(JOO) T~1r t/)(() .S.I.IJ ~ :.-----" RES9~Ref! ·· ItA TION SECTION A Magnetic Investigation·()f the Round Mountain Plug, Castle Valley, Grand County, Utah
By R. A. Black
. EP 2 3 98 * JU~ 30 19'5 Denver ~y ~.,..s<''~~ ~
Trace Elements In-..esti gations Report 342
UNITED STATES DEPAR.Tl\fENT OF THE INTERIOR GEOLOGICAL SURVEY OFFICIAL USE ONLY
Geology and Mineralogy
This document consists of 25 pageso Series A
UNITED STATES DEPARTMENT OF THE INTERIOR GEOLOGICAL SURVEY
A MAGNETIC INVESTIGATION OF THE ROUND MOUNTAIN PLUG CASTLE VALLEY~ GRAND COUNTY~ UTA~
By
R., A., Bl ack
December 1953
Trace Elements Investigations Report 342
This preliminary report is distributed without editorial and technical review for conformity with official standards and nomenclature& It is not for public inspection or quotationo
:A: This report concerns work done on behalf of the Division of Raw Materials of the Uo Sa Atomic Energy Commissiono
OFFICIAL USE ONLY OFFICIAL USE ONLY 2 USGS~TEI=342 . GEOLOGY AND MINERALOGY
Distribution (Series A) of copies Argonne National Laboratory o o o o 1 Atomic Energy Commission, Washington o o o o o o o o o 1 Battelle Memorial Institute, Columbus o o ~ ~ o o 1 Carbide and Carbon Chemicals Company, Y=l2 Area o 1 Division of Raw Materials, Albuquerque 1
Division of Raw Materials, Butte o f) C) o o 1 Division of Raw Materials, Denver o 1 Division of Raw Materials, Douglas o 9 e 0 0 0 1 Division of Raw Materials, Hot Springs o o o o 1 Division of Raw Materials, Ishpeming o o o o o o o o o 1 Division of Raw Materials, New York o o o o o o 6 Division of Raw Materials, Phoenix o o o o o o o o o 1 Division of Raw Materials, Richfield o o o o 1 Division of Raw Materials, Salt Lake City o o o o o 1 Division of Raw Materials, Washington o o o o o 3 Dow Chemical Company, Pittsburg o o 1 Exploration Division, Grand Junction Operations Office 6 Grand Junction Operations O~fice 1 National Lead Company, Winchester o 1 Technical Information Service, Oak Ri dge 6 Tennessee Valley Authority, Wilson Dam o o o o o o o 1 Uo So Geological Survey~ Alaskan Geology Branch, Menlo Parke o o o o o o 1 Fuels Branch, Washington 1 Geochemistry and Petrology Branch, Washington o 1
Geophysics Branch, Washington o o o 8 MineraJ. Deposits Branch, Washington o 1
Eo Ho Bailey, Menlo Parka o o o o o o o o 0 0 0 1 Ro Ao Black, Grand Junction o o o o o .. o o o 1 Ko Lo Buck, Denvero 1 J., Ro Cooper, Denver a o o 1 No Mo Denson, Denvero o 1 Co Eo Dutton, Madison o ' o o o 1 Ao Lo Brokaw, Grand Junction 2
Lo So Gardner, Albuquerqueo o 0 0 0 0 1 \
Co Bo Hunt, Washingtono 0 0 9 0 1 Mo Ro Klepper, Washington 1
Ao Ho Koschmann, Denver o 1 Ro Ao Laurence, Knoxville 1
Do Mo Lemmon, Washingtono o o o o o 0 0 0 0 0 0 9 1 J o Do Love, Laramie o o o o o o o o o o o 1 Vo Eo McKelvey, Menlo Park o o o o 1 Wo Lo Emerick, Pl ant City o o o o o o o o o 1 Ro Jo Roberts, Salt Lake City o o o o o o a 1 Qo Do Singewald, Beltsville o o o o o 1 Jo Fo Smith, Jro, Denvero o o o o o o 1 Ao Eo Weissenborn, Spoka.ieo 1
TEPC 0, Denver o o 0 0 0 2
TEPCO, RPS, Washingtono 0 0 0 0 0 3 (Including master) 76
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3
CONTENTS
Page
Abstract o o o o o o e o • o o • • o • • o o • " o o • o • o o • o o < 4 Introduction o " " ., " o • co o o • o o o o • o o o o • o 5 Location and topography o • o • • o • ., o o o • • o • • o • • • o o 6
Geolog:y- o o e o e o o o 9 o e e o a o e o o o e o o e o o e e e 6 Magnetic investigations o o o o o e o o o o o o e ., o e o o e ., o o 11 Field procedures o o o o o o o e o o o o o o e o o o o o o o o 11 Theoretical anomalies o o o o " o o o o o o • o o o o o o o o 13 Results o • o o e o • o o o e o o o o o o o e o o o o .. o 17 Conclusion o o o o e o o o o o o o o o o o o o o o o e o o o o e o 24 Literature cited o 0 0 0 0 0 0 0 0 0 0 0 o o o o o o o o o o o o o e 25
ILLUSTRATIONS
Figure le Index map of part of the Colorado Plateau showing the location of the Castle Valley area~ Grand Countyp
