When a potential placer is of great richness, considerable tolerance can be allowed in evaluating valuable mineral content. When a placer is very poor in valuable mineral content, it may be rejected without further work. Unfortunately, many potential placers are in the intermediate range of just good enough or almost good enough, and require very careful evaluation.
In placer evaluation, the evaluator should relate mineral content to volume. What is needed is mineral content per cubic yard or meter. Weighing bulk placer material is unrewarding and can be misleading. Weights of placer material will vary greatly with moisture content and with the nature of the material. Placer material normally weighs from 2500 lb to 3700 lb per cubic yard. For rough conversions, one cubic yard may be considered to weigh 1.5 short tons, and one cubic meter may be considered to weigh 2.0 short tons.
In evaluating placer material above the water table firm enough to stand unsupported, channel sampling is effective, requiring little equipment and more care than great expertise. As placers are deposited at low gradients, channels should be cut vertically. To limit the excavation of exposed banks, vertical cuts may be stepped, i.e., offset between elevations.
For evaluations made in traditional U.S. or English units (milligrams of gold or pounds of tin per cubic yard), channels 6 in. × 12 in. have proven effective. If material is fine (limited coarse gravel), smaller channels may be used. If material is coarser, smaller channels are unadvisable and larger channels are worth considering. Sampled material is generally collected every 1.2 m (4 ft), or by strata, and concentrated by rocker and/or panning. Tailings from an entire channel should be collected and checked for values by repanning or tabling (if such equipment is available). Checking fine tailings is especially advisable when very fine heavy minerals (gold, cassiterite, rutile, etc.) are known to be present.
Where exposed banks are not available, but placer material is above the water table, shafts or trenches may be excavated. In shallower deposits [to about 6 m (20 ft)], trenches may be dug with a backhoe, or shafts may be excavated to that depth or deeper (with a larger hoe or by benching down). Bulldozer trenches are used but are generally more expensive and less acceptable environmentally.
Channel sampling of shafts or trenches is rapid, and provides information on the stratification of mineral values. If the responsibility of the samplers is not well established, different samplers may be used on opposite sides of a shaft or trench.
Subsequent treatment of the entire volume excavated from a trench or shafts may be passed through sizing tests and a pilot plant (with a circuit similar to a commercial recovery circuit). The information obtained from bulk sample treatment will be helpful in designing a commercial plant. If material does not disintegrate readily in a small (pilot-plant-size) trommel, it may do so in a large one.
When sampling old hydraulic pits or other exposed banks, it is important to cut the banks to fresh surfaces. Heavier gold or cassiterite may fall from long-exposed surfaces, resulting in under-evaluations. Further, in old hydraulic pits, the former operators probably mined the best exposed material, and exposed faces may under-represent the remaining deposit. A few shafts excavated back from the face should provide useful supplementary data.
Sampling more than a few feet below the water table requires casing the hole or pit. Proven methods include Banka or Empire drilling, hand and mechanized churn drilling, and caisson shafting or drilling.
Banka drilling is an effective method in shallow alluvials, and where coarse and tight gravels are not prevalent. Casing is flush-threaded, and the drilling shoe at the lower end of the casing is sometimes serrated. Pressure is applied at the top of casing by men standing on a platform and by men dropping a weight on the casing head. The casing is rotated while pressure is applied. After a suitable casing advance, which is generally limited to 1 ft, or 0.25 or 0.50 m (depending upon the terrain and mineral sought) in economic ground, core removal is undertaken. In overburden or low-grade strata, longer drives may be used to increase drilling speed, but long drives generally result in poor core recovery and affect the accuracy of sampling. It is important to remember that the purpose of most placer drilling is to obtain an accurate sample, not to drill rapidly. Core removal in gravel is generally by bailer, which is a piece of pipe with a flap or ball valve at the lower end and a plunger which pulls material into the barrel. In clayey ground, clay pumps, which are often pieces of pipe, open at the bottom, with side slots for core removal, may be used. Augers have been used at shallower depths, but are generally not the favored tool of experienced churn drillers.
