Feldspar metasomatism along a Proterozoic rift-basin margin - 'Smoke' around the base metal 'fire' (HYC deposit, Australia) or a product of background diagenesis?

The relationship between regional alkalifeldspar metasomatism and the alteration around large, sediment-hosted, stratiform Zn-Pb deposits is not well understood. At the Proterozoic HYC deposit in the Northern Territory, Australia, feldspar alteration of volcaniclastic beds (∼85 thin beds within a 700-m-thick section) forms a wedge (in plan view, >2 km wide) that is zoned westward from microcline to albite away from the Emu fault zone, which forms the half-graben margin. In cross section, the base-metals deposit occurs near the bottom of the feldspar-altered wedge and has a similar width. However, similar felspar alteration occurs at several more poorly exposed points regionally along the Emu fault; base-metal sulfides are absent at these sites. Whole-rock and δ18Owhole rock geochemistry indicate that the feldspar-forming fluid was cool (32 ± 27 °C) and had a δ18O composition that is most consistent with a meteoric origin (-12‰ ± 6‰). A fluid with these characteristics is not considered suitable for base-metal transport; in addition, textural evidence indicates that the main feldspar alteration of the volcaniclastic beds predated the base-metal mineralization in most examples. To account for the activity of two fluids leading to the low-temperature feldspar alteration and the spatially coincident high-temperature base-metal deposition, I propose that meteoric ground water from the adjacent horst was focused into the shale-filled basin by a long-lived, shallow, saline fluid flow. This fluid preferentially reacted with vitroclastic rocks in the upper diagenetic zone. The extent of the feldspar and microquartz alteration was proportional to the head generated by transpression along the Emu fault zone (i.e., the basin margin). Salts were leached from a carbonate-evaporite sequence within the fault zone without substantially altering the δ18O composition of the fluid. The westward zonation from microcline to albite was achieved by K-feldspar deposition during brine migration, which isothermally increased the Na/K ratio in the fluid until albite was stabilized. Basemetal deposition occurred separately by the episodic fault release of high-temperature, deep-basin brines, producing disseminated Mn-Fe carbonate, adularia, pyrite, and basemetal sulfides in some volcaniclastic beds. The hydrogeochemical model may be generally applicable to provinces in which synsedimentary faults juxtaposed arid subaerial regions against volcaniclastic-bearing marine basins, inducing basinward saline fluid flow.