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We are grateful to Mr. Bryan Yap, President of Lepanto Consolidated Mining Company, for permission to conduct this study in the Mankayan district, for providing access to the air magnetic and radiometric survey results, and for permission to publish the outcome of this study. This work is part of AMIRA Project P765, completed in December, 2006, at CODES, University of Tasmania. The project was sponsored by Anglo American, Anglo Gold Ashanti, Gold Fields, Newcrest, Newmont, Placer Dome, and Teck Cominco, as well as Barrick in the final year, with additional funding provided by the Australian Research Council. We acknowledge the discussion and ideas provided by representatives of the sponsoring companies, and assistance by the AMIRA research coordinator Alan Goode. We thank Lyndon Bradish and Froilan Conde, plus many other Lepanto staff, including Bene, Danny, Ed, Louie, Perfecto, Perry, Ric, and Willy, for their assistance, and Dave Braxton for help in the field. Paddy Waters of Anglo American Philippines generously provided logistic support and funding for some of the whole-rock geochemical analyses. We appreciate the help from CODES staff, including Simon Stephens for sample preparation, Sarah Gilbert and Leonid Danyushevsky for LA-ICP-MS analyses, and June Pongratz for report preparation. We thank Roger Stoffregen for comments on an early version of the manuscript, and John Thompson and Raymond Jannas for useful reviews.

The fine-grained, laminated, stratiform zinc-lead ores of the McArthur River (HYC) ore deposit have been the subject of continued debate in regard to the timing of zinc-lead mineralization. Early workers (e.g., Croxford and Jephcott, 1972;Lambert, 1976) considered the ores to be sedimentary-exhalative (sedex), while more recent researchers (e.g., Williams, 1978a;Eldridge et al., 1993; have argued for a syndiagenetic replacement origin. Several lines of evidence, including studies on the regional sedimentology, sediment geochemistry, nature of sulfide layering, paragenesis, and chemistry of base metal transport and deposition, are used here to demonstrate that a synsedimentary origin is likely for the HYC deposit.Studies on the regional sedimentology and geochemistry of the Barney Creek Formation have shown that the host rocks to the ore deposit, the HYC Pyritic Shale, represent a period of water deepening. Water depth was probably in excess of several hundred meters, with the deepest water euxinic facies corresponding to the ore position at the base of the HYC Pyritic Shale. A widespread manganese halo, developed in the W-Fold Shale Member below the ore horizon, is considered to relate to the start of the basin-deepening event, with manganese-bearing connate brines released into the basin along marginal rift faults. Manganese deposition occurred in the transitional dolomitic shale facies, where shallow-water, oxidized basin fluids intermittently mixed with deeper-water, reduced fluids along a redox interface. Sedimentary manganese deposition was immediately followed by zinc-lead mineralization in the deep-water, reduced facies of the HYC Pyritic Shale Member. The intimate relationship between the eight stratiform ore lenses and inter-ore sedimentary breccias indicates that sedimentary breccia deposition and ore formation are genetically linked. It is suggested that deep seismic events led to the following ore cycle: fault movement; sediment breccia deposition sourced from faults; episodic release of hot metalliferous brines along faults accompanied by sedimentary turbidites; and metal sulfide deposition in muds and turbidites adjacent to faults. The three modes of base metal sulfide occurrence in the ores, each considered by Eldridge et al. (1993) to have formed by replacement, are shown to have separate origins. The fine-grained sphalerite-galena bands (Mode 1) are intricately interlayered with pelagic organic-rich beds and thin turbidire beds. They exhibit classic sedimentary textures, such as load casts and flames, indicative of a synsedimentary origin. Coarser-grained Mode 2 base metal sulfides show complex intergrowths with nodular dolomite bands, and are considered to form by replacement during diagenesis. The discrete patches of coarse, Mode 3 base metal sulfides are interpreted to represent clastic input from turbidity currents sourced partly by the erosion of vein-style mineralization from the faulted margins of the basin.The detailed nature of the layering in the ores suggests that the introduction of m...

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