Geological Mapping

When mapping as part of mineral exploration, prospecting is accompanied, and both activities go hand-in-hand. It is the process of collecting rock specimens from the field, usually taken across a mineralized location to get a representation of how the metal concentration varies across the potential ore zone. For example, mineralization may be confined to thin vein/lode bodies, or the metals of interest may have also leaked into the surrounding rock. No chemical analysis is undertaken in the field, as the geologist will take a small sample (< 2kg) broken-off with a hand-held rock hammer, which can be sent to the lab for subsequent analysis.

Mapping and prospecting are one of the first physical steps taken in a field area by geologists – especially where knowledge of the bedrock geology is limited. This is because it is a relatively inexpensive way to give good information on whether or not to invest in progressing exploration in a licence area any further. This is the nature of all mineral exploration methods, that initially inexpensive methods are used to test whether there is a warrant for further, more advanced exploration, or conversely ‘ticking-off’ an area as economically uninteresting.

However, it is extremely important to note, that the vast majority of all exploration campaigns – including drilling programmes, will not make it to mine stage. Due to the extremely long planning time and expenditure of mine setup (potentially >10 years and 100s of millions of dollars), very few exploration projects develop into mines. Prospecting and mapping would likely be preceded by desk-study research on the geology and historical mining activity in a licence area, with use of public data – such as made available by the Geological Surveys of the country. This information would then be used to hone-in on specific areas of interest with detailed mapping. This may be followed by and/or done in conjunction with geophysical methods and soil sampling. In areas where rock exposure is limited (much of Ireland’s countryside), soil sampling is a vital alternative.

maps are made to find out as much as possible about the geology of an area as quickly as possible. They are usually made at a scale of 1:250,000 or smaller, sometimes very such smaller. Some reconnaissance are made by photo geology, that is by interesting geology from aerial photographs, with only a minimum of work done on the ground to identify rock types and to identify structural features such as lineaments. Reconnaissance maps have even been made by plotting the main geological features from a light aircraft or helicopter with, again only brief confirmatory visits to the ground itself.

Unfortunately is countries geological mapping outstrips topographic coverage when there is a sudden economic interest in a specific area and geologist must then survey the topography themselves. An accurate geological map loses much of its importance if superimposed on an inadequate topographic base. Regional geological mapping done on the ground may be supported by systematic photo geology and it should be emphasized that photo geological evidence is not inferior to information obtained on the ground although it may differ in character.

Scales for detailed geological maps may be anything from 1:10000 and larger. Such maps are made to investigates specific problems which have arisen during smaller scale mapping or from discoveries made during mineral exploration or perhaps for the preliminary investigation of a dam site or for other engineering projects.

Specialized maps include large-scale maps of small areas made to record specific geological features in great detail. Some are for research others for economic purposes such as open pit mine plans at scales from 1:1000 to a:2500; underground mine plans at 1:500 or larger and engineering site investigations at similar scales. There are many other types of maps with geological affiliations too. They include geophysical and geochemical maps; foliation and join maps; and sampling plans. Most are superimposed over an outline of the geology, or drawn on transparencies to be superimposed on geological maps to study their relationship with the solid geology.

Traversing:

It is also a method for covering the ground in the detail required for particular project. A traverse is made by walking a more or less predetermined route from one point to another plotting the geology on the way. Traverses are an excellent way of controlling the density of your observations.

• Walking along a predetermined route plotting in the geology on the way
• The geology in between traverses is interpolated using whatever clues can be gleaned from the topographic map

• Mapping by exposures is the mainstay of much detailed mapping at scales of 1:10,000 and larger. The extent of each exposure or group of exposures is indicated on the filed map by colouring them in with the appropriate coloured pencil for that rock type or formation. Some geologists go further and mark the limits of the exposure by drawing a line around it, later inked in green, hence ‘Green line mapping’. Green fades rapidly in the tropics and a fine black dotted line can be substituted.
• Exposures ringed by green lines using a very fine nib size.
• All exposures are portrayed. There should be evidence on the map that all the ground has been covered.
• When all exposures have been mapped boundaries between the units can be drawn on the map.
• The classic technique for 1:10,000 mapping

Geological mapping is the fundamental task of a geologist on the basis of which all subsequent qualitative and quantitative geological studies are carried out leading to understanding of evolution of chosen part of the terrain in space and time. The information is representation graphically on a chosen scale by use of combination of colours/ characters, lines and symbols.

• One of the main aims in compiling a geological map
• Locate a boundary and follow it across the map
• One of the easiest methods of mapping where exposure is good
• Very effective in conjunction with aerial photographs
• Does not work well if exposure is poor

It is most commonly employed to locate formations at depth; to confirm their presence in the lack of other evidence to solve structural problems, and to sample rocks and ores. It is also used to find and exploit water, and of course oil. Basically there are two kinds of drill:

• Percussion drilling
• Rotatry drilling

Although mapping geology in underground workings, especially in metal mines or in caves is not a subject for this small book a few words may be usefully said. In the underground workings there is a convention that the geology in metal mines has normally been mapped in the walls at waist height or projected to waist height. This is because the ‘back’ (i.ie. the roof) is irregular and often too high to reach or even see properly, and the floor is covered in mud, debris or even water.

Photogeology is the systematic interpretation of geology from aerial photographs. It can be used as a method of geological reconnaissance with only limited ground checking or as an adjunct to orthodex geological mapping.

• Measuring strike and dip
• Plotting strike and dip
• Recording strike and dip
• Measuring linear features
• Folds
• Faults
• Thrust and unconformity
• Joints

• Hammer and chisels- Any geologist going into the field needs at least one hammer with which to break rock. Generally a hammer weighing less than about 3 kg (11 lbs) is of little use except for soft rocks. Hammering alone is not always the best way to collect rock or fossil specimens. Sometimes a cold chisel is needed to break out a specific piece of rock or fossil. Its size depends upon the work to be done. A 5mm (1 inch) chisel may be ideal to delicately chip a small fossil free from shale but break out large pieces of harder rock a 20-25 mm (3 inch) chisel is required.

• Compass and clinometers
• Handles
• Tapes
• Map case
• Field note book
• Acid bottle
• Global Positioning system (GPS)