Geophysics Applied to Phosphate Exploration in Northern Saudi Arabia

Jeff Wynn, U.S. Geological Survey, Reston, VA 20192

(Presented at the Society of Exploration Geophysicists Annual Meeting, 12 November 1996)

Contents: Summary || Introduction || Well-logging: Gamma-ray and caliper || Microgravity and cavities || Conclusions || References || Acknowledgements


Geophysical methods have proven useful for mapping the large phosphate deposits in the Al-Jalamid region of northern Saudi Arabia. They were used as an integral part of the mine feasibility study completed there in 1992. Friability (which affects economic value) and phosphate content can be rapidly determined using caliper and portable gamma-ray logging tools. Once the relationship between the gamma-ray count rate and phosphate content are established, integration of gamma-ray activity over the extent of the drillhole provides a rapid estimate of the phosphate resource. The limits of the economic deposit can thus be fairly easily outlined. At the weathering horizon the ratio between the gamma-ray activity and phosphate content changes substantially, and refraction seismics and the caliper log are essential for knowing where this transition takes place.

Photo of the plains of Al-Jalamid, pointing to the Iraqi border.
A view of the Al-Jalamid area on the plains in northern Saudi Arabia. In the photo the author is pointing north towards Iraq, which lies just beyond the horizon. Worked stone chips and an antler tool from a prehistoric hominid arrow-head workshop were found where the author is standing.

The Al-Jalamid area is a featureless, wind-swept, sand-covered plain; extensive drilling experience, however, has shown that this deceptively flat surface masks a karst terrane, honeycombed with cavities, as circulation is lost in 25%-30% of drillholes initiated. Microgravity profiles have proven more useful than refraction seismics for locating the hidden cavities. Since these cavities are encountered throughout the Al-Jalamid area, geophysical methods will be an essential tool in the development of a mine and its associated mill site.

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Commercial phosphate deposits are associated with the Mesozoic Tethys seaway, and are found from Morroco eastward to Iraq and beyond; large deposits are actively mined in Morroco, Iraq, and Jordan (Kluyver and others, 1981). The U.S. Geological Survey (USGS) has had a long involvement in Saudi Arabia, and initially identified phosphate deposits in northern Saudi Arabia in the early 1960's (Sheldon, 1965). The Rio Finex Mission carried out extensive prefeasibility studies there between 1978 to 1981 (Kluyver and others, 1981). From 1987 to 1993, the USGS conducted a regional phosphate resource assessment and completed a mine Feasibility Study in the Al-Jalamid area ("FS" in figure 1; Sheldon, 1991; Jacobs Engineering, 1993).

Figure 1, location of the Al-Jalamid phosphate deposit, Saudi Arabia.
Figure 1. Location map of the Feasibility Study area ("FS") of the Al-Jalamid phosphate deposit, northern Saudi Arabia. The location implies a genetic linkage with the Jordanian phosphate deposits across the border to the northwest.

Zablocki and others (1989) conducted an experimental set of telluric profiles and some audio- magnetotelluric (AMT) soundings in the area. Sheldon (1992, written comm.) reports that these are helpful in mapping the water table and the vadose zone; high-grade phosphate is generally associated with a thick vadose zone. From 1990 to 1993 the USGS geophysics group based in Jeddah carried out a series of field measurements aimed at assisting both the phosphate resource exploration, and the assessment program and mine feasibility studies. The work focused on three areas: (1) surface gamma-ray and seismic refraction surveys to map the phosphate resource and the weathered (friable) zone, (2) borehole natural gamma-ray and caliper logging on a kilometer-spaced grid to map the 3rd dimension, and (3) microgravity and refraction seismic experiments to identify subsurface cavities.

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Natural gamma-ray methods have been applied to phosphate deposits for at least four decades (Moxham, 1954, 1960; Espenshade, 1959; Habashi, 1966; Jacobs Engineering, 1993). Ammar and others (1988) report relatively constant ratios of uranium vs. phosphate for most deposits in Egypt. These efforts take advantage of the genetic association of uranium with apatite, which forms with phosphate deposits. The actual radioactivity detected is from Bi-214 and other a daughter products of the decay of uranium. Moxham (1960) reported a typical ratio of 0.018 percent uranium for 14 percent P2O5 in deposits in Florida.

Figure 2, gamma-ray counts versus percent phosphate in a drillhole.
Figure 2. Percent P2O5 versus gamma-ray counts in counts per second (cps), drillhole #755 from the eastern side of the Al- Jalamid phosphate deposit. There are two populations in the sample, which can be segregated according to lithology. This figure shows a higher uranium/P2O5 ratio in the weathered upper layer (silicified limestone) than in the dolomite beneath. This higher apparent ratio is probably due to the friability of the weathered layer and increased volume sampled by the gamma-ray tool.

