Layered mafic intrusions

Layered mafic intrusions are intrusive bodies which mainly crystallize from basaltic magma. So, they can occur in any tectonic environment where basaltic magma is generated.

They are called layered mafic intrusions (LMIs) due to the presence of igneous layering.

Layering (or stratification) is a sheet-like feature that can be distinguished by compositional and textural variation. Because of the high temperature and low viscosity, LMIs are ideally natural laboratories to study crystal-liquid fractionation during cooling and crystallization.

Some early studies about LMIs focus on big intrusions such as the Bushel Complex of South Africa, the Stillwater Complex of the western United States, and the Skaergård intrusion of eastern Greenland (Hess, 1939; Wager, 1963; Wager and Brown, 1968; Campbell et al., 1983). These studies tried to explain the origin, magmatic processes and layers of layered intrusions.

The Precambrian (2.06 Ga) Bushveld Igneous Complex of South Africa is the world’s largest layered intrusion (Eales and Cawthorn, 1996). It is about 300-400 km wide and 9 km thick. The large-scale layering forms the basis for a simple subdivision, including the

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Marginal Zone, the Critical Zone, the Main Zone and the Upper Zone (Wager and Brown, 1968). Because of clear stratification with different petrographic composition, the Bushveld Complex considers as stunning example to interpret the cooling and magmatic differentiation processes in a large magma chamber.

Layered intrusion may host magmatic ore deposits, containing some of the world's economic concentrations of platinum-group elements (PGE), Fe, Ti, Cu, Ni. Previous studies suggested the ore-forming processes of these deposits including crystal settling, convection, compaction, recharge and magma mixing.

1.2.1. Major types of magmatic Fe-Ti oxide ore in the world

Iron-titanium oxide deposits formed as results of accumulation or injection of Fe-Ti rich liquids. Global magmatic Fe–Ti oxide deposits are associated with mafic intrusions or Proterozoic anorthosite complexes and form by the concentration of Fe–Ti oxides in gabbroic or ferrodioritic magma chambers (Lister, 1966).

Anorthosite-related ore deposits

Anorthosites are large (occasionally enormous) plutonic bodies of nearly pure plagioclase. They are thus as felsic as any granite, but their mineralogy (plagioclase + pyroxene ± olivine) conforms more to mafic rocks (Ashwal and Myers, 1994). The two classic types are Archean and Proterozoic anorthosites (Emslie, 1978). The favored model for anorthosite petrogenesis involves a mantle plume that induces peridotite melting in the spinel– or plagioclase–lherzolite stability field. The resulting aluminous basaltic liquid rises and ponds at the base of the crust. Crystal fractionation produces olivine and pyroxene, which sink, and plagioclase, which floats. The upper plagioclase-rich crystal–liquid mush rises in several pulses to shallower levels, and the dense Fe-rich interstitial liquid is expelled downward, leaving adcumulus masses of anorthosite (Arndt, 2013). Some large

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related ore deposits include: the Lac Tio (Canada), Tellnes (Norway) and Damiao (China) intrusions. A range of magmatic processes may be responsible for the formation of these deposits but fractional crystallization is considered to be one of the most important processes.

Crystallization of plagioclase within a mafic melt results in the enrichment of Fe and in the residual liquid consequently the dense Fe-Ti rich liquid may sink to the bottom of magma chamber. This is a key factor for the formation of iron oxide deposits. Some additional processes can be considered as immiscibility, magma mixing and compaction (Charlier et al., 2015).

Layered intrusion-related deposits

Another type of Fe-Ti oxide can be found on the upper sequences of layered intrusions. Perhaps the most well known example is the Bushel Complex where large titaniferous iron ores are located in the Upper Zone (Reynolds, 1985). The ores are associated with gabbronoritic, ferrogabbronoritic and ferrodioritic cumulates and have titanomagnetite as the dominant oxide mineral. It is suggested that the Fe-Ti oxides crystallize at the late stage of magmatic differentiation, after accumulation of mafic minerals as olivine, pyroxene, plagioclase (Cawthorn and Molyneux, 1986). The enrichment of iron oxide from evolved magma at late stage causes the formation of ore bodies and also explains their occurrence at the upper zone of the intrusion.

1.2.2. Special characteristics of the Panzhihua deposits

The Panzhihua intrusion hosts large iron oxide deposits with special characteristics that are different than the type of deposits associated with anorothosites or layered mafic-ultramafic intrusions. There are no known anorthosites that are spatial or temporally associated with the Panzhihua intrusion. Instead, the ore deposits are hosted by a rhythmically layered gabbroic intrusion that was emplaced at the same time as the eruption of

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the Emeishan flood basalt. Specifically, the ore bodies are located in the lower part of the intrusion. The location of the ore bodies is interpreted as evidence for the early crystallization of Fe-Ti oxides. It is unusual in comparison with other oxide ore-bearing intrusion where iron oxides crystallize during the late stages of magma evolution and lie on top of cumulate layers of mafic minerals. Pang et al. (2010) suggests the main features that distinguish the Panzhihua intrusion from other layered complexes includes: (1) association with flood basalts of a large igneous province, (2) occurrences of ores in low stratigraphic positions with gabbro as host rocks and (3) presence of granitic rocks surround the intrusion.