Sedimentary and epigenetic copper mineral assemblages in the Cambrian Timna Formation, southern Israel

Copper mineralization in the Timna Formation occurs in the dolomitic, sandy, and shaly lithofacies of the Upper Sasgon Member. Scanning electron microscope, chemical, and isotopic studies of the copper mineral parageneses indicate an evolution from diagenetic, neutral reducing conditions through stages of epigenetic alteration at oxidizing and progressively more acid conditions. Primary copper sulfide minerals are djurleite (Cu_1.93S) and covellite (CuS) with minor chalcocite (Cu_2-1.93S), digenite (Cu_t.8S), and anilite (Cu_1.75S) randomly dispersed in the dolomitic lithofacies. δ³⁴S values of -14‰ are consistent with their formation by reduction of marine sulfate. Subsequent evolution of the ores involves their replacement by malachite (Cu₂ (OH)₂CO₃), which armors and replaces the sulfide minerals, followed by the development of paratacamite (Cu₂(OH₃)CI) as veins emanating from the altered sulfides. The malachite alteration stage represents a change to oxidizing conditions; paratacamite develops with a decrease in pH and in the presence of chloride ion, which allows the development of stable aqueous copper chloride complexes. The first copper mineral to form in the sandy and shaly lithofacies is chrysocolla pseudomalachite (Cu₅(OH)₄PO₄), and dioptase (CuSiO_3·H₂O). Thermodynamic calculation indicates that copper silicate formation was brought about by a decrease in pH and/or increase in aCu and is thus consistent with the evidence for epigenesis in the dolomitic lithofacies.