Mark A. Kendrick

Fluid inclusion noble gas and halogen evidence on the origin of Cu-Porphyry mineralising fluids

The naturally occurring noble gas isotopes (40Ar, 36Ar, 84Kr and 129Xe) and halogens (Cl, Br, I) have been determined simultaneously in irradiated quartz vein samples by noble gas mass spectrometry. Quartz vein samples were collected from the potassic and propylitic alteration zones of six porphyry copper deposits (PCD): Bingham Canyon, Utah; and Silverbell, Ray, Mission, Pinto Valley and Globe-Miami in Arizona. In addition, analyses of 3He/4He have been obtained from sulphide minerals hosted by the quartz veins at Silverbell, Ray, Pinto Valley and Globe-Miami. The majority of PCD fluids studied have Br/Cl and I/Cl ratios that overlap those of fluids included in mantle diamond, suggesting that the salinity had a juvenile origin. The high I/Cl (121 × 10−6 mole, M) in samples from the propylitic zone of Silverbell is attributed to the presence of sedimentary formation water. 3He/4He ratios have R/Ra values in the range 0.3 to 1.72, and provide evidence for the involvement of a crustal component in addition to mantle volatiles. 40Ar/36Ar ratios vary from meteoric values of ∼317 in the propylitic zone of Bingham Canyon, and 323 in the skarn alteration of Mission up to 3225 in the potassic zone of Pinto Valley. Fluids in both the potassic and propylitic alteration zones of every deposit are a mixture of a low salinity end-member comprising meteoric water and air, and a high salinity end-member consisting of a mixed mantle and crustal fluid. The 40Ar/Cl ratio of fluid inclusions at Pinto Valley (∼10−4 M) is similar to values obtained previously for mantle fluids. The 40Ar/Cl value is two orders of magnitude lower at Bingham Canyon, where a depleted 36Ar concentration (0.2 × 10−6 cm3/g) below that of air saturated water (ASW), and a range of highly fractionated noble gas compositions (F84Kr = 13 and F129Xe = 160) indicate that boiling and pulsed fluid flow have occurred.

Halogen and Ar–Ar age determinations of inclusions within quartz veins from porphyry copper deposits using complementary noble gas extraction techniques

Abstract Extension of Ar–Ar methodology has been used to determine mineralisation ages from mica inclusions and to simultaneously evaluate the noble gas and halogen composition of inclusion fluids within irradiated quartz vein samples from five porphyry copper deposits. Samples have been collected from the potassic and propylitic zones of Bingham Canyon, Utah, and four Arizonan deposits; Silverbell, Globe-Miami, Pinto Valley and Ray. Data obtained using three noble gas extraction techniques (laser ablation, in vacuo crushing and stepped heating) are compared with each other. Laser ablation provides a means for the analysis of individual fluid inclusions but is limited by blank levels and detection limits. Stepped heating and in vacuo crushing are bulk extraction techniques that preferentially release gases from solid and fluid inclusion phases, respectively, and can be used in combination to obtain accurate and meaningful ages of mineralisation. Ages obtained for porphyry copper deposits are as follows: Bingham Canyon 37.1±0.5, Ray 65.3±1.5, Globe-Miami 61.7±3.4, Pinto Valley 63.2±8.0 and Silverbell 55.8±1.8. The age of mineralisation at Ray is of particular interest as it enables a reported discrepancy in K–Ar ages to be understood. Halogen data for the Bingham Canyon samples gives insight into the partitioning of the heavy halogens (Br and I) between solid and liquid phases.

