Lithogeochemical and Mineralogical Methods For Base Metal and Gold Exploration

Abstract

Specific criteria used for exploration for new ore may be derived from genetic models. These might include specific lithological assemblages, fractionation trends, alteration assemblages and ore-controlling structures, for example. Three "lithogeochemical" methods of use in exploration include: diagnostic petrogenetic trends, obtained from geographical or statistical analyses of major and minor element data; diagnostic mineral assemblages, obtained through petrographic and XRD analyses; and specific elemental signatures (gains, losses, and isotopic shifts), also obtained from analytical data.Volcanogenic massive sulphide deposits formed from high-temperature metalliferous fluids generated in the sub-seafloor through heating (from a subvolcanic intrusion) of downwelling seawater. Both the subvolcanic intrusions and related volcanic rocks have somewhat aberrant petrochemical trends, caused by unusually rapid heat removal to the hydrothermal system; extensive fractionation is evident in both major element and REE trends. Alteration includes lower semi-conformable horizons, albite-epidote-actinolite-quartz zones, and under some deposits, broad carbonatized zones. Alteration pipes vary from those with cores of Fe-chlorite and silica and rims of Mg-chlorite (after smectite), through Mg-chlorite core and sericite-rim pipes, to silica-sericite ± Fe-carbonate pipes. All are Na-, Ca-and Sr-depleted.Lode-gold deposits are associated with major transgressive (typically high-angle reverse) fault zones. Vein systems typically occur either in dilational jogs, near fault terminations, or at contacts between units with high ductility contrast. Regional alteration is dominated by CO 2 addition. Iron-dolomite and/or ankerite are most common near the deposits, but dolomite or calcite form the regionally developed alteration assemblage. Sphene occurs distally, but rutile is common near vein systems. Sericite and albite or K-spar occur within a few tens of metres or less of the deposits. Magmatic sulphide deposits formed by segregation of immiscible sulphide liquid from a parent mafic or ultramafic magma. Deposits occur in intrusions and flows with unusually high Mg/Fe ratios. Nickel is depleted relative to Mg in fertile intrusions. Olivine in "barren" intrusions is rich in Ni, compared with "fertile" intrusions. Because sulphur saturation is promoted by assimilation of sulphur from an external source, Se/S ratios are higher in ore-bearing intrusions. Sulphur isotopes may deviate strongly from mantle compositions because of assimilation, particularly in post-Archean intrusions.

Geodynamic setting, crustal architecture, and VMS metallogeny of ca. 2720 Ma greenstone belt assemblages of the northern Wawa subprovince, Superior Province

“…The dominance of siderite and chloritoid alteration associated with the Vanguard VMS prospect (Farrow 1993;Lodge and Chartrand 2013) is typical of Mattabi-type alteration systems. At the Mattabi VMS deposit, the siderite and chloritoid alteration is interpreted to have formed in relatively shallow water during effervescence of CO 2 that was derived from magmatic fluids or CO 2 -enriched seawater (Morton et al 1991;Franklin 1997;Holk et al 1998). The Calvert-Stares VMS prospect is dominated by Zn-Au mineralization with muscovite and iron-carbonate alteration (Bottrill 2003) and is typical of boiling hydrothermal system that may have had input from magmatic fluids, resulting in a volatile-rich, hydrothermal fluid capable of mobilizing gold (Hannington et al 1995;Dubé et al 2007).…”

Normative minerals and alteration indices developed for mineral exploration

“…The use of MgO and K 2 O in the A variable reflects the formation of magnesian chlorite and sericite as alteration products. This choice is very specific to the VMS environment (Franklin, 1997; among others) and may not be applicable in other environments. Proper choice of F and A in Eq.…”

“…As a consequence, the lithogeochemistry of altered rocks has commonly been used to characterize the intensity of hydrothermal alteration in almost all types of deposits, including massive sulphide (Franklin, 1997), mesothermal gold (Campbell and Kerrich, 1994), epithermal (Cooke et al, 1996) and porphyry deposits (Beane, 1982). Several geochemical calculations have been developed to quantify the various types of hydrothermal alterations for mineral exploration purposes.…”

Architecture et origine du système de minéralisation polymétallique du secteur lac Line, région de Chibougamau, Québec /

“…(Barrett et MacLean, 1994b;Franklin, 1997;Gifkins et al, 2005). Dans ce genre de système, le fluide hydrothermal de base est l'eau de mer, et sa chimie (donc l'altération et la minéralisation en résultant) varie en fonction des contributions de fluides magmatiques provenant de l'intrusion sous-jacente, de la température et des réactions qui surviennent avec la roche encaissante.…”

“…Par opposition, un système hydrothermal relié au métamorphisme et à la déformation régionale est plutôt caractérisé par la circulation de fluides plus uniformes d'origine métamorphique, principalement riches en CO2 (Goldfarb et al, 2005). Ces derniers ont pour effet une carbonatation importante intimement associée à la minéralisation au sein même des veines (Franklin, 1997;Goldfarb et al, 2005), et donc des changements de masse traduisant les effets de cette carbonatation, ce qui n'est pas vraiment le cas dans cette étude. …”