Felsic Porphyry Dykes Research Articles

Petrochemistry and U–Pb (zircon) age of porphyry dykes at the McKenzie Gulch porphyry–skarn Cu–Ag–Au deposit, north-central New Brunswick, Canada: implications for emplacement age, tectonic setting, and mineralization potential

North- to northeast-trending intermediate to felsic porphyry dykes in the McKenzie Gulch (MG) area intrude Upper Ordovician through Silurian calcareous sedimentary rocks of the Matapédia Group. These dykes are spatially associated with numerous copper–silver skarn occurrences. In this area, two distinct suites of dykes are recognized: plagioclase–hornblende porphyry (P–H) and quartz–plagioclase porphyry (Q–P). These suites yielded U–Pb (zircon) ages of 386.2 ± 3.1 Ma and 386.4 ± 3.3 Ma, respectively, indicating a coeval relationship and similar genesis despite slight geochemical and petrographical differences. Geochemical data indicate that the dykes are granodioritic to granitic to slightly tonalitic in composition with I-type and slab failure signatures. When compared with other granitoids in the region, the Murdochville suite in Gaspésie, Québec, is the only intrusion that exhibits slab failure magmatic signatures similar to the MG dykes. Magmas with slab failure signatures are compositionally similar to tonalite–trondhjemite–granodiorite (TTG) suites and adakites. They exhibit low Y and Yb concentrations and subsequent high Sr/Y and La/Yb ratios, which are interpreted to be the result of melting of subducting slab and mantle during the waning stages of collision and consequent slab failure. These processes were followed by assimilation and fractional crystallization of minerals such as hornblende and clinopyroxene (± titanite), which preferentially partition Y and heavy rare earth elements (HREE) in the absence of significant plagioclase. Magmas with similar geochemical characteristics worldwide have been proposed to be an important source for metals in most porphyry Cu and skarn deposits and consequently are a target during mineral exploration.

Evaluation of crystallization and emplacement conditions of the McKenzie Gulch porphyry dykes using chemistry of rock-forming minerals: Implications for mineralization potential

Electron microprobe analyses of rock-forming minerals in the Middle Devonian (386.2 ± 3.1 and 386.4 ± 3.3 Ma) intermediate to felsic porphyry dykes of the McKenzie Gulch (MG) area, in northern New Brunswick, Canada are used to constrain important intensive parameters, such as oxygen fugacity (fO2), temperature, pressure, dissolved water (H2Omelt) content, and emplacement conditions. Biotite compositions plot just above the nickel-nickel oxide (NNO) buffer at fO2 values of ~−12.22, which together with the presence of magnetite indicate oxidizing conditions during crystallization. On a FeOtotal–MgO–Al2O3 discrimination diagram, biotite compositions plot in the calc-alkaline field, consistent with petrochemical observations, whereas in terms of log(XF/XOH) versus log(XMg/XFe), biotite plots in the moderately contaminated I-type field, suggesting possible interaction of these magmas with host rocks during their evolution.Pressure and temperature conditions of the magma estimated from aluminum-in-hornblende barometry and amphibole–plagioclase thermometry suggest that hornblende phenocrysts in these rocks crystallized at temperatures between ~700° and 810 °C and pressures between 4 and 6 kbar. Depth estimates based on these pressures suggest that these magmas resided at depths between ~13 and ≥19 km prior to final emplacement as dykes in the upper crust. The estimated water (H2Omelt) content of the magmas that fed the dykes indicates that they were highly hydrous (up to ~5.0 wt%), consistent with the presence of phases such as hornblende and biotite. High magmatic water content is a prerequisite for the formation of magmatic-hydrothermal ore deposits and can be used to assess the prospectivity of these slab failure magmatic suites for porphyry and skarn mineralization.

