Ishihara, and 6 co-authors, reported in 2011 that selected ores from the Sin Quyen allanite-rich Au-bearing chalcopyrite-magnetite deposits were studied microscopically and chemically. The ore minerals tend to occur along the NW-SE-trending sheared zone of altered host rocks; yet the ore minerals show no stress effect, implying the mineralization later than the regional shearing. The ore minerals occur associated with the metasomatic minerals consisting mainly of clinopyroxene, hastingsite, allanite, epidote, biotite, titanite, carbonate minerals and rarely quartz.
The Allanite series of minerals contain almost all of the rare earth element (REE) components of the ores occur in disseminated manner and are euhedral to subhedral. The mineral is an early crystallized mineral replaced by magnetite and chalcopyrite. The allanite contains around 16 wt.% REE and is low in mafic components having chemical composition of Mn-poor type, which tends to occur in the magnetite-series granitic rocks in Japan. The REE components could have been derived from an oxidized alkaline granitic activity of mid-Tertiary. Both chalcopyrite and magnetite are well separated by the mineral dressing, and all the allanites moved to the tailings.
The authors concluded that:
1) The magnetite and chalcopyrite ores of the Sin Quyen deposits occur with metasomatite consisting of pyroxene, hastingsite, allanite, epidote, biotite, titanite and latest-stage carbonates.
2) Both allanite and ore minerals show no stress effect; the mineralization appear to be much later than the deformation of the host rocks, possibly related to magnetite-series granitic magmatism of alkaline affinity during a Paleogene time.
3) The chalcopyrite-magnetite ores have LREE-enriched character, because allanite is the main REE mineral. Allanite is euhedral and epidotized at the rim, and is an early crystallized mineral replaced by magnetite and chalcopyrite.
4) The studied allanites are depleted in “mafic” components, and could be compared with allanites in the magnetite-series granite.
5) The allanites have chemical composition of Mn-poor type of Hoshino et al. (2007), indicating that the mineral crystallized from metasomatic fluids liberated from an oxidized magmatic activity.
6) Both chalcopyrite and magnetite are well separated by the mineral dressing, and all the allanites moved to tailings.
Therefore, the tailing pond turns out to be an excellent LREE reservoir in future.
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