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

The relationship between regional alkali-feldspar metasomatism and the alte ration around large, sediment-hosted, stratiform Zn-Pb deposits is not well understood. At the Proterozoic HYC deposit in the Northern Territory, Aust ralia, feldspar alteration of volcaniclastic beds (similar to 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, w hich forms the half-graben margin. In cross section, the base-metals deposi t occurs near the bottom of the feldspar-altered wedge and has a similar,wi dth, However, similar feldspar alteration occurs at several more poorly exp osed points regionally along the Emu fault; base-metal sulfides are absent at these sites, Whole rock and delta(18)O(whole rock) geochemistry indicate that, the feldspar forming fluid was cool (32 +/- 27 degrees C) and had a delta(18)O composition that is most consistent with a meteoric origin (-12 parts per thousand +/- 6 parts per thousand). A fluid with these characteri stics is not considered suitable for base-metal transport; in addition, tex tural evidence indicates that the main feldspar alteration of the volcanicl astic beds predated the base-metal mineralization in most examples, To acco unt for the activity of two fluids leading to the low-temperature feldspar alteration and the spatially coincident high-temperature base-metal deposit ion, I propose that meteoric ground water from the adjacent horst was focus ed into the shale-filled basin by a long-lived, shallow, saline fluid flow This fluid preferentially reacted with vitroclastic rocks in the upper diag enetic zone. The extent of the feldspar and microquartz alteration was prop ortional to the head generated by transpression along the Emu fault zone (i .e., the basin margin). Salts were leached from a carbonate-evaporite seque nce within the fault zone without substantially altering the delta(18)O com position of the fluid, The westward zonation from microcline to albite was achieved by K-feldspar deposition during brink migration, which isothermall y increased the Na/K ratio in the fluid until albite was stabilized, Base-m etal deposition occurred separately by the episodic fault release of high-t emperature, deep-basin brines, producing disseminated Mn-Fe carbonate, adul aria, pyrite, and base-metal sulfides in some volcaniclastic beds, The hydr ogeochemical model may be generally applicable to provinces in which synsed imentary faults juxtaposed arid subaerial regions against volcaniclastic-be aring marine basins, inducing basinward saline fluid flow.