In the axial metalliferous oozes of the East Pacific Rise, accessory amounts of shiny metallic grains occur. Optical microscope, SEM, microprobe and XRD analytical techniques have permitted detailed mineralogical study of these grains and established a hitherto unknown (in seafloor sediments, rocks and ores) Ag71.5Cu28.5 alloy. An endogenous hypothesis of the formation of the extremely small amounts of these alloy particles dispersed in the metalliferous sediments is suggested on the basis of a precise investigation of alloy micromorphology, chemical composition, inner texture and temporal distribution. According to the proposed hypothesis, native silver-copper grains are formed as ultra-accessory micro-segregations in a silicate matrix during the mantle pre-chamber evolution of the basic and ultrabasic systems beneath the spreading zones, under high P-T and low fO(2)-fS(2) conditions. The alloy formation was probably realized with falling temperature in the following sequence: crystallization of beta crystals (T congruent to 785 degrees C) --> crystallization of alpha + beta eutectic mixture (T = 779.4 C) --> alpha and beta exsolution (T < 779.4 degrees C) to an end composition Cu + Ag-Cu eutectic + alpha(ii) (Ag-ii). Mantle convection and plumes transport the silicate melt matrix and the silver-copper segregations contained therein to the earth's crust. The short residence time of the magma in the shallow crust chamber and its rapid crystallization in the crust and neovolcanic rift zone are responsible for the preservation of metallic grains in the silicate groundmass. During the cyclic tectono-magmatic processes in the rift zone, the host basic ridge rocks have undergone desintegration and native Ag71.5Cu28.5 alloy, together with other rock clasts, disperse into the near-axial metalliferous sediments. The maximum quantity of native silver-copper particles occurs in sediments formed during periods of intense volcanic activity.