Usability of Hematite for Dating Ore Formation

N. R. Mankopf Laboratorium für Geochronologie, Universität Heidelberg, Im Neuenheimer Feld 234,

D-69120 Heidelberg, Germany

H. J. Lippolt Laboratorium für Geochronologie, Universität Heidelberg, Im Neuenheimer Feld 234,

D-69120 Heidelberg, Germany

The mineral hematite is very well suited to be dated by the (U+Th)/He method due to its high retentivity for 4He (Lippolt et al., 1993; Bähr et al., 1994; Boschmann Käthler, 1996; Bähr, 1987), due to moderate amounts (some ppm) of parent nuclides without clustering and due to the frequent occurrences in ore formations. Hematite is an ubiquitous mineral and occurs in several very different mineralogical varieties depending on the physico-chemical conditions of formation. The differences may have consequences in behaviour of the U, Th and He subsystems of this dating method. In this contribution we refer to the experience gained with the hematite varieties specularite, mica iron, botryoidal ore and red iron ore. The research on hematite in Heidelberg was begun with the macrocrystalline hematite species specularite. The He-indices of multiple samples from a particular hematite occurrence coincide very well with an isochron at the 4He-(U+Th)-correlation diagram (Boschmann Käthler, 1996; Wernicke and Lippolt, 1994; Lippolt et al., 1995). Single datings are - with one exception - within the variance of other datings (hematites, adularia: Mertz et al., 1991, sericites: Lippolt and Kirsch, 1994) of the same region. One single dating of a hematite from an alpine joint (La Fibbia) yields an excessive He-index similar to the cogenetic
K-minerals muscovite and adularia (about 30 Ma instead of about 15 Ma) (Bähr, 1987). Iron mica - a very thin-leaved mica-like specularite - behaved differently depending on its genesis. Due to its lamellar texture a He-index reduced by loss of He should be expected (radiation of alpha particles, diffusion of 4He). Metasomatic iron micas yield He-indices concordant to paleomagnetic measurements (deposit Waldenstein/Austria) (Bähr, 1987) as well as concordant to cogenetic adularia and cogenetic coarse-grained hematites (Isle of Elba, sample RM-LI) (Lippolt et al., 1995). Iron mica from hydrothermal environments may yield erroneous - excessive - He-indices (Baden-Baden: Bähr, 1987, Gleisinger Fels/ Fichtelgebirge). The increased He-indices of the iron micas in comparision with the specularites from the Murg valley region (N-Schwarzwald) (Voigt-Kirsch, 1990) could also be the result of blastic formation of specularite. Numerous samples of cryptocrystalline fibrous botryoidal hematite from Schwarzwald (Wernicke, 1991) and Harz (Hageborn, 1992) deposits show large variations of the
He-indices of some 10 Ma. Differences in the diffusion parameters can be excluded. Samples of 3He/4He measurements at the laboratory of Prof. Friedrichsen (Berlin) allow the preliminary conclusion, that the 3He contents of the samples had been produced completely in situ due to their Li contents. Thus, the He-index variations cannot be explained by 4He excess. The variations of (U+Th)-He ages can be interpreted as reflecting multiple low-temperature mineralizations (formation and/or superimposition). Epigenetic-hydrothermally formed red iron ore (microcrystalline hematite-aggregates) could also permit dating with the (U+Th)/He method. Depending on the intensity of possible intergrowths the effort for control measurements to estimate the influence of these concomitants increases rapidly in comparison with other varieties of hematite. As in the case of hydrothermal iron mica studies a partially porous red iron ore disseminated with quartz yields instead of lowered very high He-indices.

1. conclusion : The distinction of ore deposition time between Upper Carboniferous, Mesozoic time and Tertiary which has been the major object of the study on (U+Th)/He chronometer has been achieved.

2. conclusion : Mesozoic hematite formation prevails and, for example in the Schwarzwald, coincides with other datings of adularia (Mertz et al., 1991), plagioclase sericitization (Lippolt and Kirsch, 1994) and alteration of lamprophyres (Lippolt and Siebel, 1991).

3. conclusion : In order to detect possible distortions always multiple samples of a hematite occurrence should be analyzed to provide correlation lines in the 4He-(U+Th) diagram. In the case of iron mica furthermore different grain sizes should be analyzed.


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