Summary:
Mineralization in the Rochester district exhibits characteristics of both low-sulfidation and intermediate-sulfidation precious metal systems, complicated by supergene enrichment processes and significant oxidation.
The Rochester and Nevada Packard mines are located on the southern flank of the Humboldt Range within the Basin-and-Range province, where late Tertiary extension created large listric normal faults bounding generally north–south-trending mountain ranges and adjacent downdropped valleys. A major structural feature within the southern portion of the Humboldt Range is the Black Ridge fault system, which is an extensive shear zone that is, in places, hundreds of feet wide. Most of the Rochester silver–gold mineralization occurs in hanging wall faults, splays, and cross-faults within the Black Ridge system.
Mineralization is structurally controlled. Economic mineralization is hosted mainly in the oxide zone, where the Rochester–Weaver Formation contact is the primary host for silver–gold mineralization, followed and influenced by mineralized fault zones with associated fracture, stockwork and disseminated mineralization away from the faults. The contact is extensively brecciated and healed by silica in both the Rochester and Weaver Formations. Quartz veins and veinlets typically exhibit both parallel and cross-cutting features, indicating multiple mineralizing events.
Acanthite and chlorargyrite are the most abundant oxide silver phases. Below the oxidation zone, the hypogene profile is preserved, with the main minerals including pyrite, sphalerite, galena, argentiferous tetrahedrite, chalcopyrite, arsenopyrite, and pyrargyrite.
Rochester
Mineralization is discontinuous over an area of 5,100 ft. north to south and 6,000 ft. east to west. Mineralization dips westerly at an average of 30°, and mineralization is nearly parallel with topography, with an average true depth of 700 ft.
The Rochester Formation, as exposed in the Rochester open pit, shows little continuity in the volcanic stratigraphy, either laterally or vertically, and is typically mapped as undifferentiated Rochester tuffs and flow banded rhyolites. Erratic intervals of conglomerate-breccia as thick as 100 ft, occur at various places in the stratigraphy. The Rochester Formation is extremely fractured in the mine area.
The contact between the Rochester and Weaver Formations is marked by a discontinuous lithic tuff with up to cobble-sized clasts. A discontinuous lens-shaped ash layer (W1a) that is not a favourable host for mineralization is occasionally found along the base of the Weaver Formation. volcaniclastic unit (W1c) lies stratigraphically above W1a and is relatively thin, approximately 60 ft. in thickness. Unit W1c is composed of sandstones interbedded with lithic tuffs and minor siltstone. Overlying W1c is a siltstone unit (W2), followed by the uppermost Weaver unit (W3), which is a predominately dark siltstone with a discontinuous spherulitic tuff at its base.
In the mine area, spherulitic tuffs, ash-fall and water-lain ash, shale/siltstone, fine-grained volcaniclastic rocks, tuffs, and lithic tuffs comprise the Weaver Formation. Basal units of the Weaver Formation (W1t, W1lt) are the most favorable mineralized host rocks.
The Rochester deposit geology is characterized by penetrative reverse and normal faults overprinted by a complex structural system of high-angle fracture sets. Mineralization occurs along high- and low-angle faults, related breccias and veins, and can extend as far as 500 ft. laterally away from the structures in the vicinity of the Weaver–Rochester Formation contact. Vein intersections form the largest zones of mineralization, with triple point intersections (i.e., intersecting veins in conjunction with the Weaver–Rochester Formation contact) forming the largest volumes of mineralization.
Fracture intensity is poorly developed in the upper two units of the Weaver Formation (W2 and W3). The lack of fracturing resulted in poor mineralization in these units. Basal Weaver (W1t) and upper Rochester units (Rt) are extremely fractured which prepared these units for mineral deposition by allowing hydrothermal fluids extensive access in these hosts.
Silicification is common, and well-developed in the conglomerate-breccia that occurs at the Rochester–Weaver Formation contact.
High-grade precious metal mineralization at Rochester occurs in discontinuous and anastomosing veins within compressional and extensional fault structures that range in thickness from a few inches to 3 ft. These veins are steeply dipping at the surface (>60°) but at depth become shallower dipping (<30°) and lower grade.
Lower-grade precious metal mineralization occurs in fractures, narrow veins, stockwork breccia stockwork and in disseminated zones associated with structures. In plan view, veins strike north and northeast with dominate orientations at approximately 0, 10, 30, 55 and 70° azimuth. The highest-grade, best-developed historical underground silver stopes were located on the East Vein, a conjugate 30°-striking shear between splays of the 10°- or northerly-striking Black Ridge Fault. In cross-section, mineralization associated with faults dips at 35–65° west, while mineralization occurring near the formational contact exhibits shallow dips (0–30°) both to the east and west.
Low-grade mineralization is controlled by hypogene processes and possible supergene enrichment. These low-grade systems vary in width (both along strike and down dip) from tens to hundreds of feet. Below the oxidation zone, metal grade typically drops off, but high grades of silver–gold with minor base metal content can be found in narrow quartz veins. The deposit is strongly oxidized to a depth of 200–500 ft. from the current pit bottom and partially oxidized to a depth of over 700 ft. Silver mineralization becomes erratic with increasing distance from favorable fault intersections, unit contacts, and structures. Supergene processes are responsible for the remobilization and enrichment of silver at Rochester
Nevada Packard
The Nevada Packard mineralized zones are broad, but in general, smaller than those at Rochester, typically no larger than 200 ft wide. The discontinuous mineralized zones cover an area of 2,500 ft by 2,300 ft and as deep as 600 ft. Silver and gold mineralization below 300 ft rapidly decrease in tenor.
Nevada Packard is situated in the same lithologies as described for Rochester. The structural setting and alteration styles as described for Rochester are similar to that of Nevada Packard.
Precious metal mineralization at Nevada Packard is like that at Rochester in that northeast-trending, west-dipping faults with associated disseminated metal, veins, and fractures, are the most dominant controls.
One difference in the Nevada Packard mineralization is that silver tends to be of higher grade than at Rochester, while the gold grades tend to be lower.