Ogata K.,University Center in Svalbard |
Pini G.A.,University of Bologna |
Care D.,ENI S.p.A |
Zelic M.,SJS Resource Management |
Dellisanti F.,University of Bologna
Tectonics | Year: 2012
Block-in-matrix is a common fabric characterizing highly deformed to apparently chaotic rocks originated by sedimentary, tectonic and mud-diapiric processes, in many exposed orogenic belts. A true mélange originates when this fabric is associated with mixing of rocks of different ages and provenance, as that characterizing the main décollement shear zone developed between the highly allochthonous Ligurian nappe and its substratum of foredeep deposits at the margins of the Bobbio Tectonic Window (Trebbia Valley, Northern Apennines). Mixing of rocks by both mass transport processes and synsedimentary thrusting occurs at the front and tectonic erosion at the base of the nappe during its emplacement. Evidence of polyphased deformations, spanning from liquefaction-related and hydroplastic structures, to pseudo- hydrofracturing features and mineralized veining, have been recognized within the marginal portions and along the sheared contacts of blocks encased within a scaly matrix. Crosscutting relationships testify a progressive deformation involving a transition from mesoscopic ductile to brittle conditions with the significant contribution of fluid overpressure. A novel evolutionary scheme for block-in-matrix fabric development is here proposed, involving the progressive dismembering of already highly deformed units, originally developed in non- to poorly lithified conditions, through a generalized simple shearing achieved during the evolution of the shear zone, coupled with strain hardening due to tectonic compaction, synkinematic diagenesis and fluid expulsion. Copyright 2012 by the American Geophysical Union.
Duuring P.,University of Western Australia |
Duuring P.,Geological Survey of Western Australia |
Hassan L.,Geological Survey of Western Australia |
Zelic M.,SJS Resource Management |
And 2 more authors.
Economic Geology | Year: 2016
Volcanic-hosted massive sulfide (VMS) occurrences in the Quinns district are hosted predominantly by ca. 2814-2800 Ma banded iron formation (BIF) within a sequence of rhyolite, basalt, and minor siltstone. The VMS occurrences and their surrounding rocks are folded, metamorphosed, and deeply weathered and variably covered by transported regolith. This study uses an integrated petrological and geochemical approach to map gradients in synvolcanic mineral and element abundances, with the aim of understanding the effects of postvolcanic processes on Archean VMS ore. Rhyolite-dominant footwall rocks and the BIF record kilometer-scale gradients in alteration mineral patterns and geochemistry. Rhyolite exposed throughout the district and intersected by drill holes records distal alteration assemblages of quartz-white mica ± chlorite. At the Austin deposit in the western part of the district, abundant talc and anthophyllite with minor cummingtonite and hornblende are associated with mineralization in the BIF, indicating intense magnesium metasomatism. Further away from mineralization, chlorite is the dominant alteration mineral. Adjacent rhyolite is altered to chlorite in proximal zones and white micas in more distal areas. Approaching known VMS prospects in the eastern half of the district, the rhyolite grades into a 2 × 1 km zone of schistose rhyolite with generally dispersed, but locally abundant, coarse-grained andalusite ± kyanite ± garnet rhyolitic schist. This broad Al-rich silicate alteration zone envelops two discrete 1 km × 500 m proximal alteration zones in rhyolite, defined by chlorite ± talc, with minor disseminated magnetite, pyrite, and chalcopyrite. The northernmost proximal alteration zone lies stratigraphically below the BIF-hosted Cu-Zn-rich gossan exposed near the Tasman prospect. The proximal zones are interpreted to be the result of the interaction between synvolcanic, Mg-rich fluids and rhyolite footwall at the time of VMS mineralization and BIF deposition. In contrast, outer zones of andalusite ± kyanite ± garnet suggest the removal of silica and alkali elements and residual concentration of aluminium in rhyolite footwall by acidic synvolcanic hydrothermal fluids prior to peak regional metamorphism. The mapped patterns in alteration minerals