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Well-logging: Gamma-ray and caliper

A detailed multi-year program of shallow percussion drilling followed closely by natural gamma-ray logging was conducted at the Al-Jalamid deposit simultaneously and in support of the mine feasibility study completed in 1993 (Wynn and others, 1994). Several hundred drillholes were placed in the deposit and the immediately surrounding region on a 1-kilometer grid. Drillholes were nominally completed to 20 meters in depth, but caving and debris often limited the logging to typically 18.5 - 19.0 meters. Figure 2 shows a natural gamma-ray log of drillhole #755, located on the eastern edge of the Al-Jalamid deposit; superimposed are chemical analyses (sampled over 0.5 meter intervals) of percent P2O5 content. The resolution of a gamma-ray log is on the order of a few centimeters, but is here digitized at 0.5 meter intervals for comparison.

The phosphate deposits on northern Saudi Arabia are organic sediments of Mesozoic age, overlying Paleozoic sediments that are gently folded. The unweathered phosphate ore at Al-Jalamid is dolomitic, and is never encountered at the surface. Compact calcareous ore is formed by de-dolomitization, and progressive calcite solution of the calcareous ore forms the friable surface layer of the deposit. In the U.S. Colorado Plateau and in Iraq, as well as phosphate deposits to the west of Al- Jalamid, laterite weathering breaks down the uranium-containing apatite to form aluminum phosphate minerals largely stripped of uranium, which is deposited in and around the phosphate as secondary yellow uranium minerals. This has not been observed at Al-Jalamid, and in fact the gamma-ray log in Figure 2 (typical of the area) suggests that the uranium-to-phosphate ratio in the near-surface weathered (0 - 6 meter) zone is enhanced. A more reasonable explanation is that the more porous friable ore allows uranium daughter product gases better access and more surface exposure to the gamma-ray tool, giving an artificially increased apparent uranium-to-phosphate ratio.

Extensive sampling and comparison have shown a consistent relationship between percent phosphate and gamma-ray counts in the dolomite ore below the weathered zone, on the order of 0.125 x gamma-ray counts = % P2O5. Above the weathered zone, the working ratio is between 0.04 (limestone and silicic limestone) and 0.055 (chert and limestone-chert) x gamma-ray counts = % P2O5. These ratios are determined empirically only for the Al-Jalamid deposit. The transition between the weathered (readily millable) and the dolomite ore is obvious in Figure 2 at around 6 meters depth. These ratios can be used to roughly integrate gamma-ray counts over the length of the drillhole to get a quick approximate estimate of the mineable phosphate in place. Determining the approximate depth of weathering is necessary in order that the calculation be reasonably accurate, however. This depth can be obtained most efficiently using refraction seismic methods: the velocity in the weathered zone averages 0.8 km/sec, whereas velocities in the dolomite zone range between 2.5 and 4.0 km/sec, depending on the degree of karsting (more on this later). Additionally, the caliper log can sometimes be used to identify where the weathered zone ends and the dolomite takes over. This transition is rarely, if ever, sharp.

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Microgravity and cavities

Figure 3, microgravity profile.
Figure 3. Two north-south-oriented parallel microgravity profiles (Free Air), spaced 50 meters apart, in the vicinity of drillhole #623, near the center of the Al-Jalamid phosphate deposit. The site was chosen because the first four drillholes at this site were lost due to circulation failure and loss of at least one drill-stem. The station spacing is five meters, and the data are elevation-corrected to 1 centimeter precision. No terrain corrections were made because the area is essentially flat. Residuals are shown after the removal of a regional trend by a regression fit. Evidence of a tunnel-like cavity can be seen between stations 180 and 190.

Seismic refraction surveys used to map the weathered ore zone have sometimes shown anomalous lower velocities in areas where drillholes were lost to caving or circulation failure, but the data are difficult to interpret and the horizontal resolution is poor. This is probably because the solution cavities are complex three-dimensional structures. For this reason, microgravity profiles were evaluated as a cavity-detection tool. Figure 3 shows a pair of microgravity profiles (5-m station spacing) conducted over one of these anomalous low-velocity zones. The profiles traverse north-south and are parallel, separated by 50 meters. The ground surface is essentially flat, but we made elevation corrections to 1-cm accuracy in the gravity data nevertheless. A regression fit was subtracted from each profile, to remove any regional trend, and the residuals are here compared. Between stations 175 and 190, evidence of a tunnel is apparent in the data. This was verified by circulation loss in a shallow well at this point on one of the profiles. Annecdotal evidence indicates that these cavities are very common; failure to anticipate them in 1993 caused a one-million dollar, two-week loss of time in a large waterwell drilling effort critical to the feasibility study. We believe that any mill site will have to be carefully chosen after extensive high- resolution geophysical surveys to certify their absence.

