Dating paleochannel iron ore by (U-Th)/He analysis of supergene goethite, Hamersley province, Australia

(U-Th)/He dating of late-stage authigenic goethite, combined with corrections for diffusive loss of He-4 by the He-4/He-3 methodology, reveals strong correlation between a sample's age and its depth in ferruginized channel sediments from the Yandicoogina deposit, Western Australia. Corrected ages, ranging from ca. 18 Ma near the surface to ca. 5 Ma at the bottom of the profile, indicate that ferruginization of the aggraded channels becomes progressively younger with depth. This trend is consistent with goethite precipitation at the groundwater-atmosphere interface during water table drawdown driven by the aridification of Western Australia during the Neogene. The results demonstrate that the (U-Th)/He system is ideal for dating goethite if diffusive loss corrections are applied. The approach is suitable for dating weathering reactions on Earth and should also be suitable for dating Fe oxyhydroxides in the Martian regolith.

(U-Th)/He and 40Ar/39Ar Geochronology of Weathering, Hamersley Province, Australia: Implications for weathering history and landscape Evolution

(U–Th)/He dating of goethite, when combined with quantification of diffusive 4He loss by the 4He/3He methodology, provides reliable corrected ages for minerals precipitated in weathering profiles. We have combined (U–Th)/He dating of supergene goethite with 40Ar/39Ar dating of supergene manganese oxides to study the weathering history and landscape evolution in the Hamersley Province, northwestern Australia. Incremental heating 40Ar/39Ar analysis of 187 grains of Mn oxides from 65 samples (44 hand specimens) collected from weathering profiles at seven field sites across the Hamersley Province yield precipitation ages ranging from 63.4 ± 0.9 to 1.5 ± 0.2 Ma. These results, combined with previous results of 40Ar/39Ar dating of Mn oxides (Vasconcelos, 1998 Vasconcelos, P.V., 1998. Unpub. report, pp. 1–278.Vasconcelos, 1998 and Cochrane, 2003), reveal a protracted and episodic history of weathering and landscape evolution, which was already ongoing in Late Cretaceous and spans the Palaeogene and Neogene. Seventy-three grains of goethite from 39 samples extracted from 21 hand specimens, collected from the same field sites where the Mn oxides originated, were dated by the (U–Th)/He method. Internally consistent (U–Th)/He ages, which range from 84.3 ± 12.2 to 3.3 ± 0.5 Ma, have been obtained for most samples when corrections are applied for 10% helium diffusive loss. The geochronological results obtained show remarkable similarity in the distribution of ages associated with supergene mineral precipitation. The widespread occurrence of iron oxides such as goethite in soils and weathering profiles and the successful application of (U–Th)/He dating of goethite offers great opportunities for extracting the wealth of palaeoclimatic and palaeoenvironmental information recorded by these profiles on the surface of terrestrial planets such as Earth and Mars.

40Ar/39Ar geochronological constraints on the evolution of lateritic iron deposits in the Quadrilatero Ferrifero, Minas Gerais, Brazil

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-heating 40Ar/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.

40Ar/39Ar constraints on a temporal link between gold mineralization, magmatism, and continental margin transtension in the Jiaodong Gold Province, eastern China

The Jiaodong gold province is the largest gold repository in China. Both mineralization and granitoid hosts are spatially related to the crustal-scale Tan-Lu strike-slip fault system, which developed along the Mesozoic continental margin in eastern China. A series of Ar-40/Ar-39 laser incremental heating analyses of hydrothermal sericite/muscovite from three major gold deposits (Jiaojia, Xincheng, and Wangershan) and igneous biotite from the granodiorite hosts were performed to establish a possible temporal link between gold mineralization, magmatism, and movement along the Tan-Lu fault zone. Magmatic biotite crystals yield well-defined and concordant plateau ages between 124.5+/-0.4 Ma and 124.0+/-0.4 Ma (2sigma), whereas sericite and muscovite samples (a total of 30 single separates) give reproducible plateau ages ranging from 121.0+/-0.4 Ma to 119.2+/-0.2 Ma (2sigma). An integration of our Ar-40/Ar-39 results with age data from other major gold deposits in Jiaodong demonstrates that widespread gold mineralization occurred contemporaneously during a 2-3-m.yr. period. Most gold deposits show intimate spatial associations with abundant mafic to intermediate dikes. The mafic dikes have K-Ar ages of 123.5-119.6 Ma, in excellent agreement with those of the gold deposits. These newly obtained Ar-40/Ar-39 ages, in combination with other independent geological and geochronological data on granodioritic intrusions (130-126 Ma), volcanic rocks (1243.6-114.7 Ma), and deformed rocks within strike-slip faults (132-120 Ma) in Jiaodong or adjacent areas, also support the idea that gold mineralization postdated the granodioritic magmatism but was contemporaneous with mafic magmatism and volcanism, all controlled by the transtensional motion along the Tan-Lu fault in the Early Cretaceous.

The performance of community organisers in promotion of community forestry in Leyte Province, Phillipines

A study was conducted to assess the role and effectiveness of community organisers in supporting the development of people’s organisations in achieving community-based forest management objectives in Leyte Province Philippines. Community organisers were found to be effective in forming people’s organisations (POs), motivating people to participate in voluntary activities organised by POs and encouraging cohesiveness among PO members. Community organisers manage to raise the level of environmental awareness and knowledge of members of people’s organisations, develop leadership interest and skills, create various livelihood opportunities and provide direction and facilitate the establishment of large tree plantations. However, the short duration of community organisers’ contracts (typically two years) is insufficient to establish mature and cohesive POs prepared to assume management on their own, including the management of tree plantations. Further, lack of training and funding support, low wages, delayed payment of salaries and limited time to work with people’s organisations, as well as the pressure to produce tangible outputs such the establishment of large tree plantations, prevents them from placing greater emphasis on the development and empowerment of the people.