Gold-Transporting Hydrothermal Fluids in the Earth's Crust

Gold-Transporting Hydrothermal Fluids in the Earth's Crust
Author: P.S. Garofalo
Publisher: Geological Society of London
Total Pages: 266
Release: 2014-09-16
Genre: Science
ISBN: 1862396574

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Hydrothermal ore deposits that are exploited for gold include both gold-only deposits, such as orogenic deposits, and gold-bearing examples of the common hydrothermal deposits types that are formed around upper-crustal magmatic centres, in particular porphyry and epithermal deposits. Fluid-inclusion data have shown that ore fluids of gold-only deposits are compositionally distinct compared to fluids of other deposit types. This Special Publication includes an up-to-date summary of thermodynamic parameters of aqueous Au species at high temperatures and pressures; a dataset of fluid inclusion properties and compositions from orogenic deposits of different parts of the world; several comprehensive case studies of different types of gold deposits and their fluids from USA, Brazil, Egypt, Slovakia and Bulgaria; and numerical modelling aimed to define key parameters that affect fluid flow and gold deposition at a range of scales.

Multi-scale Controls on Vein-type Orogenic Gold Precipitation and Remobilization in the Malartic-Val-d'Or District of the Abitibi Subprovince (Québec, Canada)

Multi-scale Controls on Vein-type Orogenic Gold Precipitation and Remobilization in the Malartic-Val-d'Or District of the Abitibi Subprovince (Québec, Canada)
Author: Michael Herzog
Publisher:
Total Pages: 0
Release: 2023
Genre:
ISBN:

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Brittle-ductile reverse shear zones are the most important host for orogenic gold mineralization globally. These reverse shear zones are known to form by common physical processes associated with orogenic cycles, at postpeak metamorphic conditions. Crustal shortening periods induce regional-scale reverse shear zones that act as main conduits for ascending hydrothermal fluids, forming mesothermal quartz-carbonate veins. Fluid source reservoir(s), physico-chemical processe(s) and the timing that lead to gold precipitation within these veins have been the subject of long-standing research activities, particularly focusing at the deposit and ore body-scale. Nevertheless, the nature of these parameters remains elusive at the orogenic gold district-scale, commonly encompassing regional, km wide vein fields. In this thesis, I explore through a combination of micro-analytical chemical and isotopic techniques, spatial and temporal controls on several gold ore bodies contained in the world-class Malartic-Val-d'Or district of the southern Neoarchean Abitibi subprovince (Canada). Structurally controlled gold deposit clusters in this district are distributed along the E-W striking Larder Lake-Cadillac-fault zone and hosted in lower-order structures that cut volcanic rocks and intrusions of the greenstone belt. In these structures, gold is commonly associated with two vein sets that form pre- or syn- the regional-penetrative fabric (S2) developed during major N-S shortening and peak-metamorphism. This include: (1) highly-deformed quartz carbonate veins entrained in the S2-fabric and (2) laminated brittle-ductile quartz-tourmaline-carbonate reverse shear and sub-horizontal extension-veins that are sub-parallel to oblique to the S2-fabric, the latter form part of the Val-d'Or Vein Field (VVF), which contains the bulk of orogenic gold in the Malartic-Val-d'Or district. In this thesis, I studied nine gold ore bodies (Kiena-S50, Canadian Malartic East, Kiena-Deep, Goldex, Triangle, Plug #4, Beaufor, Pascalis Gold Trend and Akasaba West) to tackle three key topics. 1) The regional scale timing of auriferous hydrothermal events in the area was resolved by in-situ U-Pb geochronology. 2) Physico-chemical processes controlling gold saturation in auriferous hydrothermal fluids by in-situ sulfide multiple sulfur isotope and trace element compositions. 3) Hydrothermal remobilization of gold hosted in pyrite were studied at the nano-scale by transmission electron microscopy. [...] Acquired data show that district-scale orogenic gold mineralization is the result of multiple processes that operate at various scales in space and time throughout an orogenic cycle. Regionally discrete, short-lived hydrothermal fluid events during the retrograde path of an orogenic cycle are a main pre-condition to form district scale gold mineralization. During such an auriferous fluid event, bulk of the VVF is formed through a main fluid mechanism, wallrock sulfidation in meta-volcanic and intrusive rocks, which controls precipitation of Au-Ag-Te-Bi inclusions in pyrite. Subsequent hydrothermal events have the potential to shape and modify district-scaleorogenic gold mineralization, as recognized in remobilized Au-Ag-Te-Bi assemblages within pyrite fractures. Despite a well understood structural framework and common physical processes that lead to the formation of orogenic gold mineralization, in-situ micro-analytical techniques offer the unique opportunity to link structural observations with detailed geochemical data at the mineral grain scale. This approach will significantly improve the timing of gold mineralization and recognition of key physico-chemical processes acting at the disctrict-scale, as well as present different perspectives on physico-chemical processes associated with low and medium grade gold precipitation.

