Summary:
Deposit Type
Mineralization at MLP most closely resembles Carlin-type sediment-hosted gold systems of northern Nevada. The Mother Lode deposit model includes structurally and stratigraphically-controlled disseminated sulphide and oxide mineralization hosted in primarily in Tertiary sedimentary rocks and rhyolite porphyry dikes. Paleozoic sedimentary rocks comprise a smaller volume host in proximity to dikes and the basal Tertiary unconformity. Tertiary volcanic rocks and debris flow breccias are also mineralized at Mother Lode. Alteration types associated with gold mineralization include passive decalcification and illite-pyrite alteration. Unique features of Mother Lode-style mineralization compared to typical Carlin-type deposits include: the association with fluorine, the significant volume of mineralization that is rhyolite porphyry dike-hosted, elevated tellurium and bismuth, and the generally passive, innocuous, very low-silica nature of the mineralization. Carlin-type gold deposits are generally known to form at depths of >2 kilometers, at temperatures between 180-240°C. Mineralization at Mother lode may have formed at depths of 500-1000 meters or less, and at temperatures <240°C (Weiss, 1996).
Mineralization
Mother Lode is characterized as a sediment, intrusive, and locally volcanic-hosted disseminated gold deposit. Mineralization most closely resembles Carlin-type sediment-hosted gold deposits of north-central Nevada. The Mother Lode deposit formed at ~12.7 Ma, which is much younger than the typical ~40 Ma age in north-central Nevada. The nature of mineralization is rather passive and with very low introduction of secondary silica, suggesting it may have formed at a shallower depth and at lower temperature than typical Carlin-type deposits. Mineralization exhibits geochemical associations between Au and As-Sb-Hg-Tl-Te-Bi-F, with very low Ag and base metals. The Mother Lode deposit model consists of structurally and stratigraphically-controlled disseminated gold mineralization hosted primarily in rhyolite porphyry dikes (Tip) and the Sedimentary Rocks of Joshua Hollow (SRJH). Lesser volume hosts include Paleozoic sedimentary rocks (Psd and Psq), Tertiary volcanic rocks (Tlr), and debris flow breccias (Tox). Mineralization in Tip, SRJH and Tlr is mostly sulphide, but may be oxidized depending on depth. Mineralization in Psd, Psq and Tox is mostly oxide. The current interpretation is that oxide mineralization in Tox is detrital, with mineralized rock having been mass-wasted as scarp breccias into the hanging-wall of the FCF. Paleozoic rocks are most commonly mineralized in proximity to dike margins but are also mineralized along subtle non-dikefilled structures and as pseudo-stratiform jasperoid bodies.
The primary structural control feeding mineralization at Mother Lode is a series of north-trending, 50-70° west dipping rhyolite dike-filled structures. Mineralization is both semi-tabular and highly irregular as fluids ascended along dike-filled structures in the underlying Paleozoic rocks, through the Tertiary unconformity, and expanded upward into the Tertiary section. Mineralizing fluids appear to have bled out laterally away from mineralized dikes into favorable permeable lithologies and secondary structures, including the FCF. Deeper drilling has extended the deposit to the west, where it appears that the FCF plays a significant role as a high-grade structural feeder. The potential for additional mineralized dikes in the vicinity of the FCF at depth to the west below current drilling is a highly attractive target.
Rhyolite dikes along the entire Bare Mountain swarm were subjected to high-temperature, sanidine- and biotitestable alteration shortly after emplacement (Weiss, 1996). At Mother Lode, biotite- stable alteration was followed by a lower temperature pervasive illite-smectite-pyrite event that affected both the dikes and surrounding wall rocks. The illite-smectite-pyrite event penetrates well up into the Lithic Ridge Tuff, forming a large sulphidation halo around the gold mineralization. Illite-smectite-pyrite alteration typically exhibits strongly elevated As, Sb and Tl, and may be well-mineralized or only anomalous in gold. The evidence suggests an early relatively barren illite-pyrite event is followed by an overprinting main stage Au-Te sulphidation event.
Trace element geochemistry associated with gold mineralization in drill hole samples includes: As (max 9262 ppm), Sb (max 1429 ppm), Te (max 16 ppm), Tl (max 17 ppm), Hg (max 30 ppm) and Bi (max 25 ppm). The elements with the strongest correlation to gold are tellurium and arsenic. Base metals are very low at Mother Lode. Elevated base metal values are largely associated with basement rocks. Weiss (1996) reports the presence of fluorite veining and a strong association between gold and fluorine at Mother Lode. Fluorine is not included in the multi-element analysis used by Corvus, but fluorite has been identified in the Mother Lode pit. Petrographic studies by Weiss (1996) on sulphide concentrates from drill holes at Mother Lode identified growth zones in pyrite grains exhibiting concentrations of arsenic. Weiss (1996) concluded that the Mother Lode deposit formed at a depth of 500-1000 meters or less, from gold-bearing fluids with temperatures between 200-240°C. High-grade gold appears to be associated with remobilized carbon, particularly in Tjs and Tjvs in the upper portion of the deposit. Remobilized carbon appears to have accumulated in a blanket-like zone above the upper reaches of the rhyolite dikes.
Jasperoid in Paleozoic rocks appears to have both structural and stratigraphic control. Jasperoid intervals in drill holes commonly contain significant Ag values up to 100 ppm. The average Ag/Au ratio from jasperoid samples is 15:1. The average Ag/Au ratio in non-jasperoid Tertiary rock-hosted samples is <0.5:1. Jasperoid mineralization in Paleozoic rocks may be a separate mineralizing event that predates the main Mother Lode gold system. Jasperoid is typically oxidized and yields high cyanide shake leach recoveries.
Rhyolite dikes along the entire Bare Mountain swarm were subjected to a sanidine- and biotite-stable (potassic), high-temperature alteration shortly after emplacement, as evidenced by secondary hypersaline fluids inclusions in quartz phenocrysts (Weiss, 1996). At Mother Lode, rhyolite dikes and surrounding wall rocks were later subjected to lower temperature, pervasive, illite-smectite alteration and associated disseminated pyrite sulphidation as an early event associated with gold mineralization. This alteration in Mother Lode dikes is progressive in intensity, probably due to proximity to open feeder structures. Biotite goes to chlorite and eventually chlorite gets replaced by pyrite. The groundmass and feldspar phenocrysts get replaced by illite-smectite, calcite and adularia.