Summary:
Deposit Type
Mineralization at the Springpole Gold project is dominated by large tonnage, low grade, disseminated porphyry-style or epithermal-style gold mineralization associated with the emplacement of an alkali trachyte intrusion. Textures observed in the extensive repository of drill core appear to confirm that the disseminated gold-silver-sulphide mineralization, the mesothermal to epithermal lode vein gold mineralization, and the banded iron-formation hosted gold mineralization are all the result of the emplacement of multiple phases of trachyte porphyry and associated diatreme breccias, hydrothermal breccias, dikes and sills.
Barron considered the Springpole Complex to be the end product of magmatic fractionation processes and of fluids that evolved from magmatic to hydrothermal in the high level, sub-volcanic porphyry environment. These processes produced a low-grade gold-porphyry-epithermal type deposit and associated high-grade veins and breccia pipes.
Santa Fe geologists felt the nature of the mineralization at Springpole had many similarities with deposits of the Cripple Creek District, Colorado, including the Cresson Mine. Detailed mapping on the land-based portions of the property by Santa Fe geologists showed that most, if not all, of the gold mineralization at the Springpole Gold Project is spatially associated with the feldspar porphyry diatreme dikes, veins, and diatreme breccia.
Porphyry Intrusion
A polyphase alkali, trachyte intrusion displaying autolithic breccia textures lies at the heart of the Springpole Gold Project. The intrusion is comprised of a system of multiple phases of trachyte believed to be part of the roof zone of a larger syenite intrusion, as fragments displaying phaneritic textures were observed from deeper drill cores in the southeast portion of the Portage zone. Early intrusive phases consist of megacrystic feldspar phenocrysts, up to 5 cm long, of albite and orthoclase feldspar in an aphanitic groundmass. Successive phases show progressively finer-grained porphyritic texture while the final intrusive phases are aphanitic.
Pervasive alteration and metamorphism have reduced the original porphyry intrusion to a complex alteration assemblage dominated by sericite, biotite, pyrite, calcite/dolomite, and quartz. Primary igneous textures are remarkably well preserved in places and give indications to the possible genesis of the initial phase of gold mineralization. Within the country rocks to the north and east are trachyte and lamprophyre dikes and sills that source from the trachyte - or syenite-porphyry intrusive system.
Confederation Age Volcanic and Siliciclastic Rocks
The country rocks pre-date the alkali intrusion and are composed of a geologic sequence of altered and metamorphosed intermediate andesitic volcanic and associated volcaniclastic rocks, siliciclastic sedimentary rocks, chemical sediments including banded iron formation (BIF), and coarse pebble conglomerates. Devaney (2001a) indicates that the sediments are likely of the Confederation assemblage dating at around 2,740 Ma, representing the proximal portions of a mixed volcanic-sedimentary basin.
Mineralization
Three separate mineralized zones have been modelled at the Springpole Deposit, each with their own characteristics. The Portage zone, which is hosted within volcanic trachyte and heterolithic breccia dominated by potassic (biotite) alteration and pyrite; the Camp zone, largely hosted in sheared mafic volcanics with intense ankerite and sericite alteration associated with more lode gold style of mineralization; and the East Extension zone which is associated with intrusive megacrystic trachyte and heterolithic breccia, with similar potassic (biotite) alteration and pyrite mineralization style as the Portage zone.
Apart from the Camp zone, which is interpreted to be hosted in the volcanic basement rocks, the Portage and East Extension zones are interpreted to be hosted in an unconformable package of trachytic volcanics varying from aphanitic to medium-grained porphyritic with discernible volcanic sequences. The basement volcanic sequence is comprised of subaqueous mafic volcanic flows which have been intruded by gabbroic dikes/sills. Ancillary mineralization is noted within these gabbroic units and is largely characterized as ankerite-altered shear zones with sulphide replacement mineralization associated with quartz-carbonate veining. Megacrystic trachyte intrusions are largely confined to the unconformable package of trachytic volcanic rocks and represent the porphyry-style of mineralization outlined in the Portage and East Extension zones. Intermediate volcanic flows overlay the deposit and are structurally unconformable from the main package of trachytic volcanics.
Porphyry-style Mineralization
The main intrusive complex appears to contain many of the characteristics of alkaline, porphyry-style mineralization associated with diatreme breccias (e.g., Cripple Creek, Colorado). Direct comparison with drill core from the two sites shows several consistent textures and styles of mineralization. Observations made from drilling, combined with the airborne magnetic surveys, show that gold mineralization is largely coincident with discrete geophysical low anomalies. This style of mineralization is characterized by the Portage zone and portions of the East Extension zone where mineralization is hosted by diatreme breccia in aphanitic trachyte. Ductile shearing and brittle faulting have played a role in redistributing structurally controlled blocks of the mineralized rock. Work continues to outline a consistent form of porphyry style alteration zoning consisting of an outer zone of phyllic (sericite) dominant alteration with narrow zones of advanced argillic alteration characterized by illite and kaolinite, and a core zone of intense potassic alteration characterized by biotite and K-feldspar.
Lode Gold Mineralization
The intrusion of the trachyte complex into the volcanic pile, as well as the chemical and siliciclastic sedimentary rocks in a near surface environment, produced mesothermal to epithermal style lode vein mineralization. The difference between mesothermal and epithermal mineralization regimes is the temperature and pressure of the mineralizing fluids.
Higher temperature (mesothermal) fluids would have existed within the emplaced intrusive, associated with the diatreme breccias, and in the immediately adjacent wall rock/country rocks. In the porphyry intrusive, and at the contact between intrusive and wall rock in the East Extension zone, and localized within the Main zone, mesothermal style quartz-biotite-calcite-sulphide veins with occasional tourmaline are observed with occasional coarse, visible gold.
Alteration
All rocks on the Project exhibit pervasive alteration that consists of multiple overprinted phases. The country rocks and alkali intrusive rocks exhibit pervasive green-schist facies metamorphism and alteration, probably the result of burial. This manifests as chlorite, calcite, and pyrite in the intermediate volcanic rocks, pyritization of the banded iron formation, and sericitepyrite alteration within the alkali intrusive associated rocks.