The Orogenic Crixás Gold Deposit, Goiás, Brazil: A Review and New Constraints on the Structural Control of Ore Bodies

A review of the current knowledge of lithostratigraphy, geochronology, mineralogy, alteration, fluid chemistry and structural data is provided in order to discuss the main controls on mineralization in the Crixás gold deposit and the existing structural framework and evolution. Gold mineralization at Crixás represents orogenic upper mesothermal to lower hypothermal types developed within the overturned Paleoproterozoic (Rhyacian) stratigraphic sequence. The structural data indicates that upright folding of the stratigraphy and formation of a distinct S1 foliation characterizes D1 deformation. This controls the formation of laminated quartz veins, precipitation of Au-rich sulfides, and the development of NW trending orebodies. Localized F2 folding of D1 structures and strong L2 stretching lineations characterizes the D2 event. This localized F2 folding created oreshoots trending W-WNW within distinct orebodies. The D2 deformation is associated with the precipitation of native Au. The ‘bulk plunge’ of these orebodies is a combination of both plunges defined by D1 and D2 deformation events. A critical assessment of lithostratigraphic and structural data reveal two possible tectonic scenarios operating during the Paleoproterozoic. The first possible tectonic scenario considers sagduction and the development of a regional-scale synclinorium, followed by tilting and localized folding of D1 structures. The second scenario, which is similar to the current model, considers accretion producing a synclinorium, followed by thrusting. Based on the current knowledge, both models are considered to be permissible and a series of focused research studies are proposed to test both the current and the new tectonic models.

Marine Acoustic Zones of Australia

Underwater sound is modelled and mapped for purposes ranging from localised environmental impact assessments of individual offshore developments to large-scale marine spatial planning. As the area to be modelled increases, so does the computational effort. The effort is more easily handled if broken down into smaller regions that could be modelled separately and their results merged. The goal of our study was to split the Australian maritime Exclusive Economic Zone (EEZ) into a set of smaller acoustic zones, whereby each zone is characterised by a set of environmental parameters that vary more across than within zones. The environmental parameters chosen reflect the hydroacoustic (e.g., water column sound speed profile), geoacoustic (e.g., sound speeds and absorption coefficients for compressional and shear waves), and bathymetric (i.e., seafloor depth and slope) parameters that directly affect the way in which sound propagates. We present a multivariate Gaussian mixture model, modified to handle input vectors (sound speed profiles) of variable length, and fitted by an expectation-maximization algorithm, that clustered the environmental parameters into 20 maritime acoustic zones corresponding to 28 geographically separated locations. Mean zone parameters and shape files are available for download. The zones may be used to map, for example, underwater sound from commercial shipping within the entire Australian EEZ.