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GEOLOGICAL AND GEOCHEMICAL EXPLORATION Dr Ahmed Ali Madani Associate Professor Tel (off.): 64324 E-mail:email@example.com firstname.lastname@example.org Overview of Exploration Geology Exploration geology is the process and science of locating valuable mineral or petroleum deposits, ie, those which have commercial value The term “prospecting” is almost synonymous with the term “exploration” Mineral deposits of commercial value are called “ore bodies” (compared to commercially viable deposits of oil which are called “oil fields”) This course will be focused largely on mineral exploration, although many of the same techniques are used in petroleum exploration The initial signs of potentially significant mineralization are called “prospects” Through the exploration process, the prospect is investigated to acquire more and more detailed information The goal is to prove the existence of an ore body (or oil field in the case of petroleum exploration) which can be mined (or “developed”) The exploration process typically occurs in stages, with early stages focusing on gathering surface data (which is easier to acquire), and later stages focusing on gathering subsurface data, including drilling data and detailed geophysical survey data Determining the value of an ore body (or “deposit”) requires determining two main features: 1) “tonnage” (or volume), and 2) “grade” (or concentration) The volume is determined by using drill data to outline the deposit in the subsurface, and by using a geometric models to calculate the volume If the ore body is exposed at the surface, then the dimensions of length and width can be gathered at the surface, possibly with the aid of some trenching or blasting methods However, most of the volume which must be defined is typically located at depth and requires the use of extensive drilling or underground excavation methods to define The volume is difficult to delineate because ore deposits often have irregular shapes The “grade” is the average concentration determined from numerous assays of drill samples The grade can vary considerably within different parts of the same ore body Development usually consists of extensive, close-spaced drilling which outlines the geometry of the deposit in great detail The development stage will also conduct extensive testing, with some preliminary metallurgical testing, to precisely determine grade of the deposit and the “recovery” (the amount of metal possible to extract, compared to the total amount of metal present in the ore body) The final stage before actual mining or extraction is called “feasibility” During this stage, the actual mining or extraction method is proposed, taking into consideration all of the economic variables which effect the bottom-line profit (commodity price, milling cost, transportation cost, labor cost, etc ) At this stage, a decision is made whether to mine the deposit from the surface (called “open-pit mining”), or to mine the deposit by tunneling (called “underground mining”) Mineral seldom occur at the surface and are seldom obvious Most often they are buried, sometimes at considerable depth Since they are not visible we must detect their presence indirectly and extrapolate between points where data is known Many different techniques can be used to detect an ore body This class will discuss the more important techniques in some detail; others are only briefly mentioned The most important techniques used in exploration geology include geological field methods, geochemical sampling methods, and geophysical methods Exploration conducted from the surface is far less expensive than drilling or underground excavation, so thorough surface exploration usually precedes either of these activities The Exploration Process Exploration for a mineral deposit is usually conducted in a step-wise fashion which progresses through stages, each of which moves closer to making a valuation of the ore body Geological reconnaissance and surface geochemical sampling prevail in the earliest stage Simultaneously or afterwards, geophysical surveys are typically conducted Following surface exploration, the project moves into the drilling stage Drilling may begin with a small number of exploratory drill holes on select targets After this drilling stage, extensive, close-spaced drilling (called “development drilling”) is conducted Finally, pending good results, “reserve drilling” is conducted, which is the type of drilling which makes the final assessment of the deposit before actual mining begins Generally, some amount of drilling will continue throughout the life of the mine, as further definition is required and new information is obtained and used to refine the deposit model Exploration Methods If bedrock is exposed anywhere at or around a prospect, then surface bedrock mapping is an essential beginning step for an exploration program This would include mapping and sampling (field geologic methods) This work focuses on identifying and mapping outcrops, describing mineralization and alteration, measuring structural features (geometry), and making geologic cross sections Geochemical methods involve the