References and geophysical mapping.However.as the exploration process moves progressively rtoa potential orebodyfrom regionto prject to prospect to target drilli aha ciple The reaso at ore b of unusual con be cheap and plentiful and you and I would be working in some other profession. When interpreting the geology of a mineral prospect,the aim is to identify posi- tions where ore bodies might occur and to target them with a drilling program Almost always,a number of different geological interpretations of the available data are possible.Interpretations that provide a target for drilling should be preferred over interpretations that yield no targets,even although the latter might actually ore likelv nario,or be tter satisfies occam However this is not ence for interpretatic be dr n by mere wish-fulfilme nt.All interpreta of geolog ave to be feasible,that is,it they must satisfy rul f geology There still has to be at least some geological evidence or a logically valid reasoning process behind each assumption.If unit A is younger than unit B in one part of an area,it cannot become older in another;beds do not appear or disappear,thicken or thin without some geological explanation;if two faults cross,one must displace the other;faults of varying orientation cannot be simply invented so as to solve each detail of complexity.And so on. it is relatively e to find a number of good reasons why a prope erty might not do that).but it akes an expert explorationist to he one good reason why it migh References Handley GA.Henry DD(1990)Porgera gold deposit.In:Hughes FE(ed)Geology of the mineral deposits of Australia and Papua New Guinea.Australasian Institute of Mining and Metallurgy. Helm y HH.Ke:107 zH.Loizenbauer J(2004)The Sukari gold mine,Eastern Desert,Egypt and fluid inclusions.Miner De sin34g5_51 wSeicpA tion gold mines 10 Naa Foon Coo ecial issue devoted to barite and Zn-Pd-Ag deposits D)Threa ding the eye of the sons from the search for another Koehler GF.Tikkanen GD (191)Red Dog.Alaska:Discovery and definition of a major zinc- lead-silver deposit.Econ Geol Monogr 8:268-274 Institute of Mining and Metallu 1793-1805 PerelloCox D.Garamjay D.Diakov S.Schissel D.Munkhbat T.Oyun G(2001)Oyu Tolgoi Mongolia:Siluro-Dew with a cretaceous
References 11 and geophysical mapping. However, as the exploration process moves progressively closer to a potential orebody – from region to project to prospect to target drilling – the successful explorationist has to be prepared to abandon the principle of economy. The reason for this is that ore bodies are inherently unlikely objects that are the result of unusual combinations of geological factors. If this were not so, then metals would be cheap and plentiful and you and I would be working in some other profession. When interpreting the geology of a mineral prospect, the aim is to identify positions where ore bodies might occur and to target them with a drilling program. Almost always, a number of different geological interpretations of the available data are possible. Interpretations that provide a target for drilling should be preferred over interpretations that yield no targets, even although the latter might actually represent a more likely scenario, or better satisfies Occam. However, this is not a licence for interpretation to be driven by mere wish-fulfilment. All interpretations of geology still have to be feasible, that is, it they must satisfy the rules of geology. There still has to be at least some geological evidence or a logically valid reasoning process behind each assumption. If unit A is younger than unit B in one part of an area, it cannot become older in another; beds do not appear or disappear, thicken or thin without some geological explanation; if two faults cross, one must displace the other; faults of varying orientation cannot be simply invented so as to solve each detail of complexity. And so on. It is relatively easy to find a number of good reasons why a property might not contain an orebody (any fool can do that), but it takes an expert explorationist to find the one good reason why it might. References Gresham JJ (1991) The discovery of the Kambalda nickel deposits, Western Australia. Econ Geol Monogr 8:286–288 Handley GA, Henry DD (1990) Porgera gold deposit. In: Hughes FE (ed) Geology of the mineral deposits of Australia and Papua New Guinea. Australasian Institute of Mining and Metallurgy, Melbourne, 1073–1077 Helmy HH, Kaindl R, Fritz H, Loizenbauer J (2004) The Sukari gold mine, Eastern Desert, Egypt – Structural setting, mineralogy and fluid inclusions. Miner Deposita 39:495–511 Holiday J, McMillan C, Tedder I (1999) Discovery of the Cadia Au–Cu deposit. In: New generation gold mines ’99 – Case histories of discovery. Conference Proceedings, Australian Mineral Foundation, Perth, 101–107 Kelley KD, Jennings S (2004) Preface: A special issue devoted to barite and Zn–Pd–Ag deposits in the Red Dog district, Western Brooks Range, Alaska. Econ Geol 99:1267–1280 Kerr A, Ryan B (2000) Threading the eye of the needle: Lessons from the search for another Voisey’s Bay in Northern Labrador. Econ Geol 95:725–748 Koehler GF, Tikkanen GD (1991) Red Dog, Alaska: Discovery and definition of a major zinc– lead–silver deposit. Econ Geol Monogr 8:268–274 Moyle AJ, Doyle BJ, Hoogvliet H, Ware AR (1990) Ladolam gold deposit, Lihir Island. In: Hughes FE (ed) Geology of the mineral deposits of Australia and Papua New Guinea. Australasian Institute of Mining and Metallurgy, Melbourne, 1793–1805 Perello J, Cox D, Garamjav D, Diakov S, Schissel D, Munkhbat T, Oyun G (2001) Oyu Tolgoi, Mongolia: Siluro-Devonian porphyry Cu–Au–(Mo) and high sulphidation Cu mineralisation with a cretaceous chalcocite blanket. Econ Geol 96:1407–1428
