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Abstract

The Timmins-Porcupine gold camp, with a total production of more than 2,125 tonnes (75 Moz) Au to date, represents the largest Archean orogenic greenstone-hosted gold camp worldwide in terms of total gold production. The gold deposits of the camp are distributed over 50 km of strike length along the Destor-Porcupine fault zone, including the giant Hollinger-McIntyre and Dome deposits. These two deposits are archetype examples of large Archean orogenic gold systems. The Dome mine, where the ore is centered on a folded unconformity between Tisdale volcanic rocks and Timiskaming sedimentary deposits, also illustrates the spatial relationship between large gold deposits and a regional unconformity. Gold-associated hydrothermal activity in the camp spanned a long period of time, as illustrated by early stage, barren to low-grade ankerite veins formed between ca. 2690 and 2674 Ma, i.e., prior to or very early in the development of the regional unconformity and sedimentation of the Timiskaming assemblage. Such early carbonatization may represent a key hydrothermal event in the formation of large orogenic gold deposits and illustrates the protracted nature of the large-scale CO2-rich metasomatism occurring before and during gold deposition. The bulk of the gold is, however, younger than the Three Nations Formation in the upper part of the Timiskaming assemblage (i.e., ≤2669 ± 1 Ma) and consists mainly of syn-main regional shortening deformation (D3) networks of steeply to moderately dipping fault-fill quartz-carbonate ± tourmaline ± pyrite veins and associated extensional, shallow to moderately dipping arrays of sheeted and sigmoidal veins hosted in highly carbonatized and sericitized rocks.

Formation of the gold deposits of the Timmins-Porcupine camp can be related to several key factors. The Destor-Porcupine fault zone represents a first-order control on the location of the camp as this major fault zone allowed large scale CO2-rich hydrothermal fluid upflow. The fault zone also controlled the location of the Timiskaming clastic basin, which is thought to have been developed as a result of early-stage synorogenic extensional faulting. Several of the orogenic gold deposits of the camp are spatially associated with the regional unconformity separating folded submarine volcanic rocks of the Tisdale assemblage form the syn-orogenic sedimentary deposits of the Timiskaming assemblage. The current level of erosion is deep enough to expose the unconformity and to maximize the chance of discovery of the orogenic deposits or their footprint, but allowed for preservation of at least part of the gold deposits that are mainly hosted in the highly reactive Fe-rich Tisdale basalt. Additional key factors include the presence of komatiitic and/or basaltic komatiite flows, of competent intrusions that predate the main phase of shortening of the belt and the occurrence of bends in the trace of the Destor-Porcupine fault zone that may have facilitated focus to ore-forming fluid upflow. Furthermore, the camp is characterized by complex structural and rheological discontinuities, competency contrasts, and early stage folds with associated fracture and fault networks that provided highly favorable ground preparation conditions. The exceptional gold enrichment of the camp requires that the hydrothermal fluids originated from favorable source rocks, lending support to the concept of provinciality, which may best explain the exceptional gold fertility of the southern Abitibi greenstone belt.

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