Felsic Porphyry Research Articles

Post‐collisional Adakitic Porphyries in Tibet: Geochemical and Sr‐Nd‐Pb Isotopic Constraints on Partial Melting of Oceanic Lithosphere and Crust‐Mantle Interaction

Abstract The distribution of Neogene felsic porphyries intruding in earlier granitic batholiths was mainly controlled by north‐south‐tending rifting zones and normal faults. The main rock types of the felsic porphyries include granodiorite‐porphyry, monzonitic granite‐porphyry and quartz monzonitic porphyry. The porphyries are characterized by high SiO2 (≥64.26%) and Al2O3 (>15% at 70% SiO2), low Y and HREE (Yb) contents, strong enrichment of LILE and LERR, especially K and Sr. Geochemical features of the porphyries show distinct adakitic magma affinity. Nd, Sr and Pb isotopic compositions of the porphyries form a linear alignment from MORB to EM2, suggesting a mixing of the MORB reservoir with the metasomatized mantle reservoir. Considering also the geochemical characteristics of the porphyries and the sequence of observable structural‐thermal‐magmatic events at Gangdise, it is thought that the Neogene porphyries were formed by partial melting of dead subducted oceanic crust in a post‐collision setting. K‐enrichment in the porphyries is attributed to the interaction of slab‐derived melts, i.e., adakites, with the metasomatized mantle during the ascent There might be a delamination of residual eclogites or amphibole eclogites before the eruption of potassic lava on the Tibetan plateau since 13 Ma.

Extending the ancient margin outboard in the Canadian Cordillera: record of Proterozoic crust and Paleocene regional metamorphism in the Nicola horst, southern British Columbia

The Nicola horst exposes plutonic and amphibolite-grade metamorphic rocks and is surrounded by low-grade arc rocks of the Late Triassic Nicola Group. We present new geological mapping and U–Pb, Nd, and metamorphic data for the Nicola horst near Bob Lake, ~40 km south of Kamloops, British Columbia. The Bob Lake assemblage includes felsic to intermediate metavolcaniclastic rocks, metaconglomerate, schist, phyllite, and other rock types. From U–Pb zircon analysis, a felsic metaporphyry clast in metaconglomerate is 1.04 Ga old. The oldest detrital zircons in metaconglomerate and schist are also near 1 Ga. The Bob Lake assemblage was intruded by 230 Ma tonalite, 219 Ma diorite, and 64 Ma leucogranite and includes 161 Ma felsic porphyry and 157 Ma rhyodacite. Amphibolite-grade metamorphism and ductile flattening and stretching affected all rocks except crosscutting Paleocene granite and granodiorite. The high-grade rocks may be exposed as a result of latest Cretaceous – Eocene extensional ductile flow beneath a thin brittle upper crust. A thickness of ~20 km of juvenile crust beneath the proposed Quesnel terrane is inconsistent with the evidence of Proterozoic source rocks at surface. We infer that most of the crustal lithosphere in this part of the Intermontane Belt is continental, which does not preclude possibly thick arc rocks in other parts. The local thinness of the Nicola Group, however, is inconsistent with emplacement as a regional allochthon and thus with their inclusion in a Quesnel "terrane." The Nicola arc succession appears to have been built on the ancient continental margin.

Dispersion of gold and associated elements in the lateritic regolith, Mystery Zone, Mt Percy, Kalgoorlie, Western Australia

Primary Au mineralization in the Mystery Zone, Mt Percy, occurs in fuchsite–carbonate alteration zones at the contact with felsic porphyries intruding the Hannan’s Lake Serpentinite. The mineralized sequence is concealed beneath an almost complete lateritic regolith over 60 m thick. The regolith consists of 50 m-thick saprolite, which is clay-rich in the top 10 m, plasmic and mottled clays, lateritic residuum (duricrusts and gravels) and pisolitic soils; the surficial horizons contain pedogenic carbonates. Elements occurring with primary Au mineralization are S, Ag, W, As, Sb, Te, K, Rb and, possibly, Ba and Pb, but except for Ag and Te, none has a mineralogical association with Au. The Au distribution in the regolith is typical for the region, with minor enrichment and lateral dispersion in lateritic residuum, a strong association with pedogenic carbonates, leaching and depletion in the underlying clay-rich horizons and some secondary concentration in the saprolite. Primary and saprolitic Au mineralization is indicated by a broad Au anomaly (100 to >1000 ppb) in the soils and lateritic residuum, and by high concentrations of W (5 to >40 ppm), Sb (7 to >16 ppm) and As (10–200 ppm). High K contents, corresponding to resistant muscovite, give surface expression to the alteration zone. Although Au contents are <100 ppb in the underlying clay-rich horizons, Sb, W and, to a lesser extent, As remain anomalous; similarly, Ba and K contents remain high, indicating the porphyries and alteration zones, respectively. The porphyries and ultramafic rocks can be discriminated geochemically throughout much of the regolith by relative abundances and ratios of Ti, Zr, Ba and K. However, the lateritic duricrusts overlying the talc–chlorite rocks display abnormal geochemical signatures. They have low Cr contents (<1000 ppm), because primary Cr is present in weatherable chlorite rather than resistant chromite, but are enriched in ‘immobile’ elements (e.g. Zr) derived from the porphyries. The element distribution patterns are products of deep weathering under warm humid conditions in the early Tertiary or earlier, followed by semi-arid conditions that continue to the present. Chemical and physical mobilization of gold was restricted to the surficial lateritic residuum in the humid phases, but depletion and supergene enrichment in the saprolite, and concentration with pedogenic carbonates, are the result of chemical mobility under semi-arid conditions.

