16 2 Geological Mapping in Exploration Style 1:The systematic data collector(the mindless slogger) Day one:20 data points Day two:40 data points Day three:60 data points Style 2:The ide ent mappe ■■ vie Fig.2.1 Com parison of geological ma The task will eventually be completed.but this is not the most efficient procedure.The intelligent mapper on the er h d continuously assesses th Many small structural features can be observed in individual outcrop or hand specimens that allow predictions to be made about large structures occurring at the scale of a map.Most useful of such observations are the predictable geometrical relationships that occur between bedding.cleavages,lineations and folds.as well as mo ndica ors tha n be used to deduc he sense of me ment on brittle faults and ductile sh ear zones Where such structures as these occur,they are a boo
16 2 Geological Mapping in Exploration Style 1: The systematic data collector (the mindless slogger) Style 2: The ideas-driven intelligent mapper Day one: 20 data points Day three: 60 data points Day one: key outcrops Regional overview Preliminary interpretation Day two: 40 key outcrops Focus on contacts and major structure Day three: 60 key outcrops Job Complete! Day two: 40 data points Fig. 2.1 Comparison of geological mapping styles. In the first case, the “systematic data collector”, driven by a pre-determined inflexible strategy rather than ideas, regularly traverses the ground. The task will eventually be completed, but this is not the most efficient procedure. The intelligent mapper on the other hand continuously assesses the significance of each outcrop against evolving ideas about geology, and then determines strategy in the search for the next significant outcrop. The job is completed more quickly, and better too Many small structural features can be observed in individual outcrop or hand specimens that allow predictions to be made about large structures occurring at the scale of a map. Most useful of such observations are the predictable geometrical relationships that occur between bedding, cleavages, lineations and folds, as well as movement indicators that can be used to deduce the sense of movement on brittle faults and ductile shear zones. Where such structures as these occur, they are a boon
21 General Considerations 17 to the field mapper,and he should learn to recognize and make use of them.A detailed description of these structures is beyond the scope of this book but they are treated in many standard geology texts.Some useful references will be found i Appendix E Anothe r aspect of rocks is the way the features and relationships seen in hand specimen or outcrop often exactly mirror features occurring at map scale.This has been informally called "Pumpelly's Rule"after Raphael Pumpelly,the nineteenth century USGS geologist who first described it.Once again the intelligent mapper will be on the look out for such potential relationships in outcrop as a means of developing ideas as to the map scale geological patterns. With geochemistry having a major role in most modern exploration programmes ical ma of surf p will us play a lage part in the plan standin ampling programmes In order to fulfil t exploration geolog mapping in most case w need to ca the di tribution of superficial and weathered rock units(the regolith),as well as bedrock Observations are thus not made randomly,nor are they collected on a regular grid or according to a fixed search pattern;rather they are selected to most effectively prove6 or disprove the current ideas.Geological mapping is a scientific process and when carried out properly corresponds to the classic scientific method:theoriz- tions)to te ediction kproceeds.In oth earlier phase from data interpretation;these two aspects are inextricably linked and must proceed together.Above all,observation and interpretation should never come to be regarded as“field work"and“office work ST arison in shap between a rock pool and the coastline of which it is an element.Pumpelly's Rule is an early recognition of this type of relationship (see Pumpelly et al. 1894). ctually,as por out by th ds the e range of c ons unde it can n wve it 7all theorie and that ineludes ideas an st h formulated in such a way tha they are capable of being falsified.For example,for field mapping purposes it is not very useful to postulate that"these outcrops constitute a metamorphic core comple use there is unlikely to be a s which can falsify that statement.R postulate n th hypothesis may need revision. 8In our society from the earliest training we are unfortunately conditioned to think indoors,and to enjoy less cerebral pursuits outdoors.It is a syndrome that the field geologist must learn to break
2.1 General Considerations 17 to the field mapper, and he should learn to recognize and make use of them. A detailed description of these structures is beyond the scope of this book but they are treated in many standard geology texts. Some useful references will be found in Appendix F. Another aspect of rocks is the way the features and relationships seen in hand specimen or outcrop often exactly mirror features occurring at map scale. This has been informally called “Pumpelly’s Rule” after Raphael Pumpelly, the nineteenth century USGS geologist who first described it.5 Once again the intelligent mapper will be on the look out for such potential relationships in outcrop as a means of developing ideas as to the map scale geological patterns. With geochemistry having a major role in most modern exploration programmes, the geological map will usually play a large part in the planning and understanding