Pleistocene Glacial Transport of Nephrite Jade from British Columbia, Canada, to Coastal Washington State, USA

Abstract

Since prehistoric times, indigenous residents of southwest British Columbia, Canada, collected water-worn nephrite specimens from the gravel bars along the Fraser River, using the stone for the manufacture of tools that were widely traded with other tribes. Allochthonous nephrite occurs in another geologic setting. Late Pleistocene continental glaciers transported nephrite and many other rock types from western Canada to northwest Washington State, producing extensive sediment deposits that border the Salish Sea coast in Whatcom and Island Counties, Washington. This material was little utilized by indigenous residents, but “black jade” specimens are prized by modern collectors. The depositional history and mineralogy of this material has received little attention. X-ray diffraction and SEM/EDS analyses indicate that the Salish Sea “black jade” is a form of impure nephrite that probably originated from metamorphism of a mafic igneous parent material (metabasite). The texture consists of prismatic amphibole crystals (ferro-actinolite) set in a matrix rich in plagioclase feldspar. Pyrite inclusions are locally present. A second material, sometimes erroneously labelled “muttonfat jade” by amateur collectors, consists of an intermixture of quartz and sillimanite.

1. Introduction

Western Washington is known as a jade-producing region because of the abundance of nephrite associated with serpentinite bodies in the North Cascade Range that were brought to the near-surface along deep-seated faults. Metamorphic terranes in western Washington State and British Columbia are linked to the Cascadia Subduction Zone, where oceanic plates and associated island arcs collided with the North American continent [1,2]. The oceanic materials were generally in the form of ophiolite complexes, which represent the various components that were present at mid-ocean rift zones (Figure 1).

Emplacement of ophiolite complexes typically involves major deformation that causes individual components to become intermixed. The coexistence of ultramafic rocks and neighboring formations provides a favorable setting for the paragenesis of nephrite. Many of Washington’s nephrite occurrences are associated with the Darrington–Devils Mountain Fault Zone (DDMFZ) [2], as well as a few other ultramafic bodies (Figure 2).

Nephrite occurrences in the foothills of the North Cascades have been known since the 1930s, but most discoveries were made after extensive logging road networks were constructed beginning in the 1960s [3,4]. The earliest discoveries of nephrite and associated minerals were made in the Fraser River area of southwest British Columbia, Canada, and in coastal locations of the Salish Sea, a name now given to marginal Pacific Ocean waters of British Columbia, Canada, and northwest Washington State (Figure 3). This report is the first time that the origin of this glacially transported coastal nephrite has been described in detail.

2. Historical Importance of Glacially Transported Nephrite

Because of its ability to withstand abrasion and impact, nephrite has been used for making stone tools since Neolithic times. These properties, combined with the attractiveness of color and pattern, also led to the use of nephrite for decorative purposes, including intricate works of art. One of the earliest known locations for artistic use of jade is in China, where the Xinglongwa Culture (6200–5400 BC) marked the beginning of jade preeminence in East Asia [5]. Other early locations are in the Baikal Lake area, Republic of Sakhal, Siberia, Russia, where nephrite adzes and ornamental rings come from a site dated at 4880 BC [6].

In North America, native people have been using nephrite for tools and decorations for thousands of years. Early published reports of nephrite use in British Columbia date from 1851 to 1923 [7,8,9]. In the Pacific Northwest, the earliest use of nephrite for fabrication of sharp-edged celts began about 3,500 BP, with an abundance of celts recovered from sites dating 2500–1000 BP. The rise in celt production correlates with increased woodworking activities that included carving canoes, construction of plank houses, and fabrication of wood storage boxes [7,10,11,12].

Nephrite tool production was centered at two localities along the Fraser River where alluvial nephrite is abundant: the entrance of the Fraser Canyon at Hope, B.C., and the Lytton–Lillooet area in the mid-Fraser River region [12]. Nearly all sawn nephrite cores have been found at these two relatively restricted locations. In contrast, the greatest number of nephrite celts have been found at sites in the lower Fraser River and Salish Sea coast, two areas that have no evident history of nephrite tool production. Celts made from alluvial Fraser River nephrite were also traded to tries in the Canada Interior Plateau. These celts are relatively large, they presumably served as indications of personal wealth rather than being useful tools [11,12]. Manufacture of a single sawn nephrite celt is estimated to have required 40–100 hours [13]. Specialization in tool production probably explains why the glacially transported Salish Sea nephrite described in this report was apparently not exploited by local indigenous communities.