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Statistical analysis of the gamma-ray counts versus the phosphate content in 88 drillholes supports the conclusion that gamma-ray counts can be converted to percent phosphate if the host-rock lithology is known or can be reasonably guessed (See Figure 4).

Figure 4, relative phosphate from borehole geophysics, Eastern Al-Jalamid.
Figure 4. Relative phosphate content from borehole logging, eastern Al-Jalamid area. This figure represents the results of integrating the gamma-ray counts over ~60 drillholes, each averaging about 20 meters deep. The integrand was then scaled using analytical chemistry results from several of the holes to obtain an approximate estimate of phosphate in the ground.

If the weathered zone is mapped using seismic refraction methods, the gamma- ray logs can be integrated over the depths of the drillholes to provide a rough phosphate-in-the-hole number for each drillsite. This evaluation was done as part of this study, and the numbers were used to roughly outline the phosphate deposit. These numbers depend, of course, on a careful calibration of gamma-ray counts to phosphate over a number of holes whose stratgraphy is well understood and representative of the site as a whole. With certain assumptions about friability (obtained from seismic refraction profiling and caliper logs), rough economic boundaries could be drawn around the Al Jalamid deposit. Microgravity surveys proved to be more useful than refraction seismic methods in identifying zones with cavities in the underlying karst terrane that could cause potentially serious problems with site development.

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Ammar, A.A., Abdelhady, H.M., Soliman, S.A., and Sadek, H.S., 1988, Aerial radioactivity of phosphates in the western central eastern desert of Egypt: Journal of King Abdulaziz University - Earth Science, 1, pp. 181-203.

Espenshade, G.H., 1959, Geologic features of areas of abnormal radioactivity south of Ocala, Marion County, Florida: U.S. Geological Survey Bulletin 1046-J, p. 205-220.

Habashi, Fathi, 1966, Radioactivity in phosphate rock: Economic Geology, 61, pp. 402-407.

Jacobs Engineering, 1993, Mine feasibility study for the Al-Jalamid phosphate deposit: Confidential report submitted to the Deputy Ministry of Mineral Resources, Kingdom of Saudi Arabia, 13 volumes.

Kluyver, H.M., Bege, V.B., Smith, G.H., Ryder, J.M., and van Eck, M., 1981, Sirhan-Turayf phosphate project - results of work carried out under the phosphate agreement, 29 Dhul Hijah 1398 - 30 Jumad Thani 1401 (29 Nov 1978 - 4 May 1981): Technical Record, Rio Finex Mission in Saudi Arabia, RF-TR-01-5, 77 p., 4 plates, 10 tables.

Moxham, R.M., 1954, Airborne radioactivity surveys for phosphate in Florida: U.S. Geological Survey Circular 230, 20? p.

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Moxham, R.M., 1960, Airborne radioactivity surveys in geologic exploration: Geophysics, 25, pp. 408- 432.

Sheldon, Richard P., 1965, Discovery of phosphate rock in Saudi Arabia and recommended program for further study: U.S. Geological Survey Saudi Arabian Project Technical Letter 22, 9 p.

Sheldon, R.P., 1991, Calculation of stoichiometric modal mineral analyses of Al-Jalamid rocks, Kingdom of Saudi Arabia: U.S. Geological Survey Saudi Arabian Mission Data-File Report USGS-DFR- 91-1, 9 p.

Wynn, J.C., Bazzari, M., Bawajeeh A., Tarabulsi, Y., Showail, A., Hajnoor, M.O., Techico, L., and Wynn, J.P., 1994, Phosphate Content Derived from Well Logging, Al Jalamid Phosphate Deposit, Northern Saudi Arabia: U.S. Geological Survey Mission Data-File Report IR-869, 9 p.

Zablocki, C.J., Showail, A, Hajnoor, M.O., Kincar, A.R., and A. Saeed, 1989, Telluric profile and audio- magnetotelluric studies of the Al Jalamid phosphorite deposit, Kingdom of Saudi Arabia: USGS Mission Administrative Report, 15 pp., 12 figures, 2 Appendices.

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The author owes much to the supportive field crew who helped acquire the data in Saudi Arabia: Abdullah Showail, Maher Bazzari, Mohammed 'Omar Hajnoor, Leony Techico, Abdullah Bawajeeh, Yahya Tarabulsi, and Jared Wynn.