Chapter 33: Neoarchean Eastern Goldfields of Western Australia

2020 ◽  
pp. 709-734
Author(s):  
Gerard I. Tripp ◽  
Richard M. Tosdal ◽  
Thomas Blenkinsop ◽  
Jamie R. Rogers ◽  
Scott Halley
Keyword(s):  
Western Australia ◽  
Spatial Association ◽  
Shear Zones ◽  
Gold Deposits ◽  
Gold Deposition ◽  
Structural Environment ◽  
World Class ◽  
Greenstone Belts ◽  
Crustal Shear Zones ◽  
Host Rocks

Abstract Neoarchean greenstone-hosted gold deposits in the Eastern Goldfields Superterrane of the Yilgarn craton of Western Australia are diverse in style, timing with respect to magmatic activity, structural environment, host rocks, and geochemical character. Geologic constraints for the range of gold deposits indicate deposit formation synchronous with volcanism, synchronous with syn- and postvolcanic intrusion, synchronous with postvolcanic deformation in faults and shear zones, or some combination of superposed events over time. The gold deposits are distributed as clusters along linear belt-parallel fault zones internal to greenstone belts but show no association with major terrane boundary faults. World-class gold districts are associated with the thickest, internal parts of the greenstone belts identified by stratigraphic preservation and low metamorphic grades. Ore-proximal faults in those regions are more commonly associated with syn- and postvolcanic structures related to greenstone construction and deformation rather than major terrane amalgamation. Using the Kalgoorlie district as a template, the gold deposits show a predictable regional association with thicker greenstone rocks overlain unconformably by coarse clastic rock sequences in the uppermost units of the greenstone stratigraphy. At a camp scale, major gold deposits show a spatial association with unconformable epiclastic and volcaniclastic rocks located above an unconformity internal to the Black Flag Group. Distinct episodes of gold deposition in coincident locations suggest fundamental crustal structural controls provided by the fault architecture. Late penetrative deformation and metamorphism overprinted the greenstone rocks and the older components of many gold deposits and were accompanied by major gold deposition in late quartz-carbonate veins localized in crustal shear zones or their higher order fault splays.

Chapter 13: Boddington: An Enigmatic Giant Archean Gold-Copper (Molybdenum-Silver) Deposit in the Southwest Yilgarn Craton, Western Australia

2020 ◽  
pp. 275-288
Author(s):  
Stephen J. Turner ◽  
Graeme Reynolds ◽  
Steffen G. Hagemann
Keyword(s):  
Western Australia ◽  
Melt Inclusion ◽  
Gold Mineralization ◽  
Extended Period ◽  
Granite Gneiss ◽  
Shear Zones ◽  
Gold Deposits ◽  
Yilgarn Craton ◽  
Local Evidence ◽  
Host Rocks

Abstract Boddington is a giant, enigmatic, and atypical Archean Au-Cu deposit hosted in a small, remnant greenstone belt within granite-gneiss and migmatite of the Southwest terrane of the Yilgarn craton, Western Australia. Primary Au and Cu (and Mo) mineralization consists of a network of thin fractures and veins, controlled by shear zones, and dominantly hosted by early dioritic intrusions and their immediate wall rocks, which comprise felsic to intermediate-composition volcanic and volcaniclastic rocks. The pre-~2714 Ma host rocks are typically steeply dipping and strongly deformed, with early ductile and overprinting brittle-ductile fabrics, and have been metamorphosed at mid- to upper greenschist facies. Features consistent with porphyry-style mineralization, classic orogenic shear zones, and intrusion-related Au-Cu-Bi mineralization are all recognized, giving rise to a variety of genetic interpretations. It is clear that Boddington does not fit any classic Archean orogenic gold deposit model, having a general lack of quartz veins and iron carbonate alteration, a Cu (Mo and Bi) association, zoned geochemical anomalism, and evidence of high-temperature, saline ore-forming fluids. Detailed petrographic, geochemical, and melt inclusion studies suggest a late-stage ~2612 Ma, monzogranite intrusion as one of the principal sources of the mineralizing fluids. However, there is also local evidence for older, perhaps protore, porphyry-style Cu (±Au) in the dioritic intrusions and patchy, locally high-grade, orogenic-style gold mineralization associated with enclosing shear zones and brittle-style deformation, which was focused on the relatively competent dioritic intrusions. The relative contributions of metals from these components to the system may not be resolvable. It appears that the Boddington deposit has been a locus for multiple episodes of intrusion, alteration, and mineralization over an extended period of time, as has been demonstrated in a number of other large Canadian and Australian gold deposits, including the Golden Mile near Kalgoorlie.

