In considering the prospective value of dredging ground there are many conditions to be taken into account besides its actual gold content; it is like all other classes of mines in this respect. As a prominent mining engineer recently said: ‘‘Each mine is a law unto itself.” Likewise each dredging scheme must be considered strictly on its own merits before deciding on the methods and machinery to be used. Failure to recognize the specific conditions affecting a property, in calculating the means and cost of working it, has entailed the loss of large amounts of money. Misrepresentation and ‘salting,’ too, have played an important part in dredging as in other kinds of mining. Besides the amount of gold and the manner of its distribution, the following points must not be overlooked:
- Characteristics of the gravel, as to clay, hardness, cementing, size of boulders, etcetera.
- Depth to bedrock; the character and contour of the rock.
- Permanent or variable water-level, and available water supply under head or otherwise.
- Costs of power, labor, transportation, and supplies.
- Another consideration, not seriously affecting dredging in the State of California, but to be taken into account in many other localities, is the climate.
The methods of determining the factors mentioned are by sinking shafts, by drilling, and by actual test with dredge. Many diverse opinions have been expressed as to the relative value of each method, but undoubtedly the most practicable is the shaft. Water, however, is a serious drawback to prospecting by shaft and therefore drilling is more common. The Keystone drill No. 3, boring a hole of 6 in. diam., is generally used in the Sacramento valley and it costs about $1700. It is a self-contained machine and consists of a walking-beam arrangement and an engine of 8 or 10 h. p. In drilling, about 52 strokes are made, while 54 strokes are made in driving. Casing is, of course, used and a shoe with steel cutting edge and weighing 800 lb. is placed on the bottom joint. The diameter of the shoe is 7½ in. outside and this diameter was used at first in calculating the area excavated.
Exhaustive tests have shown that the actual cubic content of the core brought up should be 0.27 of the linear depth bored, and though this is greatly modified one way or the other—depending on the nature of the ground—this result is theoretically true for the usual standard casing (which measures 5¾ in. inside diam.) and it has been arrived at by measuring the content removed from the sand pump after drying. Theoretically, provided that all the core drilled (and no more) is recovered, 16 in. should be drilled for each cubic foot of core produced. It will be found that this result varies from that obtained on the assumption that the diameter of the core is equal to the diameter of the outside of the shoe. The diameter should really be reckoned about midway between the outside and inside diameter of the drive shoe, thus allowing for the wear that takes place.
The practice varies as to keeping the casing below or above the bottom of the drilling. To prevent gold from a rich streak outside the area of casing being drawn in and thus making the computation valueless, the casing is kept 3 to 4 in. below drilling. On the other hand, if the casing is likely to strike a large boulder, or become plugged with clay, so as to prevent, for some distance, gold being extracted that really belongs to the core, then the drilling is kept a few inches below the casing. Probably the best method of obviating these difficulties is by drilling below the casing, but not pumping all of the material out until the casing has again been driven and drilling resumed. Conditions, such as hard or soft ground, old workings, etc., must govern this part of the practice so that sometimes it is found necessary to drive the casing much below the drilling.
The sand-pump is a hollow-steel cylinder 8 ft. long and 4 in. diam., with a valve at the bottom and a closely fitting plunger; on each side, near the top. it has an oval orifice to allow the pulp to be washed out. There are several methods of treatment, and the general practice is the same though the methods of keeping records and estimating results differ slightly. The apparatus used includes, a sluice-box 16 by 12 in. and 10 ft. long with holes at the lower end for allowing flow into a four compartment settling-tank, another settling-tank, and an ordinary rocker with several pans and tubs. A bucket is placed in the sluice and the contents of the pump are washed into this, each foot being
treated separately. The overflow runs off into the settling-tank. The content of the bucket is panned into a tub. The number and size of the ‘colors’ or particles of gold in each pan are estimated by eye and the result of each foot is noted in the log-book, the colors being classed in three sizes. The colors and black sand from each pan are kept separately; the former are segregated for each foot and then amalgamated. The gold is separated by nitric acid, washed, dried, annealed, and weighed, the resulting buttons from each hole being assayed for fineness. The surplus contents of the pannings caught in the tub are passed through the rocker, the concentrate being re-rocked. The overflow runs into another settling-tank and the contents of the two settling-tanks are roughly dried and measured, the computed yardage being thus checked.
