Sedimentological study of the western zone of the Lady Annie phosphate deposit, Queensland, Australia
Date
1981
Authors
Elgueta, Sara
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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.
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