14

0 ff cos 6 dx dy dz

P
"

ay
a, zctn¢é + x, cy -3/2
-y J zaz | ax [G+ 42) ay
zetn® + x, -00
_ 1d ctnd+ % “1 d, ctnd +%

ve { oe @+% singcos¢)tanTM -(d.+ x singcos¢)tan

4 4,

- (q+ x, sin8cos@)tanTM a + (a+ x, sin@cos6)tanTM Senet +% |

1

+ rein? g in Bt Get BY Ly stare tn SU etn + AY

d,? + (d,ctng + x, )? d,? + (d,ctn@ + x 7

The gravitational constant is y, the density is p, cos 9 = 2/r and

r =x + y? +22. The particular form of A, is readily obtained when the

relation tan~? a = 4 in Lia

— is introduced prior to the last integration and
2 l+ia

when the same relation in
Geometry of the vertical gravity (Az) due

toon infinite horizontal prism of
trapezoidal cross section

the form In (b + ia) -5 In
(b? + a?) + itanTM? . is used

following integration.

Xe This equation was pro-
gramed on a computer, and

iy the curves of figures 14 and
15 represent the theoretical
gravity over the several
mass distributions. Because
the rocks along the line
were at or near the surface,
the depth to the top of the
rocks (>) was always taken
as 1 foot.

Az

Figure 14 is a series
of curves for a 70°-dipping
zone of pyroxenite bounded
on one side by monzonite and
on the other side by gabbro.
Since the relative gravity
is all that is desired, the
FIGURE 13. - Geometry of Theoretical Gravity. density contrast of



wy
Monzonite

3405

(

1550 \ \)
Gabbro
1400 Monzonite
1300
yn \)
“
‘as°
\)
Gabbro 1 Gabbro
Pyroxenite
490
!
Gabbro \
'
3 : F : gis : : poe
E Gabbro
\ i
<n /
(Sonzonire
AN . 400
360 9
Gabbro
An
Gabbre
LEGEND
i Gobbro—_____"——
Monzonite ———— (MM
Pyroxenite ———— xo)
bo 0 a Titaniferous mognetite— a>
1550 yo)
1é00 100 1600 Andesite An
Bosalt Ba
Diorite ——————_i
Quartz monzonite QM
180
0 An 1800 Contact at titude-————4_
Za Attitude of gneissic structure—<
i Foult _—-—
2600 Di °
2200 C) 200 400 600
SCALE, FEET

2600 FIGURE 16. - Bedrock Geologic Map, Iron Mountain Titaniferous Magnetite Deposit, Fremont County, Colo.







fo)

_
Iron
Mountain

E18 120

— Survey lines with
‘geophysical stations
== Road

1090 _2000 4000

‘SCALE, FEET
Elevations in feet

ian

FIGURE 2. - Grid Location Map, Iron Mountain Titaniferous Magnetite Deposit,
Fremont County, Colo.

using the field from the calibrating coil to cancel the earth's field at each
station. The magnetic intensity was calculated from the milliamperes of coil
current necessary to zero the magnetometer, the direction of the current in
the coil, and the coil constant in gammas per milliampere. The usual proce-
dure is to record the magnetic intensity directly. The method used was ideal
for the area covered; areas of extreme magnetic relief could be traversed
rapidly without losing accuracy in the readings over areas of low magnetic
relief.

The operation required two men. Between stations the magnetometer oper-
ator carried the magnetometer, tripod, and a belt-attached potentiometer case;
the notekeeper carried the calibration coil and a case containing the milli-
ammeter, battery, and leadwires. At the station the magnetometer operator
leveled the tripod plate and oriented it with a compass. The magnetometer was
locked on the oriented plate and leveled. The calibrating coil, now connected
to the milliammeter by a plugged-in leadwire, was placed over the magnetometer.
The magnetometer operator plugged the leadwire into the potentiometer case to
complete the circuit. As the magnetometer balance was released, the


18

Geology

The map of the bedrock geology is largely a combination of the outcrop
map and the magnetic-contour map (fig. 16). The cross section along the 2,000
foot-south line is largely a blend of the surface geology and the gravity data
(fig. 17). The wedgelike broadening of the strip of pyroxenite with depth
accompanied by little or no increase in titaniferous magnetite is considered
to be a reasonable interpretation of the field data. The iron deposits appear
to be somewhat limited and discontinuous.

“alluvium Titoaiferous magnetite T ]
via . (somewhat generalized) ova
\ AN 2222 pote =

7400

7000

g

ALTITUDE, feet

NY

‘5800

10 6 2 2 6 10 14 16 18 22 East
DISTANGE, hundred feet

FIGURE 17. - Geologic Interpretation of 2,000-Foot South Cross Section.

INT, ©BU,OF MINES, PGH. PA, 3096




CONTENTS
Page

Introduction and summary...scsssecsessscccsccvceccaceccens 1
Acknowledgments ...cccsccccccecccccccecccseececesnsvcseeens 1
Location and accessibility... ..ccccccceccceccreserveseees 2
Physical features and climate.....ccccceccccccececeeceeeee 3
History and production.....cecceeccsccccceccesescverescees 3
Description Of depOSit..cccsscccsccccsscccccsecceseseeeses 4
B. (o) oo) 1b o_o
PYLOKENLCS. cere eeceececeresececcseesccssescsesesseses 5
GabDLO. cece eee e creer eee eee e eee eseeeeeeesseseseeens 5
Titaniferous magnetite... .cccssccecccccersevcecesveces 6
DIKES cece cee cece cence cece eeesccesececacesasssssesees 6
Work by the Bureau of MineS....ccesecccccececececeseeceeee 6
Survey control and geologic mapping.......eeeeseeeees 7
MagneticS..ccccsccccecvccccccceccsseesceeresesseences 7
(eo
Interpretation... .ccccccccccccccceccecceccecessesesececees 12
MagneticS...ecsecccccerccccccsevcescecceseecesesesees 12
GrAVLEY. cee cee eo cece cence cece seererescescesseeceses 13

Geology. cecccccccccccccccsvcscceccsseecsesecsssseses 18


GRAVITY, milligals

GRAVITY, milligals

Iaan

Yen tN

ToT TT

Gravity profile
3200 - south line

Reduction density =2.8

_ fPrerrain corrections aw

too high due to

low-density scarp XJ

- MN
7 “

Observation in
draw near bedrock

Gravity profile
4000 - south line

Reduction density =2.8

a
to

al
hb

a)
o

wt
©

NS

-3.0
West 20

18

16

14

2 10 86 6 4 2 0 2 4 6 8
DISTANCE, hundred feet

10 12 14 16 18 20 22 24 26 East

FIGURE 11. - Gravity Profiles—3,200- and 4,000-Foot South Lines.