Core removal or pumping usually proceeds until the casing has been cleared to the level of the drive shoe. Exceptions are: (1) when drilling in loose sand (often called rising ground) and a plug is left in the casing to reduce the inflow of alluvium, and (2) when drilling in very hard ground where it is impossible to drive the casing deeper within acceptable time. It is poor practice to pump or drill below the drive shoe; however, there are times when there is no other solution. When drilling below the drive shoe, appropriate notation must be made on the drill log, and positive corrections are not advisable.
In tight placer ground, there is an intermediate step between driving casing and pumping which consists of loosening material in the casing by dropping a chisel, star, or other shaped bit upon it. The bit breaks up placer material, including rocks, and also causes slimes by disintegrating clays. Because of the sliming effect and the possibility that stones may be driven downward, it is important to measure the height of material in the casing before and after pumping. Two or more measurements may be made to ascertain that pumping has reached the proper level before it is suspended.
The sequences in Banka or churn drilling are: (1) advance casing, (2) measure core rise in the casing, (3) loosen the core if necessary, (4) remove the core, (5) measure to ascertain that enough core has been removed, and (6) measure the core removed (volumetrically). Core removed from the casing is dumped into a trough and washed to a measuring box or pipe. For simplicity, some engineers use a pipe of drill casing for a measuring box. Overflow from the measuring box should be decanted into tubs and the slimes checked for mineral content. Slime volume may be added to the box or pipe measurement. Except in rising ground, core rise (the difference between before pumping and after pumping measurements) is generally greater than box or pipe measurement.
Hand and powered churn drills use the same basic procedures as the Banka drills, but casing is larger, especially for the powered drills, drilling shoes are not serrated, casing is rotated infrequently, casing couplings are used, and tools are lowered and removed by winch. Driving the casing is accomplished by dropping a heavy hammer on the drill (driving) head at the top of the casing. The heavy hammer is a pair of blocks bolted to the drill stem.
The standard placer exploration tool for over 50 years has been the churn drill (Keystone, Bucyrus, Speed-Star, Cyclone, etc.) with 152 mm (6 in.) casing and a 191 mm (7.5 in.) drive shoe (see Fig. 2.7.1). Thirty years ago, it was common practice to use manila rope with drill tools, but recent practice uses left lay wire rope (to keep the tool joints tight). An experienced driller will bounce the drill bit on material in the casing, using the spring in the rope to keep from driving material downward.
Figure 2.7.1.
On drills using 152 mm (6 in.) casing, cable is generally 19 mm (¾ in.) [though some use 16 mm (5/8 in.)] and cable on the bailers is 9.5 mm (3/8 in.). Crayon marks on the wire ropes indicate the distance to the end of the drill bit or bailer, and are used to measure core levels in the casing. Casing is marked at 1 ft (or appropriate meterFootnote 01) intervals from the drill shoe before use.
Eighty-eight feet per cubic yard is considered the theoretical drive required to provide one cubic yard of core; however, drill shoes wear and experience has shown that the theoretical evaluation may be adjusted to provide greater accuracy. In the last century, an engineer named Radford began using 100 feet of drive to equal one cubic yard of
material, and it has been accepted by many in the industry.
Dividing drive shoe area by drill pipe area:
Correcting this factor for shoe wear:
After studying production and drilling results of several companies for several years, the author proposed that 1.4 ft of core rise for 1 ft of drive be accepted in place of the several variations in use, and this practice was adopted and has been in use by several companies for 25 years. Different factors are obviously in use for casing of substantially different sizes, but the principle is valid.
An area where engineers and companies have differed is in the correction of drilling results. One major company did not use positive correction of drilling results for deficient cores and production results averaged 10 to 15% above drilling estimates. Another group of companies used positive corrections and obtained results (for five to seven dredges) of very close to 100% of drilling estimates. Monthly recovery/prospecting factors would vary, but annual and longer term factors were extremely close to 100%. The method of treating exceptionally high holes (correcting for the nugget effect) also affects recovery factors.
Four methods used by drilling organizations are: 1. No corrections: seldom used at present. 2. Corrections for rises or volume in entire hole. 3. Corrections for rises or volume in pay streaks. 4. Corrections for rises or volume in individual drives.
Generally, the most conservative correction, whether it be from box measurement or from core rise measurement, is used, i.e., the correction giving the greatest negative correction is used.