Hydrothermal Alteration and Exploration Vectors at the Island Gold Deposit, Michipicoten Greenstone Belt, Wawa, Ontario

Hydrothermal Alteration and Exploration Vectors at the Island Gold Deposit, Michipicoten Greenstone Belt, Wawa, Ontario
Author: Tyler Justin Ciufo
Publisher:
Total Pages:
Release: 2019
Genre:
ISBN:

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The Island Gold deposit is a high-grade orogenic gold deposit located in the Goudreau Lake Deformation Zone (GLDZ), within the Michipicoten greenstone belt. This belt is situated within the larger Wawa-Abitibi terrane which is an area of significant gold endowment in the Superior Province. This underground mine is operated by Alamos Gold Inc. (formerly operated by Richmont Mines Inc.). At the end of 2016, the Island Gold Mine had estimated indicated and measured resources of 478,800 tonnes at 5.94 g/t Au (91,400 oz) as well as inferred resources estimated at 3,041,800 tonnes at 10.18 g/t Au (995,700 oz). Simultaneously, proven and probable mineral reserves were estimated at 2,551,000 tonnes at 9.17 g/t Au (752,200 oz). This thesis integrates petrology, geochemistry, electron probe microanalysis, U-Pb zircon geochronology, and multiple sulphur isotopic study to characterize the nature and timing of alteration associated with the auriferous zones at this deposit. The protoliths of pre-gold mineralization lithologies identified at the Island Gold deposit, listed from oldest to youngest, include dacitic volcanic rocks, iron formation, gabbro, and tonalite-trondhjemite. VGD extensional and V1-V2 shear-related quartz (± carbonate) veins, that often contain visible gold, post-date and alter these lithologies. Protoliths of post-mineralization gold lithologies include quartz diorite, gabbro/low-K lamprophyre (spessartite), and silica-poor diorite-monzodiorite. Late V3 extensional quartz-carbonate veins result in local, inconsistent alteration and remobilization of certain elements. Later tourmaline ribbons invade pre-existing structures. Lastly, diabase-quartz diabase dykes sharply cut all lithologies and vein types. Greenschist facies metamorphism has variably affected all lithologies, resulting in metamorphic minerals that mainly include actinolite, carbonates (mainly calcite), chlorite (ripidolite), chloritoid, epidote, hematite, plagioclase, quartz, and white mica. Archean U-Pb LA-ICP-MS ages of 2735 ± 8 Ma and 2738 ± 9 Ma obtained from zircons in altered and least-altered dacitic samples are consistent with previous geochronological studies. Analysis of uniform cathodoluminescent zircons and rims in the altered sample yielded anomalous Proterozoic ages ranging from c. 975 to 1499 Ma, which are attributed to unidentified alteration event(s) that post-date auriferous quartz veining. Alteration related to auriferous quartz veining at this deposit results in the significant and consistent enrichment of Au, K2O, Rb, S, and Te as well as typically the depletion of Na2O in alteration envelopes hosted by dacite, gabbro, and the tonalite-trondhjemite. Alteration minerals associated with V1-V2 auriferous quartz veining include biotite, Ca-Mg-Fe carbonates, chlorite (ripidolite), plagioclase, quartz, sulphides (pyrite ± pyrrhotite ± chalcopyrite), and white mica (muscovite ± phengite). The alteration associated with V1-V2 veining remains relatively consistent with varying distance parallel to the GLDZ and elevation. Aside from auriferous alteration resulting in Na2O depletion, alteration-derived biotite, and insignificant proportions of alteration-derived potassium feldspar at the Island Gold deposit, the gold-related alteration at orogenic gold deposits east of the Kapuskasing Structural Zone is similar to the alteration identified at the Island Gold deposit. This overall consistency of the alteration between these deposits and the Island Gold deposit supports a common genetic process for these orogenic gold deposits. Multiple sulphur isotope study of sulphide minerals from auriferous quartz veins and associated ore zones dominantly suggests a mantle/igneous parentage. The auriferous fluid that produced these veins was likely derived from the mantle, exsolved from a cooling crustal intrusive body, or the dehydration of igneous rocks due to metamorphism.

