Overview of Naturally Occurring Asbestos in California and Southwestern Nevada

R. MARK BAILEY* Asbestos TEM Laboratories, Inc., 600 Bancroft Way, Suite A, Berkeley, CA 94710

ABSTRACT

Naturally occurring asbestos (NOA) is being discovered in a widening array of geologic environments. The complex geology of the state of California is an excellent example of the variety of geologic environments and rock types that contain NOA. Notably, the majority of California rocks were emplaced during a continental collision of eastward-subducting oceanic and island arc terranes (Pacific and Farallon plates) with the westward continental margin of the North American plate between 65 and 150 MY BP. This collision and accompanying accretion of oceanic and island arc material from the Pacific plate onto the North American plate, as well as the thermal events caused by emplacement of the large volcanic belt that became today’s Sierra Nevada mountain range, are the principal processes that produced the rocks where the majority of NOA-bearing units have been identified.

INTRODUCTION

California is among the most important states in terms of the extent and diversity of rock units bearing naturally occurring asbestos (NOA). Particularly noteworthy are two serpentine and serpentinized peridotite belts that cover approximately 3,200 km 2 (Frazell et al., 2009), including in 42 of 58 California counties. Additionally, areas of serpentine-bearing gravels and soil shed from outcrop exposures are found in large alluvial fans and other drainage systems. Numerous other rock types described in the following paragraph also contain asbestos. An overview of the wide extent of asbestos occurrences is provided in a map compiled by the U.S. Geological Survey (USGS) and the California Geological Survey, as shown in Figure 1 (Van Gosen and Clinkenbeard, 2011). Ultramafic rocks crop out in several distinct geomorphic provinces: (1) the Coast Range, (2) the western Sierra Nevada (referred to as the Sierra Nevada Foothills), and (3) the Klamath Mountains. Additionally, a considerable number of asbestos occurrences

*Corresponding author email: mark@asbestostemlabs.com

not related to ultramafic rocks occur in the Basin and Range/Mojave Desert provinces.

A general summary of California rock types known to contain asbestiform minerals and references to documents describing these types of occurrences include the following:

Serpentinite (Van Gosen and Clinkenbeard, 2011): chrysotile, tremolite/actinolite Sedimentary serpentine deposits (Wakabayashi, 2011): chrysotile Metabasalt/metagabbro (blueschist) (Erskine and Bailey, 2018): glaucophane, winchite, riebeckite Metabasalt/metagabbro (greenstone) (Sacramento Metropolitan Air Quality Management District, 2004): tremolite/actinolite Meta-granitoids (Buck et al., 2013; Metcalf and Buck, 2015): actinolite, Mg-riebeckite, winchite/richterite Other meta-volcanic (Van Gosen et al., 2004; Wakabayashi, 2011): Na amphibole Fe-rich meta-chert (Van Gosen and Clinkenbeard, 2011): riebeckite, grunerite Iron-ore deposits replacing dolomite (Van Gosen and Clinkenbeard, 2011): tremolite/anthophyllite Meta-carbonates (Van Gosen et al., 2004; Van Gosen and Clinkenbeard, 2011): serpentine, tremolite/actinolite, winchite/richterite Phyllites (sodium-metasomatized) (Albino, 1995): riebeckite Schists (Higgins and Clinkenbeard, 2006): tremolite/actinolite, anthophyllite Shonkinite carbonatite and syenite (Olson et al., 1954): riebeckite Talc deposits (Van Gosen et al., 2004): tremolite, winchite/richterite Colluvium/alluvium and other rock/soil shed off hills containing the above rock types

CALIFORNIA COAST RANGE

The Coast Range province is a belt of rocks ∼500 miles long by as much as 100 miles wide trending in a northwest-to-southeast direction and dominated by rocks of the Franciscan Complex (hereafter

Figure 1. California maps. (A) Ultramafic rocks (Harrison et al., 2004). (B) Asbestos mines and known occurrences of NOA (Van Gosen and Clinkenbeard, 2011). (C) Geomorphic provinces (California Geological Survey, 2016).

Franciscan) containing abundant outcrops of ultramafic rocks emplaced onto the west coast margin of the North American plate ∼65-150MY ago during subduction of the East Pacific and Farallon plates (Wakabayashi and Unruh, 1995). The type section of the Franciscan runs directly through the major population center of San Francisco (see Figure 2) as well as the Greater San Francisco Bay Area, and the ultramafic rocks it contains are responsible for a major effort and expense by local environmental, engineering, and construction firms as well as government regulatory bodies to control dust emissions during construction activities.

