Digitized by the Internet Archive in 2012 with funding from University of California, Davis Libraries http://archive.org/details/suggestionsforex17bail STATE OF CALIFORNIA EARL WARREN. Governor DEPARTMENT OF NATURAL RESOURCES WARREN T. HANNUM, Director DIVISION OF MINES FERRY BUILDING. SAN FRANCISCO 11 OLAF P. JENKINS, Chief SAN FRANCISCO SPECIAL REPORT 17 FEBRUARY 1952 SUGGESTIONS FOR EXPLORATION AT NEW ALMADEN QUICKSILVER MINE, CALIFORNIA By EDGAR H. BAILEY Prepared in cooperation with the United States Geological Survey LIBRARY UNI\ -' JtA n / EXPLANATION Verticol shoft Portal of caved adit Bottom of cut Dump motenol without torrr 4 Londshde from dumps Underground workings on the 800 I Contours on the upper surface the upper serpentine sill d where known; dotted where projected. Edgar H Bailey, August 1950 Coordinates and datum are those used by the mining companies. MAP AND CROSS SECTION OF PART OF THE NEW ALMADEN MINE AREA, SANTA CLARA COUNTY, CALIFORNIA SHOWING STRUCTURES FOR SUGGESTED EXPLORATION Contour interval 10 f / SUGGESTIONS FOR EXPLORATION AT NEW ALMADEN QUICKSILVER MINE, CALIFORNIA * By Edgar H. Bailey ** OUTLINE OF report programs, such as can be carried out only by well-financed ag .® mining: companies. This report calls to attention certain t t ^ct'o 3 structures that seem to be unusually promising for ex- Geology _. I~ZI 3 ploration in the oldest and largest of the California quick- Structures suggested for exploration 4 silver mines. Although the abnormally high prices make ... .. exploration particularly timely, nevertheless it is quite ™ . i tit i »• f » f t ,,„ y„„, Ai,„n-io„ possible that the proposed exploration might discover ore Plate 1. Man and eross-xection of part of the .Now Almaden f -,. »«••,• in • ■/» mine area, Santa Clara County, California, showing bodies of sufficient size and grade to be economic even if structures for suggested exploration 2-3 the price of quicksilver again falls to a low level. The ore bodies mined at New Almaden were among the Abstract mi , . . f t , xt U i -„i -i „ m -„„ largest found in anv domestic quicksilver mine, and the The geologic investigation of the New Almaden quicksilver mine J ' in Santa Clara County. California, resulted in the location of sev- structures recommended for further exploration are ex- eral areas believed to he favorable for the discovery of new ore tensive enough to contain ore bodies comparable to those bodies. Because ore bodies are found in silica-carbonate roei< along previouslv mined. The average quicksilver content of the the margins of serpentine sills intrusive in Upper Jurassic (?) rocks ,„.,. furnaced at New Almaden from the time of its dis- of the Franciscan group, the search tor ore should be guided by a ,., 1(ln A , . . , ,, i 4 - i knowledg ■ the form of the serpentine bodies. Tw tential ore '-"Very unttl 1900 was about 4 percent ; and the relatively structures were discovered by contouring the surface of the principal Small amounts 01 lnwer-prade ore taken t rom Underground serpentine mass. The mass is now believed to consist of two main workings ill the early 1900's. although lint so accurately sill- joined in the central area; if this is so. other contact areas that ,.,,,., ,,., !,.,]_ would not reduce this average V( . rv milch. Dlir- may contain ore bodies remain little explored. ing World War I I . however, when quicksilver prices were Introduction h i trh . it was possible to mine profitably from open cuts The Xew Almaden mine, about 10 miles south of San "< vs containing as little as 3 pounds of quicksilver to Jose, California, has yielded more than a third of the *'"' t ""- quicksilver produced in the United States. Between 194] The New Almaden mine area has been explored by about and 1948 the [7. S. Cieological Survey made a detailed 35 mUes of underground workings, some oi them near the study of the entire Xew Almaden district in an effort to surface and some extending to a depth o about 2,400 feet determine the relation of the quicksilver ore to the geo- below the top oi Mine Hill, or about ,00 feel below sea logical structure, and to discover guides that might aid level. The recenl geologic study mdicates, however, that in future exploration and development work. A compre- tll,MV ■" stlU a Ee ™ blo< *s "I unexplored yet easily tested hensive report on the results of this study is in prepara- ground thai are geologically favorable for the discovery tion (1951), bul this preliminary report has been pre- of ore bodies. TI,e V remained unexplored during the de- pared to describe some of the more easily tested places veloptnent of the mine, between 1850 and 1900, largely that are believed to be favorable for the discovery of new because the geologic environment ,,| the known ore bodies ore bodies. It is not exhaustive, and does not pretend to had not bee,, adequately studied and understood. be an evaluation of the possibilities of the entire district. Geology which also contains other notable quicksilver mines. Thi , (i](1( .