The structural history and mineralization controls of the world-class Geita Hill gold deposit, Geita Greenstone Belt, Tanzania

The Geita Hill gold deposit is located in the Archean Geita Greenstone Belt and is one of the largest gold deposits in East Africa. The Geita Greenstone Belt experienced a complex deformation and intrusive history that is well illustrated and preserved in and around the Geita Hill gold deposit. Deformation involved early stages of ductile shearing and folding (D1 to D5), during which episodic emplacement of large diorite intrusive complexes, sills, and dykes occurred. These ductile deformation phases were followed by the development of brittle-ductile shear zones and faults (D6 to D8). The last stages of deformation were accompanied by voluminous felsic magmatism involving the intrusion of felsic porphyry dykes, within the greenstone belt, and the emplacement of large granitic bodies now forming the margins of the greenstone belt. Early, folded lamprophyre dykes, and later lamprophyre dykes, crosscutting the folded sequence are common, although volumetrically insignificant. The gold deposit formed late during the tectonic history of the greenstone belt, post-dating ductile deformation and synchronous with the development of brittle-ductile shear zones that overprinted earlier structural elements. The main mineralizing process involved sulfide replacement of magnetite-rich layers in ironstone and locally the replacement of ferromagnesian phases and magnetite in the diorite intrusions. The intersection between the brittle-ductile (D6) Geita Hill Shear Zone and different structural elements of ductile origin (e.g., fold hinges), and the contact between banded ironstone and folded diorite dykes and sills provided the optimal sites for gold mineralization.

Boron, sulphur and copper isotope systematics in the orogenic gold deposits of the Archaean Hattu schist belt, eastern Finland

The Hattu schist belt is located in the western part of the Archaean Karelian domain of the Fennoscandian Shield. The orogenic gold deposits with Au–Bi–Te geochemical signatures are hosted by NE–SW, N–S and NW–SE oriented shear zones that deform 2.76–2.73Ga volcanic and sedimentary sequences, as well as 2.75–2.72Ga tonalite–granodiorite intrusions and diverse felsic porphyry dykes. Mo–W mineralization is also present in some tonalite intrusions, both separate from, and associated with Au mineralization. Somewhat younger, unmineralized leucogranite intrusions (2.70Ga) also intrude the belt. Lower amphibolite facies peak metamorphism at 3–5kbar pressures and at 500–600°C temperatures affected the belt at around 2.70Ga and post-date hydrothermal alteration and ore formation. In this study, we investigated the potential influence of magmatic-hydrothermal processes on the formation of orogenic gold deposits on the basis of multiple stable isotope (B, S, Cu) studies of tourmaline and sulphide minerals by application of in situ SIMS and LA ICP MS analytical techniques.Crystal chemistry of tourmaline from a Mo–W mineralization hosted by a tonalite intrusion in the Hattu schist belt is characterized by Fe3+–Al3+-substitution indicating relatively oxidizing conditions of hydrothermal processes. The range of δ11B data for this kind of tourmaline is from −17.2‰ to −12.2‰. The hydrothermal tourmaline from felsic porphyry dyke swith gold mineralization has similar crystal chemistry (e.g. dravite–povondraite compositional trend with Fe3+–Al3+ substitution) and δ11B values between −19.0‰ and −9.6‰. The uvite–foitite compositional trend and δ11B ‰ values between −24.1% and −13.6% characterize metasomatic–hydrothermal tourmaline from the metasediment-hosted gold deposits. Composition of hydrothermal vein-filling and disseminated tourmaline from the gold-bearing shear zones in metavolcanic rocks is transitional between the felsic intrusion and metasedimentary rock hosted hydrothermal tourmaline but the range of average boron isotope data is essentially identical with that of the metasediment-hosted tourmaline. Rock-forming (magmatic) tourmaline from leucogranite has δ11B values between −14.5‰ and −10.8‰ and the major element composition is similar to that of the metasediment-hosted tourmaline.The range of δ34SVCDT values measured in pyrite, chalcopyrite and pyrrhotite is from −9.1 to +8.5‰, which falls within the typical range of sulphur isotope data for Archaean orogenic gold deposits. In the Hattu schist belt, positive δ34SVCDT values characterize metasediment-hosted gold ores with sulphide parageneses dominated by pyrrhotite and arsenopyrite. The δ34SVCDT values are both positive and negative in ore mineral parageneses within felsic intrusive rocks in which variable amounts of pyrrhotite are associated with pyrite. Purely negative values were only recorded from the pyrite-dominated gold mineralization within metavolcanic units. Therefore the shift of δ34SVCDT values to the negative values reflects precipitation of sulphide minerals from relatively oxidizing fluids. The range of measured δ65CuNBS978 values from chalcopyrite is from −1.11 to 1.19‰. Positive values are common for mineralization in felsic intrusive rocks and negative values are more typical for deposits confined to metasedimentary rocks. Positive and negative δ65CuNBS978 values occur in the ores hosted by metavolcanic rocks. There is no correlation between sulphur and copper isotope data obtained in the same chalcopyrite grains.Evaluation of sulphur and boron isotope data together and comparisons with other Archaean orogenic gold provinces supports the hypothesis that the metasedimentary rocks were the major sources of sulphur and boron in the orogenic gold deposits in the Hattu schist belt. Variations in major element and boron isotope compositions in tourmaline, as well as in the δ34SVCDT values in sulphide minerals are attributed to localized involvement of magmatic fluids in the hydrothermal processes. The results of copper isotope studies indicate that local sources of copper in orogenic gold deposits may potentially be recognized if the original, distinct signatures of the sources have not been homogenized by widespread interaction of fluids with a large variety of rocks and provided that local chemical variations have been too small to trigger changes in the oxidation state of copper during hydrothermal processes.

