Electromagnetic, magnetic, and gravimetric surveys at the Bi'r Jarbuah gold prospect, Kingdom of Saudi Arabia

Bi'r Jarbuah is a potential gold-bearing site located east of the Ishmas gold district in the southeastern part of the Arabian Shield. Surface rocks are mostly diorite and granodiorite plutons and the metavolcanic and metasedimentary rocks they have intruded. Extensive thin alluvial deposits cover approximately 40 percent of the area studied. Diorite, granodiorite, aplite porphyry, granite, granite pegmatite, and mafic dikes intrude the plutons and the metavolcanic and metasedimentary rocks. Veins of quartz, carbonate materials, and associated carbonate-altered rocks containing limonite and hematite cut all rock types. The veins were extensively mined by ancient people, and recent trenching has revealed small amounts of free gold associated with these veins and adjacent altered country rocks. A detailed search for gold and associated minerals was begun in the Bi'r Jarbuah area in 1988. Crone electromagnetic (CEM), magnetic, and gravimetric surveys were run in the areas of greatest interest. Anomalous areas are most interesting in the southern part of the area where linear magnetic and gravity anomalies trend east-northeast and overlap in large part. They are most prominent at or near the south end of a diorite pluton where some quartz veins mined by the ancients also trend northeast. A second area, at the extreme southern end of the survey, contains a large CEM anomaly that coincides with northeasttrending magnetic and gravity anomalies. Although this second area is largely overlain by alluvium, a major quartz vein strikes to the northeast in the adjacent bedrock. TT^TRODLJCTION Quaternary sedimentary deposits. Most of the exposed rocks are metamorphosed sedimentary and « > T u v. /\fnr\c IACA\ \A u volcanic rocks that have been intruded by diorite and Bi'r Jarbuah (MODS 1454) is a gold-bearing ,. . . ,, _ ^ : . . . f . .. , . , ° . , ,,° granodiorite plutons. The Proterozoic intrusive, area of potential economic value. It is in the e ,. v , . . . , ,, J ^ e .. A ,. ,. ., ,_,. , N metasedimentary, and metavolcanic rocks are all cut southeastern part of the Arabian Shield (Figure 1), , J ,. ... ... , , , .. T f ,, ,. . . . , . ., .1 by numerous granodionte, aplite porphyry, granite, east of the Ishmas gold district and in the northern J ^ *:., , JT ,., r ' 3 *f ! * f *t T u i f v «7 jT. »ti-*t. u L i* granite pegmatite, and mafic dikes. Quartz and part of the Jabal Ishmas-Wadi Tathlith gold belt . » * « i ^ /«7 i imnx TV i * n-» T if i. carbonate veins cut all rock types. (Worl, 1979). The rocks at Bi'r Jarbuah are -^ . . , JV j n 4. j ^ j u Extensive ancient exploration and mining is Proterozoic in age, and are partly covered by.,.,,, rr ,, & 0 * J J indicated by the presence of many trenches, dumps, and particularly the scattered milling stones that imply the presence of gold. More recent geologic investigations of the Bi'r Jarbuah ancient mine sites have determined that Bi'r Jarbuah may still contain ore of economic value (Worl, 1979). Consequently, geologic mapping, geochemical and geophysical surveying, trenching (during which a small amount of free gold was found), and drilling were begun in 1988. This report describes the geophysical surveys and the geologic interpretation of the geophysical data from these investigations. In order to conduct systematic geophysical studies at Bi'r Jarbuah, a station grid was surveyed that encompassed most of the ancient mine sites and contiguous areas. The grid was 2 km long by 1/2 km wide, and was marked by stakes placed on 50-m centers. The grid origin (0,0) is at lat 20°52.1T N., long 43°35.01' E. All directions in this report are referenced to true north. Grid north is N. 25°E., but station coordinates are given in terms of simply N-S and E-W on this grid. Electromagnetic, magnetic, and gravimetric surveys were then carried out over the gridded area. Both the gravity and the magnetic surveys occupied all the 50-m surveyed grid points. The gravity survey occupied additional stations at 25-m intervals along the N-S baseline. The electromagnetic (EM) surveys occupied all the grid points to the east; some EM grid points over the diorite pluton to the west were not used because the data in this part of the grid are relatively featureless. Additional EM stations were added to the southeastern part of the grid to help define significant anomalies. The various reduced geophysical data are presented on individual contour maps of equal scale. A summary geophysical map with generalized geology is also included for the mutual comparison of the geophysical anomalies and their correlation with geology. The geophysical surveys were conducted in accordance with the Ninth Extension to the Work Agreement between the Directorate General of Mineral Resources (DGMR), Jeddah, Saudi Arabia, and the United States Geological Survey (USGS) Saudi Arabian Mission under Subprojects 6.22.03 and 3.11.29.