Lục Yên District, Yên Bái Province, Vietnami

Regional Level Types
Lục Yên District	District
Yên Bái Province	Province
Vietnam	Country

There are numerous quarries spread along a 12 km line in this area, all generally labelled "Luc Yen", although some should probably be renamed.

Chauviré et al. (2015) give an good overview of the geology in the Luc Yen area: “The rich tectonic history of Southeast Asia is inherited from several deformation episodes related to the closure of the Paleo-Tethys Ocean and, later, to the Himalayan orogeny. The geology of northern Vietnam is dominated by metamorphic rocks inherited from these two major orogenic events. The first one, the Indosinian orogeny, led to the collision of the main shields (Yangtze and Indochina) during the Permo-Triassic at about 240–245 Ma. In the later orogeny, the Himalayan collision during the Tertiary period, the terrains were strongly reworked.”

The gem deposits are hosted by platform carbonates metamorphosed to the amphibolite facies. Near Luc Yen, these marbles occur in the Lo Gam tectonic zone in the Cambrian Thac Ba and An Phu metasedimentary sequences. The up to 300 m thick marble units are intercalated with biotite-garnet-sillimanite or -kyanite gneisses, and granitoids. The Lo Gam zone is limited by two left-lateral faults which belong to the Red River Shear Zone and is intruded by Triassic granitoids.

The marble horizons show different parageneses, and Van Long et al. (2018) publish data showing that the spinel is Permian (256 Ma) and ruby Triassic (38 MA), both having seen similar metamorphic condition. They refer to Hauzenberger et al. (2001) which considered a minimum temperature of about 700°C for the formation of spinel based on the mineral assemblage Chu-Cal-Fo-Dol- Spl at water-rich fluid composition and compared to the retrograde temperatures for the formation of ruby between 620 and 670°C.

Humite-group minerals are found in Mg-rich associations, consisting of calcite, dolomite, spinel, forsterite, clinohumite, apatite, margarite, pargasite, chlorite, ± graphite, ± pyrite, ± pyrrhotite. It is believed (Garnier et al., 2008), that the marble horizons were metamorphosed as closed systems, i.e. that no fluid or metasomatic element influx took place. The Mg, Al and other elements needed to form the various minerals originated as aluminous clay, mafic volcanics (ash?) and evaporate layers within the Cambrian sediments.

NOTE: All of the analysed 'fluor-cannilloites' are pargasite (see http://www.mindat.org/mesg-106-259756.html, http://www.mindat.org/mesg-7-69925.html and http://www.mindat.org/mesg-7-198029.html). Hauzenberger et al. (2005) report intense green pargasite in association with red spinel from Luc Yen. This pargasite has been confirmed by EPMA and single-crystal XRD.

Fossils

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Other Databases

Wikipedia:	https://en.wikipedia.org/wiki/L%E1%BB%A5c_Y%C3%AAn_District
Wikidata ID:	Q6695817
GeoNames ID:	1905666

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References