Structural and hydrothermal alteration evidence for two gold mineralisation events at the New Celebration gold deposits in Western Australia

The New Celebration gold deposits (∼2 Moz Au) are located within the Boulder segment of the first-order, transcrustal Boulder-Lefroy fault zone about 35 km southeast of Kalgoorlie, in the Kalgoorlie Terrane of the Eastern Goldfields Province, Western Australia. The gold deposits are hosted by ultramafic rocks (komatiite), differentiated gabbro/dolerite and variably thick (0.5 to 5 m, locally up to 80 m) felsic porphyry dykes that have intruded the ultramafic–mafic rock contact. Host rocks have undergone regional metamorphism to upper greenschist facies.At least four deformation events are recorded at New Celebration: (1) D1NC is represented by vertical stratigraphic contacts, the expression of regional scale upright folds. (2) D2NC deformation is expressed as a well-developed, NNW-trending, steeply WSW-dipping penetrative shear foliation (S2NC) and foliation-parallel, boudinaged quartz–carbonate veins (V2NC), which are representative of the regional D2 Boulder fault zone; movement on the fault is sinistral oblique slip. (3) D3NC deformation resulted in NNE-trending, WNW-dipping, short strike length (<50 m) faults, quartz–carbonate–epidote–chlorite fault fill veins and widely spaced S3NC foliation. Subhorizontal L3sNC slickenline lineations on D3NC fault planes indicate strike-slip movement during D3NC (no kinematic indicators are observed), whereas the geometry of S–C fabrics in the S3NC foliation suggests sinistral movement. (4) D4NC deformation is poorly developed by west-dipping, curviplanar faults that cross cut the S2NC foliation. Lineations formed during D2NC deformation include: (a) moderate to steep, SSE to SSW-plunging L2mNC mineral elongation lineations, (b) moderate, NW-plunging L2iNC intersection lineations between S- and C-foliation planes, and (c) moderate to steep, SSE to SSW-plunging L2sNC slickenline lineations. Moderate to steep SSW-plunging quartz L2mNC elongation lineations, in conjunction with the orientation of S–C fabrics, constrain the sense of movement on the shear zone as sinistral oblique-slip. The S2NC foliation is also developed in thin (1–5 m width) M1 plagioclase-rich porphyry dykes, thus indicating their emplacement pre- to syn-D2NC deformation. The mylonitic fabric in the first-generation Magmatic 1 (M1) porphyry dykes contrasts with the ‘fresh,’ undeformed igneous textures of second generation Magmatic 2 (M2) quartz–feldspar porphyry dyke, which preferentially intruded along the mafic–ultramafic rock contacts. The lack of internal ductile deformation fabrics and a predominance of brittle structures (e.g. fracture network at the margins) in the M2 porphyry dyke indicate its emplacement into a brittle deformation environment.Two gold events are interpreted at the New Celebration deposit. (1) An ‘early’ Mylonite-style gold event in high-strain, quartz–ankerite–biotite–sericite mylonite in mafic rocks and M1 porphyry dykes, which is interpreted to be synchronous with the D2NC deformation. Minute inclusions of gold (<100 μm) are hosted within pyrite that is aligned parallel to the S2NC foliation planes. Gold in mafic mylonite is in equilibrium with ankerite–sericite ± biotite ± pyrite and is also spatially and temporarily associated with gold and non-gold-bearing tellurides such as calerverite, petzite, hessite, altaite, melonite and a bismuth telluride. (2) A ‘late’ gold event that consists of Contact and Fracture-infill styles. The Contact-style gold mineralisation is located in high-Mg basalt within S2NC foliation planes that wrap around the M2 porphyry dyke during late D2NC deformation. Gold is in equilibrium with pyrite–sericite–ankerite. TheFracture-infill-style gold mineralisation is located in brittle fracture networks formed at the margin of the M2 porphyry dyke. ‘Free’ gold and inclusions of gold in pyrite are in equilibrium with pyrite–sericite2 ± ankerite. No telluride species are observed in either Contact or Fracture-infill-style mineralisation. The exact timing of the Fracture-infill-style gold mineralisation is equivocal; it may have formed late-D2NC or during D3NC or even during a later deformation event.The New Celebration gold deposits are a rare example of an orogenic gold system located in a first-order, transcrustal fault system, the Boulder-Lefroy fault. Its complex fault zone architecture and long-lived nature of fault movement is interpreted to be at least in part responsible for the high gold endowment when compared with the typically barren first-order fault zones. Magnetite and biotite in the outer alteration zones are spatially related to M1 dykes, and the spatial and temporal restriction of tellurides to the M1 dyke suggest that the orogenic New Celebration gold deposits were sourced from magmatic fluids, at least during the early stages of the formation of the deposits.