New Evidence for Multiple Periods of Gold Emplacement in the Porcupine Mining District, Timmins Area, Ontario, Canada

The Porcupine mining district is the largest lode gold-producing district in North America, with production in excess of 62,000,000 troy oz. Nearly all of the Au has come from quartz-carbonate lode systems hosted by metamorphic rocks of greenschist facies. Field relationships and radioisotope ages of intrusions indicate that gold emplacement occurred over a span of at least 7 m.y. Depositional events that predated formation of the Timiskaming unconformity surface are manifested by auriferous clasts of detrital pyrite, banded quartz sulfide, and quartz sulfide in basal Timiskaming conglomerate. Pyritic clasts contain on average 2.54 ppm Au and are conclusive evidence for the existence of pre-Timiskaming-aged auriferous pyritic deposits. These clasts may have been derived from pyritic bodies in carbonaceous argillites of the Tisdale Group, from sulfide facies iron-formation of the Deloro Group, or from pyritic deposits of unknown genesis. Auriferous banded sulfidic quartz clasts were probably derived from iron-formation of the Deloro Group and auriferous sulfidic quartz clasts possibly from epigenetic quartz sulfide veins. Intrusion-related and post-Timiskaming gold deposition is evidenced by Cu-Au-(Mo) mineralized porphyry and younger quartz-carbonate gold veins. The magmatism that emplaced feldspar and quartz-feldspar porphyries was accompanied by Cu-Au-(Mo) mineralization at the Dome mine, where a conformable quartz-feldspar porphyry intrusion and its host greenstones are pervasively and atypically copper and gold mineralized, and the porphyry is anomalously molybdenum enriched. A new U-Pb zircon date on this mineralized porphyry fixes its emplacement and the maximum age of Cu-Au-(Mo) deposition at 2688 ± 2 Ma. This mineralization was overprinted by later gold mineralization in the form of planar, sheeted, and en echelon auriferous quartz veins and by mineralized shear and/or fault zones. Field relationships indicate that these veins significantly postdate emplacement of the porphyries and Timiskaming sedimentation. The latest vein systems form the bulk of the ore at the Dome mine and are similar to the most economically significant veins of the district. The largest gold deposits of the district are spatially associated with, but not in, porphyries similar to those exposed at the Dome mine. This association has led to considerable speculation regarding the genetic relationship of felsic porphyry emplacement to ore formation. The results of this study show that although some Au was deposited during a Cu-Au-Mo event temporally linked to porphyry emplacement, most of the Au at the Dome mine was emplaced significantly later than the porphyries. The documentation of at least three broad periods of Au emplacement, at least two of which are premetamorphic, is permissive and supportive of genetic hypotheses that invoke metamorphogenic remobilization of Au from earlier formed deposits. Alternatively, the coincidence of early gold enrichments and later economic concentrations may reflect a single, unidentified gold source from which successive but unrelated hydrothermal activity mobilized, transported, and deposited Au, with no direct genetic link between early and late episodes of gold deposition.

Tower Hill gold deposit, Western Australia: An atypical, multiply deformed Archaean gold‐quartz vein deposit

The Tower Hill gold deposit is distinguished from most Archaean lode deposits of the Yilgarn Craton by virtue of its formation early in the regional deformation history and its consequent deformation. The deposit is located in ultramafic schist, adjacent to the contact with a small pluton of biotite monzogranite that intrudes pervasively foliated granodiorite, the dominant component of the Raeside Batholith. Gold, accompanied by local concentrations of bismuth minerals and molybdenite, occurs in a number of quartz vein ‘packages‘. Mineralised quartz veins at Tower Hill lie within an envelope of potassic alteration (talc‐biotite‐chlorite‐pyrite schist), up to several hundred metres wide. They are spatially and temporally associated with the biotite monzogranite and felsic porphyry intrusions, and their deformed equivalents. The deposit lies in a broad zone of ductile deformation (the Sons of Gwalia Shear Zone). Within the altered ultramafic schist, thin units of felsic schist, derived from biotite monzogranite and felsic porphyry, provided sites of contrasting competency that localised quartz vein formation. The mineralised quartz veins were subsequently deformed during alternating periods of shortening and extension, probably related to the syntectonic, solid‐state emplacement of the Raeside Batholith. These deformations pre‐dated strike‐slip movement on the Cemetery Fault, which truncates the ductile fabrics of the Sons of Gwalia Shear Zone, south of Tower Hill. In terms of the regional deformation history, gold mineralisation at Tower Hill formed during early D2 (regional upright folding); subsequent deformation of the orebody pre‐dated D3 (strike‐slip movement on the Cemetery Fault). The nearby Sons of Gwalia and Harbour Lights deposits also probably formed at an early stage, in contrast to most lode gold deposits in the Yilgarn Craton, which formed during or after D3.