Gold Deposits of the ~15-Moz Ahafo South Camp, Sefwi Granite-Greenstone Belt, Ghana: Insights into the Anatomy of an Orogenic Gold Plumbing System

2021 ◽  
Author(s):  
Quentin Masurel ◽  
Paul Morley ◽  
Nicolas Thébaud ◽  
Helen McFarlane
Keyword(s):  
Rock Mass ◽  
Shear Zone ◽  
Gold Mineralization ◽  
Hanging Wall ◽  
Spatial Association ◽  
Sedimentary Rocks ◽  
Shear Zones ◽  
Gold Deposits ◽  
Orogenic Gold ◽  
Plutonic Rocks

Abstract The ~15-Moz Ahafo South gold camp is located in southwest Ghana, the world’s premier Paleoproterozoic gold subprovince. Major orogenic gold deposits in the camp include Subika, Apensu, Awonsu, and Amoma. These deposits occur along an ~15-km strike length of the Kenyase-Yamfo shear zone, a major tectonostratigraphic boundary juxtaposing metamorphosed volcano-plutonic rocks of the Sefwi belt against metamorphosed volcano-sedimentary rocks of the Sunyani-Comoé basin. In this study, we document the geologic setting, structural geometry, and rheological architecture of the Ahafo South gold deposits based on the integration of field mapping, diamond drill core logging, 3-D geologic modeling, and the geologic interpretation of aeromagnetic data. At the camp scale, the Awonsu, Apensu, and Amoma deposits lie along strike from one another and share similar hanging-wall plutonic rocks and footwall volcano-sedimentary rocks. In contrast, the Subika gold deposit is hosted entirely in hanging-wall plutonic rocks. Steeper-dipping segments (e.g., Apensu, Awonsu, Subika) and right-hand flexures (e.g., Amoma, Apensu) in the Kenyase-Yamfo shear zone and subsidiary structures appear to have represented sites of enhanced damage and fluid flux (i.e., restraining bends). All gold deposits occur within structural domains bounded by discontinuous, low-displacement, sinistral N-striking tear faults oblique to the orogen-parallel Kenyase-Yamfo shear zone. At the deposit scale, ore-related hydrothermal alteration is zoned, with distal chlorite-sericite grading into proximal silica-albite-Fe-carbonate mineral assemblages. Alteration halos are restricted to narrow selvages around quartz-carbonate vein arrays in multiple stacked ore shoots at Subika, whereas these halos extend 30 to 100 m away from the ore zones at Apensu and Awonsu. There is a clear spatial association between shallow-dipping mafic dikes, mafic chonoliths, shear zones, and economic gold mineralization. The abundance of mafic dikes and chonoliths within intermediate to felsic hanging-wall plutonic host rocks provided rheological heterogeneity that favored the formation of enhanced fracture permeability, promoting the tapping of ore fluid(s). Our interpretation is that these stacked shallow-dipping mafic dike arrays also acted as aquitards, impeding upward fluid flow within the wider intrusive rock mass until a failure threshold was episodically reached due to fluid overpressure, resulting in transient fracture-controlled upward propagation of the ore-fluid(s). Our results indicate that high-grade ore shoots at Ahafo South form part of vertically extensive fluid conduit systems that are primarily controlled by the rheological architecture of the rock mass.