Stress analysis, post-orogenic extension and 3.01 Ga gold mineralisation in the Barberton Greenstone Belt, South Africa

Gold deposits in the Barberton Greenstone Belt (BGB) have formed along a common network of brittle–ductile shear zones that developed during extension and emplacement of felsic porphyry dykes at ∼3015Ma. At a deposit scale, mineralised shears occur in en-echelon arrays, distributed as Riedel, P- and anti-Riedel shears, with envelopes that connect groups of mine workings along NNW, ENE, NE and ESE trends. The dip of the mineralised shear envelopes is generally moderate to steep (50–80°). The existence of the directional alignment of gold deposits at the scale of the greenstone belt has been demonstrated with Fry analysis.Stress inversion for mineralised shear zones from workings near 28 mines using a Bingham axial distribution and an optimized right dihedron approach provide near identical results with σ1 vertical and σ3 horizontal NW. The stress regime is generally radial extensional, or pure extensional. At the scale of the greenstone belt, the mineralised shear envelopes comprise two sets of near-orthogonal trends, positioned symmetrically with respect the measured far field stress: 1. A NW and NE trending pair with the latter accommodating pure normal slip and the former both dextral or sinistral strike-slip; and 2. A NNW and ESE trending pair accommodating dextral transtensional and sinistral transtensional movements respectively.The fact that all mineralised shears developed systematically in response to a similar far field stress suggests that mineralisation formed during a single tectonic stage. 207Pb/206Pb ages of 3017.1±17.8Ma and 3008.8±16Ma from a syn-mineralisation, porphyry dyke near Golden Quarry indicates gold deposition at ∼3015Ma; i.e. ∼85Ma after the craton had stabilised. Older porphyry dykes intruded near Sheba and Consort mines have been dated at ∼3100Ma and ∼3260Ma respectively, and provide a minimum and maximum age constraint on accretionary deformation in the area.We propose that the main phase of gold enrichment in the BGB occurred after tectonic and thermal stabilisation of the crust at a time the craton was acting as a stable terrane. Mineralised brittle–ductile shear zones allowed ingress of deeply sourced fluids during cratonic extension. Mine camps display a cluster spacing of ∼12km, which suggests a regular distribution of controlling shears, possibly as a function of the elastic rigidity of the lithosphere. Rather than linking mineralisation to orogenic processes during greenstone belt accretion, we link gold mineralisation to the opening of fault-bounded intracratonic basins including the Pongola and Witwatersrand Basins, and concomitant igneous activity.

