Structural geology of the eastern Nadaleen trend, Yukon Territory, Canada: Implications for recently discovered sedimentary rock-hosted gold (original) (raw)
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Bulletin of the Geological Society of Finland, 2007
The Satulinmäki and Riukka prospects located in the Häme Schist Belt in southern Finland are dominated by mafic and intermediate to felsic metavolcanic rocks of the Forssa Group formed in a continental arc setting. This magmatic belt formed some 1890-1880 Ma ago and has been deformed during the Svecofennian orogeny. The dominant penetrative foliation is represented by S n+1 , which is axial planar to cm-to dm-scale iscolinal F n+1 folds and thus parallel to the layering S n. Associated ductile shearing might reflect early thrusting. D n+2 postdating peak metamorphism is characterized by small-scale to regional-scale refolding of F n+1 folds around ~SW-NE fold axes. This phase is transitional to development of SW-NE to WSW-ENE and NW-SE striking shear zones and faults formed due to dextral transpression during D n+3 at retrograde conditions crossing the brittle-ductile transition. Many mineralized quartz veins have formed during this event. Later faults and quartz veins and reactivation of pre-existing structures during D n+4 indicate rotation of the stress field to ~NE-SW oriented compression. A clear ~SW-NE trend of sulphide mineralization and elevated gold contents and the spatial association to D n+3 quartz veins, shear zones and faults suggest a strong structural control, typical of orogenic gold deposits, and that mineralization took place during D n+3. The controlling structures, (i) WSW-ENE to SW-NE shear zones and faults and (ii) NW-SE oriented fault, are second and third order structures to major regional-scale shear zones. The fault zones and their intersection points impart a directional permeability so that the mineralising fluids were channelled along dilatant zones. Approximately (W)NW-(E)SE trending faults being (re)activated as extensional faults or dilatant shear planes during D n+3 transpression with WNW-ESE to NW-SE oriented compression direction could have acted as conduits for fluids during upward flow from deeper crustal level. Future studies in this area are necessary to test if other prospects show similar timing and structural control, and future exploration for finding new targets should consider sites of (W)SW-(E)NE and NW-SE-striking second and third-order structures branching from major shear and fault zones.
Mineralium Deposita, 2006
Northern Nevada hosts the only province that contains multiple world-class Carlin-type gold deposits. The first-order control on the uniqueness of this province is its anomalous far back-arc tectonic setting over the rifted North American paleocontinental margin that separates Precambrian from Phanerozoic subcontinental lithospheric mantle. Globally, most other significant gold provinces form in volcanic arcs and accreted terranes proximal to convergent margins. In northern Nevada, periodic reactivation of basement faults along this margin focused and amplified subsequent geological events. Early basement faults localized Devonian synsedimentary extension and normal faulting. These controlled the geometry of the Devonian sedimentary basin architecture and focused the discharge of basinal brines that deposited syngenetic gold along the basin margins. Inversion of these basins and faults during subsequent contraction produced the complex elongate structural culminations that characterize the anomalous mineral deposit "trends." Subsequently, these features localized repeated episodes of shallow magmatic and hydrothermal activity that also deposited some gold. During a pulse of Eocene extension, these faults focused advection of Carlin-type fluids, which had the opportunity to leach gold from gold-enriched sequences and deposit it in reactive miogeoclinal host rocks below the hydrologic seal at the Roberts Mountain thrust contact. Hence, the vast endowment of the Carlin province resulted from the conjunction of spatially superposed events localized by long-lived basement structures in a highly anomalous tectonic setting, rather than by the sole operation of special magmatic or fluid-related processes. An important indicator of the longevity of this basement control is the superposition of different gold deposit types (e.g., Sedex, porphyry, Carlin-type, epithermal, and hot spring deposits) that formed repeatedly between the Devonian and Miocene time along the trends. Interestingly, the large Cretaceous Alaska-Yukon intrusion-related gold deposits (e.g., Fort Knox) are associated with the northern extension of the same lithospheric margin in the Selwyn basin, which experienced an analogous series of geologic events.