Two ages of porphyry intrusion resolved for the super-giant Chuquicamata copper deposit of northern Chile by ELA-ICP-MS and SHRIMP

Zircon U-Pb ages measured in situ by excimer laser ablation–inductively coupled plasma–mass spectrometry (ELA-ICP-MS) and verified by sensitive high-resolution ion microprobe (SHRIMP) on ore-bearing felsic porphyries from the Chuquicamata porphyry copper-molybdenum deposit, northern Chile, identify two discrete igneous events. The volumetrically dominant East porphyry has an age of 34.6 ± 0.2 Ma, whereas the Bench and West porphyries yield ages of 33.3 ± 0.3 Ma and 33.5 ± 0.2 Ma, respectively. The age of the East porphyry is indistinguishable from a Re-Os age for early molybdenite mineralization (35 Ma) and the oldest reported 40 Ar- 39 Ar ages for hydrothermal alteration, confirming a genetic link with mineralization. Previous geological studies and 40 Ar- 39 Ar and Re-Os geochronology identify two main hydrothermal events: high-temperature potassic alteration with chalcopyrite at 33.4 ± 0.3 Ma followed by lower temperature quartz- sericite alteration with pyrite at 31.1 ± 0.3 Ma. The ages of the West and Bench porphyries match the ages for potassic alteration. Younger quartz-sericite alteration may reflect an additional fourth intrusion concealed at depth. The anomalously large size of Chuquicamata appears to be due to a protracted igneous history resulting in the superposition of at least two temporally distinct magmatic-hydrothermal systems.

Change in the magnetic properties of bituminous coal intruded by an igneous dike, Dutch Creek Mine, Pitkin County, Colorado

Magnetization measurements have been made on natural coke–coal samples collected at various distances from a felsic porphyry dike in a coal seam in Dutch Creek Mine, Colorado to help characterize the nature and distribution of the iron-bearing phases. The magnetization passes through a maximum at the coke-to-coal transition about 31 cm from the dike contact. The magnetic measurements support the geochemical data indicating that magmatic fluids along with a high-temperature gas pulse moved into the coal bed. Interaction of the magmatic fluids with the coal diminished the reducing power of the thermal gas pulse from the dike to a point about 24 cm into the coal. The hot reducing gas penetrated further and produced a high temperature (∼400–525°C) zone (at about 31 cm) just ahead of the magmatic fluids. Metallic iron found in this zone is the principal cause of the observed high magnetization. Beyond this zone, the temperature was too low to alter the coal significantly.

Timing of gold mineralisation in western and central Victoria, Australia: New constraints from SHRIMP II analysis of zircon grains from felsic intrusive rocks

Age constraints provided by felsic intrusive rocks indicate that gold mineralisation in western and central Victoria, Australia occurred over a prolonged period of time ranging from prior to the Late Silurian to the Late Devonian. An early phase of gold mineralisation associated with regional deformation pre-dated the emplacement of Late Silurian felsic porphyry dykes at 413±3 Ma in the central lode system of the Magdala deposit, Stawell. Gold mineralisation associated with the final stages of the main regional deformation also pre-dated the intrusion of Early (ca. 400 Ma) to Late Devonian (ca. 370 Ma) plutons at Stawell (Wonga deposit), Mt. Bute, Tarnagulla, Tallangalook, Maldon and Heathcote in both the Stawell, Pyrenees and Ballarat metallogenic provinces, consistent with preliminary dating studies of hydrothermal micas from major gold deposits. Later phases of gold mineralisation throughout western and central Victoria were either temporally associated with, or post-dated the intrusion of Late Silurian to Late Devonian felsic dykes. Examples of this style of mineralisation include the Wonga deposit at Stawell, Mafeking, Fosterville, Maryborough, minor occurrences near Linton and the Woods Point region. Preliminary constraints from Ballarat West indicate that Late Devonian dykes intruded at ca. 386 Ma were pervasively altered by interaction with an auriferous fluid similar to that responsible for gold deposition within the Guiding Star Lode, but that this was probably related to remobilisation during emplacement of the dykes into a pre-existing mineralised structure. With the exception of the Woods Point region and Ballarat West, these later phases often have a magmatic affiliation, indicated by an association with Cu, Mo, Bi, Sb and W. These complex timing relationships reflect the diachronous nature of deformation, metamorphism and magmatism in the western Lachlan Fold Belt of southeast Australia and suggest at least two discrete periods of gold mineralisation in the region.

Geology of the Hammer Down mesothermal gold deposit, Newfoundland Appalachians, Canada

The Hammer Down gold deposit is one of the most significant mesothermal vein-type gold deposits in the Canadian Appalachians. It is located within a complex sequence of Ordovician, mafic-dominated tholeiitic and calc-alkalic and arc-related volcanic rocks, which was intruded by Silurian felsic porphyry dykes. The host rocks have undergone complex polyphase deformation. At least three deformational events influenced vein emplacement and overall geometry of the deposit. A Taconian deformation (D1–2) was responsible for the development of a 250 m wide zone of high-strain deformation (HSZ1) at the interface between two blocks of Ordovician rocks: the Catcher's Pond Group and the Lush's Bight Group. Rocks included within the HSZ1, represent "exotic" slabs of volcanic rocks that were tectonically juxtaposed, intensively foliated (S1), and folded (F2). Gold occurs in high-grade, sulfide-rich, fault-fill quartz veins that occur within the HSZ1. At the outcrop scale, these veins are hosted by discrete centimetre- to metre-wide ductile–brittle D3 high-strain zones (HSZ3) of Silurian or younger age. The development of the gold-hosting structures (HSZ3) is genetically related to layer anisotropy induced by intrafolial F2 folds, and most importantly by the presence of felsic porphyry dykes, which were competent compared to the intensively foliated and incompetent mafic volcanic rock sequence. A postmineralization D4–5 deformation, which included two generations of folds (F4 and F5) and late brittle faulting, is responsible for the actual geometry of the deposit.

