Abnormal concentrations of boron, gold and heavy metals in the pleistocene and recent terraces at Wadi al Hamd, Northwestern KSA

Abstract EN

The contents of boron, gold and some other heavy metals in the Quaternary terraces at Wadi Al Hamd area are abnormally high.

The boron content can be used as paleo-indicator of salinity and pH conditions during deposition.

These B-rich terraces lack detrital tourmaline which is the commonest source of the element.

The Quaternary terraces of Wadi Al Hamd comprise a lower consolidated very coarse part of Pleistocene age followed by a much finer Recent upper unconsolidated alluvial terraces.

The Pleistocene terraces contain abnormal B content (161-345 ppm) in their coarse carbonate cement of conglomerate which is a strong indication of up to 12 during cementation.

A relatively lower paleo-pH value of 8-9 is assigned to the topmost part of the Pleistocene terraces as B content in its quartz-arenite is lower (161-185 ppm).

The upper Recent alluvial terraces are composed of silt and mud with kaolinite / illite (K / I) ratio in the range of 2.5 to 3 and have lower B content (16-40 ppm) owing to the dominance of kaolinite.

The given boron content in the latter suggests low pH (amounting 5-6.5).

It is believed that B is incorporated into the crystal structure of calcite cement possibly in the form of B (OH)-4 which is dependant on the pH value, being more enhanced by the incorporation of H+ into the calcite structure.

It is also believed that the organic matter plays a significant role in B fixation on plant remains of the clayey parts of the upper terraces.

The Ba and Sr contents show lowest values in the quartz-arenite at the top of the Pleistocene terraces suggesting elemental fractionation in more reducing conditions.

This also suggests sea transgression after the deposition of very coarse conglomerate in a lacustrine and hence the quartz-arenite was deposited under reducing conditions.

There is also distinct variation in the contents of two analyzed REE (namely Ce & La) and geochemically-related Y.

The silt-sized fractions of the mud cracks and the upper unconsolidated terraces are characterized by the presence of free native gold.

Considerable amount of economic gold amounting up to 14 and 6.1 g / t are assigned to both lithology's, respectively.

On the other hand, the exposed older Pleistocene terraces lack gold that might be accumulated at unexposed depths due to gravity settling.