Mineralogy and geochemistry of the Paleoproterozoic, tin-mineralized Bom Jardim granite of the Velho Guilherme Suite, eastern Amazonian craton

The Bom Jardim granite is located to the south of São Felix do Xingu town and is intrusive in the intermediate to felsic volcanic rocks of the Uatumã Group. It is formed dominantly of coarse- to medium-grained isotropic monzogranite and syenogranite, both affected by intense late- to post-magmatic alteration. Biotite, generally chloritized, is the main primary mafic phase, with rare amphibole being found in the monzogranite. Hydrothermally altered and greisenized rocks, containing small primary concentrations of cassiterite + wolframite, as well as quartz veins with millimeter- to centimeter-sized crystals of wolframite + pyrite + fluorite occur in pervasively altered cupolas. Presently, alluvial cassiterite and wolframite (±columbite, tantalite) are mined in the Pedra Preta mine, located in the northern part of the pluton. SEM data showed that Sn–W mineralization is dominantly associated with syenogranite and greisenized rocks. EDS Semi-quantitative analysis revealed that the zircon crystals of the Bom Jardim granite are characteristically enriched in Hf, Y, U, and Th and display Zr/Hf ratios decreasing from monzogranite/leucomonzogranite toward syenogranite and greisenized syenogranite rocks, suggesting that magmatic differentiation significantly contributed for this particular feature. The Bom Jardim granite is slightly peraluminous and displays geochemical affinities with A-type granites. The Bom Jardim granite varieties evolved dominantly by fractional crystallization and their REE patterns are similar to those of the tin-specialized granites of the Velho Guilherme suite. It is concluded that the more evolved granites and associated greisenized rocks of the Bom Jardim pluton are tin-specialized granites. The similarities observed between the granites of the Velho Guilherme suite and the Bom Jardim granite allow to include the latter in this important granite suite.

► The Bom Jardim granite was affected by intense late- to post-magmatic alteration.
► It is slightly peraluminous and display geochemical affinities with A-type granites.
► Their zircons display Zr/Hf ratios decreasing towards syenogranite and greisens.
► Sn and W mineralization are associated with the syenogranite and greisens.