Gold mineralization in the Paleozoic Appalachian Orogen; constraints from geologic, U/Pb, and stable isotope studies of the Hammer Down Prospect, Newfoundland

The Hammer Down gold prospect occurs in an accretionary tectonic setting in the Paleozoic Appalachian orogen. It has many geologic and geochemical features in common with other shear-hosted, mesothermal gold deposits. The gold prospect is located about 25 km to the east of an exposed continental-oceanic suture zone and is near a major fault (the Green Bay fault) of regional extent. The main mineralized zones consist of structurally controlled quartz veins with abundant pyrite, chalcopyrite, sphalerite, and carbonate, hosted by volcanic and sedimentary rocks of the Ordovician Catchers Pond Group. Gold-bearing veins postdate a regional subgreenschist metamorphism and an early phase of deformation. Mineralization occurred synchronously with a locally intense S 2 fabric within narrow brittle-ductile high strain zones in, and adjacent to, Early Silurian felsic porphyry dikes. Carbonate and chlorite alteration is widespread and, in part, predates the mineralization; however, there is increasing chloritization near the gold-bearing zones and the mineralized veins contain abundant carbonate. Strong sericite-pyrite-rutile hydrothermal alteration is restricted to narrow zones adjacent to veins. Isotopic compositions of quartz (delta 18 O =11.4 + or - 0.7%) and carbonate (delta 13 C =-6.2 + or - 0.7%) from gold-bearing veins at Hammer Down are typical of shear-hosted, mesothermal gold deposits of Archean and younger age. Oxygen isotope thermometry using quartz, chlorite, and muscovite yields temperatures of vein formation between 245 degrees and 320 degrees C. The ore-forming fluids have calculated delta 18 O and delta D values between 1.1 and 5.8 and -36 and -54 per mil, respectively, and are interpreted to be of mixed origin, dominantly from a metamorphic reservoir with a small component of low 18 O meteoric water. In the Hammer Down area, felsic tuff in the host arc-related sequence, the Catchers Pond Group, has an igneous U-Pb zircon age of 480 (super +4) (sub -3) Ma. This allochthonous sequence is intruded by felsic dikes which are correlative with granodiorite of the Burlington Intrusive Suite, a major regional unit of the eastern Baie Verte...

Time constraints on the evolution of the Tennant Creek Block, northern Australia

Recent ion-microprobe UPb zircon age determinations on crustal components of the Palaeoproterozoic Tennant Creek Inlier have identified a relatively short (∼ 30 m.y.) history for the major magmatic events in the area, including those related to the Barramundi Orogeny. This work independently confirms the revisions of the Tennant Creek stratigraphy proposed by Donnellan et al. (1991). The oldest known rocks from the area are the volcaniclastic turbidites of the Warramunga Formation, which were deposited in a rapidly subsiding basin at 1862 ± 9 to 1859 ± 13 Ma. Subordinate zircon populations in the greywackes indicate earlier magmatic activity at 1930 ± 12 and 1908 ± 15 Ma. The regional shortening (D1) and contemporaneous intrusion of voluminous and chemically homogeneous granites (and associated felsic porphyry dykes), known across northern Australia as the Barramundi Orogeny, occurred here between 1858 ± 12 and 1845 ± 4 Ma. Following uplift, the felsic volcanics and volcaniclastics of the Flynn Subgroup were extruded between 1845 ± 4 and 1827 ± 9 Ma. During this interval, felsic porphyry dykes and minor granite bodies probably related to the Flynn Subgroup volcanism intruded the Warramunga Formation sediments. Magma source regions contain components as old as 3.0 Ga, as revealed by zircon inheritance in most of the analysed rocks. The youngest recognised igneous activity in the area are the intrusion of the “Warrego granite” at 1700-1650 Ma and the Gosse River East granite at 1712 ± 5 Ma. The chronostratigraphy presented here is remarkably similar to that of parts of the well-exposed Palaeoproterozoic Trans-Hudson and Wopmay orogens of northern America.

