Study On The Mineralogy And Chronology Of The Nianzigou Rutile Ore Deposit In Daixian County, Shanxi Province

As an important industry mineral, rutile is a major resource for metal titanium. In China, although Ti resource is very rich, Ti resources from rutile, however, take up only 2%, since 98% Ti resources are from ilmenite. Hence, it is very important to study rutile mineral resource. The Daixian rutile deposit, with reservoir of 3.6967 million tons, is one of the largest rutile deposits in China. Located on western part of the Henshan Mountain, the Daixian rutile bodies occur as 11km long belts within NE-strike ultramafic rock belt. Till now, data about forming age, evolution and protolith of the rutile deposit have not acquired. In this investigation, combined studies of petrography, mineralogy and geochronology were conducted on this deposit.According to mineral components, ores of the Daixian rutile deposit can be classified into the following types: actinolite- bearing anthothyllitite, actinolite-garnet- bearing anthothyllitite, staurolite- actinolite- bearing anthothyllitite, actinolite-biotite-bearing anthothyllitite, kyanite-bearing anthothyllitite, talc-actinolite- bearing anthothyllitite and chlorite-actinolite-bearing anthothyllitite, etc. According to color of rutile, ores of the Daixian rutile deposit, the second biggest rutile deposit in China, can be classified into two types: red and black ore; the red ore generally occurred at upper part of the deposit, overlying the black ore without distinct border. These two ores have contrasting mineral associations: anthothyllite (Ath) + plagioclase (PI) + rutile (Rut) + magnesiohornblende (Mhb) + alkline fledspar (Afs) for red ore and anthothyllite + phlogopite (Phi) + epsode (Ep) + Rutile for black ore. A chemical reaction was suggested as Mhb + Afs → Ath +/- PI +/- Phi (?) based on petrographical observations in red rutile ore, PI is possibly the transitional phase and complete reactants are Ath + Phi (?). The mineral associations of black ore are thus infeered to be more complete reactants of this reaction. As for rutiles, red rutiles are homogeneous in composition while black ones bear ilmenite exsolution lamellae of up to 2 μm in thickness. It is concluded that existence of the lamellae causes color difference between these two rutiles. The rutile-hosted rocks are metamorphic rocks with amphibolite facie, but more work should be done to discuss features of their protolith.Characteristics of zircons revealed by cathodeluminescence images from rutile ores with different colors are incredible similar. There are three stages upon formation sequence of the zircons. Stage-I and stage-ll zircons have oscillating zones with darker luminescence, Th/U ratios are mostly more than 1.00, indicative of igneous zircons. These zircons have nearly the same ²⁰⁴Pb -corrected ²⁰⁷Pb/²⁰⁶Pb ages, with weighted mean age of ca 2.5Ga, interpreted as forming or anatexis age of protolith of the rutile ores. Stage-Ill zircons occur as rims surrounding or as veins crosscutting stage-l or/and II zircons, have lower U, Th and lowest Th/U ratios. They yield a weighted mean age of ca 1.8 Ga, interpreted as metamorphic age of the rutile ores.Moreover, step-heating ⁴⁰Ar/ ³⁹Ar geochronology on these rutile ores shows that the red-color rutile ores are stable without experiences of more than 350°C thermal events since ca 1.8 Ga, whereas the black-color rutile ores had undergone a thermal event of more than 350°C, but less than enclosure temperature of zircon at ca 1.4 Ga after metamorphised. It is inferred that the event at ca 1.4 Ga resulted in color changes of the rutile ores.