Under certain conditions, methods 2 and 3 may be adequate, or nearly adequate, but method 4 is the only one believed applicable in all circumstances.
When evaluating a gold property in a district where recent mining or professional prospecting information is not available, representative gold samples must be sized and weighed; average weights are then assigned to No. 1, 2, 3, and 4 colors. These estimated weights are used for corrections to individual drives, and the algebraic total of the corrections adjusted as follows:
Permissible Corrections
Maximum Positive Corrections in general use are given as follows:
Co. A = 42%
Co. B (recent) = 50%
Co. C (old) = to about 200%
Co. C = later 0%
Co. D = 240% (lower river areas)
= 100% (upper river areas)
Co. E = 0%
If no boulders are present, the use of positive corrections to 100% may be acceptable. If boulders are present, the use of positive corrections must be reduced or eliminated. Experienced engineers regard excessive positive corrections with caution.
Where hard, irregular bedrock is present, it is generally advisable to delete gold or tin recovered from the last foot or two of hole.
Similar corrections may be applied for cassiterite (tin) evaluations, but additional attention must be given to: (1) checking panners’ tailings for fine tin, (2) checking coarse material for locked tin [passing + 9.5 mm (3/8 in.) material over heavy liquids is useful], and assaying samples. To avoid an excessive number of assays, material from various drives may be combined. If formations are relatively constant, errors should be
minimal. If core recoveries and mineral content vary substantially, more assays are advisable.
If a 1-ft drive yields a core rise of 1.4 ft, there is no core correction. If a 1-ft drive yields a core rise of 1.7 ft, excess core has been recovered and a negative correction is applied of 1.4 ÷ 1.7 or 0.82 to the values recovered. If a 1-ft drive yields a core rise of 1.1 ft, deficient core has been recovered and a positive correction is applied of 1.4 ) 1.1 or 1.27 to the values recovered for that drive.
In gold prospecting, it is customary for the driller to estimate the quantity of gold recovered for each drive, and individual drives are corrected on that basis. Upon completion of the hole and log, all gold recovered from the hole is weighed and checked against the panner’s estimate. A single correction of actual weight of gold-estimated weight of gold is then applied to cumulative corrections. If the driller is consistent in his estimating (high or low), there will be no error. To assist the panner in his estimating, sample colors (size 1, 2, 3 and 4) are weighed and displayed on a small backboard. In a new gold field, drillers and panners may err in their first estimates, and extra care must be taken; however, if estimates are high or low, there will be no residual error if estimating is on a consistent basis.
In estimating tin values, and especially where other heavy minerals of similar appearance are present, it is difficult to eyeball estimate tin content for individual drives. In this case various drives may be combined and the concentrate assayed. Some prospectors use the zinc block test to estimate tin content, but the author has found this practice inaccurate. If cores are consistent, several drives may be combined for corrections. If they are inconsistent, caution and probably more assays are required. Most placer engineers amalgamate gold samples. Common practice is to put the sample of gold and black sand into a porcelain or agate mortar with chemically pure (C.P.) mercury, and to grind with a porcelain or agate pestal until all visible and, hopefully, all commercially recoverable gold has been amalgamated. Amalgam and residual mercury is then placed in a small crucible with nitric acid and heated on a hot plate. This operation should be conducted under a hood with exhaust fan. The remaining gold is annealed (by additional heating on a burner), cooled, washed, and weighed. Samples may be collected from a group of holes and assayed to determine gold fineness. Most placer gold (bullion) will average from 850 (85%) to 950 (95%) fine gold. Alloying metals are generally silver and copper.
Churn drilling by experienced drillers has yielded satisfactory results for placer mining for many years, but drilling expertise is needed, and is often in short supply. In new fields there is a need for additional supporting information on the nature of the mineral and material to be mined. Also, despite the use of heavier drills, hydraulic assists, welded casing, and other modest improvements, churn drilling remains a slow process. To check and supply supporting information to churn drilling, caisson shafts have been used, and they are effective to depths of 12 or 15 m (40 or 50 ft), especially if labor costs are reasonable. One practice has been to cut a 0.3-m (1 ft) cube of material every 0.3-m (ft) of advance (using a knife-edged box), and later to screen and process all material from the shaft. Air pumps in the shaft will dewater and provide needed ventilation. Marsh gases can accumulate in placer shafts. If telescoping casing is used, a
51 mm (2 in.) reduction in diameter for each 1.2 m (4 ft) section has been effective. A 102 cm (40 in.) diameter is about the smallest in which a man can work effectively. During the past decade, caisson drills have been available and they provide very reassuring data on both mineral content and the nature of material to be mined. The data obtained can help avoid expensive mistakes and will permit the development of an efficient mining plan and the use of efficient equipment. The only problem with caisson drilling is the high cost of the equipment and the relatively high cost of its operation, at least until depreciated.