Thermochronology of the Cornubian batholith in southwest England: Implications for pluton emplacement and protracted hydrothermal mineralization

The metalliferous ore deposits of southwest England are associated with biotite-muscovite granites that intruded upper Paleozoic sediments and volcanic rocks at the end of the Hercynian Orogeny. The hydrothermal mineralization can be subdivided into four stages: 1. (1) exoskarns 2. (2) high-temperature tin and tungsten oxide-bearing sheeted greisen bordered veins and Sn-bearing tourmaline veins and breccias 3. (3) polymetallic quartz-tourmaline-chlorite-sulfide-fluorite-bearing fissure veins, which represent the main episode of economic mineralization 4. (4) late-stage, low-temperature polymetallic fluorite veins. U-Pb dating of monazite and xenotime and 40Ar 39Ar dating of muscovite were used to determine emplacement ages and cooling times for individual plutons within the Cornubian batholith, as well as separate intrusive phases within the plutons. In addition, 40Ar 39Ar ages from hornblende and secondary muscovite and Sm-Nd isochron ages from fluorite were employed to determine the relationship between pluton emplacement and different stages of mineralization. The U-Pb ages indicate that granite magmatism was protracted from ~300 Ma down to ~275 Ma with no evidence of a major hiatus. There is no systematic relation between the age of a pluton and its location within the batholith. The U-Pb ages for separate granite phases within a single pluton are resolvable and indicate that magma emplacement within individual plutons occurred over periods of as much as 4.5 myrs. Felsic porphyry dike emplacement was coeval with plutonism, but continued to ~270 Ma. The geochronologic data suggest that the Cornubian batholith originated from repeated melting events over 30 myrs and was formed by a series of small coalescing granitic bodies. Cooling rates of the main plutons are unrelated to emplacement age, but decrease from the southwest to the northeast from ~210°C myr −1 to ~60°C myr −1 with a mean of 100°C myr −1. These slow cooling rates appear to reflect the addition of heat from multiple intrusive episodes. The mineralization history is distinct for each pluton and ranges from coeval with, to up to 40 myrs younger than the cooling age for the host pluton. Stage 2 mineralization is broadly synchronous with the emplacement of granite magmas, is dominated by fluids expelled during crystallization, and may be repeated by the emplacement of younger magmas within the same pluton. Sm-Nd isochrons for fluorite from stage 3 polymetallic mineralization give ages of 259 ± 7, 266 ± 3 and 267 ± 12 Ma, postdating stage 2 mineralization by up to 25 myrs within the same deposit. The similarity in age of the main polymetallic mineralization hosted by the oldest and youngest plutons, suggests that this stage of mineralization is unlikely to be related to hydrothermal circulation driven by the emplacement and cooling of the host granite. The mineralization is more likely the product of regional hydrothermal circulation driven by heat from the emplacement and crystallization of younger buried pulses of magma.

Contrasting styles of epithermal precious-metal mineralization in the southwestern Nevada volcanic field, USA

The southwestern Nevada volcanic field contains epithermal precious-metal deposits hosted by Miocene volcanic rocks and pre-Tertiary sedimentary rocks with production+reserves greater than 60 t of gold and 150 t of silver. The volcanic rocks consist predominantly of ash-flow tuffs erupted between 15 and 7 Ma during three major magmatic stages: the main stage (ca. 15-13 Ma); the Timber Mountain stage (ca. 13-9 Ma); and the late stage (ca. 9-7 Ma). Hydrothermal activity and precious-metal mineralization in the southern part of the field took place between ca. 13 and 8.5 Ma, coinciding with portions of all three magmatic stages. Regional extension during this period produced imbricate normal and detachment faulting that provided structural control for some of the mineralization. Contrasts in the style and geochemistry of mineralization, together with stratigraphic and radiometric age data and differences in geologic setting reflect the variable nature of hydrothermal activity during development of the southwestern Nevada volcanic field. During the main magmatic stage, silver-rich vein mineralization of the adularia-sericite type occurred in an intermediate volcanic center at Wahmonie. Secondary high-salinity fluid inclusions in felsic subvolcanic intrusions, a trace element suite that includes bismuth and tellurium, and geophysical data support the presence of a buried porphyry-type magmatic system at Wahmonie. Hydrothermal activity at Bare Mountain took place during the main magmatic stage, and may have continued into the Timber Mountain magmatic stage. Bare Mountain contains gold-rich, disseminated Carlin-type deposits with high arsenic, antimony, mercury and fluorine in sedimentary and igneous rocks. In northern and eastern Bare Mountain, mineralization is associated with felsic porphyry dikes that contain secondary high-salinity fluid inclusions. A genetic relationship between porphyry magmatism and shallow Carlin-type gold deposits seems likely at Bare Mountain. Sedimentary-rock-hosted mineralization at Mine Mountain is spatially associated with a thrust and was apparently deposited, in part, by a hydrothermal system active during the Timber Mountain magmatic stage. The silver:gold ratio is high and base-metal, arsenic, antimony, mercury and selenium contents are very high. Mine Mountain mineralization shares features with vein and disseminated silver deposits at Candelaria, Nevada. Gold-silver deposits in the areally extensive Bullfrog district comprise the largest known precious-metal resource in the volcanic field. They are mainly quartz-carbonate±adularia veins with alteration and mineralization styles similar to other adularia-sericite-type deposits in the Great Basin. Deposits in the Rhyolite area and at the Gold Bar mine have very low contents of arsenic and mercury compared to other epithermal deposits in the Great Basin, although copper and antimony are locally elevated. Similarities in mineralization style and assemblages, which include two occurrences of the rare gold-silver sulfide uytenbogaardtite, indicate deposition under similar conditions in different parts of the district. Hydrothermal activity in the Bullfrog district was coeval with extensional tectonism and may have continued from the Timber Mountain stage into the late magmatic stage. Mineralization at some deposits in the Bullfrog and Bare Mountain districts is spatially associated with, and, in part, structurally controlled by a regional detachment fault system. However, significant differences in age, mineralization style and geochemistry indicate that mineralization in the two districts is unrelated.