NOA within ultramafic bodies in the Coast Range tends to be dominated by a short-fiber variety of

Figure 2. Geologic map of San Francisco. Note purple belt of ultramafic rock extending from Hunters Point (southeast) to the Golden Gate Bridge (north) along the Hunters Point Shear Zone.

chrysotile (length <5 μm) while also containing occasional tremolite, actinolite, glaucophane, winchite, and hornblende fibrous amphiboles. In some ultramafic bodies, chrysotile is visible in hand samples and present in concentrations of 10% or more, commonly as a stockwork of veins. Curiously, although the veins are often several millimeters or more in thickness, they do not typically yield an abundance of long fibers. A possible explanation is given by Andreani et al. (2004) with veins growing through a small-scale (<5 μm) repetitive crack-seal cycle.

The highest concentrations of asbestos in serpentine generally occur where it is invisible to the unaided eye. The defining example of this type of occurrence is in the New Idria serpentinite asbestos mining district (see Figure 3), which has been designated by the USGS as having the largest reserves of asbestos in the country (Van Gosen and Clinkenbeard, 2011). The ore material (Figure 4A; Rice, 1963) is rather non-descript with little obvious fibrosity. Rather, it is a pale waxy light greenish color. While the New Idria serpentinite had been known since the mid-1800s, it was not recognized until the mid-1950s when geologists analyzed the material by X-ray diffraction and discovered an exceptionally high purity level of sub-microscopic chrysotile (>50%). Notably, when fiber bundles are observed in the ore, they have been found by transmission electron microscopy to consist of collections of very short, generally <5-μm fibers that form interwoven aggregates that appear longer (Ilgren, 2008).

Besides containing serpentine, the Franciscan in the Coast Range has numerous other types of NOAcontaining rocks with various fibrous amphiboles, including actinolite, tremolite, crocidolite, and winchite.

Figure 3. Atlas mine, Coalinga, California. The largest of numerous asbestos mines in the area (∼450 acres) is now an EPA Superfund site.

A recent unusual occurrence of fibrous amphiboles of a previously unrecognized form of asbestos discovered by the author is within the Calaveras Dam Replacement Project site (Erskine and Bailey, 2018) located in the hills east of Fremont, California. Here, both asbestiform glaucophane (a sodic amphibole closely related to regulated crocidolite) and winchite (a sodiccalcic amphibole similar to that found at Libby, MT) co-occur within altered block-in-matrix rocks composed of metabasalt mix greenstone and blueschist in

Figure 4. Asbestos ore. (A) Coalinga chrysotile, not obviously fibrous, from the Coast Range. (B) Calaveras chrysotile in stockwork veins from the Sierra Nevada Foothills.

Figure 5. Right abutment of the Calaveras Dam in Fremont, California, in Franciscan mélange. Note the arrows pointing to an assortment of block-in-matrix rocks, three of which (underlined) were found to contain different varieties of NOA.

a tectonic mélange (Figure 5). To complicate matters at the site, chrysotile-bearing serpentine and fibrous actinolite in an actinolitic schist are also present. The project site may well be the largest privately operated NOA management project in history with tens of thousands of transmission electron microscopic air samples collected and analyzed.

SIERRA NEVADA FOOTHILL BELT

Extending over 500 km in length, the Sierra Nevada Foothill serpentinite belt runs in a northwest-tosoutheast line along the western edge parallel to the Sierra Nevada mountain range, much of it contained within a the Foothill metamorphic belt group. It is generally rather narrow, varying from ∼10 km to as much as 50 km in width with numerous gaps. Although more remote than the San Francisco Bay Area, it has seen its population rise significantly due to its proximity to the growing Sacramento Valley area. Concern over NOA exposure first arose due to dust generated by vehicles traversing over NOA-bearing serpentinite gravels with a number of studies undertaken (Berman, 1994; Volpe Center, 2004; and Department of Toxic Substances Control, 2005) to assess the problem. The concern was well founded, resulting in considerable efforts to control hazardous dust, including paving existing roads and ceasing the use of offending material; implementation of the latter led to the shutdown of numerous serpentine rock quarries.

