New textural and mineralogical constraints on the origin of the Hongge Fe-Ti-V oxide deposit, SW China

Abstract

The Hongge magmatic Fe-Ti-Voxide deposit in the Panxi region, SW China, is hosted in a layered mafic-ultramafic intrusion. This 2.7-km-thick, lopolith-like intrusion consists of the lower, middle, and upper zones, which are composed of olivine clinopyroxenite, clinopyroxenite, and gabbro, respectively. Abundant Fe-Ti oxide layers mainly occur in the middle zone and the lower part of the upper zone. Fe-Ti oxides include Cr-rich and Cr-poor titanomagnetite and granular ilmenite. Cr-rich titanomagnetite is commonly disseminated in the olivine clinopyroxenite of the lower parts of the lower and middle zones and contains 1.89 to 14.9 wt% Cr 2 O 3 and 3.20 to 16.2 wt% TiO 2 , whereas Cr-poor titanomagnetite typically occurs as net-textured and massive ores in the upper middle and upper zones and contains much lower Cr 2 O 3 (<0.4 wt%) but more variable TiO 2 (0.11 to 18.2 wt%). Disseminated Cr-rich titanomagnetite in the ultramafic rocks is commonly enclosed in either olivine or clinopyroxene, whereas Cr-poor titanomangetite of the net-textured and massive ores is mainly interstitial to clinopyroxene and plagioclase. The lithology of the Hongge intrusion is consistent with multiple injections of magmas, the lower zone being derived from a single pulse of less differentiated ferrobasaltic magma and the middle and upper zones from multiple pulses of more differentiated magmas. Cr-rich titanomagnetite in the disseminated ores of the lower and middle zones is interpreted to represent an early crystallization phase whereas clusters of Cr-poor titanomagnetite, granular ilmenite, and apatite in the nettextured ores of the middle and upper zones are thought to have formed from an Fe-Ti-(P)-rich melt segregated from a differentiated ferrobasaltic magma as a result of liquid immiscibility. The dense Fe-Ti-(P)-rich melt percolated downward through the underlying silicate crystal mush to form net-textured and massive Fe-Ti oxide ores, whereas the coexisting Si-rich melt formed the overlying plagioclase-rich rocks in the intrusion.

Origin of the early Permian Wajilitag igneous complex and associated Fe–Ti oxide mineralization in the Tarim large igneous province, NW China

“…In this fashion, Cr-rich and Cr-poor titanomagnetite formed in early and later stages from a continuous fractionation process. Given the absence of conjugate felsic rocks at Wajilitag, We think liquid immiscibility may not have happened during the evolution of magmas, as was invoked for the Hongge intrusion of the Emeishan LIP (Wang and Zhou, 2013).…”

Immiscible Fe- and Si-rich silicate melts in plagioclase from the Baima mafic intrusion (SW China): Implications for the origin of bi-modal igneous suites in large igneous provinces

“…Eruption of excessive low-FeO t magmas (Bai et al, 2012) and fractional crystallization for the formation of silicic and mafic units (Shellnutt et al, 2009) have been proposed to account for Fe enrichment of these layered intrusions. However, these models cannot explain the net-textured Fe-Ti-P oxide ores and the Si-rich melt inclusions hosted in cumulus apatite in these Fe-Ti oxide-bearing mafic layered intrusions, which were considered to be important evidence of liquid immiscibility (Zhou et al, 2005Wang and Zhou, 2013;Liu et al, 2014b).…”

Silicate Liquid Immiscibility in Layered Intrusions

“…Unmixing would have been triggered by the addition of fluids from upper crustal wall-rocks (Zhou et al 2005). This model is supported by textural and mineralogical constraints from Hongge Fe-Ti-V oxide deposit showing the presence of two populations of Cr-rich and Cr-poor titanomagnetite (Wang and Zhou 2013). Cr-rich titanomagnetite is interpreted as an early liquidus phase whereas clusters of Cr-poor titanomagnetite, ilmenite, and apatite are thought to have formed from an Fe-Ti-(P)-rich immiscible melt.…”