DEPARTMENT OF TEE INTERIOR UNITED STATES GEOLOGICAL SURVET

GROUND-FOLLOWUP STUDIES OF THE 1977 AIRBORNE

ELECTROMAGNETIC SURVEY IN THE ASSIFAR AND MULHAL

AREAS, WADI BIDAH DISTRICT, KINGDOM OF SAUDI ARABIA

by

V. J. Flanigan and Hamdy Sadek

U.S. Geological Survey . ^ Open-File Report 83- 3 6 /

Prepared for:Ministry of Petroleum and Mineral Resources

Deputy Ministry for Mineral ResourcesJiddah, Kingdom of Saudi Arabia

1403 AH 1982 AD

This report is preliminary and has not been reviewed for conformity with U.S. Geological Survey editorial standards.

1983

CONTENTS

Page

ABSTRACT................................................. 1

INTRODUCTION............................................. 1

Purpose.............................................. 1Data storage................................. 2Location............................................. 2Previous investigations.............................. 2

PRESENT INVESTIGATIONS................................... 4

Assifar area......................................... 4Anomaly B-37......................................... 8Mulhal No. 2......................................... 9

SUMMARY OF GROUND-FOLLOWUP STUDIES....................... 10

CONCLUSIONS.............................................. 16

REFERENCES CITED......................................... 17

APPENDIX 1. Data profiles from Assifar study area...... 192. Data profiles from Mulhal No. 2 study

area..................................... 42

ILLUSTRATIONS

Plate 1

3.

Figure 1.

2.

[Plates are in back pocket]

Map showing locations of airborne electro­ magnetic anomalies detected in the 1977 airborne input survey, Wadi Bidah district

Maps showing in-phase and out-of-phase electromagnetic response (888 Hz), self- potential response, and location of inter­ preted geophysical anomalies in the Assifar area

Maps showing in-phase and out-of-phase electromagnetic response (888 Hz) and self-potential response, Mulhal No. 2 area

Index map of western Saudi Arabian showing location of the Wadi Bidah district......

Plots of electromagnetic/self-potential and magnetic responses along traverse 215 S, Assifar area.............................

GROUND-FOLLOW-UP STUDIES OF THE 1977 AIRBORNE

ELECTROMAGNETIC SURVEY IN THE ASSIFAR AND MULHAL

AREAS, WADI BIDAH DISTRICT, KINGDOM OF SAUDI ARABIA

by

V. J. Flaniganl/ and Hamdy Sadek

ABSTRACT

Parts of four airborne electromagnetic (AEM) anomalies were selected for study in order to determine the cause of high conductivity of Precambrian rocks underlying extensive areas in the southern Wadi Bidah district, Kingdom of Saudi Arabia. In the Assifar area, which contains an ancient mine or prospect having the same name, geophysical data suggest that a mineralized body may lie beneath and immediately south of the ancient workings. Many other conductive zones detec­ ted during the course of the geophysical survey are thought to be related to metavolcanic rocks containing carbonaceous materials. Detailed geologic mapping, and possibly diamond core drilling, will be necessary to fully evaluate the area.

In the Mulhal No. 2 area, located about 2 km south of the Mulhal ancient mine, geophysical studies suggest that miner­ alized rocks extend about 500 m along strike beneath outcrops of gossanous material.

A brief review of the AEM ground-followup studies in the Wadi Bidah district suggests that most, if not all, of the AEM conductors are carbonaceous rocks. Secondary causes of conductivity are intense faulting and shearing.

INTRODUCTION

Purpose

This report presents results of part of the geophysical studies conducted during 1979-80 field season in the Wadi Bidah district, Kingdom of Saudi Arabia, including reconnais­ sance geophysical surveys conducted in the southern part of the district to locate and delineate the geophysical charac­ teristics of anomalies detected by the 1977 airborne electro­ magnetic (AEM) survey of the district. In addition, results of studies in the Mulhal No. 2 (MODS 02702) area are pre­ sented. Results of other geophysical work conducted during the 1979-80 field season have been presented by Flanigan and others (1982).

I/ U.S. Geological Survey, Denver,CO

The work on which this report accordance with a cooperative Geological Survey (USGS) and the Petroleum and Mineral Resources.

is based was performed in agreement between the U.S.

Sau-di Arabian Ministry of

Data storage

Mineral localities referred to in this report are re­ corded in the Mineral Occurrence Documentation System (MODS)data bank and identified by a uniqu 5 five-digit sample num­ber, study.

