Abstract

The Sterling Hill deposit is an isoclinically folded sequence of zinc-, iron-, and manganese-rich strata surrounded by the Franklin Marble. The most metal-rich layers are composed of combinations of willemite, franklinitc, zincitc, and calcite; other layers contain calc-silicate minerals and calcite. Structural, petrologic, and geochronologic evidence indicates that the deformation and mineral assemblages observed today were produced during granulite-grade Grenville metamorphism of the deposit at about 1.0 Ga.In general, the oxygen isotope and chemical compositions of individual minerals vary from lithologic layer to lithologic layer but are uniform within layers which suggests that mineral compositions were controlled by the bulk compositions of the rocks. Important exceptions to this rule occur near faults and fractures where the deposit was infiltrated by retrograde metamorphic fluids. In these areas, the rocks commonly contain hydrous minerals which have replaced anhydrous assemblages, and the minerals have been exchanged to larger delta 18 O values.Willemite and zincite are stable at higher f (sub O 2 ) /f (sub S 2 ) than pyrite and pyrrhotite which implies that the ores are not chemically equilibrated with the surrounding Franklin Marble but are higher in oxidation state and/or lower in sulfidation state. The deposit also contrasts with the surrounding marble in oxygen isotope composition. Bulk delta 18 O values for ore layers range from about 6 to 13 per mil, and thin marble layers interbedded with the ores are consistently about 15 per mil. The Franklin Marble is 20 to 25 per mil.Models for the protolith are presented in which the bulk oxygen isotope compositions and bulk chemical compositions of the rocks are assumed to reflect protolith compositions modified by metamorphic devolatilization. The mineralogy of the protolith is inferred from the bulk chemical compositions of the rocks and by analogy with the mineralogy of other metal-rich oxidized rocks, and the minerals are assumed to have equilibrated isotopically with the fluid which formed them. The application of the models which require the fewest assumptions is to the marbles interbedded with the ore layers. Their isotopic compositions appear to require either equilibration with 18 O-depleted meteoric water at the earth's surface temperatures, or equilibration with a more 18 O-enriehed water at temperatures characteristic of hydrothermal or metamorphic processes. Application of the models to the ore layers suggests that the last fluid-dominated event took place at 150 degrees + or - 50 degrees C rather than at the earth's surface temperatures. Structural features of the deposit also support the higher temperature formation in that they appear to be incompatible with an origin by weathering.Exposure to seawater is the most plausible alternative to subaerial weathering for producing the high oxidation state-low sulfidation state of the protolith. An extremely large volume of seawater would have been required to oxidize the entire metal inventory of the deposit. Thus, the formation of the oxidized protolith is best inferred to have taken place on the Proterozoie sea floor or in shallowly buried sediments which were well irrigated by seawater. The closest modern analogues for Sterling Hill, and by analogy the nearby Franklin Furnace deposit, are the sulfide-poor strata in the metalliferous sediments beneath the Red Sea brine pools. There are, however, significant differences between the two occurrences.

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