3.4 Dating ore deposits

Metallic ore deposits have always been notoriously difficult to date reliably. The most common approach to dating such deposits is to analyse gangue mineralisation and hope that this material was deposited in the same episode as the associated metallic ores. Another alternative has been to date fluid inclusions believed to form part of a hydrothermal ore-forming system. One or two successful attempts at this technique have been reported (e.g. Sheppard and Darbyshire, 1981), but fluid inclusion populations may represent more than one stage in the evolution of a hydrothermal system, leading to complex mixtures that have no age significance.

Typically, the large ion lithophile (LIL) elements which comprise most long-lived decay systems do not partition into metal sulphides, preventing direct dating of such ores. However, LIL elements may partition onto some sulphide ores in just sufficient abundances to allow analysis. One sulphide ore mineral which has been successfully dated by this means is sphalerite.

Nakai et al. (1990) made the first successful Rb–Sr isochron determination on sphalerite samples from a Mississippi Valley Type (MVT) lead-zinc deposit from Tennessee. The sphalerite grains were found to have low Sr contents averaging only 1 ppm. As a result, fluid inclusions in the sphalerite grains, estimated to make up only 300 ppm by weight of the host mineral, actually contained more Sr than the host. Therefore, it was necessary to remove these inclusions by crushing the samples and leaching with deionized water before dissolving the sphalerite host for analysis.

This procedure gave a suitable range of Rb/Sr ratios and generated the errorchron shown in Fig. 3.18. One outlier, believed to have been disturbed during a deformation event, was excluded from the data set, after which the remaining seven points gave an age of 377 " 29 Myr (2F). This is a ‘scatter error’, determined by expanding the analytical errors to reduce the original MSWD of 62.6 to unity (section 2.6.3). The fluid inclusions leached during crushing were also analysed, and were found to lie on the isochron defined by the host phase (Fig. 3.18); however, these analyses were not included in the isochron calculation. The age of 377 " 29 Myr suggested that MVT mineralisation occurred during the Acadian orogeny (380 - 350 Myr ago), which caused the expulsion of basin brines from strata within the deformation zone in the Appalachians. These fluids were then transported to the west, causing ore deposition when they mixed with other fluids during their return to the surface.

Fig. 3.18. Rb–Sr isochron diagram for sphalerite grains from the Coy mine, Tennessee. ( !) = sphalerite host; ( + ) = fluid inclusions. Open symbol was excluded from age calculation. After Nakai et al. (1990).

Brannon et al. (1992) applied this method to other MVT deposits. However, the range of Rb/Sr ratios in the ores themselves was not sufficient for the determination of a precise isochron. Therefore, it was necessary to combine analyses of the host sulphide with fluid inclusions (Fig. 3.19a). This procedure yielded a precise age (269 " 6 Myr, 2F), but was effectively a ‘two-point’ isochron, raising fears that if the host sulphide and the inclusions were not co-genetic, the calculated age might be geologically meaningless.

Fig. 3.19. Rb–Sr isochron analysis of sphalerite host ( !) and extracted fluid inclusions ( ") from MVT lead-zinc deposits: a) West Hayden, Wisconsin; b) Polaris, arctic Canada. After Brannon et al. (1992) and Christensen et al. (1995a).

Further studies by Nakai et al. (1993) revealed two more examples (from the Pine Point MVT deposit in Canada, and the Immel mine in east Tennessee) where the inclusions lay on a well-defined host isochron. However, analysis of the Polaris MVT deposit in arctic Canada provided an example where the inclusions lay off a well-defined host isochron (Christensen et al., 1995a). In this case the host (ore minerals) gave an age of 366 " 15 Ma, in good agreement with the age of the wall-rocks from paleomagnetic evidence, whereas the inclusions defined a cloud of points above the isochron (Fig. 3.19b). Seven of these leachate samples lay just outside error of the sphalerite isochron, whereas four were more radiogenic, suggesting that the inclusion population included primary inclusions which were co-genetic with the ores, along with more radiogenic secondary inclusions.

To avoid possible complications arising from mixing between host ores and fluid inclusions, Christensen et al. (1995b) tested for mixing relationships when they dated sphalerites from the Canning Basin MVT deposit of western Australia. They found that Sr concentrations in the sphalerite host grains showed no correlation with ⁸⁷Rb/⁸⁶Sr, suggesting that the sphalerite residues after crushing and leaching were not significantly contaminated by Sr from unopened inclusions (Fig. 3.20a). On the other hand, Rb contents were found to be strongly correlated with ⁸⁷Rb/⁸⁶Sr. Since the fluid inclusions contain negligible Rb, these Rb contents must have originated from the host sulphide ore itself. Therefore the isochron must also date the sulphide ore itself.

Fig. 3.20. Plots to test for mixing relationships between sphalerite hosts and inclusions: a) ⁸⁷Rb/⁸⁶Sr versus Rb and Sr concentrations in the Canning Basin MVT deposit, Australia; b) Sr isotope ratio versus 1/Sr from the West Hayden MVT deposit, Wisconsin. After Pettke and Diamond (1996).

Pettke and Diamond (1996) used a similar approach to test the possibility of mixing in the sphalerite-inclusion Rb–Sr isochron of Brannon et al. (1992). They plotted the data on a graph of Sr isotope ratio against the reciprocal of Sr concentration, on which mixing processes generate straight lines (Fig. 3.20b). On this graph, the fluid inclusions have Sr contents that are essentially infinite (relative to the low abundances in the host), so they are plotted on the y axis. The results of this analysis showed that one of the isochrons determined by Brannon et al. (1992) was probably a mixing line generated by sampling of sphalerite grains with a few un-released inclusions.(sample 58-B). Therefore, this age determination is only meaningful if the host and the fluid were cogenetic. However, the other isochron (sample 10-C) does not show the mixing effect, so this age is more reliable. Since both isochrons gave results within error (269 " 6 and 270 " 4 Myr), it was concluded that this is a reasonable estimate of the age of ore deposition.

Another sulphide mineral successfully used to date ore deposition is the mercury sulphide galkhaite. This hydrothermal mineral was found associated with Carlin-type gold mineralisation in Nevada, and was used to estimate a date of 39 " 2 Myr for gold mineralisation at the Getchell deposit in northern Nevada (Tretbar et al., 2000).

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