NATIVE SILVER-COPPER ALLOY IN METALLIFEROUS SEDIMENTS FROM THE EAST PACIFIC RISE AXIAL ZONE (20-DEGREES 30'-22-DEGREES 10'S)

In the axial metalliferous oozes of the East Pacific Rise, accessory a mounts of shiny metallic grains occur. Optical microscope, SEM, microp robe and XRD analytical techniques have permitted detailed mineralogic al study of these grains and established a hitherto unknown (in seaflo or sediments, rocks and ores) Ag71.5Cu28.5 alloy. An endogenous hypoth esis of the formation of the extremely small amounts of these alloy pa rticles dispersed in the metalliferous sediments is suggested on the b asis of a precise investigation of alloy micromorphology, chemical com position, inner texture and temporal distribution. According to the pr oposed hypothesis, native silver-copper grains are formed as ultra-acc essory micro-segregations in a silicate matrix during the mantle pre-c hamber evolution of the basic and ultrabasic systems beneath the sprea ding zones, under high P-T and low fO(2)-fS(2) conditions. The alloy f ormation was probably realized with falling temperature in the followi ng sequence: crystallization of beta crystals (T congruent to 785 degr ees C) --> crystallization of alpha + beta eutectic mixture (T = 779.4 C) --> alpha and beta exsolution (T < 779.4 degrees C) to an end comp osition Cu + Ag-Cu eutectic + alpha(ii) (Ag-ii). Mantle convection and plumes transport the silicate melt matrix and the silver-copper segre gations contained therein to the earth's crust. The short residence ti me of the magma in the shallow crust chamber and its rapid crystalliza tion in the crust and neovolcanic rift zone are responsible for the pr eservation of metallic grains in the silicate groundmass. During the c yclic tectono-magmatic processes in the rift zone, the host basic ridg e rocks have undergone desintegration and native Ag71.5Cu28.5 alloy, t ogether with other rock clasts, disperse into the near-axial metallife rous sediments. The maximum quantity of native silver-copper particles occurs in sediments formed during periods of intense volcanic activit y.