4 500 feet
Computed curves: =~ >| [eae rY
Field curve, 2000 S.line,

(relative gravity)

ep
[yest
eer .
&
z, / \
> ~
£ / d= 2000 feet ~.
a a
, Po
ra -

t—__| 42=1000 feet

—
° 6 r
t 70 30r]
3 s
8 Monzonite Pyroxenite Gabbro
v4
ee Density Scan oan aug,
8 Density dotum contrast jensity contrast = 0.4 g./ec.
Q + 0.8.9./cc)
West 363086 82 18 @ é 01818 2226 ost

10 6 2 0 2
DISTANCE, hundred feet

FIGURE 14. - Computed and Field Curve Gravity Comparison, Part I.


2.8

2.6

2.4

2.2

2.0)

Magnetite
outcrops

{ Gravity profile
2200 - south line

v4 Reduction density =2.8

GRAVITY, milligals

[ee ee

2.6

2.4

2.2

2.0

Hl + Gravity profile
f Magnetite outcrops \ 2400 ~ south line

{ / \ Reduction density=2.8

Base check
discrepancy ?

GRAVITY, milligals

A

/ \\

Base check 4
discrepancy?

a

ES

\ Gravity profile
2800 - south line

c
is)

GRAVITY, milligals
6

/ \, Reduction density =2.8

/

fo)
West20 18 16 14 12 10 8 6 4 2 0 2 4 6 8B 10 12 14 16 18 20 22 24 26 East

DISTANCE, hundred feet
FIGURE 10. - Gravity Profiles—2,200-, 2,400-, and 2,800-Foot South Lines.


Computed curves:

Field curve, 2000 S.line,
(relative gravity) “~~~ %2= 2005

——~ i

S.

GRAVITY, milligals

\_\\

° be °20'10" be
ae 70° 34° 20'10' 90°]
J oo
2
°
S
‘w
73 ; ;
Monzonite Pyroxenite Gabbro
°
oy Vd
8
or Density datum Density contrast = 0.8 g./cc. Density contrast =0.4 g./cc.
3
West 34 30 26 22 8 14 10 6 20 2 6 10 14 18 22 26 East

DISTANCE, hundred feet

FIGURE 15. - Computed and Field Curve Gravity Comparison, Part Il.

lens in gabbro

Anomaly due toa
o A titaniferous magnetite

17

Density contrast =1.6 g./cc.




12

TABLE 3. - Specific gravity determinations

Titaniferous
Measurement Monzonite | Gabbro | Pyroxenite | magnetite
1.. 2.59 3.05 3.33 4.33
2. 2.64 3.13 3.62 4.90
Becccececeee 2.67 3.13 3.41 4.39
Aeseseee 2.56 3.31 3.19 4.63
2.59 2.99 - -
- 2.95 - -
- 2.92 - -
- 3.05 - -
- 2.93 - -
Mean. cecseeceescccsoveeees 2.61 3.05 3.39 4.48
Standard deviation.........++++++ +039 118 +156 +265

INTERPRETATION

Magnetics

Quantitative calculations of magnetic anomalies over highly magnetic
materials are not necessarily accurate because the intensity of magnetization
(I) may not be proportional to the magnetic susceptibility (k). This fact
limits the utility of these calculations as an interpretive tool for deposits
composed largely of magnetite. In addition, at the deposit being considered,
there is considerable titaniferous magnetite float near the magnetite lenses.
This float tends to obscure and perhaps broaden the anomaly caused by magnet-
ite in place. The magnetic interpretation is, therefore, largely qualitative.

Anomaly calculations (if carried out) would involve (1) making a volume
integration because the reduction to a surface integral is not necessarily
valid for highly magnetic material or (2) taking the derivative of the
vertical-gravity field in the direction of the intensity of magnetization (I).

The average and the standard deviation of the susceptibilities for the
major rock types are given in table 2. The monzonite is much more uniform
magnetically than the gabbro-related rocks. For instance, no magnetic con-
tours cross the west end of the zero-south line which is over monzonite. The
gabbro is also fairly uniform magnetically on the north and northeast side of
Iron Wash. This uniformity is best illustrated by the lack of magnetic con-
tours on the 400-north line which is adjacent to gabbro outcrops. Gabbro in
the rest of the area and the rocks closely related to the gabbro, the pyroxen-
ite, and titaniferous magnetite are quite variable magnetically.

The magnetic contour map reveals the small size and discontinuous nature
of the more highly magnetic zones (the lineated and crosshatched areas). Three
of these areas probably represent single lenses of titaniferous magnetite;
they are labeled A,?° B, and C on the magnetic contour map. Areas D and E
each probably represent several lenses. These five areas constitute the major
magnetic anomalies covered by the survey.

10These areas are not to be confused with Hammer zones given on page 10.


The Colorado Fuel and Iron Co. sold its Iron Mountain holdings in 1945.
Subsequently, several transfers of the property occurred. In 1947, the owner-
ship passed to A. C. and H. L. Ellison of Hillside, Colo. In 1959, the deposit
was owned by six members of the Ellison family.

The Fremont Construction Co. and its successor, the Florence Aggregate

Co., produced magnetite aggregate in 1955 and 1956. These companies operated
a screening and crushing plant capable of producing 150 tons per day of minus
%-inch aggregate. The crushed product was trucked to Texas Creek, Colo., and
shipped by rail to Texas for use as a heavy aggregate in the concrete coating
for pipelines to be sunk underwater. An aggregate having a minimum specific
gravity of 4.25 was required for this use. There was no production from the
deposit during 1957-60.