Nature of alkali-carbonate fluids in the sub-continental lithospheric mantle

Mantle xenoliths sampled by kimberlite and alkali basalt magmas show a range of metasomatic styles, but direct evidence for the nature of the metasomatising fluids is often elusive. It has been suggested that carbonate-rich melts produced by partial melting of carbonated peridotites and eclogites play an important role in modifying the composition of the lithospheric mantle. These mantle-derived carbonate melts are often inferred to be enriched in alkali elements; however, alkali-rich carbonate fluids have only been reported as micro-inclusions in diamonds and as unique melts involved in the formation of the Udachnaya-East kimberlite (Yakutia, Russia). In this paper we present the first direct evidence for alkali-carbonate melts in the shallow lithospheric mantle (∼110–115 km), above the diamond stability field. These alkali-carbonate melts are preserved in primary multiphase inclusions hosted by large metasomatic ilmenite grains contained in a polymict mantle xenolith from the Bultfontein kimberlite (Kimberley, South Africa). The inclusions host abundant carbonates (magnesite, dolomite, and K-Na-Ca carbonates), kalsilite, phlogopite, K-Na titanates, and phosphates, with lesser amounts of olivine, chlorides, and alkali sulfates. Textural and chemical observations indicate that the alkali-carbonate melt likely derived from primary or precursor kimberlite magmas. Our findings extend the evidence for alkali-carbonate melts/fluids permeating the Earth mantle outside the diamond stability field and provide new insights into the chemical features of previously hypothesized melts. As metasomatism by alkali-rich carbonate melts is often reported to affect mantle xenoliths, and predicted from experimental studies, the fluid type documented here likely represent a major metasomatising agent in the Earth’s lithospheric mantle.

Tracking halogens through the subduction cycle

The flux of halogens into the Earth’s mantle at subduction zones is a critical yet poorly constrained parameter in the geochemical evolution of the planet. Here we report the first ever combined high-precision measurements of chlorine, bromine, and iodine for backarc basin basalt (BABB) and ocean island basalt (OIB) glasses. The measurements were undertaken in order to evaluate the depth and extent of the halogen subduction cycle by comparing: (1) melts formed in the Manus Basin (Papua New Guinea) proximal to a modern subduction zone, and (2) melts formed from enriched mantle (EM) reservoirs that have been linked to ancient subduction recycling [EM1 and EM2 sampled by the Pitcairn and Society seamounts (central Pacific Ocean), respectively]. As expected from previous studies, the BABBs are strongly enriched in chlorine relative to other trace elements and mid-oceanic ridge basalts (MORB); however, the combined Br/Cl and I/Cl data provide additional insights. The BABBs have I/Cl weight ratios of up to 5.3 × 10–4, that are up to five times higher than typical MORB; and the BABBs with the highest I/Cl have Br/Cl ratios of 2–3 × 10–3, that are lower than typical MORB, and significantly lower than either iodine-rich sediments or seawater-derived sedimentary pore fluids. The final breakdown of iodine-rich serpentine is considered the most likely source of the halogen enrichment in the BABB, suggesting that subduction of serpentinized peridotites enables transport of strongly incompatible, fluid-mobile, volatile elements, like iodine, beyond zones of arc-magma generation. The Pitcairn and Society melts exhibit a remarkable correlation between K/Cl and 87Sr/86Sr. The K/Cl ratios vary from MORB-like values of ~15 to maxima of ~40 in the isotopically most enriched EM end members. The trend reflects the lower subduction efficiency of halogens compared to K and other lithophile elements. Melts formed from EM and MORB mantle reservoirs have very similar Br/Cl and I/Cl weight ratios of 3.6 ± 0.8 × 10–3 and 85 ± 42 × 10–6 (2 ) respectively, that could indicate that subducted volatiles have been mixed throughout the mantle.

Timing of gold mineralisation in the western Lachlan Orogen, SE Australia: A critical overview