Utah o o o o o o o o o o o o o o o o o c o o o o o 0 " 7
Map of the Castle Valley area, Grand County~ Utah o o 0 0 8 Map showing the location of the magnetic traverses with respect to Round Mountain~ Grand County~
Utah 0 0 0 e 0 0 G 0 e " 0 " 0 " 0 G 0 0 9 e " o o e o 9 Theoretical vertical intensity magnetic anomalies over a vertical dike buried at depths of
(A) 200 feet 9 (B) 400 feet~ and (C) 1»000 feet c 0 0 " 15 Theoretical vertical intensity magnetic anomalies over a horizontal cylinder buried at depths of · (A) 200 feet 9 (B) 400 feet~ and (C) 1~000 feet 0 0 0 0 16 6o Diagram showing some of the magnetic profiles in the Castle Valley area9 Grand County~ Utah o o o o o o 18 Diagram showing magnetic profile obtained over Round Mountain9 Castle Valley area~ Grand County, Utah o o • o o o o o o o o o o o o o o o o o 19 Magnetic vertical intensity contour map of the area around Round Mountain~ Castle Valley are~, Grand County9 Utah o o o o o o ~ o o o o ~ o o o 0 0 20
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A MAGNETIC INVESTIGATION OF THE ROUND MOUNTAIN PLUG7 , CASTLE VALLEYJl GRAND COUNTY, UTAH
By Ro Ao Black
ABSTRACT
A magnetic survey was made in Castle Valley~ Grand County~ Utah~ to determine if the Round Mountain plug was fed laterally from the La Sal
Mountains or vertically from a source at deptho Additional measurements were made around the exposure of igneous rock to obtain the shape of the plug at deptho
No indication of a buried lateral igneous feeder was detected on the magnetic profileso The magnetic contour map showed that the plug was much larger than indicated by the diorite porphyry outcrop, and that the buried part of the plug southeast of Round Mountain has a higher magnetic susceptibility than the diorite outcropo Theoretical anomalies~ calculated using the susceptibility of the diorite outcrop, indicate that a lateral feeder at a depth of greater than 200 feet could not be de= teetedo The size and shape of the magnetic anomaly as shown on the con= tour map suggest that the plug was fed from a source at deptho
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INTRODUCTION
Discussions during the winter of 1952 with Co Bo Hunt of the U.. So
Geological Survey concerni~g the intrusives and volcanism phase of
Cenozoic studies led to a proposal that a geophysical study be made of the Round Mountain plug in Castle Valley~ Grand County~ Utah, to de= termine if the Round Mountain plug was connec.ted to the La Sal Mountains by a relatively shallow lateral feeder~ or if it was fed from an indi vidual source at depth~ and to obtain information on the shape of the intrusiono This survey was part of Colorado Plateau geologic studies being carried out by the Uo So Geological Survey on behalf of the Atomic
Energy Commissiono
During a series of aeromagnetic traverses by the Uo S .. Geological
Survey plane in the La Sal Mountain area in February and March 1952~ an anomaly of 150 gammas had been detected at ~ altitude of 2~000 feet above a point half way up the slope of Round Mountain~ indicating that the diorite porphyry of the plug is fairly magnetico From this information and from the knowledge of the geology of the ~rea~ ·it was decided that a ground magnetic survey could best be used to obtain the desired informa= tion about the Round Mountain plug o A vertical intensity magnetic sur= vey was carried out in Castle Valley9 Utah~ during the period May 5 to
June 18~ 1953~ by a field party under the direction of R.. A.. Blacko At various times during this period~ Po E.. Byerly and Ro Eo Warrick of the
Geological Survey assisted in obtaining the m~gnetic measurementso
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LOOATION AND TOPOGRAPHY
The Castle Valley area is in Grand County~ Utah~ {figo 1) approxi= mately 18 miles northeast of Moab 9 and can be reached by Utah Highway 128.,
Figure 2 is a more detailed map after Baker (~933) which shows the Valley outline and the location of the Round Mountain stock.
