Abstract

The Baid al Jimalah tungsten deposit in Saudi Arabia (lat 25 degrees 09'N, long 42 degrees 41'E) consists predominantly of swarms of steeply dipping, subparallel, tungsten-bearing quartz veins and of less abundant, smaller stockwork veins. It is spatially, temporally, and genetically associated with a 569 Ma, highly differentiated, porphyritic, two-feldspar granite that intrudes Late Proterozoic immature sandstones.Paragenetic data from crosscutting veins demonstrate unambiguously a single cycle of magma intrusion and hydrothermal mineralization. Hypogene mineralization can be divided into three periods: (1) early quartz-molybdenite stockwork veining, (2) wolframite- and scheelite-bearing, greisen-bordered veining, and (3) late, quartz-carbonate-fluorite veining. The first two of these three periods can be further divided into several stages that are transitional to each other. The greisen-bordered veins, in particular, show replacement of earlier mineral assemblages by later ones. Precious and base metal veins at Baid al Jimalah East, approximately 1.5 km east of the Baid al Jimalah tungsten deposit, are genetically related to the tungsten deposit and probably formed contemporaneously with the greisenized tungsten-bearing veins.Fluid inclusion and oxygen isotope data indicate that the Baid al Jimalah deposit formed over a temperature range of 120 degrees to 550 degrees C, from low salinity magmatic and metamorphic fluids, and at a depth of about 4.2 km. Early stockwork veins (period 1) formed at low magmatic temperatures (ca. 550 degrees C) from magma-derived (delta 18 O = 9.6-9.7ppm), low-salinity (1-2 wt % NaCl equiv) fluid. This hydrothermal fluid was generally low density and CO 2 rich. All other veins were formed from regionally derived fluid in equilibrium with metamorphic rocks (delta 18 O = 7.9 + or - 1.0ppm at the site of deposition). This fluid probably scavenged most of the period 2 ore-mineral components from a postulated granite batholith whose existence is indicated by a 6-mGal gravity low centered on the deposit. The greisen-bordered tungsten veins (period 2) formed from fluids in the liquid state at temperatures mostly between 380 degrees and 440 degrees C with salinities between 4.5 and 10.9 wt percent NaCl equiv. Late, barren veins (period 3) formed from liquids with salinities between 0.0 and 3.5 wt percent NaCl equiv at temperatures as low as 120 degrees C. The veins at Baid al Jimalah East formed from liquids with salinities between 0 and 4.2 wt percent NaCl equiv at temperatures mostly between about 340 degrees and 390 degrees C. Important volatile constituents in some hydrothermal fluids were CO 2 and CH 4 , in addition to H 2 O and HF. The delta 18 O data on mineral separates of fresh and altered Bald al Jimalah granite, and whole-rock delta 18 O data on country-rock samples as far as 16 km from the deposit, indicate that the rocks in the Bald al Jimalah area were pervasively infiltrated by a fluid with relatively high delta 18 O values. Interaction and exchange of the country rocks with this delta 18 O fluid led to an increase in the delta 18 O values of volcanic rocks of the Jurdhawiyah Group but to a decrease in the delta 18 O values of the high value delta 18 O Murdama Group sandstones, resulting in a hydrothermal anomaly exceeding 100 km 2 in area. This fluid had an estimated delta 18 O value of about 6 to 8 per mil, essentially identical to that of the metamorphic water calculated from the vein quartz, thus strongly supporting the conclusion that all of the mineral deposits at Baid al Jimalah (except for the early-stage quartz-molybdenite veins), as well as the 12-km 2 geochemical anomaly surrounding the deposit, were from the same metamorphic fluid.Bald al Jimalah is similar in character and origin to Phanerozoic tungsten-tin greisen deposits throughout 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 proportions of molybdenum and tungsten mineralization reversed, primarily owing to differences in oxygen fugacity. This similarity in mineralization styles and fluid histories indicates that metallogenic processes in granite-related deposits in the late Precambrian were similar to those seen in the Phanerozoic.

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