DEPARTMENT OI? THE INTERIOR

BULLETIN

UNITED STATES

GEOLOGICAL SURVE17

No. 178

WASHINGTON

GOVERNMENT PRINTING OFFICE 

1901

UNITED STATES GEOLOGICAL SURVEY

CHARLES g). WALCOTT, DIRECTOR

THE

EL PASO TIN DEPOSITS

BY

WAJLTER HARVEY WEED

WASHINGTON

GOVERNMENT PRINTING OFFICE

1901

Page.

Letter of transmittal.....'..................................-.............. 9

Location of deposits....................................................... 11

Geological structure and formations. ........................................ 11

Ores and veins ........................................................... 12

Development............................................................. 14

Continuance of veins in depth........................ . .................... 15

ILLUSTRATIONS.

Page. 

PLATE .1. Map of a portion of the El Paso quadrangle, showing the Franklin

Mountains, on the east flank of which the tin deposits lie ....... 1 11

FIG. 1. Cross-section of Franklin Mountains 10 miles north of El Paso, Texas. 12

2. Section of tin vein exposed in open cut (north vein)................. 13

3. Section of north vein (6 feet across)................................ ' 14

4. Greisen bowlder in tin vein (shaft on middle vein).................. 14

LETTER OF TRANSMITTAL.

DEPARTMENT OF THE INTERIOR,

UNITED STATES GEOLOGICAL SURVEY,

Washington, D. C., March 19,1901,

SIR: In December, 1900, specimens of tin ore were submitted to the 

Survey for examination. They proved to contain abundant cassiterite 

and wolframite in a quartz gangue. The}7" were said to come from the 

vicinity of El Paso, Texas, and in view of the infrequency of wellauthenticated occurrences of tin ore in the United States it was judged 

wise to have their provenance and manner of occurrence verified at 

tlje earliest opportunity. This opportunity presented itself when, in 

January, 1901, Mr. W. H. Weed had occasion to pass through El Paso, 

and he was instructed to make a reconnaissance examination of the 

locality whence the specimens were said to come. The result of his 

examination is presented briefly in the following .report, the immediate 

publication of which I recommend.

S. F. EMMONS, 

Geologist in Charge of Section of Metalliferous Ores.

Hon. CHARLES D. WALCOTT,

Director of United States Geological Survey.

U. S GEOLOGICAL SURVEY BULLETIN No.178 PL. I

PORTION OF THE El. PASO QUADRANGLE

Contour Interval 50 feet
THE EL PASO TIN DEPOSITS.

By W. H. WEED.

LOCATION OF DEPOSITS.

The El Paso tin deposits lie on the east flank of the Franklin Mountains, the southern extension of the Organ or San Andreas Range, 

about 10 miles north of the city of El Paso. The ores were discovered in 1899 and have been prospected b}^ several open cuts and pits, 

the deepest of which is about 50 feet below the surface. The propert}7 

belongs to Judge C. R. Moorhead, of El Paso, to whom I am indebted 

for many courtesies during my visit to the deposits. The place is distant about 14 miles by wagon road from El Paso, 12 miles of excellent 

road across the flat mesa being succeeded by 2 'miles across the foothills. The White Oaks Railroad crosses the flat 3 or 4: miles east 

of the property, and the main line of the Southern Pacific lies 10 

miles' to the south. There is a good spring one-fourth of a mile from 

the ledges, but there is no large supply of water nearer than the Rio 

Grande. The mesa is underlain by water, the city of El Paso being 

supplied from driven wells sunk in the mesa gravels. The mesa is 

scantity grassed and covered with the usual desert vegetation of small 

3T ucca and cactus, while the mountain slopes show cedar bushes, with 

mesquite, yucca, sotol, and other arid-land plants. The mountains 

show a very regular crest of bedded rocks surmounting smoother 

basal slopes of a prevailing red-brown color dotted by green sotol 

bushes.

GEOLOGICAL STRUCTURE AND FORMATIONS.

The geological structure is very simple and is easily made out, as the 

mountains are not wooded, but show outcropping edges of the upturned 

limestones and bare slopes of red granite. The mountain range consists of Cambrian and other Paleozoic limestones, upturned by and 

resting upon an intrusive mass of coarse-grained granite that forms 

the central core of the range. This granite is well exposed for a distance of 4 or 5 miles along the eastern side of the mountains, forming 

the lower half of the mountains proper, and in places extending out to 

the foothills. The crest of the range consists of steeply tilted, heavity 

bedded, dark-grajr limestones dipping westward. The basal quartzites

11
12 THE EL PASO TIN DEPOSITS. [BULL. 178.

were observed in the drift seen in arroyos, so that the granite is 

probably intruded between the base of the Cambrian rocks and the 

underlying Archean complex.

