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Geochemistry and Genesis of 1.69 Ga Iron Formations Spatially Associated with the Giant Broken Hill Pb-Zn-Ag Deposit, Curnamona Province, Australia

Erica Serna1, Adriana Heimann1, and Paul Spry2
1Department of Geological Sciences, East Carolina University, Greenville, NC 27858
2Department of Geological and Atmospheric Sciences, Iowa State University, Ames, IA 50011

Iron formations (IFs) near the giant Broken Hill Pb-Zn-Ag deposit are among the youngest (1.69 Ga) in the Precambrian. The IFs and sulfide deposit experienced five periods of deformation at the granulite facies. Their origin is controversial but modern geochemical studies of the IFs are limited. Here we use mineralogy, mineral chemistry, and bulk-rock major and trace element compositions to reconstruct the physicochemical conditions of the fluid from which the IFs precipitated.

The IFs are either banded or massive. Banded IFs consist of magnetite, hematite, Mn-Fe garnet, quartz, and accessory apatite and in some cases biotite, whereas massive samples contain magnetite, hematite, quartz, accessory apatite, and in some cases minor sphalerite, chalcopyrite, and pyrite. For both rock types, Fe/Ti and Al/(Al+Fe+Mn) ratios show mixing of mostly hydrothermal and minor detrital components. Banded IFs contain a 75-95% hydrothermal input whereas massive rocks have >90% hydrothermal input and higher Fe/Ti ratios and Fe content. This, along with very low Co+Ni+Cu contents, indicates that all the IFs are similar to hydrothermal sediments. Banded IFs have bulk-rock chondrite-normalized REE patterns with positive Eu anomalies, which indicates the protoliths formed from reduced hydrothermal fluids at T >250oC. Negative Eu anomalies in the massive IFs reflect a distal source to the hydrothermal vent at T <250oC. These results suggest that the IFs formed as a mixture of Fe-Mn oxyhydroxides and minor clays at the base of a rift basin where the oxygen content was low, even after the great oxidation event.

AAPG Search and Discovery Article #90182©2013 AAPG/SEG Student Expo, Houston, Texas, September 16-17, 2013