Tectonic subsidence analysis in the characterization of sedimentary ore deposits; examples from the Witwatersrand (Au), White Pine (Cu), and Molango (Mn)

Abstract

It is common to construct a subsidence model for a sedimentary basin when exploring the basin for oil and gas, but subsidence models rarely are used in the search for mineral deposits. To demonstrate the potential of this technique for mineral exploration, we examined geochronologic data from basins with exceptionally large mineral deposits hosted by sedimentary rocks. We chose as examples three basins: one thought to be dominated by foreland thrust loading, one by rifting, and one by strike-slip motion. As a foreland example, we chose the Witwatersrand basin of South Africa, host to the world's largest Au accumulation and regarded by many workers as a rift that subsequently evolved into a ret-roarc foreland basin. Application of a tectonic subsidence model, however, shows that the flexural subsidence expected in the later stages of evolution of a foreland basin did not occur; instead, the basin is dominated by the initial mechanical subsidence of the rift, followed after a long break by renewed rapid subsidence suggestive of a pull-apart basin. The end of this second subsidence episode saw very slow tectonic subsidence and the deposition of the Au-bearing conglomerates. As a rift basin example, we chose the Keweenaw basin, host to the large Cu deposit at White Pine, Michigan. The subsidence model shows two episodes of early rapid subsidence accompanied by great thicknesses of basalt. This volcanic phase was followed by sedimentary infilling of the basin that we equate to the thermal subsidence phase. The model output shows interruption of the thermal phase by two episodes of enhanced subsidence rate, each associated with epigenetic introduction of Cu. The first event was dominantly extensional, the second dominantly compressional. As a basin in a strike-slip terrane, we examined the Huayacocotla basin of eastern Mexico, host to the giant syngenetic Mn deposit at Molango. The model output shows highly episodic subsidence, consistent with deposition in a basin marginal to a major zone of strike-slip activity. Mineralization occurred when sea-level rise and basement subsidence combined to allow Mn-bearing, reducing bottom waters from the Gulf of Mexico to enter the basin...