TESTING MODELS OF LARGE-SCALE CRUSTAL FLUID-FLOW USING DIRECT DATING OF SULFIDES - RB-SR EVIDENCE FOR EARLY DEWATERING AND FORMATION OF MISSISSIPPI VALLEY-TYPE DEPOSITS, CANNING BASIN, AUSTRALIA

The Rb-Sr dating of sphalerite is a powerful method for evaluating the timing of large-scale crustal fluid flow events and testing hydrological models. The Mississippi Valley-type Zn-Pb deposit at Blendevale, Western Australia, is the largest of several such deposits situated on the northern margin of the Canning basin on the Lennard shelf. It is hosted by the reefal facies of the Devonian (Frasnian) Pillara Limestone, flanked to the south by the Fitzroy trough, a deep half-graben. Rb-Sr isotope analysis of six sphalerites and their fluid inclusion leachates form an isochron indicating an age of 357 +/- 3 Ma ((Sr-87/Sr-86)(0) = 0.7129 +/- 2), in good agreement with the average age, 355 +/- 14 Ma, of the leachate-sphalerite pairs and an isochron consisting of a sphalerite sample, its leachate, and associated host carbonate (362 +/- 12 Ma). This age renders as untenable a recently proposed model calling for basinal brine expulsion in response to a hydrocarbon generation event triggered by eustatic sea-level drop in the late Carboniferous. Instead, mineralization occurring only 30 to 50 m.y. after initiation of extension of the associated Fitzroy trough is consistent with models of episodic basin dewatering from overpressured zones.