The search for asbestos within the Peter Mitchell Taconite iron ore mine, near Babbitt, Minnesota

Abstract

Asbestos crystallizes within rock formations undergoing intense deformation characterized by folding, faulting, shearing, and dilation. Some of these conditions have prevailed during formation of the taconite iron ore deposits in the eastern Mesabi Iron Range of Minnesota. This range includes the Peter Mitchell Taconite Mine at Babbitt, Minnesota. The mine pit is over 8 miles long, up to 1 mile wide. Fifty three samples were collected from 30 sites within areas of the pit where faulting, shearing and folding occur and where fibrous minerals might occur. Eight samples from seven collecting sites contain significant amounts of ferroactinolite amphibole that is partially to completely altered to fibrous ferroactinolite. Two samples from two other sites contain ferroactinolite degraded to ropy masses of fibers consisting mostly of ferrian sepiolite as defined by X-ray diffraction and TEM and SEM X-ray spectral analysis. Samples from five other sites contain unaltered amphiboles, however some of these samples also contain a very small number of fiber bundles composed of mixtures of grunerite, ferroactinolite, and ferrian sepiolite. It is proposed that the alteration of the amphiboles was caused by reaction with water-rich acidic fluids that moved through the mine faults and shear zones. The fibrous amphiboles and ferrian sepiolite collected at the Peter Mitchell Mine composes a tiny fraction of one percent of the total rock mass of this taconite deposit; an even a smaller amount of these mineral fragments enter the ambient air during mining and milling. These fibrous minerals thus do not present a significant health hazard to the miners nor to those non-occupationally exposed. No asbestos of any type was found in the mine pit.

Introduction

Asbestos crystallizes under very special conditions that occur within rock formations that are undergoing intense deformation characterized by folding, faulting, shearing, and dilation. Such deformations are often accompanied by the intrusion of magmatic fluids that solidify to form dikes and sills. The fibers crystallize in high strain environments, such as within folds, shear planes, faults, dilation cavities, and at intrusion-host rock boundaries. Fibers can crystallize from solutions moving within the fault and shear zones; the fibers being oriented parallel to the two rock faces that compose the shear or slip plane—thus the term slip fiber. Slip fiber growth can also occur during folding of layered rocks for the folding process causes a shearing between adjacent layers. Fibers can also crystallize from a fluid phase within cracks formed when the rock undergoes dilation due to tectonic stress, a process in which parallel cracks and fissures form open spaces within the rock. The process of folding in layered rocks can also produce openings or dilation cavities between adjacent layers. Such fibers nucleate on a wall of the crack or cavity and grow across to the opposite wall—thus the term crosses fibers (Ross and Nolan, 2003).

Cross and slip fiber growth of crocidolite and amosite asbestos occurs in the intensely folded Precambrian banded ironstones of the Transvaal and Cape Provinces of South Africa. The asbestos deposits in the Cape Province are generally found within monoclinal folds. The crocidolite asbestos deposits of Western Australia are also found Precambrian banded ironstones, the deposits generally confined to the northern limb of a broad syncline of the Hamersley Range. Here, crocidolite asbestos grew within dilation cavities between folded iron formation bands. Asbestos-bearing serpentinites, such as those found in Quebec, Canada, commonly contain cross-fiber chrysotile asbestos that crystallized within dilation cavities. Slip fiber chrysotile asbestos is less common but is ubiquitous in the serpentinite located in Eden Mills, Vermont (Ross, 1981, Ross and Nolan, 2003).

In our examination of numerous mines and construction sites within igneous and metamorphic terrain located in the eastern United States, Arkansas, Michigan, California, Cyprus, and Ontario and Quebec, Canada, we observe that asbestos occurs within faults and shear zones, in folds, or at contacts between igneous intrusions and the host rock. In any of these geologic features it is possible that other fibrous minerals can form as describe above. However, in most rock formations, other than those exploited for asbestos production, such non-asbestos fibers are restricted to a very small volume of rock.

With the above mentioned criteria in mind we made a detailed survey of the rocks within the Peter Mitchell Mine to see if any rocks contained asbestos or asbestos-like minerals; our results are reported below. The history of the asbestos controversy over exposure to mineral particulates, including those suspected to be asbestos, released during the mining and milling of taconite iron ore is given by Berndt and Brice (2008) and Wilson et al. (2008). This asbestos controversy is particularly related to litigation involving the Reserve Mining Company which previously operated the Peter Mitchell Mine (Reserve Mining Company v USEPA, 514 F.2d 492, 8th Cir. 1975).