Figure 3 shows the location of the magnetic traverses with relation to the Round Mountain plugG The indi vidual stations occupied on and around the plug are also showno The area northwest of traverse 5 is a relatively flat valley floor covered with alluvium except for the small circular outcrops of the Paradox member of the Hermosa fo~ation around
I the base of the pl ugo The Pa.radox aureole is sharply upthrust around the base of the plug and displays very steep dips6 Southeast of traverse 5~ roughly between 0 and 5500 on traverses 5 to ~ 9 there are small rounded hills composed of 30 to 80 feet of Pleistocene gravels covered with thin alluviumo The area between 5500 and 8500 on ~raverses 5 to 8 is a sagebrush=covered alluvial sl opeo
GEOLOGY
The major structural feature of the Castle Valley area is the Castle I Creek anticline which trends about N ~ . 50° Wo and plunges northwest from the
La Sal Mountains~ flattening near the mouth of Castle Creeko The crest of the anticline has been cut by Castle Creek to form a wide deep valley i floored with alluviumo Near the center of the valley is Round MountainJ a steep-sided roughly cylindrical mass of igni ous material surrounded by I
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MESA
~ \ \ '\__ \ (', -- · ·~ ~ :· ·t-',~- -.,... ~,~--~-,1,_ \ . SAN 'l ···~ ··· ' \ .(/I I/ .
Ji U ronium-vonodium mill· True k trai I
0 10 :1 • SCALE OF ~ILES DOLORES
FIGURE l. INDEX llAP OF PART OF THE CotoRAOO PLATEAU SHOWING THE LOCATION OF THE CASTLE VALLEY AREA, ORAND COOHTY, UTAH 8
EXPLANATION
Qol ALLUVIUM
TPhp PARADOX MEMBER OF PENNSYLVANIAN HERMOSA FORMATION R 2 2 E . R 23 E. DIORITE PORPHYRY
0 I MILE
OF
~
rJ) -- If) N 1-' I I ,· ------~------~------+-· ______---L______,_L------~~~
Qol I I ·---+------r I .. .r--v" · -...... -""' .. f ·-.... i I -- -~ ... J'--- -ir -- -....__ ;-""' ... - ... ..,__... - - - ~ '. ) +--US 160 I I I .. -v· ·~ · -- ' _ , _} ------====,_ ____ J______J______~------~------L------~------L------L------~L------~
MOA ( atter Baker, 1933)
FIGURE 2. JUP OF THE CAS'l'll VALIJrr AREA, GIWW CrotfTY, UTAH 9
TRAV. J TRAV. H TR AV. G TR AV. F TRAV C TRAV. B TRAV. A EXPLANATION 0 TRAV. I
MAGNETOMETER • TRAV. 2 TRAVERSES • 3A • • • • • ' • 2A • •. -1000 e!IA 4A • • DIORITE PORPHYRY •• MOUNTAIN lPhp PARADOX MEMBER Of PENNSYLVANIAN HERMOSA FORMATION _...,v- 1 ·-2000 • MAGNETOMETER STATIONS
500 0 1000 FEET
-3000
'k_ MINE AOIT
TRAV. 5
- TRAV. !lA
~-----~--~~L-----~------~-----r----~~------4------4------~ --5000 TRAV. 6
-6000 TRAV. 7
------~------~------' - 7000 FEET TRAV. 8
FIGURE 3. llAP SHCWING THE LOCATION OF THE lldh'TIC TRAVERSES WITH RESPECT TO iOOND MWNT AIN, GRABD COONTY, UTAH OFFICIAL USE ONLY
10 upthrust Paradox beds through which it has been i~trudedo The lower slopes .of the mountain are covered with taluso The valley floor is estimated to be underlain by 109 000 feet of salt, anhydrite~ and gypsum of the Paradox formation~ either in the for.m of a single body or as several adjacent smaller bodies of salta
Part of the area between magnetic traverses 5 and 8 is underlain by 30 to 80 feet of Pleistocene gravelso These gravels are river terrace gravels derived from the igneous rocks of the La Sal Mountainso The
La Sal Mountains are composed primarily of diorite porp~~o
The Round Mountain plug has been described by Baker (1933) as com~ posed of a fine=grained soda trachyte made up of soda=rich plagioclase feldspars with phenocrysts of hornblende» augite, and small amounts of apatite$ titanite, and magnetiteo
Butler and others (1920) suggested that the Castle Creek anticline might be underlain by igneous materialo Baker finds no evidence to
