier to precious metal exploration, as sites that contain profitable levels of precious metal can be incorrectly diagnosed as comprising insufficient concentrations to make excavation worthwhile.

-2It is common to pulverise hard rock samples prior to analysing their chemical composition in order to attempt to minimise the nugget effect. The aim is to grind the material as fine as possible (e.g. to a homogeneous powder) such that the precious metal particles are more evenly distributed throughout the sample. Commonly, a sample is defined as pulverised if the average particles size is 75 pm or less.

The pulverised material is then assayed to analyse the precious metal content. Different assaying methods, for example titration or cupellation, are used to analyse the concentration of different precious metals (e.g. gold, silver and platinum) in a sample. Some assaying techniques involve the application of heat and/or chemicals such as acids and result in the complete destruction of the sample, such that the method cannot be repeated.

There are many disadvantages to the above techniques. In practice it is difficult to fully pulverise precious metal particles, as the precious metal (e.g. gold) will usually be beaten into flakes, rolled into cylinders and/or smeared onto the grinding surfaces during preparation, rather than being reduced to particles as fine as the host rock. Thus the pulverised sample is not homogeneous and so the nugget effect has not been negated.

In addition, each sample extracted from a particular site usually has to be sent to an external laboratory to be pulverised and/or assayed. The laboratory is often very far away from the extraction site and it takes a number of weeks for results to be received. This process is expensive and time consuming, particularly where large volumes of samples are required.

In order to completely negate the nugget effect the sample would have to be assayed to extinction and the results used to calculate the overall concentration, or a screened assay performed in which gold particles or nuggets are screened out and assayed separately. This is even more costly and time consuming than the above techniques.

X-ray fluorescence (XRF) is a non-destructive method for analysing the elemental content of a material. An XRF analyser is a type of spectrometer comprising an x-ray source which emits primary x-rays onto a sample. The atoms within the sample absorb the primary x-rays

-3and are excited and an electron is ejected. To stabilise the atom an electron from a higher orbital drops down to fill the hole left by the ejected electron, thereby releasing a fluorescent (or secondary) x-ray. The energy of the fluorescent x-ray is equal to the specific difference in energy of the two electron states (i.e. before and after the transition to fill the hole). Each element (e.g. gold, silver, platinum) has a unique set of energy levels and thus produces a set of characteristic fluorescent x-rays. Therefore, by measuring the energy of the fluorescent x-rays emitted, the XRF analyser can quantitatively determine the amount of each element present in the sample.

It is well known that XRF analysers can only be used to accurately determine the precious metal content of a sample if the precious metal concentration is very high, such as in jewellery. XRF analysers are not generally used to analyse the precious metal content of geological samples as the nugget effect would result in unreliable readings.

It is an object of the present invention to provide improved methods and apparatus for assessing the precious metal content of a geological sample.

Summary

In a first aspect of the invention there is provided a method of assessing the precious metal content of a geological sample, comprising:

using a spectrometer device to direct primary electromagnetic (EM) radiation at a plurality of positions on the sample to excite atoms and to detect secondary EM radiation emitted from the sample at each position; and processing the detected secondary EM radiation to assess the composition of the sample.

Advantageously, the present invention only requires a spectrometer device to conduct a spectroscopic analysis at multiple positions on the sample. This method is therefore much simpler and cost effective than traditional precious metal assaying techniques.

-4Moreover, as the equipment required is relatively affordable and easy to use, the method of the present invention does not require the sample to be sent to an external laboratory. This is because the spectroscopic analysis of the sample can be done on site (e.g. at the sample extraction site) and/or by the geologists themselves. However, the present invention is not designed to replace the need for assaying of samples. Rather, it is envisaged that the method of the present invention is to be used in conjunction with traditional precious metal assaying techniques.

For example, the method of the present invention can be used to assess whether the sample is likely to contain any precious metal or not. In other words, the method may be used to assess (e.g. semi-quantitavely) whether the sample is likely to comprise an amount or concentration of a precious metal above a given threshold. The samples which are of further interest (e.g. likely to comprise any precious metal, or an amount of a precious metal above the threshold) may then be analysed further, for example at an assaying laboratory.

Thus, the method may be an initial assessment method used to reduce the number of sample which are sent to be assayed. This is time and cost effective, as less samples will need to be transported and paid for and the wait for quantitative results may therefore be reduced. As such, the feasibility of a potential precious metal mining site can be determined more rapidly.

