Mineral systems: Their advantages in terms of developing holistic genetic models and for target generation in global mineral exploration

Abstract

• Single deposit class mineral system model for orogenic gold deposits. • Application to conceptual exploration. • Multiple deposit class model for porphyry-high sulfidation-skarn Cu-Au (Mo, W, Ag). • Combined geodynamic and preservation factors for craton margin deposits. Since the concept of mineral systems was first proposed, there have been growing calls for their use in deriving holistic genetic models and in assisting in exploration targeting in a world of diminishing mineral discoveries. This is because the mineral systems approach provides a means of integrating information over a range of time and terrane scales using the broad critical components of Geodynamics, Fertility, Architecture, and Preservation. Although their adoption is limited, the value of such mineral systems for single deposit classes and closely related deposit groups is demonstrated and numerous disparate deposit classes are shown to occupy equivalent tectonic niches in terms of their Geodynamic and Preservation components. The value of a mineral system model for a single deposit class is demonstrated for orogenic gold deposits using the premise that their genesis is explained in terms of a single global model rather than a series of disparate local models. If all orogenic gold deposits define a coherent mineral system, there are only two realistic sources of auriferous fluid, based on their syn-mineralization geodynamic settings. These are from devolatilization of a subducted oceanic slab with its overlying sulfide-rich sedimentary package, or release from mantle lithosphere that was metasomatized and fertilized during a previous subduction event, particularly adjacent to craton margins. This orogenic gold mineral system can be applied to conceptual exploration by first identifying the required settings at Geodynamic to deposit scales. Within these settings, it is then possible to define the critical gold mineralization processes in the system: Fertility, Architecture, and Preservation. These among other parameters, dictate that the structures controlling ore fluid advection must be lithosphere-scale faults that can be identified in magneto-telluric surveys, and that amphibolite-facies metamorphic domains are prospective exploration search spaces. The porphyry-high sulfidation-skarn Cu-Au (Mo, W, Ag) deposit group is an example where the mineral system comprises several deposit classes that are commonly spatially and temporally associated. These occur in strike extensive, curvilinear volcanic, continental, island, and post-collisional arcs, where Geodynamics and province-scale Architecture are controlled by arc-parallel continental-scale faults and intersections of oblique crustal-scale accommodation structures, commonly in shallow subduction slab arc segments. Fertility of the arcs is indicated by geochemistry of volcanic components that reflect residence and replenishment of H 2 O-rich hybrid source magmas in upper crustal magma chambers. In terms of district-scale Architecture, the porphyry-related systems are sited within and above small finger-shaped porphyritic bodies that intrude the roof zones of oxidized granitic plutons. The mineral systems comprise vertically and laterally zoned orebodies, alteration envelopes, and metal ratios developed from long-lived, originally high salinity and low pH, evolving magmatic-hydrothermal fluids exsolved from the fertile porphyry intrusions. Preservation potential is limited because of high uplift and erosion rates in the host arcs, with most deposits restricted to the Cenozoic, although there are Mesozoic examples in post-collisional arcs. At district scale, the presence of lithocaps identifies systems at a suitable exhumation level for exploration, with a range of mapping, remote sensing spectral, and geophysical methodologies applicable to detect critical responses from extensive porphyry-related footprints. Craton and thick lithosphere margins represent Geodynamic settings with high Preservation factors for an incredibly large range of deposit classes enriched in Ag, Au, Bi, Co, Cu, Fe, Mo, Ni, P, Pb, PGE, REE, Sb, Sn, Te, W, and Zn, and including diamonds. These are sited within ∼100 km of the margins due to adjacent metasomatized and fertilized mantle lithosphere, development of lithosphere-scale fault arrays, or fertile marginal basins on these margins. Thus, although Fertility and Architecture components vary widely between component mineral systems, the combined Geodynamic and Preservation factors provide specific restricted exploration search spaces for a variety of deposit classes. This is shown in spectacular fashion for the North China and Yangtze cratons in eastern China where 66 diverse giant or world class deposits are sited within ∼100 km of their margins.