THE ORIGIN OF UNUSUAL PHOSPHATE DEPOSITS ON THE VENEZUELAN ISLAND OF GRAN ROQUE, LEEWARD ANTILLES

The bedrock geology of the Leeward Antilles island of Gran Roque has long been interpreted to be exposures of the Caribbean Plate, which formed when existing Pacific oceanic crust was thickened by oceanic plateau mafic magmatism. The mafic complex on Gran Roque consists of older amphibolite facies rocks derived from a diabase protolith and younger coarse-grained gabbros containing a low-grade static metamorphic overprint of prehnite ± actinolite. The gabbros have yielded an 87.0 ± 4.1 Ma U-Pb micro-zircon age. The age, petrography, and geochemistry of the mafic complex are consistent with a Caribbean Plate origin, supported by their similarity to those of other Caribbean Plate exposures on nearby islands Aruba and Curaçao. In the mafic complex, the older amphibolites may represent an earlier phase of plateau magmatism or the pre-existing Pacific oceanic crust. We interpret the intrusion of the gabbros to have been the heat source for amphibolite facies metamorphism. The mafic complex was then intruded by a quartz diorite (65.6 ± 1.4 Ma U-Pb zircon age) with island arc geochemistry and associated dikes and sills of hornblende dacite, pegmatite (65.3 ± 0.91 Ma U-Pb zircon age), and granophyre.

Also within the mafic complex are visually striking phosphate deposits. The best exposures are on the western part of the island, where rinds of green phosphate surround spheroidally weathered corestones of gabbro. X-ray diffraction indicates that the deposits principally consist of the unusual minerals variscite, phosphosiderite, and strengite. The quartz diorite and its associated dikes and sills are not phosphatized and sharply cross-cut the phosphatized rocks of the mafic complex. Thus, the spheroidal weathering and phosphate formation occurred between ca 87-66 Ma. This period of subaerial weathering is broadly consistent with those documented on Aruba and Curaçao. We attribute the phosphate formation to rainwater leaching Cretaceous guano deposits and subsequently reacting with the weathered mafic complex. Similar modern processes have been observed and described from other localities.