D. F. Sangster | USGS Science

USGS2004

Reply to discussion on “Tectonic controls of Mississippi Valley-type lead–zinc mineralization in orogenic forelands”

Bradley and Leach (2003) focused on the tectonics of a subset of Mississippi Valley-type (MVT) lead–zinc deposits. Age assignments followed those given in a review of MVT deposits worldwide (Leach et al. 2001)—a paper that provoked a discussion by Kesler and Carrigan (2002) and reply by Leach et al. (2002), similar to the present exchange. While the new discussion by Kesler et al. (2004) does address tectonic issues, it also continues the old debate about the pros and cons of Rb–Sr isotopic and paleomagnetic dating of MVT deposits. We address these dating techniques first.

Mineralium Deposita↗

USGS1996

Paleomagnetism of the Mississippi Valley-type Zn-Pb deposits of the Silesian-Cracow area, Poland

Paleomagnetic analysis of zinc-lead ore and host rocks from 35 sites in three mines and several quarries was completed using alternating field and thermal step demagnetization and saturation isothermal remanent magnetization methods. Paleomagnetic conglomerate, breccia and fold tests were used to test for remanence stability. Most limestone and dolostone sites of the Middle Triassic Muschelkalk Formation carry a dual-polarity primary or diagenetic Middle to Upper Triassic remanence. Late dolomite and Mississippi Valley-type (MVT) mineralization from 14 sites carry a dual-polarity Tertiary magnetization that is shown to be post-brecciation and syndeformational by the paleomagnetic tests. This age supports gravity-driven fluid flow models for ore genesis that are associated with the Alpine orogeny.

Prace - Panstwowego Instytutu Geologicznego↗