By the 1850s, nephrite collected from the Fraser River became a global resource. The Kunun Mountains of China have long been a major source of nephrite, but by the 1700s, Chinese jade sources were becoming depleted. In the mid-1800s, Chinese placer miners in British Columbia were collecting Fraser River nephrite boulders, sometimes sending them to China in the coffins of those who died and were sent home for burial [14].

Modern mining of British Columbia nephrite dates to 1938 when boulders were discovered in placer deposits in the Cassiar District. In the early 1950s, nephrite boulders weighing up to 10 tons were found along the Fraser and Bridge Rivers near Lillooet, B.C. Later, bedrock deposits in the Cassiar and Omineca areas became major localities for Canadian nephrite mining [15,16,17]. In Washington, commercial nephrite activity has been focused on bedrock occurrences of nephrite on the western side of the North Cascade Range, with most prospecting focusing on serpentinite bodies exposed along the Darrington–Devils Mountain Fault zone in Skagit and Snohomish Counties, Washington [18]. Rock and mineral occurrences on Salish Sea beaches have largely been the domain of amateur collectors.

3. Salish Sea “Black Jade”

Dense homogenous masses of “black jade” occur in relative abundance on cobblestone beaches in northwest Washington State (Figure 4).

Nephrite specimens on these cobblestone beaches commonly occur as rounded masses that have been semi-polished from abrasion. The color is typically very dark green; but the material is commonly described as “black jade” (Figure 5). The stone is commonly homogeneous in composition, but some specimens show faint swirl patterns. The gemology grade is typically low because of the opacity and dark color, but samples have been used for artistic carving.

Sizes of individual specimens commonly range in size from pebbles (8–64 mm diameter) to cobbles (>64–256 mm diameter), but large boulders are locally present (Figure 6).

3.1. Late Pleistocene Sediment Transport

The original geologic setting of the Salish Sea nephrite is enigmatic, but the evidence is clear that this material was delivered to coastal locations as a component of sediment that arrived in the late Pleistocene, when a continental glacier flowed south from British Columbia to bury Puget Sound lowlands under an ice thickness of 1.2–1.4 km [19,20,21,22]. Beaches where nephrite is found are backed by bluffs of sediment that were deposited by a combination of glacial, glaciomarine, and interglacial fluvial processes [23,24]. Ice-transported sediment is typically poorly sorted, with clast sizes that range from sand and gravel to large erratics. Pebbles and cobbles in these beds are commonly well-rounded, providing evidence of fluvial transport during some stage of their travel from a Canadian source. The probable explanation is that earth materials were picked up during advance of a continental glacier and deposited mid-journey during subsequent recession. This sediment became available for fluvial transport during an interglacial interlude, becoming entrained as glacial sediment during a later glacial advance.

Late Pleistocene glacier flow directions are shown in Figure 7. Many of the larger clasts, including most erratics, have granitic compositions that are characteristic of the B.C. Coast Range (Figure 8). However, the presence of quartzite clasts suggests that another source area was the quartzite-rich peaks of the Monashee and Selkirk Mountains in eastern British Columbia. The dispersed source areas explain why glacial deposits in northwest Washington contain a wide range of rock types. The presence of nephrite can best be explained if the source of the material came from the ancestral Fraser River watershed, based on the abundance of nephrite in the riverbed. At its final deposition in western Washington, the coastal sediment included a large component of glacial outwash, as evidenced by the presence of sand rich layers. Whidbey Island beach cliffs include thick sequences of interglacial sediment dominated by silt, sand and woody peat; “black jade” clasts do not occur in these strata.

“Black jade” pebbles and cobbles can be observed in situ in the glacial strata at Point Whitehorn in Whatcom County and in the interglacial sediment on Whidbey Island (Figure 9).

