Petrochemistry and hydrothermal alteration within the Tyrone Igneous Complex, Northern Ireland: implications for VMS mineralization in the British and Irish Caledonides
Steven P. HollisStephen RobertsG. EarlsRichard HerringtonMark CooperStephen J. PierceySandy M. ArchibaldMartin Moloney
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The Saxby and Mt. Angelay Igneous Complexes represent the volatile rich, post ~1530
Ma granitoids of the Williams Batholith, and have been considered to be a possible
source of magmatic-hydrothermal fluids that produced IOCG deposits in Cloncurry
district. The evolution of these igneous complexes is studied in this thesis with the
ultimate aim to understand the chemistry of magmatic fluids and their involvement in ore
genesis. These igneous complexes contain a variety of intrusions including mafic,
intermediate and felsic members and their fluid evolution was examined by several bulk
and micro-analytical techniques.
The distribution of rock units and their contact relations were obtained from field
observations and regional and detailed mapping. The Saxby (SIC) and Mt. Angelay
(MAIC) Igneous Complexes are dominated by metaluminous, potassic, magnetite bearing
intrusive rocks, which intruded into the calc-silicate rocks of the Mary Kathleen
Group and psammo-pelitic rocks of the Soldiers Cap Group between 1530 and 1500 Ma.
The major rock types in the SIC include granites and a large number of mafic intrusions,
with limited pulses of intermediate magmas, typically observed at the magma
mixing/mingling locations. The MAIC apparently represents a more evolved pluton,
which has limited mafic intrusions with more intermediate rock types and abundant
felsic rocks. Other major rock types and structures include magmatic-hydrothermal
transition veins and ‘brain rocks’ of Mt. Angelay, mixed/mingled rocks and explosive
breccias of the SIC, and late igneous phases of pegmatites and aplites. Intense sodic/
sodic-calcic alteration is abundant in both complexes, complicating geochemical
interpretation.
Petrographic and geochemical studies were used as tools to distinguish various rock
types and magmatic crystallization processes from sub-solidus hydrothermal processes.
The major and trace element studies together with rare earth element (REE) patterns and
field observations suggest different magma sources for the mafic and felsic rocks. The
REE patterns, depletion in Eu, Sr, P and Ti, and Y-undepleted nature of K-rich, abundant
felsic intrusions suggest a crustal source which is more likely depleted in garnet, titanite,
apatite, pyroxenes and/or amphiboles and enriched in plagioclases. In mafic and
intermediate intrusions, the decrease in CaO, Nb, Sr, Sc, V and TiO2 with increasing
SiO2, together with negative Eu anomalies, suggested that fractional crystallisation of
plagioclase and amphibole were prominent processes involved in the formation of the
more silicic phases from mafic magmas. REE patterns also suggest that this mafic source
region was enriched in pyroxenes, amphiboles, apatite and titanite and depleted in
garnet.
The volatile evolution of the SIC and MAIC intrusions was particularly estimated from
halogen (F/Cl) abundances and ratios of hydrous minerals including biotite, hornblende
and apatite, and from calculated halogen activities of magmatic fluid in equilibrium with
biotite. The F and Cl concentrations of ferromagnesian minerals highly depend on Fe and
Mg contents; however, they show variable rates of compatibility with fractionation that
may have influenced the halogen concentration of the final magmatic-hydrothermal
fluid. The halogen contents of both whole rocks and minerals show high F and Cl
contents in mafic rocks and gradual loss in Cl with crystallization. The majority of F
analyses in the whole rocks are below detection, but the minerals show major increase in
F contents from mafic to intermediate rocks. The halogen variability in intrusions depends on a number of factors including bulk rock chemistry, wall rock alteration and
Fe-Mg avoidance.
Fluid inclusions were used as a tool to understand the magmatic-hydrothermal evolution
of the SIC and MAIC and the intrusions contain a variety of primary and secondary fluid
inclusions. The primary magmatic fluids of the SIC and MAIC include a common,
abundant CO2 rich fluid phase, which may have been sourced from mafic magma. High
salinity, primary, multisolid inclusions of Mt. Angelay brain rocks represent magmatichydrothermal
fluids; in which their magmatic origin is also confirmed by PIXE halogen
ratios. The multisolid inclusions show very high salinities (38-60wt% NaCl equivalent)
and high homogenization temperatures ranging from 450-600°C and more. Secondary
inclusions of L+V+S (16-46wt% NaCl equivalent) and L+V (1-30wt% NaCl equivalent)
are present in all the SIC and MAIC rocks including granites, brain rocks and breccias,
and they homogenize in between 140-300°C and 100-250°C respectively.
