HR: 0800h
AN: V21B-0603    [Abstracts]
TI: Trace Element Evidence for a Hydrothermal/Magmatic Origin of Stratiform Magnetite Deposits in the Crystal Spring Formation
AU: * Thiagarajan, N
EM: nivedita@rice.edu
AF: Rice University, 6100 Main Street, Houston, TX 77005 United States
AU: Lee, C
EM: ctlee@rice.edu
AF: Rice University, 6100 Main Street, Houston, TX 77005 United States
AB: The Beck Spring Magnetite Deposit is a stratiform body located within the stromatrolitic limestone deposits of the Neoproterozoic Crystal Springs Formation in San Bernardino County, California. There is still some debate regarding its origin and age. Its stratiform nature suggests that it could be a product of metamorphism of a Fe-rich sedimentary layer. Alternatively, it may have formed through hydrothermal processes associated with the 1.0 Gy diabase intrusions or perhaps even younger magmatism, such as the ~14 Ma magmatism responsible for the nearby Kingston Peak pluton. The magnetite deposits are two layers radiating from an ore body characterized by an intergrowth of massive fine-grained magnetite, calcite and pyrite, while the surrounding country rock is a limestone consisting of an orangish white calcite. Here we report the trace element abundances of the magnetite deposits and surrounding limestone in an effort to constrain its origin. We find that the magnetite and limestone wallrock have distinctive primitive-mantle normalized trace-element abundances. In particular, the magnetite has a distinct negative Eu anomaly, anomalously low Th/U ratios, general enrichments in REEs and HFSEs and extreme depletions in Ni and Cr. These features are unlike Fe-rich sedimentary deposits. Instead, the negative Eu anomaly strongly suggests that fractional crystallization of plagioclase took place, while depletions in Ni and Cr are indicative of a highly evolved compositions. The low Th/U ratios also imply that transport of these elements may have occurred via aqueous fluids. These characteristics suggest a magmatic or hydrothermal origin for the magnetite. This is supported by the fact that the immediate surrounding limestone has trace-element systematics very similar to that of the magnetite layers although there is little or no petrographic evidence for sulfide and magnetite infiltration into the limestone. The trace-element systematics of the limestones thus indicates that the fluids associated with the formation of the magnetite deposits metasomatized the surrounding limestone. Thus, we propose that magmatism in the area caused the formation of the magnetite ore deposits and that associated hydrothermal fluids metasomatized the surrounding limestone resulting in the trace element characteristics we have reported.
DE: 1065 Trace elements (3670)
SC: Volcanology, Geochemistry, Petrology [V]
MN: 2004 AGU Fall Meeting