No new MODS entries were made as a result of thisInquiries regarding this

through the Office of the Technical. Advisor, Saudi Arabian Deputy Ministry for Mineral Resources, Jiddah.

No data files were established for this report.

Location

The Wadi Bidah district is located about 350 km by road southeast of Jiddah (fig. 1) in a north-trending belt of Pre- cambrian metavolcanic and metasedimentary rocks approximately 15 km wide and 80 km long. The Wadi Bidah district as de­ fined in this report covers a somewhat larger area than that suggested by Earhart and Mawad (1970), who described an area 15 km wide and 45 km long. The present investigation includes the area suggested by Earhart, as well as an addi­ tional area extending 35 km to the south.

Previous investigations

data bank may be made

Many scientists have contributed to the present geologicunderstanding of the Wadi Bidah district through geologic, geochemical, and geophysical studies;. Only a few are men­ tioned in this report; for a more complete list, refer to Kiilsgaard and others (1978), Flanigan and others {Hll,tfli ) and Smith and others (unpub. data, 1982).

In the middle to late 1960 f s, Earhart and Mawad (1970) conducted reconnaissance geologic mapping, geochemical sam­ pling, and diamond core drilling at several of the more prom­ ising prospects in the district. Their work indicates that although there are several massive sulfide deposits in the district, they are marginal both in grade and tonnage. Sub­ sequent studies by Jackaman (1972), Roberts and others (1975), and Roberts (1976) suggest two possible genetic origins. Earhart and Mawad (1970) and Jackaman (1972) both proposed a volcanogenic origin for the deposits, whereas Roberts and others (1975) and Roberts (1976) suggested a postvolcanic-replacement origin. Kiilsgaard and others (1978) indicated that the massive sulfide deposits may well be volcanogenic in origin and that the rocks have undergone

35

31'

46* 47* 48* 49*

ARABIAN SHIELD

100 200 300KM

46' 47' 48*

Figure 1. Index map of western Saudi Arabian showing location of the Wadi

district.

Bidah

varying amounts of postdepositional shear and fracture zones.

remobilization along

Between 1972 and 1976, the work of Earhart and Mawad (1970) was expanded upon when detailed geologic, geochemical, and diamond core drilling studies were made at the Sha'ab at Tare (MODS 00464), Gahab (MODS 00468), and Rabathan (MODS 00463) prospects (Kiilsgaard and others, 1978). In both the earlier and later phases of geologic; investigations, a limi­ ted amount of geophysical work was done in support of these studies. Most of this work is summarized by Kiilsgaard and others (1978).

In 1977, a districtwide AEM survey was made using aflight-line spacing of about 250 m.eated more than 50 discrete electromagnetic (EM) conductorsranging from a few hundred metersstrike length (Wynn and Blank, 1979). In 1978, ground- followup investigations of the sources of the EM anomalieswere made in both reconnaissance andof these latest studies are reported by Flanigan and others

and Worl and Wynnj 1982), Smith and others (unpub. data). -

This AEM survey delin-

to more than 20 km in

detailed modes. Results

PRESENT INVESTIGATIONS

Assifar area

The Assifar area, so named for (MODS 01341; Greenwood, 1975), covers B-42, B-44, B-45 and B-46 (Geoterrex area is located about 30 km northwes and is accessible by road from the (plate 1).

the Assifar ancient mine parts of AEM anomalies Limited, 1977). The

; of the town of Al Bahah At Taif-Al Bahah highway

The area is underlain by predominantly fine clastic meta- sedimentary rocks consisting mostly of siltstone, chert, and argillite that grades into both siltstone and chert in some places. Minor amounts of volcanic rock (mostly andesite por­ phyry flows, dikes, and sills) are also present. Smith and others Q2ri£^tffjXsugges ted that the structural fabric of the area is complex, such that extensive mapping would be re­ quired to understand the local structure and geologic rela­ tionships. The most noteworthy structural feature, however, is the pronounced difference between the strike of layered rocks west of the Assifar ancient mine (N. 5° W.) and that of the rocks east of the mine (N. 20° E,, ).

Twenty reconnaissance traverse l:,nes were made across the area to establish ground control for geophysical, geologic, and geochemical studies (plate 2). (EM) and self-potential (SP) measurements were made by the Arabian Geophysical and Surveying Company (ARGAS) under con-

tract to the USGS through the Directorate General of Mineral Resources (DGMR). In addition, magnetic measurements were made over the area by Mohamed Nur Jama (USGS). The geophys­ ical data from these surveys are included in this report as appendix 1.