In contrast to the prior art, the method of the present invention does not try to eliminate or negate the nugget effect. Accordingly, the method of the present invention does not require the sample to be pulverised before it is analysed. This again reduces time delays and costs. Instead, the present invention requires a spectroscopic analysis of the sample to be conducted at a plurality of positions on the sample. Therefore, if a ‘nugget’ of a precious metal is only present at one of these positions, it should still be detected. Whereas, if the same nugget was pulverised and evenly distributed across the whole sample the concentration of precious metal may be too low to be detected and/or incorrectly deemed negligible or not viable for extraction.

-5Spectroscopic analysis of the sample may be non-destructive so that the method can also be repeated, for example to analyse a particular portion of the sample in more detail. This would not be possible using some traditional precious metal assaying methods.

Optionally, the plurality of positons are arranged such that the entire sample is analysed by the spectrometer device. For example, the plurality of positions may be distributed across the entire upper surface of the sample.

Optionally, the geological sample is a rock sample.

Optionally, the rock sample is a crushed rock sample. For example, the crushed rock sample may be extracted using percussion drilling, such as a rotary air blast or reverse circulation drill. These drilling methods are commonly used in mineral exploration and these methods produce a crushed or partially crushed rock sample, rather than a solid section of rock.

It is important to note that crushing is not the same as pulverising in geological terms. A crushed sample is much coarser than a pulverised sample, typically having a grain size of the over of a millimetre rather than 0.01 millimetres. As mineral exploration often results in a crushed sample being extracted, the use of a crushed rock sample is straight forward and advantageously does not require any additional work.

In some embodiments, the crushed rock sample may have an average grain or particle size of between 0.1 mm and 5 mm. In particular embodiments, the average grain size may be between 1 mm and 2 mm.

Alternatively, the sample may be a solid rock sample. For example, the sample may be extracted using diamond drilling which extracts a solid rock core sample, rather than a crushed rock sample.

Optionally, the rock sample is cut into slices before it is analysed.

-6The primary EM radiation will only be able to penetrate to a given depth, thus the depth of the sample should not exceed the maximum penetration depth of the EM radiation or else not all of the sample will be analysed. In some embodiments, the depth of the sample will already be within these limits before it is received for analysis.

The method may therefore include measuring the depth of the sample. If the depth of the sample exceeds the maximum penetration depth of the EM radiation then the method may include the step of reducing the depth of the sample to be less than or equal to the maximum penetration depth.

In some embodiments the method comprises reducing the average depth of the sample to 10 mm or less.

Optionally, the average depth of the sample is reduced to 2 mm or less.

The method may include ensuring that the sample has a uniform depth at each of the plurality of positions. This may ensure that the spectrometer device can analyse the same amount of material at each position. For example, the sample may be levelled off or the crushed rock sample may be distributed in an even layer of a given depth.

Optionally, the primary EM radiation comprises x-rays. In other words, the spectrometer may be an x-ray spectrometer device comprising an x-ray source.

Optionally, the spectrometer device is an x-ray fluorescence (XRF) analyser.

Optionally, the spectrometer device is a laser emission spectroscopy device. Instead of emitting primary x-rays, a laser emission spectroscopy device emits a primary laser. The primary laser may be focused onto the sample to form a plasma. The plasma atomizes and excites the sample. This is also known as laser induced breakdown spectroscopy.

The use of laser emission spectroscopy may be advantageous as it is much faster than x-ray spectroscopy using an XRF analyser, thus the time taken to analyse the sample at each of

-7the positions is reduced. However, laser emission spectroscopy is at least partially destructive, as the laser bums or ablates away material from the surface of the sample, typically the top 0.5 mm or so of material.

The method may include placing the sample in a receptacle before it is analysed. For example, the receptacle may be a tray, a box, a bag (such as a sealable bag), or other container. The receptacle may encase the sample.

Optionally, the receptacle comprises a series of markers indicating the plurality of positions at which the sample is analysed.

The markers may be applied to a surface of the receptacle, for example by printing, embossing, painting or adhesive. Alternatively, the markers may be integral to the receptacle.

Optionally, the markers divide the receptacle into a grid and the sample is analysed at each segment of the grid. For example, the markers may comprise a 2D grid or matrix formation applied to a surface of the receptacle.

Optionally, the receptacle is transparent to visible light.