Fluid-chemical Studies of Gold Systems

Fluid-chemical Studies of Gold Systems
Author: Mitchell James Kerr
Publisher:
Total Pages:
Release: 2019
Genre:
ISBN:

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Despite several decades of study, the source(s) of gold and mineralizing fluids, and the mechanisms responsible for gold precipitation remain controversial. In particular, there is a need to improve: (i) the identification of secondary gold upgrading processes that lead to very highgrade gold zones; (ii) the understanding of precipitation mechanisms in carbonaceous material (CM)-bearing metasedimentary systems (i.e., slate belts); and (iii) the exploration criteria for the cryptic, finely disseminated gold mineralization in CM-bearing metamudstone lithologies. The combined results of an extensive in situ microanalytical protocol (SEM, confocal Raman microspectroscopy, microthermometry, decrepitate mound analysis, LA-ICP-MS, cathodoluminescence, SIMS) has idenitified evidence of secondary gold enrichment at the Madrid deposit, Hope Bay Greenstone Belt, NU, Canada, resulting from the addition of new gold onto an earlier orogenic quartz-carbonate vein deposit by magmatic-hydrothermal fluids. This conclusion is based on robust textural, mineralogical, and fluid inclusion evidence. The geological setting and mineral-chemical features suggest an intrusion-related (i.e., porphyry), or intermediate-sulfidation epithermal mineralization style for the later event. This work provides another example of the importance of compositionally distinct cumulative hydrothermal events in the development of high-grade gold deposits in orogenic settings. The Dufferin deposit, NS, Canada, is a vein-type system hosted in CM-bearing metasedimentary rocks. A detailed fluid inclusion study strongly suggests that Au mineralization occurred through coupled fluid fO2 reduction (via interaction with CM) and pH increase, leading to efficient Au precipitation from an Au-undersaturated aqueous-carbonic fluid. This study presents one of the few examples of measured Au concentrations in ore fluids in an economic orogenic Au deposit and reports direct evidence for the genetic relationship between Au and CM in metasedimentary lithologies. The results of this study show that, despite mineralizing fluids being Auundersaturated, they may still produce economic deposits through highly efficient precipitation processes. The Moose Rive anticline, NS, Canada, hosts a number of, commonly inconspicuous, metasediment-hosted disseminated Au deposits. An initial study using an on-line, rock-crushing gas chromatographic technique identified bulk fluid differences (CO2 and hydrocarbon abundances) between gold-bearing and gold-barren parcels of metasedimentary rock. This work suggests that bulk fluid characteristics may be a viable exploration vector for disseminated gold systems.