EM and SP data were compiled in contour maps and are shown on plate 2, along with a comprehensive interpretive map showing the location and apparent width of the rock unit con­ taining the source of the EM and SP response. Local magnetic anomalies interpreted as indicating near-surface concentra­ tions of magnetic minerals are shown on the interpretive map as arrows. Local geology, as mapped by Smith and others {25£!^7#.)forms the base for the interpretive geophysical map.

The interpreted location and width of the EM conductor is based on one-half the coil spacing (50 m) from each side (zero or inflection point) of the EM response. Where the in­ dividual conductor could not be resolved, the zone containing the multiple conductors is shown. The location and width of the SP response are based on the width of the half-amplitude of the SP response. As is the case for the EM interpreta­ tion, if multiple SP sources combine to give a complex re­ sponse, the entire zone is indicated on the interpretive map.

Several EM-SP anomalies were detected along each tra­ verse. Most of the anomalies are complex; that is, the responses indicate interaction between two or more conductive sources spatially located such that their individual respon­ ses cannot be resolved at. the 200-m coil separation used for the slingram-EM survey. Semiquantitative estimates of con­ ductance and depth to the conductor are not considered valid in areas that produce such complex responses, and, in many cases, estimates of conductor width and location are only approximate. However, because the SP data indicate a broad SP response having several peaks or minima located over the source rocks, it is possible to partially resolve the EM data, at least qualitatively, by using the SP minima as the location for the source rocks.

The amplitudes of the SP anomalies range from -200 to -450 mV, the highest values occurring over rocks that almost certainly contain a large percentage of carbonaceous mate­ rial. Given the apparent complex structure of the host rocks, most of the SP anomalies are coincident with an EM conductor, except in a few cases that will be discussed later.

From the Assifar ancient mine eastward (between traverses 215 S and 5 S), most of the geophysical anomalies lie along the crest of a east-trending anticline (plate 2). On the northern limb of the anticline, the EM anomalies in partic­ ular follow a northeast-trending dikelike outcrop of cherty

manganese-iron oxide metasedimentary rocks. A similar lens of metasedimentary rocks on the southernmost limb of the anticline has no geophysical anomalies associated with it. According to Smith and others("S2g£f'/? &) both exposures of the cherty manganese-iron oxide lens are of the same formation, and, although moderately high copper values are associated in places with the northern exposure, no significant gossans are exposed. Two small gossans are associated with the formation at the southern exposure. Although it appears that the cherty manganese-iron oxide formation is the source of the ob­ served geophysical anomalies, it is :iot known why the forma­ tion produces strong EM-SP responses in some places and little or no response in others. Several possibilities are suggested below.

500-From traverse 50 S to 100 S ( northwest of the base line, strong are observed. A second area of high tween traverses 135 S and 215 S (800 of these zones of high EM-SP response jor fault zone. The conductive zones concentrated shearing along the fault lized mineral concentration along parts of the fault zone, or the rocks lying beneath the manganese-i similar manganese-iron oxide zone in high EM-SP responses was tested by (RAB-2), and carbonaceous schist was of the anomalies (Flanigan and drill hole (R-3) located .several tested the same zone and intersected body (Earhart and Mawad, 1970).

others, hunc red

Traverse 215 S crosses the area

-m strike length) and complex EM-SP responses EM-SP response is be-

m strike length). Both are just south of a ma- may represent areas of zone, areas of remobi-

the more highly sheared response of carbonaceous on oxide formation. A the Rabathan area having diamond core drilling found to be the source

1982). However, a meters to the south

a small massive sulfide

of the ancient Assifarmine (fig. 2). Based on the in-pt.ase response along the traverse, three zones of high conductivity can be delineated. The zones are each as wide as 100 m and are centered 100 m west, 275 m east, and 600 m east of the base line. Because the last zone is not completely traced out, its location is not shown on figure 2. The EM anomaly located 100 m west of the base line has an associated SP response of about -200 mV. A second SP anomaly (25 m east of base line) is thought to be associated with the ancient mine workings; no EM conductor is associated with this SP response. There is a good possi­ bility that a mineralized body lies at depth in the area of this SP anomaly. A third SP anomaly is 150 m east of the base line at station 275 E and is offset northwest from the associated EM anomaly. It seems likely that both the SP and EM responses are related to the same geologic source, but only very tentative relationships can be surmised because of limited knowledge of the local geologic and structural con­ trol. A fourth SP anomaly of about -100 mV is near tjtie end of traverse 215 S at station 600 E. Again, the fact that the

200

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Figure 2. Plots of

electromagnetic/self-potential re

spon

ses

alon

g traverse 21

5 S, Assifar

area

. Re

, real

or in

-pha

se E

M response;

Im,

imaginary

or ou

t-of

-pha

se electromagnetic

resp

onse

. The

downward-

poin

ting

ar

row

indi

cate

s the

interpreted

loca

tion

of

a

loca

l magnetic an

omal

y.

