Geochemical relationship between REE and PGE in mafic and ultramafic rocks and their petrogenetic significance - Rare Earth Elements in Ultramafic and Mafic Rocks and their Minerals - page 244

10.2 NARANSKY MASSIF (WEST MONGOLIA)

This chromite-bearing massif located 15 km south-east of the Gobi Altai city is one
of the largest mafic-ultramafic massifs in the territory of West Mongolia. It is a part
of Dzabkhan ophiolite belt that traces the zone of abyssal fracture. The massif is
extended to the west-northwest direction at 38 km at a width of 0.5-10 km. Its struc-
tural position, internal structure, petrographic, mineralogical, petrochemical, geo-
chemical and metallogenic features were discussed in many publications [Pinus et al .,
1976, 1984; Lesnov, 1979, 1981, 1982; Lesnov & Simonov, 1989; Dmitrenko et al .,
1991]. Below, we discuss the previously obtained data on the distribution of REE and
PGE in rocks of the massif [Lesnov & Tsimbalist, 1983; Lesnov et al ., 1990; Lesnov
et al ., 1993], as well as new materials related to this issue.
Naransky massif is a complex plutonic body, the structure of which contains three
main elements [Lesnov, 1982]: 1) the large steeply dipping ultramafic restite protru-
sion, and 2) intruding through this protrusion along its southern tectonic contact,
much inferior to it in size and also steeply dipping gabbro intrusion of elongated form,
and 3) the contact-reaction zone localized along the boundaries of ultramafic protru-
sion and gabbro intrusion, which is composed of hybrid rocks of ultramafic and mafic
composition. To the south of the massif, among the host strata there are several small
tectonic blocks of ultramafic restites and gabbro stocks intruding through them and
the host strata. The host strata is represented by metamorphosed terrigenic-volcanic
and carbonate rocks of the Vendian-Cambrian age. Naransky massif and its host
strata are transgressive overlain by horizons of volcanites of average composition and
conglomerates of Devonian age. The later formation of gabbro intrusion in relation
to ultramafic protrusion is confirmed by the contact-reaction zone located along their
borders, xenoliths of ultramafic restites in the composition of gabbro intrusion found
in several places, as well as their veined bodies of gabbro and pyroxenites injecting
them. Ultramafic restites are represented by harzburgites and by less common lher-
zolites and dunites, stripe-like bodies of which alternate with harzburgites. All these
rocks are serpentinized and dinamometamorphosed on different levels, especially
when close to the faults. Gabbro intrusion and its stock-like satellites are composed of
gabbro-norites and gabbro, as well as olivine-bearing varieties of them. The structure
of the contact-reaction zone involves websterites that alternate with lenticular and
stripe-like isolations of wehrlites and olivine gabbro of variable quantitative-mineral
composition. Among the ultramafic restites many diluvial-eluvial and as well as some
indigenous manifestations of ore chromitites there were found. In websterites of the
contact-reaction zone there was scattered sulphide mineralization identified, which
contains microparticles of copper-bearing minerals of Pd and Pt. In many schlich
samples from the alluvium and proluvium of temporary channels draining the massif
there were grains of PGE minerals of various composition, as well as Au were found
[Lesnov, 1982; Dmitrenko et al ., 1991, Lesnov et al ., in print].
The distribution of REE, PGE and Au in rocks of Naransky massif has been
studied using atomic absorption method, as well as RNAA and ICP-MS methods
(Table 10.6, 10.7). According to the available analyses, the lowest total REE contents
are characteristic of dunites (0.18 ppm). Note that approximately the same average
total REE content (0.20 ppm) was determined in dunites from the Twin Sisters mas-
sif (USA) [Frey et al ., 1971], as well as in the international geochemical standard of