MINERALOGY OF THE NEAR-SURFACE EXPRESSION OF AU-AS-CU MINERALIZATION IN AN ARID ENVIRONMENT

In the arid, Late Precambrian terrain of southern Israel, a complex su ite of minerals and amorphous species were deposited in host gneiss fr om fluids under near-neutral conditions within 1 m of the surface. The morphology of secondary gold appears to relate to its host mineral (s keletal-dendritic with quartz; multi-faceted crystals with arsenates; spherical droplets with iron oxide). The gold is very fine-grained, an d was most likely complexed as a thiosulphate. Three amorphous phases are present (iron oxide, chrysocolla, Cu-Mn-(Fe-As) silicate), At leas t in part, gold and baryte appear to have crystallized out of a metal- Fe-oxide gel. Other minerals, including apatite, anglesite, and conich alcite, may have grown from appropriate crystallites present in the ge l. The conichalcite occurs mainly as bladed to acicular radial spherul ites. In the presence of lead, a solid solution phase between duftite and conichalcite ('Pb-conichalcite') was formed.