For

deta

iled

ex

­ planation, see

appendix 1.

SP response is offset northwest of tor indicates either a very complex environment or separate geologic responses.

the associated EM conduc- geologic and structural sources for the two

they ;icn only

conductors the

In the area west of the Assifar B-42), three or more EM anomalies we verse by the ground survey. Because tors are less than 200 m apart, patterns and the interpreted position the source conductors are at best some of the more isolated traverse line to traverse line, on teristics it is nearly impossible to alies with a reasonable degree of the EM anomalies are in areas of me dominantly chert and grading into anomalies range from -200 to -400 m1 ficult to recognize carbon in hand the SP response suggests that carbon age of the cherty metasedimentary zones below the weathered layer, correlate at least in part with as seen on traverses 10 N, 40 N, however, as in the eastern half of gossans are associated with this unit metal values are low (Smith and others

Results of the geochemical and suggest a very complex environment carbonaceous rocks, and there is no gest that any of the EM conducto sulfide-mineralized rocks in the area.

ancient mine (AEM anomaly re detected on each tra- many of the EM conduc-

form complex anomaly and apparent width of approximate. Although may be traced from

basis of their charac- trace many of the anom-

certainty. Nearly all of tasedimentary rock, pre-

ai-gillite. Associated SP Although it is dif-

samples, the amplitude of composes a high percent- ocks in the conductive he EM-SP anomalies may

-iron oxide zones, N, 115 N, and 135 N; Assifar area, no major

and copper and precious , ( unpub. data, 1982)

manganese- 95the

almost

western

geological observations certainly involving

strong evidence to sug- s are associated with

part of the Assifar

Anomaly B-37

AEM anomaly B-37 located north of the Assifar area was visited in a brief geologic reconnaissance (plate 1). Rocks in the area are mostly fine grained, bedded metasedimentary clastic rocks and calcareous mudstones. Black shales ranging to carbonaceous shales underlie much of the area. In most places, carbonaceous rocks form wadi bottoms and are the loci of shear zones. Some evidence of mineralization is indicated by malachite staining and at least one ancient prospect pit. Two north-northwest-trending geophysical traverses were made at the northern and southern ends of AEM anomaly B-37. The data produced (not included in this report) are of little value in locating the AEM anomaly f,or two reasons: first, the profiles are parallel with the strike of the AEM anomaly and do not appear to have crossed it and, second, the datawere collected in an area crossed power lines and hence show only power

8

by recently constructed -line response. Because

of the negative geochemical results, further geophysical work was abandoned.

Mulhal No. 2

Mulhal No. 2, named by Smith and others(^K^^W \lies about 2.5 km north of Bilajimah (AEM anomalies B-25 and B-26; plate 1). No airborne electromagnetic anomaly was detected over the area probably because the survey aircraft was too high above the ground to record a response. A gossan crops out intermittently in an area more than 375 m long; host rocks are mostly quartz crystal pumice tuff interlayered with thin andesite beds. A syntectonic felsic intrusion lies about 1 km west of the gossan, and in the gossan zone aplite dikes extend outward from the intrusion.

Five geophysical traverses, each 500 m in length, were made across the gossan outcrop area. The geophysical data profile plots are included in this report as appendix 2. Anomalous SP values form a pattern that closely follows gos­ san outcrops over a length of at least 500 m along strike (plate 3). The amplitude of the SP response is less than -100 mV, a value which almost certainly precludes the pres­ ence of carbonaceous rocks as the source of the anomaly. As seen in profile form (appendix 2), the anomalous SP zone forms a broad response, several hundred meters long, coupled with several discrete minima; this pattern suggests multiple source zones, as would be expected over a shallow, vein-type mineral deposit.

The EM data (plate 3) reflect a multiple source response similar to that of the SP data. The interpreted widths of the conductors suggest the primary conductive zone is between 75 and 100 m wide and dips steeply to the east. Depth to the top of the EM conductor is estimated to be 20 m. The EM data profile (appendix 2) indicates a moderate in-phase (real) and out-of-phase (imaginary) EM response over the gossan outcrop area and suggests that a moderate to good conductor lies beneath the gossan.

