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Mining and Scientific Press, Vol. 10, No. 12, p. 182 (March 25, 1865)


Upon the northerly flank of Mount Diablo may be seen a spur of primitive formation, similar to that of which the main body of the mountain is composed, thrown out some two or three miles in a northerly direction from the principal, cone-shaped upheaval of the main summit. The easterly declivity of this slope is deeply indented by two cañons, known as the Mitchell and Bagley* canons. The former almost completely severs the northerly portion of this spur into an isolated summit, which is known as Mount Zion, and which overlooks the town of Clayton.

Near the peak of this summit, and on the westerly slope, we have the Summit of Zion mine, and on the easterly slope is located the Mount Zion Copper Company. Of these companies we shall speak more at length next week. In passing through the Mitchell canñon, upon either hand, but chiefly upon its northerly aclivity, may be noticed numerous other companies working for copper. Although very fine specimens and bunches of sulphurets, together with some native copper are found, no very extensive operations have yet been carried on there, for the reason that no well-defined veins have yet been discovered; the bunches of ore having the appearance of being isolated deposits.

This cañon and Mount Zion, already described, contain the chief copper deposits yet found in and about Mount Diablo. Following along the spur above indicated, toward the main Mount Diablo summit, and passing the mouth of Mitchell cañon, a short distance, we come to the mouth of another ravine, known as Bagley cañon. This runs nearly parallel with Mitchell cañon, and segregates, but at a higher level, another portion of the spur already mention from the main portion of the mountain. Ascending to the head of this cañon we come to the mining ground of what is known as the Open Sesame Mining Company. This company was incorporated August 5, 1863, and was consolidated from thirteen other companies.

The consolidated company now owns what is considered to be nine different ledges, named as follows: Cascade, Black, Henriquita, Carmel, Santa Domingo, Herman, Cortez, San Pedro, La Verdad, La Feliz and White Diamond; they also own the first extensions of the Cascade, Black, Henriquita and Cortez. The three principal mines of the company are the White Diamond, the La Feliz and the San Pedro. These mines are located on the right hand side of Bagley cañon, just before reaching its source. They are located one above the other in the order named, and are each plainly exposed to view by the wearing away by the waters of this cañon.

As these were the only mines belonging to the Open Sesame Company which we had time to examine during our visit at Mount Diablo the past week, we shall, of course, be compelled to confine our remarks to them exclusively. With the exception of these three and the Carmel and Santa Domingo ledges, all the other mines of the company are located in Mitchell cañon. The three which we shall particularly describe, together with the Carmel and San Domingo, are worked for gold and silver; the others, in Mitchell cañon, for copper. The chief work done by the company has been expended in opening these three mines, and in constructing a road to them through the cañon. We come first to the WHITE DIAMOND LEDGE.

This is the lowest in the series of supposed gold and silver ledges, which in geological position overlie the copper deposits of Mitchell cañon, and, if we are not mistaken in our hasty examination, of Mount Zion, also. The stratification of the summit at this point is unmistakable, and is in perfect conformity to that of Mount Zion, a little further north. The trend or direction is nearly north and south, and the dip is to the westward, at an angle of about forty-five degrees. But little or nothing has yet been done towards developing this vein, which has merely been opened. It is massive, some eight feet thick on the surface, and the croppings stretch away to the right (and northward) as far as the eye can reach, and are as distinctly traced as a stone-wall along a brown side-hill. We were informed that it could be thus traced for more than a mile.

The vein-stone of this ledge is of a bluish color and exceedingly hard. According to the superintendent's last report to the company, 113 pounds of this rock was sampled and assayed by Riehn, Hemme & Co., of this city [San Francisco], showing a yield of $119 to the ton, in gold and silver. THE LA FELIZ LEDGE. This ledge, although it does not show as distinctly upon the surface as the White Diamond, is nevertheless quite well defined. It contains 9,800 feet of ground, and is about 150 yards in vertical height above the Black [sic] Diamond, to which it is parallel. The first tunnel commenced upon this ledge has been continued a distance of 140 feet.

