Late-kinematic timing of orogenic gold deposits and significance for computer-based exploration techniques with emphasis on the Yilgarn Block, Western Australia

Orogenic gold deposits no a widespread coherent group of epigenetic ore dep osits that are sited in accretionary or collisional orogens. They formed ov er a large crustal-depth range from deep-seated low-salinity H2O-CO2 +/- CH 2 +/- N-2 ore fluids and with Au transported as thio-complexes, Regional st ructures provide the main control on deposit distribution. In many terranes , first-order faults or shear zones appear to have controlled regional flui d flow, with greatest ore-fluid fluxes in, and adjacent to, lower-older fau lts, shear zones and/or large folds. Highly competent and/or chemically rea ctive rocks are the most common hosts to the larger deposits. Focusing of s upralithostatic ore fluids into dilatant zones appears to occur late during the evolutionary history of the host terranes, normally within D-3 or D-4 in a D-1-D-4 deformation sequence. Reactivation of suitably oriented pre-ex isting structures during a change in far-field stress orientation is a fact or common to many deposits, and repeated reactivation may account for multi ple mineralization episodes in some larger deposits. Absolute robust ages o f mineralization support their late-kinematic timing, and, in general, sugg est that deposits formed diachronously towards the end of the 100 to 200 m. y. long evolutionary history of hosting orogens. For example, in the Yilgar n Block, a region specifically emphasised in this study, orogenic gold depo sits formed in the time interval between 40 and 90 m.y., with most about 60 to 70 m.y., after the youngest widespread basic-ultrabasic volcanism and t owards the end of felsic magmatism. The late timing of orogenic gold deposi ts is pivotal to geologically-based exploration methodologies. This is beca use the present structural geometries of: (i) the deposits, (ii) the hostin g goldfields, and (iii) the enclosing terranes are all essentially similar to those during gold mineralization, at least in their relative position to each other. Thus, interpretation of geological maps and cross-sections and three-dimensional models can be used to accurately simulate the physical c onditions that existed at the time of ore deposition. It is particularly si gnificant that the deposits are commonly related to repetitive and predicta ble geometries, such as structural heterogeneities within or adjacent to fi rst-order structures, around rigid granitoid bodies, or in specific "locked -up" told-thrust structures. Importantly, the two giant greenstone-hosted g oldfields, Kalgoorlie and Timmins, show a remarkably similar geometry at th e regional scale. Computer-based stress mapping and GIS-based prospectivity mapping are two computer-based quantitative methodologies that can utilize and take advantage of the late timing aspect of this deposit type to provi de important geological aids in exploration, both in broad regions and more localized goldfields. Both require an accurate and consistent solid geolog y map, stress mapping requires knowledge of the far-field stresses during m ineralization, and the empirical prospectively mapping requires data from a significant number of known deposits in the terrane. The Kalgoorlie Terran e, in the Yilgarn Block, meets these criteria, and illustrates the potentia l of these methodologies in the exploration for orogenic gold deposits. Low minimum stress anomalies, interpreted to represent dilational zones during gold-related deformation, coincide well with the positions of known goldfi elds rather than individual gold deposits in the terrane, and there are add itional as-yet unexplained anomalies. The prospectivity analysis confirms that predictable and repetitive factors controlling the siting of deposits are: (i) proximity to, and orientation and curvature of, granitoid-greenstone contacts, (ii) proximity to segments of crustal faults which strike in a preferred direction, (iii) proximity t o specific lithological contacts which have similar preferred strike, (iv) proximity to anticlinal structures, and (v) the presence of preferred react ive host rocks (e.g., dolerite). The prospectivity map defines a series of anomalous areas, which broadly conform to those of the stress map (> 78% co rrespondence). The most prospective category on this map covers less than 0 .3% of the greenstone belts and yet hosts 16% of the known deposits, which have produced > 80% of known gold. Thus, it discriminates in favour of the larger economically more-attractive deposits in the terrane. The successful application of stress mapping and prospectively mapping to geology-based e xploration for orogenic gold deposits indicates that more quantitative anal ysis of geological map data is a profitable line of research. The computer- based nature of these methodologies is ideal for the production of an ultim ate, integrated, deposit target map, which can be compared to other, more c onventional, targetting parameters such as geophysical and geochemical anom alies. Such an integrated strategy appears the way toward in the increasing ly difficult task of cost-effective global exploration for orogenic gold de posits in poorly exposed terranes. (C) 2000 Elsevier Science B.V. All right s reserved.