Geology and metallogeny of Jordanian uranium deposits

摘要

Uranium mineralisation has been known within the Hashemite Kingdom of Jordan for a long time and in 2014 maiden mineral resources were estimated for the Central Jordan Uranium Project (CJUP) following the reporting principles of the JORC Code (JORC 2012). Based on this estimate the CJUP contains 269mt of mineralisation at an average grade of 135ppm U_3O_8 (36-4kt of contains U_2O_8) within an Inferred Resource. The objective of this paper is to describe the geology and the metallogeny of uranium mineralisation in Jordan, providing new insights into this mineralisation that remains largely unknown outside of the country. Most of Jordan's territory is covered by platform sedimentary rocks that generally formed during the Cretaceous and Paleogene. The majority of uranium mineralisation is confined to Upper Cretaceous sediments, in particular the Al Hisa Phosphorite (AHP) and the Muwaqqar Chalk Marl (MCM) formations. The MCM formation hosts surf icial type-mineralisation that is mainly concentrated in a narrow, approximately 4-5m thick layer that is distributed close to the topographic surface and is independent of phosphorite beds. This mineralisation, termed 'surf icial', is the economically most important type of uranium in Jordan and currently represents the main exploration target in this area. The main uranium minerals within the surficial layer are uranium vanadates, colloquially referred as carnotite group minerals. The average uranium grade rapidly decreases below a depth of 4-5m, although mineralisation continues to depths of 30-40m. Beneath the surficial layer, uranium mineralisation is distributed as lenses that are generally small and discontinuous. This mineralisation, termed 'Deep', has many common features with the surficial mineralisation, including similar mineralogy and chemical composition and may represent a transition between surficial and phosphorite-hosted types. The metallogeny of surficial uranium mineralisation is well studied, although several important questions remains unanswered. In particular, close spatial relationships between pyrometamorphic marbles and the uranium mineralisation can be observed on regional geological maps and in the field. Two different models have been proposed to explain the formation of the pyrometamorphic marbles in central Jordan, namely the burning of bituminous marls and an alternative model involving the combustion of deep reservoirs of hydrocarbon gases relating to mud volcanoes. Tectonic faults have also possibly played a role in the distribution of uranium mineralisation in central Jordan. This is in particular relevant for the Khan Azabib area where higher grade mineralisation and their associated gamma anomalies are broadly coincident with regional faults. These faults may have acted as permeable zones, facilitating migration of the hyperalkaline solutions that leached uranium from the host rocks. Eventually, the liberated uranium was redistributed by supergene processes towards the surface, where uranium minerals precipitated along the contact between saprolite and fresh to weakly weathered rocks. In summary, the uranium deposits of Central Jordan resulted from the interplay of sedimentary, tectonic, and metamorphic events, with the final impact made by climatic factors and chemical weathering processes. The AHP formation hosts phosphorite-hosted type mineralisation that formed as a result of the synsedimentary concentration of uranium in apatite and is abundant in the Hasa-Qatrana area of central Jordan. The surficial parts of the phosphorite-hosted uranium mineralisation have also been partially modified by weathering, which has released uranium from apatite and redistributed and precipitated it as 'carnotite'. This type of mineralisation is present at the Sahb El Abyad prospect located in the south-eastern part of Jordan. Uranium mineralisation also occurs in hydrothermal veins that cross-cut Cambrian rocks exposed along the plate margin in the Dead Sea rift val