Damien Gaboury

Does gold in orogenic deposits come from pyrite in deeply buried carbon-rich sediments?: Insight from volatiles in fluid inclusions

The origin of volatiles in fluid inclusions was reviewed for testing the involvement at depth of carbonaceous-pyritic sedimentary rocks as the source for orogenic gold mineralization. Fluid inclusions from selected deposits were analyzed by solid-probe mass spectrometry. Fluids are mostly aqueous-carbonic, with variable amounts of N 2 , CH 4 , C 2 H 6 , Ar, H 2 S, H 2 and He. For fluids with CH 4 and C 2 H 6 , their ratios (C 1 /C 2 ) range from 2.6 to 25.5, indicating that C 2 H 6 is sourced from thermally degraded organic matter. Proportions of CO 2 , CH 4 , C 2 H 6 and H 2 are highly variable and can be explained by hydrothermal reactions where C 2 H 6 is degraded to CO 2 by water consumption. Such reactions may account for the problematic CO 2 -rich, H 2 O-poor fluids associated with some of the richest gold districts. Conditions needed for C 2 H 6 degradation are also fundamental for forming gold deposits, such as HS – -enriched fluids for carrying gold and local weakly oxidizing conditions for promoting gold precipitation. The C 2 H 6 content is recorded in fluids from Mesoarchean to Cretaceous gold deposits, providing support for a general model where fluids and gold were sourced from deeply buried, carbon-rich, and pyrite-gold–bearing sedimentary rocks.

The Key Tuffite, Matagami Camp, Abitibi Greenstone Belt, Canada: petrogenesis and implications for VMS formation and exploration

The Key Tuffite is a stratigraphic marker unit for most of the zinc-rich volcanogenic massive sulfide deposits of the Matagami Camp in the Abitibi Greenstone Belt. This 2- to 6-m-thick unit was previously interpreted as a mixture of ash fall (andesitic to rhyolitic tuffaceous components) and volcanogenic massive sulfide (VMS)-related chemical seafloor precipitate (exhalative component). Previous attempts to develop geochemical exploration vectoring tools using metal content within the Key Tuffite were mostly inconclusive due to the complex nature of the Key Tuffite unit and a poor understanding of its composition, origin and relationship with the VMS-forming hydrothermal systems. Detailed mapping and thorough lithogeochemistry of the Key Tuffite in the vicinity of the Perseverance and Bracemac-McLeod deposits indicate that the Key Tuffite is a homogeneous calc-alkaline, andesitic tuff that was deposited before the VMS deposits were formed. The unit is mostly devoid of exhalative component, but it is strongly hydrothermally altered close to orebodies. This is characterized by a strong proximal chloritization and a distal sericitization, which grades laterally into the unaltered Key Tuffite. Neither the Key Tuffite nor the ore was formed by seafloor exhalative processes for the two studied deposits. This probably explains why previously proposed exploration models based on metal scavenging proved unsuccessful and suggests that a re-evaluation of the exhalative model should be done at the scale of the mining camp. However, as shown in this study, hydrothermal alteration can be used to vector towards ore along the Key Tuffite.

Chlorine in mantle-derived carbonatite melts revealed by halite in the St.-Honoré intrusion (Québec, Canada)

Mantle-derived carbonatites are igneous rocks dominated by carbonate minerals. Intrusive carbonatites typically contain calcite and, less commonly, dolomite and siderite as the only carbonate minerals. In contrast, lavas erupted by the only active carbonatite volcano on Earth, Oldoinyo Lengai, Tanzania, are enriched in Na-rich carbonate phenocrysts (nyerereite and gregoryite) and Na-K halides in the groundmass. The apparent paradox between the compositions of intrusive and extrusive carbonatites has not been satisfactorily resolved. This study records the fortuitous preservation of halite in the intrusive dolomitic carbonatite of the St.-Honore carbonatite complex (Quebec, Canada), more than 490 m below the present surface. Halite occurs intergrown with, and included in, magmatic minerals typical of intrusive carbonatites; i.e., dolomite, calcite, apatite, rare earth element fluorocarbonates, pyrochlore, fluorite, and phlogopite. Halite is also a major daughter phase of melt inclusions hosted in early magmatic minerals, apatite and pyrochlore. The carbon isotope composition of dolomite (δ13C = −5.2‰) and Sr-Nd isotope compositions of individual minerals (87Sr/86Sri = 0.70287 in apatite, to 0.70443 in halite; eNd = +3.2 to +4.0) indicate a mantle origin for the St.-Honore carbonatite parental melt. More radiogenic Sr compositions of dolomite and dolomite-hosted halite and heavy oxygen isotope composition of dolomite (δ18O = +23‰) suggest their formation at some time after magma emplacement by recrystallization of original magmatic components in the presence of ambient fluids. Our observations indicate that water-soluble chloride minerals, common in the modern natrocarbonatite lavas, can be significant but ephemeral components of intrusive carbonatite complexes. We therefore infer that the parental magmas that produce primary carbonatite melts might be enriched in Na and Cl. This conclusion affects existing models for mantle source compositions, melting scenarios, temperature, rheological properties, and crystallization path of carbonatite melts.

Flat vein formation in a transitional crustal setting by self-induced fluid pressure equilibrium — an example from the Géant Dormant gold mine, Canada☆

Abstract Gold-bearing veins grossly define a bimodal distribution within the Earths crust as demonstrated by epithermal- (0 to 2 km) and mesothermal-type (∼5 to 15 km) deposits. Vein formation in the epithermal and mesothermal environments is commonly attributed to the suction pump and the fault-valve mechanisms, respectively. Characteristics of the Geant Dormant gold-bearing vein network are compatible with neither mechanism. In this paper, vein morphology and geometry, alteration styles, and host rock characteristics are used to constrain the following empirical parameters: tectonic regime, fluid flow vectors, crustal depth, host rock permeability, fluid pressure, mineral precipitating conditions and scale of filling processes (zoning). Based on these parameters, a three-stage model for vein formation is envisaged. The self-equilibrating mechanism involves the formation of mostly flat veins stacked along cross-stratal dikes within an impermeable volcanic pile. The dikes served as flow-restricted fluid feeders and as conduits for fluid discharge to the paleosurface. During the stable prefailure stage, the dike conduits acted as dampers in controlling the fluid discharge rate and in keeping fluid pressure at a constant level needed for the opening of preexisting fractures for vein formation (Pf≈σ1+0.3T) at specific vertical intervals. The formation of veins corresponds to an equilibrating process that releases differential fluid pressure (ΔP) built up vertically in the flow-restricted conduits. The ΔP is induced by the decrease of the lithostatic pressure as long as the hydrothermal fluids move upward at a low velocity. At a critical state, when the deepest veins cannot physically absorb more fluid pressure accumulation, the excess fluid pressure (ΔP) is then transferred upwards along the QFP dikes, leading to the failure of the equilibrium process for vein formation at the network-scale (failure stage). The postfailure stage involves draining to the paleosurface of the underlying pressurized hydrothermal reservoir. At an advanced state, hydrothermal self-sealing leads progressively to the restoration of the initial, prefailure, flow-restricted conditions of the dike conduits. The proposed model involves a crustal depth of 2–5 km and a near-neutral tectonic regime. These characteristics are intermediate to those involved for the suction pump and the fault-valve mechanisms and suggest that each tectonic regime has an optimal crustal depth for the formation of gold-bearing veins.