Petrogenesis of the panzhihua-type gabbroic layered intrusions and associated Fe-Ti-V oxide deposits : insights from mineral chemistry and numerical modeling

The Panzhihua-Xichang district, SW China, is located within the central part of the Emeishan Large Igneous Province (ELIP). The district hosts the world largest titanium resources and the third largest vanadium resources. Ti and V are hosted by magnetite and ilmenite, forming the unique Panzhihua-type deposits amounting to ~ 20 billion tonnes of Fe-Ti-V oxide in potential reserves. This project is mainly focused on understanding the formation mechanism of the Panzhihua gabbroic layered intrusion and associated Fe-Ti-V oxide deposit, and also on the neighbouring Taihe Intrusion. Lithological and stratigraphic investigations by a wide range of researchers over the last 50 years have subdivided the Panzhihua Intrusion into a marginal zone (MGZ), lower zone (LZ), lower middle zone (MZa), upper middle zone (MZb) and upper zone (UZ). The Fe-Ti- V oxide ores are concentrated below the MZb, which is marked by the appearance of cumulus apatite. This study is centred on the rocks from the Lanshan and Zhujiabaobao sections of the Panzhihua intrusion. Samples were collected from the Lanshan and Zhujiabaobao open-pit mines, representing the lower ~ 700 of the 2000 meters of the total stratigraphy. The rocks are composed of cumulus clinopyroxene (Cpx), plagioclase (Plag), magnetite (Mgt) and ilmenite (Ilm). Olivine (Ol) and apatite (Ap) occur as the main cumulus minerals below and above the ore bodies, respectively. Hornblende (Hbl) and Fe-rich sulfide (pyrrhotite- Po) are found throughout the intrusion. In general, the abundances of Fe-Ti oxides decrease up the section. Ilm becomes more abundant up the studied section. Fe-Ti oxides occur as massive and disseminated ores, as intercumulus minerals in a wide range of rocks, and as inclusions in silicate minerals in various rock types. Fe-Ti oxide inclusions are common in Ol and Cpx, also occasionally in Plag. Po occurs as discrete, inclusion and droplets, which arecharacterized by linear distribution of round-subround Po grains. Some discrete intercumulus Po grains coexist with Fe-Ti oxides in triple junction between silicate minerals. Po inclusions appear commonly in Cpx, Plag and Fe-Ti oxides, however, no Fe-Ti oxides are included in Po. Po droplets are only found in Cpx. Pentlandite (Pn), chalcopyrite (Cpy) and pyrite (Py) are subordinate minerals. Cpx and Mgt are characterised by extensive exsolution textures. The whole-rock major element composition were obtained using XRF and solution ICP-MS at CODES, Univeristy of Tasmania. It may be well explained by a mixture of an ox ide end-member and a silicate end-member. The former consists of Mgt and Ilm in a constant ratio of approximately 4:1. The latter, consisting mainly of Cpx and Plag, is more variable due to large variations in modal proportions and occurrence of Ol, Hbl and Ap in some rocks. Trace element abundances also show strong correlations with the modal proportion of one or more minerals of either end-member. Rocks from the LZ and MZa contain less than 5% interstitial melt, which have little effect on the whole rock chemistry. Rocks from the MGZ have significant amounts of interstitial melt, which is likely the result of rapid cooling. Mineral major and trace element compositions were analyzed using EMPA and LA-ICP-MS at the CSL and CODES respectively, University of Tasmania. The Panzhihua and Taihe Fe-Ti oxides are mostly depleted in Cr and Ni. Cr-rich Mgt and Ilm only appear in three wehrlite and one microgabbro from the MGZ. Re-equilibration with Mgt has resulted in an increase in Fo content of Ol. A significant difference in Fo content in Ol and Mg#Cpx for coexisting Ol and Cpx indicates subsolidus re-equilibration. Temperatures calculated using a) an Ol-Cpx thermometer and b) an experimentally derived relationship between An content in Plag and temperature, indicate a small temperature difference (~20vÄv¿C) between gabbro located below and above the first and last massive ore, suggesting Fe-Ti oxides were deposited from compositionally similar magmas. The whole rock and mineral compositions show little variation throughout the LZ and MZa, which is inconsistent with in-situ fractional crystallization. This strongly suggests that the parental magma for all rocks in the intrusion had a nearly constant composition. A major reversal is recorded in Plag An content in the lower MZa, however, this is not observed in other minerals (e.g. Mg#Cpx, Fo, Ni and V contents in Mgt and Ilm). Hence, it may not represent new primitive magma replenishment event as previously suggested. Instead, it is more likely a result of a localised increase in H2O activity, which would significantly increase An content in Plag but would have little effect on other parameters. Numerical modelings with the Petrolog3 algorithm are performed under various conditions. The highly consistency of mineral compositions and modal proportions between calculated and observed results reflect high quality of the modelings. We propose instead a new magma replenishment hypothesis. The MGZ rocks display large vertical variations in rock chemistry. Solidification at high cooling rates coupled with likely upward migration of residual melts, are responsible for compositional reversals within the MGZ. Later intrusion of less evolved crystal-laden magma formed dyke-like wehrlite, sometimes with porphyritic texture. In the proposed model for the petrogenesis of the Panzhihua intrusion, the intrusion acted as an open plumbing system, where crystal-rich magmas with similar composition continuously flow through. In detail, the silicate minerals were crystallized in equilibrium during ascending and intrusion. These crystal-loaded parental magmas emplaced into a weakened zone of crustal dolomitic wall rocks. CO2 released during reaction with wall rocks at the time of emplacement oxidized the parental magma, trigging Fe-Ti oxide fractional crystallization. Dense Mgt and Ilm accumulated on the floor by gravitational settling. Removal of Fe-Ti oxides from the magma also caused sulfide saturation (probably as a Fe-rich monosulfide liquid) due to the rapid decrease in melt Fe content. This immiscible sulfide liquid was segregated from the magma during Fe-Ti oxides fractionation, subsequently trapped and crystallized in the solidifying crystal piles. The density differences between minerals and melt may have resulted in different settling rates and formation of variety of rock types in the Panzhihua intrusion. Silicate minerals were accumulating at both the bottom and top of the chamber, forming layers with the intrusion, whereas Fe-Ti oxides were preferentially accumulated near the bottom. The magmatic system was closed after depositing approximately 1100m-thick cumulate at the bottom of the intrusion, and at least 800m-thick cumulate at the roof, leaving the entrapped liquid differentiated subsequently in-situ.