Archaean evolution of the Wongan Hills Greenstone Belt, Yilgarn Craton, Western Australia

Ion microprobe and conventional U‐Pb analyses on single zircons from a felsic porphyry from the Wongan Hills Greenstone Belt in the Western Gneiss Terrain of Western Australia define an age of 3010 ± 7 Ma. This is a minimum age for volcanism in the greenstone belt. A sample of gneiss from near the belt contains two populations of zircons with ages of 2997 ± 47 Ma and 2800 ± 9 Ma. The ca 3.0 Ga zircons are interpreted as dating the formation of the granitic parent of the gneiss, and the ca 2.8 Ga zircons are interpreted as dating gneiss formation. This latter event may be an early manifestation of the major 2.65–2.75 Ga granite‐greenstone episode which affected the entire Yilgarn Craton and is expressed at Wongan Hills by the emplacement of post‐tectonic porphyritic granite dated at 2651 ± 4 Ma, and greenschist facies retrogressive metamorphism, dated at 2646 ± 11 Ma by U‐Pb analyses of cogenetic sphene. The present results confirm the significance of ca 3.0 Ga volcanism in the north and western Yilgarn Cr...

The scale of ore-depositional systems: an important restraint on epigenetic vs remobilized syngenetic origins for Archaean mesothermal gold deposits

There is general consensus that the majority of gold deposits in the important group hosted by volcanic or intrusive host rocks within Archaean granitoid-greenstone terrains have an epigenetic origin. However, there is controversy surrounding those stratabound deposits hosted by Fe-rich sedimentary rocks, such as banded iron formation (BIF), for which epigenetic, syngenetic and remobilized syngenetic (lateral secretion) origins have all been proposed. In Western Australia, such controversial gold deposits in Fe-rich sedimentary units have similar ore mineralogies, ore-element ratios and, in places, alteration assemblages and, on the craton scale, show very similar structural controls to the other deposits: equivalent relationships are recorded for similar deposits in other cratons. All Western Australian deposits occur along kilometre-scale shear or fault zones linked to trans-craton, obliqueto strike-slip shear zones that were a focus for: i) carbonation with a mantle-like isotopic signature, and ii) emplacement of high-level A- and I-type granitoids, felsic porphyries and/or calc-alkaline lamprophyres. The scale of the mineralizing systems and the broad contemporaneity of mineralization, as indicated by radiogenic isotope studies, is much larger than that envisaged in conceptual secretion models which involve volumetrically small, locally-enriched source rocks. Instead, Archaean gold mineralization in Western Australia, at least, is probably the result of high fluid-flux during deformation, hightemperature metamorphism and magmatism which may be related to tectonism at convergent plate margins, as in modern examples. There is strong evidence for similar tectonic and mineralizing processes from the Canadian Shield.

Recent developments in understanding the Archaean granitic basement of the Barberton Mountain Land and adjacent witwatersrand basin hinterland

Two komatiite emplacement styles are recognized in the Boorara Domain of the Kalgoorlie Terrane, Eastern Goldfields Superterrane, Yilgarn Craton, Western Australia. (1) Large volume, dominantly cumulate-textured komatiite bodies, interpreted as high-level intrusions based on the presence of symmetrical spinifex-cumulate textural zonation, together with apophysy injection and xenolith incorporation along the top and basal margins of the komatiite. (2) Extrusive komatiite lavas exhibit asymmetrical textural zonation, with the top chilled flow margin preserving a fine-grained, fractured crust that coarsens downward into a supercooled former olivine spinifex zone and then into pseudomorphed olivine (now serpentine) orthocumulate texture above a thin basal chilled margin.The intrusive komatiite unit is observed to be intercalated with quartz- and feldspar-phyric dacite. The two lithologies exhibit primary irregular contacts against each other; komatiite apophyses have been injected into the dacite and magma mingling has occurred. These features are indicative of liquid–liquid interaction between the two units and komatiite magmatism is interpreted to have been contemporaneous with dacitic magmatism. The dacite was emplaced first and the komatiite intruded the dacite before it solidified.The reconstructed stratigraphy of the Boorara Domain comprises a basal unit of quartzofeldspathic turbidite beds, conformably overlain by high Mg tholeiite basalt, the intercalated dacite and komatiite units overlie the lower tholeiitic basalt with an unobserved contact, an upper komatiitic basalt unit overlies the komatiite and at the stratigraphic top of the sequence are felsic volcaniclastic turbidite units with felsic porphyry intrusions. These greenstone lithologies were emplaced within a deep submarine basin as indicated by carbonaceous metapelite units intercalated throughout the sequence.The tectonic-scale processes operating in, and around, the Boorara Domain depocentre include derivation of the komatiite unit from a likely mantle plume source. The contemporaneous dacite magmatism is interpreted to be the result of melting of pre-existing Archaean crust during plume ascension, although the exact composition of the primitive crust is yet to be determined.