Another method is as follows: The material from the hand-pump is received in a pan held in a sluice-trough similar to that mentioned above. The slime is allowed to flow away and the coarsest material is panned off in the tank. The finer portion of the material in the pan is panned over a large metal tub. The estimation of ‘colors’ is made in the usual manner, and everything in tub and trough is then passed over a rocker.
Often the rich material in the interstices of the bedrock, or a rich seam will continue to be pumped into the hole, giving higher pannings than are warranted. To prevent this, when within a foot or so of the bottom, the results are panned as usual and from the foot just above and below bedrock the concentrate is caught in separate pans and if the results appear unduly high they are disregarded. All this is noted in the log-book, as well as the character and amount of gold content, and other particulars relating to the character of the gravel.
In one instance I observed an ingenious method of preventing salting. The work was being done on single shift, and on leaving for the night two panfuls of barren tailing were dropped down the hole. The heavy bit with rope attached was then lowered into the hole and the iron blocks were put on. To move these it would have been necessary to re-fire the boiler. In the morning the blocks were removed and the rope and bit washed into the hole and pounded to knock off any ‘salt’ (gold flakes) that might have adhered to them on being dropped down. The pans and sluice, etc., were thoroughly washed and the barren tailing at the bottom was tested. It might be a safer plan to keep a watchman on the spot.
A clever case of salting the drillings occurred recently at Oroville. The ‘salter’ mixed some finely divided gold with pipe-clay ; he became so adroit that he could mark a piece of casing on the inside so that the streak would contain a given number of cents per cubic yard to be drilled. In case the driller noticed the marks at all, he would simply think that they were initials or shop numbers, and during the process of drilling, the streak with its fine gold was washed into the pump and unwittingly recovered with the gold in the gravel.
In connection with salting, several cases were related where the drillers wilfully produced (at the instance of their employer, it is said) misleading records by the following method: The bulk of the gold content was known to lie in a stratum 3 ft. thick and between 17 and 21 ft. from surface; each hole was drilled to about 30 ft., or 10 ft. deeper than was necessary. To illustrate the result we will assume that each of the 3 ft. between 17 and 21 ft. contained 60c. per cu. yd. and the cost of drilling was 7c. Thus the whole ground necessary to be dredged (20 ft. deep) would average 9c. per yd., leaving a profit of 3c. per yd. worked. The crafty party who attempted to wreck the negotiations for a fair sale, handed in his report, giving the depth of the ground as 30 ft., and the average yield per yard as 6c. Taking the vendor’s admitted cost of working (7c.), there was an apparent loss of 1c. per yd.; and so the sale was declared off.
The following examples show the headings used in the log book kept by drillers of the Central Gold Dredging Co. and from the records of which the value of the property is computed. A summary statement is attached to the bottom of the report and is made up of details under the headings shown.
A great deal has been said about the extreme care necessary in making estimates from drill-records. At the best, the method is but an approximation and the accuracy with which it is done should bear an exact ratio to the care taken in measuring the bank and calculating the cubic content dredged each month; otherwise, the results are not only misleading, but useless.
Thus the value per cubic yard of the ground at each particular hole is arrived at; the yardage of the ground necessary to be dredged is calculated from the depths to which the ‘pay’ is shown to exist, by the drill-records, and the actual average value per yard is determined by simply multiplying the value per cubic yard by the number of dredgeable cubic yards in the property. To estimate the purchasable value of the tract the first cost, depreciation, interest, and total cost per yard of operation must be deducted and, of course, a certain percentage allowed for the prospected value being lower than the recoverable value. One of the largest operators in the State told me that he always deducted at least 40% of the prospected value in purchasing dredging tracts.
As to the number of holes necessary to test the ground, this also is arbitrary. If the gold is known to be evenly distributed, the tract should be divided into five to ten acre squares. A flag at the centres of these marks the site of the hole. If the gold is in narrowing or widening channels, the ground should be first crossed by series of holes at long intervals and then closer together if favorable results are indicated.
The cost of drilling differs with the conditions, but it may be said that the price per foot will vary between $1 and $3.75 per ft.—a wide range. In one case at Oroville 13 holes were put down at a cost of $3.48 per ft., and in the same district at another property seven holes cost $2.40 per ft. Some contract work at Oroville was done for $2.50 per ft. The following recent examples of costs and speed cover a large area, and will give a practical idea of the variation: On the Yuba, five holes were drilled to an average depth of 93 ft. at an average cost of $3.85 per ft. The work was done during the winter rains and the roads were extremely heavy—freighting was difficult, transportation charges excessive, and there was great delay in having repairs made at Marysville (the nearest point), the fixed charges having to be sustained all the time. Moreover the work had to be finished within a time limit, so as to secure results before an option on the property expired.