The eastern foothills consist mainly of limestones, but near the tin 

deposits these bedded rocks have been cut through and granite now 

forms the surface, remnants of the limestone cover showing as isolated 

masses capping the hillocks. These relations are shown in the diagram, 

fig. 1, which is a rough sketch of the range, representing a cross section at the tin mines. North of the place where this section was made 

a transverse ridge of the range shows the granite to be sheeted b)7 wellmarked planes, dipping eastward at an angle of about 45° to 50°. The 

granite is very much altered by surface decomposition, and crumbles 

readily to a coarse sand. No fresh material was observed anywhere on 

the surface, but fairly good material was obtained from the dump heap 

of the shaft on the north vein. The granite is sheeted near the veins, 

the planes 'of sheeting being parallel to the veins themselves. The 

general sheeting, however, is in a different direction, the average strike

FIG. 1. Cross section of Franklin Mountains 10 miles north of El Paso, Texas.

being N. 20° E, and the dip 70° SE. A thin section of this granite 

has been examined under the microscope by Mr. Lindgren, who furnishes the following notes:

The rock is a coarse-grained normal granite. It shows much anhedral quai-tz with 

anhedral feldspar, largely rnicroperthite, with some few grains of microcline. A few 

small flakes of brownish-green hornblende and some small grains of magnetite were 

also seen. The rock is a soda granite.

White aplite-granite occurs in veinlets and irregular masses intrusive in the granite, but none was observed close to the veins. The 

mesa is underlain bjT cemented gravels, which, form also the lower 

slopes of the foothills.

ORES AND-VEINS.

The ores consist of cassiterite, or oxide of tin, with wolframite 

(tungstate of iron and manganese) in a gangue of quartz. Specimens 

of nearly pure cassiterite weighing several pounds have been found on 

the surface, and this mineral occurs in the quartz, either alone or associated with wolframite. The most abundant ore is a granular mixture
WEED.] ORES AND VEINS. 13

of tin ore and'quartz which resembles a coarse granite and corresponds to the greisen ore of European tin deposits. Pyrite occurs 

rarely in the eastern exposures of the vein, but appears to constitute 

the bulk of the metallic contents in exposures seen in the westernmost 

openings. These ores occur in well-defined veins, which run up the 

slopes nearly at right angles to the direction of the range, the strike 

being approximately.east-west and the veins dipping steeply to the 

north. Three veins have been discovered, all of which have been 

exposed by open-cut work and by pits for several hundred feet in 

length. The most northerly vein is traceable along the surface for a 

distance of about 1,200 feet. The middle vein lies about 300 feet south 

of the east end of the northern one, but apparently converges westwafd toward the northern vein. The southern vein, which is the 

smallest of the three, lies about 600 feet farther south. 

The veins exhibit the usual characters of the European tin veins,

notably those of Cornwall, England, their clearly denned fissures

showing a central core or lead of coarse 

quartz, sometimes containing tin ore, and 

flanked on either side by altered rock in 

which the tin ore replaces the feldspar of 

the granite. Where this metasoinatic replacement is complete the ore shows a 

mixture of cassiterite, with or without 

wolframite and quartz. Where the replacement is only partial the greisen ore 

fades off into the unaltered granite. A 

cross section of the veins shows, therefore, the same phenomena seen at Cornwall. The diagram, fig. 2, shows an ideal 

representation of the conditions existing 

in the veins, and has been drawn from sketches made in the field. 

The central mass of quartz corresponds to the "leader" of the Cornish 

veins. It is composed of massive, coarsely crystalline quartz, sometimes showing comb structure, and it is clearly the result of the filling 

of the open fissure by quartz. The adjacent ore-bearing material is 

a replacement deposit in which the mineral solutions have substituted 

ore for the feldspar of the granite by metasoinatic action; in other 

words, the main mass of the ore occurs alongside of a quartz vein, and 

is due to the alteration of the granite forming the Avails of the fissure. 

In general, the ore passes into the granite by insensible transition and 

there are no distinct walls.

A thin section of the greisen ore has been examined by Mr. Lincl. 

gren, who furnishes the following notes:

The thin section of the tin ore shows it to be a quartz-cassiterile rock. It is a 

coarsely granular rock consisting of anhedral quart/, with which is intergrown 

grains of slightly brownish cassiterite. The quartz is full of fluid inclusions and makes

FIG. 2. Section of tin vein exposed 

in open cut (north vein).
14 THE EL PASO TIN DEPOSITS. [BULL. 178.

FIG. 3. Section of north vein (6 feet across).

up about 75 per cent of the mass. The cassiterite grains are, along the edges, intimately intergrown with quartz. If this is a metasomatic form of the granite a silicification has taken place. The microscope affords no direct evidence, however, that 

this ore is metasomatic. One small grain of tourmaline and a few flakes of sericite 

were seen. Neither topaz nor mica occurs in the section, and no remains of feldspar 

were observed.

