Geology of the North Bullion Gold Deposit: Eocene Extension, Intrusion and Carlin-style Mineralization, the Railroad District, Carlin Trend, Nevada

摘要

The North Bullion gold deposit is located within Gold Standard Venture Corp.'s 100 percent controlled, 115 km~2 Railroad property, centered on the 4th and southernmost dome on the Carlin Trend. North Bullion was discovered in 2010, when an integrated process of detailed gravity and vectoring from new geologic and assay drill hole data, led to intercepts of 32 m of 1.39 g/t Au and 43.6 m of 1.21 g/t Au in RR10-8. The gold system remains open in all directions and spans an area of 400 m by 1000 m. North Bullion occurs in a triangular shaped horst in the footwall of the major north-striking, steeply east-dipping, North Bullion Fault Zone (NBFZ). The western edge of the horst is bounded by a northeast-striking, northwest-dipping fault. The deposit is blind with the gold system capped by gently east-dipping dacite sills. In general, gold is hosted in two zones: a gently to moderately dipping upper zone of strongly sheared siliciclastic/carbonate rocks (a mixed composite of Mississippian Webb Formation and Tripon Pass Limestone); and a flat lying, lower zone of dissolution collapse breccia developed above and within variably dolomitized micrite and calcarenite of the Devonian Devils Gate Limestone. Between strands of the NBFZ, breccia with both collapse and tectonic features propagated upwards through the Mississippian section incorporating silty mudstone of the Webb Formation, micrite of the Tripon Pass Limestone and sandstone of the Chainman Formation. Gold zones range from 105 to 400 meters in depth, and steepen from flat (10°) to moderate (45°) dips to the east, as they approach the western strand of the NBFZ. Gold is associated with sooty sulfides, silica, carbon, clay, barite, realgar and orpiment in addition to elevated As, Hg, Sb and TI. High-grade (> 6 g/t Au) gold occurs in both the upper and lower gold zones. Some of the best gold intercepts include: RR12-01 upper zone: 163.7 m of 3.38 g/t Au from 237.5 to 401.2 m; including 42.7 m of 9.40 g/t Au. RR12-10 upper zone: 124.1 m of 4.05 g/t Au from 218.0 to 342.1 m; including 16.5 m of 15.09 g/t Au. RR13-11 lower zone: 98.2 m of 3.26 g/t Au from 313.4-411.6 m; including 17.1 m of 9.98 g/t Au. Hydrothermal alteration in breccias is characterized by a spatial zonation of three types: distal breccia with calcareous clasts and calcite cement; intermediate breccia with dolomitized clasts and dolomite cement; and proximal breccia with silica/sulfide cement and replacement of clasts. Dolomitization of the Devils Gate Limestone thickens and strengthens towards the NBFZ. Veinlets show a general zonation around the deposit: an outer calcite shell; an intermediate ferroan dolomite zone; and barite ± quartz immediately above, within and below the gold zones respectively. Intrusive relationships and tilting of units indicate the deposit formed during an Eocene event with synchronous intrusion, hydrothermal activity and extensional movement on graben-bounding faults. Dacite sills, dated at 38.8-38.2 Ma, intruded steeply dipping faults within the NBFZ and low angle, bedding parallel faults, capping the gold system. The margins of dacite dikes and sills are commonly sheared and some dacite occurs as clasts within mineralized dissolution collapse breccia, indicating continued movement along faults and hydrothermal activity after emplacement of the dacite. In fault steps within the NBFZ, the Eocene Elko Formation has the same moderate eastward dip as the underlying Paleozoic rocks. The collapse breccia generally exhibits a flat-tabular textural fabric subparallel to today's surface. All of this evidence supports the formation of North Bullion during a very dynamic, focused Eocene event with synchronous extension, intrusion and Carlin-style mineralization.