EXPLORATION FOR ALTERATION HALOS AROUND THE AG-W DEPOSITS IN SOUTHEASTERN MISSOURI

The St. Francois Mountains in southeastern Missouri are an extensive terrane of anorogenic granitic ring complexes and associated rhyolites. They have been an important source of silver, tungsten and tin in the past and were mined from the mid-1800s until the 1950s. The silver-tungsten-tin ores occur in the Silvermine Granite and rhyolite roof rocks within the Silver District. The deposits are classified as a xenothermal ore system that formed at high temperatures (>400°C) and shallow depths (≤1500m). Mineralization occurs in quartz veins and surrounding (up to 2.5m) greisen.

Although there are currently no mining activities in the Silver District, the area is a well characterized natural laboratory that allows for development and testing of novel exploration tools for these ore systems. Traditionally, exploration for hydrothermal ore deposits often focuses on the identification of alteration halos in the surrounding host rocks. The extent of alteration can range from meters away from the ore veins to several kilometers. However, in xenothermal ore deposits, such as in the Silver District, extensive alteration halos are often very small (cm to m) at the local scale and difficult to identify due to the rapid cooling experienced by the ore bearing fluids. Although the Silver District ore veins have been extensively studied in the past, the nature and extent of the alteration halos at the regional scale around these deposits remains to be established.

Here we present recent results of a comprehensive study that investigates these alteration halos using petrographic observations, bulk rock chemistry, and magnetic susceptibility measurements. Samples were collected in incremental steps of 1m, 3m, 5m, 10m, 15m, 25m, 50m, 100m, and 200m away from the most prominent vein in the area, the Einstein vein. Apart from the ore-vein itself, silver concentrations in the surrounding host rocks are generally below detection limit (gravimetric fire assay; 3.13 ppm). However, petrographic observations show distinct color changes within the rock, most notably at the 1m, 5-10m, and 50m increments that are interpreted to reflect chemical alteration which will be further discussed during this presentation. The results will be used to reflect on the use of geochemical exploration halos in the exploration for xenothermal Ag-W –Sn deposits in the Midwest.