The western sub-province of the Paleozoic Lachlan Orogen in Victoria is dominated by thick turbidite sequences overlying Cambrian basement volcanics. The region was subjected to multiple Cambrian to Late Devonian regional deformation events, followed by extensive post-tectonic granitic magmatism. The western Lachlan Orogen is considered a typical ‘orogenic’ gold province and hosts a large number of goldfields, including the world-class Bendigo–Ballarat goldfields. A variety of geochronological methods (e.g. U–Pb zircon; Re–Os sulfide; 40Ar/39Ar whole-rock, mica) have been used to constrain the timing of gold mineralisation, and the relationship to metamorphism/deformation/magmatism. Regional granitic magmatism is relatively well constrained from U–Pb zircon dating, with the timing of deformation/metamorphism and gold mineralisation reliant largely on 40Ar/39Ar dating results. Owing to inconsistencies in the available 40Ar/39Ar data and recent revisions to 40Ar/39Ar monitor ages and decay constants, we recalculate and re-evaluate all existing 40Ar/39Ar age results. These revisions confirm that the western Lachlan Orogen is characterised by multiple deformation/metamorphism events, with the Stawell structural zone deformed during the ca 500 Ma Delamarian and ca 445 Ma Benambran orogenies, the Bendigo Zone deformed during the Benambran orogeny (with minor Tabberabberan overprinting), and the Melbourne Zone affected by the ca 380–370 Ma Tabberabberan orogeny. Post-tectonic granitic magmatism occurred in two main time intervals, the Early Devonian (ca 400 Ma) and the Late Devonian (ca 380–370 Ma), with the former limited to the Stawell and northwest Bendigo Zones, and the latter distributed throughout the Bendigo and Melbourne Zones and southeast Stawell Zone. Gold mineralisation occurred in two main episodes at ca 445 Ma and ca 380–370 Ma, with another possible (minor) event at ca 410–400 Ma. The ca 445 Ma event is prevalent across the Stawell and Bendigo Zones, with Late Devonian gold mineralisation restricted to the Melbourne and eastern Bendigo Zones. The timing of the two main events is supported by geological constraints, the reproducibility of 40Ar/39Ar results and, in the case of the Bendigo goldfield, coincidence with Re–Os data. Suggestions of a single Devonian age gold mineralisation event are not supported by the available data. The two main gold mineralisation episodes (ca 445 Ma; ca 380–370 Ma) coincide with the waning stages of the Benambran and Tabberabberan orogenies, respectively. Crustal thickening and consequent metamorphic devolatilisation during the Benambran orogeny may have been the main cause of fluid flow related to gold mineralisation at ca 445 Ma. In contrast, crustal anatexis is considered responsible for metamorphic fluid generation and Early Devonian gold mineralisation.

Noble Gases and Halogens in Fluid Inclusions: A Journey Through the Earth’s Crust

1. Introductory Chapter.- 2. Noble Gases in ice cores: Indicators of the Earths climate history.- 3. Ocean circulation traced by noble gases.- 4. Groundwater circulation and palaeotemperatures by noble gas geochemistry.- 5. Noble gases as environmental tracers in porewater of lacustrine or oceanic sediments and in fluid inclusions of stalagmites.- 6. Extraterrestrial He in sediments: From recorder of asteroid collisions to timekeeper of global environmental changes.- 7. Application of noble gases to the viability of CO2 storage.- 8. Noble gases in oil and gas accumulations 9. The analysis and interpretation of noble gases in modern hydrothermal systems.- 10. Inclusion trapped fluids: tracing ancient fluids using noble gases.-.11. Noble gases as tracers of of mantle processes and magmatic degassing.

Kinematics of the Høybakken detachment zone and the Møre–Trøndelag Fault Complex, central Norway

The tectonic disintegration of the Caledonian orogen through combined extension, contraction and strike-slip was characterized by spatial and temporal strain partitioning through a period of at least 30 Ma. Early to Mid-Devonian exhumation of the Central Norway basement window was associated with retrograde, top-to-the-SW extensional shearing in the Høybakken detachment zone, sinistral shearing along the Møre–Trøndelag Fault Complex, and formation of extension-parallel folds. Progressive exhumation led to increasing strain localization and to the transition from ductile to brittle deformation. In the interval between c. 370 and 320 Ma, the Høybakken detachment fault cut previously folded detachment mylonites, capturing mylonites in its hanging wall. 40Ar/39Ar mica and K-feldspar ages indicate a Late Devonian or younger age for the uppermost parts of the adjacent ‘Old Red’ basin. Gentle folding of this stratigraphic level attests to the continuation of shortening and orogen-oblique extension into Late Devonian–Carboniferous time. Shortening was intensified along strands of the Møre–Trøndelag Fault Complex, as shown by mutually cross-cutting reverse and strike-slip faults. ‘Flower structures’ may be particularly common in constrictional strain systems where strike-slip faults develop parallel to the principal elongation trend, but normal to the principal axis of shortening.