The geophysical response of the Mulhal No. 2 area is quite similar to that recorded over the Sha'ab at Tare pros­ pect (plate 1), which is considered to be a low-grade massive sulfide deposit (Kiilsgaard and others, 1978). It seems very likely that further exploration in the Mulhal No. 2 prospect area would confirm the interpreted presence of a massive sul­ fide body at depth. Inasmuch as all of the known deposits of copper in the Wadi Bidah district are of low grade and lim­ ited in size, it seems most likely that such a body would be similar in size and grade.

SUMMARY OF GROUND-FOLLOWUP STUDIES

The ground-followup studies of i;he 1977 AEM survey of the Wadi Bidah district included three phases: 1977-8 (phase 1 studies), 1978-9 (phase 2), and 197JJ-80 (phase 3). Classifi-

based partly on the pre-cation of the 51 AEM anomalies wassurvey knowledge of the mineral potential of the source areas and partly on how readily apparent the source of the AEM anomaly was (Flanigan and others Wjtfs*-). In general, the ground-followup studies included a brief geologic examination of the source area by a geologist-geophysicist team and lim­ ited geochemical sampling in at least those anomalous areas for which the source rock was not roadily apparent. Based on these reconnaissance studies, the work was expanded in areas believed to have fair mineral potential to include reconnaissance geophysical traverses, which would accurately locate the anomalous AEM zones on the ground, to be followed by a somewhat closer examination by the exploration team. In two areas considered to have a somewhat greater mineral potential, detailed, geologic, geochemical, and geophysical studies were made, followed in two of the areas by diamond- core drilling and reexamination by the exploration team.

The following listing of the some pertinent facts about each, followup studies of the 1977 AEM district (plate 1).

AEM anomaly B-l

AEM anomalies, including summarizes the ground-

survey in the Wadi Bidah

This anomaly represents a conductive zone probably relat­ ed to the contrast in conductivity between a major fault zone and surrounding country rocks.other signs of mineralized rocks were observed (ARGAS, Flanigan and others, 1981, 1982).

AEM anomaly B-2

Reconnaissance geophysical surveys located an EM conduc­ tor, but no further followup studies, such as geologic map­ ping or geochemical sampling, were made. The zone contains the ancient mine of Mulgatah (MODS 00467); see previous re­ ports on the area by Earhart and Mawad (1970) and Kiilsgaard and others (1978). Phase 1 ground-followup studies were discussed by ARGAS (1978) and Flanig;an and others (1981, 1982).

AEM anomaly B-3

A long linear anomaly on the eastern side of Wadi Bidahis partly covered in the north by flow rocks. The zone is formationalthe meandering channel of Wadi Bidsih, and most probably re­flects conductive sediments in the

a thin layer of volcanic , following quite closely

wadi itself or possibly

10

high conductivity along a major fault zone thought to control the location of the wadi. The anomaly area is believed to have little or no mineral potential.

AEM anomaly B-4

This westward extension of anomaly B-3 is most likely related to the same source rocks, and it is thought to have little mineral potential.

AEM anomaly B-6

The anomaly lies within a north-trending zone, 1 to 2 m long, of quartz-sericite schist; a small gossan, from 4 to 5 m wide and about 100 m long, was found in the zone. The area is accessible by helicopter only. Geochemical sampling re­ sults, the limited gossan exposure, and difficult access dis­ couraged further work at this site. Although the gossan out­ crop is small, mineralization at depth cannot be ruled out.

AEM anomalies B-7 and B-8

Two reconnaissance geophysical traverses were made at each of these anomalies (ARGAS, 1978; Flanigan and others, 1981,/9?^). Both anomalies are on strike with anomaly B-l and probably reflect the same shear or fault zone. Geochemical sampling produced negative results.

AEM anomaly B-9

This anomalous area is associated with Gahab prospect (MODS 00468). No further work was done as part of this study; see Kiilsgaard and others (1978) for a discussion of this prospect.

AEM anomaly B-10

This anomaly is thought to be related to the same major fault zone as anomalies B-l, B-7, and B-8.

AEM anomaly B-ll and B-l8

Two geophysical profiles were made that cross both anom­ alies (Flanigan and others, 1981,/%^. No geochemical samples were taken, mineralized rocks are not evident on the surface, and no gossans are present in the area.

AEM anomaly B-12

This anomaly is thought to be formational and is most probably related to carbonaceous rocks similar to those in the anomaly B-13 area (see below).

11

AEM anomaly B-13

This anomaly is formational and extends at least 20 km in strike length. The conductive zone is composed of at least three individual conductors. The area has been studied in some detail and was discussed by Smith and Flanigan and others (1982).