9 support Butler s suggestion9 and seems to be more in favor of Gouldvs idea (1926) that the salt anticlines$ although not caused by the lacce= lithic intrusions that formed the La Sal Mountains, served to some ex= tent to localize the laccoliths at the edges of the La Sal Mountainso
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MAGNETIC INVESTIGATIONS
No previous ground geophysical work in the Castle Valley area is knowno The only geophysical information available was from the single airborne magnetometer flight line that the Geological Survey plane had flown over the edge of the Round Mountain plug. As previously mentioned, in this flight, at an altitude of 2,000 feet, an anomaly of 150 gammas was detected over the plugo This provided the basis for the choice of the magnetic method as the ground geophysical method to use in investi= gating the plugo
Geologic conditions in the Castle Valley area seemed favorable for a ground magnetic surveyo The igneous plug~ a diorite porphyry~ could be expected to have a high magnetic susceptibility and the surrounding
Paradox member~ to be of quite low susceptibility~ owing to its salt~ gypsum~ and anhydrite Contento
The area was an excellent one for magnetic investigations because of the lack of artificial sources of magnetic disturbance such as wire fences, buried iron pipes, drill=hole casings, and grounded direct= current machineryo
Field procedures
The magnetic measurements were made with a temperature compensated
Askania vertical magnetometer with a scale constant of 10 gammas per scale divisiono
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A three=man crew was used on the survey: an instrument operator,
a recorder, and a third crew member who was responsible for laying out
the traverse lines and marking the station locationso The traverses were laid out as shown in figure 3, with marker stakes put in at 500- foot intervalso Traverses 5 to 8 were laid out at right angles to the
postulated trend of the igneous feeder and were considered sufficient to
provide evidence as to its existenceo The other traverses were laid out
to determine the attitude and lateral extent of the igneous plugo
The lack of roads in the area and the presence of numerous rocky
washes in the valley floor prohibited the use of vehicles and made it
necessary to run the traverses on footo These facts, plus the length of
the traverses, made it impossible to obtain more than four to six base
station readings per day for purposes of determining the base and diurnal
correctiono After a number of traverse lines had been completed, a
~ $ystem of moving bases was employedo The original base was read at the beginning and end of the day, and at various intervals during the day stations were occupied along previously run traverseso This system per=
mitted closer control of the diurnal variationo In general, magnetometer stations were occupied at 200 foot intervals along the traverses, and where anomalous readings were 'obtained, more detailed observations were madeo
In addition to the regular grid, stations on traverse F' Ftt and
eleven individual magnetometer stations on and around the diorite porphyry outcrop were occupiedo The steep slopes of the plug made it difficult to
obtain any unifor.m pattern of stations~ their location was decided by
accressibilityc QfFICIAL USE ONLY QEFICIAL USE ONLY
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Theoretical anomalies