Geology of the shear-hosted Brookbank gold prospect in the Beardmore–Geraldton belt, Wabigoon subprovince, Ontario

2007 ◽  
Vol 44 (7) ◽  
pp. 925-946 ◽  
Author(s):  
Jerry C DeWolfe ◽  
Bruno Lafrance ◽  
Greg M Stott
Keyword(s):  
Shear Zone ◽  
Hanging Wall ◽  
Shear Zones ◽  
Gold Deposits ◽  
Iron Formation ◽  
Alteration Zone ◽  
Gold Deposition ◽  
Low Grade ◽  
Metasedimentary Rocks ◽  
Polymictic Conglomerate

The Beardmore–Geraldton belt consists of steeply dipping, intercalated panels of metavolcanic and metasedimentary rocks along the southern margin of the granite–greenstone Wabigoon subprovince in the Archean Superior Province, Ontario. It is an important past-producing gold belt that includes classic epigenetic iron-formation-hosted deposits near Geraldton and turbidite-hosted deposits, north of Beardmore. The Brookbank gold prospect belongs to a third group of related gold deposits that formed along dextral shear zones localized at contacts between panels of metasedimentary and metavolcanic rocks. The Brookbank prospect occurs along a steeply dipping shear zone at the contact between footwall polymictic conglomerate and hanging-wall calc-alkaline arc basalt. Early during shearing the basalt acted as a structural and chemical trap that localized brittle deformation, veining, and gold deposition, ankerite–sericite–chlorite–epidote–pyrite alteration, and the replacement of metamorphic magnetite and ilmenite by gold-bearing pyrite. This produced a low grade (≤5 g/t Au) ankerite-rich alteration zone that extends up to 20 m into the hanging-wall basalt. Later during shearing, gold was deposited within higher grade (≤20 g/t Au) quartz–orthoclase–pyrite alteration zones superimposed on the wider ankerite-rich alteration zone. Auriferous quartz–carbonate veins oriented clockwise and counter-clockwise to the shear zone walls are folded and boudinaged, respectively, consistent with dextral slip along the shear zone. A key finding of the study is that different groups of gold deposits in the belt, including epigenetic iron formation gold deposits near Geraldton, formed during post-2690 Ma regional dextral transpression across the belt.

Chapter 34: The Paleoproterozoic (Rhyacian) Gold Deposits of West Africa

2020 ◽  
pp. 735-752
Author(s):  
Nicolas Thébaud ◽  
Andrew Allibone ◽  
Quentin Masurel ◽  
Aurélien Eglinger ◽  
James Davis ◽  
...  
Keyword(s):  
Sedimentary Basins ◽  
Shear Zones ◽  
Gold Deposits ◽  
Gold Deposition ◽  
Quartz Veins ◽  
Free Gold ◽  
High K ◽  
Refractory Gold ◽  
Close Relationship ◽  
Transcurrent Deformation

Abstract Paleoproterozoic terranes of the Man-Leo Shield in the southern part of the West African craton host one of the world’s largest gold provinces with an overall endowment >10,000 metric tons (t). Although gold deposition commenced by ca. 2170 Ma, most deposits formed later, either during the inversion and metamorphism of intraorogenic sedimentary basins between ca. 2110 and 2095 Ma, or during later transcurrent deformation and associated widespread high K plutonism following docking of Archean and Paleoproterozoic domains within the craton at ca. 2095 Ma. Deposits formed between ca. 2110 and 2095 Ma include those with free gold in quartz veins and refractory gold in arsenopyrite and/or pyrite, and are associated with halos of carbonate, sericite, chlorite, and albite alteration. Most are located in bends and intersections between shear zones, minor faults, folds, and entrained blocks of relatively reactive igneous rock. Conglomerate-hosted gold deposits of the Tarkwa district formed early in the 15-m.y.-long period. Gold deposits that formed subsequently between ca. 2095 and 2060 Ma have a wider variety of styles, geologic settings, and metal assemblages. District-scale albite, carbonate, and tourmaline alteration, hydrothermal breccias, and a close relationship to high K granitoids characterize some of these deposits, whereas others are more typical orogenic gold deposits that are similar to those formed earlier during the craton evolution.