AEM anomaly B-14

This anomaly was visited briefly carbonaceous schist is thought to be conductors. Geochemical sampling produced and no further work is thought worthwhile

by an exploration team; the source of the EM

negative results,

AEM anomaly B-15

A geologic examination within the ed a zone of highly altered schist siliceous gossanous material with ent for a strike length of more than pling indicated negative results, thought worthwhile.

from

100

AEM anomaly B-16

weatheredThis area contains highly having much the same appearance as area. Carbonaceous schists are the the EM conductor. Geochemical results

AEM anomaly B-17

anomalous area indicat- 5 to 10 m wide; some

abundant limonite is pres- m. Geochemical sam- no further work isand

dark-gray schist ocks seen in the B-14 most probable source of were negative.

A brief geologic visit revealed no obvious reason for this anomaly. A zone of altered schist, about 40-50 m wide, with abundant limonite might be the possible source. No vis­ ible signs of mineralized rocks were seen, and the geochem- ical sampling results were negative; therefore, no further work is thought worthwhile.

AEM anomaly B-18

See discussion of anomalies B-ll and B-18

AEM anomaly B-19

This anomaly is associated with the pect. Previous work at this site was and Mawad (1970) and Kiilsgaard and others

Sha'ab at Tare pros- described by Earhart

(1978).

12

AEM anomaly B-20

Anomaly B-20 is many tens of kilometers in strike length and is on the western edge of the Wadi Bidah AEM survey. It is thought to be formational, but no geologic reconnaissance was made in the area during these investigations»

AEM anomaly B-21

A brief geologic visit revealed a 100-m-wide zone of dark-gray to black carbonaceous schist, thought to be the source of the anomaly.

AEM anomaly B-22

The rocks in the area are predominantly metasedimentary rocks of volcanic origin(?); geochemical sampling produced negative results. Carbonaceous rocks are the most probable source of the EM conductors.

AEM anomaly B-23

Dark-green chlorite schist forms resistant ridges in the center of the anomalous area, and there is no apparent reason for the anomaly. Ground-geophysical methods are needed to determine the exact location of the AEM anomaly, but because little evidence of surface mineralization was seen and no anomalous metallic minerals were found by geochemical sam­ pling, no further work at the site is thought worthwhile.

AEM anomaly B-24

Carbonaceous schist is thought to be the major source of anomaly B-24. Previous work in the southern end of the anom­ aly was discussed by Allcott (1970) and Flanigan and others (1982).

AEM anomalies B-25 and B-26

Studies of these anomalies included reconnaissance and detailed mapping, geochemical sampling, and geophysical sur­ veying, accompanied by two drill holes. The results were discussed by Flanigan and others(/??(//9f«L) and Smith and others ( unpub. data, 1982).

AEM anomaly B-27

This anomaly was not investigated. An ARGAS report (1980) indicated a series of reconnaissance traverses over AEM anomaly B-27; these were, in fact, made over the Mulhal No. 2 prospect discussed earlier in this report.

13

AEM anomaly B-28

The B-28 anomaly is a single-line southern end of anomaly B-13. It has ground-EM methods. Its close proximity gests that it may be related to relationship is not known for certain.

carbonaceous

AEM anomaly B-29

Results of geologic, geochemical, and diamond drilling were discussed (1981, 1982) and Smith and others (unpab

AEM anomaly B-30

In a visit to the area, the exploration team found dark- gray schist that is very likely carbonaceous in content. Be­ cause no evidence of mineralization was seen and the geochem­ ical sample results were negative, no further work is thought worthwhile.

AEM anomaly at the not been located using

to anomaly B-13 sug- rock, but this

and geophysical studies by Flanigan and others

data, 1982).

AEM anomaly B-31

tostiear

Anomaly B-31 lies along strike B-29 and is most likely related to ceous rocks similar to those at anomaly undertaken in the ground-followup expected to yield positive results.

AEM anomaly B-32

A brief geologic visit indicated area are mostly dark gray to green

program

the south of anomaly zones and carbona- B-29. No work was because it was not

that the rocks in the chloritic schist. The

most likely source of the anomaly is carbonaceous-bearing schist. The geochemical samples yielded no anomalous metallic values, and no further work is thought worthwhile in the area.

AEM anomaly B-33

A geologic reconnaissance of this aly revealed no obvious source for th ical sampling produced no anomalous other work is thought worthwhile in

weak airborne EM anom- 3 anomaly. The geochem- metallic values, and no

area.t!ie

AEM anomalies B-34, B-35 and B-35A

Reconnaissance geophysical traverses located airborne EM anomalies, and detailed mapping by Kiilsgaard and others 0?ff>n82^ revealed that carbonaceous rocks correlate with con­ ductive zones delineated by the geophysical methods.

