BULLETIN OF THE GEOLOGICAL SOCIETY OF AMERICA V o l. 19, pp. 93-98 Ju n e 1, 1908

OCCURRENCE OP PROUSTITE AND ARGENTITE AT THE CALIFORNIA MINE, NEAR MONTEZUMA, COLORADO1

BY FRANK R. VAN HORN

(Read before the Society December SO, 1907)

CONTENTS Page Introduction and briei description of mine...... 93 Principal ore of mine and assays of same...... 94 Occurrence of the proustite...... 95 Occurrence of the argentite...... 96 Paragenesis of the ores...... 97 Conclusion ...... 98

I ntroduction a n d b r ie f D e s c r ip t io n o f M i n e

During the fall of 1902 the officers of the Mine Developing Company of Cleveland presented the geological department of Case School of Ap­ plied Science with several vein sections and specimens of high grade ore from their California mine in Colorado. The writer has obtained most of his information concerning the mine from the officials of the company, whom he wishes to thank for both information and specimens. The California mine, formerly known as the Bell property, is situated on Glacier mountain, about 3 miles from Montezuma, Summit county, Colorado. The ore occurs in a fissure vein having a dip of approximately 30 degrees to the northwest and a strike which varies from 30 to 50 de­ grees northeast and southwest. The country rock is a coarsely foliated granitic gneiss in which the biotite has altered to chlorite near the vein; this alteration was probably due to normal weathering, and not to the action of the solutions which deposited the vein matter, as no cases of impregnation of ore minerals were noticed outside the vein wall in any

1 Manuscript received by the Secretary of the Society December 30, 1007. X—Boll. Geol. Soc. Am., Vol. 19, 1907 (93)

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of the specimens under observation. In hand specimens the foliation of the gneiss is not always distinct, and the rock is locally known as granite. The vein varies in width, and frequently has a massive granular appear­ ance, but sometimes possesses a banded structure due to symmetrical deposition of minerals; at other times a more or less brecciated appear­ ance is noticeable, which was probably caused by movements of the vein walls on each other. The surrounding region is more or less faulted, and at one place in the mine, the main ore body has been dislocated and shows slickenside surfaces. The vein is separated from the gneiss by a wide clay selvage, or gouge, which causes more or less trouble by caving in.

P r i n c i p a l O r e o f M i n e a n d A s s a y s o f S a m e

The ore occurs in streaks in the vein, varying from 1 to 21 inches in width, and consists principally of argentiferous galena, although in places large amounts of sphalerite are mixed irregularly through it. As a rule, both galena and sphalerite are coarsely granular, the of each vary­ ing in size up to 3 inches in diameter. Some assays were made, which show an interesting variation of silver and lead values in the different pay streaks of the vein as follows:

No. 1, 1 inch w ide; silver, 86.5 ounces per ton; lead, 34 per cent. No. 2, 2-3 inches w ide; silver, 11.0 ounces per ton; lead, 33 per cent. No. 3, 4 inches w ide; silver, 17.0 ounces per ton; lead, 65 per cent. No. 4, 4-5 inches w ide; silver, 34.0 ounces per ton; lead, 64.5 per cent. No. 5, 5 inches w ide; silver, 19.0 ounces per ton; lead, 47 per cent. No. 6, 14 inches w ide; silver, 25.8 ounces per ton; lead, 80 per cent

An inspection of the above assays shows that the which ran high­ est in silver values contained the lowest percentage of lead, and that the one with highest lead contents gave results low in silver. This shows that the amount of silver does not depend on the quantity of galena present. It might be suggested that the variations in silver values could be ex­ plained by the presence of proustite and argentite mixed with the galena and sphalerite. However, the above assays were made long before the discovery of the rich proustite-argentite ore which will be described later in this article, and, as far as the writer can learn, those minerals were never found in the mine before the present discovery. Therefore it would seem that the variations in silver were probably caused by argentiferous galena in which the silver contents varied considerably. Possibly there is not sufficient evidence to warrant the speculation, but it occurs to the author that the different streaks might possibly have been deposited from

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different solutions. This explanation would account for the variation of silver and lead values shown in the assays.

O c c u r r e n c e o f t h e P r o u s t it e

In September, 1902, the largest pay streak of the vein widened to about 21 inches and assumed a distinctly banded structure, with galena and sphalerite irregularly mixed on each side; these were followed by siderite, also symmetrical, while in the center was a streak of massive proustite with finely intermingled quartz which was more or less drusy. This streak was usually about 2 inches in width, but in one instance amounted to 14 inches. The proustite possesses all its usual mineralogical charac­ teristics, which require no further description. For a very short distance the central portion consisted of argentite and finely disseminated quartz or a mixture of the two with proustite. When the latter predominated along the vein, no argentite could be observed, whereas in many of the argentite specimens proustite was detected. The proustite ore was fol­ lowed along the strike for a distance of 30 feet with an upward stope of 20 feet, at which point it disappeared. By means of a winze and cross­ cut, it was also followed downward 15 feet until mining operations were suspended in the winter. The average specific gravity of four speci­ mens of the proustite-quartz ore was found to be 4.17 as compared with 5.60 for the pure proustite. Several quantitative blowpipe determina­ tions showed an average of about 20 per cent silver or about 6,000 ounces to the ton for the specimens examined. This would indicate about 33 per cent of the ruby silver, while the average specific gravity obtained would indicate a little over 50 per cent proustite. Mr J. C. Sharp made two quantitative analyses of carefully selected proustite, the average of which gave as follows: Theoretical Found (Ag3ASS3) Ag ...... 67.60 65.5 As ...... 13.85 15.1 Sb ...... 93 S ...... 17.40 19.4

