Textural relationships and mineral compositions of Fe-Ti oxide ores of the Panzhihua layered intrusion, SW China: Insights for magma emplacement and lateral extension of Fe-Ti oxide ore bodies

Abstract

The Panzhihua gabbroic layered intrusion in the Emeishan large igneous province (SW China) hosts economically important Fe-Ti-(V) oxide ore deposits. The intrusion consists of six sections from northeast to southwest. To constrain the emplacement of parental magma across the sections and the magmatic processes associated with Fe-Ti oxide ore deposits, a combined study on field relations, rock textures, mineral compositions, and late-stage magmatic microstructure was carried out for the Fe-Ti oxide ores from the Zhujiabaobao, Lanjiahuoshan, Jianshan, and Nalaqing sections. A ∼7-meter-thick ultramafic portion comprising peridotite, olivine (Ol)-rich net-textured and massive ores is firstly recognized at the bottom of the Zhujiabaobao section in this investigation. The magnetite grains in the ultramafic portion are Cr-rich with 0.58 to 1.83 wt.% Cr2O3, and may have crystallized from an earlier injection of Fe-Ti-Cr-rich magma. On the other hand, in the predominate clinopyroxene (Cpx)/plagioclase (Pl)-rich net-textured to massive ores of the intrusion, the magnetite grains contain <0.2 wt.% Cr2O3, and may have crystallized from late injections of Fe-Ti-rich, Cr-depleted magma. We proposed that the parental magma may be emplaced into an originally “tilt-spoon” structure and initially unloaded in the Zhujiabaobao section, then infilled the Lanjiahuoshan to Jianshan sections and extended further to the Nalaqing section. The thick, massive Fe-Ti oxide ore layers in the lower part of the intrusion are attributed to the silicate immiscibility of interstitial liquid within the crystal mush, and the large amounts of Fe-Ti oxides likely crystallized from immiscible Fe-rich melt aggregated in the lower part of the magma chamber.