Rare earth element phosphate minerals from the Olympic Dam Cu-U-Au-Ag deposit, South Australia: recognizing temporal-spatial controls on REE mineralogy in an evolved IOCG system

Abstract

Florencite, [REEAl₃(PO₄)₂(OH)₆], is the most abundant REE-phosphate mineral in the giant Olympic Dam Cu-U-Au-Ag deposit, South Australia. Florencite typically occurs as fine-grained crystals and occasional aggregates in the matrix of the granite-dominant breccia that hosts the majority of the copper mineralization. Olympic Dam florencite, with the compositional range and extended formula (Ca₀.₀₁-₀.₂₄SR₀.₀₃--₀.₄₀La₀.₁₄-₀.₄₉Ce₀.₂₀-₀.₄₇Pr₀.₀₀-₀.₀₃Nd₀.₀₀-₀.₀₅)Σ₀.₄₃-₀.₉₆Al₂.₈₉-₃.₃₃(P₁.₄₂-₁.₉₆S₀.₀₅-₀.₃₄As₀.₀-₀.₂₀)Σ₁.₇₇-₂.₂₁O₄(OH)₆], is LREE-enriched, typically La-dominant, while HREEs are minor. There is also compositional variability with respect to Sr, Ca, SO₄, and AsO₄ components. Chondrite-normalized fractionation trends are steeply downwards-sloping with a relatively low and flat HREE segment. Such a fractionation trend is markedly different from that seen for REE-fluorocarbonates, the dominant REE host at Olympic Dam, which contain relatively higher MREE and HREE components. Xenotime is relatively rare at Olympic Dam and is most commonly seen as overgrowths on zircon. Compositional data for xenotime show HREE concentrations in which Y > Yb > Er > Dy > Ho. Thorium, U, and Pb are minor components in both xenotime and florencite. Monazite is a minor phase and co-existing monazite and florencite are never observed. Texturally, florencite appears to belong to a later stage of LREE mineralization at Olympic Dam, following and less prominent than the main REE-fluorocarbonate stage. Olympic Dam florencite is more La-rich and Ca-poor than it is in most of the localities worldwide for which compositional data have been published. The REE mineralization trends of florencite are compared with published REE trends for other mineral groups from Olympic Dam. Uraninite, brannerite, coffinite, and apatite show patterns of progressive LREE depletion from generation to generation, suggesting enhanced solubility of LREE over geological time. All LREE minerals (fluorocarbonates and phosphates), on the other hand, display progressive LREE enrichment correlating with a paragenetic sequence from REE-fluorocarbonates with essential Ca, through bastnäsite, to florencite. The late-stage florencite reported here is the most La-rich of all REE-bearing phases and is interpreted to record either the end of a continuous fluid evolution, or a later separate event at changed physicochemical conditions under which REE-aluminum phosphate minerals are stable.