The genesis of gold mineralisation hosted by orogenic belts: A lead isotope investigation of Irish gold deposits

Lead isotope analyses have been performed on 109 gold and 23 sulphide samples from 34 Irish gold occurrences, including 27 placers, and used to shed light on the sources of mineralising fluids and metals associated with gold mineralisation hosted by orogenic belts. The Pb isotope ratios of lode and placer gold range from 206Pb/204Pb=17.287-18.679, 207Pb/204Pb=15.382-15.661, and 208Pb/204Pb=37.517-38.635, consistent with the Pb isotopic data on previously reported Irish sulphide mineralisation. There is no evidence that gold mineralisation is associated with distinctive source regions, and it appears to have been derived from similar sources to those responsible for the widespread sulphide mineralisation in Ireland. It is inferred that the principal controls on the Au mineralisation are structural and not related to the distribution of Au in their source rocks. The range of Pb isotope ratios favours the interaction of multiple source reservoirs predominantly during the Caledonian Orogeny (c. 475-380Ma). Underlying basement was the primary control on two key sources of Pb. Gold occurrences located to the south-east of the Iapetus Suture are characterised by Pb compositions that derive predominantly from the Late Proterozoic crustal basement or overlying Lower Palaeozoic sediments, whilst those located north-west of the Iapetus Suture are characterised by less radiogenic Pb signatures derived predominantly from Late Proterozoic or older crustal basement. A third source, relatively enriched in radiogenic Pb, also played a role in the formation of a number of Irish gold occurrences, and may have been associated with syn- to post-orogenic intrusives. Magmatic processes may therefore have played an important role in the formation of some orogenic gold occurrences

U-series dating of bone using the diffusion-adsorption model

U-series dating of bone has suffered problems of reliability since its inception because bone remains an open system with respect to uranium. Commonly applied a priori assumptions of U uptake, such as early uptake or linear uptake, are inadequate because they have no physical or chemical bases, no means of demonstrating which model is suitable for a particular bone, and no intrinsic tests of reliability. Despite this and numerous examples of anomalous U-series dates, such assumptions are still routinely applied. We address this problem using the diffusion-adsorption (D-A) model of U uptake (Millard and Hedges, 1996), which incorporates a physicochemical description of U uptake. Using this model, we show how the U uptake of a bone responds to geochemical changes in the burial environment, which can lead to phenomena such as the removal of U from bones (“leaching”) or U uptake late in their burial history (“recent uptake”), and we show how the overall uptake history is reflected in distributions (profiles) of U and U-series isotopes across a bone section. We present measurements of U concentration profiles, and 230Th/234U profiles on archeological bone from a number of different sites and burial environments and compare the results to profiles predicted by the D-A model. Bones that have undergone complex uptake histories (which include U leaching or recent uptake) are identified on the basis of these profiles and rejected as unsuitable for dating. For bones that appear to have undergone uptake under constant geochemical conditions, the D-A model is applied to calculate U-series dates, with much improved reliability

Diagenetic arsenic uptake in archaeological bone: Can we really identify copper smelters?

In a recent paper Oakberg, Levy & Smith (2000) reported measurements of arsenic concentrations in bone from the Chalcolithic site of Shiqmim, Israel. They inferred that since elevated levels of As had been found in the bone of modern copper smelter workers in Sweden, high concentrations in archaeological bone could be used to infer copper smelting activities in the past. However, their argument ignores the effects of post-depositional mobility of arsenic between soil, groundwater and bone. Here, we outline the processes influencing mobilization of As and post-depositional As incorporation in bone. We argue that the As concentrations reported by Oakberg, Levy & Smith (2000) can be accounted for by diagenetic uptake and are unlikely to reflect biogenic As concentrations. We therefore conclude that the data presented by Oakberg, Levy & Smith (2000) cannot be used to infer past copper smelting