Magnetic anomalies result chiefly from dif~erenees in magnetic susceptibility between the geologie feature causing the anomaly and the surrounding mediumo These anomalies are interpreted by relating their size and shape to geologi.e conditions which could be responsible l for theme A knowledge of the type and size of anomaly to be expected from a certain known set of seologie conditions can be of considerable aid in interpreting magnetic datao One of the more convenient methods of computing the surface magnetic effect of a buried geologic body is to assume that it roughly approximates one of several simple geometric shapes for which the magnetic e ~ ffect can be computedo
The geologie problem investigated in the Castle Valley are~ was essentially that of determining the presence or absence of a lateral igneous feeder connecting Round Mountain and the La Sal Mountainso To obtain some idea as to the magnitude and shape of the anomaly to be ex= pected from this igneous feeder9 it was assumed to have either of two simple two=dimensional shapes for which the theoretical anomalies were then computed.,
The igneous feeder was first assumed to approximate the shape of a vertical igneous dike of infini.te depth extento To simplify the ealcula= tions the dike was assumed to be vertically polarized and unifonnly mag= netizedl) induced pol arization "'ras further assumedo Calculations were made for a dike of a width of 400 feet~ buried at depths of 200~ 400~ and 1,000 feeto The normal vertical inte~nsity of the earth ns magnetic field at this OFFICIAL USE ONLY 14 latitude is about Oo5 gauss~ and the magnetic dip is about 65 degreeso
Fourteen rock samples were taken from the Round Mountain plug and the
surrotmding sediments for suscelptibility determinationso The magnetic
susceptibility of fresh samples taken from the igneous rocks averaged 6 about 400 x 10= cgs units and 1:;he sedimentary samples showed a magnetic
susceptibility of practically zeroo On the basis of this information a
susceptibility difference of 400 x 10=6cgs units was assumed to exist be= tween the postulated lateral feeder and the surrounding sedimentso
Figure 4 shows the theoretical anomalies that would be produced by a vertical dike of infinite depth extent9 a width of 400 feet~ a sus ceptibility of 400 x 10=6 in contrast to Oo5 x 10=6 for the surrounding
sediments9 buried at depths of 200~ 400 9 and 1~000 feet respectivelyo The second shape assumed for the igneous feeder was that of a hori=
zontal cylindero Again for simplification the cylinder was assumed to be vertically polarized and uniformly magnetizedo Calculations were made for a horizontal cylinder of radius 200 feet~ buried at depths of 200~
400~ and 1~000 feet~ using the same assumptions about the intensity of the- ea hvs magnetic field, and magnetic susceptibilities of the cylinder
and surrounding rocko Figure 5 shows the theoretical anomalies from such a body ~
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-20 GAMMAS
/ ~~·-'-..._,. ~/'/ c ~------_____ .------0
---40 GAMMAS
-20 B
--- -·-··------·------~~=- -==-=------0 \ -60 GAMMAS
-40
-20
-·------·-- --r --·--··-· ------·r -- - - · · ·~ - ---==-==~, - ---0 3000 2000 1000 0 1000 2000 3000 FEET
FIGURE 4'. THEORETICAL VERTICAL Ir..TTENSITY l-~AGNETIC ANOl·~ALIES OVER A VERTICAL DIKE !3URIED AT DEPTHS OF (A) 200 FEET, (B) 400 FEET, AND (C) 1,000 FEET
6 3~ ~ 7 ---+5 GAMMAS
------~------~-==--===-===-==-====~~:-=--= ---0 ------
---5 GAMMAS
--+15 GAMMAS
. -+10
I 8 \
----5 GAMMAS
(\ - -+30 GAMMA~ / \ I \
I '\
--- +20
A
+10 I I I I I I ---o -==---==-=--====--___:____:_------r------~-- ---·.------====~ ====--=== ----- ' -.-- b -----~ \~/- /--- . -· --5 GAMMA~ I I l 3000 FEET 3000 2000 1000 0 1000 2000
FIGURE 5. THEORETICAL VERTICAL INTENSITY MAGl\TETIC ANONALIES OVER A HORIZONTAL CYLINDER BURIED AT DEPTHS OF {A) 200 FEET, (B) 400 FEET, AND (C) 1,000 FEET OFFICIAL USE ONLY 17
