The basal gabbro subdivision and associated magmatic nickel-copper sulphide mineralization of the Pants Lake intrusion, Labrador, Canada : a combined geological, petrological, geochemical, and metallogenic study

Smith, Roderick L. (2006) The basal gabbro subdivision and associated magmatic nickel-copper sulphide mineralization of the Pants Lake intrusion, Labrador, Canada : a combined geological, petrological, geochemical, and metallogenic study. Masters thesis, Memorial University of Newfoundland.

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Abstract

The Pants Lake Intrusion (PLI), located in northern Labrador, represents an early phase of the 1.34 to 1.29 billion-year Nain Plutonic Suite that is located amongst the 1.45 to 1.46 billion-year old Harp Lake Intrusive Suite, the 1.85 billion-year Nain-Churchill Province suture zone, and the 2.78 to 1.74 billion-year Churchill Province which actually hosts the intrusion. The early evolution of the Pants Lake Intrusion is similar to that described for the Voisey's Bay Intrusion. -- The PLI is composed of two separate intrusions termed the North and South Pants Lake Intrusion. A U/Pb zircon and baddeleyite igneous crystallization age of 1322.2 ± 2 million years was derived from the north intrusion, whereas a U/Pb baddeleyite igneous crystallization age of 1337 +4/-2 million years was derived from the south intrusion. Apart from geographical proximity, and common origin and host, the north and south intrusions do not share many characteristics; in fact, the north intrusion intrudes the south intrusion. -- The South Pants Lake Intrusion, located in the southern part of the study area, is a weakly layered mafic body that intrudes the Harp Lake Intrusive Suite. The south intrusion is characterized by olivine cumulate-textured peridotite and troctolite compositions, high MgO abundances (up to 21 wt %), elevated light rare earth elements, positive Eu anomalies (Eu/Eu* = 1.19), and Ce/Yb ratios that resembles the range reported for the troctolites of the Voisey's Bay Intrusion (Ce/Yb = 26-31). -- Disseminated, blotchy, net, and les abundant massive textured sulphides are widespread in the south intrusion and are dominated by pyrrhotite with lesser chalcopyrite and pentlandite. Nickel tenors of the sulphide minerals are slightly elevated compared to those in the north intrusion due to the greater percentage of nickel available to partition into the localized sulphide liquid (as indicated by the elevated modal olivine). Sulphur isotopic ratios of sulphide mineral separates from the south intrusion range from -5.25 to -1.99 ‰, indicating the influence of the local sedimentary sulphide source. Partially digested gneissic inclusions are widespread and closely resemble inclusions present in the north intrusion, as well as inclusions described from the Voisey's Bay Intrusion. -- The North Pants Lake Intrusion is spatially much larger (up to 60 km²) than the south intrusion (10 to 15 km²). The north intrusion is a layered, horizontal to sub-horizontal, mafic sill-like body that is divided into the Upper and Basal Gabbro Subdivisions. These two subdivisions are present only in the north intrusion. Collectively the two subdivisions range from 100 to 400 meters thick, with the thickest parts present in the north part of the intrusion. The footwall contact of the intrusion is undulating and locally brecciated which aided the deposition of semi-massive to massive sulphide. Not much is known about the upper contact of the north intrusion, only that there was very little assimilation with the country rock gneiss. -- Unlike the south intrusion, the north intrusion pooled in a secondary magma chamber located in the upper crust prior to its final ascension and lateral emplacement. While in the secondary magma chamber, the magma fractionated, producing a plagioclase and olivine cumulate silicate liquid. With increasing assimilation of the surrounding sulphide-bearing Tasiuyak paragneiss, as indicated by sulphur isotopic ratios of -3.33 to -1.99 ‰, sulphide saturation and subsequent production of an immiscible sulphide liquid was induced within the silicate liquid. This was followed by partitioning of nickel from the silicate magma into the available immiscible sulphide liquid (< 650 ppm Ni in olivines of the north intrusion). The first pulses of fractionated silicate liquid to enter the sill chamber formed the Upper Gabbro, followed by the denser olivine-rich liquid that formed the Basal Gabbro. The point of entry for the silicate liquid feeding the Upper and Basal Gabbro as it entered the sill was near the northwest mapped boundary of the north intrusion. Magma injection into the northern part of the intrusion continued well after the main volume of Basal Gabbro had flowed into the sill. -- As the name implies, the Upper Gabbro forms the uppermost part of the north intrusion and is the more voluminous of the two subdivisions. Variations in the mineralogy and texture of the Upper Gabbro allow for the designation of four primary subunits. These are the Altered Gabbro, Coarse- to Medium-Grained Gabbro, Pegmatitic Gabbro, and Olivine Gabbro. These subunits constitute variations in a medium- to coarse-grained gabbro, containing cumulate plagioclase with intercumulate olivine and clinopyroxene, which are typically void of visible foreign contaminants and sulphides. Chemically, the Upper Gabbro is less varied than its basal counterpart. The Upper Gabbro has slightly elevated SiO₂, Na₂O, A1₂O₃, CaO, Rb, and Sr contents, but much lower FeO, MgO, Ni, Cu, and S concentrations compared to the Basal Gabbro. Rare earth element patterns exhibit slightly decreasing light to heavy rare earth element patterns and an average Europium anomaly of 1.24 which is comparable to the Basal Gabbro. -- The Basal Gabbro, present at the base of the North Pants Lake Intrusion, is approximately 27 meters thick and is distinguished primarily by the presence of partially digested gneissic inclusions coupled with the occurrence of magmatic-textured sulphides similar to the varied textured troctolites and basal breccia sequences described from the Voisey's Bay deposits. The Basal Gabbro is further subdivided into four primary subunits. These constitute the Transition Gabbro, Gabbro Hybrid, Olivine Gabbro, and Leopard Gabbro that collectively forms a recurring sequence present throughout the entire north intrusion. Overall, the major, trace, and rare earth element chemistry of the Basal Gabbro subunits mimics the chemistry of the Upper Gabbro, and where variations are present, they are most typically a direct consequence of gneissic and sulphide influences. -- Magmatic sulphide textures observed in the Basal Gabbro vary from disseminated, blotchy, leopard, to semi-massive and massive. The leopard texture, not present in the northern part of the intrusion, indicates areas of less turbulent flow. The largest massive sulphide intersections discovered to date is 15.7 meters of 1.13 % Ni, 0.78 % Cu and 0.20 % Co. Typical massive sulphide intersections are much smaller (< 1 meter) but have similar to slightly higher grades. Pyrrhotite is the most common sulphide present in the intrusion followed by chalcopyrite and pentlandite. Pentlandite is present as small (< 1 cm) inclusions and exsolutions within the pyrrhotite. Sulphides observed in the footwall are of two types, that is, sulphides remobilized from the main sulphide mass in the Basal Gabbro and unrelated sedimentary sulphides. One remobilized footwall sulphide intersection assayed 11.75 % Ni, 9.70 % Cu and 0.43 % Co over 1.1 meters. -- The North Pants Lake Intrusion is the closest analogue to intrusive rocks at the Voisey's Bay deposits recognized to date in Labrador. The present lack of identification of Voisey's Bay-like mineralization may be a function of observation level within the magmatic system. The northwestern margin of the intrusion offers the best exploration potential due to the recognition of new magma pulses, the potential feeder entry-point, and of course, spatial proximity to the most significant sulphide intersections encountered to date.

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