Text.2 working of terrace and alluvial deposits
It is not always feasible or possible to work river terrace and other deposits of this class by hydraulics, in which case ordinary sluicing must be employed, the direct loading method being adopted. As deposits of this class are nearly always encountered on the slopes of hills, it is necessary to make certain that any payable alluvial deposits, in the valley below, are first worked out to prevent the covering over of any workable ground with tailings from the operation of a river terrace or alluvial deposit. As deposits of this class are usually of low grade, it is necessary to operate them on a large scale, by washing as great a yardage as possible, to make the deposit a payable proposition. The method employed is as follows (fig.33).
Fig. 33. Method of working river terrace and eluvial deposits
A leat is excavated above the deposit, for the water supply necessary for sluicing purposes; trenches are then excavated in the deposit, for its full width, down to bedrock, and at right angles to the general direction of the valley in which the deposit is located; in these trenches, sluice boxes are rigged up resting on the bedrock and for the full length of the deposit, so that the tailings are discharged into the valley below. Alongside these sluices, paddocks are excavated the material being dumped into the boxes by shovel, no transport being used. As the paddocks advance, so the sluices are moved, as it is necessary to keep these as close as possible to the working faces. The number of sluices that can be utilized depends entirely on the quantity of water available; this is sometimes augmented by means of a pump. Sufficient grade is obtainable for the sluices by resting them on the bedrock, as the bedrock is usually fairly steeply inclined. By this method, some 7 to 10 cubic yards per man per day should be treated, per line of sluice boxes.
Ground sluicing. Ground sluicing may be employed where the gravel to be worked is shallow, say. 6 feet deep, where the water supply is plentiful, where the grade is steep enough to enable the water to carry the disintegrated gravel to the sluice boxes, and where sufficient dumping ground is available for the disposal of tailings.
This method of sluicing consists in excavating a shallow trench on one side of the deposit to be worked, into which a stream of water is diverted, until the trench is deepened to bedrock, the gravel thus obtained being washed down by the flow of water into the sluice boxes, which have already been rigged up the lower end of the workable ground. It may be necessary to aid the flowing water in its work, by picking the ground, so as to loosen it. When the shallow trench has reached bedrock, its banks are picked and barred down into the stream of water, and small dams that are constructed cause the water to be forced against the banks, thus aiding in the excavation of gravel by undercutting. When the banks have receded far enough, the bedrock is thoroughly cleaned up by means of scrapers and hoes, as the coarse gold usually remains on the bedrock, only the fine gold being washed down with the flow of water into the sluices.
For ground sluicing auriferous gravels in Uganda, the following method was employed. A sluice or tail race was excavated, well down to bedrock, in the paddock to be worked and for its entire length, the bottom of which was lined with rocks and stones, with drops every 3 feet down the race in the direction of the flow, a bank of gravel 2 feet thick being left standing on either side of the race to form its walls. Feed races were then cut at right angles to the tail race at 9-foot intervals (all cuts were kept 9 feet wide) on the upper side of the paddock and into these the excavated material was dumped. All the available water was brought to a point just above the head of the tail race by means of a leat, and was turned into the feed races, down which the excavated material was washed into the tail race, where boys with forks were stationed to fork out large stones and to work "over" the bed of sand to prevent its "packing".
After a 27 feet wide paddock (that is three cuts) had been excavated, the water was turned off, the 2 feet wide banks on either side of the race were dug up and transported to a sluice box, similar to that described together with the sands and concentrates from the head of the tail race for the final clean-up. During this operation, the upper 27 feet of the tail race remaining was repaved with large stones, as this now formed the head of the ground sluice, sluicing was recommenced, and the procedure repeated until the whole area was worked out.
Generally speaking a minimum of 400 gallons of water per minute (65 cubic feet per minute) is required for ground sluicing and, working on the assumption that it required 2,400 to 2,500 gallons of water per minute to move 1 cubic yard of material, it will be seen that the minimum quantity of water will take 6 minutes to remove 1 cubic yard, or 10 cubic yards per hour.
Hushing or booming. This material is employed where the quantity of water is insufficient for continuous sluicing; in this case, water is damned up above the deposit and then released at intervals, by means of automatic gates, the rush of water excavating the gravel and washing it down into the ground sluices.
Records. During sluicing operations it is essential to keep a record, for each area, of the work done and of labor employed, etc.; this is done in a "log" book, a specimen page of which is given in Fig. Overburden and gravel is measured daily, measurements being taken by means of a graduated staff, at necessary intervals; thus, if a paddock is 9 feet wide, and measurements of overburden and gravel are taken at 12- foot intervals, every foot of depth read on the graduated staff will represent 4 cubic yards. Similarly for a cut 10-8 feet wide, if measurements are taken at 10-foot intervals, every foot in depth will represent 4 cubic yards.
Exploitation plans. Such plans are necessary to show what yardage reserves are available, month by month, besides which they enable a comparison to be made between pitting and operating results. A tracing of the original pitting plan, showing pits and pitting lines, should be made, and on this should be plotted the area exploited during the month, as shown in fig. 34.
