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Book Chapter

An Immiscible Sulfide Melt from Hawaii1

By
Brian J. Skinner
Brian J. Skinner
Kline Geological Laboratory, Yale University, New Haven, Connecticut, and U. S. Geological Survey, Washington, D. C.
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Dallas L. Peck
Dallas L. Peck
Kline Geological Laboratory, Yale University, New Haven, Connecticut, and U. S. Geological Survey, Washington, D. C.
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Published:
January 01, 1969

Abstract

The basaltic lava trapped in Alae pit crater from the August 1963 eruption of Kilauea volcano, Hawaii, produced an immiscible sulfide melt during a late stage of the cooling of the lava. A differentiated siliceous liquid confined within interstices of the upper crust of the lava lake became saturated with respect to sulfide sulfur at a temperature of 1,065° C and a sulfur content of 0.038% S by weight. Precipitation of two sulfide-rich phases, one an immiscible sulfide-rich liquid, the other a copper-rich pyrrhotite solid solution, followed.

The sulfur-rich liquid, which quenched to a mixture of pyrrhotite, chalcopyrite, and magnetite, is estimated to have a composition of approximately 61% Fe, 4% Cu, 31% S, and 4% O by weight. The copper-rich solid sulfide that coexisted with the sulfide liquid has an estimated composition of 53% Fe, 9% Cu, 3% Ni, and 35% S by weight.

The basaltic liquid from which the sulfide-rich phases precipitated does not contain abnormally high contents of either Cu or Ni. The important conclusions to be drawn from the samples are:

1) Immiscible sulfide-rich liquids separating from basic magmas are always FeSrich and always contain a significant oxygen content, an important factor in depressing the sulfide liquidus surface. The oxygen content will appear as an iron oxide, generally magnetite in the cooled and crystallized sulfide mass, and all ores formed in this manner should therefore contain significant amounts of an iron oxide.

2) The compositions of most sulfide-rich liquids are such that the first phase to crystallize is a copper- and nickel-rich pyrrhotite solid solution, leaving the remaining liquid progressively enriched in oxygen and depleted in Cu and Ni until its composition reaches a cotectic between pyrrhotite and magnetite, when a pyrrhotite-magnetite mixture in the approximate ratio 2:1 will crystallize.

3) The pyrrhotite solid solutions crystallizing from and coexisting with the immiscible sulfide melts preferentially concentrate copper and nickel, so the early formed phases in a crystallizing immiscible sulfide melt will form the richest ore pockets.

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Contents

Society of Exploration Geophysicists Geophysical Monograph Series

Magmatic Ore Deposits

H. D. B. Wilson
H. D. B. Wilson
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Society of Economic Geologists
Volume
4
ISBN electronic:
9781934969991
Publication date:
January 01, 1969

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