The walls of the vein were distinctly defined from the mouth of the tunnel some fifteen feet, at which point a fault was encountered, where the vein matter appeared to have been considerably disturbed. From that point onward the vein walls are quite well defined to the extreme end of the tunnel. The ore taken from this mine bears a strong resemblance to much of the Washoe silver ore, with the exception of some slight presence of copper, as carbonates, yellow sulphurets and the vanadate or peacock ores. The ore is highly quartzose in character carrying spar. Numerous assays of rock from this mine have been made, which have generally assayed about $45 to the ton ; some have gone as high as $100, and in one instance an assay, fairly assorted from some sixty pounds of ore taken from a "pocket," went as high as $291 to a ton, (incorrectly printed at the time in the superintendent's report at $201 to the ton.) One lot of two tons, taken as it came from the mine, was brought to this city [San Francisco] and worked, a little over a year ago, which yielded at the rate of $26 to the ton. The brick taken from this rock may still be seen at Martinez.

The "tailings" from this working assayed at the rate of $10 per ton. The above reports of assays are taken from the superintendent's report. Below this first tunnel another, intended as the main working tunnel, has been commenced, which will strike the vein eighty feet deeper. A blacksmith forge has been constructed upon the esplanade, or level floor, which has been constructed in front of this tunnel, and upon which the ore is to be placed as it is brought to the surface. THE SAN PEDRO LEDGE. Going still further up the mountain we come to the San Pedro Ledge, upon which the Open Sesame Company hold 9,800 feet.

This vein is equally well defined with those already noticed, and exhibits the heaviest outcrop of any yet opened, measuring about thirty feet in width at the surface. The company commenced work by clearing off an esplanade in front of the tunnel. After which a tunnel was run in about fifty feet, keeping the wall rock well defined upon the left. From this point a drift was run to the right, across the vein and thence downwards nearly in a right line twenty-two feet, at which point the vein matter was very much concentrated, exhibiting an almost uniform mass of sulphurets, which entirely covered the bottom of the shaft seven by eight feet.

Some thirty tons of first-class ore was taken out in sinking this shaft, besides a large quantity of second and third class, that was thrown into the dump. This shaft was full of water at the time of our visit and of course we could not examine it ; but we have gathered the particulars from conversation with one of the proprietors and the superintendent, Felix A. Mathews, also a large stockholder. We have examined a large quantity of ore from this shaft. It appears to be remarkably uniform in character, and consists of nearly forty per cent. of light gray sulphurets, of a somewhat friable nature, with a quartzose matrix carrying considerable spar. It is said that some very fine specimens of sulphide of silver were found near the surface.

There is no free gold visible to the eye ; but from numerous assays made the rock appears to yield from $40 to $45 per ton. A working process yielded, at the North Beach works, in this city [San Francisco], $37 per ton. The ore, for a sulphuret, appears to be one which should be easily worked--there being but little or no indications of antimony or any other particularly stubborn metal present. Of course, the ore will require roasting, but so long as it carries so large a quantity of sulphurets as it now appears to do there will be no necessity for any concentration. The quantity of quartz mixed with the sulphurets will present no material hindrance to a roasting of the entire mass. CONCLUSION.

From the description given of these veins--their position, their well-defined character and the favorable appearance of much of the ore, judging from observation and reported analyses, we see no reason for entertaining very serious doubts of an ultimately fair paying vein in either the San Pedro or La Feliz ledges. The White Diamond has not yet been sufficiently opened to form any very definite conclusions as to its value. We have said that no fine gold is to be seen in either of the mines,, but we are told that small prospects may be obtained from the ravine below. The probability, however, is that when worked in depth, copper will be found to gradually come in, and perhaps entirely change the character of the vein rock. The presence of spar in the matrix of the two upper veins is a favorable indication in this direction.