The field and analytical studies suggest that the Saxby breccia pipes and Mt. Angelay
brain rocks represent the release of magmatic fluids at the final stages of magma
evolution (Chapter 2 & 6). It is suggested that the process of magma mingling and the
variable CO2 input from mafic intrusions have played major role in the formation of
breccias and brain rocks. The fluid inclusion P-T estimations from these magmatic hydrothermal
locations together with geochemical and mineral chemical observations
also provide clues to the overall volatile evolution of the mafic and felsic magma, and
their possible role in IOCG genesis.
The metal and element budget of some Cloncurry ore deposits and SIC and MAIC
intrusions are compared as the fluid inclusions provide a direct correlation. The primary
fluid inclusions assemblage in Mt. Angelay brain rocks (CO2 inclusions + multisolid
inclusions) is similar to that found in the most obviously granite-related IOCG deposits
(especially Ernest Henry), and is verified in detail by PIXE and LA-ICP-MS analysis.
The element concentrations, ratios and Fe, Cu, Mn and Zn contents of multisolid
inclusions from these two settings show similarities, which suggest a magmatic
involvement in the IOCG ore genesis of Cloncurry. However, fluid mixing is also
suggested as a major process for the formation of ore deposits.
Although many previous studies supposed that granites were crucial in the magmatic-to-
IOCG connection, the data collected during this study suggest that mafic intrusions
played major roles in the evolution of Saxby and Mt. Angelay Igneous Complexes and in
the formation of some of the Cloncurry ore deposits.
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The Pilbara region of Western Australia, covering some 500 km × 500 km, provides a diversity of Archaean to Proterozoic igneous rocks in a relatively compact area that records a younging southward crustal history of igneous activity, sedimentation, early life, tectonics, and metamorphism from the Archaean (3.6–2.7 Ga) to Proterozoic (2.5–1.8 Ga). The igneous rocks are variable in age, types of rocks, and mode of occurrence and, throughout the Precambrian, record varying igneous rock activity that appear related to several age-related geological settings: to north, the Archaean Pilbara Craton consists of a granitoid-and-greenstone complex; in the central region, there are Proterozoic sequences of volcanic rock, volcaniclastic rock, ironstone, chert, dolomite, shale, and intrusive dolerite sills and cross-cutting dolerite dykes; to the south, there are Proterozoic shale, dolomite, and chert with isolated granitic batholiths. Igneous activity begins in the Archaean with mafic and ultramafic volcanism alternating with sedimentation, and then granitoid cratonisation. This was followed by Proterozoic volcanic crustal accretion with mafic volcanic and volcaniclastic rocks, and by dolerite and gabbro sill and dyke intrusions, ending with isolated granite batholithic intrusions. Igneous rocks in the Pilbara region are diverse: komatiite; mafic volcanic/volcaniclastic rocks; basalt; tuff/volcanic breccia/accretionary lapilli; dolerite, gabbro, leucogabbro, pegmatitic gabbro, granite, and adamellite; xenolithic dolerite/gabbro; andesite, dacite, rhyodacite, rhyolite; granitoids: adamellite, monzogranite, syenogranite, granodiorite, tonalite, granite; granophyre; felsic dykes; and felsic porphyry. They are expressed as granitoid batholiths, komatiite and basalt sheets/lenses, mafic volcanic/volcaniclastic rocks in sheets, sills of dolerite, gabbro, ultramafic rocks, and diorite, dykes of dolerite, gabbro, and felsic rocks, structurally-oriented dolerite dyke swarms, tuff/volcanic breccia/accretionary lapilli in sheets/lenses, sheets of dacite, rhyodacite, rhyolite, and andesite, gabbroic plugs, apophyses, and a variety of host-rock to xenolith relationships. Today, the Pilbara region is arid, hence outcrop is excellent and many of these geological features are well exposed. The diversity of Archaean to Proterozoic igneous rocks in a relatively compact and well-exposed area and qualifies it as a globally unique potential Precambrian igneous-rock geopark.