DESCRIPTION OF DEPOSIT

Four principal Precambrian rock types are in the immediate area of the
deposit. A large mass of monzonite occurs along the western edge of the
mapped area. A strip of pyroxenite, as much as 600 feet wide, commonly
borders the monzonite and grades into gabbro, which covers most of the remain-
ing area. Within the gabbro are lenses of magnetite, 5 to 25 feet wide and as
much as several hundred feet long. In addition, dikes of intermediate compo-
sition transect the gabbro, pyroxenite, magnetite lenses, and possibly the
monzonite.

The lenses of titaniferous magnetite are restricted to several discon-
tinuous, subparallel zones in the gabbro which trend N. 25° E. to N. 15° W.
Most of the lenses have steep westerly dips. Scattered occurrences of magnet-
ite have been mapped for 2,200 feet along the strike of these zones. Near the
outcrops the overburden contains much magnetite float.

The contact of the magnetite with the gabbro is normally sharp or grada-
tional over several inches; the percentage of plagioclase decreases and iron
content increases from the gabbro toward the magnetite. Along the south wall
of the main pit, the transition zone includes several alternating layers of
magnetite and gabbro that concordantly underlie the main magnetite lens. This
layering of magnetite and gabbro was also observed elsewhere. In addition to
the normal contact between magnetite and gabbro, three fault or shear contacts
were observed in two pits. Away from the pits the gabbro-magnetite contact is
commonly obscured by overburden.

The transition from gabbro to pyroxenite is gradational but may extend
several hundred feet; the percentage of ferromagnesian minerals gradually
increases towards the pyroxenite. The only outcrop showing the contact
between gabbro and monzonite is in the southwest part of the mapped area. The
contact is a shear zone with fragments of gabbro and titaniferous magnetite
included in the monzonite. In other places the pyroxenite facies of the
gabbroic rock adjoins the monzonite. Although the contact is obscured by
float, it may be traced easily for 2,000 feet by the sharp line of demarcation
between the dark-colored pyroxenite and light-colored monzonite float.


GRAVITY, milligals

GRAVITY, milligals

GRAVITY, milligals

2.4

2.2 \
2.0 } i
pat,
8 Magnetite Y.
outcrop

4 in
: \ Gravity profile
2 O - south line _|

Reduction density=2.8

N
Para

oS)

iS)

co}

jo
|

N |
\ Gravity profile

400 - south line
Reduction density = 2.8

Pot fj f ff ts

@

Magnetite
outcrops

=
Hl

o

b

re)
°
b>

- f i"
20 \
:

1.4 ,-— _| \
he | \ [Gravity profile “|
1.0 800 - south line _|
8 \ Reduction density =2.8 _|
6 /

4

/ \
:
iN

2
West20 18 16 14 12 10 8 6 4 2 0 2 4 6 8 10 (2 14 16 18 20 22 24 26 East
DISTANCE, hundred feet

FIGURE 7. - Gravity Profiles—0-, 400-, and 800-Foot South Lines.





IRON MOUNTAIN TITANIFEROUS MAGNETITE DEPOSIT,
FREMONT COUNTY, COLO.'

by

R. M. Becker,? S. S. Shannon, Jr.,? and C. K. Rose4

INTRODUCTION AND SUMMARY

As part of the Bureau of Mines program for examining the Nation's iron
and titanium resources, magnetic, gravity, and geologic surveys were made to
permit an estimate of the extent and magnitude of the Iron Mountain, Colorado,
titaniferous magnetite deposit.

Gabbro is the dominant rock type of the area. Within the gabbro are
lenses of titaniferous magnetite, 5 to 25 feet wide and as much as several
hundred feet long. These lenses comprise a zone which has scattered occur-
rences of magnetite for 2,200 feet. To the west, the gabbro grades into a
strip of pyroxenite that in turn borders a large monzonite mass.

A wedgelike broadening of the pyroxenite with depth accompanied by little
or no increase in the titaniferous magnetite is considered a reasonable inter-
pretation of the field data. The iron deposits appear to be limited and
discontinuous.

ACKNOWLEDGMENTS

This investigation was made under a cooperative fellowship agreement,
dated July 1, 1956, between the Bureau of Mines and the Colorado School of
Mines and was the subject of a master's thesis in geophysics. The School of
Mines provided the gravity meter and the magnetometer used in this survey, and
Prof. Paul Rodgers of the geophysics department offered much valued advice and
assistance.

The Bear Creek Mining Company furnished their field susceptibility meter.
The Burroughs Corporation installed a demonstration computer at the Denver

1Work on manuscript completed April 1960.

2Mining methods research engineer, Denver Mining Research Center, Bureau of
Mines, Denver, Colo.

8Mining engineer, Division of Mineral Resources, Bureau of Mines, Denver,
Colo.

4Supervising mining engineer (deceased), Division of Mineral Resources, Bureau
of Mines, Denver, Colo.


ii

11.
12.

13.
14,

15.

16.

17.

ILLUSTRATIONS

Location map, Iron Mountain titaniferous magnetite
deposit, Fremont County, Colo......scccccceeseeees

Grid location map, Iron Mountain titaniferous
magnetite deposit, Fremont County, Colo........e0e

Topographic map of grid area, Iron Mountain titani-
ferous magnetite deposit, Fremont County, Colo....

QOutcrop map, Iron Mountain titaniferous magnetite
deposit, Fremont County, Colo.....eseeeeeeeceeeere

Magnetic-contour map, Iron Mountain titaniferous
magnetite deposit, Fremont County, Colo...........

Gravity profiles over gabbro using several reduction
densities. ccrsecsecececcreceessceseencesasceseeees

Gravity profiles--0-, 400-, and 800-foot south lines

Gravity profiles--1,200-, 1,400-, 1,600-, and 1,800-
foot south Lines... ccessee cece eee eeeeseeeeeeeees

Gravity profile--2,000-foot south line..............