14

AEM anomaly B-36

Anomaly B-36 was not investigated but perhaps should be included in future exploration of the area, especially if a relationship is found between AEM anomalies and nearby mineralization.

AEM anomaly B-37

Reconnaissance geophysics surveys were not successful in delineating the airborne anomaly, as discussed in this report.

AEM anomaly B-38

This anomaly is very near anomaly B-37 and may well be part of the same conductive zone as B-37. However, few in­ dications of mineralized rocks were seen and the geochemical sampling results were not encouraging; therefore, no further work is recommended in this area.

AEM anomaly B-39

A geologic traverse in the B-39 area suggests that dark- gray fine-grained metavolcanic rocks predominate. A zone of highly altered schist, 50 to 70 m wide, crosses the area and marks the loci of a major fault zone. No gossans or other surface indications of mineralized rocks were observed. Geo­ chemical analysis of selected samples from the area produced negative results; no further work is thought worthwhile.

AEM anomaly B-40

Dark-gray quartz-sericite schist, which may contain sig­ nificant amounts of carbonaceous material, is the most prob­ able source of the EM anomaly. Geochemical results were .negative, and no further work is planned.

AEM anomaly B-41

Anomaly B-41 was not visited; it is on strike with anom­ aly B-42 and is most likely also related to carbonaceous rocks.

AEM anomalies B-42, B-44 and B-45-

Results of geologic, geochemical, and geophysical inves­ tigations in the northern parts of these anomalies were dis­ cussed earlier in this report.

15

AEM anomalies B-43, B-46 to B-51

These anomalies are thought to be related to formational and cultural sources rather than to mineralized zones; how­ ever, no work has been done to confirm this assumption.

CONCLUSIONS

Detailed geologic, geochemical, and geophysical studies in the Assifar area indicate moderate to low potential for economic mineralization. Self-potential values, coupled with moderate metallic contents of geochemical samples, suggest a possible mineralized zone beneath and immediately to the south of the Assifar ancient mine. Electromagnetic and self- potential anomalies detected with ground-geophysical methods are spatially associated with manganese-iron oxide-bearing metasedimentary rocks that may be the source of the geophys­ ical anomalies. However, it seems much more likely that car­ bonaceous rocks not recognized at the sur:ace are the source of the conductive zones producing the anom/.lies.

Geophysical data in the Mulhal No. 2 area are spatially associated with gossan outcrops and moderate geochemical cop­ per contents and suggest the possibility of mineralized rocks at depth over a strike length of 500 m. An exploratory drill hole is necessary to ascertain this assumption.

By far the highest percentage of magnetic anomalies detected during Wadi Bidah district are caused by a bonaceous material in some lithologic

the 51 airborne electro- the 1977 AEM survey in the very high content of car-

units.

16

REFERENCES CITED

Allcott, G. H., 1970, Diamond drilling at the Ma'dan ancient mine, in Mineral resources research 1968-69: Saudi Arabian Directorate General of Mineral Resources, p. 34- 39.

Arabian Geophysical and Surveying Company (ARGAS), 1978, Wadi Bidah (geophysical report), May-June 1978: Arabian Geo­ physical and Surveying Company (ARGAS) Report 78-ARG-10, 28 p.

Arabian Geophysical and Surveying Company (ARGAS), 1980, Ground follow-up at Wadi Bidah, April-May 1980: Arabian Geophysical and Surveying Company (ARGAS) Report 80-ARG- 07, 24 p.

Earhart, R. L. , and Mawad, M. M. , 1970, Geology and mineral evaluation of the Wadi Bidah district, Southern Hijaz quadrangle, Kingdom of Saudi Arabia: U.S. Geological Survey Saudi Arabian Project Report 119, 100 p.; also, 1970, U.S. Geological Survey Open-File Report (IR) SA-119.

Flanigan, V. J., Wynn, J. C., Worl, R. G., and Smith, C. W., 1981, Preliminary report on geophysical ground follow-up of the 1977 airborne survey in the Wadi Bidah district, Kingdom of Saudi Arabia: U.S. Geological Survey Saudi Arabian Mission Technical Record 9 (Interagency Report 348), 54 p.; also, 1982, U.S. Geological Survey Open-File Report 82-202.

