ZIRCON from the HARTS RANGE, NORTHERN TERRITORY, AUSTRALIA by Maxwell Faulkner and Tames E
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ZIRCON FROM THE HARTS RANGE, NORTHERN TERRITORY, AUSTRALIA By Maxwell Faulkner and Tames E. Shigley Gem-quality zircon from a relatively un- long-known but underdeveloped locality in Austra- derdeveloped locality in the Harts Range A lia's Harts Range is now producing some magnificent of central Australia is described. While gem zircons in an attractive variety of yellow, brown, pink, exhibiting many properties of other gem and purple colors (figure 1) in sizes typically of several zircons, this material is unusual in its carats. Occasionally, even near-colorless crystals are almost total lack of radioactive trace ele- found. Of particular significance is the fact that these inents. Thus, there is little or no radia- tion-relatedstructural damage as is the zircons contain little or no detectable amounts of radioac- case with softie other gem zircons. The tive trace elements. Thus they display no evidence of the Harts Rmge material occurs in a size, structural damage that is common in some gem zircons quality, and color range suitable for from other localities. This article briefly describes the faceting. geologic occurrence and gemological properties of these interesting zircons. WHAT IS ZIRCON? Zircon is a widely distributed accessory mineral in ig- neous rocks, particularly granites and syenites (Deer et al., 1982; Webster, 1983).It is a fairly common detrital mineral in some sediments due to its resistance to chemical attack. Zircon also occurs in certain metamorphic rocks such as marbles, gneisses, and schists. While often found as small, ABOUT THE AUTHORS rounded grains, zircon can occur as large, well-formed Mr. Faulkner is a gem cutter from New South prismatic crystals. Because of its relatively high refractive Wales, Australia. Dr. Shigley is director of re- index, dispersion, and hardness, zircon has long been used search at the Gemological Institute of America, as a gemstone. Santa Monica, Calilornia. Chemically zircon is zirconium silicate (ZrSi04);how- Acknowledgments The authors thank Dr Elton L ever, there is always a small amount (usually about l %)of McCawley of Portland, Oregon, for his contnbu- lions to this study Waldo Winterburn, of Stanford the element hafnium present (Deer et al., 1982).A number University carried out the XRF and XRD analyses of trace elements can also occur in zircon, including John Kotvula of GIA documented the gemological uranium and thorium. When present, these two trace properties and took the photomicrographs Dr Emmanuel Fntsch of GIA assisted in the inter- elements undergo radioactive decay, thereby giving off pretation of the absorption spectra Bill Hagan of energetic alpha particles that can cause extensive struc- Rubyvale, Queensland, provided information on tural damage. As a result of this internal radiation bom- heat treatment Microprobe analyses were per- formed by Paul F HIava of Sandia National Labo- bardment, the initially crystalline zircon (referred to as rafones high zircon) progressively changes into an amorphous, Gems & Gemology, Vol. 25, No. 4, pp. 207-215 noncrystalline (ormetamict) state (lowzircon;see Holland 0 1990 Gemological Institute of America and Gottfried, 1955).Transition to the metamict condition Zircon from Harts Range GEMS & GEMOLOGY Winter 1989 207 Figure 1. These faceted zir- cons from the Harts Range illustrate some of the at- tractive colors in which this material occurs. They range up to 5 ct in wei* Photo by Robert Weldon. is accompanied by changes in physical and chemi- damage. Like most gem zircons, stones from the cal properties (such as a decrease in density, Harts Range belong to the high type. refractive index, and transparency, and an increase in water content). Thus, gemological properties LOCATION AND ACCESS such as refractive index or specific gravity can lie The Harts Range lies in the south-central portion anywhere between the values for high and low of the Northern Territory, toward the center of zircons (see table 1). Australia (see figure 2). The zircon occurs at a site While crystalline zircons occur in a range of aptly called Zircon Hill, which can be reached colors, metamict zircons are typically green or from the town of Alice Springs by driving 69 lzm brown. Some low and intermediate zircons can be (43 mi.)north on Stuart Highway, and then 77 lzm transformed back into high zircons by heating them (48 mi.) east on Plenty Highway. The turnoff from to 1450° for six hours (Chuboda and Stackelberg, Plenty Highway to the zircon deposit is marked by 1936))which heals the radiation-induced structural a large windmill and several concrete storage containers at a livestock watering station called Mud Tank Bore. From this turnoff, the last 9 km TABLE 1. The three types of zircon.3 (5.6 mi.) south to Zircon Hill is a gravel road, suitable for passenger cars. The digging area for Property Normal (High) Intermediate Metamict (Low) zircons is in open savannah country crossed by a Color Colorless, Brownish Green, orange, few dry stream beds. brown, green, green, brown yellow, red- brownish GEOLOGY brown, orange, red blue (heat The low-lying hills of the Harts Range extend east- treatment) west for approximately 150 lzm (93 mi.). They are Structural Crystalline, Slightly Amorphous, composed of schists, gneisses, and other strongly state undamaged damaged damaged metamorphosed sediments and volcanic rocks Refractive which have been intruded by occasional peg- indices 0 matite~(for details of the local geology, see Jolzlilz, â 1955).The area has long been known as a source of Birefringence mica, but it also contains numerous separate small Specific deposits of gem minerals such as ruby, aqua- gravity marine, garnet, and amethyst that have been Radioactivity Low Medium High mined sporadically in the past (see McColl and Warren, 1980). In addition, gem-quality iolite, "Properties compiled from Anderson (1941), Webster (1983), Liddicoal (1987). epidote, sunstone feldspar (called "rainbow lat- tice" sunstone in the trade), as well as kornerupine 208 Zircon from Harts Range GEMS & GEMOLOGY Winter 1989 Plenty Hwy. Figure 2. Zircon Hill is lo- cated in the Northern Terri- Mud Tank Bore tory of Australia, approx- imately 155 km by road northeast of Alice Springs. Neighboring Specimen Hill also produces zircons, but few are of gem quality Art- work by Jan Newell. \ L and sapphirine (McColl and Warren, 1984) are phlogopite, magnetite altered to martite, and apa- found in small amounts. A guidebook to localities tite are present. in this area has been published by the Northern While there are many sites in Australia where Territory Department of Mines and Energy sediments rich in fine-grained zircons occur, the (Thompson, 1984). area around Zircon Hill produces the mineral in Brown et al. (1989) briefly describe the gem- large crystals. Zircons up to 2.5 kg (5.5 lbs.) have quality zircons that are found at Zircon Hill both been recovered from the decomposed carbonatite. as large crystals and crystal fragments. The entire Although these large crystals are invariably crest of the hill, a slight, brush-covered promi- cracked and flawed, they may contain small areas nence that rises some 50 m above the surrounding of material suitable for faceting. Much more com- plateau (which is 1,500 m above sea level), is mon are crystals or crystal fragments that range up covered by small diggings (figure 3).Zircon is also to several carats in weight. Only recently has the found at a nearby location called Specimen Hill, gem potential of this material begun to be recog- but these crystals, although better developed than nized (Brown et al., 19891. those from Zircon Hill, are generally unsuitable for faceting because of internal fracturing. Both MINING locations have been known by local miners and According to guidelines issued by the Department mineral collectors for about 40 years. of Mines and Energy, digging at these deposits can At these two localities, the zircon occurs in only be done with hand tools, and only after one carbonatite, a carbonate-rich magmatic igneous has obtained a prospecting license. Use of explo- rock that intruded the country rock to form a sives or mechanical equipment is prohibited. series of low hills. The carbonatite is late Pro- Hence, there has not been any large-scale mining terozoic, and has been age-dated between 1.50 and in this area. 1.78 billion years. In addition to calcite and zircon, On Zircon Hill and along a nearby creek bed, Zircon from Harts Range GEMS & GEMOLOGY Winter 1989 209 zircon from other mineral and rock frogmen ts stands in the center of the photo- graph. crystals and fragments are recovered by hand Some of this material may be enhanced by heat digging and dry sieving of the weathered soil that treatment. No accurate figures are available for the covers the carbonatite. Because of zircon's subada- quantity of zircon that has been recovered from mantine luster, the glistening but often highly this deposit thus far, nor has any estimate been fractured crystals and fragments are easily recog- made of possible reserves. However, the potential nized. After six hours of easy digging, about 0.5 kg seems quite good. (1 lb.) of mine-run zircons can usually be recov- ered. Pick-and-shovel mining of the underlying CHARACTERIZATION OF weathered but still intact carbonatite can yield HARTS RANGE ZIRCON zircon matrix specimens. For this study, we examined eight rough zircons As is usual for many gem mineral deposits, less and two faceted stones. The rough pieces weighed than 5% of the mine-run material is facet grade. between 6.6 and 37.1 ct (figure 4). With the exception of the largest sample, the rough pieces were rounded crystals or fragments. The largest Figure 4. These four of the eight rough zircons sample exhibited some crystal faces and a recog- examined for this study are representative of nizable tetragonal habit.