Rare-earth element mobility around the volcanogenic polymentallic massive sulfide deposit at Que River, Tasmania, Australia

Abstract

The Que River volcanogenic base- and precious-metal deposit is located in the Cambrian Mt. Read Volcanics of western Tasmania. The host andesites and dacites adjacent to mineralization have been hydrothermally altered during mineralization and the entire sequence has undergone Devonian regional prehnite-pumpellyite facies metamorphism. Rocks petrographically recognizable as andesites exhibit chondrite-normalized REE patterns similar to modern orogenic high-K andesitic rocks with ( Ce Yb ) n ratios ranging from 6.4 to 8.6. Dacites are more fractionated with ( Ce Yb) n ranging from 10.6 to 16.0. Like the andesites, the dacites are characterized by negative Eu anomalies ( Eu Eu ∗ = 0.73–0.85 ). Except for a more extreme Eu anomaly in one sample ( Eu Eu ∗ = 0.57 ), polymict volcaniclastic rocks have patterns very similar to those of the andesites. The chondrite-normalized REE patterns for the hydrothermally altered volcanics fall into three groups that can also be distinguished petrographically. A single chloriterich sample has a substantial Eu anomaly and shows less relative depletion in the middle REE than do either the andesites or the dacites. Highly silicified samples are all characterized by large negative Eu anomalies ( Eu Eu ∗ = 0.37–0.46 ) and many are relatively unfractionated with ( Ce Yb ) n ratios ranging from 3.5 to 7.7. A third group of samples, characterized by the development of phengitic mica and carbonate show REE patterns similar to the andesites, although their Eu anomalies are generally smaller. A sample of massive Zn/1bPb ore has very low REE abundances showing light REE enrichment and a small negative Eu anomaly ( Eu Eu ∗ = 0.82 ). Massive pyrite has a similar pattern except for higher overall abundances and a small positive Eu anomaly ( Eu Eu ∗ = 1.10 ). Except for the Eu anomalies, the andesites and dacites show no evidence for REE mobility. Eu anomalies in the andesites are probably due to alteration of phenocrystic plagioclase and subsequent removal of divalent Eu. Substantial REE mobility has occurred in the hydrothermally altered volcanic rocks, the degree of mobility correlating with alteration style and intensity. Eu has been leached from both former plagioclase phenocrysts and the groundmass. Some of the Eu leached from zones of the most intense alteration may have been redeposited in lower-temperature, more distal alteration zones.