The number and arrangement of the plurality of positions at which the sample is analysed depends on the properties of the material being assessed. For example, if due to the geographical location it is expected that the precious metal particles (or nuggets) will be relatively coarse or large, then the positions can be spaced further apart on the sample (less dense).

If it is expected that the precious metal particles will be finer (smaller) then more measurements will have to be taken and each of the plurality of positions will have to be positioned closer together. This is because there is more chance of the precious metal particles being missed (falling between positions) if the nuggets or particles are smaller. As

-8mentioned above, it may be preferred that the entire sample is analysed by the spectrometer device.

The method may include repeating the analysis with an increased number of positions (reduced spacing between each of the plurality of positions). For example, if the initial analysis of the sample is inconclusive or potentially anomalous results are detected, then the method can be repeated by analysing the sample at a higher number of positions.

Optionally, the sample may be washed before it is analysed by the spectrometer device.

Washing the sample may remove some of the minerals or particles that do not contain any precious metals. This increases the detection limit (the lowest quantity of a precious metal that can be distinguished from the absence of that substance).

The method may comprise moving the spectrometer device relative to the sample to each of the plurality of positions on the sample.

Additionally or alternatively, the method may comprise moving the sample relative to the spectrometer device to allow the spectrometer device to analyse the sample at each of the positions.

Optionally, the sample is placed on a conveyor belt or moveable platform. In some embodiments, the sample may be placed directly on the conveyor belt or the moveable platform, in other embodiments the receptacle containing the sample may be placed on the conveyor belt or the moveable platform.

Optionally, the sample and/or the spectrometer is moved using a robotic tool.

In some embodiments, the sample may be inserted into a tube or channel. The tube or channel may be inclined such that the sample moves under the force of gravity relative to the spectrometer device. The angle of inclination of the tube or channel may adjustable to control the speed of the sample.

-9In some embodiments a plurality of spectrometer devices may be used to analyse a single sample. This would reduce the time taken to analyse the whole sample. In addition, it may provide more information regarding the sample, if different types of spectrometer devices are used.

Optionally, the spectrometer device records the intensity of the secondary EM radiation emitted at each position on the sample as a function of the energy of the secondary EM radiation.

The step of processing the detected secondary EM radiation may then comprise determining whether any peaks in intensity of the secondary EM radiation are generated by a precious metal.

The step of assessing the composition of the sample may comprise determining whether the sample contains any precious metal or not. The method may include determining whether the concentration of at least one precious metal in the sample is likely to exceed a given threshold.

For example, the method may include calculating the average concentration of one or more precious metals in the sample as determined by the spectrometer device. The threshold at which the sample is deemed to be of interest by anything above 0 ppm (parts per million). In other examples, the threshold may be 0.5 ppm, or 1 ppm.

The method may include determining whether the sample should be analysed further. For example, sending the sample to be assayed (i.e. by an external laboratory) if it is determined that the sample is likely to comprise a significant concentration of precious metal(s), or an amount of precious metal above the given threshold.

The precious metal may be one or more of silver, gold and platinum.

-10In a second aspect of the invention there is provided a system for assessing the precious metal content of a geological sample, comprising:

a receptacle or surface for receiving a geological sample; and a spectrometer device, wherein the spectrometer device is configured to move relative to the sample and/or the receptacle or surface is configured to move relative to the spectrometer device, wherein the spectrometer device is configured to direct primary electromagnetic (EM) radiation at a plurality of positions on the sample to excite atoms and to detect secondary EM radiation emitted from the sample at each position.

The technical advantages of the second aspect of the invention are as defined above in relation to the first aspect of the invention.

The second aspect of the invention may comprise any feature described in relation to the first aspect of the invention, and vice versa.

For example, the receptacle may be a tray, a box, a bag (such as a sealable bag), or other container. The receptacle may encase the sample.

Optionally, the receptacle may be transparent to visible light.

Optionally, the spectrometer device is an x-ray fluorescence (XRF) analyser.

In some embodiments the spectrometer device and/or the receptacle may be moved by hand (i.e. manually by a user). For example, the XRF analyser may be a handheld device.

Optionally, the spectrometer device is a laser emission spectroscopy device. In some embodiments, the laser emission spectroscopy device may emit an infrared, visible or UV laser.

The system may further comprise a conveyor belt or moveable platform onto which the receptacle is placed.

-11Optionally, the surface for receiving the sample may be a conveyor belt or moveable platform.

The conveyor belt or moveable platform may be controlled by a control device.