Economic Geology, 1993
The Francoeur 3 gold deposit, located approximately 15 km southwest of Rouyn-Noranda, Quebec, is one of several geologically similar Archeart mesothermal gold deposits occurring along the Francoeur-Wasa shear zone, a second-order reverse structure possibly related to the Cadillac-Larder Lake break. The principal gold mineralization, which is completely hosted by the Francoeur-Wasa shear zone, is associated with disseminated pyrite and occurs in tabular zones of intense hydrothermal replacement. The mineralized zones consist of a buff-or gray-colored, very fine grained, carbonate-albite-pyrite assemblage which occurs in very different lithologies, including sheared andesite, red albitite dikes, and gabbro. Three types of hydrothermal alteration occur in the deposit. In chronological order, they are (1) a widespread pervasive preore carbonate-hematite-muscovite alteration associated with the intrusion of late kinematic albitite dikes, (2) an albite-pyrite alteration closely associated with the gold mineralization, and (3) a postore sulfate-bearing alteration. The distribution of alteration assemblages and replacement textures suggests evolution of fluid-rock interaction. The initial hydrothermal alteration involved the hydrolysis of metamorphic silicates and oxides to form carbonate, muscovite, and hematite with net additions in CO2, K, Rb, and TiO2. Subsequently, muscovite and hematite were replaced in the ore zones by albite and pyrite, respectively, resulting in K•O and Rb depletion and large enrichments in Na•O, S, W, and Au. These two alteration types are crosscut by late gypsum and anhydrite veinlets which are associated with late brittle extensional faults and possibly mark the last product of hydrothermal activity in the Francoeur-Wasa shear zone in this area. It is proposed that gold deposition occurred as a result of changes in physicochemical conditions of ore-forming fluids in response to progressive fluid-rock interaction. The combined effects of an increase info• and a(Na+ or K +) and reductions in both fH•s and pH controlled Au solubility, and very efficiently partitioned its deposition in the albite-pyrite alteration facies. This interpretation is consistent with preliminary sulfur isotope data which indicate that pyrite coexisting with gold is highly depleted in b34S (-10.5 _+ 0.8%0) compared with pyrite elsewhere in the shear zone, as a result of reduced S oxidation. The absence of an auriferous quartz-carbonate vein system and the occurrence of disseminated replacement mineralization at Francocur suggest that this hydrothermal system remained relatively permeable through much of the hydrothermal process such that fluid flow operated under near-constant hydraulic gradient. Such a fluid regime is very different from that generally proposed for the formation of mesothermal gold quartz veins in which episodic fluid-pressure elevations above lithostatic pressure are thought to control fluid migration, wall-rock alteration, and gold deposition. * Minist&re de l'P_;nergie et des Ressources du Qu6bec contribution 91-5130-22. is an important tool in understanding the genesis of hydrothermal deposits. Hydrothermal alteration is a ubiquitous and important characteristic of Archean mesothermal Au deposits and has been the subject of numerous detailed studies. Although the type of alteration is lithologically dependent, in general these studies document increases of CO•, H•O, S, and alkalis in host lithologies from a fluid (e.g., Kerrich, 1983; Colvine et al., 1988). In many deposits, alteration zones are characterized by K•O enrichment and Na•O depletion. In some deposits, however (e.g.,
2002
The Longline gold occurrence is located within the Tintina Gold Belt, in the Moosehorn Range area of west-central Yukon Territory. Gold occurs in sheeted, high-grade (-30 g/t), shallowly dipping mesothermal quartz veins hosted within mid-Cretaceous intrusions of the Dawson Range batholith. Felsic to intermediate intrusions in the Moosehorn Range area are probably closely related; they were emplaced at -96-100 Ma, and have similar geochemical and lead isotopic compositions. Geochemistry of the intrusive phases suggests the magmas were mantlederived, subduction-related, and extensively contaminated by continental crust. The goldbearing quartz veins post-date all of these intrusions and are cut by younger mafic dykes. Metallic minerals inside the veins include galena, sphalerite, arsenopyrite, pyrite, boulangerite, tetrahedrite, native gold, and scheelite. The alteration assemblage includes muscovite, sericite, iron carbonate, pyrite, arsenopyrite, minor clay, quartz, and tourmaline. V...
