File:Quartz-gold hydrothermal vein rock (Neoarchean, 2.67 Ga; Hollinger Mine, Timmins, Ontario, Canada) 5.jpg

English: Quartz-gold hydrothermal vein rock (gold ore) from the Precambrian of Ontario, Canada.

This high-grade gold ore sample is from the famous Hollinger Mine in Timmins, Ontario, one of the richest gold mines on Earth. The whitish-gray material is quartz and the buttery, metallic-lustered material is native gold (Au). Occasional, very small pyrite crystals are present. Not sure what the blackish material is (it’s actually very dark brown) - other minerals reported from the quartz-gold veins in the area include ankerite, albite feldspar, pyrite and other sulfides, scheelite, tourmaline, and various tellurides. Several gold mines in the Timmins area, called the Porcupine Gold Camp, targeted auriferous rocks in and immediately adjacent to a hydrothermal vein system about 5 x 2 kilometers in areal extent. The vein system and its host rocks are Late Archean in age, between 2.8 and 2.6 billion years old.

The gold occurrence at the Hollinger Mine site was discovered in 1909 by Benjamin Hollinger as he removed vegetation from a mound on his claim - this exposed a vein with native gold. Mining occurred from 1909 to 1968. After a half-decade of inactivity, operations resumed at Hollinger in 1973. Gold mining continues today, after several corporate ownership changes in recent decades. The current operator of the Hollinger Mine is a company called Newmont. Remote-controlled, automated drilling equipment is currently used at the site.

The gold-bearing hydrothermal vein system in the Hollinger Mine area occurs in a northeast-southwest striking shear zone that has intensely deformed the vein’s host rocks. The stratigraphy of the area’s host rocks includes three major units (from youngest to oldest): Porcupine Group Tisdale Group Deloro Group The Porcupine Group is a ~3 kilometer thick succession of metamorphosed siliciclastic sedimentary rocks, including turbidites and coarse-grained fluvial deposits. The Tisdale Group is a ~4 kilometer thick succession of 2.703 billion year old ultramafic to intermediate volcanic rocks, including komatiites, basalts, and alkaline dacitic volcaniclastics. The Deloro Group is a ~4.5 to 5 kilometer thick succession of 2.725 billion year old mafic to felsic to alkaline volcanics, plus some iron formation. These rocks have been subjected to regional greenschist-facies metamorphism and structural folding - this occurred during an Archean mountain-building event (orogeny). Various igneous intrusions are also present. Diabase dikes cut, and therefore postdate, the quartz-gold veins. The diabase dikes are ~2.633 Ga, but that date has significant error bars. Albitite dikes in the Porcupine Gold Camp cut and are cut by the gold veins. Emplacement of the albitite dikes occurred at 2.673 Ga. Molybdenite samples from vein rocks at nearby mines date to 2.670-2.672 Ga. These ages are likely good approximate dates for emplacement of the gold veins - albitite dike intrusions apparently generated hydrothermal activity and gold mineralization.

Most gold at the Hollinger Mine (about 95%) comes from altered wall rocks adjacent to the quartz veins. It principally occurs as micron-scale blebs of native gold within pyrite masses. Macroscopic native gold in the veins themselves contributes about 5% of the mine’s gold production. Both vein gold and wall rock gold is naturally mixed with silver (Ag), plus traces of other elements such as iron (Fe) and copper (Cu). Hollinger Mine’s gold is about 10 weight percent silver.

Geologic unit: Hollinger-McIntyre-Coniaurum-Vipond-Moneta-Crown Gold-Quartz Vein System, Abitibi Greenstone Belt, Superior Province, Canadian Shield, Neoarchean, ~2.67 Ga

Locality: unrecorded site at the Hollinger Mine, eastern side of the town of Timmins, Porcupine Gold Camp (Porcupine Mining District), eastern Ontario, southeastern Canada (vicinity of 48º 28’ 15.85” North latitude, 81º 19’ 06.21” West longitude)

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Info. mostly synthesized from:

Brisbin (2000) - World class intrusion-related Archean vein gold deposits of the Porcupine Gold Camp, Timmins, Ontario. pp. 19-35 in Geology and Ore Deposits: the Great Basin and Beyond Symposium Proceedings.

MacDonald & Piercey (2019) - Geology, lithogeochemistry, and significance of porphyry intrusions associated with gold mineralization within the Timmins-Porcupine gold camp, Canada. Canadian Journal of Earth Sciences 56: 399-418.

Wood (1991) - The Hollinger-McIntyre Gold-Quartz Vein System, Timmins, Ontario: Geologic Characteristics, Fluid Properties and Light Stable Isotope Geochemistry. Ontario Geological Survey Open File Report 5756. 289 pp.