Abstract <p>The Sn(-Cu) Tam Dao ore district is located at the southern margin of the South China Block within the Lo Gam structure (northeastern Vietnam). Genetically, ore mineralization of this ore district is associated with the homonymous massif of Middle Triassic biotite granites of the Pia Bioc complex and includes a number of different-sized tin deposits. The Ngoi Lem deposit (4700 t of Sn and 300 t of Cu) is the largest and most studied one. Despite the widespread distribution of Early to Middle Triassic granitoid magmatism within the Lo Gam structure, tin mineralization was identified only near the Tam Dao massif and is practically absent in other massifs of the Phia Bioc complex. Zircon geochemical features (rare-earth elements, Ti, and U contents) from biotite granites of various massifs indicates the similarity of the oxygen fugacity and degree of fractionation of the parental melts. The obtained zircon characteristics are fully consistent with realistic genetic models developed for tin deposits related with reduced intrusions. The partial melting temperature of the metasedimentary substrate is the reason of the various metal fertility potential of coeval and geochemically similar granitoids. The relatively high zircon crystallization temperatures (∼>780–800°C) of tin-bearing biotite granites of the Tam Dao imply that they were produced at a higher temperature by biotite-dehydration melting, which requires additional heat from the mantle. The interaction between mafic and granitic magmas led to the enrichment of acidic melts in copper. The low oxygen fugacity led to the dominance of the sulfide form of sulfur in melts, which in turn determined its low concentration. The low S/Cu ratio in the granite melt prevented crystallization and fractionation of sulfides. Therefore, the residual melts experienced some Cu enrichment. The proposed scenario of saturation of the granite melt in Cu and the absence of its significant sulfide fractionation explains the contradictory Sn–Cu metallogeny of the Tam Dao ore district. The newly obtained data on geochemical features of zircons from the Early to Middle Triassic Pia Bioc granites propose key zircon geochemical indicators of tin fertility of reduced granitic magma: (1) <i>T</i> ∼> 780–800°C, (2) ΔFMQ <InlineEquation ID="IEq1"> <InlineMediaObject> <ImageObject Color="BlackWhite" FileRef="11477_2025_9388_Article_IEq1.gif" Format="GIF" Height="12" Rendition="HTML" Resolution="72" Type="Linedraw" Width="21" /> </InlineMediaObject> <EquationSource Format="TEX">\( \ll \)</EquationSource>  </InlineEquation> 0, and (3) Eu/Eu* < 0.08.</p>

U–Pb Age and Zircon Geochemistry of Fertility-Reduced Granite Using the Example of the Tam Dao Tin Ore District, Northeast Vietnam

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2025年6月1日

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P. A. Nevolko,
T. V. Svetlitskaya,
Pham Thi Dung,
Nguyen Thi Hau,
Tran Trong Hoa,
P. A. Fominykh,
Ngo Thi Phuong

摘要

Abstract

The Sn(-Cu) Tam Dao ore district is located at the southern margin of the South China Block within the Lo Gam structure (northeastern Vietnam). Genetically, ore mineralization of this ore district is associated with the homonymous massif of Middle Triassic biotite granites of the Pia Bioc complex and includes a number of different-sized tin deposits. The Ngoi Lem deposit (4700 t of Sn and 300 t of Cu) is the largest and most studied one. Despite the widespread distribution of Early to Middle Triassic granitoid magmatism within the Lo Gam structure, tin mineralization was identified only near the Tam Dao massif and is practically absent in other massifs of the Phia Bioc complex. Zircon geochemical features (rare-earth elements, Ti, and U contents) from biotite granites of various massifs indicates the similarity of the oxygen fugacity and degree of fractionation of the parental melts. The obtained zircon characteristics are fully consistent with realistic genetic models developed for tin deposits related with reduced intrusions. The partial melting temperature of the metasedimentary substrate is the reason of the various metal fertility potential of coeval and geochemically similar granitoids. The relatively high zircon crystallization temperatures (∼>780–800°C) of tin-bearing biotite granites of the Tam Dao imply that they were produced at a higher temperature by biotite-dehydration melting, which requires additional heat from the mantle. The interaction between mafic and granitic magmas led to the enrichment of acidic melts in copper. The low oxygen fugacity led to the dominance of the sulfide form of sulfur in melts, which in turn determined its low concentration. The low S/Cu ratio in the granite melt prevented crystallization and fractionation of sulfides. Therefore, the residual melts experienced some Cu enrichment. The proposed scenario of saturation of the granite melt in Cu and the absence of its significant sulfide fractionation explains the contradictory Sn–Cu metallogeny of the Tam Dao ore district. The newly obtained data on geochemical features of zircons from the Early to Middle Triassic Pia Bioc granites propose key zircon geochemical indicators of tin fertility of reduced granitic magma: (1) T ∼> 780–800°C, (2) ΔFMQ \( \ll \) 0, and (3) Eu/Eu* < 0.08.