METALLOGENY OF PHANEROZOIC INTRACRATONIC RIFT BASINS

Sedex deposits account for more than 50% of world's Zn, Pb, and barite resources. These enormous deposits formed in restricted sedimentary basins within intracratonic/epicratonic rift systems from submarine venting of hot (ca. 100-200 °C) hypersaline (10-30 % NaCl eq.) basinal brines that are geochemically analogous to those that formed MVT deposits. Rift basins also host a wide range of other sediment-hosted ores including Au-Ba, Fe, Mn, Ba, P, and Ni-Mo-PGE-Au deposits. Similar stratigraphic/tectonic settings and, in some cases, a temporal and spatial correspondence of these diverse metal accumulation suggests they share a common genesis. Resent ore genesis studies and chemical modeling suggests many of these metallogenic differences reflect a combination of chemical variations of generative hydrothermal fluids in source basins (P, T, redox, amount and redox state of S) and different marine conditions during fluid venting.

The immense flux of fluid and metal accompanying large sedex systems were sufficient to effect global ocean chemistry. Evidence presented here suggests that several sedex hydrothermal systems had the proper (87)Sr/(86)Sr composition, timing, duration, and magnitude to explain prominent positive Sr-spikes in the secular record. This indicates the Sr-curve is a unique tool for ore genesis studies because, in concert with deposit studies, it can constrain the age, duration, and fluxes of fluids and metals vented by these hydrothermal systems. Moreover, Sr-spikes on the curves may also be an effective tool in assessing the mineral potential of sedimentary basins of different ages, as they identify periods of intense fluid venting. The clustering of Sr-spikes and synchronous worldwide formation of sedex deposits during short intervals of Phanerozoic time suggests anomalous periods of basin fluid venting. For example, >100 sediment-hosted deposits in Canada, US, Mexico, Poland, and Kazakhstan formed in < ca. 5 m.y. during Upper Devonian time. The tectonic triggers of these anomalous metallogenic periods remain enigmatic, yet the synchronous formation and global distribution of these deposits suggests they represent abrupt periods of widespread extension and rifting of the earth’s crust.