collection and geochemical analysis of geological materials, including rocks, soils and stream sediments The results mapping and sampling may suggest patterns indicating the direction where an ore deposit could be present underground or at the surface Geophysical methods focus on measuring physical characteristics (such as magnetism, density or conductivity) of rocks at or near the earth’s surface The measured values are then used to compare with the values and models of known ore deposits EXPLORATION GEOLOGY TERMS Ore: the rock material or minerals which are mined for a profit Ore Minerals: the specific minerals within the ore which contain the metals to be recovered Gangue Minerals: the minerals having no commercial value, they just happen to be mixed up with the ore minerals Prospect: potential ore deposit, based on preliminary exploration Mine: Excavation for the extraction of mineral deposits, either at the surface (open pit mine) or below (underground mine) Orebody or Ore Deposit: naturally occurring materials from which a mineral or minerals of economic value can be recovered at a reasonable profit Mineral Deposit: similar to an ore deposit, but is implied to be subeconomic or incompletely evaluated at present Mineral Occurrence: anomalous concentration of minerals, but is uneconomic at present Grade: this means the concentration of the substance of interest, usually stated in terms of weight per unit volume Cut-off Grade: the lower limit of concentration acceptable for making a profit when mining Host Rock: the rock lithology (type) which contains the ore May or may not comprise ore Country Rocks: the rocks of no commercial value surrounding the host rocks and/or the ore Anomalous: above or below the range of values considered to be normal SAMPLING AND CALCULATION OF TONNAGE AND GRADE The dominant means of chemical breakdown of minerals in the near surface environment is oxidation Oxidation has dramatic effects on the behavior of iron and sulfur, which happen to key elements in many types of ore deposits After decomposition, the elements from the minerals are released into groundwater or surface water, which carries the elements outward Halos caused by secondary dispersion are usually much more widespread than those caused by primary dispersion For this reason, sampling of soils, stream sediments or plants can detect the presence of a mineral deposit from a much further distance Groundwater and surface waters migrate and transport metallic ions away from ore deposits Weathering, oxidation and water migration also produce and transport iron and manganese ions, which are paritcularly abundant in and around ore deposits Iron and manganese ions tend to precipitate easily once they leave acidic water conditions around a weathered ore deposit and come into contact with normal pH water conditions They precipitate as hydroxides forming solid particles which are abundant in soils and silt size stream sediments These hydroxides are negatively charged, and behave like magnets to metallic cations in solution, causing them to be precipitated also This process, called adsorption, leads to small accumulations of metallic ions in soils and stream sediments (Figure 12 – 1) Figure 12 – Dispersion of metallic ions in soils near ore body (SME Mining & Engineering Handbook) Dispersion results in the transport of metallic ions away from a source Some of these ions are precisely the ones sought after, and others are called “pathfinder” metals or elements Pathfinder elements are those which are closely associated with the metal of interest High values of pathfinder elements may be more significant because they have better mobility, resulting in greater dispersion For example, arsenic and bismuth are good pathfinders for gold Stream Sediment Sampling Surveys Stream sediment surveys are very useful for mineral exploration because of greater dispersion in the stream environment Greater dispersion means greater ability to detect an ore body from a greater distance A drainage basin is an area with a network of streams like the branches of a tree: smaller streams join together leading into larger and larger streams It is assumed that the values will decrease downstream from the source, so following the “path” of increasing values upstream may lead to mineralization (Figure 12 – 2) Mechanical erosion leads to the breakdown of host rocks containing ore minerals Consequently, tiny grains of the minerals occur in the suspended load of the stream Turbulence of the water keeps the particles in suspension Turbulence is greatest in steeper areas where the stream water flows faster Downstream where the topography is gentler the stream waters move slower, thereby decreasing turbulence This causes the suspended load to drop out, resulting in deposition of the mineral grains in the stream sediments Heavy minerals, like ore minerals, tend to drop out first because less turbulence is needed to keep them in suspension Studies have shown that the preferred material to collect for a stream sediment sample is the –100 mesh size fraction, which corresponds with silt size About ½ to cup of this size material is sufficient in most cases If gravel or organic