Optionally, the system may include a robotic tool configured to move the sample relative to the spectrometer device.

Optionally, the system may include a robotic tool configured to move the spectrometer device relative to the sample.

The system may comprise a control device to control movement and/or operation of the spectrometer device.

The control device may comprise a processor and a programmable memory. The control device may be integral (i.e. built in) to the spectrometer device.

Optionally, the same control device controls the conveyor belt or moveable platform and the spectrometer device.

Optionally, the receptacle comprises a series of markers indicating the plurality of positions at which the spectrometer device measures the sample.

-12Optionally, each segment of the grid has an area of 20 mm² or less. Optionally, each segment of the grid has an area of 10 mm² or less. The smaller the segments are the more measurements are taken of the sample and the less likely it is that a nugget of precious metal, if present, will be undetected.

The system may comprise a plurality of spectrometer devices. Each of the spectrometer devices may be the same, or they may be different. For example, the system may comprise two or more XRF analysers spaced apart to analyse the sample at different positions substantially simultaneously.

In other embodiments, the system may comprise at least one XRF analyser and at least one laser emission spectroscopy device. This would allow different types of readings to be taken of the same sample, which may improve the reliability and/or accuracy of the analysis.

It will be appreciated that the system may be configured to carry out the method of any embodiment of the first aspect of the invention.

Specific Description

Illustrative embodiments of the invention will now be described by way of example only and with reference to the accompanying drawings, in which:

Figure lisa flowchart illustrating an embodiment of the method of the present invention;

Figure lisa schematic diagram showing a plan view of a system according to the present invention;

Figure 3a is a schematic diagram showing a plan view of a further system according to the present invention;

Figure 3b is a side on perspective view of the receptacle in Figure 3a;

-13Figure 4 is a schematic diagram showing a plan view of a further system according to the present invention; and

Figure 5 is an example of what a recorded secondary EM radiation spectrum looks like (not based on real data).

It will be appreciated that the drawings are schematic diagrams and none of the features are shown to scale.

Figure lisa flowchart illustrating an embodiment of the method of the present invention. A geological sample which may contain precious metal is extracted from an extraction site (not part of the claimed method). The composition of the sample needs to be assessed to determine whether it does contain precious metal and, if so, whether it should be sent for further analysis (e.g. to be assayed).

Step 1 of the method is to place the crushed or solid rock sample on a surface. In some embodiments the surface may be a conveyor belt or a moveable platform. The surface may form part of a receptacle.

Step 2 is to ensure that the depth of the sample does not exceed the maximum penetration depth of the primary x-rays used by the XRF analyser. For example, if the sample is a crushed sample it may be distributed over a layer having a depth less than the maximum penetration depth.

In some embodiments, the depth of the sample will not need reducing or altering.

Step 3 is to direct primary x-rays at a first position on the sample. In this embodiment the method uses x-ray fluorescence (XRF) spectroscopy to analyse the sample. In other embodiments different types of EM radiation may be used, for example to conduct laser emission spectroscopy.

-14The primary x-rays excite atoms at the first position of the sample. These atoms then eject an electron and emit secondary x-rays. Step 4 of the method is to detect and record the secondary x-rays emitted from the sample. This can take around 20 seconds. Steps 3 and 4 are achieved using an XRF analyser.

Steps 3 and 4 are then repeated at a plurality of positions on the sample (see step 5 in Figure 1). For example, the XRF analyser is moved relative to the sample, or the sample is moved relative to the XRF analyser, to conduct the same measurements at a given number of positions on the sample.

Step 6 of the method is to process the recorded secondary x-rays to assess the chemical composition of the sample at each of the positions. The XRF analyser can use the data gathered to indicate the concentration of any precious metals (such as gold, silver and/or platinum) at each position on the sample. There results can then be used to determine whether the sample is of interest and should be sent for further analysis, such as assaying.

Figure 2 shows an example of a system for analysing the precious metal content of a geological sample. The system comprises a receptacle 12 for receiving a geological crushed rock sample 10 and a spectrometer device 15.

In this example the receptacle 12 is a sealable, transparent plastic bag. A series of markers 14 are applied to a surface of the bag 12 to form a 2D grid. The markers 14 are lines that are painted, embossed, adhered, mounted, engraved or otherwise applied to the upper surface of the bag 12. The markers 14 divide the sample 10 into a series of segments 13. Each segment 13 defines one of the plurality of the positions at which the spectrometer device 15 should analyse the sample.