The genesis and tectonic control on archaean gold deposits of the Western Australian Shield — A metamorphic replacement model

Gold deposits occur in greenstone belts world wide, and contribute to anomalously high gold production from Archaean terranes. As in other cratons, Archaean gold mineralization of Western Australia represents a complex array of deposit styles. Despite this, most deposits are clearly epigenetic, and large deposits have a number of features in common, including their strong structural controls, distinctive wallrock alteration (Fe-sulphide, K-mica±albite, CaMgFe carbonates), consistent metal associations (AuAgAsSbWB; low base metals), commonly Fe-rich host rocks, great depth extension and lack of appreciable vertical zonation. These shared characteristics, combined with their ubiquitous occurrence, indicate that Archaean gold deposits had a common origin related to the tectonic evolution of greenstone belts. Auriferous hydrothermal systems were broadly synchronous with regional metamorphism and emplacement of synkinematic granitoids and felsic (porphyry) intrusions. Although these gold systems involved low-salinity, lowdensity, reduced, near-neutral H 2OCO 2 fluids carrying gold as reduced sulphur complexes, the origin of the fluids is equivocal. Most timing evidence and stable isotope data cannot distinguish metamorphic from magmatic (granitoid or felsic porphyry) orggins, but the lack of consistent spatial relationships between specific, volumetrically significant intrusive phases and large gold deposits in a number of cratons strongly favours metamorphic derivation of fluids. The metamorphic-replacement model for gold mineralization involves devolatilization of the lower portions of the greenstone pile, with high geothermal gradients inhibiting significant melting. CO 2 possibly formed by the decarbonation of early alteration, related to mantle degassing along crustal-scale, synbasinal fault zones. Auriferous fluids were channelled along greenstone-scale faults, in part developed during reactivation of crustal-scale faults in a strike-slip regime. Gold deposition occurred largely under greenschist facies conditions (about 300–400°C, 1–2 kb) in response to decreasing gold solubility with declining temperature. However, a major control on gold deposition was fluid/wallrock interaction. Many large deposits formed by sulphidation of Fe-rich host rocks, with synchronous deposition of Fe-sulphides and gold. However, the variable nature of gold-depositing reactions, including lowering of fO 2 and pH, allowed a multitude of small, and some large, deposits to form wherever that fluid circulation occurred. In consequence, several of the relatively small deposits currently worked from open pit are hosted by ultramafic and felsic rocks. There are few constraints on the source of components (Au, S, K, CO 2) added to gold deposits, but even giant deposits such as the Golden Mile, Kalgoorlie could have formed from a realistic greenstone source volume (ca. 8×8×5 km). Convective circulation of fluids could have contributed to the generation of high fluid-rock ratios. On the regional scale, the markedly heterogeneous distribution of large gold deposits, gold productivity and host rocks to deposits can be accommodated by the metamorphic-replacement model. The most favourable conditions for development of auriferous hydrothermal systems operated in younger (ca. 2.7±0.1 Ga) rift-phase greenstones where greatest extension and crustal thinning produced high geothermal gradients, crustal-scale synbasinal faults, and rapid extrusion and burial of volcanics, including abundant komatiites. Iron-rich tholeiitic basalts and dolerites were preferred host rocks for large gold deposits. The least favourable conditions existed in older (ca. 3.5-3.4 Ga) platformphase greenstones, where gentle sagging on submerged continental crust produced eruption of mainly mafic volcanics with few komatiites, commonly in very shallow-water environments. This allowed intense synvolcanic alteration of both gold source rocks and potential host rocks. The generally smaller gold deposits formed mainly in ultramafic or greywacke hosts. Younger (ca. 3.0 Ga) platform-phase greenstones appear intermediate in nature but, unlike other greenstones, have significant epigenetic gold deposits in originally oxide-facies BIF, which were deposited on relatively deep-water platforms. Similar controls appear to exist on a world scale, with gold mineralization peaking at ca. 2.7±0.1 Ga in response to development of major rift zones in thickened, relatively mature continental crust. Interestingly, the giant Witwatersrand goldfield formed at about the same time.