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Abstract The Darran Complex in northeast Fiordland, New Zealand, was investigated for its mineral occurrences. Geological mapping, detailed petrography, and geochemical studies of selected areas, were carried out. Rocks that make up the Complex are subdivided into a series of consanguineous plutonic suites ranging from peridotite, anorthosite and gabbro, through to granites. Petrological and geochemical composition of the rocks show them to be of I-type and possibly derived from a common mafic-intermediate igneous source. A normal fractionation trend from mafic to felsic chemistry, with a lateral and temporal dispersion, is proposed. A petrogenetic theory is also proposed in the light of current views on the crustal structure of Fiordland. Sulphide mineralisation in the Complex is of 3 types. One is of primary magmatic segregation and occurs in layered complexes of gabbroic parentage. The other 2 are of replacement contact metasomatic and hydrothermal porphyry types in dioritic rocks intruded by granitoids of the Hut Plutonic Suite. These types are structurally controlled as evidenced by their proximity to fractures and lineaments. The geochemistry and field relationships indicate that the Hut Plutonic Suite is a late leucocratic differentiation product of the Complex plutonic suites and is also genetically related to the contact metasomatic and hydrothermal mineralisation. The latter type includes a well-defined copper-molybdenum specialisation and is considered to be further evidence of the crustal nature of Fiordland. Keywords: Darran ComplexFiordlandgranitegeochemistrypetrogenesismetasomatismhydrothermal alterationmineralisation
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The original Aldermac mine near Noranda contained several Cu–Zn massive sulfide lenses hosted by felsic to mafic volcanic rocks of the late Archean Blake River Group. The original Nos. 3–6 orebodies, which consisted of massive pyrite, with lesser magnetite, pyrrhotite, chalcopyrite, and sphalerite, contained 1.87 Mt of Cu–Zn ore that averaged 1.47% Cu (Zn was not recovered). The orebodies occurred within felsic breccias and tuffs up to 100 m thick that are stratigraphically overlain by an extensive dome of mainly massive rhyolite and rhyodacite (up to 250 m thick and at least 550 m across). Most of the volcanic rocks that laterally flank and overlie the felsic dome are dacitic to andesitic flows, breccia, and tuff, with minor rhyolites, and associated subvolcanic sills of quartz-feldspar porphyry and gabbro.The new massive sulfide deposit, discovered in 1988, lies 150–200 m east of the mined-out orebodies, at a similar stratigraphic level within altered felsic breccia and tuff. The sulfides are mainly in the No. 8 lens, which contains 1.0 Mt at an average grade of 1.54% Cu, 4.12% Zn, 31.2 g/t Ag, and 0.48 g/t Au. Pyrite forms porphyroblastic megacrysts in a groundmass of pyrrhotite, sphalerite, magnetite, and chalcopyrite. A funnel-shaped, chloritized stockwork zone underlies the No. 8 lens and contains Cu-stringer mineralization. The No. 8 lens appears to be zoned, with overall decreasing Cu:Zn ratios from the core to the fringes of the lens. Massive sulfides in this lens have high Ag, Cd, and Hg contents relative to other massive sulfide deposits near Noranda.Ti versus Zr trends for least-altered Aldermac volcanic rocks indicate a more or less continuous magmatic fractionation trend ranging from high-Ti andesite to andesite, dacite, rhyodacite, and two distinct rhyolites (A and B). Most volcanic rocks were derived from a common parental magma that was transitional between tholeiitic and calc-alkaline compositions, as indicated by Ti–Y–Zr–Nb data and rare-earth-element distributions.Ti versus Zr trends in altered volcanic rocks indicate that silicification (mass gain) has affected some of the andesitic to rhyodacitic rocks, whereas chloritization (mass loss) has affected many of the rhyolitic rocks. Intermediate to mafic volcanic rocks above and lateral to the felsic dome are commonly silicified, possibly the result of hydrothermally remobilized silica derived from underlying felsic volcanic rocks.The orebodies appear to have formed at an eruptive hiatus between mafic → felsic and felsic → mafic cycles, during explosive activity and accumulation of felsic breccia and tuff. Ore was deposited mainly within a felsic fragmental sequence (rhyolite A), but before emplacement of the dome of rhyolite B. In compositionally diverse volcanic terrains, the contact between successive mafic–felsic and felsic–mafic cycles may be a good exploration target, in particular specific geochemical contacts within the felsic stratigraphy.
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The origin of Fe oxide-(Cu-Au) deposits and the relative role of played by magmas (both felsic and mafic) versus evaporite-rich country rocks as a source of fluids and/or metals remains controversial. A popular model for the formation of IOCG deposits in the Mt Isa Eastern Succession involves fluids derived from the late orogenic granites mixing with a second external fluid source forming Fe(commonly magnetite-) rich alteration zones that contain vein stockwork, breccia, dissemination or replacement style mineralization (Oliver et al., 2000). This is assumed to be commonly spatially and temporally associated with felsic pluton emplacement and cooling around 1540-1500 Ma. This contrasts with an alternative model in which the fluids are entirely intra-basinal and amagmatic in origin (Barton and Johnson, 1996). Recent dating studies at Osborne have highlighted a potential syn-peak metamorphic timing to mineralization (based on 1595 Ma Re-Os age dates on molybdenite and a 1595 ± 6 Ma U-Pb age date on hydrothermal titanite), with no apparent proximal major intrusion (Gauthier et al., 2001). There is also a potential link between mineralization and widespread mafic intrusive activity that occurs in the Eastern Succession for the entire range of known mineralization ages. Futhermore, at some deposits (276 orebody at Starra) intra-ore mafic intrusives have been recorded.
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