mirror bulk rock geochemical gradients in rhyolite; proximal alteration zones are enriched in Cu, Zn, Ag, Au, Bi, Fe2O3, In, MgO, Mo, S, Se, and Te, with local enrichments in As, Cd, MnO, Pb, and V. These rocks are depleted in Ba, K2O, Li, Na2O, and Rb relative to least-altered rhyolite. Rhyolite located up to 100 m from proximal alteration zones is enriched in Ag, Fe2O3, In, K2O, MgO, S, and V. Banded iron formation hosting massive sulfides is altered to talc, anthophyllite, chlorite, and magnetite; these rocks are enriched in Cu, Zn, Pb, Ag, and Sn, with local enrichments in Bi, In, and MgO. Positive Eu anomalies are associated with mineralized BIF in the Austin VMS deposit and contrast with the mostly flat REE slopes for the BIF sampled from less mineralized areas in the district. The kilometer-scale gradients in hydrothermal alteration minerals and bulk rock geochemistry in rhyolite and the BIF are interpreted to be mostly controlled by synvolcanic hydrothermal alteration related to the development of VMS-related hydrothermal fluid pathways. Metamorphism of the synvolcanic alteration was isochemical, apart from localized metasomatism related to later shear zones or fault zones. The defined alteration mineral and geochemical gradients serve as useful tools for detecting fluid alteration pathways related to VMS systems in complexly deformed and metamorphosed districts globally. © 2016 Society of Economic Geologists, Inc.
Bright S.,SJS Resource Management |
Conner G.,SJS Resource Management |
Turner A.,SJS Resource Management |
Vearncombe J.,SJS Resource Management
Transactions of the Institutions of Mining and Metallurgy, Section B: Applied Earth Science | Year: 2014
This paper summarises the technologies available in exploration and mining and describes techniques of core orientation, marking-up, structure measurement and the visual representation of structural data. We provide a critical comparison of tools and methods available at each stage of the process. © 2014 Institute of Materials, Minerals and Mining and The AusIM.
Vearncombe J.,SJS Resource Management |
Zelic M.,SJS Resource Management
Transactions of the Institutions of Mining and Metallurgy, Section B: Applied Earth Science | Year: 2015
Structure is the principal control on gold mineralisation, and the paradigms reflect the punctuated evolution of our understanding of that control. Since the 1950s, structure paradigms for gold control have been a mechanism for gaining research funding, building a public front, enabling publication, communication between government and academia, and building interaction between research groups and industry. Critically, the authors ask if the paradigms have helped us find and mine gold? The 10 paradigms discussed here are: (1) syngenetic (1950s–1980s), (2) late compression (1980s–1990s), (3) brittle strike-slip (1980s), (4) complexity (mid-1980s to present), (5) stress mapping (1990s), (6) earthquake paradigm (1990s–2000s), (7) detail and micro-detail in texturally chronological sequence (2000s), and (8) conglomerate-related (1990s–2000s). The most recent, innovative initiatives are more fluid- than structure-centric, such as the rapid energy transfer of the (9) lightning paradigm, and (10) the non-linear/disequilibrium paradigm. In this paper, the authors overview these paradigms, the critical geology that supports the paradigm and practical exploration implications. Authors confront each paradigm’s premise and results with empirical facts in gold exploration. Our study indicates that the structural paradigms are more descriptive than predictive, and consequently few if any have genuinely found gold. However, many paradigms encourage structural geology and mostly produce quality science an essential component of all mineral exploration. Finally, the authors recommended a simple, critical, and knowledge-based modus operandi. In order to be successful in finding more gold, the authors emphasise the need for more, not less, structural geology and this should be early in the exploration program. An interpreted 3D image of the structural geology and mineralization (not necessarily computer-generated, but based on high-quality information) is a critical tool from the earliest stages of exploration and mining for gold. © 2015 Mineral Deposits Study Group of the Geological Society of London