Tin-bearing　magnetite　is　reported　from　several　types　of　magmatic-hydrothermal　ore　deposits.　The　question　of　whether　tin　is　incorporated　within　solid　solution,　as　Sn4+,　or　as　nanoinclusions　remains　open,　however.　We　report　a　micron-　to　nanoscale　investigation　of　Sn　(Mg,　Si)-bearing　magnetite　from　serpentinite　in　the　Dulong　Zn-Sn-In　skarn,　South　China,　with　the　dual　aims　of　understanding　the　mechanisms　involved　in　accommodating　Sn　and　associated　elements　into　the　Fe-oxide,　and　the　inferences　that　this　carries　for　constraining　the　early　stages　of　skarn　formation.　Magnetite　preserves　a　range　of　textures　that　record　the　evolution　of　metasomatism　during　prograde　growth　of　grain　cores　and　retrograde　rim　replacement.　Observations　reveal　the　presence　of　chondrodite　and　sellaite　(MgF2)　as　nanoscale　inclusions　preserved　in　magnetite.　This　implies　initiation　of　the　Dulong　mineralizing　system　during　a　humite-bearing,　magnesium　skarn　stage.　Magnesium-Si　defects,　forming　along　(110)　planes　prior　to　Sn-enrichment,　are　recognized　for　the　first　time.　Release　of　high　volatile,　F-rich　fluids　is　interpreted　to　lead　to　precipitation　of　cassiterite　inclusions　along　＜　111*＞　directions　in　magnetite.