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Stratigraphy
Paleozoic sedimentary rocks form the regional basement throughout the Cortez District and have undergone a complex history of sedimentation and deformation. These rocks are divided into two different stratigraphic packages deposited indifferentenvironments and juxtaposed by the Roberts Mountains thrust complex. Marine clastic rocks, or western assemblage rocks (upper plate), were deposited in deep water to the west while carbonate rocks, or eastern assemblage rocks (lower plate), were deposited within shallow waters along a passive continental margin to the east.
The formations associated with the Western Assemblage are primarily deep-water siliceous sedimentary rocks with little or no carbonate. These formations include the Ordovician Valmy and Vinini Formations, Silurian Elder Sandstone, and Devonian SlavenChert. Formations associated with the Eastern Assemblage are predominately carbonate-rich reef, shelf, slope and basin facies and include the Cambrian Hamburg Formation, Ordovician Eureka Quartzite and Hanson Creek Dolomite, Silurian to Devonian Roberts Mountains Formation and DevonianWenban Limestone and Horse Canyon Formation. The Hanson Creek Dolomite, Roberts Mountains Formation, Wenban Limestone and Horse Canyon Formation are the primary host rocks for Carlin-type gold mineralization within the district.
Structure and Magmatism
Within the district, at least four major tectonic episodes are recognized from the period between sediment deposition and the formation of Carlin-type gold deposits. Magmatism is associated with three of these tectonic events. Beginning in the late Devonian, the Antler orogeny led to extensive deformation across the region, primarily related to the Roberts Mountains Thrust. During the Mesozoic, two periods of compression and coeval magmatism are evidenced by Jurassic and Cretaceous stocks and associated deformation, including thrust faulting crosscutting the Roberts Mountains Thrust. Eocene extension and associated siliceous volcanism appears to be spatially and temporally related to Carlin-type mineralization, especially based on observation from the Cortez Hills Complex. Following formation of Carlin-type ore bodies, Basin and Range extension began in the Miocene with early rifting and bimodal volcanism, and associated low sulfidation epithermal gold deposits.
During the late Devonian to early Mississippian Antler Orogeny, sedimentation was interrupted and Paleozoic sedimentary rocks were uplifted, folded, and faulted to create a pervasive structural architecture of low-angle, often bedding sub-parallel, faults. The Roberts Mountains Thrust, a regionally extensive system of low angle thrust faults is the main expression of the Antler Orogeny. Movement along the Roberts Mountain Thrust resulted in the displacement of the siliciclastic Western (upper-plate) Assemblage up to 90 miles eastward over the carbonate (lower-plate) Eastern Assemblage. In carbonate rocks, low-angle faults preferentially propagated along formation boundaries then ramped upward and eastward, resulting in development of duplex structures and imbricate fault zones. Within carbonate rock, most Antler-related folds have north- to northwest-striking axial planes and are upright or overturned to the east.
Compression persisted through the Mesozoic. Several intrusive bodies of Jurassic and Cretaceous ages outcrop in the southern Cortez and northern Toiyabe Ranges near Cortez. Aeromagnetic surveys indicate that intrusive rocks, presumably Mesozoic in age, underlie most of the Cortez Mountains; a Jurassic quartz monzonite known as the Mill Canyon Stock is exposed at surface to the northeast of the Cortez Hills deposit (Muffler 1964). Minor Jurassic-aged lamprophyres dikes are also observed in the Cortez Hills area. Jurassic--Cretaceous magmatism produced contact metamorphism of surrounding sedimentary rock. Jurassic--Cretaceous intrusive rocks include the Gold Acres and Mill Canyon stocks as well as a variety of equigranular felsic to mafic dikes primarily found intruding along north to northwest-striking faults, including thrust faults.
Beginning in the Eocene, tectonic activity shifted from compressional to extensional, with numerous events of explosive volcanism and caldera formation. In the northern Toiyabe Range, south and west of the Cortez Hills deposit, the basement rocks are covered by up to 2,400 meters (8,000 feet) of Oligocene rhyolitic and dacitic ash flows and volcanic debris of the Caetano Tuff. The Caetano Tuff accumulated in a deep caldera and contains minor interbeds of water-laid tuff and pebble conglomerate derived from the nearby Paleozoic rocks. These silicic volcanic rocks are slightly younger than intrusives present within and around the Cortez Hills deposits. These pre-caldera intrusives have been dated between 35.4 and 38 Ma. Most porphyritic intrusives are found in north--northwest-striking shallow to steeply west-dipping structures. At Cortez Hills, porphyry dikes and sills are spatially related to mineralization, most often as mineralization-bounding, post-mineral features. The latest siliceous magmatism within central Nevada ended at approximately 20 Ma.
Following the cessation of widespread siliceous magmatism, localized extension and bimodal volcanism associated with the Northern Nevada Rift began during the Miocene. Basalt and andesite flows and dikes occur within the Cortez Range, north and east of the Cortez Hills area, where basalt flows up to 60 meters (200 feet) thick are attributed to rifting. A northwest-trending magnetic anomaly that connects these basalt exposures suggests that basalt flows underlie alluvium in portions of Crescent Valley.
Post-mineralization Miocene tectonism associated with the Basin and Range extension is evidenced by the north--northwest-trending Northern Nevada Rift with associated basaltic dike swarms located east of the sediment-hosted Carlin-type gold deposits. Mioceneepithermal gold mineralization is locally found along rift-related structures as typified by the Buckhorn and Mule Canyon gold deposits. Post-mineralization extensional tectonics displaced or even dissected deposits, with minor post-mineral displacement commonly seen along northeast-striking, steeply dipping faults. Post-mineralization displacement across district-scale basin-bounding faults such as the Crescent and Cortez Faults is likely between 1,500 meters to 2,200 meters (5,000 feet to 7,000 feet).
Project Geology and Structural Setting
The Garden Gate Pass project is covered by post mineral volcanics and alluvium. The targets identified on the project are beneath cover and therefore will be blind discoveries. Rye Patch had a significant knowledge base of the area since its geologists worked at Cortez, Gold Acres, Pipeline and Cortez Hill prior to working for Rye Patch Gold. Mapping in the district shows several key geologic and structural features trending toward the GGP property. These ore controlling features are the basis for drilling on the project.
First and foremost, Carlin-type gold systems have one characteristic that makes them an attractive target. Carlin-type deposits tend to cluster along trends and within districts. The Carlin trend best exhibits this phenomenon in the fact that over 180 million ounces of gold have been discovered, mined or processed. Similarly, along the Cortez trend new deposits have been discovered that total 35 million ounces of gold since 1990.
The most recent discovery and mine is the Cortez Hills deposit which is located approximately 14 kilometres to the north of the Garden Gate Pass project. The Cortez Hills mine has a surface open pit and underground mine with resources and reserves that total over 15 million of gold. In addition, Barrick Gold is drilling on a new deposit just 4 kilometres north of Rye Patch's ground. ET Blue is a new discovery that has potential for 5 to 10 million ounces of gold.
Drilling on Garden Gate Pass is planned for July 2011, and permits are in and being processed. A total of 8 to 12 drillholes are planned in a budget totaling $750,000. The goal is to make a Carlin discovery before the end of the summer.
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