Abstract
3D geological modeling of lithogeochemical and geological data provides insight into the role of the sulfide ore horizon and associated footwall hydrothermal alteration in localizing shear strain in the Flin Flon volcanogenic massive sulfide deposits, Canada, as deformation evolved from brittle-ductile to ductile regimes during collisional stages of the 1.9–1.8 Ga Trans-Hudson orogeny. 3D spatial characterization of hydrothermal alteration based on the Ishikawa index (AI) and normative corundum percentages outline sericite + chlorite-rich high strain zones, consisting of Al-enriched and Na-depleted felsic and mafic volcanic rocks in the footwall of the sulfide ore horizon. The hydrothermal vent complex, from which these sheared alteration zones originated, was stacked together with the ore horizon by W-vergent thrust faults during an early collisional deformation regime, imbricating molasse-type clastic sediments with the ore-hosting volcanic and volcaniclastic rocks of the Flin Flon arc assemblage. Chlorite-rich planar zones marked by high values of the Carbonate–chlorite–pyrite index (CCPI) are laterally more extensive and outline a later system of ductile shear zones, in which phyllosilicates, quartz and chalcopyrite in stringer zones localized shear strain and enhanced transposition of the hydrothermal vent stockwork. The contrasting deformation styles of these two thrusting events and their localization within the ore horizon and hydrothermal vent stockwork have important implications for vectoring towards undiscovered ore in this mature mining camp that are possibly also relevant to other strongly deformed VMS ore systems.
Highlights
The identification, classification and zoning of hydrothermally-altered rocks play a prominent role in vectoring towards volcanogenic massive sulfide (VMS) deposits
In VMS deposits metamorphosed to grades below lower amphibolite facies, these upflow zones are identified by intensely chloritized feeder zones with local mineralized and silicified quartz-sulfide stringers that are enveloped by broader zones of aluminous phyllosilicates, including sericite, pyrophyllite, and phengite [2,3]
Trend 1: This trend is typical of weak sericitization at the margins of the hydrothermal system [4]. This trend is not present in the samples of mafic volcanic rocks and is ambiguous for some of the felsic volcanic rock samples. This trend is poorly represented in the Flin Flon lithogeochemical data and is likely due to the truncation of the laterally extensive conformable hydrothermal alteration zone at deeper levels in the footwall by D3 thrust faulting
Summary
The identification, classification and zoning of hydrothermally-altered rocks play a prominent role in vectoring towards volcanogenic massive sulfide (VMS) deposits. In VMS deposits metamorphosed to grades below lower amphibolite facies, these upflow zones are identified by intensely chloritized feeder zones with local mineralized and silicified quartz-sulfide stringers that are enveloped by broader zones of aluminous phyllosilicates, including sericite, pyrophyllite, and phengite [2,3] This zoning in hydrothermal alteration minerals results predominantly from variations. Many alteration indices for these sericite-producing (Na depleting) and chlorite-by-sericite-replacement (Mg–Fe gain–K depletion) reactions, have been proposed [4,5,6,7], while their effectiveness as proxies for mapping the intensity of hydrothermal alteration has been convincingly demonstrated in many mining districts [4,8,9] In addition to these phyllosilicate alteration assemblages, carbonate is a common constituent of altered felsic and mafic volcanic rocks, in the immediate footwall and hanging wall of VMS ore zones [4]
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