The spectrometer device 15 is a handheld XRF analyser which a user manually moves to take a measurement at each segment 13. In other embodiments, the spectrometer device 15 may be automatically programmed to move to each segment 13 in turn, for example the device 15 may be programmed to detect the markers 14.

-15The spacing and number of markers 14 is determined based on the material properties of the geological sample, such as the typical grain size of particles formed in the region where the sample 10 was extracted.

In a particular example of the invention, the apparatus in Figure 2 was used to assess geological samples that had previously been sent to an external laboratory to be assayed (or analysed) to determine the concentration of gold in the samples. This allowed direct comparison between the results of the present invention and traditional assaying techniques. The samples used were raw drill hole samples (crushed rock) from a rotary air blast drill rig. The samples were not processed before analysing. Each sample typically had rock chips or grains of less than 1cm. The samples were placed into the bag 12 and distributed such that the depth of the sample was kept at 3mm or less.

In one example (see column 4 of Table 1), ten marker lines 14 were drawn onto the bag 12 to divide the bag into a 10 xlO grid, the spacing between the lines 14 being 1 cm. Thus, the samples were divided into 100 segments, each segment having an area of 1 cm². The XRF analyser was manually moved to analyse the sample at each segment.

In another example (see column 3 of Table 1), the bag 12 was divided into a grid comprising 468 segments. Thus, each segment was smaller than in the 10x10 grid. Again, the XRF analyser was manually moved to analyse the sample at each segment.

Sample	ALS Average	Matrix Average (Olympus Delta)	Difference(ALSOlympus)
M03216	1.06	1.13	0.07
M03805	0.57	1.03	0.46
M06405	2.59	0.25	2.34
M02214	1.41	0.31	1.1
M01114	3.94	0.18	3.77
M01517	3.95	0.38	3.47

The spectrometer device used for the samples in Table 1 was an Innov-X Alpha 6500 handheld XRF analyser.

-16The ‘average Au concentration (ppm) external lab’ in column 2 of Table 1 corresponds to the parts per million gold in the sample obtained by the external laboratory assaying the sample. Sometimes the external laboratory will do a duplicate analysis on anomalous results, which is why the average result has been taken.

Table 1 shows that the method of the present invention, using a grid of 100 segments or 486 segments, can be used to indicate whether or not a sample contains any gold. It is generally considered that a concentration of around 0.4 ppm gold or less is not of commercial interest.

For example, sample A0010283 which the external laboratory found to contain only 0.02 ppm gold, which is a negligible amount, was also assessed as containing no gold by the ‘nugget effect’ assessment of the present invention. In addition, the ‘nugget effect’ assessment of the present invention, taking 100 readings or 486 readings, indicated that all of the other samples did contain gold, which was supported by the external laboratory results.

Thus, although there are differences between the calculated concentration of gold in the sample between the external laboratory results and the method of the present invention, the method of the present invention could reliably be used to deduce which samples are not of interest and should not be sent for analysis by an external laboratory. This would also apply to other precious metals, not just gold.

Another example of a system for analysing the precious metal content of a geological sample is shown in Figure 3a. The system comprises a spectrometer device 25 and a receptacle 22 for receiving the sample.

The receptacle 22 is a container having a transparent upper surface 23. A series on markers 24 are provided on the upper surface 23. Each marker 24 defines one of the plurality of the positions at which the spectrometer device 25 analyses the sample.

The receptacle 22 is shown in more detail in Figure 3b. In this example the receptacle 22 is a cuboid shaped container comprising the upper surface 23 as shown in Figure 3a. The upper surface 23, or one of the other sides of the receptacle 22, may be removable so as to allow

-17the sample to be placed in the receptacle 22. Alternatively, the surface 23 (or other side) may be hinged or pivotable relative to the receptacle 22.

The spectrometer device 25, which for example may be an XRF analyser or a laser emission spectrometer, is mounted to a moveable platform 26. The moveable platform 26 is configured to move the spectrometer device 25 relative to the receptacle 22. The platform 26 may be mounted on at least one rotatable element (such as a wheel). This ensures that the spectrometer device 25 can analyse the sample at each marker 24 of the receptacle without the user having to manually adjust the position of the spectrometer 25 or the receptacle 22.

The motion of the moveable platform 26 is controlled by a control device 27. The control device 27 may be a remote or wireless controller. The control device 27 may allow a user to program the motion of the spectrometer device 25, such that the analysing method is at least partially automated.