Results
The magnetic observations obtained in the Castle Valley area were corrected for diurnal and base variation and~ where necessaryD auxiliary magnet corrections were appliedo The corrected data were first plotted as magnetic profiles as shown in figures 6 and 7~ and then a magnetic contour map (figo 8) was preparedo The magnetic contour map was pre= pared in the following mannero The corrected magnetic values for each station were plotted at the appropriate station location on a planimetric map with a scale of 1 inch to 1~000 feeto Each set of three consecutive magnetic values along each traverse line was averaged and the average value was plotted at the middle station locationo The erratic dis= position of the station locations within the area enclosed by traverses
2~ G~ 5~ and E~ made it necessary to use a different system for obtaining average values for use in contouring this areao A transparent template of the area was prepared with a series of collinear circles~ half an inch in radius and tangent to each othero This template was laid over the area in question9 and all station values that lay within any given circle were averaged and this average was plotted on the map at the center of the circlee The template was then offset by half an inch and the averaging process was repeatede A second offset was then made at a right angle to the first offset and the averaging process was repeatedo The average values obtained were used to contour the area bounded by traverses 2~ G9
5~ and Ee The averaging process smoothed the erratic values due to wa~hes and boulders and made the values easier to contour without changing the general magnetic picture of the areao
OFFICIAL USE ONLY 18 TRAV. F TRAV. E TRAV. C TRAV.B TRAV. J TRAV. H TRAV. G TRlA I I I I I I I 8500 fEET 8000 7000 6000 5000 4000 3000 2000 1000 0 I ~ I I I I I I I I
--.- orO -TRAV. I EXPLANATION
-TRAV. 2
• ... +: MAGHETIC PROFILES -1000 +200 GAMMAS
+200 PROFILE SCALE -200 -2000 -200
- -3000
-4000 -TRAV. 5
-5000 -TR.AV.6
-6000 -TRAV. 7
-1000 FEET- TRAV.8
FIGURE 6. DIAGRAM SHa'IING SCI.IE OF THE MAGNEI'IC PROFI~ IN THE CASTLE V.AU.EY ARAA, GRAND COONTY, UTAH - 700 GAMMAS 19
-600 GAUMAS
I . -500 GAMMAS
-400 GAMMAS
~' '--- Ai\ -300 GAMMAS
1 ! 1\ / \ /\ ~
I j I ~a i' y y y \ -200 GAMMAS ,I I • """: 0~ (f)l cf" il w ... ~ I I nl :r' / ~ ~ I ~ , - 1 -100 GAMMAS I >- " Wj ::;, ..... tr 1l · a: I 0 : /~vvv-- ; . 4:-.Y//.fi~VJv~~~ 1 / .-vv vvvvv'v _. -""Vvvvv vvv'!/£v.vvvvv·, 1 / VVVVWVVVVVVVVVVVVV VVVVVVVVVVI/V\ ., 1 / YVV VV I/VVVVVV 'IV'I VVVVVVVVVVV V VV V V \ : j / VVVI/VV VVVVVVVVVVVVVVVVVI/VVVVVVI/VV/ -,. / VVVVVVVVVWV V OIORITE OUTCROP VVVI/ ..-· ' ./VVVVVVVVVVVVVVVV VVVVVVVVV VVVV'j. \ vvvvvvvvvvvvvv vvv vvvvvvvvvv v . '\ VVVVVVVVV V VVVVVVV VVVVVVVVVY - / VVVVVVVVVVVVVV WVVVVVVVI/IN-- ', ./ vvvvvvvvvvvvvvvvvvvv. - ' STEEP TALUS SLO PE I. / ~-VVVV'!_Y_~~ VV~'}_VY_'j _.. ~T~L-US SLOPE ~ v / "'ARAOOX UEMBER AND TALUS ------·------··· ______f30~.N_Q_ - ~.9~_ . TA I_t) ______-·------·-----·---·------ FIGURE 7. DIAGRMI SH - 0 -TRAV.I -TRAV. 2 -1000 EXPLANATION - 2000 500 0 1000 FEET ____J CONTOUR INTERVAL 50 GAMMAS -3000 0 f'J; OIORITi PORJ>tiYRY OUTCROP - 4000 - TRAV 5 f" ., I \ ' - 5000 -TRAV 6 ~------+ 50 ._ __ .... / - 6000 - - TR AV 7 0 \ --·------·--. ______k_+.:=~~L---- - 7000 FEET - TRAV. 8 FIOURK .g • lUONETIC VERTICAL INTE.t;SITY CONTOOR MAr OF THE AREA ARaJND InJND KOONTAIN, CASTLE VALLEY ARAA, GRAND COUNTY, UTAH OFFICIAL USE ONLY 21 • ~en the magnetic profiles shown in figure 6 are considered9 a number of interesting features are notedo On traverses 