Genesis and evolution of the Baid al Jimalah tungsten deposit, Kingdom of Saudi Arabia

Abstract

The Baid al Jimalah tungsten deposit flat 25°09 N., long 42°41' E.) is a swarm of steeply dipping, sheeted, tungsten-bearing quartz veins. It is spatially, temporally, and genetically associated with a 569 Ma, highly differentiated, porphyritic granite that intrudes late Proterozoic, immature sandstones of the Murdama group. The bulk of the vein constituents came from hydrothermal fluids exsolved from a granite cupola at a depth of about 3.1 km during a single cycle of magma intrusion and hydrothermal mineralization. Hypogene mineralization can be divided into 3 main periods: early quartz-molybdenite stockwork veins, wolframite- and scheelite-bearing greisen veins, and late, barren veins. Each of the three periods can be divided into several stages that are transitional to each other. The greisen veins, in particular, show replacement of earlier mineral assemblages by later ones. The veins at Baid al Jimalah East, approximately 1.5 km to the east of the Baid al Jimalah tungsten deposit, are genetically related to it and probably formed while the greisen mineralization was being deposited. Early stockwork mineralization was formed near magmatic temperatures (580°-700°C) from low salinity fluids (1-2 weight percent NaCl equivalent). Two fluids were present, one low density and CO 2 rich, the other high density and H 2 O rich. Greisen mineralization was formed from fluids in the liquid state at temperatures mostly between 390° and 430°C with salinities between 4.5 and 10.9 weight percent NaCl equivalent. Late, barren mineralization formed from liquids with salinities between 0.5 and 3.0 weight percent NaCl equivalent and at temperatures at least as low as 238°C. The veins at Baid al Jimalah East formed from liquids between 0 and 4.2 weight percent NaCl equivalent at temperatures largely between 300° and 375°C. Depth of mineralization was greater than 3.1 km. The temperatures given above have been corrected for pressure assuming this depth, and are 75-80°C higher than the fluid inclusion filling temperatures. Important volatile constituents of the hydrothermal fluids were CO 2 and CH 4 , in addition to H 2 O and HF. Baid al Jimalah is similar in character and origin to other tungsten-tin greisen deposits in the world, especially the Hemerdon deposit in Devon, England. It is also analogous to Climax-type molybdenum deposits, which contain virtually identical mineral assemblages, but with the relative intensities of the molybdenum and tungsten mineralization reversed.