12 1 Prospecting and the Exploration Process Sillitoe RH (2004)Targeting under cover:The exploration challenge.In:Muhling J.Goldfarb N. Vielreicher N oves D SEG 2004 Global Metallogeny.Nedlands.WA.16-21 Van Leeuwen TM(1994)25 years of mineral exploration and discovery in Indonesia.J Geochem Explor 50:13-90
12 1 Prospecting and the Exploration Process Sillitoe RH (2004) Targeting under cover: The exploration challenge. In: Muhling J, Goldfarb N, Vielreicher N, Bierlin E, Stumpfl E, Groves DI, Kenworthy S (eds) Predictive mineral discovery under cover. SEG 2004 extended abstracts, vol 33. University of Western Australia, Centre for Global Metallogeny, Nedlands, WA, 16–21 Van Leeuwen TM (1994) 25 years of mineral exploration and discovery in Indonesia. J Geochem Explor 50:13–90
Chapter 2 Geological Mapping in Exploration 2.1 General Considerations 2.1.1 Why Make a Map? A geological map is a graphical presentation of geological observations and inter- pretations on a horizontal plane.A geological section is identical in nature to a map except that data are recorded and interpreted on a vertical rather than a horizontal surface.Maps and sections are essential tools in visualizing spatial,three dimensional,geological relationships.They allow theories on ore deposit controls to be applied and lead (hopefully)to predictions being made on the location,size grade of pote tialore bodies.Theyarethe ential tool to aid in devel- concepts about geology and mineralisation at all scale S.A John Proffett-widely regarded as one of the most skilled geological mappers in the exploration industry of recent decades-has written(Proffett,2004): g is a method of recordin of its power in targeting lies in providing conceptual insight of value.Conceptual toos ca adjacent to covered areas Making.or otherwise acquiring.a geological map is invariably the first step in mineral exploration me.and it ains a ontrol docu for all sub equent stages expl istry,geophysics,geostatistics and mine planning.In an operating mine,geologica mapping records the limits to visible ore in mine openings,and provides the essen- tial data and ideas to enable projection of assay information beyond the sample points. Making a geological map is thus a fundamental skill for any exploration or mine geologist. R.Maroribanks.Geological Methods in Mineral Exploration and Mining.2nd ed. 13 DOI 10.1007/978-3-540-74375-0_2.Springer-Verlag Berlin Heidelberg 2010
Chapter 2 Geological Mapping in Exploration 2.1 General Considerations 2.1.1 Why Make a Map? A geological map is a graphical presentation of geological observations and interpretations on a horizontal plane.1 A geological section is identical in nature to a map except that data are recorded and interpreted on a vertical rather than a horizontal surface. Maps and sections are essential tools in visualizing spatial, threedimensional, geological relationships. They allow theories on ore deposit controls to be applied and lead (hopefully) to predictions being made on the location, size, shape and grade of potential ore bodies. They are the essential tool to aid in developing 3-dimentional concepts about geology and mineralisation at all scales. As John Proffett – widely regarded as one of the most skilled geological mappers in the exploration industry of recent decades – has written (Proffett, 2004): Because geological mapping is a method of recording and organising observations, much of its power in targeting lies in providing conceptual insight of value. Conceptual tools can then help in the interpretation of isolated outcrops and drill hole intercepts that might be available in and adjacent to covered areas. Making, or otherwise acquiring, a geological map is invariably the first step in any mineral exploration programme, and it remains an important control document for all subsequent stages of exploration and mining, including drilling, geochemistry, geophysics, geostatistics and mine planning. In an operating mine, geological mapping records the limits to visible ore in mine openings, and provides the essential data and ideas to enable projection of assay information beyond the sample points. Making a geological map is thus a fundamental skill for any exploration or mine geologist. 1The ground surface is, of course, not always horizontal and, although this can usually be ignored in small-scale maps, it can have profound effects on the outcrop patterns of large-scale maps. R. Marjoribanks, Geological Methods in Mineral Exploration and Mining, 2nd ed., 13 DOI 10.1007/978-3-540-74375-0_2, C Springer-Verlag Berlin Heidelberg 2010