1 and 2 9 the magnetic profiles are relatively flat 9 averaging about 40 gammas out to 39700 to 4 9 000 feet, at which point they increase sharply and flatten off again for the remainder of the traverse at an average value of about 100 gammaso This same plateau effect is evident on traverse 59 and to a lesser extent on traverse 6o Traverses A9 B9 and C average 20 to 40 gammas from 0 to 59 000 feet~ while traverse E displays much higher values in the same intervalo Traverses G» H9 and J also exhibit this plateau effecto On traverse 5 a large positive anomaly of 600 gammas was found over the talus slope near the base of Round Mountaino No igneous rock is ex= posed~ but this area is certainly underlain by igneous materiale No other large anomalies are noted on any of the rest of the profiles shown in figure 6o The parts of traverses 5 to 8 (figure 6) which cross the gravel de= posits are quite irregular.9 as could be expectedo Even so 9 the general level of the profiles is low~ and anomalous readings can be attributed to the presence of large boulders of igneous material and steep stream bedso In almost every case~ the stream beds produced a sharp negative magnetic anomaly~ while boulders produced sharp positive or negative anomalies~ no doubt according to their orientatione The plateau=like effect on the magnetic profiles could be ex= plained by a talus sheet of diorite buried under the alluvium~ a sill= • like intrusion surrounding the central plug~ or simply by a larger buried OFFICIAL USE ONLY OFFICIAL USE ONLY I 22 plug which woUld naturally have a steeper magnetic gradient on the north sideo The smooth nature of the profiles and the geology of the area suggest that a buried plug much larger than the diorite exposure is the most logical suppositiono Assuming the susceptihili ties used in calculating.- the _:th:eoretical anomalies to be reasonably correct, it seems apparent from examination of figures 4 and 6 that a feeder of the shape of a vertical dike could not be detected at a depth greater than 200 feeto The corrected mag""' netic profiles (figo 6) show irregularities due to dry washes and magnetic float which often exceed the maximum calculated anomaly that 'WOuld be prO= duced by a dike=shaped feeder buried at a depth of 200 feeto It is prob able, however, that the broad anomaly produced by the dike coUld be distinguished from the narrow sharp anomalies due to surface irregulari= tieso The magnetic profiles (fige 6) show no broad anomalies which could ' be considered to be indicative of a buried igneous feeder shaped like a vertical dike o Figure 5 shows that the maximum anomaly to be expected from a buried horizontal cylinder at a depth of 200 feet is approximately 30 gammas o It is very doubtful that such a small anomaly could be distin= guished from the anomalies produced by surface effectso Figure r::t~ shows the magnetic profile obtained on traverse F w F't over the _Round Mountain plugo The section of the profile between 600 and 1,900 feet exhibits the same general plateau-like effect previously noted on the northeastern side of the plugo A sharp increase is noted between OFFICIAL USE ONLY OFFICIAL USE ONLY I 23 1,900 and 2,600 feet which corresponds to the exposed diori outcropo The somewhat erratic nature of the profile in this interval os no doubt due to local concentrations of magnetiteo From 2,600 to 4~ 50 feet on the profile~ a definite positive anomaly is noted which doe not corre= spond to any surface featureso The magnetic contour map (figo 8) prepared from the av ~ged mag~ netic values shows no indication of a .lateral feeder igneous plug in any directiono