"Rock of the Month #205, posted for July 2018" ---

CHROMITE needs little introduction here. A cubic oxide of the spinel family (of which the most famous examples are arguably the eponymous pink Mg-Al "spinel" from Burma and the ubiquitous iron ore, magnetite), chromite or chrome spinel is an ore mineral that varies widely in composition. The ideal formula is FeCr₂O₄. The Cr and Fe in its structure are variously substituted by percent levels of other elements, notably Mg and Al. It is the principal, and essentially the only important ore mineral of chromium, and may occur as layers or pods of rock in which it is very abundant, so-called chromitites. Another chromium mineral of note is crocoite, lead chromate (see an example from Tasmania).

OPHIOLITES are named for the Greek for snake, because they are lithological associations rich in serpentine, and the green and shiny appearance of rocks rich in magnesian minerals such as serpentine and talc reminds people of serpents! Ophiolites are essentially slices though the ocean crust. The oceans are much thinner than the continents, and the thickness of rock from the surface (of the continent, or the sea bed) down to the base of the crust (the Mohorovicic discontinuity, or "Moho") is typically tens of km on the continents (thicker under high mountain ranges) but just a few km under the oceans. Because the convection of magma in the upper mantle is such that lava erupts at mid-ocean ridges (as in Iceland, for example), the structure of ophiolites is dominated by igneous rocks. Though the deep ocean floor has a thin layer of sediment, the ophiolite beneath is largely erupted basalt lavas, underlying "sheeted dykes" formed from innumerable basalt dykes injected into the crust, and at depth the coarser-grained intrusions of gabbro and more "primitive" (magnesium and iron-rich) plutonic rocks such as peridotites and pyroxenites (respectively rich in olivine and pyroxenes).

A few examples of ophiolite complexes, as these crustal cross-sections are named, appear in Table 1. The top three entries are relevant to this article, and below are a few other examples, famous in the history of Earth science. A typical ophiolite provides samples of both ocean crust and upper mantle. Submarine eruptions of basaltic lava generate extensive outcrops of pillow lavas. Though ultramafic and gabbroic intrusive rocks are the norm, there may also be smaller volumes of siliceous igneous rocks, the quartz- and plagioclase-dominant plagiogranites (Miyashiro, 1975; Dilek and Robinson, 2004; Hebert, 2007; Dilek and Furnes, 2014). The examples listed in Table 1 are of Phanerozoic age (younger than 540 Ma), but Precambrian examples are well-known, though they may be altered by deformation and metamorphism (Kusky, 2004). The plate-tectonic setting of individual ophiolites can be determined from the distribution of minor and trace elements in the volcanic suites (Pearce, 2014).

Podiform chromitites that attain the scale of economic Cr deposits are associated with harzburgite (olivine- orthopyroxene) mantle rather than lherzolite (olivine-rich rocks with essential clinopyroxene and orthopyroxene) mantle (Leblanc and Nicolas, 1992). The deposits form in the transition zone at the top of mantle units, immediately below the Moho, and only rarely are they >1 km below it. The chromite deposits tend to be larger and more numerous in mantle diapir zones,"which channeled most of the basaltic liquid supplied to the crust" PGE contents are generally low, 0.1-0.5 ppm total PGE), and the few with >1 ppm PGE contain relatively more sulphides and may contain traces of arsenides (Leblanc and Nicolas, 1992).

Here is a brief history of work on the Luobusa ophiolite and associated topics, 1979-2016, with something of a Canadian perspective, as seen in articles and conference abstracts. Luobusa is located some 200 km ESE of Lhasa. There are two east-west belts of ophiolites across the Tibetan plateau. Luobusa lies in the south belt, with Xigaze and Dazhu. The north belt includes the Donqiao ophiolite (Zhou et al., 1997).

Regional context

IGCP project 39, "ophiolites of continents and comparable oceanic rocks" was something of a worldwide gazetteer, including entries on Tibet, Bhutan, Nepal, India, Pakistan and Afghanistan (Working Group of Ophiolite Project, 1979). Tibetan ophiolites and lithological associations in neighbouring regions were noted in context of the wider Himalayan and Tethyan contexts (e.g., Burke and Kidd, 1980; Stocklin, 1980; Burg and Chen, 1984; Windley, 1985; Frank et al., 1987; Searle et al., 1992; Searle, 1993). The breadth of modern China contains a range of styles and ages of magmatic Ni- Cu- PGE and ophiolite-hosted Cr deposits (Pirajno, 2013, pp.457-467).

