Summary:
Deposit Type
The Dublin Gulch intrusion is part of the mid-Cretaceous Tombstone Intrusive Suite of Alaska-Yukon granitoids, and the Eagle Zone belongs to the RIRGS class (Reduced Intrusion-Related Gold Systems) of mineral deposits. Gold mineralization in the Dublin Gulch intrusion shows strong similarities to the Fort Knox deposit in Alaska, including the presence of sheeted quartz veins and elevated levels of bismuth, arsenic, tellurium, and tungsten. The veins in the Eagle Zone consist of early quartz-scheelite with varied occurrences of pyrrhotite, pyrite and arsenopyrite, and are associated with K-feldspar and minor albite alteration envelopes. These are overprinted by sericite-carbonate and occasional chlorite alteration. The metasediments marginal to the intrusion are mineralized as well, but the bulk of the gold is hosted within the intrusive. The Dublin Gulch Stock is an elongate body trending 070°, with surface dimensions of approximately 6 x 2 km. Exploration for additional gold deposits is ongoing with significant potential for further discoveries.
The Dublin Gulch intrusion is composed of mainly biotite hornblende granodiorite. Minor phases of diorite and granite occur within the intrusion. The overall low sulphide content of the rock, commonly less than 0.5%, and the presence of carbonate (Calcite 1 to 4%) make the rock non-acid generating. In a report prepared by SRK for Stantec in April 2011 (SRK, 2010: Geological Characterization and Water Quality Predictions Eagle Gold Project), SRK states that acid rock drainage (ARD) is not anticipated for the project.
Mineralization
The Eagle Zone is the principal concentration of mineralization within the property. Within the Eagle Zone, gold occurs in extensional quartz veins that are most abundant on the hanging and footwall contacts of the narrowest portion of the Dublin Gulch granodiorite near its known western limits. Subordinate quantities of gold mineralization occur in quartz veins within the adjacent metasediments. Veins strike at azimuths of 060° to 85°, sub-parallel to the intrusive contact and are commonly fractured by repeated movement along the host fractures.
The Eagle Zone is irregular in plan and is approximately 1,600 m long (east-west) and 600 m wide northsouth. The Eagle Zone is near-vertical and has been traced for about 500 m below surface. Current drilling indicates that the mineralization is relatively continuous along this length and is open in several directions, including to depth.
Mineralization occurs as elemental gold, both as isolated grains and most commonly in association with arsenopyrite, and less commonly with pyrite and chalcopyrite.
The sulphide content in the veins is typically less than 5%; and is less than 0.5% within the deposit overall, with 1 to 4% carbonate (calcite) present as a buffer, acid generation from the ore and waste rock is not expected to be an issue (Stantec, 2011).
In descending abundance, the principal sulphides present are pyrrhotite, pyrite, arsenopyrite and chalcopyrite. Minor sphalerite, galena and molybdenite are also present. Scorodite and limonite are common weathering products.
The Olive Zone is a narrow-elongated zone sub-parallel to the intrusive-metasediment contact; located approximately 2.5 km northeast of the Eagle Zone. Olive measures approximately 20 to 80 m in width, 900 m in length, and has been drilled to approximately 175 to 250 m in depth. Compared to Eagle, the Olive mineralization is more associated with sulphides and quartz-sulphide veining in an interpreted shear-zone setting; with veining having an orientation at angles to the general northeast mineralized trend.
The Olive Zone differs from Eagle is some respects. Olive has more sulphide mineralization as both disseminated pyrite with moderate to strong sericitic alteration, and sulphide and quartz-sulphide veins, and is more tightly structurally controlled along the granodiorite-metasediment contact. Pyrite plus arsenopyrite (or arsenical pyrite) and quartz-pyrite veins to several centimetres in width have an average strike trend of azimuth 120°, and dips of 60° to 80° south, within the overall NE trending zone of mineralization. Vein densities vary significantly; however, trench exposures and assays indicate that good grade mineralization typically hosts multiple centimetre wide sulphide veins, on metre or less spacings, within areas of moderate to strong sericitic alteration with 3 to 5% disseminated sulphides. The most commons sulphides noted are pyrite, arsenopyrite, with minor to trace amounts of sphalerite, chalcopyrite, galena, bismuthinite, and molybdenite. Olive also has higher levels of silver than Eagle.
Multi-element geochemistry for Olive, based on over 17,300 analyses, shows the following:
• A good Au-Ag-As correlation; with Au correlation coefficients of 0.50 with Ag, and 0.42 with As;
• A strong Au-Bi correlation coefficient of 0.74;
• A strong Ag-Bi-Cu-Fe correlation; and
• Overall levels of associated elements at Olive are relatively low, as shown in the box-whisker plots. Similar multi-element associations at perhaps lower levels are indicated at Eagle, based on a less complete database.
Several other mineralized showings occur within the property. Most of these are related to the Dublin Gulch granodiorite and are in part characteristic of RIRGS deposits similar to the Eagle Gold deposit. Others are more characteristic of later structurally hosted overprinting mineralization. The Wolf (formerly Mar) tungsten deposit is located approximately 3 km east-northeast of the Eagle Zone. Scheelite occurs in a calc-silicate skarn in metasedimentary rocks adjacent to the Dublin Gulch granodiorite.
A number of gold-bearing quartz-sulphide veins occur around the margins of the Dublin Gulch Stock. These veins are narrow (centimetre-scale), steeply dipping and generally strike at about 070°. Silverquartz-sulphide veins also occur. These distal veins are infrequent relative to the sheeted vein system within the Dublin Gulch Stock and due to their small size, they are not a significant part of the Mineral Resource, with the exception of the Olive Zone.