Petrology of the neoproterozoic Cape Granite Suite hosted W-Mo-REE Riviera endoskarn deposit with special reference to allanite characteristics

[摘要] ENGLISH ABSTRACT: The Riviera W-Mo deposit is located near Piketberg in the Western Cape and is hosted by I- and A-type granitoids of the Neoproterozoic to Palaeozoic Cape Granite Suite that intruded the meta-volcano-sedimentary Malmesbury Group. Scheelite and molybdenite are the principal ore minerals and are associated with endoskarn- and vein-type alteration of the granitoids. It is the 6th largest deposit of its kind in the world with a grade of 0.216% tungsten (WO3) and 0.02% molybdenum (Mo), and a tonnage of 46 Mt. The discovery of rare earth elements in addition to W and Mo in this deposit makes it attractive to the global market as there is currently a demand for tungsten and rare earth elements in the West.This study comprises a mineralogical and geochemical investigation of a representative suite of drill core samples to identify the various mineral phases, their spatial distribution and textural features. This allowed definition of the relationship between hydrothermal alteration facies and enrichment. In addition this knowledge contributes to unravelling the genesis of this unusual deposit.Results were obtained by means of optical microscopy, scanning electron microscopy, as well as whole rock geochemical XRF and LA-ICP-MS analyses.The deposit consist of primary and secondary mineral assemblages; the latter as a result of superimposed skarnification and hydrothermal alteration. Primary minerals include quartz, feldspars (plagioclase and alkali), micas (biotite) and accessory titanite. The process of hydrothermal alteration formed skarn assemblages which include prograde metasomatic minerals like bastnaesite, scheelite, garnet, pyroxene, epidote, titanite, vesuvianite, apatite, and allanite. Other prograde minerals include secondary biotite, white mica, secondary albite and alkali feldspar. A later phase of pervasive hydrothermal alteration of the granite formed retrograde minerals like chlorite, carbonates, amphiboles, sericite (white mica), goethite and clay. Enrichment is mainly in the roof of the Riviera granite cupola.Three rock types have been identified based on the modal mineralogy, they include firstly and closest to the granite wall rock contact a quartz porphyry monzogranite, followed by a biotite granite to monzogranite and lastly an aphanitic granite to monzogranite at depth in the pluton. Whole rock geochemical data is generally unreliable due to the effect of hydrothermal alteration, although it is speculated that this pluton is largely A-type, metaluminous to peraluminous in composition and subduction-related.The different hydrothermal alteration types that were identified include argillic and advanced argillic alteration, phyllic and advanced phyllic alteration, and potassic alteration. The spatial relationship between these alteration types appear to be interlayered and follows the contour of the pluton. The entire pluton has been affected by phyllic alteration and potassic alteration occurs mainly towards the roof of the granite cupola. The other types of alteration occur sporadically.The vertical distribution of various elements, based on geochemical data, allowed for the identification of a mineralized zone, close to the granite wall-rock contact, and a non-mineralized zone at depth. The mineralized zone constitutes the bulk of the enrichment and is rich in calcic skarn minerals and potassic alteration products. Although altered, the non-mineralized zone reflects the original rock assemblage more clearly with sporadic occurrences of enrichment as skarnified granite patches.Significant concentrations of allanite, a light rare earth element enriched mineral of the epidote group, were recently discovered in the endoskarn part of the Riviera pluton and could constitute an economically important by-product. Allanite is a complex mineral with poikilitic textures, irregular zonation patterns, metamictization, medium- to coarse grain sizes and no apparent consistent association with other minerals. The two most common types are allanite (Ce) and ferriallanite (Ce).Chondrite-normalized REE patterns for the whole rock analyses are similar to those of the single grains and suggest that most of the rare earth elements are hosted by allanite. The steep slope of the pattern demonstrates LREE-enrichment and Eu-anomalies vary from slightly negative, neutral to slightly positive. A high ΣLREE content of allanite correlates with a positive Eu-anomaly whereas negative anomalies are associated with low concentrations. This suggests that the partitioning of REE in allanite is redox-sensitive.Various evolutionary stages gave rise to this complex deposit; from prograde skarn formation to retrograde alteration. The main influence is due to the variability of metasomatizing fluids that evolved from early prograde, high temperature and reduced, to late retrograde, low temperature, and oxidized. A similar study of zoned scheelite from this deposit supports these observations and indicates genetically early scheelite and late stage allanite.

[发布机构] Stellenbosch University