Abstract

The Al Wajh gold district contains small deposits of gold-bearing quartz veins located in sheared and altered Neoproterozoic mafic lavas and volcaniclastic sandstone and siltstone. The veins formed during multiple episodes of deformation and have structural and mineralogic features characteristic of mesothermal, low sulfide, gold-bearing quartz veins. Three early deformation episodes (D 1 , D 2 , D 3 ) are interpreted as progressive phases of a major deformation event that culminated about 660 Ma; episode D 4 makes up part of a later event about 620 Ma. The bulk of the gold-bearing veins in the district are located in D 2 and D 3 structures. D 1 veins consist of thin quartz veins in phyllite. D 2 veins are located in throughgoing, steeply dipping sinistral shear zones and in the crests of folds along the shears. Da veins, typically associated with carbonate alteration, are concentrated in small thrust faults that in places modify earlier D 2 shears. Umm al Qurayyat, the largest deposit in the district (155,634 metric tons averaging 3.8 g/t gold), comprises shallowly to moderately steeply dipping sheeted quartz veins in the hanging wall of a D 3 thrust in basaltic andesite and dacite. It is centered on an inner sericite alteration zone and an outer orange-weathering carbonate alteration zone. Structural evidence indicates that sericite alteration was contemporaneous with D 2 deformation, predating the main quartz veining event, whereas carbonate alteration was contemporary with D 3 thrusting and emplacement of the deposit's principal gold-bearing quartz veins. Quartz at Umm al Qurayyat contains an average of 3.1 g/t gold and sericite-altered wall rock contains an average of 3.9 g/t. Basaltic country rock outside the deposit contains as much as 1 g/t gold, the higher grades occurring adjacent to the prospect.

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