In the southern part of the Oroville district 50 holes were put down averaging 35 ft. each and two drills were employed working simultaneously. The cost in this case averaged the extremely low price of 97c. per ft. The work was done in summer and all conditions, including soft ground, were most favorable. Water was close at hand and the fact that both drills ran under the same management reduced expenses. The labor included 1 foreman, 2 drillers, 2 helpers, 1 panner, 1 two-horse team and 1 teamster; the last also acted as water carrier.
At Oroville nine holes were put down about June, 1904, and totaled 258 ft. in depth. One machine was used on single shift and 33 days were employed in the work with 4 days more for moving.
The cost was divided as follows:
In panning and for the boiler 800 gal. were used per shift. Cost of fuel per shift $1.90 (using coal).
In the following case a month’s work is selected out of the seven months continuous work done just previous to June 1906 and in which 15 holes were bored to an average depth of 28 ft. During the month selected 5 holes aggregating 172 ft. of drilling were put down and cost $514.86 or an average of $3 per ft.
Just 31 days were employed in the work and 5½ ft. were drilled per day. The above work was done in extremely wet weather and the ground was very tight. At some drilling operations in the Yuba bottom during the present summer, oil was used as fuel in the boiler and 55 gal. per day were consumed at a cost of about $1 per gal. The cost of prospecting a tract by drill-holes varies with the thoroughness with which it is done and for practical purposes the following simple calculation prepared in tabular form will show the range:
With regard to the ratio of recovery between dredging and drilling much, of course, depends on the method and care employed, the relative efficiency of the gold-saving appliances and the manner in which the clean-up is accomplished. Most experienced operators contend that however carefully the operation is carried out (within the practical economic limits) and using the best of gold-saving appliances the results by drilling can never approximate the work of sampling and estimating an orebody underground. It might be supposed that by using an arbitrary method of (over or under) estimating the drilling results, a fair idea of the probable recoverable value might be arrived at and if the results bore a consistent relation (greater or less) to the recovery, this would be an effective method, but such is unfortunately not the case in practice. The
ratios are quite irregular and results vary both above and below the returns from prospecting. Only one general rule seems to hold, and its application is found to be generally true, namely, that when very high results—70c. to $3 say—are got by drill, the recovery from that place is sure to be lower. Likewise it is true in practice, that ground giving very low results from the drill, say from 1 to 5 or 6c., generally dredges considerably higher. Some experienced operators go so far as to say that drilling is practically useless except as a means of ascertaining whether gold is actually present in the ground or not. On the other hand at Folsom, where several thousand holes have been put down and the most careful records kept, it is asserted positively that an average approximating about 90% of the gold shown by drilling has consistently been obtained in dredging. Be that as it may, when it is explained that roughly a drill-sample will only represent something like 1/200000 to 1/1,000000 of the body of material to be worked while in sampling a mine probably from 1/5200 to 1/10000 taken of the orebody (which is not, as a rule, less homogeneous than the gravel beds), there should be at least a proportionate difference in the working results. Moreover, the method of estimation and selection cannot be compared for accuracy. The following authentic cases are extremely interesting in this connection : In front of the Boston & California dredge No. 1, 22 holes were put down in one acre, and the results showed that the ground contained 60c. per cu. yd. When this particular acre was dredged, just 30c. per cu. yd. was recovered. In another instance, 25 holes were put down on one acre and the dredge recovered 95% of the estimated amount. On the Delancy tract, the results of dredging recovery came very close to the drill estimate, the latter being very carefully done.
Several holes were checked by dredge at Oroville and the report illustrates the practical application of the rule laid down above, as follows:
Of course, the closer together the holes are put down and the more of them there are on a given tract, the closer the general result for the whole tract should come to the actual recovery; if the work is properly done, a fairly accurate idea of the value of the property may be obtained.
