40Ar/39Ar geochronological constraints on the evolution of lateritic iron deposits in the Quadrilátero Ferrífero, Minas Gerais, Brazil
40Ar/39Ar geochronological constraints on the evolution of lateritic iron deposits in the Quadrilátero Ferrífero, Minas Gerais, Brazil
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DOI：
10.1016/j.chemgeo.2006.04.006
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发表时间：
2006-10
期刊：
影响因子：
3.9
通讯作者：
C. Spier;P. Vasconcelos;Sonia M.B. Oliviera
中科院分区：
文献类型：
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作者：
C. Spier;P. Vasconcelos;Sonia M.B. Oliviera
巴西米纳斯吉拉斯的Sapecado、Pico和Andaime铁矿QuadriláTero FerríFero(QF)上覆的风化剖面深达150-400米，蕴藏着世界级的表生铁矿。除了拥有具有全球经济意义的表生矿体外，QuadriláTero FerríFero和其他地方(如卡拉贾·S、哈默斯利)的风化条带状铁建造被认为是地球上一些最古老的持续暴露的风化剖面的基础。激光增量加热40Ar/39Ar结果显示，从Sapecado、Pico和Andaime矿床深度为5至150米的23个样品中提取的69个霍兰特组锰氧化物的年龄为约62至14 Ma。较老的锰氧化物出现在表层附近，而较年轻的锰氧化物出现在深部。然而，在风化-基岩界面收集的许多样品获得的年龄在51-41 Ma之间，表明QuadriláTero FerríFero的风化剖面在古近纪已经达到现在的深度。QuadriláTero FerríFero风化剖面的年代可与巴西卡拉贾S地区和西澳大利亚哈默斯利省带状铁建造的风化剖面年代相媲美。研究中获得的年龄-深度分布表明，新近纪QuadriláTero FerríFero红土剖面几乎没有进一步推进风化前锋的作用，但在其他含有类似表生铁矿的地区却没有。结果表明，一些古地貌的风化不受风化锋随时间的稳定推进的控制，而可能反映了遥远过去的气候和地貌条件。地质年代学的结果也证实，在几千万年的时间里，QuadriláTero FerríFero的古老地表可能不会受到侵蚀。深部风化作用，主要发生在古近纪，加上低的剥蚀率，解释了该地区表生铁、锰、铝矿体的丰富和广泛分布。
Weathering profiles overlying the Sapecado, Pico and Andaime iron ore deposits, Quadrilátero Ferrífero (QF), Minas Gerais, Brazil, reach depths of 150–400 m and host world-class supergene iron orebodies. In addition to hosting supergene ore bodies of global economic significance, weathered banded iron-formations at the Quadrilátero Ferrífero and elsewhere (e.g., Carajás, Hamersley) are postulated to underlie some of the most ancient continuously exposed weathering profiles on earth. Laser incremental-heating40Ar/39Ar results for 69 grains of hollandite-group manganese oxides extracted from 23 samples collected at depths ranging from 5 to 150 m at the Sapecado, Pico and Andaime deposits reveal ages ranging from ca. 62 to 14 Ma. Older Mn-oxides occur near the surface, while younger Mn-oxides occur at depth. However, many samples collected at the weathering–bedrock interface yield ages in the 51–41 Ma range, suggesting that the weathering profiles in the Quadrilátero Ferrífero had already reached their present depth in the Paleogene. The antiquity of the weathering profiles in the Quadrilátero Ferrífero is comparable to the antiquity of dated weathering profiles on banded iron-formations in the Carajás Region (Brazil) and the Hamersley Province, Western Australia. The age versus depth distributions obtained in this study, but not available for other regions containing similar supergene iron deposits, suggest that little further advance of the weathering front has occurred in the Quadrilátero Ferrífero lateritic profiles during the Neogene. The results suggest that weathering in some of these ancient landscapes is not controlled by the steady-state advance of weathering fronts through time, but may reflect climatic and geomorphological conditions prevailing in a remote past. The geochronological results also confirm that the ancient landsurfaces in the Quadrilátero Ferrífero probably remained immune to erosion for tens of millions of years. Deep weathering, mostly in the Paleogene, combined with low erosion rates, account for the abundance and widespread distribution of supergene iron, manganese, and aluminum orebodies in this region.