Ore Geology Reviews, 2017
The structural setting of the Pontiac Subprovince in the vicinity of the world-class Canadian Malartic gold deposit has been revisited by combining and reinterpreting airborne geophysical surveys together with a century of structural observations. Felsic-intermediate intrusive bodies are a key component of this deposit. Defining the regional and local favorable structural setting for intrusive rock emplacement within the clastic meta-sedimentary rocks of the Pontiac Group, south of the Cadillac Larder Lake Deformation Zone, may further assist gold exploration in similar tectonic environment. Three structural domains are interpreted in the area based on the geometry of the bedding, folds and structural fabrics related to the three major phases of deformation. During these events four phases of magmatism and one metamorphic episode occurred. The North domain, which hosts the Canadian Malartic deposit, is characterized by highly variable bedding orientations produced by the interference patterns of isoclinal F 1 folds overprinted by open to tight, steeply dipping, F 2 folds. The bedding in this domain is cut by a penetrative S 2 biotite foliation, which possibly built on rheological changes enhanced by metasomatism in the footprint of the Canadian Malartic deposit. By contrast, the Central and South domains display homogeneous bedding orientations cross-cut by a discrete S 2 biotite foliation and syn-to late-D 2 metamorphic porphyroblasts. In all domains the L 2 stretching lineation consistently plunges at about 60 degrees toward the east. D 3 is a minor deformation event in the Pontiac Subprovince, which possibly correlates with late dextral transcurrent movement along the Cadillac Larder Lake Deformation Zone. Intrusive bodies were emplaced throughout the first and second deformation events. Phase 1 monzonite, quartz-monzodiorite and granodiorite plutons (ca. 2683-2680 Ma) intruded into consolidated Pontiac sedimentary rocks during D 1. Phase 2 quartz-monzodiorite bodies (ca. 2679-2676 Ma) predominantly formed in the North Domain along F 1 fold axial surfaces prior to or at the onset of D 2. Phase 3 basic dykes (ca. 2675-2673 Ma) cross-cut earlier felsic-intermediate intrusions across all domains and subsequently developed an S 2 metamorphic hornblende foliation. Phase 4 magmatism (ca. 2672-2662 Ma) is related to the Décelles Batholith S-type granite and pegmatite, which is interpreted to be contemporaneous with the peak of regional metamorphism. The Décelles Batholith may project at depth underneath the Canadian Malartic deposit and could be associated with magmatic-hydrothermal mineralizing fluids in the Canadian Malartic deposit. Gold mineralization at Canadian Malartic is spatially located on the contact of Phase 2 quartzmonzodiorite bodies. The proximity to the Cadillac Larder Lake Deformation Zone combined with the rheological contrast between steeply dipping clastic meta-sedimentary rocks and quartz-monzodiorite intrusions favored the protracted failure of the contacts between these two rock masses, thus forming favorable conduits for hydrothermal fluids (e.g., the Sladen Fault Zone). Such specific rheological behavior is demonstrated by domains of structural complexity, emphasized by the variance of the bedding dip. These structurally complex zones systematically host gold mineralization in the Canadian Malartic district of the Pontiac Subprovince proximal to the Cadillac Larder Lake Deformation Zone.
