Summary:
East Deposit
The East Deposit, located 7,000 ft east of the North Deposit, measures approximately 2,000 ft by 1,200 ft and consists of flat-lying to gently dipping, bedding-controlled, stacked skarn-IOCG mineralized zones within the limestone of the Mason Valley Formation at depths of 1,400 to 2,200 ft. The East Deposit is defined by drill holes spaced approximately 175 to 250 ft apart.
Higher grade copper occurs mainly in dolomitized limestones and skarn-chalcopyrite-magnetite-pyrite breccias immediately adjacent to diorite or diorite endoskarn. Much of the high magnetite rock that grounds the East Deposit is within diorite endoskarn. A good amount of continuity is evident in the East Deposit with mineralization possibly continuing and thickening to the west. The area between the East Deposit and the North Deposit still requires detailed exploration to accurately determine the lateral extent of each Deposit.
E2 Deposit
The E2 Deposit is a steeply northwest-dipping lens of high grade copper-magnetite skarn-IOCG breccia within the Mason Valley limestone, which lies on the hanging wall of an endoskarn sill. The lens has been explored along approximately 1,200 ft of strike length, is 40 to 120 ft thick, and is locally continuous for at least 1,600 ft down-dip.
The chalcopyrite-magnetite mineralization follows the marble front, similar to the East Deposit. A major east-trending rotational fault appears to exist between the two deposits and results in a significant variation in the deposit orientation. Analysis of the drill hole intersections suggests that the E2 Deposit may, in fact, be a series of steeply dipping, plunging shoots, as much as 400 ft wide and 3,000 ft long on dip. The main portion of the mineralization starts approximately 800 ft below the present surface and extends 2,400 ft below the surface.
Mineralization
Granodiorite to diorite rocks belonging to the Jurassic Yerington Batholith intrude the limestones of the Triassic Mason Valley Formation and calcareous argillites and siliceous shales, siltstones and limestones of the Gardnerville Formation. Associated with this intrusive episode is the development of large areas of IOCG mineralization, which is dominantly skarn with associated copper and magnetite mineralization with varying levels of gold and silver. The skarn occurs primarily in the middle to lower portion of the Gardnerville Formation and the upper part of the Mason Valley Formation, as well as within the intrusive granitoid itself.
The geological environment reflects a classic copper skarn in one of its type localities (Basin and Range) where deposition is associated with I-type, magnetite series, calc-alkaline quartz diorite granodioritic stocks of hypabyssal character, proximity to stock contacts, assemblages of andradite garnet with diopside pyroxene, magnetite with hematite and moderate to high sulfide content consisting of pyrite, chalcopyrite, minor tennantite, and sphalerite. The large copper skarns are generally associated with altered and mineralized porphyry copper stocks and display extensive retrograde alterations.
Sulfides and iron oxides occur as disseminations, as massive streaks and in veins in skarn, and as massive replacements of marble at the skarn front. As skarns are commonly zoned, with massive garnetite near the pluton, increasing pyroxene and finally idocrase near the marble contact, sulfide mineralogy and metal ratios may also be systematically zoned relative to the pluton. Pyrite, chalcopyrite and magnetite are most abundant near the pluton with increasing chalcopyrite and finally bornite near the marble contact, with decreasing magnetite reflecting an outward decrease in total iron.
Initial contact metamorphism altered Gardnerville sediments to diopside-garnet, calc-hornfels and siliceous hornfels, and converted the limestone of the Mason Valley Formation to calcite ± dolomite ± tremolite marble. The intrusive rock is commonly altered to diopside-plagioclase endoskarn. Later metasomatism formed diopside-garnet-magnetite ± sulfide replacement zones.
Hydrothermal fluid movement and alteration were enhanced by fracturing and brecciation along and near diorite contacts, within the diorite, within beds of limestone and along fractures at a high angle to bedding in hornfels. Higher copper values are generally on the fringe of magnetite zones near a mineralized limestone contact (marble front) or within bodies associated with late stage retrograde actinolite-epidotegarnet-magnetite skarn.
Retrograde skarn development was accompanied by deposition of pyrite, pyrrhotite and chalcopyrite, and may also be synchronous with the intrusion of altered and weakly mineralized andesite and quartzmonzonite porphyry dikes. There is a general overall zonation to the mineralization, with magnetite decreasing and copper increasing in relative content away from the intrusive. These two end member zones are commonly referred to as proximal and distal zones.
Retrograde skarn development was accompanied by deposition of pyrite, pyrrhotite and chalcopyrite, and may also be synchronous with the intrusion of altered and weakly mineralized andesite and quartzmonzonite porphyry dikes. There is a general overall zonation to the mineralization, with magnetite decreasing and copper increasing in relative content away from the intrusive. These two end member zones are commonly referred to as proximal and distal zones.
All deposits are believed to be directly associated with the intrusive granodioritic mass. There is a strong possibility that all Deposits were originally part of a single unit peripheral to and within the intrusive but have since been displaced and separated to their present locales by a complex post-mineralization tectonic history.
In general, each of the Deposits displays an early pro-grade mineralogy consisting of diopside-garnetcalcite-dolomite-tremolite and plagioclase, which represents the initial metamorphism followed by diopsidegarnet-magnetite ± sulfide replacement along fractures. Retrograde alteration (actinolite-epidote-garnetmagnetite-calcite-dolomite) was accompanied by deposition of pyrite, pyrrhotite, and chalcopyrite.
All deposits are "blind" and are known strictly from drilling results. The uppermost oxidized portions of the North and South deposits lie within 100 ft of the surface but the bulk of the mineralization lies 300 to 400 ft below barren hanging wall rock. The top of the Southeast Deposit lies beneath 300 to 500 ft of barren granodiorite porphyry. The East Deposit is located beneath 1,500 ft of Tertiary Volcanics (TV), Quaternary gravels and barren hornfels units while the E2 Deposit is covered by 1,000 ft of TV and Quaternary gravels. The discoveries were made by drill evaluation of magnetic anomalies associated with large magnetite-rich bodies. The South and Southeast Deposits represent areas of large magnetite deposition within the Mason Valley FM associated copper mineralization of lower tenor, whereas the Northwest, East, and E2 Deposits, although less explored, contain localized zones of much higher grade copper mineralization. The North Deposit is a low to moderate grade copper deposit with weak magnetite content and is primarily within hornfels and silicate skarns of the Gardnerville Formation.