Summary:
The Silvertip mineralization is a silver-zinc-lead Carbonate Replacement Deposit (CRD) with metals content, polyphase mineralization, abundant replacement textures, pyrite pseudomorphing pyrrhotite, and wallrock alteration reminiscent of many of the manto-chimney CRD’s of Mexico and the western US.
These economically attractive, polymetallic systems can stretch continuously from copper-gold enriched skarns near intrusion contacts in the “proximal” part of the system, to massive sulfide manto and chimney deposits with no exposed igneous relationship in the “distal” areas. Traditionally these deposits have been considered difficult exploration targets due to a paucity of peripheral indicators to mineralization such as hydrothermal alteration or consistent relationships to breccias or structures.
Mineralization
There are two 'layers' of mineralization identified on the Silvertip Property. A series of older sediment hosted Pb, Zn exhalite deposits hosted in Earn Group sediments overlie a deeper, and younger carbonate replacement Ag, Pb, Zn, Au deposit. This 'lower' deposit is the focus of exploration on the Silvertip Property.
The targeted Silvertip mineralization, termed the 'Lower Zone', consists of silver-leadzinc massive sulphide, formed by hydrothermal replacement processes in McDame Group limestone and takes the form of either shallow dipping mantos or sub-vertical chimneys. This lower zone replacement deposit terminates against impermeable shales at the base of the overlying Earn Group sediments. The lower zone mineralization does not outcrop. It extends between about 30m depth and several hundred meters depth beneath the surface, covered by the Earn Group sediments. The chimney zones vary in size but typically extend down from the manto horizon up to 70m. The chimneys are strongly structurally controlled and are composed of mineralized breccia and massive sulphides zones.
The Lower zone has been further sub-divided into four areas of contiguous mineralization. The ‘Silver Creek’ area is the shallowest and furthest west zone and is bounded to the west by the Camp Creek Fault. Directly adjacent to the east of Silver Creek zone is the Central Zone. The Central Zone is displaced from the Discovery Zone to the east by faulting. The ‘Discovery’ area lies farther east and at greater depth. The 65 Zone lies furthest to the south and is interpreted to be analogous to the chimney structures in Silver Creek.
Sedex Deposits (Upper Zone)
Early Mississippian syngenetic ‘Sedex’ deposits associated with siliceous to baritic exhalate subunits in unit 2A of the Earn Group (see section 9, above) overlie the Lower zone mineralization. These predominantly Pb, Zn deposits pre-date the carbonate replacement deposit in the Lower Zone and were the original exploration target on the property in 1980. They are not considered economic due to their limited size, lower grades, and proximity to the poor rock quality, acid generating Earn Group. They are of interest because they contain a sulphide overprint that may be related to the muc younger hydrothermal event that mineralized the McDame carbonates structurally below.
Carbonate Replacement Deposits (Lower Zone)
The main Lower zone deposits formed by the interaction of hot, magmatically derived, metal enriched hydrothermal fluids with McDame carbonate rocks (see Figure 7-8 above). The source of the fluids has not been found, but an area of quartz-sericite-pyrite alteration on the surface south and southeast of Silvertip Mountain might indicate a buried intrusion below. This alteration has a fluorine signature and has been dated at around 70 Ma (Late Cretaceous), the same age as felsic intrusions exposed elsewhere in the region. On this basis, the mineralizing event is assumed to be Late Cretaceous in age, although it may be slightly older.
Most of the mineralization defined so far occurs at the top of the McDame limestone, at or near the unconformable contact with the Earn Group, although significant sulphides are also present much deeper in the McDame. The massive sulphides are in the form of gently plunging tubes, or cape shaped mantos, up to about 20m thick and 30m wide, and in places extending for at least 200m in length. Narrower and thicker bodies of massive sulphide between 20 and 30m thick have been intersected locally by past drilling and are probably discordant, vertically oriented (minor) chimneys connecting mantos at different levels. Sulphide intersections deeper in the McDame are much less well defined; most are probably also mantos, but some might be parts of structurally hosted chimneys (no scale implied) or connections between stacked mantos.
Contacts between the massive sulphides and the host limestone can be remarkably sharp. Transitional zones of alteration (silicification, dolomitization) and recrystallization and brecciation are common (not all the dolomitization is related to the mineralization event – some is much older). The mineralization consists of early-formed pyrite, pyrrhotite and sphalerite and lesser galena, and a slightly younger, higher temperature, sulphosalt-sulphide suite of minerals. The latter contain the main silver-bearing phases including pyrargyrite-proustite, boulangerite-jamesonite and tetrahedrite (freibergite), as well as silver-rich galena. Quartz and calcite are the main gangue minerals and locally fill late-stage vugs and cavities.
Brecciation of sulphides, mixed with limestone vein quartz and calcite, attest to multiple phases of fluid infusion and intra-mineral, solution collapse processes. Unmineralized, crackle or rubble brecciated limestone is common, as are tectonic stylolites. Some rubble and matrix breccias are of Late Devonian paleokarst origin, although they too may be infiltrated by sulphide replacement, as they represent suitable ‘ground preparation’. Paleokarst is probably only an indirect controlling factor with respect to mineralization.
The main control on the mineralization in the deposit area is the Earn unconformity which formed a relatively impermeable cap to the upwelling fluids, concentrating the development of mantos at or near the top of the McDame. The Silvertip mantos are believed to have been fed from depth, at some point in the system, by structurally controlled chimney feeders. These feeders were possibly channeled in faults such as the Camp Creek fault and numerous subsidiary fractures, or along other faults such as those in the Discovery area where the unconformity steps down to the east. Many intralimestone mantos, which occur 100 m or more vertically below the unconformity, probably formed by lateral fluid flow branching off from the feeders and were controlled by a combination of structural and stratigraphic permeability contrasts. The main zone of chimney development, if it exists, has not yet been discovered. It is believed to occur closer to the thermal source of the system.
Structure
The basic structure of the Silvertip area is not complicated. Like the rest of the immediate region, it is dominated by faulting rather than folding. Strata generally strike north to northeast and dip gently to moderately east to southeast. There are no fold closures affecting the local map pattern, which is characterized by a general younging of units eastwards, broken up by faults.