3.2. Age of Glacial Transport of Nephrite

Late Pleistocene delivery of nephrite and other materials from Canada occurred over a long time span. On Whidbey Island, “black jade” occurs in the lower part of the Whidbey Formation, an interglacial sequence. The deposition age exceeds the range of ¹⁴C dating [26], but ⁴⁰Ar/³⁹Ar dating of pumaceous interbeds suggests an age of approximately 120 ka [27]. Nephrite-bearing glacial sediments in the Point Whitehorn/Bellingham area are younger, deposited during the final glacial episode, where the continental glacier crossed the Canada–USA border at ~18,000 ka, and retreated at ~11.5 ka [19,20,21]. Therefore, the arrival time for “black jade” spans ~100,000 y.

3.3. Bedrock Source of Transported Sediment

Nephrite occurs over a long span of the Fraser River (Figure 6), but bedrock sources remain enigmatic because of the topographically rugged and heavily forested slopes of the watershed. The regional geology is similar to the rest of the northwest Pacific coast, where bedrock is largely composed of Paleozoic and Mesozoic exotic terrane components, in combination with younger igneous intrusions and local Cenozoic terrestrial sediments.

4. Analytical Methods

A variety of methods were used for mineral identification. These include optical microscopy, X-ray diffraction, and SEM/EDS analyses. Petrographic thin sections were studied using a Leitz petrographic microscope equipped with a 5-megapixel CMOS microscope camera. SEM images were obtained using a Tescan Vega III SEM, with elemental microanalyses done with an Oxford energy-dispersive X-ray detector using AzTek 4.0 analytical software, analyzing regions on polished blocks mounted horizontally on 1 cm diameter aluminum stubs. Because XRF oxygen peaks are poorly quantifiable, oxygen values for silicate minerals were calculated based on stoichiometry.

X-ray patterns were made on packed powders using Ni-filtered Cu K-α radiation on a Rigaku Miniflex 6G diffractometer using Smartlab II software.

All analyses were performed by the author using facilities at Western Washington University, Bellingham, WA, USA.

5. Mineralogy of the Salish Sea “Black Jade”

Prior to this investigation, the Salish Sea “black jade” received very little study. There has been uncertainty regarding the composition of this material. Ream [3] examined six specimens, reporting them as amphibolite, argillite, and meta-argillite. He concluded “…these black rocks have some physical characteristics that are similar to, but mostly different than the characteristics of jade”. Ream’s conclusion may have resulted from analysis of specimens provided by a local collector that may have included a variety of dark, dense stones of varying geologic origin. For the present study, the author personally collected more than a dozen specimens that have homogeneous composition, green-black color, waxy luster, and densities of approx. 2.9 g/cm³. As described below, these specimens were identified as nephrite.

5.1. X-ray Diffraction

X-ray diffraction patterns from six specimens gave similar results. The diffractograms (Figure 10) show the major constituents. The most abundant mineral is a member of the actinolite/tremolite series. The compositions of naturally occurring nephrite specimens typically fall somewhere between tremolite and ferro-actinolite end members; XRD patterns are commonly not definitive for identification. A similar uncertainty exists with XRD determinations of plagioclase, where diffraction patterns do not show a clear indication of where specimens fall on the albite–anorthite continuum. In the present study, XRD-based mineral identifications were combined with SEM/EDS data, as described below.

5.2. SEM/EDS

SEM images from six specimens show that the Salish Sea “black jade” primarily consists of clusters of prismatic microcrystals of amphibole set in a fine granular matrix that is primarily composed of plagioclase feldspar (Figure 11 and Figure 12). Pyrite inclusions are locally present. Small magnetite inclusions are present in some specimens (Figure 11B). Mineral compositions were evaluated from elemental maps (Figure 13), and from semiquantitative elemental compositions determined from EDS spectra obtained from individual crystals of amphibole and plagioclase (

Table 1. Major element composition of individual amphibole crystals in one Salish Sea “black jade” specimen.

