Rare earth element systematics in scheelite from hydrothermal gold deposits in the Kalgoorlie-Norseman region, Western Australia

Scheelite (CaWO4) is a widespread accessory mineral associated with hydroth ermal veins in the Archean gold deposits of the Kalgoorlie Norseman region of Western Australia. Rare earth element (REE) and other trace element abun dances in scheelite have been determined in situ by excimer laser ablation- inductively coupled plasma-mass spectrometry. (ELA-ICP-MS) in order to cons train the composition and sources of the mineralizing fluids. The scheelite s can be grouped according to the two distinct types of chondrite-normalize d REE (REEN) patterns which they exhibit: hump-shaped (type I) and flat (ty pe II). Type I scheelites have higher total and Na concentrations than type TI samples and can be further subdivided into type Ia which has maximum RE EN concentrations between Sm and Gd, and type Ib which has maximum REEN con centrations displaced toward Dy. Type I scheelites are dominant at Coolgard ie, Kalgoorlie, and Kambalda, whereas type II samples are most abundant at Norseman. A few scheelite grains exhibit both type I and type II characteri stics. The two types of REEN patterns with different Na abundances suggest that th e trivalent REE substituted into the Ca site of scheelite by mio different mechanisms: 2Ca(2+) = REE3+ + Na+ for the type I scheelites and 3Ca(2+) = 2 REE(3+) + square(Ca) (where square(Ca) is a Ca site vacancy) for the type I I samples. The parabolic shape of REEN patterns in the type I scheelites ca n be used to calculate the ionic radius of the REE3+ that preferentially su bstitutes into the scheelite structure. When type I REE patterns are normal ized to account for variations in light REE abundance, the average value ob tained is 1.060 Angstrom for type Ia and 1.055 Angstrom for type Ib scheeli tes. These values are smaller than the value of 1.12 Angstrom for the Ca si te but are in excellent agreement with the value of 1.06 Angstrom predicted by the coupled Na+-REE3+ substitution mechanism, assuming size as well as charge compensation. This result implies that Na+ and REE3+ substitute into adjacent sites in the crystal structure of type I scheelites. In contrast, the incorporation of REE into type II scheelites appears to be independent of size. It is suggested that in this type of scheelite, involvement of a Ca site vacancy relaxes the size constraint on the substitution of the REE3 +. The influence of REE speciation in the hydrothermal fluid on the systema tic difference between scheelite-fluid partition coefficients for type I an d II scheelites is of secondary importance compared with crystal chemistry effects. The distinctive REEN patterns and high Na contents of type I scheelites imp ly that they crystallized from hydrothermal fluids with higher Na activitie s than those that formed type II samples. Type I scheelites exhibit no chan ges in the size of the Eu anomaly with REE concentration, implying a predom inance of Eu3+ and crystallization under relatively oxidized conditions. Ty pe II scheelites have variable Eu anomalies and trivalent REE concentration s and thus appear to contain mostly Eu2+ and to have formed under reduced c onditions. Type Ia and II scheelites have (Ce/Lu)(N) > 1 and are interprete d to have crystallized from light REE-enriched fluids, whereas type Ib sche elites with (Ce/Ln)(N) < 1 formed from light REE-depleted fluids. The fluid characteristics inferred from the REE and trace element systemati cs in the scheelites can be attributed to the rock types and estimated phys ical conditions in the Kalgoorlie Norseman region at the time of hydrotherm al activity High Na activities are consistent with buffering of fluids by g reenschist facies mafic rocks, whereas low fluid Na activities are a produc t of fluid rock reactions at higher metamorphic grades. Oxidized fluids are consistent with near-surface derivation, whereas reduced fluids are indica tive of generation at elevated temperatures and pressures. The range and di stribution of fluid (Ce/Ln)(N) ratios are similar to those of the major roc k types in the region.