scholarly journals Intrusion-Associated Gold Systems and Multistage Metallogenic Processes in the Neoarchean Abitibi Greenstone Belt

Minerals ◽  
2021 ◽  
Vol 11 (3) ◽  
pp. 261
Author(s):  
Lucie Mathieu
Keyword(s):  
Greenstone Belt ◽  
Gold Deposits ◽  
Hydrothermal Systems ◽  
Magmatic Fluid ◽  
Gold Deposition ◽  
Alkaline Magmatism ◽  
Abitibi Greenstone Belt ◽  
Metamorphic Fluids ◽  
Greenstone Belts ◽  
Gold Bearing

In gold-endowed greenstone belts, ore bodies generally correspond to orogenic gold systems (OGS) formed during the main deformation stage that led to craton stabilization (syntectonic period). Most OGS deposits postdate and locally overprint magmatic-hydrothermal systems, such as Au-Cu porphyry that mostly formed during the main magmatic stage (synvolcanic period) and polymetallic intrusion-related gold systems (IRGS) of the syntectonic period. Porphyries are associated with tonalite-dominated and sanukitoid plutons, whereas most IRGS are related to alkaline magmatism. As reviewed here, most intrusion-associated mineralization in the Abitibi greenstone belt is the result of complex and local multistage metallogenic processes. A new classification is proposed that includes (1) OGS and OGS-like deposits dominated by metamorphic and magmatic fluids, respectively; (2) porphyry and IRGS that may contain gold remobilized during subsequent deformation episodes; (3) porphyry and IRGS that are overprinted by OGS. Both OGS and OGS-like deposits are associated with crustal-scale faults and display similar gold-deposition mechanisms. The main difference is that magmatic fluid input may increase the oxidation state and CO2 content of the mineralizing fluid for OGS-like deposits, while OGS are characterized by the circulation of reduced metamorphic fluids. For porphyry and IRGS, mineralizing fluids and metals have a magmatic origin. Porphyries are defined as base metal and gold-bearing deposits associated with large-volume intrusions, while IRGS are gold deposits that may display a polymetallic signature and that can be associated with small-volume syntectonic intrusions. Some porphyry, such as the Côté Gold deposit, demonstrate that magmatic systems can generate economically significant gold mineralization. In addition, many deposits display evidence of multistage processes and correspond to gold-bearing or gold-barren magmatic-hydrothermal systems overprinted by OGS or by gold-barren metamorphic fluids. In most cases, the source of gold remains debated. Whether magmatic activity was essential or marginal for fertilizing the upper crust during the Neoarchean remains a major topic for future research, and petrogenetic investigations may be paramount for distinguishing gold-endowed from barren greenstone belts.

Chapter 9: Orogenic Gold Deposits of the Kibali District, Neoarchean Moto Belt, Northeastern Democratic Republic of Congo

2020 ◽  
pp. 185-201
Author(s):  
Andrew Allibone ◽  
Carlos Vargas ◽  
Etienne Mwandale ◽  
Justus Kwibisa ◽  
Richard Jongens ◽  
...  
Keyword(s):  
Democratic Republic ◽  
Gold Mineralization ◽  
Democratic Republic Of Congo ◽  
Shear Zones ◽  
Gold Deposits ◽  
Iron Formation ◽  
Banded Iron Formation ◽  
Republic Of Congo ◽  
Lithostratigraphic Units ◽  
Host Rocks

Abstract The Kibali district in the Democratic Republic of Congo hosts the large Karagba-Chaffeur-Durba (KCD) deposit and smaller satellite deposits that together contained 20 million ounces (Moz) of gold when mining recommenced in 2013. An additional 3 Moz of gold was probably mined from the district before 2013. Gold deposits in the Kibali district are located along the KZ trend, a series of folds, contractional shear zones, and altered lithostratigraphic units that coincide with the margin of an earlier 2630 to 2625 Ma intraorogenic basin within the Neoarchean Moto belt. Fluids first responsible for barren carbonate-quartz-sericite alteration, and later for siderite and/or ankerite (±quartz, magnetite, pyrite, and/or chlorite) alteration with associated auriferous pyrite ± rare arsenopyrite veinlets, infiltrated and replaced the siliciclastic, banded iron formation (BIF), and chert host rocks via fold axes, shear zones, and reactive BIF horizons. The complex shape and gentle northeast plunge of the lodes across the Kibali district reflect the shape and plunge of coincident folds that formed during early barren alteration. Many other folded BIF horizons across the wider Moto belt remain barren or only weakly mineralized, suggesting deep extensional structures that may have developed in the vicinity of the KZ trend during basin opening and prior to gold mineralization, were important fluid pathways during later contractional deformation and mineralization.

