Sedimentological study of the western zone of the Lady Annie phosphate deposit, Queensland, Australia

Abstract

A detailed sedimentological study of part of a major phosphate deposit within the Middle Cambrian Georgina Basin phosphogenic province was undertaken in order to establish a depositional model, to determine the sedimentary parameters controlling phosphorite deposition and to identify the factors influencing phosphate grade. The study was concentrated on the Western Zone of the Lady Annie deposit which, together with the Lady Jane deposit, occurs in an elongate north-south outlier surrounded by Precambrian basement rocks on the eastern margin of the Georgina Basin. Field investigations were directed principally towards understanding the spatial relationships between phosphorites and associated non-phosphatic sediments within the same overall palaeoenvironment. Field mapping and measurement of stratigraphic sequences and palaeocurrent data resulted in the recognition of eight major sedimentary lithofacies: a basal conglomerate-sandstone facies, an undifferentiated carbonated facies, a laminate facies, an oncolite facies, a skeletal facies, a phosphorite facies, a chert-terrigenous facies and a siltstone facies. The detailed description of twenty-three vertical sections from six trenches which expose part of the phosphorite facies shows that the phosphorites consist of a relatively monotonous succession of well bedded to laminnted grainstone and packstone phosphorite interbedded with chert and minor lenticular beds of fine siliciclastic sediments. Wackestone and mudstone phosphorites form minor components in the trenches and characteristically occur in the upper portion of the phosphorite sequence. Thin section petrography of about 74 selected samples from the trenches shows that the grainstone-packstone phosphorite varies in grain size between very coarse and very fine sand. Grains resemble carbonate clastic grains and consist of varying proportions of rounded and relatively well sorted fossil fragments (mainly fragments of calcisponges and echinoderms), ovulitic grains of indefinite but probably skeletal origin, rounded intraclasts, and very minor nucleated pellets and oolites. Chemical and X-ray diffraction analysis of over 150 samples indicates that the Lady Annie phosphorites have a composition similar to that of other deposits of the Georgina Basin, which consist almost entirely of finely crystalline carbonate fluorapatite. X-ray diffraction revealed that silica is the dominant gangue mineral in all phosphorites, clays are minor and micas and dolomite are very minor. Comparisons are made between the facies observed in the study area and similar facies associations described from the modern carbonate environments of Shark Bay (Western Australia) and the Trucial Coast (Persian Gulf) . The essential climatic, oceanographic and morphological factors determining Holocene sedimentation in these areas may be applied to the Western Zone of Lady Annie. Although Holocene models are applicable .to the sedimentological features of the study area, wholly acceptable Holocene models of phosphorite accumulation closely similar to the Lady Annie phosphorites are not known at present. HowPver, the sedimentary processes controlling the distribution of phosphorites on the Santo Domingo platform of Baja California do have some relevance. It is proposed that the phosphorite deposit is the result of the trapping of phosphatic allochems in a semi-protected microtidal tectonic depression marginal to the Georgina Basin epeiric sea. The analysis of grain type, textural fabrics and sedimentary structures of the phosphorites indicates that reworking, transport and winnowing played a major role in the formation of the deposit and in determining phosphate grade. Facies distribution within the environment is complex and principally reflects the palaeotopography and the local hydrological and water depth conditions. The close spatial relationship between phosphorite, carbonate and oncolite facies provides strong evidence that the deposit formed in a water depth of only a few metres, probably in a subtidal-intertidal system. Phosphorite formation and accumulation was concomitant with the introduction of nutrient-rich transgressive marine waters and terminated with the flooding of the area by fine siliciclastic sediments. It is evident from this study that an understanding of the sedimentology of phosphatic intervals is important if the factors responsible for the formation of phosphate deposits of potential economic interest are to be fully elucidated.