Mineral inclusions and SHRIMP U–Pb dating of zircons from the Alamas nephrite and granodiorite: Implications for the genesis of a magnesian skarn deposit

摘要: Abstract Extending approximately 1300 km and located in the Western Kunlun Mountains, Hetian nephrite belt is largest world contains 11 major deposits more than 20 orebodies including Alamas deposit. can be classified as Mg-skarn with Precambrian dolomitic marble host rock green, green–white white zones are distributed gradually zone of a granodiorite pluton. The green mainly predominately composed tremolite generally minor to trace constituents diopside, grossularitic garnet, actinolite other minerals. Also has higher content TFe2O3, nephrites have. We subdivided zircons from into four types, depending on their internal textures, mineral inclusions, SHRIMP U–Pb ages. Type I round instead idiomorphic shape lack obvious zoning. II IV have broad, clear oscillatory zoning hypidiomorphic or shape; they contain inclusions tremolite, chlorite calcite. Most III narrow rims (  0.1), similar REE element patterns, Ce anomaly (Ce/Ce* > 5), ΣREE contents 454 922 ppm 102 3182 ppm averages 627 ppm 855 ppm, respectively. geochemical signatures, morphologies, ages indicate that most (or fragments zircon) came some experience partially recrystallized during skarnization. This consistent field observation original granodiorite–dolomitic boundary now represented within sequence, close white/white–green adjoining marble. Typical skarn prograde retrograde metasomatism stages. According observations petrographic studies, both early altered stages two main for formation deposits. replacements coarse-grained by fine-grained (nephrite) lead nephrite. Based processes 1) diopside ← dolomite; 2) (nephrite) ← diopside; 3) chlorite ← tremolite (nephrite). Thus, timing later Mg-skarn.