Gravity profiles--2,200-, 2,400-, and 2,800-foot
south lines.....ccesseeeeeeeceeeeee

Gravity profiles--3,200- and 4,000-foot south lines.

Gravity-contour map, Iron Mountain titaniferous
magnetite deposit, Fremont County, Colo...........

Geometry of theoretical gravity...

Computed and field curve relative gravity compari-
SON, PALt Levesecceccccsceccccevescsvevcsvsvesvere

Computed and field curve relative gravity compari-
son, part II..

Bed-rock geologic map, Iron Mountain titaniferous
magnetite deposit, Fremont County, Colo.........6+

Geologic interpretation of 2,000-foot south cross
section........

TABLES

Partial analyses and location of samples..........+.

Apparent susceptibility measurements, K = 10-¢
C.G.S. UNIS. cc ccceccececesesereeeenvsevvsvervnres

Specific gravity determinations... ...cececeeeseeeeee

Page

Pocket

Pocket

Pocket

11
Pocket

Pocket
Pocket.

Pocket

Pocket

Pocket
14

15

7

Pocket

10
12


13

Area A is well substantiated by outcrops. This appears to be one contin-
uous tabular lens of titaniferous magnetite, 20 to 25 feet wide and about 800
feet long, with a dip of about 70° to the northwest. The flatter dip of the
intensity-of-magnetization vector (normal for the area) accounts for the
observed southwest shift of the magnetic high off the line of magnetite
outcrops.

Several outcrops within areas B and C suggest that each of these areas
also represents a single, narrow lens of titaniferous magnetite. Within areas
D and E there are some exposures of both magnetite and gabbro; however, the
considerable amount of magnetite in the surface sands and gravels tends to
mask the occurrences and make the interpretation more difficult.

If the width of the magnetite structures increased considerably and they
perhaps merged with depth, a broad magnetic high might be expected from east
to west across the grid. Superimposed on this broad high would be the local
observed anomalies due to the near-surface material. No broad high seems to
be present across the map; instead, the anomalies are quite localized. This
suggests little or no increase in the size of the magnetite structures with
depth.

Gravity

Quantitative calculations are extremely useful in interpreting gravity
data. When combined with known and probable geological conditions, the possi-
ble subsurface interpretations often may be narrowed to several choices. In
the present case the gravity anomaly is rather large, elongated along the base
line, and high near the magnetite lenses. Along the 2,000-foot south line the
anomaly exceeds 8.5 milligals, and the peak of the anomaly is 1,400 feet east
of the monzonite-pyroxenite contact. The question is: Can the anomaly be
explained reasonably by differences in rock type along, or must there be con-
siderably more titaniferous magnetite at depth to explain both the magnitude
and shape of the anomaly?

The gravity profile along the 200-foot south line was selected as the
curve to try to match with the gravity profile of a reasonable subsurface
distribution of monzonite, pyroxenite, and gabbro. The surface geology was
fairly well determined by field mapping. The 2,000-foot line extension to
the west off the main grid area was entirely over monzonite. Where exposed,
the strip of pyroxenite adjacent to the monzonite ranges from 300 to 600 feet
in width. The contact between the pyroxenite and monzonite dips about 70° to
the east. The width of pyroxenite selected for computation was 500 feet. The
remainder of the line is almost entirely over gabbro. With the gravity
anomaly elongated normal to the section and the section about midway along the
length of the anomaly, two-dimensional gravity computations are appropriate.
Finally, the densities of monzonite, pyroxenite, and gabbro are taken as 2.6,
3.4, and 3.0 grams per cubic centimeter, respectively.

The vertical component of gravity for a prism of infinite length along
the y-axis and of a trapezoidal cross section may be derived in the usual way.
The geometry is illustrated in figure 13. The vertical component of gravity
(Az) is:


Topographic coverage of the area is provided by an advance print of a
U.S. Geological Survey 73-minute quadrangle (Canon City No. 2 SW.) with a
scale of 1:62,500. The area is also covered by the Forest Service map of the
San Isabel National Forest published in 1948 by the U.S. Department of
Agriculture.

PHYSICAL FEATURES AND CLIMATE

The topography is partly mountainous with relief seldom exceeding 1,200
feet; intermittent streams dissect the area. Pinons grow along the slopes and
hills, and grass sparsely covers the lower areas. Grazing in the lower eleva-
tions is the major land use.

The titaniferous magnetite deposit is in the northwest part of a mile-
long alluvial basin just south of Iron Mountain. The grid area ranges from
7,000 to 7,350 feet in altitude.

Pine Creek, which borders the alluvial basin on the east, has a limited
flow of water during dry periods. Adequate water for a sizable mining opera-
tion could probably be obtained by drilling a shallow well in the alluvial
gravel of Pine Creek near its intersection with Iron Wash. Additional water
could be obtained from Grape Creek, approximately 4% miles southeast of the
deposit.

The climate of the Iron Mountain area is characterized by moderate
winters and cool summers. The mean annual temperature is 42.9° F. at
Westcliffe, Colo., 15 miles south-southwest of the deposit at an altitude
of 7,860 feet. The average annual precipitation at Westcliffe is 16.65 inches,
most of which occurs in the spring and summer. The mean annual snowfall is
78.5 inches; maximum depths accumulate in March and April. The precipitation
at Westcliffe probably exceeds that at Iron Mountain because of the proximity
of Westcliffe to the Sangre de Cristo Mountains.

HISTORY AND PRODUCTIONS

Claims on the Iron Mountain titaniferous magnetite deposit were patented
in 1872, and small-scale production began in 1873. Ore was hauled by wagon to
Canon City and then by rail to the steel mill at Pueblo. Prior to 1880,
ownership of the deposit passed to the interests controlling the Pueblo mill--
predecessors of The Colorado Fuel and Iron Co. In 1880, a branch line of the
Denver and Rio Grande Western Railroad was constructed from Canon City up
Grape Creek to Silver Cliff. This line, abandoned in 1888, passed within 4%
miles of the Iron Mountain deposit. Plans made to construct a spur to the
deposit were never carried out, probably because of the discovery that the
high titanium content made the ore unsuitable for iron and steel. Total pro-
duction from the deposit prior to 1955 probably did not exceed a few thousand
tons.

