Lalor is a recently discovered Au-Zn-rich volcanogenic massive sulphide (VMS) deposit. It is located in the Paleoproterozoic Snow Lake arc assemblage, host to numerous past producing Cu-Zn and Zn-Cu VMS deposits. Lalor is the largest deposit of the Snow Lake camp and also the richest in gold with reserves of 14.4 Mt grading 1.86 g/t Au, 24 g/t Ag, 0.6 wt.% Cu and 7 wt.% Zn and resources estimated at 12.6 Mt grading 3.85 g/t Au, 27.3 g/t Ag, 0.9 wt.% Cu and 2.3 wt.% Zn, for a total size of approximately 27 Mt and potentially containing 75 t Au. The deposit consists of distinct Zn-Cu-Pb±Au-Ag semi-massive to massive sulphide lenses and zones of disseminated Au-Ag-Pb-Cu sulphides. The ore zones are stratigraphically and/or structurally stacked in a complexly deformed and metamorphosed succession of intensely hydrothermally altered rocks of the Chisel mature arc sequence that hosts other Zn-rich VMS deposits. Preliminary mapping and lithogeochemistry results indicate that the stratigraphic footwall is composed of at least three distinct but highly altered mafic to felsic volcanic (and perhaps sedimentary) units. The alteration of the footwall is both extensive and intense. At least 11 distinct alteration assemblages have been defined based on the distribution and relative abundance of specific metamorphic minerals such as amphiboles, chlorite, cordierite, biotite, muscovite, pyrite, staurolite, garnet, kyanite, sillimanite, diopside and epidote. The various alteration assemblages may be in part due to varying protolith compositions, together with the superposition of several hydrothermal events. The hanging wall does not show any extensive alteration and may be in structural contact with the deposit. Five ore types can be defined. They include Zn±Cu-rich massive sulphide lenses and three distinct ore types that contain significant gold: (1) Cu-rich massive sulphides; (2) low sulphide calc-silicate zones with high Ag-Pb-Cu±As-Se-Te and; (3) anthophylliterich alteration zones with trace of finely disseminated pyrrhotite. The numerous alteration assemblages and the various ore styles result from a complex hydrothermal history and possible modifications during subsequent deformation and metamorphism. The gold endowment of the deposit, its size and its distinctive features compared to known anomalous and gold-rich VMS deposits make Lalor an ideal site to document and better understand gold enrichment processes in the VMS environment.
The Flin Flon – Athapapuskow Lake area, situated in the Flin Flon Greenstone Belt, Manitoba, consists of ocean-floor and island-arc assemblages, deformed and metamorphosed during the Trans-Hudson Orogeny (∼1.86–1.69 Ga). A new map of metamorphic mineral assemblages and isograds has been compiled that reveals a largely coherent regional metamorphic sequence increasing in metamorphic grade from prehnite–pumpellyite to amphibolite facies. Regional metamorphism postdates most of the deformation within the area, with the exception of the reactivation of major block-bounding faults. The regional prograde sequence has been subdivided into 10 metamorphic zones, separated by 9 isograds, that describe the transition from prehnite–pumpellyite to greenschist to amphibolite facies. The formation of contact metamorphic aureoles, pre-dating regional metamorphism, record conditions up to amphibolite facies. Equilibrium phase diagrams for the island-arc (low-Mg) and ocean-floor (high-Mg) assemblages were calculated and allow for the evaluation of the modelling techniques and determination of pressure–temperature conditions. Discrepancies between the modelling predictions and natural observations occur due to (1) limitations in the thermodynamic models for some of the complex minerals (e.g., amphibole); and (2) metastable persistence of some minerals to higher grade due to sluggish reaction kinetics. Notwithstanding these discrepancies, the modelling suggests that metamorphosed mafic rocks in the Flin Flon – Athapapuskow Lake area reached about 430–480 °C and 3.0–4.5 kbar. Peak metamorphic conditions within contact aureoles that preceded regional metamorphism did not exceed 500 °C (at a pressure between 2.7 and 4.4 kbar). The metamorphic field gradient records a transition from 250–300 °C/1.5–2.3 kbar to 430–480 °C/3–4.5 kbar (100–150 °C/kbar), defining a geothermal gradient of approximately 25–31 °C/km.
Lalor is a recently discovered auriferous Zn-Cu volcanogenic massive-sulphide deposit. It is located in the Paleoproterozoic Snow Lake arc assemblage, host to numerous past producing Cu-Zn and Zn-Cu volcanogenic massive-sulphide deposits. With an estimated tonnage of 25 Mt of ore (reserves+resources) including 73 t Au, Lalor is the largest volcanogenic massive-sulphide deposit in the Snow Lake area and its Au-rich nature provides a unique opportunity to document processes responsible for precious-metal enrichment in volcanogenic massive-sulphide systems. The Lalor deposit host rocks are predominantly volcanic (± intrusive) rocks that have been variably altered, deformed, and metamorphosed to amphibolite grade. A combination of immobile element geochemistry and petrographic observations is necessary to properly characterize the volcanic rocks due to major postemplacement modifications. Seven distinct chemostratigraphic units and two postvolcanogenic massive-sulphide intrusive (dyke) units are present in the Lalor host succession. Mafic to felsic volcanic units have calc-alkaline to transitional magmatic affinities. Some of these units are compositionally similar to the Moore basalt (units M1a and M1b) and Powderhouse dacite (unit F2) which represent the footwall of the Chisel, Chisel North, Ghost, and Lost volcanogenic massive-sulphide deposits; this suggests that the Lalor deposit is located within the volcanogenic massive-sulphide-fertile uppermost portion of the lower Chisel subsequence. The presence of massive-sulphide ore lenses in calc-alkaline mafic rocks lying above the Powderhouse dacite-like unit indicates the continuation of volcanogenic massive-sulphide-forming hydrothermal activity after the cessation of felsic volcanism in the lower Chisel subsequence. The presence of dykes with a trace-element signature similar to that of the Threehouse basalt, which is present immediately above the other volcanogenic massive-sulphide deposits of the lower Chisel subsequence, suggests the presence of this unit at a higher stratigraphic position in the now structurally truncated sequence.