14 2 Geological Mapping in Exploration 2.1.2 The Nature of a Geological Map A geological map is a human artefact constructed according to the theories of geology and the intellectual abilities of its author.It presents a selection of field observations and is useful to the extent that it permits prediction of those things which cannotbe be There ar ifferent kinds of geological map.With large scale maps.the geolo gist gene y a ms to visit and outl cant rock atcrop in the area o the map.For that rason these are ofepathoughration or simply"outcrop"map is a much better term.In a small-scale map,visiting every outcrop would be impossible;generally only a selection of outcrops are examined in the field and interpolations have to be made between the observation points.Such interpolations may be made by simple projection of data or by making use of fea tures seen in remote sensed images of the area,such as satellite or radar imagery,air sand so on.Small-scale maps thus generally ha ent than la The differen cale maps ce between th a家兰Em two map typesi map,eve at th selection of the available geological observations and no observation is ever entirely free from interpretational bias.Even what is considered to represent an outcrop for mapping purposes is very much scale dependent.In practice,what the map-maker does is to make and record a certain number of observations,selected from the almost infinite number of observations that could be made.depending on what he regards as impor tant given the pose in constructing the map.These decisions by the e geologis nd will r b made with an unbiased mind.It is ofter thought that a w ess to be avoided at all costs but bias s is h que used by every sci ho seeks to separate a meaningful signal fro noise.If we were not biased,the sheer volume of possible observations that could be made in the field would overwhelm us.An explorationist has a bias which leads her to find and record on her map features that are relevant to mineralisation.This will not be to the exclusion of other types of geological observation,but there is no doubt that her map will (or at any rate,should)be different from a map of the same area made by.sav.a stratigrapher.or a palaeontologist.However.you can only use our bias toadvantage if you are aware it of and acknowledge itotherwise yo Tisk foolin A geolog cal map is thus different from other types of map data that the explo rationist might use.Although typical geochemical or geophysical maps can contain interpretational elements and bias,they in general aim to provide exact presentations of reproducible quantitative point data.The data on such maps can often be collected s with a small scale ratio (that is.a larg enrallthnvshd be bt terms are relative
14 2 Geological Mapping in Exploration 2.1.2 The Nature of a Geological Map A geological map is a human artefact constructed according to the theories of geology and the intellectual abilities of its author. It presents a selection of field observations and is useful to the extent that it permits prediction of those things which cannot be observed. There are different kinds of geological map. With large-scale2 maps, the geologist generally aims to visit and outline every significant rock outcrop in the area of the map. For that reason these are often called “fact” maps, although “observation” or simply “outcrop” map is a much better term. In a small-scale map, visiting every outcrop would be impossible; generally only a selection of outcrops are examined in the field and interpolations have to be made between the observation points. Such interpolations may be made by simple projection of data or by making use of features seen in remote sensed images of the area, such as satellite or radar imagery, air photographs, aeromagnetic maps and so on. Small-scale maps thus generally have a much larger interpretational element than large-scale maps. The difference between the two map types is, however, one of degree only. Every map, even at the most detailed of scales, can only present a small selection of the available geological observations and no observation is ever entirely free from interpretational bias. Even what is considered to represent an outcrop for mapping purposes is very much scale dependent. In practice, what the map-maker does is to make and record a certain number of observations, selected from the almost infinite number of observations that could be made, depending on what he regards as important given the purpose in constructing the map. These decisions by the geologist are necessarily subjective and will never be made with an unbiased mind. It is often thought that being biased is a weakness, to be avoided at all costs – but bias is the technique used by every scientist who seeks to separate a meaningful signal from noise. If we were not biased, the sheer volume of possible observations that could be made in the field would overwhelm us. An explorationist has a bias which leads her to find and record on her map features that are relevant to mineralisation. This will not be to the exclusion of other types of geological observation, but there is no doubt that her map will (or at any rate, should) be different from a map of the same area made by, say, a stratigrapher, or a palaeontologist. However, you can only use your bias to advantage if you are aware it of and acknowledge it – otherwise you risk fooling yourself. A geological map is thus different from other types of map data that the explorationist might use. Although typical geochemical or geophysical maps can contain interpretational elements and bias, they in general aim to provide exact presentations of reproducible quantitative point data. The data on such maps can often be collected 2By convention, large-scale refers to maps with a small scale ratio (that is, a large fraction) – e.g. 1:1,000 scale or 1:2,500 scale. Small-scale refers to large scale ratios (a small fraction) such as 1:100,000 or 1:250,000. Generally, anything over 1:5,000 should be considered small-scale, but the terms are relative
