Geology of the Ishmas gold prospect, Kingdom of Saudi Arabia

الملخص EN

T he area of the Ishmas go ld pro spe ct is und erl ain hy th e Uppe r Proterozoi c rocks of the Arabian Shield.

Th ese arc here int erpreted.

It1 part.

to represent an o phio litic assemblage a nd include serpentinized websterite and othe r alt ered ultramafics.

cumulate and massive gabbros , mctabusalts, rnctarhyol itc s and related rocks.

quartzites.

rnctadolornitic limeston e and ska rn.

all intruded hv granite bodies and dykes.

The cont act s betwe en the different rock units are 'gc ne rally tectonic.

The volc anic rocks ar c bimodal in natu re .

and were formed from a low K-tholciitic magma m a spread ing ocean floor environment.

Th e gahbros arc of a n oceanic type and the ultramafic rocks arc ophiolitic cumulates .

ch ar acterized by low contents of lithophile e le me nts .

So.

on the hasi s o f thi s.

and other field ob serv ation s, it is bel iev ed that these ro cks represent an ophiolite sequence, autochtho nous in nature.

formed as a result of rifting and subseque nt closure of a n ocean.

ROCks of the area ar c cut bv faults striking in a number o f directions.

nam ely: N-S.

45-60.

140-160 al{d ISS.

- Quartz vein s in the ar ea a rc beli eved to represent three stages o f vein format ion : an early stage (s triking N-S) .

an intermediate s tage (striking 140160 and 45-(0) and a late stage (striking l IS).

Th e intermediate stag e of vein formation was the most important gold-forming ep isode.

The go ld deposits are probably related spatially to the Najd fault system.

They arc hydrothermal.

e pige netic precious metal vein-type (bonanza) deposit s .

The sour ce of gold and associate d chalcophile clements is most probably the altered ultramati c ro cks.