Earlier work was conducted by pioneers such as Augusto Gansser, especially on the Indian side of the Himalayas (Srikantia, 2012). This includes parts of the Indus-Tsangpo (Yarlung-Zangbo) suture zone, ophiolite nappes such as the Spogtang (Shilakong) nappe, the Nidar and Saltoro ophiolites and the Shyok ophiolitic melange (e.g., Upadhyay, 2002; Jain and Singh, 2009). The Indus-Yarlung-Zangbo suture zone extends east from India across southern Tibet to Burma. The Xigaze ophiolite segment of the Indus-Yarlung- Zangbo suture zone contains four massifs (from west to east: Jiding, Xialu, Dazhuqu and Zedang) with a range of lower crustal and mantle rocks: harzburgites, gabbro and diabase intrusions, orthopyroxenite, peridotites, lherzolite and epidosite (Hebert et al., 1999). The podiform chromitites such as those at Luobusa soon attracted attention (Edwards et al., 2001).

The Luobusa and Dongqiao ophiolites

Some ophiolites such as Dazhu have basalt chemistry indicative of a supra-subduction zone setting (Xia et al., 2003), whereas others retain deep mantle signatures.

More detailed studies of rocks in ophiolites in Tibet uncovered some unusual features. In particular, diamonds were found in heavy mineral concentrates of mantle peridotites and podiform chromitites of the Luobusa and Donqiao ophiolites. An unusual high-pressure reduced assemblage is associated with diamonds: moissanite (SiC) and graphite, native chromium and native nickel, nickel-iron alloy as well as more commonplace minerals in mafic-ultramafic rocks, such as chromite. The ophiolites lie along two major belts: the Indus-Yarlung-Zangbo suture (with the Luobusa, Xigaze and Dazhu ophiolites) on the south, which separates the Indian subcontinent from the Lhasa block, and is thus the locus of continental collision between India and Asia (Makovsky et al., 1999), and the Bangong-Nujiang suture (with Dongqiao) on the north, separating the Lhasa and Tanggula blocks.

The ophiolites are allochthonous bodies that may contain diamondiferous peridotites and podiform chromitites, harzburgite and diopside-bearing harzburgite, with abundant lenses of dunite and chromitite, including some cumulates. The mineralogy, aspects of which are more familiar in meteorites, indicates a high-pressure, reducing setting, associated with the collision of the Indian and Eurasian plates. The mineralogy is very exotic, with SiC in the Donqiao massif occurring as colourless, frequently euhedral grains up to 300 microns across (Bai et al., 1993a,b).

Diamonds were found in heavy mineral separates of mantle peridotites and podiform chromitites from the Luobusa and Donqiao ophiolites, and placer diamonds have also been found in the region. A high-pressure setting is indicated by associated phases such as SiC and graphite. Native Cr occurs in cracks and may be a late secondary mineral. Diamond formation is thought to have occurred during deep subduction of oceanic crust into the mantle (Zhou et al., 1993). The Luobusa ophiolite contains the largest known chromite deposit in the region, with podiform chromitites hosted in harzburgite or diopside-bearing harzburgite. The ore-forming chromites are very Cr-rich with a Cr number of 78, whereas disseminated chromites in harzburgite are much richer in Al₂O₃, with a Cr number of 50 (Zhou and Robinson, 1994).

Ophiolites such as Xigaze, 200 km west of Luobusa, have features suggestive of a mid-ocean ridge origin. Luobusa has depleted mantle peridotites indicative of high degrees of partial melting above a subduction zone, with a high volatile content. Luobusa peridotites have low Al₂O₃ (mean 1.16 wt.%) and CaO (mean 1.25 wt.%) and host abundant podiform chromitites (mean Cr number 82). In contrast, residual chromites in the peridotite are quite Cr-rich, with Cr numbers 22 to 82. Mantle peridotites at Xigaze are quite enriched and lack the podiform chromitites. Sparse volcanics at Luobusa are basalt and basaltic andesite, of chemistry compatible with an arc-related setting. The cumulates at Luobusa are mainly dunite, pyroxenite and gabbro, whereas troctolite is dominant at Xigaze (Zhou et al., 1995).

Exotic mineralogy

Most of the type specimens of more than 100 new mineral species first found in China reside at the Geological Museum of China, in Beijing. Tibetan phases include anduoite, (Ru,Os)As₂, from the Anduo chromite deposit and luobusaite, Fe_0.83Si₂, a new silicide from chromitite (de Fourestier, 2007). Further exotic minerals, such as as qingsongite (BN, a boron nitride, found between coesite crystals) have been described from Luobusa (Robinson et al., 2015; noted in the great compilation of Bernard and Hyrsl, 2015). This cubic BN, qingsongite, is named in honour of Qingsong Fang (1939-2010), discoverer of the first diamond in the chromitite of the Luobusa ophiolite. The mode of formation of the tiny nitride crystals, at exceedingly high pressure and temperature, probably involves a pelitic rock fragment subducted to mid-mantle depths, and and then exhumed by entrainment in chromitite (Dobrzhinetskaya et al., 2014).