the results of surface geochemical sampling programmes. In order to fulfil this role, exploration geological mapping in most cases will need to carefully show the distribution of superficial and weathered rock units (the regolith), as well as bedrock features. Observations are thus not made randomly, nor are they collected on a regular grid or according to a fixed search pattern; rather they are selected to most effectively prove6 or disprove the current ideas. Geological mapping is a scientific process and when carried out properly corresponds to the classic scientific method: theorizing, making predictions from the theories, and designing experiments (planning the required field observations) to test the predictions.7 An aspect of this technique is that thinking and theorizing are constantly being done while field work proceeds. In other words, data collection is not a separate and earlier phase from data interpretation; these two aspects are inextricably linked and must proceed together. Above all, observation and interpretation should never come to be regarded as “field work” and “office work”.8 5Today we recognize that geological processes are essentially chaotic (i.e. non-linear). Such systems typically exhibit what is called “scale-invariance”, meaning there is a repetition of characteristic patterns at different scales – the example often quoted being the comparison in shape between a rock pool and the coastline of which it is an element. Pumpelly’s Rule is an early recognition of this type of relationship (see Pumpelly et al., 1894). 6Actually, as pointed out by the philosopher of science Karl Popper (1934), an experiment either falsifies a hypothesis or expands the range of conditions under which it can be said to hold good: it can never prove it. 7All theories in science, and that includes ideas on geology, must be formulated in such a way that they are capable of being falsified. For example, for field mapping purposes it is not very useful to postulate that “these outcrops constitute a metamorphic core complex” because there is unlikely to be a simple observation which can falsify that statement. Rather postulate “this outcrop is felsic gneiss, that outcrop is sandstone, this contact is a mylonite” – if these turn out to be false then the hypothesis may need revision. 8In our society from the earliest training we are unfortunately conditioned to think indoors, and to enjoy less cerebral pursuits outdoors. It is a syndrome that the field geologist must learn to break
18 2 Geological Mapping in Exploration 2.1.4 Choosing the Best Technique The mapping technique used depends upon the availability of suitable map bases on which to record the field observations.A summary of the different techniques is given in Table 2.1. The ideal base photograph or high satelite image.these offer the recise positioning o andscape i cal struc ctures that cannot be seen from the ground 1:100,000)rei ote sensed image are virtually the only really suitable mapping base,although if good topographic maps are available at these scales they can be used as a second-choice substitute.In Third World countries,where there is often no aerial photography available at any suitable scale,satellite imagery can provide a suitable base for regional geological mapping.Radar imagery,whether derived from satellite systems or special aircraft can also be used as a geological mapping base in much the same way as of the mir ope ning prepared by the mine surveyor and suppleme d by accu- rately established survey points from which distances can be taped.In open-cut mines,most available rock surfaces are vertical or near-vertical;observations are thus best recorded onto sections and afterwards transferred to the standard level plans,a composite open-cut plan or mine sections.In underground mines,obser- vations can be made on the walls.roofs and advancing faces of openings,and are then recorded and compiled onto a section or plan.These mapping techniques are detailed in subse que ction photography is often not available or is only avail e to permit photo er rgement for de tailed mappi In many cases also,air photographs are diffcult to use for precise field location because of vegetation cover or simply because of a lack of recognizable surface features.In areas of very high relief,photos can also be difficult to use because of extreme scale distortions.In these cases,alternative techniques are available to provide the or detailed mapping.Inrder of decreasingaccuracy (and increasing speed of execution)these ma on a pegged grid,tape and c ompas ss mapping.and pace and compas is seldom done nowa days because it is slow and the alte native use of pegged grid control can provide all the surveying accuracy that is normally required for a geological map.Further disadvantages of the plane table technique are the requirement for an assistant and the fact that geological obser vation and map-making usually have to be carried out as two separate processes However,plane tabling provides great survey accuracy and is an invaluable tech- nique where precision is needed in mapping small areas of complex geology.Such situations often arise in detailed pr ect map The plane table technique ated grid cannot readily h
