Abstract

Following systematic exploration, extensive deposits of phosphate rock were discovered near Duchess in northwestern Queensland, Australia. The phosphorite occurs in the lower Middle Cambrian Beetle Creek Formation, within an embayment of the Burke River Outlier, an appendage of the Georgina Basin. Siliceous and calcareous facies are developed within the phosphorite member and the lithological variations were controlled by basement structures active during sedimentation. Similar lithological variations were reflected in overlying and underlying formations. The majority of the phosphorite is pelletal. Sedimentation was essentially non-clastic and was dominated by carbonaceous black chert, fetid limestone and phosphorite. Petrographic and chemical similarities of the phosphorites with other major deposits of the world are described. Phosphorite deposition occurred in a shallow marine environment with restricted access to open sea. The form and composition of the phosphorite are considered to reflect the paleosalinity and redox potential of the depositional environment.

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