^ npr] . s nf th(> x ,, w Almaden lim „. area a re a The price of quicksilver fluctuates with greater rapidity par1 ,,,• the heterogeneous Franciscan -roup of Upper than the price of any other of the useful metals. During Jurassic e These rocks are folded, and are intruded World War II it was at a high level, and about 200 quick- by sills of serpentine, the margins of which are altered to a silver mines in the United States were in production. After quartz-magnesite rock called silica-carbonate rock. Ore the war period it declined to a value that was probably , )i)iiit . s .,,,, ,■„,„„, in this s dica-carbonate rock, close to the lower than at any tune in the last KM) years. ,1 the chang- original intrusive contacts of the sills. Most of the ore ni"; value of the dollar is taken into account, and as a |ini!j( , s that , |;1V( . ,„,,,, min ,,,, extended along the Upper result all but one oi the major domestic mines were shut sj(|( . s n) - s|] | s ,.| ||S( , Dene ath a capping of sheared shale down. In mid-1950 the price again rose to a level a little ( .. il]ril « a lta"; but others not quite so large but equally higher than prevailed during World War II. With this nr|| exten ded .,]„„,, tne under sides of sills and above price, the quicksilver mining industry has undergone .,,,., where the contacts are relatively flat the ore bodies BOmewhat of a revival. Nevertheless, because oi the llll- .,,.,, J,, ))l( , erestg of domeg ,,,. arches. Where the co,, certainty of a high price continuing beyond the period ;|lv steeper tne ore bodies are in places where sleep north- of time necessary to carry out major exploration, develop- east-trending veins of quartz and carbonate are particu- ment, and the construction oi a plant to furnace ore on |ai . lv abundant< \ n both situations tl innabar replaced a large scale, much oi the current activity is endued to the silica-carbonate rock along the vein fractures, but the small groups of men seeking small supplies oi rich ore to V( , ms tnemS elves are for the most part virtually barren. treat in retorts. Unfortunately, such operations yield only u 1]l( , (iI . ( , oeeurs a i ong tne altered mar-ins of the ser- asmall percentage of the quicksilver n led by our highly pe ntine the search for additional ore should be guided industrial nation, ami do not result in the development b knowledge of the form of the serpentine bodies. The of a self-sustaining domestic quicksilver industry. The principal serpentine mass is exposed at the surface in the search for large new ore bodies in the old quicksilver mines cen tra] partofMine Hill, and in the past it has been inter- m Califo rnia in general will require extensive exploration pre ted as having the form of a cedar tree that expanded •Publication authorized bv the Director, U. S. Geological Survey. irregularly downward, being steep but jagged along its ••Geoloe^U^S^S November. 1951 south ^^ am , rath(ir sm „„ th .„„, gently inclined on the (3) Special Report 17 north and east. Most of the ore bodies were found as the result of exploration along the margin of this body, and contouring of its surface indicates that at least two struc- tures that might contain ore remain unexplored. These (structures A and B of plate 1) can be probed by drilling near-vertical holes from the surface to depths of 250 to 600 feet. Other potential ore-bearing areas are suggested by a reinterpretation of the shape of the intrusive mass of serpentine. This is now believed to consist chiefly of two main sills that are joined in the central area near the sur- face of Mine Hill. In this block, where the early workings of the mine were run, the serpentine has the appearance of a single intrusive mass, and, in spite of later develop- ment revealing its twofold character, the original concept of a single body was closely followed in the search for ore. The new interpretation, involving two sills, introduces three additional extensive, but little-explored, contact areas that can be expected to contain ore bodies in favor- able parts. Structures along two of these (structures C and D of plate 1) appear especially promising and can be reached by drilling holes from the 800-level Day tunnel. To reach the suggested drill stations, however, will require reopening the Day tunnel, which is now caved at the portal and is also caved tight in at least two other places farther in. Structures Suggested for Exploration Structure A. Structure A is an arch in the upper mar- gin of a serpentine sill, at an altitude ranging from 1,000 to 1,150 feet above sea level, under and east of the camp area (see plate 1). The upper part of the sill has been explored to the south by the Harry workings and to the west and northwest by the Velasco and North Randol workings. A projection of the contact as explored in these workings indicates that between them there is a northwest- trending arch, with its apex a little east of the main camp area (see plate 1). Similar structures in other parts of the mine have contained ore bodies. That ore-forming solu- tions have penetrated the area