Once a favorable area is selected and the lands are acquired, a drilling program is formulated and undertaken. Drilling is commonly conducted in three stages.
1. During the first stage, holes are drilled on a wide spacing to delineate favorable ground and to obtain geologic information. Holes may be spaced from 800 to 1600 m (one-half to one mile) apart for sandstone-hosted uranium targets. Initial spacing may be closer for a Precambrian metamorphic or vein host where a more specific target area has been mapped.
2. During the second stage, holes are drilled on a closer spacing in the more favorable areas based on results of the first stage for the purpose of intersecting uranium mineralization. Holes may be spaced 300 to 900 m (1000 to 3000 ft) apart for a sandstone target, or closer for a metamorphic or vein target. Some programs use a grid pattern for drilling in the first two stages. However, if the targets sought are randomly located, a random drill pattern is preferred.
3. During the third stage, holes are closely spaced in mineralized areas to intercept ore-grade uranium mineralization. Once a discovery is made, four equidistant holes are drilled to offset the discovery hole (Fig. 2.4.1), and the resulting ore holes are again offset (Fig. 2.4.2). The purpose of offset drilling is to obtain a group of adjacent ore holes that is as large as possible, bordered by a rim of non- ore holes (Fig. 2.4.3). Barren or slightly mineralized holes are not offset. This is regarded as the most efficient method of determining an ore body outline that cannot be predicted before drilling.
Figure 2.4.1.
Figure 2.4.2.
Figure 2.4.3.
Spacing between offset holes and the discovery hole varies depending on drill depths and continuity of mineralization. For shallow, open-pit uranium deposits, an initial offset pattern of 45 m (150 ft) is commonly used. If ore is difficult to intercept at this spacing, the pattern is moved in to 23 m (75 ft) or 15 m (50 ft). Commonly, one drill will be placed on offsets and a second drill is moved to widely spaced locations to seek additional deposits.
Choice of drilling equipment is based on the type of rock being tested and the accessibility of the area. For sandstone-hosted deposits, a truck-mounted rotary rig capable of drilling 12 or 12.7 cm (4: or 5 in.) diameter holes with a tricone rock bit is commonly used (Fig. 2.4.4) (Bailey and Childers, 1977b). A 1500 series rig is well suited for shallow drill holes and is capable of drilling to depths greater than 300 m
(1,000 ft). The drill should have a dual circulation system for drilling with compressed air, or water and mud. The upper part of a hole is drilled by air as far as possible and the remainder of a hole is drilled by water and mud. An efficient method that is becoming increasingly common is stiff-foam air drilling. New types of detergent are mixed with high volume air to lift cuttings from a wet drill hole. Compressed air or water is pumped through a swivel connection at the top of the drill string, down the drill string, and through openings in the drill bit. The air or mud lifts drill cuttings up the hole to the surface where samples are caught. A portable steel mud pit is commonly used for efficiency and to eliminate the need for excavating earthen mud pits. A water truck with a capacity for 5,680 to 11 350 L (1,500 to 3,000 gal) is used to haul drilling water to the location. If a drill hole is completed entirely by air, it is generally filled with water to obtain a complete geophysical log.
Figure 2.4.4.
Rotary drill samples are generally caught on 1.5 m (5 ft) intervals and placed on the ground in rows of 30 m (100 ft) (Fig. 2.4.5). A portion of each sample is placed in a sample bag and marked with the hole number and depth interval. Samples should be washed if collected from drilling mud. Considerable experience is required by the drillers to obtain representative samples.
Figure 2.4.5.
In some areas, it may be decided to core (Bailey and Childers, 1977c), selected mineralized zones with a diamond core bit to obtain samples for stratagraphic information, chemical assay, or metallurgical testing. In most cases where core intervals are selected at shallow depths of less than 120 m (400 ft) it is economical to drill the hole by rotary methods to the core point, and then convert to a drilling mud suitable for coring. The tri-cone bit is then replaced with a core bit and core barrel, and coring performed through the mineralized zone and an additional 3 m (10 ft) for logging purposes. Choice of core diameter depends upon the volume of sample needed, and cost considerations as larger core is more expensive. Typically, NX sized core 4.45 cm (1.75 in.) diameter or NXWL 6.07 cm (2.39 in.) diameter is considered adequate.
Drill core is removed from the core barrel and placed in a wooden, V-shaped trough by the drillers. The core may be removed by gently tapping the core barrel, or if there is abundant clay, a pressure hose from the mud pump may be connected to one end of the barrel and the core pumped out with gentle pressure from the pump. The core is placed in a core box as shown by Fig. 2.4.6.