A positive closure is vicinity of the diorite porphyry outcrop~ with the highest of the anomaly offset to the southeast of the outcrop of igneous r The contour map suggests that the plug is much larger indi= cated by the surface exposure and further suggests that the 450-gamma anomaly is underlain by a part of the plug which has a high r magnetic susceptibility than the exposed igneous materialo This co plained by magmatic differentiation within the plug with a esultant concentration of heavy magnetic minerals in one part of the Ap= plication of depth rules to the 450-gamma anomaly shown o the contour map gives a depth estimate of 600 feet to the top of the so ce of the anomalyo Although it is difficult to defin.e the slope of a buried feature from the anomaly it produces, the lOO=gamma contour line on figure 8 could be considered a reasonable estimate of the 1° ts of the buried plugo OFFICIAL USE ONLY OFFICIAL USE ONLY • 24 CONCLUSIONS The magnetic investigation made in the Castle Valley area was in= tended primarily to determine the presence or absence of a postulated buried igneous feeder connecting Round Mountain to the La Sal Mountainso This feeder had been estimated to be buried not more than 200 feet in the vicinity of the plug~ but possibly as much as 2,000 feet at the base of the La Sal Mountainso No indication of a buried lateral igneous feeder was detected on the magnetic profiles~ but this does not definite ly eliminate the possibility that a lateral feeder is presento The mag~ netic susceptibility contrast between the igneous material and the Para= dox formation was considerably less than anticipatedo Theoretical anomalies computed from the measured susceptibility values show that a low susceptibility feeder in the shape of a horizontal cylinder would not have been detected, and a low susceptibility dike=like feeder would not have been detected at a depth greater than 200 feeto The magnetic contour map shows that the igneous plug. is much larger than indicated by the surface outcrop, The lOO=gamma contour line could be considered as roughly outlining the plugo The contour map also indicates that the plug contains a material of higher suscepti= bility than the exposed diorite porphyryo This could be due to magmatic differentiation with resultant concentration of heavy magnetic minerals in the southeastern part of the plugo The large size of the plug makes it seem unlikely that it was fed laterally from the La Sal Mountains~ and the presence of a higher OFFICIAL USE ONLY OFFICIAL USE ONLY 25 • present~ susceptibility at depth offers a possibility that$ if a lateral feeder would be of higher susceptibility than the exposed dioriteo Cal= culations show that a feeder of ten times the susceptibility of the exposed diorite could be detected as a dike at nearly 19000 feet and a horizontal cylinder could be detected to a depth of 300 feeto No indi= cation of such a feeder is shown on the magnetic profileso The possibility of a deep low=susceptibility feeder cannot be • eliminated~ but a shallow high=susceptibility feeder is certainly not present~ and the shape and large size of the plug would seem to indicate that a vertical feeder is more likely than a lateral oneo LITERATURE CITED Baker~ Ao Ao~ 1933, Geology and oil possibilities of the Moab district, Grand and San Juan Counties~ Utahg Uo So Geolo Survey Bulle 84lo Butler, Bo Sol> Laughlin£' G~ Fo . ~ and others 9 1920, The ore deposits of Utah~ Uo So Geolo Survey Profo Paper 111, po 612., Gould, Lo Mo, 1926~ The role of orogenic stresses in laccolithic intrusionsg Amo Jouro Scio~ 5th sero9 Vo 12, po 119=129o OFFICIAL USE ONLY