Rock geochemistry in the Mahd adh Dhahab district, Kingdom of Saudi Arabia

Abstract

Ten sets of geochemical samples from country rock, altered country rock, and quartz veins were collected to determine metal abundances and distributions in the Mahd adh Dhahab district. Gold, silver, copper, lead, and zinc are present throuhgout the district in veins and altered country rock. Although four generations of quartz veins are defined, there is complete gradation between types. The metals present are the same in all four, only the abundances vary. The concentrations of gold, silver, copper, lead, and zinc in altered country rock are zoned around known or projected mineralized-vein zones. Similarities in metal populations of all quartz veins, altered rock, and mineralized zones, plus the distribution of metals and some major elements, suggest that all formed as part of a single hydrothermal system. Gold in abundances greater than 1 ppm does not correlate with any other element and its distribution is very erratic. The variance in gold values between replicate samples collected in 0.5 m square panels is very high. Radio-element analyses (radium-equivalent uranium, thorium, and potassium) for 40 widely distributed sample localities indicate little variation in these elements throughout the district. If the hydrothermal system had an effect on the distribution of these elements, the system must have extended beyond the limits of the sampled area. Anomalous values of gold, silver, lead, and to a lesser extent copper and zinc in surface rock samples clearly delineated the northern mineralized zone in the upper agglomerate, and an east-vein area and west-vein area of the southern mineralized zone in the lower agglomerate. A third geochemically anomalous area occurs farther to the west in the lower agglomerate, suggesting that mineralization may have extended at least to this area along the lower agglomerate-lower tuff contact, and possibly even further to the west. U.S. Geological Survey, Denver, Colorado 2 U.S. Geological Survey, Jeddah, Saudi Arabia 8 Univ. of Michigan, Ann Arbor, Michigan * Marathon Oil Company, Denver, Colorado