Thus, Luobusa chromitites host a very unusual mineral assemblage, including SiC and diamond, graphite and corundum, sphene (titanite), rutile, sulphides,platinum-group minerals (PGM), native Fe, and native Au, almandine and uvarovite garnets, kyanite, Cr diopside and Cr chlorite. All the unusual minerals occur as inclusions in the chromite, or as interstitial material between chromites. The chromites have high Cr numbers and moderate Mg numbers, suggesting crystallization from a boninitic melt. The diamonds are octahedral and cubic crystals, typically 200-400 microns across. The moissanite is typically euhedral, up to 1 mm in diameter (Hu et al., 1998).

In detail, there is a bewildering diversity of PGM and base-metal alloys in the podiform chromitite. There are Os- Ir and Os- Ir- Ru alloys, Pt- Fe alloys, Ir- Ni- Fe alloys, Fe- Ni- Cr alloys and Fe- Co alloys. Other minerals include FeSi (an Fe silicide), native Fe, native Ni, native Cr, native Au, native Cu and native Si. The alloys and native elements were recovered primarily from heavy mineral separates of the chromitites, but some are included within or attached to magnesiochromite crystals. The rare-mineral grains are mostly subhedral to anhedral, 50-500 microns in size. Some may be secondary phases formed by alteration of PGE-sulphide minerals. Others, such as Fe silicide and maybe native Si are thought to be xenocrysts from the mantle (Bai et al., 2000). A more recent find is native Ti (Robinson et al., 2015).

The wider context

While the occurrence of economic primary deposits of diamonds in kimberlites and lamproites is well-known (Nixon, 1995), there are assorted other modes of occurrence, such as the Tibetan ophiolites, garnet pyroxenites, metadunites, eclogite facies rocks in the Dabie Shan (China) and in Norway, and others. A review of non-kimberlitic diamondiferous igneous rocks (Kaminsky, 2007) includes lamproites, assorted lamprophyres and unusual alkaline rocks, and ultramafic rocks from around the world, Tibet included. Yang et al. (2014) review the occurrence of diamonds in ophiolitic chromitites.

The Luobusa ophiolite soon yielded evidence for ultra-high-pressure (UHP) metamorphism in the mineral assemblages of podiform chromitites. The UHP minerals are thought to have been transported from the lower mantle in a mantle plume, and incorporated into the ophiolite during sea floor spreading during the lower Jurassic, dated at 176 Ma (Robinson et al., 2004).

Notably, the Luobusa, Dongqiao, Semail and Ray-Iz ophiolites all contain SiC (moissanite), and Luobusa and Ray-Iz also contain in- situ diamonds. Most zircons are subrounded grains with complex internal textures indicative of polyphase growth, with inclusions indicative of an origin in continental crust. Minerals such as zircon suggest derivation from metasediments subducted to mantle depths. Such minerals may have survived by encapsulation in chromite grains. The chromite was then carried up in melt and deposited as chromitites near the Moho.

The complexity of these rock assemblages is explained in part by the existence of collision zones such as the Dabie-Sulu orogen in China, where continental crust is subducted into the mantle, subject to UHP metamorphism at depths >100 km, and then rapidly exhumed (Robinson et al., 2015). These rocks are mostly gneiss and eclogite, with diamond and coesite inclusions in zircon and garnet, and the zircons typically have old cores with younger overgrowths.

Forty zircon grains from Luobusa yield ion-microprobe U-Pb ages of 549 to 1675 Ma, all much older then the host ophiolite. All the podiform chromitites in the ophiolites listed in Table 1 are in upper mantle sections of ophiolites, within a few km of the crust-mantle boundary (Robinson et al., 2015). In recent years, a group at the Chinese Academy of Geological Sciences in Beijing has focused on mantle studies, and systematically considered the ophiolite belts of Tibet (Xiong et al., 2015, 2017a,b, 2018) and elsewhere. The existence of inclusions of diamond, coesite and other high-pressure and/or reduced phases, within magnesiochromite crystals, affirms a deep origin for some chromitites, and subduction processes extending some hundreds of kilometres into the upper mantle.

See also illustrated examples of the UG-2 chromitite of the Bushveld layered intrusion in South Africa and other spinels such as franklinite.

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