Augsten et al. (1986) detailed sulphide mineralogy study of the MacLellan deposit found that in addition to the main sulphide minerals (pyrite, pyrrhotite, arsenopyrite, sphalerite and galena) that chalcopyrite, magnetite, tetrahedrite and freibergite are minor constituents of the mineralization. Gold is present in the mineralization as native gold, electrum and auriferous silver. The average size of the gold grains is 20-30 microns, although grains and veinlets greater than 150 microns were observed.
STYLES OF MINERALIZATION
Gold and / or silver-rich minerals are rare in the MacLellan deposit. Most of the gold occurs within iron-bearing sulphide minerals. Samson and Gagnon (1995) recognise five texturally and / or mineralogically distinct types of sulphide mineralization and an additional two sulphide-free hydrothermal assemblages:
Sedimentary sulphides
Quartz-sulphide-biotite veins
Arsenopyrite ± quartz veins
Quartz-amphibole veins and amphibole alteration
Vein and disseminated sulphides
Carbonate ± quartz veins and carbonate alteration
Quartz ± sulphide veins
STAGES OF MINERALIZATION
The earliest recognizable mineralizing evening in the deposit is thought to be represented by sedimentary sulphides, which were deposited contemporaneously with the volcano-sedimentary sequence and may be considered examples of volcanogenic massive sulphides or sulphide-facies iron formation (Samson and Gagnon 1995). However, these sulphides carry low precious metal values. The first episode of significant gold and silver mineralization accompanies the formation of the quartz-sulphide-biotite veins that predate peak metamorphism, as indicated by the deformed state of the veins and the textural relationship between the veins, their alteration minerals and the metamorphic fabric. The next stage was the formation of auriferous arsenopyrite veins that postdate the quartz-sulphide-biotite veins. With respect to metamorphism, the evidence indicates that the arsenopyrite veins are syn- to late-metamorphic and similarly the amphibole-stage mineralization postdates the arsenopyrite stage.