Chapter 3: Gold Deposits of the World-Class Timmins-Porcupine Camp, Abitibi Greenstone Belt, Canada

2020 ◽  
pp. 53-80
Author(s):  
Benoît Dubé ◽  
Patrick Mercier-Langevin ◽  
John Ayer ◽  
Jean-Luc Pilote ◽  
Thomas Monecke
Keyword(s):  
Fault Zone ◽  
Early Stage ◽  
Spatial Relationship ◽  
Gold Deposits ◽  
Main Phase ◽  
Gold Deposition ◽  
Key Factors ◽  
The World ◽  
World Class ◽  
Complex Structural

Abstract The Timmins-Porcupine camp, with >2,190 metric tons Au (70.5 Moz) produced between 1906 and 2019, is the world’s largest Archean orogenic gold camp. The gold deposits of the camp are distributed over ~50 km of strike length along the Destor-Porcupine fault zone. This includes the world-class Hollinger-McIntyre and Dome deposits, which represent archetypal examples of large orogenic quartz-carbonate gold systems. The Dome deposit, where the ore is centered on a folded unconformity between Tisdale volcanic rocks and Timiskaming sedimentary units, also illustrates the spatial relationship between large gold deposits and a regional unconformity. Ore-forming hydrothermal activity in the camp spanned a prolonged period of time, as illustrated by early-stage, low-grade ankerite veins formed between ca. 2690 and 2674 Ma. This was prior to or very early relative to the development of the regional unconformity and sedimentation of the Timiskaming assemblage, and subsequent main-stage gold deposition. The bulk of the gold in the district is younger than the Three Nations Formation of the upper part of the Timiskaming assemblage (i.e., ≤2669 ± 1 Ma) and was deposited syn- to late-main phase of shortening (D3) in the Timmins-Porcupine camp from about 2660 to 2640 ± 10 Ma. The early carbonatization represents a significant early-stage hydrothermal event in the formation of large structurally controlled gold deposits such as Dome and illustrates the protracted nature of the large-scale CO2-rich metasomatism occurring before and during gold deposition. Ores in the Timmins-Porcupine camp mainly consist of networks of steeply to moderately dipping fault-fill quartz-carbonate ± tourmaline ± pyrite veins and associated extensional, variably deformed, shallowly to moderately dipping arrays of sigmoidal veins hosted in highly carbonatized and sericitized rocks and formed during main regional shortening (D3). In contrast, at the Timmins West mine, the Thunder Creek and 144 GAP deposits are early- to syn-Timiskaming intrusion-associated deposits that slightly predate to overlap the main phase of D3 horizontal shortening in which the associated intrusions mainly played a passive role as an older mechanical and chemical trap rock. The formation of the gold deposits of the Timmins-Porcupine camp is due to several key factors. The Destor-Porcupine fault zone represents a deeply rooted first-order structure and tapped auriferous metamorphic fluids and melts from the upper mantle-lower crust. The fault zone has channeled large volumes of auriferous H2O-CO2-rich fluids to the upper crust late in the evolution of the belt. Several of the gold deposits of the camp are spatially associated with the regional Timiskaming unconformity. The current level of erosion is deep enough to expose the unconformity and to maximize the chance of discovering the quartz-carbonate style of orogenic deposits or the associated hydrothermal footprint, but also allowed for preservation of at least part of the gold deposits that are mainly hosted in the highly reactive Fe-rich basalt of the Tisdale assemblage. Additional key factors include the presence of komatiitic and/or basaltic komatiite flows, competent pre- and syn-Timiskaming subalkaline and alkaline intrusions that predate the main phase of shortening, and the occurrence of a flexure in the trace of the Destor-Porcupine fault zone that may have further facilitated and focused the ore-forming fluid upflow in the most endowed part of the camp. The complex structural and rheological discontinuities, competency contrasts, and early-stage folds with associated fracture and fault netorks in the camp provided highly favorable ground-preparation conditions.

