Summary:
The Dawson gold system occurs along a major, syntectonic structural discontinuity which has significant strikeslip motion. In such a strike-slip regime, compression and dilational zones develop and result in pinch and swell of the relatively incompetent mineralized package. Intrusive into these dilational zones are two major gold-bearing peraluminous map units: 1) a banded biotite-granodiorite aplite having accessory garnet and attendant coppergold mineralization associated with greisenous amphibole, muscovite and biotite. This unit is considered a magmato-hydrothermalite; and 2) a late quartzoid-limonite fracture filling event that locally contains Bonanza gold grades overprinting unit above in tensile structures. This bonanza gold is inferred to be an extreme magmatic fractionate. A pink peraluminous granite and allied pegmatite cross-cut the mineralization described above and are interpreted to be the source of the gold-bearing hydrothermal solutions.
Gold mineralization occurs in all the shear zone hosted rock types with the highest grades—up to 225 g/t gold (DDH GC 44), occurring predominantly in late stage hydrothermalite and limonite veins ranging in thickness from 2 mm to 5 cm. The high grade gold mineralized hydrothermalites are hosted in biotite aplite and leucogranite that host lower grade gold—0.5 to 10 g/t. Gold mineralized zones attain thickest widths of up to 15 m, in dilational or “buckle” zones within the Dawson Shear Zone and show a spatial relationship with northeast trending faults. Gold grains vary from 50 to 200 µm in size.
Footwall Rocks
The footwall rocks consist mainly of distinctively pink to grey, fine- to coarse-grained quartz-biotite-plagioclase feldspar gneiss with subordinate magnetite and local garnet in the upper portions. The pink banded unit (Pbu) is moderately to strongly foliated with characteristic alternating bands of pink feldspar gneiss and dark grey, quartz-feldspar-biotite gneiss. The more massive unit (pink massive unit; Pmu) is granoblastic to porphyritic; it cross cuts and intrudes the Pbu with evidence of partial melting. The distinctive pink colour is due to hematitization of the feldspars.
This stratigraphy has been extensively intruded, metasomatized and partially melted by very hydrous Pikes Peak quartz-potassium feldspar granite and pegmatite (Swan, 1991).
Potassium content decreases and garnets become more abundant west of Windy Gulch (grey banded unit, or Gbu); the unit exhibits finer grain sizes, better development of gneissic banding and more compositional variation.
Mineralized Horizon
The auriferous horizon on the Property is made up of interleaved mineral assemblages with numerous discontinuous sulphide lenses that are laterally continuous along strike for about a mile, but which pinch and swell along strike and range from approximately about an inch to 49 ft in true thickness. Contacts with the hanging wall and footwall gneisses have been mapped as both gradational and sheared. In more folded parts of the Property (e.g. Windy Point), the mineralized horizon comprises southwest- plunging grade shoots. The origin of the mineralized plunging grade shoots has been variously ascribed to the D1, D2, or D3 period of deformation, with migration of gold mineralization into F1, F2, or F3 fold hinges.
The stratigraphically lower part of the mineralized zone is a quartz-biotite gneissic aplite with variable amounts of garnet, sillimanite, magnetite, other silicates, and minor sulphides. It is light grey to black in colour and is typically fine- to coarse-grained, poorly to strongly foliated and gneissic to schistose in texture. Banding may be present, defined by alternating quartz-rich and biotite-rich layers. Garnet occurs as fine- to coarse-grained, pinkishred to red-brown porphyroblasts. Plagioclase feldspar occurs in the lowest portions of the zone, increasing in abundance with depth. The upper portions contain subordinate gahnite, cordierite and 1 to 5 modal percent disseminated pyrite and chalcopyrite. Native gold occurring in this unit is typically associated with quartz, biotite, sillimanite, and coarser-grained garnet.
Stratigraphically above the biotite aplite is the sulphide portion of the horizon. Sulphides make up from 1 to 50 modal percent, consisting mainly of pyrite, with lesser chalcopyrite, pyrrhotite, sphalerite, and galena. They can form very fine-grained disseminations, clots, and discontinuous hairline veinlets parallel to foliation. Gangue mineralogy is quite variable and is made up of amphibole (mainly anthophyllite), biotite, chlorite, quartz, and feldspar with lesser gahnite, magnetite, cordierite, and sillimanite. Gold in this unit is typically associated with the amphibole-rich zones. Textures vary from foliated to schistose to fragmental, with amphiboles locally forming radiating clusters.
Mineralization
Gold and base metal mineralization occur within an east-northeast trending, south-southwest-dipping fault in amphibolite-grade, Proterozoic-age metamorphic rocks. Strike-length of the entire mapped horizon is about 3.4 mi, of which about the eastern 1.6 mi, which is gold prospective, occurs within the Property.
East of the Windy Point Segment, mineralization is dominated by iron and copper sulphides with minor gold. The mineralized zone is apparently terminated on the east by the Front Range Reverse Thrust Fault, which juxtaposes Proterozoic rocks over Mesozoic rocks; it is terminated on the west by the Marsh Gulch Fault. The mineralized horizon is quite recognizable on the surface as a gossan, with limonite and hematite-stained siliceous rock from about 2 inches to about 10 ft wide and local malachite-azurite fracture-fill.
The mineralized host rock is predominantly a quartz-biotite gneissic aplite with variable amounts of iron-rich garnet and other silicates with local zones enriched in sulphides; it lies stratigraphically above the quartz-biotite-feldspar gneiss (Pbu/Pmu). The horizon has an average thickness of about 9.1 m, with gold occurring predominantly at several stratigraphic positions below the sulphide-bearing zones, but also at the base of the sulphide unit itself. The mineralized parts of the horizon are relatively discrete and range from approximately an inch to 49 ft in true thickness, with a sharp drop-off in metal values away from the contacts (Wilson, 1982).
The sulphide-rich zones are typically 10 to 50 modal percent base-metal sulphides in a fragmental chloritebiotite matrix with local quartz- or anthophyllite-rich zones. Typical gangue mineralogy includes quartz, biotite, phlogopite, garnet, magnetite, amphiboles, sillimanite, cordierite, anthophyllite, gahnite, staurolite, sericite, talc, hematite, limonite, and calcite. Sulphides include pyrite and chalcopyrite with lesser amounts of pyrrhotite, sphalerite, and galena plus rare gold, molybdenite, and bismuth sulphosalts. Later-stage talc and serpentine fill fractures in both the sulphide and auriferous zones.