material is mixed with the silt, then a larger sample needs to be collected Steep areas may lack the hydrologic conditions which allow silt and fine grained sediments to settle, which can make sample collection very difficult The downstream sides of large boulders are sometimes the best place to look in these areas Moss growing on boulders within the stream can act as a filter, trapping finer grained sediments, and can be collected to provide samples from these more difficult areas The material needs to be collected from the active stream channel, not dried up side channels A single sample taken at the mouth of a large drainage basin may be a good way to quickly evaluate potential of a large area, but it provides little detail of the location of a source of mineralization By sampling the entire stream network of an area, the location of mineralization can be narrowed down considerably This can be done by collecting samples at close spacings (approximately ¼-mile spacing is common) and by sampling both sides of every stream fork In this manner, if an anomaly occurs on one side and not on the other, only the fork with the anomaly needs to be considered in locating the source The trail of anomalies forms a path upstream towards the source Generally the values will increase upstream towards the source and reach a maximum value in close proximity to the source, and then drop to background values further upstream from the source Another type of survey which relies on collection of alluvium is the “pan concentrate” survey In a pan concentrate survey, coarse materials (generally pebble-sized) are collected and screened to ¼ inch or smaller and placed in a gold pan The screened material is then panned using a standardized method, down to a volume size of approximately ½ cup This will be further processed in a laboratory setting and then analyzed Pan concentrate samples give an indication of the types of heavy minerals present in an area Due to inherent inconsistencies in sample collection and panning methods, results from these surveys are difficult to evaluate statistically To help remedy this problem, special methods are sometimes employed in the field which use screening and collection of specified volume of material, and minimize or eliminate the use of actual panning of the materials (ie, concentration of heavy minerals) Figure 12 – Stream sediment anomaly pattern (SME Mining & Engineering Handbook) Soil Sampling Surveys Soils are the product of weathering of bedrock, decomposition of organic material at the surface, and deposition of other materials which have been transported Generally speaking the soils tend to form certain layers called “horizons” The lowermost horizon consists largely of decomposed bedrock and is called the “C” horizon The uppermost horizon, called the “A” horizon, is variable in composition In vegetated areas the “A” horizon consists largely of organic material The “B” horizon is between the “A” and “C” horizons, and is essentially a mixed zone Dispersion is generally greatest in the “A” and “B” horizons For this reason, soil samples collected from the “B” horizon can detect a mineral deposit from a greater distance In arctic regions, the “B” horizon tends to be poorly developed (if present at all) It is best to collect soil samples from the “C” horizon in these regions Soil surveys are typically situated to investigate target areas outlined by previous geophysical survey or stream sediment surveys, or they may be positioned to cover certain structural features or rock units which are known Generally close spacing (< 500 feet) is needed to detect subsurface mineralization, because large spacings may miss the target The pattern which usually emerges is one which shows highest values directly over the ore, and a broad area surrounding these with highly elevated values corresponding to alteration in the host rocks adjacent to the main ore zone (Figure 12 - 3) The strategy most often employed is to collect samples at set line or grid spacings The tighter the spacing, the more likely it will be to locate a soil anomaly over a buried ore deposit A grid survey has a big advantage over a line survey because the anomalies which are discovered may form a trend indicating the trend of the buried mineralization An anomaly discovered along a line survey gives no indication of trend, and usually must be followed up with a grid survey Geostatistics Geostatistics is the use of statistics to evaluate geochemical data Numerous samples of different types of rocks and other materials comprising the earth’s crust have been analyzed As a result, the average abundance of trace elements in these materials is fairly well established The average value for a specified rock is called the “background” value We are interested in values which are much greater than average or “anomalous” because these values may indicate the presence of an ore body A cutoff value, or “threshold value”, is the value above which all values are considered anomalous The threshold value can be selected arbitrarily by simply viewing the data, or it can be selected by statistical methods Geologists endeavor to select which values of a data set