18 2 Geological Mapping in Exploration 2.1.4 Choosing the Best Technique The mapping technique used depends upon the availability of suitable map bases on which to record the field observations. A summary of the different techniques is given in Table 2.1. The ideal base is an air photograph or high resolution satellite image, as these offer the advantages of precise positioning on landscape/cultural/vegetation features combined with an aerial view of large geological structures that cannot be seen from the ground. For small-scale maps (say 1:5,000–1:100,000) remote sensed images are virtually the only really suitable mapping base, although if good topographic maps are available at these scales they can be used as a second-choice substitute. In Third World countries, where there is often no aerial photography available at any suitable scale, satellite imagery can provide a suitable base for regional geological mapping. Radar imagery, whether derived from satellite systems or special aircraft surveys, can also be used as a geological mapping base in much the same way as aerial photography. In the special case of mine mapping, the mapping base is usually a survey plan of the mine opening prepared by the mine surveyor and supplemented by accurately established survey points from which distances can be taped. In open-cut mines, most available rock surfaces are vertical or near-vertical; observations are thus best recorded onto sections and afterwards transferred to the standard level plans, a composite open-cut plan or mine sections. In underground mines, observations can be made on the walls, roofs and advancing faces of openings, and are then recorded and compiled onto a section or plan. These mapping techniques are detailed in subsequent sections. For surface mapping, suitable photography is often not available or is only available at too small a scale to permit photo enlargement for detailed mapping purposes. In many cases also, air photographs are difficult to use for precise field location because of vegetation cover or simply because of a lack of recognizable surface features. In areas of very high relief, photos can also be difficult to use because of extreme scale distortions. In these cases, alternative techniques are available to provide the control for detailed mapping. In order of decreasing accuracy (and increasing speed of execution) these mapping techniques are: plane table mapping, mapping on a pegged grid, tape and compass mapping, and pace and compass mapping. Plane table mapping is seldom done nowadays because it is slow and the alternative use of pegged grid control can provide all the surveying accuracy that is normally required for a geological map. Further disadvantages of the plane table technique are the requirement for an assistant and the fact that geological observation and map-making usually have to be carried out as two separate processes. However, plane tabling provides great survey accuracy and is an invaluable technique where precision is needed in mapping small areas of complex geology. Such situations often arise in detailed prospect mapping or in open-cut mine mapping. The plane table technique is also indicated where a pegged grid cannot readily be
2.1 General Considerations 勿 Table 2.1 Comparison of mapping techniques Mapping technique Ideal scales Indications Advantages Disadvantages Pace and compass 1:100-1:1,000 roueh prospec Ouick.No Poor survey accuracy. map.Infill assistance and especially on between survey minimal uneven ground points Tape and compass 1:100-1:1.000 Detailed prospect Quick.Good May need assistance. maps.Linea Slow for large erse map Pegged grid 1:500-1:2500 Detailed maps of Fair survey Expensive.Requires established advance prospects grid controls/ dense scrub or hilly correlates all terrain Plane table 1:50-1:1,000 Detailed prospect High survey Slow.Requires mapping in areas accuracy.No ssistanc grou Geolo cuts required are eparate steps GPS and DGPS 1:5.000-1:25,000 Regional an Quick Encourage dable mapping.First digital sur pass prospect data.Good point data mapping kup for techniques at similar scales renced reconnaissance. man base Areas of steep Height contours geology.Not topograp for plotting GPS Remote sensed 1:500-1:100.000 Pr Geological Scale dist rtion (ai imagery ng base at directly from if new su need all scales image.Stereo to be acquired newing.Eas feature location
2.1 General Considerations 19 Table 2.1 Comparison of mapping techniques Mapping technique Ideal scales Indications Advantages Disadvantages Pace and compass 1:100–1:1,000 Rough prospect map. Infill between survey points Quick. No assistance and minimal equipment needed Poor survey accuracy, especially on uneven ground Tape and compass 1:100–1:1,000 Detailed prospect maps. Linear traverse maps. Mine mapping Quick. Good accuracy. No preparation needed May need assistance. Slow for large equidimentional areas Pegged grid 1:500–1:2,500 Detailed maps of established prospects Fair survey accuracy. Relatively quick. Same grid controls/ correlates all exploration stages Expensive. Requires advance preparation. Poor survey control in dense scrub or hilly terrain Plane table 1:50–1:1,000 Detailed prospect mapping in areas of complex geology. Open cuts High survey accuracy. No ground preparation required Slow. Requires assistance. Geological mapping and surveying are separate steps GPS and DGPS 1:5,000–1:25,000 Regional and semi-regional mapping. First pass prospect mapping Quick, easy downloadable digital survey data. Good backup for other techniques at similar scales Encourages geological mapping as collection of point data Topographic map sheet 1:2,500–1:100,000 Regional mapping and reconnaissance. Areas of steep topography. Mine mapping. Base for plotting GPS observations Accurate georeferenced map base. Height contours Difficulty in exact location. Irrelevant map detail obscures geology. Not generally available in large scales Remote sensed reflectance imagery 1:500–1:100,000 Preferred choice. Ideal geological mapping base at all scales Geological Interpretation directly from image. Stereo viewing. Easy feature location Scale distortion (air photos). Expensive if new survey needs to be acquired