SAdapted from Singewald, J. T., The Titaniferous Iron Ores in the United
States: Bureau of Mines Bull. 64, 1913, pp. 128-129.


FIGURE 12. - Gravity-Contour Map, Iron Mountain Titaniferous Magnetite Deposit, Fremont County, Colo.

LEGEND

Contours of relative gravity shown
in milligals

Contour interval = 0.2 milligals

Survey lines with

geophysical stations

oO 200 400 600
————
SCALE, FEET


Al

Leer.

b f mi
pitiere a iereategnlicis 5864

IRON MOUNTAIN TITANIFEROUS
MAGNETITE DEPOSITS, FREMONT
COUNTY, COLO.

By R. M. Becker, S. S. Shannon, Jr.
and C. K. Rose

ae
CALIFORNIA]
INSTITUTE OF
NOV 9 196) “epacer

metadc38690

UNITED STATES DEPARTMENT OF THE INTERIOR

BUREAU OF MINES

1961


Titaniferous Magnetite

Titaniferous magnetite occurs in a series of subparallel, discontinuous
lenses along an arc that strikes from N. 25° E. to N. 15° W. The lenses, as
much as 25 feet thick and over 400 feet long, generally dip steeply to the
west. Scattered outcrops have been mapped along the arc for 2,200 feet.

The massive, dark-gray titaniferous magnetite is marked by laths of
light-gray plagioclase (Ang,). Magnetite, the chief mineral, is accompanied
by a variable amount of plagioclase and is closely associated with ilmenite
and spinel. Other accessory minerals include augite, biotite, and olivine.
Under the microscope, rims of light-brown pleochroic hornblende commonly
separate magnetite from plagioclase. Sericite, limonite, and much of the
hornblende occur as alteration products.

The plagioclase laths in the magnetite often have a preferred orienta~
tion. Where the attitude of this orientation can be determined, it is always
approximately parallel to the attitude of the enclosing magnetite lens. The
longest lens of magnetite was traced by following the strike of this orienta-
tion from outcrop to outcrop.

Dikes

Numerous dikes of diorite, andesite, quartz monzonite, phonolite, and
basalt occur in the mapped area. They generally have northwesterly strikes
and steep dips. They transect the gabbro, pyroxenite, and magnetite lenses
and perhaps to a lesser degree the monzonite; however, little mapping was done
beyond the monzonite contact.

The light-gray diorite dikes have three distinct textures: coarse-,
medium-, and fine-grained. The coarse-grained to pegmatitic diorite approaches
anorthosite in mineral composition. The andesites are characterized by scat-
tered phenocrysts in pale brown, gray, or red ground masses. The quartz mon-
zonites are light brown to reddish orange, and the one phonolite dike is a
medium-gray aphanite. Megascopically, the basalt differs from the phonolite
in having a darker gray color and calcite-filled cavities.

WORK BY THE BUREAU OF MINES

J. T. Singewald of the Bureau of Mines examined the deposit in 1912 dur-
ing a study of the titaniferous iron deposits of the United States.® In
1948-49 the Bureau of Mines conducted a limited dip-needle survey of the
deposit. A 2,100-foot north-south base line was established, and east-west
crosslines, generally less than 500 feet long, were set on 100-foot intervals.
The dip-needle survey consisted of recording observations every 5 feet along
the crosslines within the area of magnetite; it indicated, within reasonable
limits, the outline of most ore occurrences. Six surface samples were taken

®singewald, J. T., The Titaniferous Iron Ores in the United States. Their
Composition and Economic Value: Bureau of Mines Bull. 64, 1913, 145 pp.


The plagioclase laths in the magnetite are often alined or oriented along
a preferred plane. In places it is possible to determine the attitude of this
gneissic structure which always approximately parallels the attitude of the
enclosing magnetite lens. The gneissic structure in the gabbro, caused by the
alinement of ferromagnesian minerals, is somewhat less distinct but is occa-
sionally mappable. Within several hundred feet of the magnetite lenses, the
gneissic structure in the gabbro is also subparallel to the attitude of the
lenses. These orientation features proved helpful in tracing the occurrences
of magnetite.

Monzonite

Monzonite is mainly restricted to a large mass that borders the western
edge of the map area. The weathering of this coarse-grained rock forms a
distinctive buff-colored rubble composed of subhedral crystals of feldspar.
Pinkish-gray orthoclase and light-gray plagioclase are the chief minerals.
Dark-gray to black biotite, hornblende, and magnetite are the principal acces-
sory minerals; sericite and limonite occur as alteration products.

Pyroxenite

Pyroxenite occurs on Iron Mountain and on a line of prominent knobs
extending southwestward from Iron Mountain along the western margin of the
mapped area. Weathering of the pyroxenite produces dark-gray blocks and
rubble along the slopes and dark-gray pyroxenite sand in the washes.

This medium- to coarse-grained rock is marked by a few plagioclase crys-
tals and occasional limonite spots. The principal mineral is augite. Plagio-
clase (Ang, ), biotite, magnetite, and spinel are common accessory minerals;
apatite, hornblende, and olivine are less common. Alteration products include
limonite, sericite, and perhaps some hornblende.

The pyroxenite grades into gabbro. In the field, this transition zone
was mapped as pyroxenite if the mafic minerals appeared predominant and as
gabbro if the plagioclase appeared predominant. The gneissic structure in the
pyroxenite transition zone is mappable in places.

Gabbro

Gabbro is the principal rock cropping out in the mapped area and is the
dominant rock type except along the western margin,

The gabbro is commonly a medium-grained gray phanerite. One specimen,
however, has a fine granular texture. The essential minerals are augite and
plagioclase (An, - An,,)+ Magnetite, olivine, and biotite are the principal
accessory minerals. Hornblende and spinel are less common. Sericite and
limonite occur as secondary minerals.