are truly significant and therefore worthy of follow-up geochemical sampling or other types of exploration Most element concentrations in geological materials follow a lognormal distribution This is demonstrated by plotting of histograms which show a skewed distribution of values towards either the high or low values Plotting the log values instead of the real values yields a typical “bellshaped” distribution Plotting the of geochemical values using geostatistical methods helps define the following types of values: Threshold Value: the value chosen above which values are considered anomalous Anomalous Values: any value above the selected threshold value Background Values: “normal” values for the given environment; majority of values are background values Threshold values can be selected in several different ways Arbitrary threshold – find the highest value, find the median value (the value at which half of the samples have higher values and half of the samples have lower values), and select a value in between, but closer to the highest value Cumulative frequency diagrams – line up values in by rank; determine class intervals; determine frequency percent and cumulative frequency percent; plot a graph with class intervals on the X axis and concentration on the Y axis using log probability paper Then specify the percentile to use as the threshold value This often selected at the 97.5 percentile value (second standard deviation), however, lower cutoffs may be selected to highlight a greater number of anomalous values This method also highlights the presence of different “populations” of values which may be related to different geologic features or rock types The evaluation of results depends largely of the type of samples being studied For stream sediment, pan concentrate, and in some cases soil samples, the procedure is often to plot all the values on a map, determine an arbitrary or statistical threshold and highlight the anomalous values This will suffice to look for general mineralization trends For soil sample grids: 1) contour the data; look for trends 2) make a thematic map which color codes the samples according to specified class intervals; look for patterns and trends One method is to assign a color code system or use symbols for specified ranges of values This type of map is called a “thematic” map (Figure 12 – 4) The advantage of thematic maps is that they are simple to make and provide the reader with a quick understanding of the distribution of anomalies in an area Another method is to create a “geochemical contour” map (Figure 12 – 5) Here the values are contoured: lines of equal value (called isopleths) are extrapolated between every data point and the adjacent points This type of map accentuates possible mineralization trends but is much more tedious to construct Figure 12 – Thematic geochemistry map showing highest values in red and lowest values in blue Figure 12 – Geochemical contour map showing highest values in red and lowest values in gray Trench/Adit Mapping Trench or adit mapping is the process of creating a geologic map, which shows the geology of the floor and walls of the trench or adit Adit mapping emphasizes mapping of the walls more than the floor because the floor is often poorly exposed due to the presence of a layer of debree which results from blasting and mucking Trench mapping emphasizes floor mapping because: 1) the floor is usually scraped as clean as possible with a dozer or backhoe, and 2) because floor mapping shows a “map view” Trench or adit mapping always involves setting up a base line using a tape Footage or meter marks are then painted or flagged and labeled The base line and footage marks are then drawn to scale on the map page to facilitate mapping Often the same base line is used to accomplish a chip channel sampling program One approach is to first draw the outline of the floor, which will be oriented with respect to true north and drawn to scale The geology of the floor is then mapped just as an ordinary geologic map is made The corner of the trench or adit matches the edges of the strip showing the geology This is the “map view” (looking straight down) of the geology of the floor The edges of the “strip map” represent the two bottom corners of the trench The walls of the trench or adit are mapped adjacent to the strip map such that the right wall is mapped as if looking at the vertical on the right, and the left wall is mapped as if looking at the vertical wall on the left These can be labeled to indicate they represent the geology of the walls, even though it is usually obvious This gives a 3-D perspective of the geology, which greatly facilitates the interpretation of the geometry of features For example in determining the dip of layers, faults, joints, etc on the floor of the trench, it is useful to show where the feature trends as it intersects the adjacent walls Structural measurements can be put directly on the map, in notation form next to the appropriate footage mark Another simpler approach used to make mapping more rapid is to sketch the floor outline at a standard, average width and not worry about the exact width The outline is drawn parallel to the edge of the map sheet without regard to actual geographic orientation The azimuth of the axis of the trench or adit floor is carefully measured and noted on the map If the trench or adit contains bends, then the new orientation is noted at the appropriate footage mark on the map The alteration style can be added to one side or the other of the map if desired The alteration can be mapped using colors, patterns or other designators, in the same way the rock types are mapped Figure 13 – Example of Trench map oriented to true north Figure 13 – Example of Trench map with trench axis parallel with map page edges [...]