A comparison of alteration assemblages associated with Archean gold deposits in Western Australia and Paleozoic gold deposits in the southeast United States

1990 ◽  
Vol 27 (12) ◽  
pp. 1560-1576 ◽  
Author(s):  
G. Todd Ririe
Keyword(s):  
United States ◽  
Western Australia ◽  
Regional Metamorphism ◽  
Spatial Association ◽  
Gold Deposits ◽  
Striking Similarity ◽  
High Alumina ◽  
Acid Sulfate ◽  
Southeast United States ◽  
Hydrothermal Origin

A comparison of high-alumina mineral assemblages spatially associated with the Haile and Brewer gold deposits in the Paleozoic Carolina slate belt (CSB) of the southeast United States and the Sons of Gwalia and Mount Celia gold deposits in the Archean Norseman–Wiluna greenstone belt (NWGB) in Western Australia suggests a similar hydrothermal origin and subsequent metamorphic and deformational history. A common hydrothermal origin is supported by the striking similarity in whole-rock chemistry, even though there were probably significant variations in original unaltered protoliths. An analysis of rocks from each of the deposits that contain high-alumina minerals demonstrates that the protoliths were leached of alkalis with respect to aluminum and have a ratio of Al2O3/Na2O + CaO + K2O greater than three. Although the rocks contain abundant high-alumina minerals, the aluminum content in these rocks is not unusually high, and it does not appear that there has been any significant transport of aluminum either into or out of the rocks that were altered.The most common high-alumina minerals found in rocks affected by the feldspar destructive alteration event include andalusite, kyanite, pyrophyllite, kaolinite, and sericite. Other minerals present in variable amounts include diaspore, chloritoid, alunite, natroalunite, paragonite, and topaz. It is possible that some of these minerals formed during the early hydrothermal alteration event and were recrystallized during metamorphism. Regardless of when the high-alumina minerals formed, textural relations suggest that the rocks had been hydrothermally altered by the time the metamorphic minerals formed. A comparison of alteration from the deposits studied in the CSB and the NWGB suggests there are many similarities to acid-sulfate alteration associated with geothermal areas, such as Yellowstone Park, Wyoming, and with acid-sulfate gold deposits, such as Goldfield, Nevada. Thus, it is possible that the protolith of the metamorphosed rocks in the CSB and NWGB contained an alteration assemblage that included alunite, pyrophyllite, and kaolinite.A generalized paragenetic sequence determined from petrographic and field observations, beginning with regional metamorphism, follows: (i) formation of andalusite, kyanite, chloritoid, and topaz during prograde metamorphism, depending on whole rock chemistry, (ii) formation of pyrophyllite and quartz-rich pods during silicification of aluminosilicate-bearing rocks, (iii) bedding parallel schistosity and fracturing produced by a deformational event, (iv) fractures filled by quartz, sericite, pyrophyllite, or calcite, (v) folding of early layering in the rocks to form a crenulation cleavage accompanied by introduction of quartz veins at high angles to foliation; and (vi) retrograde metamorphism of andalusite–kyanite-pyrophyllite to produce an assemblage of kaolinite ± diaspore.The spatial association of the acid-sulfate alteration with gold mineralization, together with comparison of analogous alteration associated with younger unmetamorphosed acid-sulfate gold deposits, suggests that at least some of the gold was introduced during the early premetamorphic alteration event. The present location of gold in each deposit is a result of local changes brought about by later metamorphic and deformational events.

Sign in / Sign up

Export Citation Format

Share Document