In other embodiments, the moveable platform 26 may be replaced by other devices configured to adjust the position of the spectrometer device 25, for example a robotic arm.

In other embodiments, the spectrometer device 25 may be configured to move relative to the receptacle 22 without requiring the moveable platform 26 or indeed any external movement device. Instead, the spectrometer device 25 may be intrinsically programmable to be moveable. For example, the spectrometer device 25 may have a built-in motor (not shown).

Another example of a system for analysing the precious metal content of a geological sample is shown in Figure 4. The system comprises three spectrometer devices 35. The spectrometer devices 35 may be the same or different types of devices. For example, they may all be XRF analysers, or a mixture of XRF analysers and laser emission spectrometers.

The spectrometer devices 35 are depicted in an arbitrary position relative to a sample 10 and it will be appreciated that they could be arranged in a variety of different positions. In this example, the sample 10 is a solid rock sample, such a core sample extracted using diamond drilling techniques.

-18The sample 10 is placed on a movable platform or conveyor belt 36. The motion of the movable platform or conveyor belt 36 is controlled by a control device 37. This allows the sample 10 to be moved relative to the spectrometer devices 35.

In use, the spectrometer devices 35 are arranged to analyse the sample 10 at a first position. There are no markers required on the sample 10 to indicate the positions. The platform or conveyor belt 36 then moves the sample 10 along and the spectrometer devices 35 analyse the next position on the sample.

The control device 37 may move the platform or conveyor belt 36 in a series of distinct steps (i.e. step-wise or index-wise translations). The time delay between each movement of the platform or conveyor belt 36 may be determined by the time taken by the spectrometer devices 35 to complete a single analysis. For example, the time delay may be set to a multiple of the time taken by the spectrometer devices 35 to analyse one segment of the sample. In other embodiments, the platform or conveyor belt 36 may move continuously.

Additionally or alternatively, one or more of the spectrometer devices 35 may be moveable relative to the sample 10 (or the conveyor belt 36). For example, the spectrometer devices 35 may be programmable to adjust their position, and/or their motion may also be controlled by the control device 37.

It will be appreciated that more than three spectrometer devices 35 may be provided.

Figure 5 shows an example of what a secondary x-ray radiation spectrum recorded by a spectrometer device can look like. This is not based on real data.

The spectrometer device (e.g. 15, 25, 35) records the energy, usually in keV, of each secondary x-ray detected. The device then outputs the number of secondary x-rays per second detected, known as the counts per second, as a function of the energy of the x-rays.

-19As shown, there are distinct peaks A-D in energy spectrum. The energy of these peaks is equal to the specific difference in energy of two electron states in an element present in that segment of the sample. Each of the chemical elements present in a sample will produce a set of characteristic secondary x-rays, or peaks that is unique for that specific element. As the difference in energy between quantum electron states is known for each element, this means that each peak A-D can be used to identify the presence of a specific element (such as gold) in the sample.

The height of the peak is indicative of the concentration (or amount) of that element in the given segment of the sample.

It should be noted that the above-mentioned embodiments illustrate rather than limit the invention, and that those skilled in the art will be capable of designing many alternative embodiments without departing from the scope of the invention as defined by the appended claims. In the claims, any reference signs placed in parentheses shall not be construed as limiting the claims. The word comprising and comprises, and the like, does not exclude the presence of elements or steps other than those listed in any claim or the specification as a whole. In the present specification, “comprises” means “includes or consists of’ and “comprising” means “including or consisting of’. The singular reference of an element does not exclude the plural reference of such elements and vice-versa. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage.

Claims

1. A method of assessing the precious metal content of a geological sample, comprising:

2. The method of claim 1, wherein the plurality of positions are arranged such that that entire sample is analysed by the spectrometer device.

3. The method of claim 1 or claim 2, wherein the sample is a rock sample.

4. The method of claim 3, wherein the sample is a crushed rock sample.

5. The method of claim 4, wherein the crushed rock sample is a sample that has been extracted using percussion drilling or rotary air blast drilling.

6. The method of claim 4 or claim 5, wherein the crushed rock sample has an average grain size of between 0.1 mm and 5 mm.

7. The method of any preceding claim, further comprising measuring the depth of the sample and, if the depth of the sample exceeds a maximum penetration depth of the EM radiation then reducing the depth of the sample to be less than or equal to the maximum penetration depth.