Ore Geology Reviews, 2017
The Wasamac deposit is an example of Archean greenstone-hosted gold deposit located in the Abitibi Belt, 15 km southwest of Rouyn-Noranda. The deposit is hosted by a second-order ductile shear zone of the Cadillac–Larder Lake Fault Zone (CLLFZ), known as the Francoeur-Wasa Shear Zone (FWSZ). It regionally sits at the boundary between the orogenic gold district of Noranda and the Kirkland Lake gold district dominated by intrusion-related gold systems. This specific location in-between two different gold mineralization environments sets the Wasamac deposit apart as a prime candidate for investigating hydrothermal processes along the CLLFZ. Within the Wasamac deposit, gold distribution is constrained to the altered mylonitized portion of the FWSZ; lode systems are absent. Hydrothermal alteration and associated disseminated mineralization occurs as a replacement of the Blake River Group metavolcanic units. The hydrothermal signature displays two distinct alkaline alteration assemblages: potassic and albitic, each associated with specific gold characteristics. (1) Potassic alteration is characterized by the crystallization of microcline, carbonates and quartz. Within this assemblage gold is associated with porous pyrite enriched in Te-Ag-Au-Mo-Pb-Bi-W, deposited under oxidizing conditions. Such characteristics are widely described in the Kirkland Lake area, and are found in examples of syenite-related mineralization, such as the Beattie and Malartic gold deposits. (2) The albitic alteration assemblage, composed of albite, sericite and carbonates, reflects more reduced hydrothermal conditions with mineralization characterized by free native gold. This hydrothermal event is coeval with the brecciation of early gold-rich pyrite reflecting a structural overprint that controlled late-stage gold characteristics. These alteration and structural features are common in orogenic gold deposits both worldwide and regionally, particularly at the neighbouring Kerr-Addison and Francoeur deposits, and in lode-gold systems such as in the Sigma-Lamaque deposit. The gold mineralization at Wasamac has similar characteristics to both intrusion-related gold systems and structurally controlled orogenic gold deposits. Hydrothermal and structural crosscutting relationships at Wasamac indicate that a structurally controlled hydrothermal event overprinted earlier potassic magmatic-hydrothermal alteration. This observation supports a multistage process of gold concentration during which new gold characteristics, metal anomalies, fluid conditions and alteration assemblages replaced earlier stages of gold enrichment, in places completely obliterating previous signatures. We propose that the Wasamac deposit was originally related to an alkaline intrusion buried at depth beneath the Francoeur-Wasa Shear Zone.
Spatial and Temporal Distribution of Gold Deposits in theUrals
Economic Geology, 2001
During the past 10 years, the Paleozoic Urals mountain belt has been the subject of internationally based, deep seismic, and ancillary geologic studies aimed at improving our knowledge of the lithospheric evolution of the orogenic belt, in general, and its prominent crustal root, in particular. In view of its wide-ranging mineral endowment, especially for gold, this work also provides important keys to help clarify relationships between collisional tectonic processes and gold mineralization. Following an outline of current ideas on the tectonic evolution of the Urals, we present an overview of the distribution of the majority of known gold deposits. This information, together with the available isotopic and geologic age data of associated alteration assemblages, is compared to available geochronological data for various magmatic and deformation episodes. Spatial and temporal relationships suggest that hydrothermal mineralizing processes and the geodynamic evolution of the orogen are linked. In an attempt to analyze the deep structural framework of the orogen, down to the mantle, we have also examined a number of subcontinental-and regional-scale continuous geophysical datasets.
Geological and structural controls on gold mineralization in the Tanami District, Northern Territory
2007
Gold mineralization in the Tanami district is hosted within moderately northwest dipping turbiditic sedimentary and basaltic volcanic rocks of the Paleoproterozoic Mt. Charles Formation. The gold occurs within a complex sinistral wrench-fault array and associated veins and alteration haloes. The main mineralized faults have a northerly trend and dip steeply east. Subsidiary structures trend at 030°and 070°and dip towards the southeast. Paleostress calculations based on fault striation populations and geometry (strike and dip) of faults indicate that at the time of the mineralizing event, σ 1 was sub-horizontal and SE-NW directed with σ 2 subvertical. Structural studies indicate that the mineralization occurred after the regional folding event and synchronous with the emplacement of felsic dykes into the mine sequence. Gold veins in the Tanami district are interpreted to be part of an outer thermal aureole gold system that formed during the emplacement of granitoids in the nearby ∼1,815 to ∼1,799 Ma Frankenia and/or Coomarie domes. Economic gold mineralization occurred late in the paragenetic history of the district. Gold is hosted by quartz-carbonate veins within shear zones, and also in the surrounding sericite-quartz-pyrite ± carbonatealtered wallrocks. Gold-mineralized veins precipitated at depths of 3 to 6 km from high temperature (∼300°C), low salinity (∼5 wt% NaCl equivalent) fluids with low CO 2 contents. Barren quartz, dolomite and calcite veins that occur in pre-and post-mineralization thrust faults formed from high salinity (∼20 wt% NaCl equivalent), low temperature (∼120-150°C) basinal brines. Pyrite in the gold mineralized veins and alteration halos has lower δ 34 S values (6.8 to 12.5‰) than local diagenetic pyrite (17.8 to 19.2‰) or pyrite in pre-mineralization thrust faults (31.7 to 37.1‰). The mineralizing fluids are inferred to have contained a well-homogenized mixture of magmatic and sedimentary-derived sulfur.