The alinement of mafic minerals occasionally produces a gneissic struc-
ture that is mappable. In the vicinity of magnetite lenses, the gneissic
structure is subparallel to the attitude of the lenses. This structure,
however, is not apparent in hand specimens.






yg
o oO

GRAVITY, milligals

GRAVITY, milligals

GRAVITY, milligals

ng
rS

Nd
i

GRAVITY, milligals
~
np

p
o

7

W

Magnetite
outcrop

id
°

@

Gravity profile
1200 - south line

.Reduction density =2.8

+I

o

3

3.0

28

2.6

Gravity profile
1400 — south line

Reduction density =2.8

rs

2.8

Ys

[ Large terra

in

2.6

vin

correction

24

|

2.2

\

20

i

°

c

Gravity profile
1600 - south line

Reduction density =2.8

f
@

f
o

{

fp
b

Magnetite
outcrop

nv
to

/

i
°

vi

\

Gravity profile
1800 - south line

Reduction density =2.8

| |

@

NA

\

| |

16
West 20 18 16 14 12 10 8

6

4

2

lo} 2
DISTANCE,

4 6 8
hundred feet

10

12 14 16

FIGURE 8. - Gravity Profiles—1,200-, 1,400-, 1,600-, and 1,800-Foot South Lines.

18 20 22 24 26 East


Mining Research Center in the summer of 1959; it proved helpful in the inter-
pretation of the gravity data.

Special acknowledgment is due the owners and lessee of the property for
their interest and cooperation in the field investigation.

LOCATION AND ACCESSIBILITY

The Iron Mountain titaniferous magnetite deposit is in the south-central
part of Fremont County, Colo., 12 miles southwest of Canon City and 15 miles
north-northeast of Westcliffe. The area investigated by the Bureau of Mines
is northwest of Pine Creek in secs. 3 and 4, T. 20 S., R. 72 W., 6th principal
meridian (fig. 1).

A line of the Denver and Rio Grande Western Railroad follows the Arkansas
River from Canon City to Texas Creek, 16 miles northwest of Iron Mountain.

Texas Creek

" cc TS.
a Iron ‘,
SA) gg eMountain |

‘y ae - iW or

° 4 ramet ae
-

Xx
5 h
z y ob
Ke, wiCiy 4 ‘Sex by-
Va 3 ise
ay g
T. | T. oe »
la7 |2o ‘ . a
n |S. Dx. a SCALE, MILES

FIGURE 1. - Location Map, Iron Mountain Titaniferous Magnetite Deposit,
Fremont County, Colo.


16

pyroxenite (0.8) and gabbro (0.4) with monzonite is used in the calculations.
The curves are for the base of the pyroxenite and gabbro at depths (a) of
1,000, 2,000, and 9,500 feet. As noted in the figure, the magnitude but not
the shape of the field anomaly may be duplicated by this geometry and density
contrast. There is always a peak over the pyroxenite with a second high
developing to the east of the field high as the base (d,) of the pyroxenite
and gabbro is increased. The calculated curve for this geometry is not an
acceptable approximation of the shape of the field curve.

As a second try in approximating the shape of the field curve, the curves
of figure 15 were obtained. The monzonite-pyroxenite contact was kept at 70°,
but the rather indefinite pyroxenite-gabbro contact was reduced to about 34°.
The two curves have values of d, of 1,000 and 2,000 feet. An anomaly due to a
50-foot-wide tabular lens of titaniferous magnetite in gabbro (density con-
trast = 1.6) is also plotted in figure 15. This is a larger mass than any
known lens of titaniferous magnetite at the deposit; it gives the order of
maximum anomaly that could occur over the magnetite. Perhaps 0.5 to 0.7 milli-
gals of the field curve near the baseline is due to the presence of several
smaller lenses of titaniferous magnetite between 250 feet west and 250 feet
east. With this allowance for magnetite, the computed curve with d, equaling
2,000 feet approximates the shape of the field curve fairly well east of the
coordinate 1,000-foot west, and the magnitude of the computed anomaly is
sufficient.

From 1,000 to 3,400 feet west the arc of the computed curves is much more
pronounced than the field curve. The flatter characteristic of the field
curve is probably due to end effects of the mass of pyroxenite and gabbro in
the southwest portion of figure 16. It may be significant that projected con-
tacts bring this mass of denser rock closest to the 2,000-foot south section
(within 300 feet) in the vicinity of the greatest discrepancy of the field and
computed curves (around 2,100 feet west). These end effects could reach the
order of one to two milligals and thus bring the field and computed curve for
d, equaling 2,000 feet into fairly good agreement.

The field curve drops to the east a little more than the computed (d, =
2,000 foot) curve. This suggests a pyroxenite-gabbro contact with a dip
nearer 40° than 34°, With the flat arc of the field curve to the west
explained by the end effects of the pyroxenite-gabbro mass just south of
the section, the monzonite~pyroxenite contact can be maintained between the
mapped dips of 70° to 85°. Thus, the gravity anomaly can be explained reason-
ably by a wedge of pyroxenite between the monzonite and gabbro with no more
(and perhaps less) titaniferous magnetite at depth than there is at surface.


GRAVITY, milligals

3.0

28

TT

2.6

24

2.2

a 4

fA /[~Larae terrain corrections

A [Observations on pit ice

2.0

Magnetite \
outcrops

\

Ls

+

T . .
1_-Terrain correction too

Ht high due to low density

monzonite hill

Gravity profile
2000 ~ south line

Reduction density =2.8

|

if

{

5,8
West 34 32 30 28 26 24 22 20 18 16 14 12 10 8 6 4 2 0 2 4

DISTANCE, hundred feet
FIGURE 9. - Gravity Profile—2,000-Foot South Lines.

6

8

ite)

12

14

16

18 20 East.






This publication has been cataloged as follows :

Becker, Robert M
Tron Mountain titaniferous magnetite deposits, Fremont
County, Colo., by R. M. Becker, S. S. Shannon, Jr., and C. K.
Rose. [Washington] U. S. Dept. of the Interior, Bureau of
Mines [1961]

ii, 18 p. illus., tables. 27cm. (U. S. Bureau of Mines. Report
of investigations, 5864)

Based on work done in cooperation with the Colorado School of
Mines.