Many of the occurrences in the Sierra Foothills lie on fault zones, particularly the Melones fault system. Within this belt lies the Calaveras mine in Copperopolis, California (Rice, 1963), which in the 1960s and 1970s was the largest producer of asbestos in the United States and operated until 1987 (Figure 6). Here, asbestos occurs as veins within an ore body of serpentine (Figure 4B). Currently, the mine is operated as a hazardous waste dump for asbestos products.

After the initial concern over the presence of asbestos in roadways abated, another type of more widespread NOA occurrence in the region was identified as asbestiform tremolite/actinolite or hornblende. Unlike the Coast Range, which experienced low- to high-pressure regional metamorphism at low temperature, the Sierra Foothills has experienced greenschist facies metamorphism, leading to the formation of amphiboles in many rocks, most notably in the El Dorado Hills area, where the U.S. Environmental Protection Agency and the USGS conducted detailed sampling and analysis as well as a full-scale risk assessment (Ladd, 2005; Meeker et al., 2006). During a series of studies of the area, fibrous tremolite was identified primarily in metamorphosed ultramafic rocks and actinolite-magnesiohornblende in mafic meta-volcanic rocks and meta-sedimentary rocks. The results of the studies led to the local high school athletic fields and playgrounds being buried with several feet of non–NOA-containing soil, new protocols being developed for dealing with NOA by the local air pollution control district, and warnings to residents on how to minimize exposure to NOA. An additional major NOA monitoring effort related to amphiboles in the area occurred during the emergency rebuilding effort at the Oroville Dam, which was severely damaged during the winter of 2016–2017, during which fibrous tremolite/actinolite was identified in metamorphosed mafic volcanic rocks that were being disturbed as repairs were made.

KLAMATH MOUNTAINS

The Klamath Mountains have the highest overall percentage of area of exposed serpentinite in the state, which occurs at the far northernmost intersection of the Coast Range and Sierra Nevada Foothills. However, the region is so remote and virtually nonpopulated that little knowledge exists of NOA in the area, and risk of exposure is concomitantly low.

BASIN AND RANGE/MOJAVE DESERT (INCLUDING SOUTHERNMOST NEVADA AND SOUTHWESTERN ARIZONA)

The Basin and Range/Mojave Desert area contains completely different rock types than the Coast Range, Sierra Foothills, and Klamath Mountains. The rocks are dominated by Precambrian though Paleozoic continental shelf and shallow sea sedimentary deposits that have been intruded and altered by various more recent volcanic and intrusive

Figure 6. The Calaveras mine in Copperopolis. The most productive asbestos mine in California is now an asbestos landfill operation.

rocks. Asbestos-bearing alterations most commonly occur at contacts of dolomitic rocks immediately adjacent to or nearby igneous intrusives ranging from basaltic/gabbroic (i.e., Death Valley; Van Gosen et al., 2004) to granitic/rhyolitic (i.e., Mazourka Canyon; Ross, 1966) and leading to a predominance of tremolite/winchite in the former and chrysotile/tremolite in the latter.

Two particularly unusual occurrences of NOA have been identified within the last 5 years in silicic granitic rocks in southern Nevada/far western Arizona that have been subjected to thermal alteration, about as far chemically from the more common situation of ultramafic rock alteration described above. In the Boulder City area, the quartz monzonite Boulder City pluton (Miocene, ∼14 MY BP) was subjected to a recent thermal event that appears to have transformed through retrograde alteration pre-existing hornblende lathes into asbestiform actinolite (Buck et al., 2013; Metcalf and Buck, 2015). A few miles to the east lies the Wilson Ridge pluton (Miocene, ∼13 MY BP), which is hypothesized to have experienced an influx of sodium from hydrothermally mobilized saline evaporite deposits during cooling after intrusion, causing albitization of feldspars and deposition of fibrous sodic amphiboles mg-riebeckite and sodic-calcic winchite/richterite (Buck et al., 2013; Metcalf and Buck, 2015).

CONCLUSION

In summary, NOA in California and southwestern Nevada is found in an exceptionally wide range of rock types, with significant unexpected new deposits seeming to come every few years. Serpentine/ultramafic rocks are the primary source of NOA for chrysotile and/or tremolite/actinolite depending on the type and degree of metamorphism to which the rocks have been subjected. However, NOA has been found not only in ultramafic rocks but also in metamorphosed mafic and silicic igneous rocks as well as altered dolomites and a variety of others. This creates a serious challenge to geologists and others involved in assessing the potential for the presence of NOA at a given site where grounddisturbing activities will occur.

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