The highly metaliferous character of the San Pedro, in particular, would indicate a very valuable deposit of copper ore in depth, and that that mineral will predominate over silver and gold. These mines are admirably located for extensive working. They each present a linear front on the steep ascent of a high mountain, dipping away into the depths of the hill almost at right angles with the approach. But little dead work will be required to be done in thoroughly opening up the veins. Most of this has already been accomplished.

A fine roadway, of moderate grade, has been partially constructed along the banks of the cañon, to connect the same with the open valley below. A large amount of money, has been spent upon this road. Many of the stockholders have been of the opinion that the money so expended should have been devoted to a development of the mines, and the roads left until after the company had become fully satisfied that they would have a permanent use for them. Probably those through whose instrumentality this work has been done are fully satisfied on this point. We trust that future developments may prove the correctness of their judgment. It is better, however, in all cases, that general improvements about a mine, not absolutely needed at the outset, should be deferred until the necessity for the accomplishment has actually appeared.

The action of the Summit of Zion Company, near by the Open Sesame, is one worthy of much commendation in this particular, and presents a lesson of practical economy which has a wonderful effect in producing a willingness on the part of shareholders to come forward promptly with their assessments. The work upon these mines, as a general thing, appears to have been most judiciously expended, and reflects much credit upon the managers and the Superintendent, Felix A. Mathews, Esq., of Martinez.

Since writing the above, O. C. Coffin, President of the Open Sesame Company, has handed us the following report of working and assays, which have been made the past week in this city [San Francisco] and which he has desired us to publish for the benefit and information of the shareholders in the company : Working result by Kimball & Murphy, of the European Metallurgical Works, on Bryant street : A lot of 1,700 pounds of rock from the La Feliz vein yielded at the rate of $2.87 in gold and $2.25 in silver ; total, $5.12 per ton. A lot of 1,900 pounds from the San Pedro vein yielded at the rate of $4.60 in gold and $2.55 in silver ; total, $7.15 per ton. In each of the above cases the entire lot mentioned was worked. Mssrs. Kimball & Murphy append to their certificate an opinion that the rock from the San Pedro yields a percentage of copper sufficiently large to warrant further development of the vein for that mineral, and we understand that the company intend to send down another lot of rock from this mine to be crushed and assayed especially for copper.

One hundred pounds of ore from the San Pedro was worked raw, without producing any result. In addition to the above working test, Capt. Coffin also requests us to publish the following assays : By B. E. Moore--the San Pedro rock was returned as yielding, per ton, $2.41 in gold, with a trace only of silver. The same assayer returns for the La Feliz--"Mere traces of gold and silver, of no practical value." Riehn, Hemme & Co. returned as an assay of ore from the San Pedro--$3.15 in silver, with a mere trace of gold. By comparing the "opinion" of Messrs. Kimball & Murphy, appended to their assay, it will be seen that the views we have expressed of the character of the San Pedro mine, written and submitted to Capt. Coffin before receiving the returns of Messrs. Kimball & Murphy, are remarkably confirmed.

We are now more fully assured that the mineral obtained near the outcrop of the San Pedro is simply "mundic," overlying a vein of copper. The expense of proving this will not be heavy, as the vein may readily be opened at a great depth in the lower tunnel. We would recommend that this depth should be reached before any further testing. With regard to the difference noticed between the assays given in the former portion of this article and taken from the superintendent's published reports of 1863, and those obtained, in part from the same parties, during the past week, and furnished us by Capt. Coffin, we have merely to say, in lack of positive knowledge, that the former might have been taken from near the croppings of the mines, a short distance below. At all events, this discrepancy goes to show the necessity of moving with the utmost care and circumspection, especially in the beginning of all mining operations.

"OPEN SESAME MINING COMPANY."--At a meeting of the Board of Trustees of the Open Sesame mining company, held on Thursday last, T. L. Bibbing was unanimously chosen Secretary of said company--Office, No. 302, Montgomery street.

[Notes] * [Now known as Back Canyon.--ed.]