FORELAND BASINS AND SEDIMENTARY-ROCK-HOSTED ORE DEPOSITS

No two foreland basins are alike. They form in a variety of convergent tectonic settings at all latitudinal belts, range from narrow flysch troughs to wide, nonmarine clastic wedges, and evolve through stages. Moreover, the character of foreland basins has changed through Earth history, in step with secular changes in tectonic style, continental positions, climate, the atmosphere, and so on. In this context, we discuss the spatial and temporal patterns of four types of ore deposit that occur in orogenic forelands.

Two types are products of regional fluid flow in sediment-filled foreland basins. Mississippi Valley-type lead-zinc (MVT) deposits have been forming since 2.3 Ga in carbonate rocks on the cratonward flanks of foreland basins. A drift history through low latitudes is needed to create suitable host rocks and evaporative brines to transport the metals. MVT deposits are known from collisional, retroarc, and transpressional forelands (Tri-State, Pine Point, and Cevennes, respectively). Forebulge unconformities and flexure-induced normal faults are common ore controls. Mineralization, commonly the product of gravity-driven fluid migration, is a late- or post-orogenic phenomenon that requires a topographic gradient away from the orogen. Some sandstone-hosted uranium deposits also form in overfilled foreland basins. Examples include the Jurassic deposits of the Colorado Plateau and the Permian deposits of the Karoo (S. Africa)—both retroarc foreland basins. Sandstone-uranium deposits are mainly Carboniferous or younger.

Two other types of deposit—bedded barite and Superior-type BIF—can be linked, in some cases, to underfilled forelands at the onset of arc-passive margin collision. Barite deposits of this type include the Carboniferous barites of Arkansas, which formed along the Ouachita foredeep axis, and the Devonian barites of Nevada, which bear a similar relationship to Antler orogenesis. Prior to 1.8 Ga, BIFs of the Lake Superior region, the Labrador Trough, and Wopmay orogen were deposited in platform-drowning successions at the onset of arc-passive margin collisions. These BIF and barite deposits thus were formed in distal and proximal parts of the same types of forelands, albeit at different times in Earth history.