OCCURRENCE OF NATIVE COPPER, SILVER AND GOLD IN BASALTIC LAVA FLOWS

Native copper, silver and gold were found in lava flows from Kilauea, Mauna Loa volcanoes, Hawaii, and pillow basalt from Chile Ridge. This is the first finding of native Au in Kilauea basalt after report of native gold bleb in submarine basanite, and the first discovery of native Ag in basalt. Cu is present as native metal, Cu-(Fe)-oxide, and a rare, poorly defined Cu-sulfide-oxide phase. Cu-oxide was most common in Mauna Loa samples, in which they are intergrown with Fe-(Ti)-oxide. Native copper metals in Hawaiian and MORB samples range about 20-40μm in diameter and have a spheroidal shape. Silver is present mainly as native metal, and two silver sulfides were found in Hawaiian samples. Native silver metals in Hawaiian samples are generally small in size (<20μm), but can reach 40μm in diameter in MORB sample. Minor Cu (<8 wt%) was detected in most native silver in the Kilauea and MORB samples. Small amount of Cl (<1wt%) was also detected in most native silver in MORB sample. Native gold metals are most abundant in Kilauea samples. They range from 5μm to 50μm in size and some of them appear to occur inside cracks of groundmass glass. The smaller grains have a spheroidal shape and the larger ones were more irregular. No other elements are alloyed with native gold.

Except for the coupled Cu-Ag occurrence in Kilauea and MORB samples, the three metal elements rarely precipitate in native forms together, suggesting different precipitation mechanisms. Both Cu and Ag are strongly chalcophile, and Ag or Cu sulfides found in Hawaiian samples reflect residual sulfide liquid after minor shallow-depth degassing. Cu-oxide phase is most likely altered Cu-sulfide following extensive sulfur degassing. Native Cu precipitation occurred after nearly complete S-loss in Cu-bearing sulfides. Under sulfide saturation in MORB, sulfide liquid is minor in melt and most Cu will remain in melt and form native metal late in differentiation (with <22% melt). A two-phase partition model under equilibrium crystallization suggests that in both Hawaiian and MORB samples, Au and Ag can saturate in silicate melt and precipitate native metals, after 99% liquid has crystallized ± S degassing. In the MORB, native Ag precipitation is also likely aided by a Cl-rich hydrothermal fluid after late volatile saturation in the liquid.