Drilling is a critically important activity in the exploration and evaluation of an iron ore deposit. Drilling usually follows detailed surface geological and geophysical studies that have shown the possible occurrence of a valuable deposit. A drill program to investigate an ore target has three basic purposes and steps: (1) to show the occurrence of a potentially valuable deposit and give basic information on broad geological relationships and nature of the iron ore; (2) to determine the ore dimensions, chemical and physical nature of the ore, its concentratability, if the ore requires processing to yield a salable product, and to acquire other positive information that will permit a generalized evaluation of the tonnage potential and a preliminary estimate of its commercial feasibility, (3) to obtain sufficient detailed information, by a continuation of drilling and sampling, concerning the distribution of ore types and grades so a fully reliable feasibility study and economic evaluation can be made.
Planning a drill program requires judgments on the appropriate drill pattern, hole spacing, type of drilling and equipment, sampling procedure, elements to be determined by assay, and the research testing needs. A principal concern should be the accurate and efficient determination of the nature of the ore and of the ore body on a cost-effective basis.
The locations of the first few drill holes in a target area are critically important as a common perception within an exploration company is that these drill holes are situated in areas of maximum opportunity for ore discovery. Thus the success or failure of the first few drill holes may determine whether the project is continued or abandoned unless a very well-documented geological evaluation of all factors is presented. At this stage there should be a careful and reasonably complete geological appraisal of the ore potential before a decision is made regarding a partly drilled prospect. This evaluation should include a statement of the likely economic feasibility of the project. It is a waste of time and money to extend work on a deposit that is without commercial merit or to drop a property with a reasonable possibility of commercial value. The need for an early geological view of exploration potential is as valid for iron ore deposits as for other ores.
The importance of a well-considered start should be recognized in preplanning. The preplanning aspect includes the selection of drilling and sampling methods, equipment and drilling pattern, whether vertical or angle holes will be drilled, and if core drilling is done, the size of the core to be taken. The primary concern should be to insure that the drilling products give the proper information. Many geologists believe it is preferable to err on the side of too much detailed information rather than on the side of too little and possibly questionable results. Other geologists are very cost-conscious and emphasize the need to keep the costs low and to show rapid progress in the ore body study and target evaluation. A cost-effective approach to exploration is always needed but this must be coupled with reliable information. Lake Superior type iron ores are often soft and friable with mixed hard and soft layers which makes sampling and data gathering difficult. Thus, preplanning becomes very important so that accurate data may be obtained. In compact, rather uniform solid ores such as contact metasomatic ores and magnetite taconite ores, a major problem may be the selection of the most cost-effective approach. Since some iron ores are very hard, the critical decision may be how to best obtain accurate core samples using diamond drills that give adequate amounts of material for analysis and testing.
Three general methods are used in iron ore exploration drilling and sampling: core, rotary, and percussion. For each method there is a considerable range in equipment and in the way it is applied. All three systems may be considered in shallow drilling, generally less than 90 m (300 ft) in depth. As a generalization, rotary and percussion drilling methods experience sampling problems in iron ores at relatively shallow depths because of the relatively low density of the gangue component and high density of the iron oxide minerals. The difference in density results in an easy separation and segregation of one or the other component during sample travel and recovery. There may also be a marked difference in the particle size between the gangue and ore minerals produced during rotary or percussion drilling. The drilling may yield a coarser and a finer component in either iron mineral or gangue so a sampling error may result as correct amounts of fine and coarser material may not be recovered. This can result in a considerable change in iron content in the samples obtained through the loss of either coarse or fine material. A loss of fine gangue material, often quartz, is common, which may result in a considerable apparent increase in iron content of the ore. Until modern, high core recovery diamond drill methods for sampling iron ores were developed in the 1950s and 1960s elaborate and questionable formulas were used to combine the core and sludge samples received to give the ore assay. In past years, sampling of the Lake Superior type ore for open pit mining on the Mesabi Range included percussion drilling using a variety of drill machines such as churn drills and the use of a dry, punch coring method accomplished by driving a nonrotating, hollow bit into the ore to retrieve an unwashed, unsegregated sample. The current method used in exploration or development sampling of natural iron ores or concentrating grade natural iron ore on the Mesabi Range is by diamond drilling using N size, swivel type core barrels and bentonite mud as the drill fluid with the work done by skilled drill crews. This drill system commonly yields a sample with the needed amount and size of core for analysis and testing that has an acceptable accuracy. Even using this system there are interlayered hard and soft ores that may cause severe sampling problems.
Exploration drilling and sampling of compact, massive, nonfriable iron ores is usually done by core drilling. Diamond core drilling, in what may be termed solid ores, is cost effective. In most cases the work is accomplished with a very high core recovery, often
between 95 and 100%. The wireline method is extensively used as it can give adequate sample material with excellent sample recovery at a reasonable cost. Surface set diamond bits and modern swivel type core barrels used with standard drill equipment and drilling methods commonly give satisfactory core samples, although the use of bentonite mud as the drilling fluid should be considered as it may improve drilling and sample recovery particularly in fractured zones. Detailed information on core drill equipment including wireline, core barrels, types of bits and advice on their use can be obtained from the drill equipment manufacturers and contract drilling companies. Some iron mining companies, particularly those working in areas with substantial glacial cover, use a rotary drill to penetrate the glacial drift. This equipment must be of sufficient size and power to effectively drill in the glacial materials. The rotary drill is used to reach the bedrock surface where casing is set. Bedrock drilling is then done using diamond drill equipment. In bouldery surface materials it is sometimes best to use down-the-hole percussion equipment instead of a rotary drill. Overburden that contains large, hard boulders may be drilled with larger-sized diamond drill holes with casing set in bedrock. The use of bentonite in drilling has largely overcome the difficulties of drilling and setting casing in surface materials.
Drill Hole Surveys
Surveying drill holes should be considered an essential part of an exploration drilling program whenever accurate knowledge of the position of the hole from top to bottom is important to the evaluation of the deposit and to its eventual development. Deviation of drill holes from the planned direction and angle is common in holes that are over a few hundred feet in depth and should be expected. The severity of the vertical and horizontal changes ranges considerably. Deviations of from 5° to over 20° in inclination and up to 20° in horizontal direction have been observed in drill holes less than 245 m (800 ft) deep that were drilled in steeply dipping cherty iron-formation. Thus the change in bottom hole location can be substantial. Deviations in holes drilled in dipping, bedded, cherty iron-formation with alternating harder and softer layers can be remarkably rapid. In iron ore exploration sufficient magnetite is commonly present to make magnetic borehole survey methods unreliable, so the use of a gyroscopic survey system is recommended. The first use of a gyroscopic system in a small diameter diamond drill hole was done at the Atlantic City taconite mine near Lander, WY, in 1966. Drilling was performed to obtain detailed information to permit the development of final open pit mine plans. This required that stripping be carried to the ultimate pit limit during mining. A substantial deviation in the inclination of drill holes was observed so horizontal deviation was suspected. Sperry Sun was encouraged to complete construction of a partly designed, small hole gyroscopic survey system that could enter a BX diameter hole. The resulting survey showed a substantial horizontal deviation in the exploration drill holes, requiring a relocation of the stripping limits. Mining has since shown that the gyroscopic survey results were accurate.
Gyroscopic drill hole surveys can be made on a contractual basis with well service companies. These surveys produce records that show the hole inclination and direction with a relatively small possible error. Well service company personnel are also available to assist with cost estimates and to assist in planning a drill hole survey program.