21 General Considerations 5 by non-professionals and the map can be compiled and plotted by computer accord- ing to pre-set formulae.A geological map.on the other hand.is not contoured point a bu nanalog pre senta on of ideas ideas backed up by detailed. rtheless,idea map-r er,it is nec nd,and throw idea of the geological map-maker as an objective collector of"ground truth" After all,one geologist's"ground truth"may be another geologist's irrelevant noise. 2.1.3 Intelligent Mapping Producing a geological map is a process of problem solving.One of the best ways a completely blank mind,but th ideas about the which has mapped.These ideas armed w preting air photc intuitive hunch.From these ideas or hypotheses,predictions are made:areas are then selected and observations are made which will most effectively test these predictions.Sometimes this will involve walking selected traverses across strike, sometimes following a marker horizon or contact,sometimes a more irregular search pattern.The mapping sequence depends on the postulated geology:strong linear strike continuity usually indicates that across-strike traversing is the best approach: complex folding or faulting is best resolved by following marker hori s,and on.In any case.the ing hypo will ce severa alternative scenaric may not be precisely formulate them very wide range of field observations will have to be made and a mix of diffe ent search patterns may need to be followed.The geologist at this stage must be open to all possible ideas,hypotheses and observations.If the observations do not fit the hypotheses,then new hypotheses must be constructed or old ones modified to accommodate the observations.These new hypotheses are then tested in their turn, and so the process is repeated. With h step in the ess the predictions become more e precise and the search ed on to the key areas s of int boundary condi e are the ons can b 一p快s ou outcrop wher there is most to be learned,and less time is spent in those areas where the rocks are uniform-in the latter areas a lower density of observation will serve (Fig.2.1). epts that are often employed to claim authority and stifle art of the
2.1 General Considerations 15 by non-professionals and the map can be compiled and plotted by computer according to pre-set formulae. A geological map, on the other hand, is not contoured point data but an analog presentation of ideas; ideas backed up by detailed, careful observation and rational theory but, nevertheless, ideas. To be a successful geological map-maker, it is necessary to keep this concept firmly in mind, and throw out any idea of the geological map-maker as an objective collector of “ground truth”3 data. After all, one geologist’s “ground truth” may be another geologist’s irrelevant noise. 2.1.3 Intelligent Mapping Producing a geological map is a process of problem solving. One of the best ways to approach problem solving is known as the system of multiple working hypotheses.4 In practice this means that the geologist does not start the field work with a completely blank mind, but armed with ideas about the geology which has to be mapped. These ideas are developed from looking at published maps, from interpreting air photos, satellite images or aeromagnetic data or even by following an intuitive hunch. From these ideas or hypotheses, predictions are made: areas are then selected and observations are made which will most effectively test these predictions. Sometimes this will involve walking selected traverses across strike, sometimes following a marker horizon or contact, sometimes a more irregular search pattern. The mapping sequence depends on the postulated geology: strong linear strike continuity usually indicates that across-strike traversing is the best approach; complex folding or faulting is best resolved by following marker horizons, and so on. In any case, the early working hypotheses will certainly contain several alternative scenarios and may not be precisely formulated; to check them out a very wide range of field observations will have to be made and a mix of different search patterns may need to be followed. The geologist at this stage must be open to all possible ideas, hypotheses and observations. If the observations do not fit the hypotheses, then new hypotheses must be constructed or old ones modified to accommodate the observations. These new hypotheses are then tested in their turn, and so the process is repeated. With each step in the process the predictions become more precise and the search pattern more focused on to the key areas of interest. These are the usually areas where significant boundary conditions can be defined in the outcrop. Most of the time of the intelligent mapper is thus spent in the areas of “fertile” outcrop where there is most to be learned, and less time is spent in those areas where the rocks are uniform – in the latter areas a lower density of observation will serve (Fig. 2.1). 3“Truth” and “fact” are slippery concepts that are often employed to claim authority and stifle debate. They are best not used in scientific contexts. 4The concept of multiple working hypotheses, now widely acknowledged as a basic part of the scientific method, was first enunciated by geologist Thomas Chrowder Chamberlin (1897)