...Geochemical Sampling Methods Geochemical sampling methods are methods which involve collecting and analyzing various types of geological materials (such as soils, stream sediments and rocks) or certain biological materials (such as plants) Historically these methods have been some of the most productive in of any methods used in mineral exploration Sometimes mineralization... mineralization can be extremely subtle, if not impossible to recognize, in hand specimen Without the use of geochemical sampling methods, many known ore deposits would probably not have been discovered After discovery, geochemical sampling plays a key role in the delineation of mineralization For example, geochemical sampling of soils is often employed to outline the general distribution of mineralization... procedure involves collection of materials in the field, laboratory (or field) analysis of the geochemistry of the materials, plotting of the geochemical values on maps, and interpretation of the results The materials may be analyzed for any number of elements Which elements are chosen for analysis depends on budget, the geology of the area, and the commodity which is being sought after Often there are specific... been analyzed by standard geochemical techniques rather than assay techniques TRNC Trench - a sample taken from a trench ****Unknown - This may only be used when the data is important and needs to be included but the sample type is not known Exploration Project Planning The extensive effort, high costs, and short field season require a great deal of planning for an exploration project to be... necessary to conduct geological field work Table 15 – 1 is a partial list of equipment items Obviously each different type of work activity requires a different selection of work-related equipment For example, claim staking requires different equipment than geologic mapping, and stream-sediment sampling requires different equipment than soil sampling It is the responsibility of the field geologist or assistant... which means they can be used to mobe gear and personnel to camps in very remote locations The helicopter can be used to drop off geologists at the beginning of the day at locations high on ridges, which would otherwise take many long hours of uphill hiking to access Then the geologist can design their daily reconnaissance traverses to cover a much larger area and obtain many more samples Various types... some cases may be worthwhile to sample Several different types of rock samples are collected for mineral exploration Most importantly, rock samples are collected to determine the concentration of metals, including both the major and trace metals This type of sample is most commonly referred to as a “geochem” sample Trace metal values are often useful as “pathfinders”, which means they are closely associated... and partly from projection for a reasonable distance on geologic evidence The sites available for inspection, measurement, and sampling are too widely or otherwise inappropriately spaced to outline the ore completely or to establish its grade throughout Inferred ore is ore for which quantitative estimates are based largely on broad knowledge of the geologic character of the deposit and for which there... are few, if any samples or measurements The estimates are based on an assumed continuity or repetition for which there is geologic evidence; this evidence may include comparison with deposits of similar type Bodies that are completely concealed may be included if there is specific geologic evidence of their presence Estimates of inferred ore should include a statement of the special limits within which... adjectives may include measured geological, drill indicated, or indicated Possible (PS): Ore reserves are stated in terms of mineable tonnes and grades computed on the basis of limited geoscientific data, but with a reasonable understanding of the distribution and correlation of the substance in relation to this data Other applicable reserve adjectives may include inferred, geological, mineral inventory,
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Xem thêm: GEOL GEOCH EXPLORATION , GEOL GEOCH EXPLORATION , GEOL GEOCH EXPLORATION , Figure 11 – 2. Profile of a stream showing how placer deposits form by the action of floods, and how major floods cause most placer deposits to accumulate on bedrock (after Faulkner, 1986)., Figure 12 – 1. Dispersion of metallic ions in soils near ore body (SME Mining & Engineering Handbook)., Figure 12 – 2. Stream sediment anomaly pattern (SME Mining & Engineering Handbook).