Summary:
The deposits at the Wind Mountain project and other occurrences of gold-silver mineralization in the Walker Lane and Northern Nevada (e.g., Round Mountain, Midas, Hycroft, Hog Ranch) have long been considered examples of epithermal precious metal deposits that are classified as the “low-sulfidation” type (e.g., White and Hedenquist, 1995; Cooke and Simmons, 2000; and Sillitoe and Hedenquist, 2003).
Many low-sulfidation epithermal precious metal deposits are largely disseminated bodies. Examples include the smaller deposits at the Wind Mountain project and the large deposits at Round Mountain, Nevada (>15 million ounces of gold). Disseminated deposits are generally low-grade with the gold distributed in a large body of favorable rock, which is associated with primary host rock porosity or secondary porosity due to fracturing, alteration, or other processes.
The disseminated mineralization at the Wind Mountain project is somewhat unique because the precious metals were deposited at high levels in the hot spring system compared to most other gold deposits in Nevada. Gold and silver precipitated within a package of volcaniclastic sediments and fossil reed-bearing sinter, which were presumably near-surface deposits.
Mineralization
Mineralization at the Wind Mountain project is characterized by low-grade gold (>~0.005oz Au/ton) with variable silver hosted by sinter and silicified volcaniclastic material. The silver to gold ratio is variable but averages approximately 30:1. Precious metals typically are accompanied by arsenic (20-175ppm As), antimony (20-175ppm Sb), mercury (1-10ppm Hg), selenium (20-180ppm Se), and molybdenum (1- 5ppm Mo), which increase in abundance with increasing gold grades. Base metals are locally enriched in the deepest drill holes. The highest molybdenum values (>100ppm Mo) coincide with elevated rhenium (to 0.2ppm Re) in a few deep intercepts, possibly indicative of a felsic intrusion at depth (Crist et al., 2016).
Continuity of gold mineralization within the Wind Mountain deposits is good at grades in the range of 0.005 to 0.015oz Au/ton. Higher-grade zones occur in generally flat-lying pods up to 1,000ft long by 300ft wide and 100ft thick within lower-grade mineralization. Gold occurrences continue sporadically for thousands of feet beyond the known mined deposits, and present opportunities for further exploration.
Gold and silver occur primarily as electrum in oxidized zones. Late-phase pyrite containing gold may also be present as coatings on an early barren form of pyrite where sulfides are not oxidized (Wood, 1990). The host mineral of the unoxidized silver mineralization has not been identified. Pyrite, minor marcasite, and traces of cinnabar are the most common sulfide minerals observed. Within the nearsurface oxide zone of the Wind Mountain deposits, small amounts of unoxidized pyrite are encapsulated in silica. Native sulfur is present in strongly bleached and leached zones in the deposit. Approximately 0.5 to 3% disseminated pyrite can be found in shallow bedrock beneath the pediment surrounding the Wind Mountain project.
Oxidation and leaching are developed to depths of more than 600ft in general, and up to 1,000ft along the Wind Mountain fault. Surface leaching of rocks occurred throughout the deposit area and resulted in the formation of goethite, jarosite, and hematite after sulfide minerals. The state of oxidation of gold and silver mineralization can have a significant impact on metallurgical recoveries.
Wind Pit And Deep Min Deposits
The axis of the Wind deposit is oriented north-northeast, and a crudely developed network of clay-filled vertical fractures of roughly similar orientation can be traced through the Wind Pit. The blasthole data reportedly delineates several pods of higher-grade gold mineralization that suggest lateral flow along permeable horizons. No obvious feeder structure is apparent from the data, and drilling beneath the deposits indicates that the clay-filled fractures do not contain enriched gold mineralization at depth. The fractures were not likely to have been feeder structures but rather provided permeability for hydrothermal fluid flow within favorable stratigraphic horizons.
In 2008, a new zone of gold mineralization, known as Deep Min, was partially defined by relatively deep drilling on the west, hanging wall side of the Wind Mountain fault. Nine holes penetrated relatively thick zones of gold mineralization, which have been dropped down approximately 700ft from the sub-horizontal Wind deposit by the fault. The mineralization at Deep Min extends downward, suggesting that upwelling fluids were centralized in the area.
The known precious metal-bearing material remaining at the Wind Mountain property includes mineralization in an area roughly 5,000ft north-south by 1,200ft wide by 600ft vertical thickness at the Wind deposit. The approximate dimensions of Deep Min mineralization are about 900ft long by 700ft wide by 700ft vertical thickness.
Breeze Deposit
The Breeze pit is the northern and smaller of the two open pits mined by AMAX. Silicified volcaniclastic rocks host gold and silver mineralization, although silicification is not as strongly developed as in the Wind Pit. A crudely developed fracture network trends roughly north to northwest through the pit. Feeder structures for the Breeze pit mineralization have not been identified with certainty.
Shallow south-dipping mineralization extends over 1,000ft southward from the existing Breeze pit. The known precious metal-bearing material remaining at the Breeze deposit covers an area that is 3,400ft north-south by 1,000ft wide by 200ft vertical thickness.
East of the Breeze pit, the Wind Mountain fault zone lies along the west flank of a north-trending ridge capped by silicified, precious metal-bearing rocks. No deep drilling has been conducted in the area, although mineralization has been intersected in many of the shallow holes along the ridge.
North Hill Target
A small area of mineralization known as the North Hill target, located approximately 3,000ft northwest of the Breeze pit, has been drilled. The limits to this mineralization have not been defined. The North Hill target mineralization could represent the surface expression of a shallowly south-dipping, down-faulted extension of the Breeze deposit. Considerable additional drilling will be required to fully define the target.
Zephyr Target
A similar geologic setting to that of the North Hill target occurs approximately 3,000ft to the northwest, where a post-mineral fault has dropped down and preserved favorable stratigraphy beneath alluvium and lake sediments. Five holes have been drilled in the area, including a historic 540ft drill hole that intersected 60ft of 0.009oz Au/ton at the bottom of the hole. Geophysics could be used to target follow-up drilling.