Lugu Sn-Fe deposit, Mianning County, Liangshan Yi, Sichuan, Chinai

Regional Level Types
Lugu Sn-Fe deposit	Deposit
Mianning County	County
Liangshan Yi	Autonomous Prefecture
Sichuan	Province
China	Country

The strata exposed in the Lugu Fe-Sn ore district include the Mesoproterozoic Dengxiangying Group (Pt), Neoproterozoic Suxiong Formation (Zs), and Triassic Baiguowan Formation (Tgb). The Mesoproterozoic Dengxiangying Group consists of three formations: the Chaowangping Formation (Ptcw, metasandstone and phyllite), Darezha Formation (Ptdr, dolomitic marble), and Jiuyingpan Formation (Ptdj, sericite phyllite). The Neoproterozoic Suxiong Formation (Zs) is dominated by rhyolite, while the Triassic Baiguowan Formation (Tgb) comprises conglomerate and feldspathic quartz sandstone. The main structures in the ore district are northwest-southeast-trending faults, followed by northeast-southwest, nearly north–south, and nearly east–west-trending faults. Magmatic rocks in the ore district are dominated by the Lugu granite pluton, which intruded on a large scale into the Mesoproterozoic Dengxiangying Group (Jiuyingpan Formation and Chaowangping Formation).
The orebodies of the Lugu Fe-Sn deposit occur near the outer contact zone between the Lugu composite granite pluton and the Mesoproterozoic Dengxiangying Group strata. They can be divided into multiple ore segments, with the Dadingshan and Tiekuangshan segments being the most representative.
Fe-Sn orebodies in the Dadingshan segment are mainly hosted in the Mesoproterozoic Dengxiangying Group, mostly distributed at the contact zones among the Chaowangping Formation, Darezha Formation, and Jiuyingpan Formation. Two main orebodies (No. I and No. II) have been delineated. These orebodies occur as irregular stratoid and lenticular shapes, with a northeast strike, southeast dip, and dip angle of 20°–65°. They range in length from 200 m to 900 m and in thickness from 0.9 m to 30.82 m, dominated by Fe and Sn mineralization. The average grades are w(TFe) = 37.81 % and w(Sn) = 0.63 %. A total of 10 orebodies have been delineated in the Tiekuangshan segment, with orebody ① as the primary one. Fe orebodies occur as stratoid and lenticular shapes, hosted between the dolomitic marble of the Darezha Formation and metamorphic quartz sandstone of the Mesoproterozoic Dengxiangying Group. They have a northeast strike, southeast dip, and dip angle of 30°–60°. Orebody ① is 605 m in length; its outcrop has a maximum horizontal width of 188 m, a minimum of 5.4 m, and a common width of 15 m–20 m. The orebody thickness ranges from 0.76 m to 8.42 m, with an average of 8.24 m. It is dominated by Fe mineralization, with an average grade of w(TFe) = 53.52 %.
The mineralization process of the Lugu Fe-Sn deposit is divided into two primary stages: the hydrothermal stage (primary mineralization, critical for Fe-Sn enrichment) and the supergene stage. The hydrothermal stage is further subdivided into three mineralization stages based on physicochemical conditions (Fig. 5): magnetite stage (Ⅰ, early hydrothermal), magnetite-cassiterite-apatite stage (Ⅱ, main ore-forming stage), and quartz-sulfide stage (Ⅲ). Magnetite stage (Ⅰ): It is characterized by xenomorphic granular magnetite (Mt1), accompanied by a small amount of muscovite. Hematite is scattered irregularly in Mt1 in the form of fine droplets. Magnetite-cassiterite-apatite stage (Ⅱ): This is the key stage for Sn enrichment, featuring extensive development of magnetite and cassiterite (metallic minerals). Non-metallic minerals are dominated by serpentine, with apatite, brucite, antigorite also present, and a small amount of tremolite and diopside. Magnetite in this phase is divided into two types: cataclastic-textured magnetite (Mt2) and euhedral-subhedral granular magnetite (Mt3). Mt2 is likely formed by the mechanical fragmentation and hydrothermal alteration of magnetite (Mt1) from stage Ⅰ; Mt3 is hydrothermal magnetite directly crystallized from the ore-forming fluid in this stage, often closely associated with cassiterite. Quartz-sulfide stage (Ⅲ, late hydrothermal): It mainly forms metallic sulfides such as sphalerite, pyrite, chalcopyrite, and arsenopyrite, with rare bornite, chalcocite, and covellite. Non-metallic minerals are dominated by quartz, followed by calcite and sericite, and a small amount of chlorite and epidote. The supergene stage mainly occurs in the surface weathering zone, where early magnetite and chalcopyrite are oxidized to form more stable surface minerals such as hematite and malachite.

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