Structural evolution of the gold quarry deposit and implications for development, Eureka County, Nevada

1995 Summer.Includes bibliographical references (pages 70-73).The Gold Quarry deposit, located seven miles north of Carlin, Nevada, is a bulk-minable sediment-hosted disseminated gold deposit situated along the Carlin Trend in Eureka County. A premier gold deposit in North America, Gold Quarry has total in-place reserves, and past production which exceed 15,000,000 oz. gold. Gold Quarry is hosted by lower Paleozoic eastern assemblage carbonate rocks, and lower Paleozoic siliciclastic and siliceous rocks of the transitional and western assemblage facies. The deposit is localized by a major structural intersection of the Gold Quarry and Good Hope fault systems at the southern end of the Carlin window. The Carlin window comprises an exposure of lower-plate eastern assemblage carbonate rocks through the upper-plate western assemblage siliceous rocks and the Tertiary Carlin Formation. The window is bounded by the Gold Quarry fault to the southeast and the Good Hope fault to the southwest. The Gold Quarry deposit comprises four connected mineral zones, and several other small satellite deposits within one large mineral system. The four zones are Gold Quarry Main, Maggie Creek, Deep West, and Deep Sulfide Feeder. These are believed to be genetically related, but differ from one another due to local stratigraphic and structural ore-controls. Detailed pit mapping and drill-log interpretation, aided by palynologic dating techniques integrated with research conducted by other workers has yielded a coherent tectono-stratigraphic sequence for the Gold Quarry area. Eastern-assemblage carbonate rocks, including the Ordovician Hanson Creek, Silurian Roberts Mountains Formation and a sequence of Devonian unnamed limestones, are in fault contact with upper Devonian siliciclastic rocks in the hanging wall of the Gold Quarry/Chukar Gulch fault. Locally termed the Quarry Member, this siliciclastic sequence includes laminated siltstone and lesser rhythmically bedded cherty-mudstone and siltstone. The bulk of the siliciclastic sequence has been dated as Frasnian to Famennian (uppermost Devonian); however, the upper portions of the unit have yielded Kinderhookian (lowest Mississippian) dates. Highly folded and thrusted limestone of Devonian and Silurian ages are found in thrust contact above the siliciclastic sequence. This folded and thrusted package is locally referred to as the "allochthonous limestone wedge". The next tectono-stratigraphic higher package of rocks is the upper-plate to a major thrust fault locally referred to as the "Roberts Mountains Thrust"(RMT). The locally termed RMT is only one thrust plane within a sequence of thrusts, which comprise the regionally recognized RMT zone. These upper-plate Caradocian (middle Ordovician) rocks include rhythmically bedded cherty mudstone and shale, laminated siltstone and lesser quartzite interbeds. A structural model for Gold Quarry has been developed based on an integration of detailed mapping, structural and ore-control data from the Maggie Creek sub district. A structural para-genesis, based on crosscutting relations of mapped faults and fault lineation, is proposed. Evidence for four main stages of faulting is noted at Gold Quarry: 1) Compression-driven thrusting and related folding; 2) Wrench-driven strike-slip, reverse and dilatational faulting; 3) Formation of collapse due to decarbonatization of host-rocks and associated normal faulting; and 4) Extensional tectonics resulting in normal faulting. The oldest faulting and folding events at Gold Quarry are low-angle; generally, east-dipping thrust faults and associated folds developed within the Devonian siliciclastic rocks and the overlying Ordovician siliceous rocks. The next sequence of faulting is wrench-related, driven by a north-northeast principal compressive stress. This episode is interpreted to be responsible for forming the Carlin window, developing in response to a reverse-fault accommodation of movement along the Gold Quarry left-lateral shear. The Gold Quarry deposit is located at the "cornerstone" or structural intersection of the Gold Hope reverse-fault and the Gold Quarry left lateral shear. In addition, dominant ore-control directions are controlled by N 10° W to N 200° W (350° to 340°) right-lateral faults of lesser magnitude positioned in a conjugate manner to the Gold Quarry fault system. An important structural event of the Gold Quarry main zone is volume-loss collapse of the system host rocks caused by extensive decarbonatization during hydrothermal alteration. Bakken (1990) documented 50% volume loss of the ore-host silty carbonate rocks at the Carlin deposit. A similar amount of volume-loss would be expected for decalcified and dedolomitized silty carbonate rocks in the footwall of the Gold Quarry Main zone. Volume-loss accommodation driven collapse and associated normal faulting, in addition to preexisting fracturing, rendered the otherwise poor host rocks of the siliciclastic sequence amenable to fracture-controlled ore-fluid penetration. Significant extension-driven normal faulting and associated rotation of the deposit region occurred during the development of the Basin and Range. The extension reactivated many pre-existing structures as normal faults, most notably, the northeast trending faults such as the Gold Quarry fault system. Dip-slip slickensides are common cross-cutting the low angle mullions and grooves on many of the northeast and north-northwest-trending faults. The post-ore Tertiary Carlin Formation is offset, as well, by normal faults.Plate 1. Interpretive geology of a portion of gold quarry -- Plate 2. Bench geology at gold quarry mine