20 2 Geological Mapping in Exploration used,for example,mapping a disused quarry or open cut.Plane table mapping is therefore a useful skill for a field geologist to acquire. Pegged grids are used for outcrop mapping at scales of 1:500-1:2.500 and are a commonly used survey control for making detailed maps.The technique relies on placing a close network of survey pegs into the ground at regular stations on a coordinate system(see Sect.10.5.2).The are marked ontot pegs that are the nd to oprovide trol for all stages of explo over the area.The disad danger that geologists often come to regard t of predetermined geological traverse lines,rather than a pre-positioned network of points for survey control. A measuring tape and compass or Hip-ChainTM and compass survey allows for quick production of detailed prospect maps,or maps to provide a base for location of sample points in areas where the geologist cannot spend long on site.With this ality.detailed geol p withou ided there is a -chain ailable there is pe ava ilable th n pacing allo ough map to e constructed.Pacing is better than est n an the of being quick.Pacing can even be reasonably accurate for short distances over open flat ground.Explorationists should be aware of their normal pace length by laying out a 100 m tape along flat even ground and checking pace length by walking back and forward many times(using a normal,easy stride)and taking an average.Every e in iswaked.the pace length over different types of e 2.1.5 Choosing the Best Scale The scale chosen for mapping c o the type of data which can and hence the type of observations wh nich are made in the field (see Fig.2.2).The choice of appropriate scale depends on the purpose in making the map. A small-scale map-say at 1:25,000 or smaller-shows broad regional patterns of rock distribution and major structures.From an exploration point of view this is the scale at which the prospectivity of a basin,fold belt,tectonic unit or other large geological subdivision might be determined.It is a scale appropriate for develop ingideas for new project generation.Explorationists do t often make maps thes wo re. sons for this:firstly.this is the of n by Geological Surveys and can often be bought of ff the app elf secondly Hip-ChainTM is a reel of dist on thread.As it reels from its spool.a meter reords te lengtound off and hence the disamceved The rad ranger
20 2 Geological Mapping in Exploration used, for example, mapping a disused quarry or open cut. Plane table mapping is therefore a useful skill for a field geologist to acquire. Pegged grids are used for outcrop mapping at scales of 1:500–1:2,500 and are a commonly used survey control for making detailed maps. The technique relies on placing a close network of survey pegs into the ground at regular stations on a Cartesian coordinate system (see Sect. 10.5.2). The coordinates are marked onto the pegs that are then placed in the ground to provide control for all stages of exploration over the area. The disadvantages of using a pegged grid lies in its expense, and the danger that geologists often come to regard the grid as a series of predetermined geological traverse lines, rather than a pre-positioned network of points for survey control. A measuring tape and compass or Hip-ChainTM and compass survey allows for quick production of detailed prospect maps, or maps to provide a base for location of sample points in areas where the geologist cannot spend long on site. With this technique it is possible to produce a high-quality, detailed geological map without needing any advance preparation (provided there is a tape or hip-chain available9). If there is no measuring tape available then pacing distances can still allow a rough map to be constructed. Pacing is better than estimation and has the advantage of being quick. Pacing can even be reasonably accurate for short distances over open flat ground. Explorationists should be aware of their normal pace length by laying out a 100 m tape along flat even ground and checking pace length by walking back and forward many times (using a normal, easy stride) and taking an average. Every time a pegged grid line is walked, the pace length over different types of terrain should be checked. 2.1.5 Choosing the Best Scale The scale chosen for mapping controls the type of data which can be recorded and hence the type of observations which are made in the field (see Fig. 2.2). The choice of appropriate scale depends on the purpose in making the map. A small-scale map – say at 1:25,000 or smaller – shows broad regional patterns of rock distribution and major structures. From an exploration point of view this is the scale at which the prospectivity of a basin, fold belt, tectonic unit or other large geological subdivision might be determined. It is a scale appropriate for developing ideas for new project generation. Explorationists do not often make maps at these small scales. There are two reasons for this: firstly, this is the type of mapping undertaken by Geological Surveys and can often be bought off the shelf; secondly, 9Hip-ChainTM is a reel of disposable, biodegradable cotton thread. As it reels from its spool, a meter records the length wound off, and hence the distance travelled. The thread is then simply broken and left on the ground. Other brand names for similar measuring instruments are FieldrangerTM, ChainmanTM and TopofilTM