	1	2	3
Oxide wt. %
SiO2	41.4	42.6	46.5
TiO2	0.2	0.4	0.3
Al2O3	7.1	7.8	9.2
FeO *	38.0	32.5	32.9
MgO	4.6	4.3	4.2
CaO	4.5	6.6	7.4
Na2O	0.9	1.0	1.3
K2O	0.2	0.2	0.2
Element wt. % (cations)
Si	19.4	19.9	21.7
Ti	0.1	0.2	0.2
Al	3.8	2.3	4.9
Fe	29.5	25.3	25.6
Mg	1.7	2.6	2.5
Ca	3.2	4.7	5.2
Na	0.7	0.7	0.9
K	0.2	0.2	0.2
Atomic %
O	55.0	59.9	59.8
Si	17.1	17.2	17.7
Ti	0.0	0.1	0.1
Al	3.5	4.0	4.4
Fe	13.1	10.8	11.11
Mg	2.9	2.6	2.5
Ca	2.0	3.1	3.3
Na	0.7	0.9	1.0
K	0.1	0.1	0.1
Mg/(Mg+Fe)	0.18	0.19	0.18

* Total iron calculated as FeO.

).

The association of Fe-rich amphibole in a plagioclase-rich ground mass is a characteristic that was observed in all specimens. The mineral textures are somewhat variable. Figure 10 shows a common texture where the amphibole occurs as sheaves of thin prismatic crystals. In some specimens, the amphibole crystals are blocky (Figure 11). As discussed later in this report, the Salish Sea specimens have physical properties, XRD patterns, and chemical compositions consistent with nephrite, but they lack the felted fiber architecture typical of nephrite that is associated with serpentinite. This difference brings on the choice of whether the material should be named as nephrite or amphibolite.

SEM/EDS major element maps clearly show the various minerals (Figure 13). The EDS elemental data are only semiquantitative, but they are sufficient to provide a basis for interpretation of mineralogy. A limitation of XRF analysis is that there is no distinction between ferrous and ferric oxidation states. In Table 1, total iron is calculated as FeO.

The scarcity of Na and the relatively low Mg values are evidence that the amphibole is not hornblende, edenite, or magnesiohastingite. The atomic % values for major elements (Table 1, Figure 12) are useful for distinguishing between tremolite [Ca₂(Mg_5.0-4.5Fe²⁺_0-0.5)Si₈O₂₂(OH)₂], actinolite [Ca₂(Mg_4.5-2.5Fe²⁺_0.5-2.5)Si₈O₂₂(OH)] and ferro-actinolite [Ca₂(Mg_2.5-0.0Fe²⁺_2.5-5.0)Si₈O₂₂(OH)₂]. Members of the tremolite/ferro-actinolite series can be distinguished by the Mg/(Mg+Fe) ratio [28]. Tremolite is characterized by a ratio of >0.9, 0.5–0.9 for actinolite, and <0.5 for ferro-actinolite. The Mg/(Mg+Fe) ratios of the Salish Sea nephrite (Table 1, Figure 14) are indicative of ferro-actinolite. The Rigaku Smartlab peak search program identifies the amphibole XRD peaks in Figure 10 as ferro-actinolite.

The major element compositions of three individual amphibole microcrystals in one typical Salish Sea specimen are shown in Table 1.

5.3. Physical Properties

Nephrite is characterized by its physical toughness, which is defined as a resistance to fracturing [21]. The Mohs hardness of amphibole family minerals is typically ~6.5, the extremely high durability of nephrite comes from the crystalline microtexture [29]. For nephrite that forms in association with serpentinite, the material is typically felted needles (Figure 15). For comparison, jadeite is a pyroxene with Mohs hardness of ~7, with a texture composed of prismatic microcrystals rather than fine fibers (Figure 16). This coarser structure causes the toughness to be somewhat less than that of nephrite [29].

The Salish Sea “black jade” nephrite has a different microstructure compared to serpentinite-associated nephrite (Figure 15). In the “black jade”, amphibole occurs as prismatic microcrystals (Figure 11). This texture is reminiscent of “black jade” that is commercially mined at Ninhan, Murchison region, in the Perth area of Western Australia (Figure 16). The parent material for the Australian material is probably metabasite [30], and the physical characteristics are similar to those of the Salish Sea black material. In both materials, iron levels are very high, and the texture involves interlocking crystal clusters. Like the Perth specimens, the Salish Sea nephrite presumably originated from metamorphism of a mafic igneous rock.

6. The “Muttonfat Jade” Enigma

Amateur collectors have long prized dense, whitish specimens that were believed to be a variety of jade (Figure 17). Popularly labeled as “muttonfat jade”, mineralogical analysis shows that the material is a mixture of fibrous sillimanite and quartz [16].