For the 12 months ending December, 1903, the Biggs No. 1 dredge of the Oroville Gold Dredging & Exploration Co., Ltd., worked 474,610 cu. yd., which, according to the estimate from prospecting, should have yielded 11.40c., the total recovery averaged only 8.45c., or 76% of the prospect value. The same dredge for the following 12 months of 1904 dredged 493,150 cu. yd., which yielded 12.32c. per cu. yd., whereas the prospect value of the area worked showed from the drilling tests an average content of 16.64c. per cu. yd., in other words, a recovery was obtained of about 74% of the estimated value by drilling. The following details of the above cases show that the ratios of recovery in cases of ground dredged after one or two drillings vary greatly, while the total average for the year appears to maintain a fairly uniform relation.
In another district in the Sacramento valley for the three months of March, April, and May, 1906, a certain dredge recovered an average of 25c. per cu. yd., where prospecting by drill had only indicated 18c. per cubic yard.
Naturally, careful shaft-sinking is a more satisfactory method of testing; it ascertains both the gold contents and the nature of the ground far more efficiently and thoroughly, but the cost is sometimes prohibitive in wet ground. Otherwise it costs less than drilling. The so-called China shaft is the method usually employed and unless one has seen the work, one wonders how it was ever accomplished by hand. Probably workmen of no other nationality would do the work or, in fact, could work in such a narrow compass.
The shafts are sunk, circular in section, 3 ft. in diam., and the work is done by Chinese. Two men will do from 5 to 8 ft. per day; and at Oroville the contract cost is $1 per ft. Washing and estimating content will cost about 30c. per day more. These costs are for all work above water level. Below that level special arrangements have to be made and it is often altogether impracticable because ‘John’ is decidedly averse to working while water is being hoisted over his head, and little is he to be blamed for his objection. In more than one case he has stink such shafts where it is dry, to depths of 40 ft. and over. In a large percentage of cases where water is supposed to be insurmountable, a centrifugal or some other form of pump could be installed with a small boiler and the work accomplished. A shaft 5 ft. square was sunk for a depth of 40 ft. within a few feet of the Feather river and on a level that at times was covered by the overflow. In this instance of successful prospecting, a 6 in. centrifugal pump (run by steam) was used. The cost should be little more for such work as electricity can now be successfully and economically used, and the results are so far ahead of the average drill-hole—if only for the purpose of positively knowing the nature of the ground—that were cost per foot several times more than drilling, it would still be advisable. The whole area of a certain property at Oroville could have been efficiently and cheaply tested in this manner, but for some inscrutable reason drilling was resorted to after a short period. Shaft-sinking on one property at Folsom cost $1 per ft. including panning, etc., and an average of 9 ft. per day was accomplished.
The only instances of work relating to the so-called paddock system that I am aware of in the Sacramento valley are those cases (cited in the Bulletin on ‘Gold Dredging’ published by the California State Mining Bureau) at Oroville, where Messrs. Hammon & Treat in 1895 sunk a pit about 100 ft. square down to the bedrock and used a centrifugal pump to keep the water out. The gravel was hauled in wagons to small sluice-boxes, where it was washed. In the other case the water was found to be too heavy to contend with on approaching bedrock. This work, however, was apparently done in a mining way and not as a prospect to test the ground.
Both at Oroville and Folsom most of the ground now being worked by dredge was all worked over years ago by pan, rocker, and sluice during the early days. As the grade of the gravel got lower, the white men left and Chinamen took their places and the ground was re-worked. In the dry season it is probable that these old shafts and drifts in many places reached the present bottom as shown by the timbers and portions of wing-dams that are encountered by the dredges and drills every day—much to the operator’s disgust. Even in wet weather some of the ground was probably worked to the bottom with the assistance of the ‘China’ pump—a contrivance consisting of one or two 3 in. rubber belts with wooden blocks attached which acted as elevators. This was either driven by hand or by an overshot wheel of native manufacture. By this means in some cases the water was pumped from a depth of 40 feet.
Although not thorough, this work was so general and the district was so carefully exploited that today it is said that ground where old workings are not found is generally poor, and actual values of 15c. per pan and over have been obtained from some of the old tailing beds. These abandoned workings, when encountered by the prospect drill-holes, however, form a menace which often vitiates entirely the result of the hole, as it is impossible to say from what amount of gravel the results are produced. The following authentic record from some recent prospect holes at Oroville show how variable is the distribution of the gold. In some cases it seems to lie in well-defined streaks and elsewhere it is disseminated through almost every foot of the ground to be worked. The first and second records are of two holes on the El Oro tract at Oroville and the other two are from some drilling near the centre of the district, both on the same property.