Mineral system analysis of the Mt Isa–McArthur River region, Northern Australia

The Mt Isa–McArthur region is renowned for a range of commodities and deposit types of world-class proportions. The region is described here in the context of a ‘mineral system,’ through consideration of processes that operate across a range of scales, from geodynamics and crustal architecture, to fluid sources, pathways, drivers and depositional processes. The objective is to improve targeting of Pb–Zn, Cu and Cu–Au deposits. Repeated extension and high heat flow characterise much of the history prior to 1640 Ma. The pre-Barramundi Orogeny (pre-1.87 Ga) metamorphic basement was the substrate on which a volcanic arc developed, focussed along the Kalkadoon-Leichhardt Belt. This is related to an inferred east-directed subduction between 1870 and 1850 Ma. From 1755 to 1640 Ma, three successive volcano-sedimentary basins developed, the Leichhardt, Calvert and Isa Superbasins, in an interpreted distal back-arc environment. The Isan Orogeny, from 1640 to 1490 Ma, overlapped with Isa Superbasin sedimentation, suggesting a transition from back-arc to a foreland basin setting. Most crustal thickening occurred in the Eastern Fold Belt, an area earlier characterised by thinned crust and deep marine environments. This region was deformed into nappe-like structures with high-temperature–low-pressure regional metamorphism and associated granites; the latter are absent from the Western Fold Belt. Metal deposition mainly occurred late in the history, with all known (and preserved) major base metal occurrences either hosted by Isa Superbasin rocks or formed during the Isan Orogeny. Earlier superbasins were potential fluid source regions. Sedimentary formation waters, metamorphic and magmatic fluids were present at prospect scale, while meteoric and possibly mantle sources are also implicated. The spatial distribution of metallogenic associations (i.e. iron oxide–copper–gold, Pb–Zn–Ag, U, Au) across the inlier may result from differences in the geodynamic make-up and evolution of the pre-1.87 Ga tectonic elements. Penetrative faults are interpreted as predominantly steeply dipping and to have acted as pathways for fluids, both in extension and compression. Fluid mixing was a potentially significant ore deposit control. Examples are drawn from the Ernest Henry iron oxide–copper–gold-related hydrothermal breccias in the east and from the Mt Isa Copper deposit in the west. Stress switching during late-stage deformation appears to have triggered a fluid mixing event that led to formation of the major copper deposits.

Deeply dredged submarine HIMU glasses from the Tuvalu Islands, Polynesia: Implications for volatile budgets of recycled oceanic crust

Ocean island basalts (OIB) with extremely radiogenic Pb-isotopic signatures are melts of a mantle component called HIMU (high µ, high 238U/204Pb). Until now, deeply dredged submarine HIMU glasses have not been available, which has inhibited complete geochemical (in particular, volatile element) characterization of the HIMU mantle. We report major, trace and volatile element abundances in a suite of deeply dredged glasses from the Tuvalu Islands. Three Tuvalu glasses with the most extreme HIMU signatures have F/Nd ratios (35.6 ± 3.6) that are higher than the ratio (∼21) for global OIB and MORB, consistent with elevated F/Nd ratios in end-member HIMU Mangaia melt inclusions. The Tuvalu glasses with the most extreme HIMU composition have Cl/K (0.11–0.12), Br/Cl (0.0024), and I/Cl (5–6 × 10−5) ratios that preclude significant assimilation of seawater-derived Cl. The new HIMU glasses that are least degassed for H2O have low H2O/Ce ratios (75–84), similar to ratios identified in end-member OIB glasses with EM1 and EM2 signatures, but significantly lower than H2O/Ce ratios (119–245) previously measured in melt inclusions from Mangaia. CO2-H2O equilibrium solubility models suggest that these HIMU glasses (recovered in two different dredges at 2500–3600 m water depth) have eruption pressures of 295–400 bars. We argue that degassing is unlikely to significantly reduce the primary melt H2O. Thus, the lower H2O/Ce in the HIMU Tuvalu glasses is a mantle signature. We explore oceanic crust recycling as the origin of the low H2O/Ce (∼50–80) in the EM1, EM2, and HIMU mantle domains.

Noble gas and carbon isotope ratios in Argyle diamonds, Western Australia: Evidence for a deeply subducted volatile component

The Argyle lamproite pipe of Western Australia contains diamonds formed at depths exceeding 150 km. We undertook noble gas and carbon isotope ratio (δ13C) analyses of three diamonds (likely of eclogitic paragenesis) from the Argyle lamproite to test for the possible presence of deeply subducted volatile components, and to further constrain the noble gas evolution of the Earths mantle. The Argyle diamonds are characterised by mantle 3He (with 3He/4He ratios of 0.79 R A to 0.25 R A, where R A is the atmospheric 3He/4He ratio of 1.4 × 10–6), small excess Ar and Xe isotope anomalies relative to atmospheric components, and δ13C values of –11.6 to –10.2‰ VPDB. These observations indicate that noble gas and carbon isotopic compositions of the mantle where the Argyle diamonds formed, represent mixtures of an intrinsic mantle component with sedimentary and atmospheric components that may have been introduced through subduction processes.