8. The method of claim 7, comprising reducing the average depth of the sample to 10 mm or less.

9. The method of claim 8, wherein the average depth of the sample is reduced to 2mm or less.

10. The method of any preceding claim, further comprising ensuring that the sample has a uniform depth at each of the plurality of positions.

11. The method of any preceding claim, wherein the primary EM radiation comprises x-rays.

12. The method of claim 11, wherein the spectrometer device is an x-ray fluorescence (XRF) analyser.

13. The method of any of claims 1 to 10, wherein the spectrometer device is a laser emission spectroscopy device.

14. The method of any preceding claim, comprising placing the sample in a receptacle before it is analysed.

15. The method of claim 14, wherein the receptacle comprises a series of markers indicating the plurality of positions at which the sample is to be analysed.

16. The method of claim 15, wherein the markers device the receptacle into a grid and the sample is analysed at each segment of the grid.

17. The method of any preceding claim, comprising the initial step of washing the sample before it is analysed by the spectrometer device.

18. The method of claim 17, wherein washing the sample removes particles or minerals that do not contain any precious metals.

19. The method of any preceding claim, comprising moving the spectrometer device relative to the sample to each of the plurality of positions on the sample.

20. The method of any preceding claim, comprising moving the sample relative to the spectrometer device to allow the spectrometer device to analyse the sample at each of the positions.

21. The method of claim 20, wherein the sample is placed on a conveyor belt or moveable platform.

22. The method of claim 19 or claim 20, wherein the spectrometer device and/or the sample is moved by a robotic tool.

23. The method of any preceding claim, comprising using a plurality of spectrometer devices.

24. The method of any preceding claim, wherein the spectrometer device records the intensity of the secondary EM radiation emitted at each position on the sample as a function of the energy of the secondary EM radiation.

25. The method of claim 24, wherein the step of processing the detected secondary EM radiation comprises determining whether any peaks in intensity are generated by a precious metal.

26. The method of any preceding claim, wherein the step of assessing the composition of the sample comprises determining whether the sample contains any precious metal or not.

27. The method of claim 26, further comprising sending the sample to be assayed if the concentration of at least one precious metal in the sample is likely to exceed a given threshold.

28. The method of any preceding claim, wherein the precious metal is one or more of silver, gold and platinum.

29. A system for assessing the precious metal content of a geological sample, comprising:

-23a receptacle or surface for receiving a geological sample; and a spectrometer device, wherein the spectrometer device is configured to move relative to the sample and/or the receptacle or surface is configured to move relative to the spectrometer device, wherein the spectrometer device is configured to direct primary electromagnetic (EM) radiation at a plurality of positions on the sample to excite atoms and to detect secondary EM radiation emitted from the sample at each position.

30. The system of claim 29, wherein the spectrometer device is an x-ray fluorescence (XRF) analyser.

31. The system of claim 30, wherein the XRF analyser is a handheld device.

32. The system of claim 29, wherein the spectrometer device is a laser emission spectroscopy device.

33. The system of any of claims 29 to 32, further comprising a conveyor belt or moveable platform onto which the receptacle for receiving the sample is placed.

34. The system of any of claims 29 to 32, wherein the surface for receiving the sample is a conveyor belt or moveable platform onto which the sample is placed.

35. The system of claim 33 or claim 34, wherein the conveyor belt or moveable platform is controlled by a control device.

36. The system of any of claims 29 to 32, further comprising a robotic tool configured to move the sample relative to the spectrometer device.

37. The system of any of claims 29 to 32, wherein the surface for receiving the sample comprises a tube or channel inclined such that the sample moves under the force of gravity relative to the spectrometer device.

38. The system of any of claims 29 to 37, further comprising a robotic tool configured to move the spectrometer device.

39. The system of any of claims 29 to 38, further comprising a control device to control movement and/or operation of the spectrometer device.

40. The system of claim 39, wherein the control device comprises a processor and a programmable memory.

41. The system of claim 39 or claim 40 as they depend on claim 35, wherein the same control device controls the conveyor belt or moveable platform and the spectrometer device.

42. The system of any of claims 29 to 41, wherein the receptacle comprises a series of markers indicating the plurality of positions at which the spectrometer device measures the sample.

43. The system of claim 42, wherein the markers device the receptacle into a grid such that the sample is analysed at each segment of the grid.

44. The system of any of claims 29 to 43, further comprising a plurality of spectrometer devices.