Like the Salish Sea nephrite, the sillimanite-rich specimens always show rounding that suggests an episode of fluvial activity during their transport. Sizes include specimens that have masses that exceed 10 kg, though smaller specimens are more common.

Field evidence suggests that this sillimanite-bearing rock originated in the Fraser River watershed of British Columbia, Canada. Although the bedrock source is unknown, gravel bars on the Fraser River near Hope, B.C., have long been one of the most popular collecting areas. The high aluminum content is evidence of pellitic sediment as the most likely parent material. Sillimanite is common in the Settler Schist, near Harrison Lake [2], British Columbia, Canada, where bundles of sillimanite prisms are aligned parallel to the grains of original andalusite that they replace. In cross section, these bundles may show flattened diamond shapes, representing pseudomorphed andalusite [2]. Textures in the transported coastal specimen may have had a similar metamorphic parentage. Leaming [16] reported that some specimens contain small amounts of dumortierite, but this mineral was not evident in the specimens studied for this investigation.

7. Discussion

This investigation focuses on the glacial transport of nephrite and describes the mineralogy of the material long known by residents of the region as “Salish Sea black jade”. Scientists have high standards for mineral nomenclature, but there is much of the interest in nephrite among non-scientists who enthusiastically seek to acquire jade specimens. This investigation is therefore intended to provide information that is useful both to geoscientists and the multitude of amateur jade enthusiasts.

Ultimately, in popular culture the question “what is jade?” is more a matter of art than science. The textural difference between the Salish Sea material and classic nephrite is a reminder of the variation in pathways between the formation of amphibole minerals as a solid-state alteration product in mafic siliceous rocks versus amphibole formed by metasomatism with an ultramafic association. For the Salish Sea specimens, interlocking prismatic amphibole microcrystals produce the fracture resistance that is an important characteristic nephrite. There is an absence of the matted fibers typical of nephrite associated with serpentinite. Hence, there is not a decisive basis for establishing a gemological name for the material, which could be considered either nephrite jade or amphibolite. Amphibolites are commonly relatively coarse grained, composed of crystals that are visible to the naked eye, or with a hand lens. In contrast, the amphibole needles in nephrite are microscopic, resulting in a dense, durable mass where natural abrasion produces specimens that have well-rounded shape and waxy luster. The characteristics suggest that the Salish Sea “black jade” can be considered as a form of nephrite.

8. Conclusions

Pebbles, cobbles, and boulders of dense, dark rock that occur on beaches of the Salish Sea have mineralogical compositions and physical properties that support their identification as nephrite. These specimens were transported south by a combination of glacial and interglacial processes during the late Pleistocene. Ice flow directions show that the original bedrock sources were in British Columbia, Canada, but the specific source areas are enigmatic. The region’s paleogeography suggests that the source may have been rocks bordering the Fraser River, where alluvial nephrite has been collected for cultural purposes for more than 2000 years. However, much of B.C. nephrite resulted from metasomatism of serpentinite [16], in contrast to the presumed metaigneous origin of the Salish Sea nephrite. One possibility is that source outcrops that yielded the Salish Sea nephrite are unrecognized because of the lack of adequate exploration of the topographically rugged and extensively forested uplands bordering the Fraser River. A second possibility is that the transported nephrite came from surface exposures that were present during the Pleistocene, but later occurred as a result of sediment deposition during the final glacial retreat or eroded away as a result of tectonic uplift. The present geologic and topographic character of the region bears the imprint of Holocene processes [31,32]. Pleistocene rock transport processes are difficult to interpret.

Mineralogists and gemologists recognize three main varietal names: jadeite jade, omphacite jade and nephrite jade. In contrast, the historic Chinese name for jade is Yu, which simply means precious stone [16]. This cultural perspective is evident in current commercial listings for Chinese material that is described by sellers as “quartz jade” (including aventurine and rose quartz) and even “petrified wood jade.” Geologic investigations offer a path for reducing this nomenclatural variation. Hopefully, the information contained in the present report will serve as a small step toward that goal.

SEM and XRD evidence show that the transported “black jade” is composed of prismatic ferro-actinolite microcrystals set in an albite-rich matrix. The microtexture produces the high fracture resistance typical of classic nephrite. Based on the mineralogical and physical characteristics, in this investigation the Salish Sea material is herein considered to be a type of nephrite.