1. Magnetite, 2. Iron ores—Colorado—Fremont Co. I. Title. IL.
Title: Titaniferous magnetite deposits. (Series)

[TN23.U7 no. 5864] 622.06173

U.S. Dept. of the Int, Library





magnetometer operator adjusted the potentiometers on his belt to vary the coil
current and return the balance to zero. Meanwhile, the notekeeper, on signal,
reversed the current direction and set the milliammeter on the most sensitive
range possible. When the magnetometer was zeroed, he recorded the milli-
amperes, current direction, and other pertinent data.

The magnetometer was set at 79 gammas per scale division. The nulling
accuracy was less than a tenth of a scale division, so the magnetometer accu-
racy was within about 5 gammas. Thus, the accuracy of the observation was
largely dependent on whether two or three significant figures could be read
on the sensitive multirange milliammeter. With the equipment used, the range
of the magnetometer was better than -60,000 gammas to +60,000 gammas with an
accuracy of generally three and sometimes two significant figures. Only once
was this range exceeded in the survey.

Two magnetic observations, 180° apart, were taken at each station. Over
areas of high magnetic relief, 3- to 4-minute intervals between readings were
usual, whereas over areas of low magnetic relief, the interval between read-
ings was 2 to 3 minutes. Ninety to one hundred stations were commonly
occupied each day. Base checks were taken at least every 2 hours. With a
more modern magnetometer the method would be faster; the magnetometer and
calibrating coil could be kept on the tripod between stations, and the orient-
ation could be omitted. The method may have more application in regions of
high magnetic relief. The advantages of the method are the use of one instru-
ment with a wide magnetic range, accuracy that depends on the magnitude of the
magnetic field, and fairly rapid coverage.

The observations ranged from -36,700 gammas to more than +63,200 gammas.
The field data were reduced for instrument drift and plotted on a map. The
drift corrections in regions of more extreme magnetic relief were often not
significant. The magnetic-contour map was constructed from the reduced data;
each of the plotted stations along the survey lines represents the location at
which a magnetic reading was taken (fig. 5). Because of the extreme values
caused by surface or near-surface magnetite, the contouring was restricted to
the range from -2,000 gammas to +10,000 gammas with a 2,000-gamma contour
interval. Areas above +10,000 gammas are lineated, and areas below -2,000
gammas are crosshatched.

Susceptibility measurements were made in the field using a Mooney’ -type
susceptibility meter. These measurements were made on 14 outcrops of monzo-
nite, gabbro, pyroxenite, and titaniferous magnetite. The results are sum-
marized in table 2.

Gravity

Gravity observations were made at 1,085 stations in the grid area. Grav-
ity readings were taken on all magnetic stations, and additional readings were
taken on a closely spaced grid about the main pit and along the westward line
extension off the main grid area.

"Mooney, H. M., Magnetic Susceptibility Measurements in Minnesota: Geophysics,
vol. 17, No. 3, July 1952, pp. 531-543.


11

1.00

80)

es 7150
WS

70

60

7120

50

THO

ALTITUDE, feet

2
3

7100

&

Ey

Gravity Reduction
profile density,
g/cc.

GRAVITY, milligals

6

3.
2.9
2.8, best straight line, 1150 to1ssoE
2.6

=.10}

iT iz is 14 7
800-SOUTH LINE, EAST END, hundred feet

FIGURE 6. - Gravity Profiles Over Gabbro Using Several Reduction Densities.

from two lines that crossed the pit and subsequent geologic mapping indicated
that the pit data could not be used to estimate quantitatively the remaining
tonnage of minable magnetite. The reduced gravity data are presented on
cross sections (figs. 7-11). A gravity-contour map was also constructed from
the reduced gravity data (fig. 12). The plotted stations along the survey
lines represent the location at which gravity data were used to construct the
contour map.

Density determinations were made on rock specimens brought to the labora-
tory. The results of density determinations for monzonite, gabbro, pyroxenite,
and titaniferous magnetite from different locations are summarized in table 3.


TRON MOUNTAIN TITANIFEROUS
MAGNETITE DEPOSITS, FREMONT
COUNTY, COLO.

By R. M. Becker, S. S. Shannon, Jr.
and C. K. Rose

» oe 6» oe 4 & % * » » » report of investigations 5864

UNITED STATES DEPARTMENT OF THE INTERIOR
Stewart L. Udall, Secretary

BUREAU OF MINES
Marling J. Ankeny, Director

The work upon which this report is based was done under a cooperative agreement between the Bureau of Mines,
U.S. Department of the Interior, and the Colorado School of Mines,










4000—

3405

2800

1850 f
1450
os 60°-70°
‘20°
67°
)
&
te 6°.
2
60%
¥
oP
g 5 pias
3 3 = =
« ZA
a 70° co | i
Proce 4 .
a Gg “4. : * Di 2B
"a |
#
960 oo | (A
55% AZ ze
omy yan ' ye ZB e *
ety 7 a 6
4 7 @ ' Z Bw al
f 2g i zB ” ZZ. Sou
1600 4 g 600 F p; AB 600
A AZ $"
1éd0 Z An bo Ds 7 S
nent ; yD Aa 2
2000
Z
@
2200 G A

FIGURE 4. - Outcrop Map, Iron Mountain Titaniferous Magnetite Deposit, Fremont County, Colo.

