Summary:
Deposit Types
The Soledad Mountain gold-silver deposit is best interpreted as a volcanic rock-hosted, low sulfidation, epithermal vein system of the low base metal type. Individual major veins formed by episodic deposition of quartz, adularia, sericite, calcite, and sparse sulfide minerals in open faults and fractures, coeval with adjacent sheeted and quartz-vein stockwork zones and quartz ±adularia ±sericite alteration of nearby wall-rocks. Other examples of this deposit class include districts such as Oatman (Arizona), Bullfrog (Nevada), Bodie (California), and Tayoltita (Mexico). Soledad Mountain contains an unusually large number of individual veins within a relatively small area by comparison to the examples cited. Post-mineral faulting at Soledad Mountain has extensively sheared and brecciated the veins, most likely due to mid-Miocene to present-day wrench-fault tectonism between the nearby San Andreas and Garlock fault systems.
Mineralization
Gold and silver mineralization occurs in a swarm of mainly northwest-striking, subparallel to anastomosing, low-sulfidation, epithermal quartz veins that formed in faults and fractures within the Miocene rhyolitic volcanic units. Veins occur in parallel and, locally, en echelon patterns over a total strike-length of 7,000 ft and a total width of 4,500 ft. The veins have been sheared and brecciated to varying degrees by post-mineral faulting.
More than 20 gold-silver veins and related vein splits occur at Soledad Mountain. Veins generally strike N40°W and dip at moderate to high angles to the northeast and to the southwest.
Mineralization consists of fine-grained pyrite, covellite, chalcocite, tetrahedrite, acanthite, native silver, pyrargyrite, polybasite, native gold, and electrum within discrete quartz veins, veinlets, veinlet stockworks and irregular zones of silicification. Gangue minerals include quartz, potassium feldspar (adularia), ferruginous kaolinitic clay, sericite, hematite, magnetite, goethite, and limonite. As stated by Bruff (1998, July) “At least five generations of quartz veining have been identified in hand-specimens within the major fissure-fill veins.” Calcite has also been reported by Diblee (1963) and in GQMC logging, as well as calcite replacement textures in quartz. Rhythmically banded veins are quite likely to show alternating layers of quartz and adularia (Bruff, 1998, July).
The veins formed by intense alteration of volcanic rocks and by deposition of quartz and sericite rich material in fault and fracture zones. The alteration and veins are generally low in sulfur, with total sulfide content generally being 1% or less. Vein “zones” consist of one or more central veins surrounded by either a stockwork or parallel zones of sheeted, narrow quartz veins. The effect is to have a core vein of 1 ft to 20 ft in width (with gold grades being generally greater than 0.1 oz/ton), surrounded by lower grade mineralization in the adjacent quartz-vein stockwork and sheeted vein zones. The widths of the stockwork and sheeted vein zones vary from 5 ft to 150 ft.
Native gold and electrum are generally associated with siliceous gangue and occur as particles with diameters ranging from less than 10 µm to as much as 150 µm or more. Electrum contains about 25% silver. Gold grades greater than 0.1 oz/ton appear to occur where veins exhibit multiple generations of quartz, adularia and sericite. Sheeted veins and stockwork veinlets decrease in grade laterally outward from the core veins. Silver to gold ratios vary from 1:1 in shallow portions of veins in the south half of the deposit to greater than 35:1 at deeper levels (600 Level) in the north half of the deposit. Silver to gold ratios increase generally with depth, averaging about 10:1 at the surface of the Golden Queen vein, to about 35:1 at the 600 Level in the same vein. There is also a general horizontal zonation, from relatively silver-rich in the northeastern vein systems (e.g., Queen Esther – Independence), to gold-rich structures in the southwest (e.g., the Sheeted Vein system). The district silver-to-gold ratio average ranges from 15:1 to 18:1.
Alteration within mineralized zones consists of fracture-controlled and disseminated fine-grained silica, adularia, sericite, and minor pyrite. Intense quartz-feldspar-sericite alteration reportedly occurs in zones from about 10 ft to over 150 ft wide. Volcanic rocks are weakly silicified and argillically altered between and adjacent to zones of strong silicification. Weakly silicified and argillically altered rocks grade laterally into weakly to strongly propyllitized + illitized volcanic rocks. Propylitic alteration is best developed in the quartz latite flows.
Important vein systems, from the northeast to southwest, are the Black, Reymert, Karma-Ajax, Independent, Queen Esther, Silver Queen, No. 1 Footwall, Golden Queen - Starlight, Soledad, Alphason, Gypsy, Echo, Hope, Elephant, Bobtail, Excelsior, and McLaughlin. Post-mineral offset of about 300 ft on the east-dipping, apparently listric, Main Fault has displaced the Starlight vein in the footwall, from its upper continuation known as the Golden Queen vein in the hanging wall. Portions of the Soledad and No. 1 Footwall veins are also displaced by offset on the Main Fault. Veins northeast of the Golden Queen vein dip from 40° to 70° northeast. Veins southwest of the Golden Queen Vein dip about 70° southwest.
A zone of “Flat Ore” is present between the Starlight and Silver Queen Vein, in the hanging wall of the Main Fault. Flat Ore is a complex zone of veins and stockwork mineralization that is from 100 ft to 125 ft thick and nearly horizontal that at least in part consists of blocks of the mineralized zones cut by the Main Fault. Individual, parallel and en-echelon vein systems are present over a total strike length of 7,000 ft trending northwest, and a total width of 4,500 ft. Veins and vein zones are from 5 ft to 150 ft in thickness, 325 ft to 3,000 ft long, and from 300 ft to 1,000 ft in extent along dip. The horizontal distance between individual veins is from 50 ft to greater than 400 ft.