PALEOCLIMATIC CONTROLS ON MISSISSIPPI VALLEY-TYPE LEAD-ZINC DEPOSIT FORMATION

Mississippi-Valley Type (MVT) Pb-Zn deposits have been interpreted to have formed in active foreland basins under conditions of gradually increasing groundwater flow rates associated with increasing hydraulic gradients during hinterland uplift. However, these parameters do not satisfactorily describe the distribution of these ore deposits in the rock record and a paleoclimatological control has been hypothesized.

This project involves an evaluation of numerical paleoclimate simulations spanning the past 380 my. We focused on model output from the general circulation model GENESIS versions 1 and 2 corresponding to well-dated MVT ore deposits described in Leach et. al,2001, Mineralium Deposita, v. 36, p. 711-740. We analyzed model output from the model grid cell in which an individual MVT deposit was paleogeographically located for: soil moisture, precipitation, evaporation and temperature, for summer (JJA), winter (DJF), and mean annual results. We compared the model output from a grid cell to average Earth conditions and found that precipitation-evaporation (P-E) and summer temperature for MVT grid cells showed significant departure from mean Earth values. The global mean temperature was found to be ~23.8 ºC while the local summer temperature averaged 30 ºC. Precipitation-evaporation averaged 3 mm/yr globally, but the local grid showed a mode of -0.5 mm/yr.

From earlier field and literature-based research we recognized that paleosols are often geographically and stratigraphically proximal to MVT Pb-Zn deposits, and that the paleosols most often record warm and dry paleoclimate conditions.This observation appears to support our conclusions from the paleoclimate model simulations. Furthermore, the age of many of the paleosols is better constrained than the age of the MVT ore bodies. Therefore, if the paleoclimate effects of P-E and summer temperature are valid controls on MVT ore formation, and the paleosols record these conditions, then a better age for ore genesis may be inferred from the age of the paleosols.