EDITORIAL & OPINION — GEOLOGY 101 — MVT lead-zinc deposits, Part 1

Mississippi Valley-type (MVT) deposits are named for the region of the U.S., along the Mississippi River, where these deposits were first discovered.

Deposits of this class are major producers of lead and zinc in North America. They have a relatively simple mineralogy consisting of pyrite, galena (lead sulphide), sphalerite (zinc sulphide) and, in some cases, marcasite, a low-temperature polymorph of pyrite (the same chemical composition but a different crystal structure).

Secondary calcite and dolomite are also common. Organic matter, in forms such as petroleum, is associated with base metal sulphides in many deposits, and appears to have appeared late in the genesis of the mineralization. Fluorite and barite can also be found in some MVT deposits. These deposits are hosted by undeformed, predominantly calcium and magnesium carbonate rocks.

MVT deposits are classic epigenetic types in that the sulphides formed after the host rock. In the case of MVT deposits, the sulphides precipitated from hydrothermal fluids that filled pre-existing holes and voids in the host rock. The sulphide mineralization also typically cements broken fragments of wallrock. Secondary calcite or dolomite may cover surfaces on wallrocks and fragments.

In many cases, the mineralization is said to fill paleo-karst (caverns, caves or holes formed in carbonate rocks as a result of dissolution of the rock through the action of rain or ground water) in the host carbonates. For instance, sulphide bodies in the host carbonate at the former Pine Point mine in the Northwest Territories resemble plums in a plum pudding. At the Nanisivik mine, also in the Northwest Territories, the orebodies resemble filled caves.

There is some debate about the role of the ore-forming hydrothermal fluid in this so-called ground preparation (the development of the open spaces in the carbonate). Some geologists suggest that ore fluids actually dissolved the host carbonate to create the open spaces, whereas others believe that those spaces formed before hydrothermal metal-bearing fluids flowed through them. Researchers are attempting to determine the age of the sulphide mineralization relative to the host rocks.

MVT deposits occur in rocks of Lower Proterozoic age (2-3 billion years old), though most are in rocks of Paleozoic age (600-300 million years). The ore-forming hydrothermal fluids are of low temperature (between 80 and 200 C), low pressure (that is, formed shallow in the earth’s surface) and highly saline (with up to 30% dissolved salts, known as brines).

As a result of the physical characteristics of the fluid, MVT deposits formed without the influence of magmas and igneous rocks or metamorphism. Hence, they formed in sedimentary rocks without the influence of heat external to the sedimentary basin. The metals and the fluids in which they are transported are thought to have derived from basinal shales, with the fluids migrating through permeable sediments and porous rocks, such as paleo-karsted dolomites, to the site of deposition.

In many ways, the formation of these deposits resembles that of petroleum, in which fluids are squeezed from sediments to the site of deposition. In some cases, MVT fluids and petroleum seem to have been derived from the sedimentary basin at different times.

As such, the MVT fluids and petroleum are part of a continuum in the migration of fluids in sedimentary basins, with petroleum following the the MVT fluids that are expelled first. There are two main models for the driving force behind the fluid movement.

One model posits that the fluids are gravity-driven, with saline fluids flowing with groundwater from zones of recharge at higher elevations to lower-lying regions where the fluids discharge. This view is a variant on classic hydrology models.

The other model of fluid movement is based on the premise of fold-and-thrust tectonics, in which fluids in a sedimentary basin are squeezed by external, plate tectonic-related forces. For example, the Canadian Rockies exhibit classic fold-and-thrust tectonic features.

— The author is a professor of geology at Memorial University in St. John’s, Nfld.