Origins of Nd-Sr-Pb isotopic variations in single scheelite grains from Archaean gold deposits, Western Australia

Accessory gangue scheelite (CaWO4) from the Archaean Mt. Charlotte lode Au deposit can be divided into two types with different rare earth element (REE) signatures. In some scheelite grains, specific REE signatures are reflected by different cathodoluminescence colours, which can be used to map their often complex oscillatory intergrowths. Domains with specific REE contents from two grains were sampled for Sm/Nd, Rb/Sr and Pb isotopic analyses using a micro-drilling technique. Type I scheelite is strongly enriched in middle REE (MR-EE) and Eu anomalies are either absent or slightly positive. Four fragments collected from Type I regions of two crystals have initial Sr-87/(86) Sr and epsilon(Nd) values ranging from 0.70141 to 0.70163 and + 2.5 to + 3.5, respectively, and Pb isotope ratios reflecting the composition of greenstone sequence. This may indicate that Nd and Pb have their source, either locally or regionally, in the greenstones. Basic greenstone lithologies have Sr-87/Sr-86 < 0.7015, and the radiogenic Sr signatures indicate that part of the Sr originated from felsic lithologies located either within or beneath the host greenstone pile. Alternatively, the Sr signature may have evolved from preferential leaching Z, of a Rb-rich mineral during hydrothermal alteration of the greenstone. The REE patterns of Type II scheelite are either flat or MREE-depleted and have strong positive Eu anomalies. Three fragments collected from Type II regions of the same two crystals have initial Sr-87/(86) Sr ratios and epsilon(Nd) c values between 0.70130 and 0.70146, and + 1.1 to + 2.6, respectively, and Pb isotope signatures that are once again similar to that of the greenstone. This implies that Sr-87/Sr-86 ratios in Type II fluids were closer to those of the host dolerite (0.7008-0.7013), due to more extensive fluid interaction with the dolerite. A positive correlation between Na and REE suggests that REE3+ are accommodated by the coupled substitution REE3+ + Na+ = 2 Ca2+, into both Type I and Type II scheelite. This is consistent with a fractional crystallisation model to explain the change in REE patterns from Type I to Type II, but not with a model involving different coupled substitutions and fluids from different origins. We propose that the complex REE and isotopic signatures of scheelite at Mt. Charlotte are related to small ( < m) to medium ( < kin) scale processes involving mixing between "fresh" batches of hydrothermal fluid with fluids that had already been involved in extensive wall-rock alteration. The very high-epsilon(Nd) values measured in some scheelites have been previously used to link, gold mineralisation with komatiites containing unusually high Sm/Nd ratios. However, tiny ( < 20 mum) grains of secondary hydroxyl-bastnasite were found within micro-fractures of one scheelite grain containing an extremely high-epsilon(Nd) signature. The hydroxyl-bastnasite probably formed during recent REE redistribution within the scheelite as a result of meteoric fluid circulation. The scale of this cryptic low-temperature alteration is sufficient to explain the anomalously high-epsilon(Nd) values observed in scheelite from Western Australia. (C) 2002 Elsevier Science B.V. All rights reserved.