Introduction

Snow Lake has had several volcanogenic massive sulfide (VMS) mines (operated by Hudson Bay Mining & Smelting Company), the most recent of which was the Photo Lake deposit, and shear-zone hosted gold deposits, including the New Britannia mine operated by TVX Gold. The VMS deposits were metamorphosed at 1.8 Ga and the gold deposits were created during that metamorphism.

Massive sulfide ore from the Photo Lake Cu-Zn-Au VMS deposit that is mostly chalcopyrite with cubes of pyrite in grayish pyrrhotite. These pyrite cubes display a deformational recrystallization texture. Some of the ore was high enough grade (>20 weight % Cu) that it was trucked directly to the smelter without milling.

Synmetamorphic mobilization and reconcentration
of ore within VMS deposits

The VMS deposits are clearly older than metamorphism, but amphibolite facies metamorphism changed the mineral assemblages in the wall rocks (Chl + Ms + Bt schists with Grt + St + Ky + Pl porphyroblasts), redistributed ore within the deposits, and moved some of the gold out of the deposits.

Backscattered electron image of a magnetite grain in massive sulfide ore from the Photo Lake deposit. The magnetite grain was dissected by M3 serpentine, which also surrounds gold-silver grains. Gold may have been mobile during M2 and M3.

Mobility of sulfides in wall rocks

Wall-rocks within 5 m of the massive sulfide layer are garnet + chlorite + quartz schists with minor magnetite. Curiously, the garnet contains inclusions of chalcopyrite and arsenopyrite in much greater amounts than does the matrix, suggesting that sulfides were removed from the rock after garnet had started to grow. Where the sulfides went is uncertain, but one possibility is into the massive sulfide layer, which has substantially higher Cu grades than most VMS deposits.

These are X-ray compositional maps of Fe and Mn content of garnet in a chlorite schist near the massive sulfide ore.

Pyrite and chalchopyrite occur mainly as inclusions in the core of the garnet. By contrast, magnetite occurs in the matrix and as inclusions in the garnet rim, indicating a change of assemblage during garnet growth and removal of sulfides. Those sulfide grains included in garnet were protected and are preserved as relicts of the the earlier assemblage. The white line marks the position of the microprobe traverse.

 

Quartz inclusions change from thin in the garnet core to much smaller and very thin at 1.3 mm from the rim, reflecting F2 deformation. Continued metamorphism after D2 coarsened quartz grains, and some of these are included in the garnet rim. The change of assemblage and loss of sulfides occurred during development of the S2 schistosity and F2 folding. These compositional maps are especially useful because they make it easy to correctly interpret the zones of low Mn around sulfide inclusions as a post-growth diffusional modification feature.

False color backscattered electron image of plagioclase in the same sample. Plagioclase grains in the matrix can be correlated with regions in the garnet zoning on the basis of the size of quartz inclusions. An44 in the center of this grain correlates with the zone of very thin quartz inclusions in the garnet.

With a detailed interpretation of the reaction history and the sequence of mineral compositions, we can calculate that D2 deformation was associated with 1-2 kbar of increasing pressure and was followed by another 10-30 °C increasing temperature.

This P-T path would not normally produce the observed change of assemblage (sulfides to magnetite) at garnet grade. The best interpretation is that the there was an event of whole-rock compositional alteration (loss of sulfides) during metamorphism and deformation.

Mobility of trace elements in wall rocks

Other rocks preserve evidence for cryptic alteration displayed in compositional zoning of trace elements. Two sets of laser-ablation pits are shown on this polished hand sample of garnet in a chlorite schist: a trail of large pits oriented at 45° and a nearly horizontal trail of 50 micron pits. Field of view is approximately 20 mm.

Compositions of selected elements in garnet determined by laser-ablation ICP-MS of the smaller pits illustrated above. The heavy REE are selectively incorporated into garnet, and they show the expected overall decrease from core to rim. Superimposed on this trend, however, is a spike in all the REE at the color boundary. The composition of the spike does not suggest that it is the result of consumption of some REE-rich phase, and the possibility remains that the spike is the result of REE mobility in the rocks at about the same time as sulfides were mobile (see above).

Major alteration in some zones of the VMS deposits

There are spectacular examples of synmetamorphic alteration in some chlorite schists in 2- to 5-m long sections of diamond-drill core from several alteration zones in the Snow Lake area.

These sections of drill core are chlorite (pale green) schists that contain variable proportions of porphyroblasts of plagioclase (white), amphibole (dark green), and staurolite (in other samples), together with variable amounts of biotite (brown), and sulfides and magnetite (black). Thin section, field of view is 2 mm.

Plagioclase (white) porphyroblasts in some of these chlorite (pale green) schists contain abundant inclusions of quartz and biotite (brown) in their cores, but not in the rims. Note that biotite is rare in the matrix. The texture suggests that the mineral assemblage changed during plagioclase growth. Furthermore, because biotite is the only K-bearing mineral, the textures suggest that K2O was removed from the rock during metamorphism. Na2O may also have been removed, as suggested by the increase of Xanorthite of plagioclase. Laser ablation ICP-MS analyses of trace-element zoning suggest that many other elements were also mobile. Thin section, field of view is 8 mm.

Backscattered electron image of zoned plagioclase in a chlorite schist. The cyclic zoning displayed in this grain is typical of metamorphism during episodic fluid infiltration and whole-rock compositional alteration. The original rock type before synmetamorphic alteration remains uncertain. But the occurrence of hornblende inclusions in some plagioclase porphyroblasts hints that the original rock may have been a diorite (Hbl + Bt + Pl). The synmetamorphic alteration in these rocks may be the result of fluid alteration focused along fault zones during D2. Field of view is 3 mm.

Possible relation to nearby gold deposits

An intriguing possibility is that amphibolite- (M2) and subsequent greenschist- (M3) facies metamorphism, alteration, and mobilization of gold in the VMS deposits is related to the genesis of gold deposits less than 20 km away in M2 and M3 shear zones.

M2 amphibolite-facies metamorphism of VMS deposits produced schists with a variety of somewhat pelitic assemblages. It also produced whole-rock compositional alteration that varies from minor to intense and that reflects the interaction of fluids with rocks in the alteration zones. If these metamorphic fluids had been in equilibrium with the surrounding country rock basalt, they would have been reactive with respect to the rocks in the deposit.

M3 greenschist-facies metamorphism produced minor chloritization of rocks in the VMS deposits and mobilized a variety of elements. The grade of gold in the VMS deposits is fairly high, about 0.15 oz/tonne, or similar to the grade in the New Britannia gold deposit

Gold ore from the New Britannia mine in Snow Lake. The gold occurs in an M3 mineral assemblage in shear zones that reactivated M2 faults. The resulting rock compositions are the result of alteration of basalts during M2 and M3.

Gold ore in thin section. Field of view is 1 mm. The host rock is a fine grained schist dominated by amphibole, biotite, calcite, and quartz, with lesser epidote, pyrite, arsenopyrite, and oxides. This is a typical assemblage in greenschist-facies gold deposits. Some of the elements lost during M3 from the VMS deposits were apparently gained in the the gold deposit. Is there a genetic link between the Cu-Zn-Au VMS deposits and nearby shear-zone hosted gold deposits?