Caisson drills come in various sizes from 0.500 m (19.7 in.) to 2.0 m (78.7 in.) or even larger, and are currently made in Europe and used there and elsewhere for pier, bridge, and building foundations. Properly sized, they have reached depths of 183 m (600 ft), They have been found acceptable in diamond prospecting, where large samples are required, and for gold prospecting. They have encountered and passed cemented gravels several feet thick.
One effective machine (BADE, made in Germany) uses oscillating hydraulic jacks of 136 t (150 st) to turn 900 mm (2953 ft) casing 22.5° (see Fig. 2.7.2). Raising and lowering jacks are 90-t (100-st). To date, drilling to 114 m (375 ft) has been successfully undertaken, and depths of 137 m (450 ft) are anticipated. In very deep ground, larger casing diameters may be used in the upper sections of a hole, followed by reduced diameter for the entire greater depth.
Figure 2.7.2.
Cores are normally extracted with hammer grabs of both clam shell (two-blade) and orange peel (three-blade) design. Bailers are available, but used infrequently. Heavy star bits and other drop breakers are necessary when cemented layers are encountered. It is practical to pass all cores through a pilot plant approximating a commercial recovery circuit, but cores should be measured upon extraction from the caisson. (Duplicate measuring pipes that discharge into the pilot plant are effective, and avoid drilling delays.) Caisson joints are flush and of patented design. Details are best obtained from suppliers. Progress is equal to, or better than, churn drill progress for known equipment. Figure 2.7.3 is a sample of a form used to record the calculation of drilling results.
Figure 2.7.3a. Figure 2.7.3b.
1Metric equivalents: in. × 25.4 = mm; sq in. × 645.16 = mm2; ft × 0.3048 = m; sq ft ×
Surface Mining
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1.0 Introduction
2
Exploration and Geology Techniques Richard L. Brown, Editor
2.8 Tar Sands Exploration and Geology J. Allen Hennessy
INTRODUCTION
Tar sand is a term applied to sand impregnated with oil, or more precisely, with a
viscous hydrocarbon called bitumen. A more technically correct term than tar sand is oil sand or bituminous sand. However, the term tar sand has been widely used and has been given official status by the Alberta Oil and Gas Conservation Act, which defines tar sand as “a sand containing a highly viscous crude hydrocarbon material not recoverable in its natural state through a well or by ordinary oil production methods.” In this section, these deposits will be referred to as oil sand, the term most frequently used for such deposits in scientific literature and within the industry.
These deposits of oil are very similar to conventional oil pools except in two significant ways: First, they are more viscous than conventional crude oils with an API (American Petroleum Institute) gravity of 5 to 15 degrees, compared to 25 to 40 degrees for conventional crude oil. The deposits in place have a viscosity such that the oil is immobile and will not move within the reservoir unless it is mobilized by heating or by a solvent. Secondly, the deposits of oil sand, especially in the case of those in the Athabasca region of Canada, are much larger than conventional oil pools. For example, the Athabasca deposit contains approximately 143 Gm3 (900 billion bbl) of bitumen in place, whereas the Prudhoe Bay oil field, which is one of the ten largest conventional oil pools in the world, contains only about 2.4 Gm3 (15 billion bbl) of oil. Because of their high viscosity, these oil sand deposits must be mined and the oil separated from the sand by a variety of techniques using either hot water or solvents. The grade and geometry of the known deposits are such that they lend themselves to exploitation by surface mining techniques. These deposits are also being studied for in situ recovery of oil but these efforts will not be discussed here since this volume is concerned with surface mining.
This section will discuss the exploration and evaluation of these minable oil deposits. The techniques for grass-roots exploration for these deposits are similar to those