Aeromagnetics, geology and ore environments, II. Magnetite and ore environments

“Ore environment”, in the sense that is usually used by geophysicists, signifies a region that is perceived to be favourable for certain kinds of ores and is large enough to be recognizable from airborne surveys. The aim of this article is to inquire whether or under what circumstances such regions can be identified from airborne magnetometer surveys as a result of characteristic features of their magnetic mineralogy. Greenstone belts and former greenstone belts can usually be recognized in large-scale aeromagnetic surveys due to their overall deficiency in magnetite compared with normal granitoid rocks. The low magnetite capacity of greenstone belt rocks is recognizable even under high grade metamorphic facies conditions. Massive, concordant, pyritic CuZn sulfide ores are found within greenstone belts (or former greenstone belts), usually within felsic volcanic or sedimentary zones, and are frequently associated with small beds of metasediments that are rich in iron or in iron and manganese. These beds are 1–10 m thick, continuous over distances of 100 m to a few kilometers, and may extend to distances of 10 km or more away from the ore zone along its strike. The iron may be in the form of pyrite, hematite or magnetite; but magnetite is the most common iron mineral in medium or high grade metamorphic terrains. These units often cause weak anomalies which can be detected by high quality aeromagnetic surveys. Porphyry CuMo sulfide ores are associated with felsic-intermediate plutons and are characterized by concentrically zoned patterns of alteration and mineralization. Aeromagnetic exploration guides to these deposits include the identification of potential porphyry districts by large-scale zoning of igneous rocks from mafic to felsic types as one proceeds towards the centre of the district, and the recognition of concentric patterns of alteration and mineralization having diameters 1–10 km. Some porphyry deposits have a weak “magnetite halo”, and some have magnetite-bearing skarns around the rim of the pluton. Volcanic-associated gold ore deposits occur within greenstone belts or former green-stone belts. They are spatially associated with ultramafic flows, alteration, major fault zones, and feldspar-porphyry intrusions. The ores are believed to besyngenetic-diagenetic Aeromagnetic exploration guides to deposits of this type, which include most of the Canadian deposits, include identification of greenstone belts and delineation of mafic, felsic and sedimentary volcanic phases within the belt, recognition of alteration zones and fault zones, identification of small, felsic porphyry stocks, and delineation of altered mafic/ultramafic layers within the volcanic regions. These and other examples suggest that the “ore environment” concept as previously defined, namely, a region several tens of kilometers in extent in which an experienced interpreter can recognize favourable patterns in the opaque oxide mineralogy, is valid. Used in this way, magnetite can be a useful pathfinder to certain types of ores, and the “aeromagnetic ore environment” has the potential of becoming a practical exploration concept.

U-Pb ages from the Nipigon plate and northern Lake Superior

Research Article| December 01, 1985 U-Pb ages from the Nipigon plate and northern Lake Superior D. W. DAVIS; D. W. DAVIS 1Geology Department, University of Toronto, Toronto, Ontario M5S 1A1, Canada Search for other works by this author on: GSW Google Scholar R. H. SUTCLIFFE R. H. SUTCLIFFE 2Department of Geology, University of Western Ontario, London, Ontario N6A 5B7, Canada Search for other works by this author on: GSW Google Scholar GSA Bulletin (1985) 96 (12): 1572–1579. https://doi.org/10.1130/0016-7606(1985)96<1572:UAFTNP>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share MailTo Twitter LinkedIn Tools Icon Tools Get Permissions Search Site Citation D. W. DAVIS, R. H. SUTCLIFFE; U-Pb ages from the Nipigon plate and northern Lake Superior. GSA Bulletin 1985;; 96 (12): 1572–1579. doi: https://doi.org/10.1130/0016-7606(1985)96<1572:UAFTNP>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract U-Pb ages have been measured for six lithologies that are relevant to the Proterozoic igneous activity around Lake Nipigon and the northern part of Lake Superior. The data show the presence of previously unrecognized pre-Keweenawan felsic magmatic events and provide a framework for relating events in the Nipigon plate to the Keweenawan rift.Tonalite gneiss from the Archean basement underlying the Nipigon plate has a minimum age of 2716 Ma. An anorogenic granite pluton at English Bay, Lake Nipigon, that is observed to grade into rhyolite is dated at 1536.7 + 10/−2.3 Ma. The rhyolites are intercalated with quartz arenites, and this may date the initiation of Sibley Group sedimentation. Samples (two) of the Logan diabase sills from different parts of Lake Nipigon have been dated using zircon and baddeleyite. The zircon fractions define an age of 1108.8 + 4/−2 Ma.A rhyolite from the Osier volcanics at Agate Point on the north shore of Lake Superior gives an age of 1097.6 ± 3.7 Ma. This is from the magnetically reversed sequence and provides a maximum age for the magnetic reversal. The zircons show evidence of inheritance and define a mixing line with an upper intercept of 2635 +143/−125 Ma. A porphyry, probably a flow, from the base of the Osier Group, dated at 1107.5 + 4/−2 Ma, is similar in age to the Logan sills. The presence of inherited zircons in the felsic rocks of the Osier Group indicates partial melting of Aichean crust during emplacement of basaltic magma. A variety of zircon ages from a conglomerate containing felsic porphyry clasts at the base of the Osler Group suggests that pre-Keweenawan felsic magmatism took place at about 1730 Ma and 1600 Ma in the Black Bay Peninsula area. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Early deformation in the Svecokarelian greenstone belt of the Kiruna iron district, northern Sweden

Abstract The Proterozoic iron ores of the Kiruna district, northern Sweden, occur as discontinous lenses of magnetite and hematite up to 4 kilometers in length within a sequence of felsic porphyries. The association of low-grade felsic volcanics and sedimentary rocks with basaltic pillow lavas and granitic rocks shows a strong similarity to typical Archean greenstone belts. Primary younging indicators, bedding/cleavage relationships, and minor fold symmetries through the district suggest that the area occupies the eastern limb of a major antiform, cored by granitic rocks to the west. A steep regional cleavage, inhomogeneously developed through the district, suggests one major episode of compressional deformation. The cleavage and associated flattened clasts indicate a compression direction plunging 10°WNW. Elongate clasts, boudins, and fibrous veins indicate a strong extension plunging 60°SSE, parallel to fold hinge lines. Boudinage of the major ore bodies to produce their present distribution is compatib...