99.78 100.0

The analysis shows that a small amount of the molecule (Ag3SbS3) is present, as is the ease with certain proustites from Chile and Germany. That the percentage of silver found is apparently too high, while the sulphur comes lower than the theoretical amount, is prob­ ably due to an imperfect analysis, but might have been caused by the

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presence of native silver, although none was perceptible. Silver, however, was observed on the argentite described later. Professor A. W. Smith very kindly recalculated the analysis on the assumption that the antimony replaces . It was found that .93 per cent of antimony is equiva­ lent to .58 per cent of arsenic, which makes the total of the analysis come still lower than the original, as follows:

Ag...... 67.60 A s...... 14.43 S ...... 17.40

99.43

This calculation shows the arsenic to be .67 per cent low when com­ pared with the theoretical amount, which is 15.1 per cent. Using the arsenic of the above as a basis of calculation of proportions in the theoretical formula of proustite, Ag3AsS3, we find the following requirements: Required Found A g...... 62.62 67.60 4.98 excess...... 14.43 14.43 S ...... 18.54 17.40 1.14 deficit.

95.59 99.43

The above calculation proves at least an error in the determination of sulphur, but also indicates that native silver may probably be present. According to the last results, the original substance analyzed may have had the following composition: Proustite...... 95.59 S ilv e r...... 4.98

100.57

O c c u r r e n c e o f t h e A r g e n t it e

The argentite specimens, which vary from 2 to 3 inches in width, are generally massive and finely granular, but in some cases are quite coarsely granular. As was indicated in the description of the proustite occur­ rence, the argentite seems to replace the proustite in the central portion of the vein for a short distance. In two instances a reduction of the argen­ tite to native silver in wire-like forms was observed. The coarsely gran­ ular material is evidently purer, but all specimens are completely sectile and malleable. That the , however, is not pure is shown by the

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fact that the average specific gravity of three samples was found to be 6.55 as compared with 7.28 for the pure argentite. Quartz is sometimes observable with the naked eye, and, as was mentioned above, proustite can often, though not always, be seen on the argentite specimens. The average of two analyses of the argentite made by Mr R. B. Dennis is as follows:

Ag...... 83.57 S ...... 12.66 Insoluble...... 3.62

99.85

The analysis was recalculated with the insoluble matter left out, with the following results: Found Theoretical A g...... 86.71 87.1 S ...... 13.13 12.9

99.84 100.0

The lower specific gravity (6.55) mentioned above conforms in general to the amount of insoluble matter in the original analysis and probably is due to the finely disseminated quartz.

P a r a g e n e s is o p t h e O r e s

Owing to the limited number of specimens which could be studied and the fact that the author has never visited the mine, it does not seem war­ ranted to enter into a very extended discussion concerning the order of deposition of the various ores mentioned above. One vein section was observed which was about 14 inches wide and consisted entirely of an irregular mixture of coarsely granular galena and sphalerite. No other conclusion seems justified but that both minerals were deposited at the same time. Another section described above under the occurrence of proustite shows the symmetrically banded or ribboned structure called crustification by Posephny, which is considered as showing simple fissure filling. This specimen contained the same irregular mixture of galena and sphalerite on each side, followed by siderite, likewise symmetrical, while the center was filled with the drusy mixture of proustite and quartz. In still another section the same order is followed, except that argentite is substituted for proustite. According to this arrangement, the galena and sphalerite were deposited at the same time, followed by siderite, while the

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last to be precipitated was the mixture of proustite and quartz or argen­ tite and quartz. Prom the solubility of these various minerals it would seem probable that here, also, the ores were deposited from different solu­ tions at different times, which was the conclusion drawn from the assays given above, all of which were made before the discovery of the rich silver minerals. It seems plausible that there were two silver solutions, one first fur­ nishing the argentiferous galena, while the later one deposited the prous­ tite and argentite along witty quartz. Although apparently of later origin, in no case were the silver minerals found penetrating through the siderite into the galena and sphalerite, which would indicate that there was no later fracturing or opening of the fissure while the filling of the same was taking place. The walls of the vein seem everywhere sharply defined, and no replacement of the country rock was noticed. It might be men­ tioned that the officials of the company working the mine have a decided opinion that more sphalerite has been encountered in the lower levels than above. Likewise no proustite and argentite were ever found in the older workings above. This indicates an enrichment of the vein below, which might be due to solution of upper deposits and precipitation of the same in the lower levels. There was no evidence observed which would warrant this assumption, however, and it seems more probable that the change of ores has been caused by other conditions, such, possibly, as variation in pressure, temperature, or chemical composition.

C o n c l u s io n

The chief incentive in writing this article was to record the new occur­ rence of proustite and argentite in this particular locality. The writer has also never seen or heard of such massive specimens in such large amounts from any other region. On account of these facts, this paper has been deemed worthy of presentation before this Society, and specimens of both minerals in vein sections are submitted for inspection, hoping that they may prove of interest.

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