The core is examined and scanned with a geiger counter to select sample intervals. The sample intervals should be selected with boundaries based on changes in mineralization or lithology. Thin intervals may be sampled [as small as 0.15 m (0.5 ft) thick]. In thick mineralized zones, a combination of contiguous samples is taken whereby individual samples are generally held to a maximum of 1.5 m (5 ft) thick. The core for the sample is split with a core splitter and one-half bagged for assay.
The lithology of either rotary or core samples is logged by a geologist in the field using a hand lens or binocular microscope. The data recorded (Fig. 2.4.7) include rock type, color, and grain size. Special attention is given to describing alteration features and the presence of carbonaceous matter or pyrite in the host rock. The presence and amounts of hematite and limonite are also given special attention. Notes are generally made of accessory minerals, however, detailed notes are not usually made of the percentage of the common minerals. Core recovery is also entered on the lithologic log.
Figure 2.4.7.
For drilling in metamorphic or igneous terrain (Anon., 1968) either core drilling or percussion drilling is commonly employed. For steeply dipping targets, the drill must be capable of drilling at an angle so as to intercept the target at as high an angle as practicable. Drill holes that intercept a target at less than a 20 degree angle may either glance off the target or deflect into the target resulting in an exaggerated thickness. For coring, truck-mounted drills are commonly used (Bailey and Childers, 1977d). However, in areas of difficult access, skid-mounted drills are either moved in by winch and cable, or flown by helicopter. Helicopter-supported drill programs are generally expensive and occur in areas where future development costs may be expected to be high. Most core rigs now are equipped for wire line methods. A messenger is lowered by cable through the drill string to the inner core barrel at the bottom of the hole. Once a latch is completed, the cable retrieves the inner barrel with its core to the surface. This method saves tripping out or removing all of the drill string to recover the lowermost core barrel.
Percussion drilling is also generally conducted with a truck-mounted drill, although portable units are available if required for rough terrain (Peters, 1978). A percussion or
hammer-tool rig drills by compressed air fed to a pneumatic drill tool and bit at the
bottom of the drill string. Hole diameter is commonly 15.2 cm (6 in.). Cuttings are carried to the surface and collected in a cyclone device that prevents escape of the fines. Samples are taken on a 1.5 m (5 ft) interval, split by a Jones splitter, and a manageable portion bagged and labeled for assay purposes. Lithology of the samples is logged in the field by a geologist. With a good driller, samples are generally representative to a depth of approximately 90 m (300 ft) or to the water table.
Percussion drilling is more rapid and is one-half to one-third the cost of core drilling. Percussion samples, however, are not as reliable as core. As a percussion hole reaches greater depth, there is usually slough and dilution by material above the zone being drilled.
Reverse circulation drilling is commonly used in exploration for other metals but less commonly for uranium exploration (Peters, 1978). The circulating fluid may be either air or water and mud. Drill pipe is double-walled and the fluid is pumped down the space between walls to the bit at the bottom of the hole. Cuttings are lifted up the center pipe to the surface. This method significantly reduces contamination in comparison to percussion or rotary drilling methods.
Drilling equipment with operators may be contracted from numerous drilling companies on a fixed price-per-foot basis or an hourly basis plus materials used. A less common practice is for an exploration company to own and operate its own drills. This practice nearly always requires that geologists spend a great deal of time directly supervising a drilling operation and obtaining parts and materials for the rig rather than devoting time to finding ore.
Drilling companies often have regional field offices in cities near exploration and mining activities. A listing of companies available in the area may be compiled from advertisements in the trade journals, listings in the appropriate telephone books, and phone calls to consulting geologic firms and geologists with other mining companies for recommendations. When the list is narrowed, it is advisable to call references for a recommendation of the drillers’ abilities. Prices are often determined by inviting competitive bids from three or more contractors. Once a driller is selected, a drilling agreement spelling out prices and terms is prepared and signed by the driller and the mining company.
Drilling prices may vary widely depending on the area and rock types. Typical prices in 1983 in the western U.S. are as follows:
Price per Ft (0.3 m) Hole Depth Type Host
Rotary Percussion Diamond Core
0–152 m Sandstone uranium $0.80–2 $3–62 $20–30
(0–500 ft) Carbonate host $1–4 $4–6 $20–35
Igneous/metamorphic $6–12 $15–32
Lost circulation $50–150 per hr plus material
Standby rates $50–80 per hr