Vapor transport of silver and gold in basaltic lava flows
© 2019 Geological Society of America. We documented occurrences of native copper (Cu), silver (Ag), and gold (Au) in a pāhoehoe flow from Kīlauea volcano (Hawaii, USA), an ā flow from Mauna Loa volcano (Hawaii), and a mid-oceanic-ridge basalt (MORB) from the Chile Ridge (southeastern Pacific Ocean). Native Ag in Kīlauea and MORB samples consistently contained minor Cl (< 1 wt%). Native Ag in Hawaiian basalts can occur at the center of nearly circular patches of relatively evolved minerals, which presumably formed after late-stage silicate liquid infilled pipe vesicles. Sulfur loss and oxidation of a Cu-sulfide phase can explain the native Cu, but not Au and Ag deposition. The rare occurrence of native Cu-Au-Ag alloys and the large native Au and Ag grain size suggest separate metal precipitation mechanisms. A fractional crystallization and degassing model envisions initial Au and Ag enrichment in crystallizing interstitial liquid and further enrichment in a separating vapor phase. From the flow interior, the metals ascend through ephemeral pipe vesicles as bisulfide (Au) or chloride (Ag) vapor complexes and precipitate in the transition zone below the upper vesicular zone, owing to temperature and oxidation state changes. Our results support igneous vapor transport of ore elements in mafic plutonic systems and imply preconcentration of gold during lava solidification before later hydrothermal remobilization.
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