The use of lithogeochemistry in delineating hydrothermal fluid pathways and vectoring towards gold mineralization in the Malartic district, Québec

• Published in: Ore geology reviews Vol. 120; p. 103351
• Main Authors: Gaillard, Nicolas, Williams-Jones, Anthony E., Clark, James R., Salvi, Stefano, Perrouty, Stéphane, Linnen, Robert L., Olivo, Gema R.
• Format: Journal Article
• Language: English
• Published: Elsevier B.V 01.05.2020; Elsevier
• Subjects: Applied geology; Canadian Malartic gold deposit; Earth Sciences; Exploration vectors; Geochemistry; Mass transfer calculations; Metasomatic haloes; Molar element ratio analysis; Sciences of the Universe; Whole-rock lithogeochemistry; Molar element ratio analysis; Canadian Malartic gold deposit; Whole-rock lithogeochemistry; Mass transfer calculations; Exploration vectors; Metasomatic haloes
• ISSN: 0169-1368; 1872-7360
• DOI: 10.1016/j.oregeorev.2020.103351

[Display omitted] •Mass change calculations are used to evaluate the magnitude and spatial distribution of the metasomatic processes in the Malartic gold district.•A statistical method based on Z-score calculations is employed to constrain the magnitude and extent of the lithogeochemical haloes.•Ore-associated pathfinder elements (e.g., W, Te and Ag) outline broad enrichment patterns that extend several kilometers laterally from the Canadian Malartic deposit.•Molar element ratio analysis is a reliable method for assessing the mineralogical controls on alkali metasomatism.•Whole-rock lithogeochemistry provides a valuable tool with which to define vectors toward gold mineralization in a regional exploration context. The world-class, oxidized intrusion-related Canadian Malartic gold deposit, with reserves estimated at 5.56 Moz Au grading 1.10 g/t Au, and a total geological endowment of 16.3 Moz Au, is one of the largest gold deposits in the Archean Superior Province of Canada. The gold mineralization is hosted predominantly by Pontiac Group metasedimentary rocks, Piché Group metavolcanic rocks, and quartz monzodiorite to granodiorite porphyritic intrusions. The ore takes the form of a low-grade envelope of disseminated pyrite (0.35 to 1 g/t Au) grading inwards into higher grade (>1 g/t Au) stockwork and breccia zones. Hydrothermal alteration in the metasedimentary rocks is zonally distributed around the fluid pathways. Proximal alteration is characterized by a microcline±albite-quartz replacement-type assemblage, with lesser phlogopite, calcite±Fe-dolomite, pyrite and rutile. The distal alteration assemblage comprises biotite, microcline±albite, phengite, quartz, calcite, pyrite and rutile. In this study, we assess the magnitude and distribution of fluid-rock interaction in the metasedimentary rocks of the Malartic district. The metaturbidites are separated into four lithotypes based on grain-size to reduce the effects of primary depositional processes on mass change calculations. Despite the variability in protolith composition, the metasedimentary rocks define a geochemically consistent, cogenetic sequence. The results of the mass transfer calculations indicate progressive gains in CO2, S, K2O and LOI, as well as Au, Te, W, Ag, As, Be, Sb, Bi, Mo and Pb, from background, to distal and proximal alteration zones (relative to the least-altered samples). Molar element ratio analysis (alkali/aluminum) indicates an increase in alkali metasomatism (K and Na) adjacent to the main hydrothermal fluid pathways, which is manifested by the progressive stabilization of microcline and albite at the expense of oligoclase, biotite and white mica. Ore-associated pathfinder elements delineate broad enrichment patterns around the deposit, and are used to understand hydrothermal fluid circulation in the Malartic district. A statistical approach based on a comparison of the mass change results with the background composition provides robust constraints on the magnitude and extent of the lithogeochemical haloes. Generally, the alteration forms envelopes that extend along the S2 fabric, with the largest lithogeochemical anomalies (e.g., Au, W, Te and Ag) reaching up to 10 km in length, and 2 km in width. The results of this study demonstrate that whole-rock lithogeochemistry can provide a valuable tool with which to define vectors toward gold mineralization in a regional exploration context.