Summary:
VMS deposits are also known as volcanic associated, volcanic-hosted, and volcano sedimentaryhosted massive sulphide deposits. They typically occur as lenses of polymetallic massive sulphide that form at or near the seafloor in submarine volcanic environments, and are classified according to base metal content, gold content, or host-rock lithology.
The Horne deposit is the largest Canadian Au rich VMS deposits in the Noranda district (331 tonnes of Au produced historically from 54.3 Mt of ore at 6.1 g/t Au; Kerr and Gibson, 1993).
The typical morphology of Au-rich VMS deposits consists of a lenticular massive sulphide body with associated underlying discordant stockwork-stringer feeders and replacement zones. At Horne, zones of auriferous sulphide veinlets with Fe-chlorite selvages account for some of the Aurich mineralization (Kerr and Mason, 1990), however, the deposit lacks a well-defined stringer zone (Poulsen et al., 2000).
The vertical extent of the stockwork is typically larger than its lateral extent. The lateral extent of the deposit is typically a few hundred metres, but in some cases where the deposits are overturned, the mineralization has more than 2 km of known vertical extent (Horne and LaRonde Penna deposits). The thickness of the massive sulphide lenses is highly variable, especially when subjected to deformation (shortening), but is commonly on the order of a few tens of metres.
Mineralization is typically hosted by felsic volcanic flows and volcaniclastic rocks (or their metamorphosed equivalents) near or at the interface with basaltic andesite, andesite or clastic sedimentary strata. The Horne deposit is contained within a fault-bounded block of tholeiitic rhyolite flows and pyroclastic breccias and tuffs in contact with andesite flows to the east. It is juxtaposed against andesite flows and a diorite intrusion to the south, and rhyolites to the north that contain the Quemont deposit, another auriferous massive sulphide deposit (Poulsen et al., 2000) potentially related to the same giant hydrothermal system responsible for the formation of the Horne deposit.
At the Horne deposit, most rhyolitic rocks within the fault-bounded block have been affected by weak sericitization and silicification that become more intense near the sulphide mineralization, where alteration is characterized by a quartz-sericite-pyrite assemblage (Poulsen et al., 2000). Chlorite alteration, which locally contains elevated Cu and Au values, is largely restricted to the immediate footwall and sidewall of the deposit, except for local discordant zones in the footwall (Barrett et al., 1991).
The Horne Mine orebodies (Upper H, Lower H and Horne 5 deposit) dip subvertically within rhyolitic flows, breccias, and tuffs that are bounded by the Andesite and the Horne Creek faults. Least-altered rhyolites have low K2O contents and other geochemical features that place them within the FII tholeiitic series (Lesher et al., 1986). Graded volcaniclastic beds, metal zoning in the orebodies, and locations of chloritized-mineralized rhyolites indicate that the volcanic sequence youngs to the north. The volcanics in the fault wedge are variably silicified and sericitized, and local zones in the orebody sidewalls and footwall are chloritized.
The H orebodies formed podiform masses up to 120 m wide, 100 m thick, and 300 m in downplunge extent, consisting of chalcopyrite pyrrhotite-pyrite gold ore.
A semi-continuous Cu-rich base (up to 15 m thick) exists above the footwall and adjacent to the sidewalls of the orebodies. The ore changes stratigraphically upwards from a chalcopyrite-rich base, through middle pyrrhotite-pyrite-rich zones, to upper pyrite rich zones. Gold enrichments occur in some of the Cu-rich ores but also in overlying pyritic ores and in adjacent host volcanics. Cu-Au bearing chloritized rhyolites occur mainly in the western and eastern sidewalls and at down plunge terminations of the H orebodies.
The Lower H orebody is stratigraphically overlain by a massive to semi-massive sulphide body, referred to as the Horne 5 deposit (Sinclair, 1971). This tabular zone consists of numerous lenses of massive pyrite interbedded with intensely altered felsic volcaniclastic rocks. The Horne 5 deposit extends for a strike length of more than 1,000 m to a depth of at least 2,650 m and ranges from approximately 30 m to 140 m in thickness (Sinclair, 1971; Fisher, 1974; Gibson et al., 2000).
Pyrite is the predominant sulphide in this zone, but scattered portions contain sphalerite, chalcopyrite, and gold in economic concentrations (Sinclair, 1971). Sphalerite is the second most abundant sulphide in the Horne 5 deposit, although it is virtually absent in the Lower H, but it is far less abundant than pyrite. Chalcopyrite is a common mineral in many parts of the Horne 5 deposit but usually is present in only very minor amounts. Pyrrhotite is even less abundant than chalcopyrite and galena is rare in the Horne 5 deposit. A set of drill-core samples of massive sulphides have been analyzed for Cu, Au, Zn, Ag by Barrett et al., (1991). Based on twenty-five (25) samples (thirteen (13) samples from level 27 and twelve (12) samples from level 49), the Horne 5 deposit contains, on average, 0.12% Cu, 1.8% Zn, 1.4 g/t Au, and 26.6 g/t Ag.
The Horne 5 deposit extends from above level 21 down to below level 65. It attains significant thickness and breadth only below level 23. Below level 23, the Horne 5 deposit ranges from 15 to 125 m thick.