The north vein has a course of N. 85£° W. magnetic, as determined 

from the openings at the east end. At the west end of the workings 

the course observed, looking back along the outcrop, appears to be 

N. 80° E. for the northern vein and N. 80° W. for the middle vein; 

so that if these observations are correct the veins must intersect toward

the west. The surveys by the 

owners of the property show 

a course N. 85i° W. for the 

middle and 65° W. for the 

south vein.

DEVELOPMENT.

A shaft 35 feet deep has been 

sunk on the north vein at the 

eastern end of the vein outcrop. This shaft is about 5 by 

10 feet across and shows a very 

well-defined vein about 5 feet wide, having a dip of 

about 70° to the north. The sides of the shaft show 

excellent ore, mostly of the greisen variety, extending 

down for 8 to 15 feet below the top. At this point a 

slip crosses the shaft and cuts out the ore. This slip, 

or fault, is a clay seam but one-fourth to one-half inch 

in thickness, and seems to have thrown the upper part

of the vein to the north. The lower half of the shaft 

reveals only rusty granite, scattered and showing 

films of quartz, but without recognizable ore. A 

crosscut south from the bottom of the shaft should 

reach the vein if the fault is a normal one. In the 

exposure seen in the upper part of the shaft the ore 

occurs in bunches in" altered granite and lies on the 

north side of a 15-inch streak of sheeted and rusty quartz. A 

second shaft on the north vein has been sunk at a point about 300 

feet west of the one just noted. This shaft is about 25 feet deep. 

The vein is well exposed at tlje top, and shows a dip northward, but 

the shaft passes out of the vein into the sheeted granite, forming the 

foot wall. A crosscut about 8 feet in length, driven from the bottom 

of the shaft, cuts the vein, but does not pass through it. The sheeting of the granite seen in this shaft is very pronounced, the rock being 

divided into plates from one-fourth inch to 12 inches in thickness

FIG. 4. Greisen bowlder in tin vein (shaft 

on middle vein).
WEED.] CONTINUANCE OF VEINS IN DEPTH. 15

by planes dipping 61° E. and crossing the vein at 90°. The outcrop of the vein is traceable westward up the slopes by its rusty quartz, 

and a nearly continuous ledge can be followed. This outcrop has been 

opened at intervals of a few yards by trenches, which expose the vein 

and show it to have a thickness of from 2 to 6 feet, with about half 

this thickness of ore. No samples were, however, taken, and it is uncertain whether the altered granite does not contain a percentage of tin 

oxide. The most westerly working that could be surely identified as 

being upon the north vein is a pit 6 feet deep, which shows a 6-foot 

vein in which the quartz is bluish in color and the tin ore is associated 

with much pyrite. This point is about 600 or more feet west of the 

first shaft. West of this point the ledge can not be traced across the 

slopes, but an opening north a hundred feet higher and a few hundred 

feet farther west shows a good vein, carrying much pyrite, but devoid

of any recognizable tin ore.

The middle vein is developed by a shaft 50 feet deep, which shows a 

vein having a central leader of quartz 2 feet wide at the top and tapering to 1 foot 4 inches wide at the bottom of the shaft. The dip, as 

shown by the walls of the shaft, is 70° N. The central quartz mass is 

spotted with cassiterite, and the altered granite on either side contains 

recognizable grains of tin oxide.

The south vein lies 500 to 600 feet south of the middle vein. This 

vein is much narrower than the veins on the north, having an average 

width of about 1 foot. The strike, as shown near the shaft, is N. 50° 

W. and the dip 50° N. The vein walls are sometimes defined by a clay 

selvage one-sixteenth inch wide, but more often show a gradual fading 

off into the granite.

CONTINUANCE OF VEINS IN DEPTH.

It will be noticed from what has been said that the veins are all well 

defined at the surface and cany good values in tin ore, but that the 

ore apparently dies out in depth. Further development is needed to 

establish the existence of the ore at a greater depth than 50 feet, but 

it is believed that the veins have been thrown by local slips or faults 

and will be found by crosscutting from the bottom of the present 

workings. The character of the fissures and the nature of the ore 

both indicate that the veins are the result of deep-seated agencies, and 

are not merely segregations due to descending surface waters. For 

this reason it is believed that further exploration will develop welldefined tin veins. The absence of topaz in the deposit is noteworthy, 

for this mineral is commonly associated with cassiterite veins the world 

over. In other respects the deposits closely resemble the tin veins of 

Europe, and are clearly due to metasomatic processes. The evidence 

of a pneumatolitic origin is, however, not conclusive.

O