Abstract As the functionality and speed of 3-D geologic modeling software have improved over the last 30 years, it has become a core tool for identifying, understanding, and modeling the structural controls on ore deposits. This chapter attempts to summarize some of the key considerations involved in the 3-D modeling of structurally controlled ore deposits and establishes a basic three-step workflow that can be applied to almost any deposit style: establish a geologic framework through field work and 3-D visualization, model the project-scale geology, and finally identify, model, and understand the controls on ore shoots. Importantly, the geologic understanding of a project is not a static concept. Each step in the modeling process should add to it, highlighting which aspects of the model fit the current geologic understanding, and thus increase confidence, and which require further review and possible modification. This chapter also provides guidance on preparing data for 3-D modeling, basic 3-D visualization techniques, selecting a modeling approach, and model validation, as well as commentary on some of the more common pitfalls encountered in 3-D modeling. Finally, case studies of the Tuzon gold deposit in Liberia and the Yalea gold deposit in Mali are provided as examples of the process involved in building a 3-D geologic model, from field work to final model.

Abstract A three-dimensional (3D) thermal–kinematic modelling approach based on finite-element techniques is used to study lower-crustal viscosity at transform margins during the continent–ocean transform development stage and after the ridge has passed by. Nine modelling scenarios combining different equilibrium surface heat flows and lower-crustal rheologies are studied. Modelling results indicate that substantial parts of the lower crust at transform margins have the potential to flow at geologically appreciable strain rates, which can lead to uplift/subsidence, as well as lateral variations, in upper- and lower-crustal thicknesses and Moho depth. These low-viscosity zones (i.e. parts of the lower crust with effective viscosities of less than 10 18 Pa s) make up distinct ductility distributions that vary in space and time during margin evolution. Three basic ductility patterns and related thermal processes can be identified: reduced lower-crustal viscosities originating at the continental rift and the continent–ocean boundary (COB), respectively; reduced lower-crustal viscosities along the transform caused by the migrating ridge; and the background distribution of lower-crustal ductility resulting from the equilibrium temperature field. Superposition of all three ductility patterns and the complex interaction of the underlying perturbations of the temperature field result in distinct differences in the potential of lower-crustal flow both in space (parallel and perpendicular to the transform) and with time. Thus, modelling results provide templates for understanding lower-crustal flow at transform margins in general and await further studies comparing model predictions with actual field observations.

A variety of lithostratigraphic units may be outlined within the pre-Cretaceous crystalline basement of the Atlantic and Gulf Coastal Plains. An expanse of undeformed granite occurs within central peninsular Florida; biotite within the pluton yields 40 Ar/ 39 Ar plateau dates of ca. 525 to 530 Ma. The high-level character of the granite and its emplacement age are similar to post-kinematic granitic plutons within northern portions of the Rokelide orogen in Guinea, West Africa. A sequence of calc-alkaline, felsic volcanic-volcaniclastic rocks, together with associated hypabyssal granite, occurs throughout the basement terrane. These are variably altered and display markedly discordant 40 Ar/ 39 Ar age spectra. Crystallization ages are likely best represented by ca. 675 to 690 Ma Rb-Sr whole-rock and 40 Ar/ 39 Ar hornblende plateau ages previously reported for correlative calc-alkaline granites exposed in southeastern Senegal. A Lower Ordovician–Devonian sedimentary sequence with non-Laurentian paleontological characteristics occurs within the Suwannee basin of the northern Florida subsurface. Detrital muscovite from Lower Ordovician sandstones in the succession records 40 Ar/ 39 Ar plateau ages of ca. 500 to 510 Ma. The sequence is similar to that within the Bové basin in Senegal and Guinea. A small area of high-grade metamorphic rocks occurs in the east-central Florida subsurface. Hornblende from this terrane records 40 Ar/ 39 Ar plateau ages of ca. 485 to 495 Ma. The character and post-metamorphic cooling history of these rocks are similar to that of units within the Rokelide orogen in Sierra Leone and Liberia, West Africa. In the subsurface Wiggins Arch of southwestern Mississippi, hornblende and biotite from interlayered gneiss and amphibolite record similar 40 Ar/ 39 Ar plateau ages of ca. 300 to 310 Ma. Phyllite from a lower-grade metasedimentary sequence penetrated in the Wiggins Arch of southwestern Alabama records a 40 Ar/ 39 Ar whole-rock plateau age of ca. 315 Ma. Characteristics of the pre-Cretaceous basement units suggest that they represent an extension of the Mauritanide, Bassaride, and Rokelide orogens of West Africa. There is no apparent record of Paleozoic tectonothermal activity in central and southern portions of the Atlantic and Gulf Coastal Plain subsurface basement terrane. This is in marked contrast to complete late Paleozoic reworking of basement units penetrated in the area of the Wiggins Arch. This suggests that a major dextral transcurrent fault system was likely active during the late Paleozoic, and that proximal basement units were directly involved in collisional aspects of Pangea assembly. The various units of the Atlantic and Gulf Coastal Plain basement are not correlative with any of the northern non-Laurentian terranes exposed in the southern Appalachian orogen (e.g., Carolina terrane of the eastern Piedmont) which had earlier accreted to exterior positions along the eastern margin of the North American craton. These were transported into their present structural positions along a basal décollement during late Paleozoic collision of Gondwana and Laurentia, and are separated from the stranded Gondwana units of the Coastal Plain basement by a suture approximately marked by the Brunswick-Altamaha magnetic anomaly.