126 SEDIMENTARY ROCKS/Phosphates occurrence is in the Cretaceous Rapid Creek Formation in the Canadian Yukon Here, a marine sequence of ironstone and shale containing unusual Fe- and Mn-bearing phosphates instead of collophane has been deformed and uplifted This resulted in sets of fractures in which a wealth of well-crystallized phosphate minerals have formed Four major assemblages characterized by the predominance of specific elements and related to a specific host rock have been identified, with at least five new phosphate species recognized (baricite, garyansellite, gormanite, kulanite, and penikisite) As well, remarkable crystals of arrojadite, augelite, and lazulite, amongst others, occur in this fracture-filling paragenesis In south-eastern Australia, a variety of settings for sedimentary phosphate deposits has been recognized, with several producing a range of unusual, in some cases new species Some deposits have been exploited for phosphate, but all are low grade The oldest deposits are in South Australia, where there are two main phosphatic horizons, one Late Precambrian, the other Early Cambrian, associated with limestones There has been local leaching and intermittent concentration of phosphate by replacement The Moculta deposit has been affected by regional metamorphism, which has recrystallised and brecciated the phosphatic rock A range of secondary phosphate minerals, such as wavellite, beraunite, cyrilovite, leucophosphite, variscite, crandallite, and aldermanite (for which Moculta is the type locality) has been recorded in veinlets and small cavities and probably formed during near-surface weathering An intense and prolonged weathering origin can probably be ascribed to a suite of phosphate minerals found in metamorphosed Early Proterozoic iron-rich sediments at Iron Monarch, in the Middleback Ranges Over thirty phosphate species, including bermanite, collinsite, cyrilovite, fairfieldite, kidwellite, montgomeryite, turquoise, and wavellite, have been identified A number of vanadates also occur in the assemblage In central Victoria, small, low-grade phosphate deposits within Ordovician black slate–chert host rocks exhibit a number of mineralization styles, such as phosphorite bands, intraformational breccias, and vein networks Secondary minerals resulting from weathering of the primary phosphorites include wavellite, turquoise, variscite, cacoxenite, and fluellite Amongst the world’s largest phosphate deposits are those of Morocco, where Late Cretaceous marine sediments occur on the plains fronting the Atlas Mountains (see Africa: North African Phanerozoic) These are nodular and sandy deposits riddled with fish teeth and fit well into the upwelling nutrient-rich current model outlined above Other significant world producers of sedimentary phosphate are the USA, Brazil, and China Guano Deposits Phosphate deposits derived from bird and bat guano represent only a small proportion of the total world reserves of phosphate rock Insular deposits are common in warm-arid or semi-arid regions with large bird populations either at the present day or in the recent past The most important deposits, now essentially worked out are on larger islands over 50 metres above sea-level, such as Nauru and Christmas Island, and are thought to be older than about one million years old In these deposits, solutions derived from overlying bird droppings have percolated into the bedrock, where minerals such as apatite, whitlockite, crandallite, and millisite have crystallized This phosphatized bedrock forms much of the resource Cave phosphate deposits derived from bat droppings are of more interest for the unusual minerals they may contain than for their economic value Such deposits are mostly in limestone caves, with a minority in lava-tube caves The chemical reactions involved in forming phosphate minerals are complex, but usually begin with leaching of very soluble nitrogen from the guano This leaves phosphorus to combine with whatever cations are available from the surrounding rocks The resulting sequence of minerals may be well stratified within the guano Typical cave phosphates include brushite, carbonate-hydroxylapatite carbonate-fluorapatite, taranakite, and variscite, generally occurring as powdery nodules within the guano or as coatings on bedrock or cave walls Distinct crystals of phosphate minerals, such as newberyite and struvite are rare, with a notable occurrence in lava caves at Skipton, Victoria Phosphates in Oxidized Metal Sulphide Deposits Large numbers of phosphate minerals occur in the oxidized zones of base metal orebodies Solubility phenomena play the most important role in determining which phosphates crystallize in these low-temperature environments, where generally acidic groundwaters dominate Phosphates of Pb2ỵ are generally the least soluble, so these minerals, particularly pyromorphite (Figure 9), are prominent in oxidized zones above galena-bearing ores As primary ores commonly contain a mix of lead, copper, and zinc sulphides, as well as arsenopyrite, a diverse suite of secondary phosphates and arsenates can form in oxidized zones Whether phosphates will be prominent over arsenates depends on the availability of phosphorus, usually as apatite, in either the primary ore or the host rocks – it can vary widely These differences are illustrated by the two most mineralogically diverse oxidized zones known At Tsumeb, Namibia, arsenates generally dominate the secondary assemblage, whereas at Broken Hill,