Flanigan, V. J. , Sadek, Hamdy, and Smith, C. W., 1982, Phase 3 geophysical studies in the Wadi Bidah district, Kingdom of Saudi Arabia: Saudi Arabian Deputy Ministry for Min­ eral Resources Open-File Report USGS-OF-02-45, 44 p.; also, 1982, U.S. Geological Survey Open-File Report 82- 597.

Geoterrex Limited, 1977, Airborne electromagnetic survey Barringer "Input" system: Unpublished report available from the Saudi Arabian Deputy Ministry for Mineral Resources, Jiddah, p. 76-135 and 373.

Greenwood, W. R. , 1975, Geology of the Al 'Aqiq quadrangle, sheet 20/41D, Kingdom of Saudi Arabia: Saudi Arabian Directorate General of Mineral Resources Geologic Map GM-23, 15 p., scale 1:100,000.

17

maissiveJackaman, Barry, 1972, Genetic and env

trolling the formation of the of Wadi Bidah and Wadi Wassat, Saudi bian Directorate General of Mineral Record TR-1972-1, 244 p.

ironmental factors con- sulphide deposits

Arabia: Saudi Ara- Resources Technical

Kiilsgaard, T. H. , Greenwood, W. R., ! Puffett, W. P., Naqvi, Mohammed, Roberts, R. J., Worl, R. G., Merghelani, Habib,Flanigan, V. J. , and Gazzaz, A. R ration in the Wadi Bidah district,Saudi Arabia: U.S. Geological Survey Saudi Arabian Pro­ject Report 237, 89 p. ; also, Survey Open-File Report 78-771.

Kiilsgaard, T. H., 1981, Geology of areas marked by geophysi­ cal anomalies (B-35 and B-34), Wadi Bidah district, King­ dom of Saudi Arabia: Saudi Arabian Deputy Ministry for Mineral Resources Technical Record USGS-TR-01-1, 34 p.;also, 669.

1982, U.S. Geological Survey Open-File Report 82-

1978, Mineral explo- 1971-1976, Kingdom of

1978, U.S. Geological

Roberts, R. J. , 1976, The genesis of disseminated and massivesulfide deposits in Saudi Arabia:Saudi Arabian Project Report 207, 54 p.; also, 1976, U.S.Geological Survey Open-File Report

Roberts, R. J., Greenwood, W. R. F. C. W., and Kiilsgaard, T. H. , in western Saudi Arabia: U.S. Arabian Project Report 201, 60 Geological Survey Open-File Report

U.S. Geological Survey

76-602.

Worl, R. C. , Dodge, 1975; Mineral deposits

Geological Survey Saudi p.; also, 1975, U.S. 75-654.

Wynn, J. C., and Blank, H. R., 1979, A of the 1977 INPUT survey on the A of Saudi Arabia, with guides f ground follow-up: U.S. Geological Project Report 268, 32 p.; also, Survey Open-File Report 79-1508.

preliminary assessment rabian Shield, Kingdom >r interpretation and Survey Saudi Arabian

1979, U.S. Geological

18

Appendix 1. Data profiles from Assifar study area

Explanation

L *= J?OOm- Slingram coil separation, in meters

X *= 5O ** Slingram measurement interval, in meters

4C.1 Real or in-phase electromagnetic (EM) response, in percent of the primary field

Imaginary or out-of-phase EM response, in percent of the primary field

44.4 Hi Frequency of measured EM response

| i { Slingram traverse showing station locations in meters 0 I

^M/r6/' ^^- Total-intensity magnetic response, in gammas

Self-potential (SP) response, in millivolts, measured in reference to a single fixed electrode

;TO|>OAltitude along geophysical traverse, in meters,measured from an arbitrary base elevation

Interpreted EM conductor, showing approximate location and width of anomalous zone

' '....ii.iilil Interpreted SP source, showing approximate location and width of anomalous zone

Interpreted location of local magnetic anomaly

19

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Appendix 2.--Data profiles from Mulhal No. 2

Explanation

L 20Otn- Slingram coil separation,

50

study area

in meters

Slingram measurement interval, in meters

Real or in-phase electromagnetic (EM) response, in percent of the primary Jfield

Imaginary or out-of-phase the primary field

EM response, in percent of

4.44 Hz Frequency of measured EM response

Slingram traverse showing

Total-intensity magnetic response, in gammas

response,Self-potential (SP)in reference to a singl

in millivolts, measured e fixed electrode

Altitude along geophysical measured from an arbitrary

Interpreted EM conductor, and width of anomalous

station locations in meters

traverse, in meters, base elevation

showing approximate location :one

Interpreted SP source, showing approximate location and width of anomalous zone

Interpreted location of local magnetic anomaly

42

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