Controls of Fault and Fold Geometry on the Distribution of Gold Mineralization on the Carlin Trend
Geological Society of Nevada, 2015
Mining exposures on the Carlin trend reveal a structural history which involves multiple deformation events that have contributed to the localization of gold mineralization. Subsequent to the Devonian-Mississippian emplacement of allochthonous Upat least two contractional phases are apparent. These comprise (i) Phase 2 east-northeast to southeast vergent, inclined to recumbent folds that include the Post-Tuscarora anticline, and which are associated with shallow-dipping, west-to west-northwestdipping reverse faults, and (ii) Phase 3 upright, open, north-to northwest-trending folds and northwest-trending, moderate to steeply-dipping, reverse faults. Phase 2 probably includes multiple superposed, coaxial phases of thinskinned east-to southeast-directed Paleozoic and Jurassic deformation. Phase 3 structures, which affect Jurassic intrusions, may have occurred during regional northeast-directed late Mesozoic or early Tertiary Laramide-style, and potential age, thickskinned deformation. Contractional structures are overprinted by oblique-normal faults which include the southeast-dipping Chukar-Alunite fault zone at the Gold Quarry deposit, and the north-northwest-trending Post-Gen fault, which exploits the east limb of the Post-Tuscarora anticline. These are either host to, or are spatially associated with, gold mineralization. These faults lack facies and sedimentary thickness changes across them, are oblique to the Bootstrap platform shelf, and equally offset the RMT, stratigraphy and contractional structures, collectively suggesting that most of their displacement was accommodated during Tertiary oblique extension. Other faults, such as the Castle Reef, which separates contractional deformation domains, may have a pre-Mesozoic history. Gold mineralization occurs in structural culminations and intersections produced by convergence of folds, faults, and favorable, reactive lithologies. Mineralized dissolution and hydrothermal breccias frequently overprint fault rock along mineralized normal faults and locally reactivated reverse faults. Breccias incorporate fragments of, but are also cut by, Eocene dikes, consistent with their emplacement during Eocene extensional faulting. Shallow plunging mineralized zones occur at the intersection of the Post-Gen fault with the hinge and the truncated east limb of the Post-Tuscarora anticline, often where lenses of reactive lithologies occur between fault strands, or where Phase 3 reverse faults intersect the hinge areas beneath the RMT. In the Genesis, Carlin and Pete-Bajo deposits, mineralization exploits Phase 2 reverse faults, particularly where internal thrust duplex panels are developed or at lithologic intersections, and broadens in fold hinge zones or junctions with normal
SYNGENETIC GOLD: LODE VEIN GEOLOGY AND EXPLORATION IMPLICATIONS
syngeneticgold.com
Fieldwork in the Meguma of Nova Scotia and Archean equivalents shows that bedding-parallel quartz lode gold veins (LVs) commonly occur at the top of graded clastic sequences. We define a "gold cycle" (GC) by analogy with Bouma turbidites and show that most "shear zones" and "lamprophyres" correspond to the upper pelagic-pelitic (E) division. We describe GCs from Archean Chester Twp, ON; Elmhirst, ON; Bourlamaque, QC; "intrusions" and interpret them as crystal tuffs and tuff-breccias. Extensive early Archean pyroclastic volcanism is implied: the genesis of trondjhemite-tonalite-granodiorite (TTG) rocks must be re-examined. The geopetal nature and asymmetric alteration of LVs indicates genesis from seafloor hydrothermal vents. Textures suggest that often quartz was deposited as a gel. Vent geometry (fracture, point source or diffuse seep), bottom topography and vent spacing is reflected in grade-thickness plots. GC facing directions survive metamorphism. The Bourlamaque batholith was a flat-lying sheet of quartz-feldspar-hornblende crystal tuff, and isoclinal folding has repeated one or more gold-bearing horizons. Syngenesis clarifies and simplifies LV formation with major implications for models. Exploration becomes more cost-effective. Basin analysis, whole rock geochemistry, ore lens geometry and vent spacing can be used.