LEGEND
Gobbro——¢b___ ==
Monzonite —Mn:

Pyroxenite ————— G20)
Titaniferous magnetite —@>
Andesite ————— An

Bosat ————-g0

Diori di
Phonolite- Ph

Quartz monzonite—QM
Contact attitude —— ~&
Attitude of gneissic

structure———_U
200 400 600
SCALE, FEET

a

NY 55°


& GS

Yy 3)
a

\Y
VBA WA
b,
ah
Xe
f
ult
yp)

ee : Se Le DIS
i EO, pis ae
4. \63-— G

TTT > —

th’
ft
ae

=

Wi

tL 1 7 s00 ey
p—__,_ | 4 @ 4
SS ee oe AA
| ———*- a an Ee
ed at aa Z|
4 ) < | 4 a roe
4 —- 4 LEGEND

Less than -2,000 gammas 7

—__—, 10}
bo Greater than +10,000 gammas: yl

Text reference——_________(@)

—_ 4 "800 1500 Contours shown in thousands of gammas
4 He 1550 from -2,000 gammas to +10,000 gammas
a (600 -——~z_1600 1600 _
| —
100

a

7 2600 C) 200 400 600
ts
ma SCALE, FEET

2200

FIGURE 5. - Magnetic-Contour Map, Iron Mountain Titaniferous Magnetite Deposit, Fremont County, Colo.


for partial analysis and metallurgical evaluation. Partial analyses and
approximate locations of these samples are listed in table 1. Samples 1
through 5 were titaniferous magnetite, and sample 6 was pyroxenite.

TABLE 1. - Partial analyses and location of samples

Sample | Percent | Percent Approximate location

No. Fe Tidz on present grid

1 49.0 14.0 1,050 ft. S. and 100 ft. E.
2 47.9 13.5 2,150 ft. S. and 200 ft. E.
3 48.6 13.7 2,100 ft. S. and 300 ft. E.
4 50.7 14.1 1,750 ft. S. and 200 ft. E.
5 49.4 13.9 450 ft. S. and 400 ft. E.
6 12.9 3.2 1,400 ft. S. and 1,000 ft. W.

Survey Control and Geologic Mapping

In 1958 a 4,400-foot base line was established due north along the trend
of the deposit and coordinated with the land subdivisions and the 74-minute
Canon City No. 2 SW. quadrangle. Initially, 42,800 feet of crosslines were
extended normal to the base line at 400-foot intervals. Later, 5,900 feet of
crosslines were added within the grid area at 200-foot intervals, and a
central crossline was extended 2,000 feet westward. These line extensions
and additions were made to obtain additional geophysical detail (fig. 2).

Geophysical stations were generally at 50-foot intervals along the base
line and the crosslines. Some smaller intervals were used near the main pit,
and 200-foot intervals were used along the westward extension off the main
area. All horizontal control was maintained by using a theodolite and steel
tape.

Vertical control for the grid area was carried 1 mile east from a point
of known elevation on the Canon City No. 2 SW. quadrangle. This point was
accurate within a foot, so absolute elevations will have this accuracy. Using
a self-leveling level, relative elevations of all geophysical stations were
established to within a tenth of a foot.

The grid-survey control was a great aid to the geologic and topographic
mapping. A map of the 1.2 square miles incorporating the grid and surrounding
region with a scale of 1 inch equals 200 feet and a contour interval of 10
feet was made with an alidade and planetable (fig. 3). During the topographic
mapping an outcrop map was also made (fig. 4). Rock specimens were collected
from the grid area, and their densities were determined. Thin sections were
made of the major rock types, and the mineral composition and rock classifica-
tion was determined by examination with a petrographic microscope. This work
was done in the Bureau of Mines laboratory at Denver, Colo.

Magnetics

Magnetic observations were made at 954 stations using a vertical-
intensity magnetometer. The magnetometer was used as a null instrument by


10

TABLE 2. - Apparent susceptibility measurements, K = 10-° C.G.S. units

Titaniferous
Measurement Monzonite | Gabbro | Pyroxenite | magnetite
| eee 1,470 7,720 10,700 63 ,500
Zaceceeccccsecverees +} 1,485 3,460 5,260 50,400
Becceccccccveseees +} 1,720 2,350 2,020 -
sees . - 1,963 - -
eee . - 2,445 - -
sees eee - 2,517 - -
Mean.. se eeeeees «| 1,558 3,409 5,993 56,950
Standard deviation..........+ 114.5 1,980 3,581 6,550

Two gravity observations within 0.025 milligal of each other were
recorded at each station. The average of these readings was taken as the
observed gravity at the station. The height of the instrument plate and the
time were also recorded for each station. Base checks were made at intervals
of % to 1 hour during the morning when the temperature gradient was high.
After the trend of the drift curve for the day was established, the interval
between base checks was lengthened to a maximum of 2 hours. If there was a
marked variation in the air temperature, the interval between base checks was
again shortened.

The gravity data were corrected for instrument drift, latitude, free air,
and Bouguer and terrain effects. The latitude correction was a reduction in
the data by 0.024088 milligals per 100 feet to the north. A profile across a
wash in gabbro gave a reduction density of 2.8 grams per cubic centimeter
(fig. 6). This density was used to compute the Bouguer and terrain
corrections.

The terrain corrections were made using Sandberg's® tables for an
inclined plane in conjunction with the usual Hammer zones. Generally, cor-
rections for Hammer zones A, B, often C, and sometimes D were made using the
tables for the inclined plane. The remaining zone corrections through zone H
were made using Hammer's chart as reproduced by Nettleton. For zones A
through E the topography for the corrections was based on the 1 inch equals
200 feet plane table survey; for zones F through H, the 7%-minute Canon City
No. 2 SW. quadrangle provided the topographic control. The magnitude of the
terrain corrections ranged from 0.19 milligals over alluvium in the southern
part of the grid to 1.10 milligals on the knob adjacent to Iron Mountain in
the northwestern part of the grid. Station elevations were based on 17 level
traverses; one tied the grid area to the previously mentioned quadrangle. All
level closures were within 0.1 foot. The reduction datum selected was 7,100
feet.

Not all the gravity data were reduced. This is particularly true in the
main pit area where a closely spaced grid was established. Reduction of data

8Sandberg, C. H., Terrain Corrections for an Inclined Plane in Gravity
Computations: Geophysics, vol. 23, No. 4, October 1958, pp. 701-711.

°Nettleton, L. L., Geophysical Prospecting for Oil: McGraw-Hill Book Co.,
New York, N.Y., 1940, p. 145.