Petrology of a 1.95 Ga granite-granodiorite-tonalite-trondhjemite complex and associated extrusive rocks in the Knaften area, northern Sweden

Two granite-granodiorite-tonalite-trondhjemite (TTG) complexes intruded mafic-intermediate volcanic and volcaniclastic rocks at 1954-1940 Ma in the central part of the Knaften area, which lies in the northern part of the Palaeoproterozoic Bothnian Basin, in the central part of the Fennoscandian Shield, northern Sweden. The regional metamorphism was upper greenschist to amphibolite facies. The granitoid complexes are subdivided texturally into three types of intrusions. Contact relationships show that the most coarse-grained type is the oldest, followed by a medium-grained type and finally an aplitic-pegmatitic type intrusion. Felsic dykes that cut the granitoids form the youngest intrusive event. Myrmekitic/symplectitic textures or graphic intergrowths show that the later intrusive phases were more volatile-rich. The granitoids are high-level intrusions, and formed the root-zone of a volcano. Close to, but outside the granitoid complexes, compositionally similar felsic porphyry dykes occur. A package of reworked felsic tuffaceous rocks occurs in the northern part of the Knaften area and may constitute extrusive equivalents of the intrusions. These clastic rocks display primary structures such as cross bedding, which indicate deposition in shallow water. Geochemically, the Knaften intrusions are granites, granodiorites, tonalites and trondhjemites. The northern granitoids are more fractionated than the southern ones, and some felsic dykes are geochemically similar to the southern intrusions. The tuffaceous rocks are intermediate in composition between the felsic and more mafic intrusive phases. All Knaften felsic phases have similar trace element patterns and, their trace element chemistry is similar to other rocks from collisional settings in island arc terrains.

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.

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.

Neutron activation analysis of gold in Archean igneous and metamorphic rocks of the Rice lake-Beresford lake area, southeastern Manitoba

Gold abundances determined by the method of neutron activation are reported for 26 replicate analyses of a variety of igneous and metamorphic rocks. Samples analyzed come from scattered locations in and adjacent to an Archean greenstone belt containing numerous low-grade gold-quartz veins. Most of the richer veins are enclosed by mafic intrusions. The average gold content of each of the rock types is as follows:Rock Type No. of analyses Au (ppb)Northern Granites; quartz diorite: 4 1.9 + or - 1.2Manigotagan Gneissic Belt; paragneiss: 2 0.6 + or - 0.4associated quartz monzonite: 2 0.3 + or - 0.1Rice Lake Group; metavolcanics: 3 0.3 + or - 0.2Mafic intrusives; gabbro, diabase: 2 18.2 + or - 1.5Central plutons; quartz diorite: 5 1.1 + or - 0.6related felsic porphyry dykes: 2 6.1, 1.1Altered diabase wall-rock and vein zone: 6 0.5-325Gold abundances for the quartz diorite, quartz monzonite and metavolcanic groups are fairly typical of younger rocks of similar composition. Explanations are put forward to account for the unexpectedly low gold content of the paragneiss samples and relative gold enrichment in one felsic dyke sample.There is evidence to indicate that the mafic intrusions may be the source rocks of the gold. Thus, in addition to the spatial relationship of the richer veins to the mafic intrusions the latter also contain an order of magnitude more background gold than other rocks in the area. Furthermore, gold is apparently depleted in altered diabase wall-rock adjacent to a vein in the San Antonio gold mine.Non-repeatable replicate analyses observed in some mafic samples may be due to inhomogeneous distribution of gold between powder aliquots resulting from an erratic distribution of gold-rich host minerals.