is indicated by the follow- ing facts: (1) A small amount of cinnabar occurs in a dolomite vein at the base of the Camp shaft, although most similar veins in the mine are barren. (2) The com- pany surveyors reported finding cinnabar and low-grade ore in the northern part of the Harry 800 level. (3) A little cinnabar occurs in chert at the apex of Church Hill, several hundred feet east of the crest of the arch. The structure could most easily be explored by a series of nearly vertical holes, from 250 to 600 feet deep, put down from the surface between the camp and the crest of Church Hill. Structure B. Structure B is also an arch in the upper surface of a serpentine intrusive, at altitudes between 1,100 and 1,350 feet, beneath the saddle northwest of Cemetery Hill (see plate 1). The serpentine surface has been extensively explored to the north and east by the Harry-Cora Blanca workings and to the northwest by the San Francisco workings. Although the margin of the serpentine is not altered to silica-carbonate rock in out- crops between these two groups of workings, all of the nearest underground workings are in silica-carbonate rock, which contains small, local ore bodies. The shape of the surface of the serpentine between these two places cannot be accurately predicted from the available data, but, as is shown by the structure contours in plate 1, the 53743 11-51 2M intrusive surface is believed to contain a structural nose i plunging to the southeast. As similar structures localized ore bodies elsewhere in the mine, this structure seems worth exploring. It can be tested most easily by drilling nearly vertical holes from the surface to depths of 200 to 400 feet. Structure C. Structure C, as shown on plate 1, is a postulated arch northwest of the Santa Rita shaft and from 200 to 600 feet below the 800-level Day tunnel. Ex- ploration at this place is proposed to test the silica-car- bonate rock along the two contacts labeled I and II in I plate 1. Contact I is at the bottom of the upper sill, and contact II at the top of the lower. Along each of these contacts some ore has been found at and above the Day tunnel level, but neither has been much explored at lower levels. A part of contact I exposed in a drift extending eastward from the bottom of the Santa Rita shaft on the 900 level contains silica-carbonate rock but no ore, al- though the drift was run to search for the downward con- tinuation of ore found on the level above. On the 1400 level this contact is cut 800 feet west of the section, and near the 1800 level of the Santa Rita shaft it is cut 450 feet east of the section. Although no silica-carbonate rock was found in either of these last two places, silica-car- bonate rock containing ore bodies might be found in the unexplored ground between the 900 and 1400 levels. If the lower surface of the upper sill is roughly parallel to its upper surface, it is arched between the 1000 and 1200 levels. This structure could easily be explored by drilling holes bearing N. 15° — 35° W. from the Day tunnel near its intersection with the Santa Rita shaft. As the position of the arch is uncertain, the inclination of exploratory holes should be governed by what is found in the first hole,, which should be inclined downward not less than 35°. Structure D. Structure D is a postulated broad arch in the lower contact of a serpentine sill (contact III, plate 1) in the central part of the mine, a short distance below the Day tunnel. The contact has not been reached at any point near the line of section, but its position and attitude are inferred from the arch of silica-carbonate rock that is penetrated for more than 600 feet by the Day tunnel. Elsewhere in the New Almaden district silica- carbonate rock is found only along the margins of serpen- tine sills, and as this silica-carbonate rock is overlain by serpentine it presumably is on the lower side of the ser- pentine body and is underlain by rocks of the Franciscan j group. Exploration on the 900, 1000, and 1100 levels a little farther to the south revealed Franciscan rocks be- neath the contact (see plate 1). The favorable part of the structure involving this con- tact is the crest of a dome that is inferred to lie east of' the old mule barn cutout in the Day tunnel. Indications of the probable existence of ore in this part of the mine are: (1) the Day tunnel follows several northeast-trend- ing veins similar to those that generally accompany ore: bodies; (2) the highly altered silica-carbonate rock is similar to the rock found along the margins of the stoped- out ore bodies; and (3) when the Day tunnel was driven, cinnabar fines were seen in a spring near the mule barn. A raise that was put up here in search of ore penetrated barren serpentine, but no winze was ever put down to the contact below. The best way to explore this structure is to drill several holes downward and eastward from a point near the mule barn. Any ore that is found can readily be taken out through the Day tunnel. ttrhlltd in r/UIFORNtA STATE P1UNTING OFFICE