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By Stephen Zahony

Seldom do we see a satisfactory big geologic picture within which mineral deposits occur. Even in the deepest open-pit mines, which can expose a porphyry copper deposit to its economic bottom, the entire intrusive system with which the deposit is associated remains speculative, only the high-level differentiated porphyries are exposed. The entire magmatic system, from which the ore-related porphyries and their residual ore components were derived, remains unknown below the 1-2 kilometers depth that the mine exposures and deeper drill holes have probed.

The whole geologic environment of epithermal vein systems is even more nebulous than that of porphyry systems. Rarely can ore-related intrusives be identified. The DREGS 2018 fall field trip to the Gold Hill gold-silver telluride district of Boulder County, CO evinced the handicap in understanding the overall geologic environment of a high-level epithermal system. A handful of late-Laramide-age dike types occur in the district, some of which are suspected to be ore-related, but even the 1,000 feet that the deepest mines reached, is not close to exposing the big geologic system that produced the precious-metals veins. An uplift, possibly magmatic in origin, might be responsible for the topographic high that forms Gold Hill-Bighorn Mountain, but understanding the entire system, from the gold-bearing zones of the hosting structures to the source intrusive, might require cognizance of the geology of as much as three to ten kilometers vertically. The mining geologist might have great insight into wall rock alteration and metal distribution in the veins but his understanding of the overall geologic system remains quite limited. An unusual and little publicized insight into the deep roots of a major gold deposit - Muruntau - is briefly described below. Super-deep drilling altered the classification and genetic model of the deposit, a model possibly applicable to California's Mother Lode.

Disregarding the paleo-placers of the Witwatersrand, the gold deposit at Muruntau, Uzbekistan is one of the largest, if not the largest singular primary gold deposit in the world. It has produced over 55 million ounces of gold and its total gold endowment may exceed 160 million ounces (3). Open-pit mining of ore averaging a little over 2 grams Au/metric ton is now reaching its mining depth limit, but deep drilling has shown that the same grade of ore continues down to depths of 1.5-2.0 kilometers (1). The mined ore body consists of myriad of smaller to larger gold-bearing quartz veins with an overall shape of a pipe, flared at the top and narrowing with depth. Host rocks are brittle variegated clastic sediments (sandstone, siltstone and phyllite) of probable Paleozoic age affected by intense structural activity (2). The stratigraphy is obscured by regional metamorphism ranging from lower greenschist to amphibolite facies, by transpressional and thrust faulting, and by hydrothermal alteration. A thrust-faulted Paleozoic cap of carbonate-dominated rocks may have been the upper limiting barrier for mineral depositing fluids.

There are several phases of veining that range from metamorphic grain-parallel to crosscutting, but the veins clearly overprint the deformational fabric of the host rocks. Stockwork to sheeted veins make up the majority of the veins, which combined with larger leader veins contain the bulk of the gold. Wallrocks are K-feldspar-biotite-sulfide altered. Veins formed at high temperatures (400-450o C) and consist of quartz with a low content of sulfide minerals, in the range of 2-3%, consisting mainly of pyrite and arsenopyrite (4). Scheelite is also present. Published analyses for 24 samples of ore from the expanded Muruntau pit, which is now connected with the Myutenbai pit to the southeast, returned the following average values: Au 8.26 ppm, Ag 4.5 ppm, Pd 1.22 ppm, As 1.44%, W 162 ppm, Bi 111 ppm, Te 17 ppm, Se 22 ppm, and Sb 68 ppm; Cu, Pb, and Zn all about 100-200 ppm (5).

A variety of altered granitic dikes are found in the pit that appear to be related to ore formation. The vein system at Muruntau has been touted as a locus tipicus of orogenic type gold mineralization, formed from metamorphic fluids; but a different genetic picture has been revealed by some little-known information generated during Soviet times.

Motivated mostly by geophysical data, an extremely deep exploration hole was drilled at Muruntau in 1986, information from which was published beginning only in 1996. This super-deep hole was collared along the edge of what is now the southeast rim of the 3.5 kilometer-long combined open-pit. The hole ultimately reached a depth of 4,294 meters (14,089 feet!) (4). Such a deep hole would only be possible within a political system where a regard for economy was not the driving factor. Metamorphic grade increased with great depth, culminating in a hornfels layer of about 500 meters thickness, immediately below which, at 4,005 meters, a granitic mass was intersected. Drilling was continued in the albitized granite for another 289 meters before the hole was terminated (4). U-Pb age-determination of the granite's zircon returned a date of 289 ñ4Ma while arsenopyrite from the central gold zone above returned Re-Os dates of 286 ñ5Ma (7). There is a significant variation in dates obtained by K-Ar and Rb-Sr methods for alteration minerals ranging from 260-300 Ma (4), but the Re-Os and U-Pb dates are probably most reliable, showing that the hydrothermal system is temporally related to intrusive rocks at depth.

In this case, the deep penetration, where an "orogenic" gold system was explored at great depth, showed the time association of an underlying intrusive with the pipe-shaped system of gold-bearing quartz veins. This big picture of the geologic system was only possible to ascertain from information that a +4,000-meter-deep hole was able to provide. Muruntau may be orogenic in a structural sense but is an intrusive-related hydrothermal system.

The picture from Muruntau makes one wonder about the Mother Lode system of California, where insufficient depth has been explored to allow a definitive understanding of the big geologic picture. Gold-bearing quartz veins of the Mother Lode postdate metamorphism; they clearly cut across the metamorphic grain. Like Muruntau, an outstanding feature of the Mother Lode veins is the close association of arsenopyrite with gold. The arsenic content of ore shoots is three orders of magnitude higher than gold. In Mother Lode drilling, the mining viability of a vein is more judged by its arsenopyrite and base metal sulfide content than the erratic gold grade. Thus, arsenic might be a more valuable tool in deciphering the provenance of the gold.

No vein-related intrusives have been reported in the deepest (1.3 km) mines of the central Mother Lode, but there is a long narrow Mother Lode-parallel extension of the Sierra Nevada batholith, located a little over one-mile east of the gold-bearing lode system. There is insufficient age information for this finger-like extension of the batholith; only one age date is available some miles to the north. A date of approximately 130 Ma has recently been established by a handful of Ar-Ar age dates on mariposite for the central Mother Lode gold-bearing veins (6). This age is slightly older than the western main mass of the Sierra Nevada batholith.

Mother Lode veins are not banded; unlike epithermal veins, they were not formed by flow-through fluids, but formed in a more static environment with fluids impounded, that probably circulated in the sealed and highly pressured environment and slowly clogged the fissures that hosted them. Structural movements caused wall rocks to rupture and be engulfed by quartz, forming ribbon-like pendants that were altered and partially dissolved under pressure to stylolitic bands that became loci for gold deposition. What generated the fluids; what was the post-metamorphic source of heat? Are there earlier phases of the Sierra Nevada batholith lurking below the central Mother Lode that are age-equivalent with the mariposite age dates? Just north of the true Mother Lode, at Grass Valley, in a vein system similar to the Mother Lode, the intrusive connection to gold mineralization is difficult to deny. Dream on Mother Lode experts, a super-deep deep drill hole is not yet in the works!

1.Csongradi, J. (2018) personal communication, Budapest, Hungary.
2. Drew, L.J. et al. (1996) Geology and structural evolution of the Muruntau gold deposit, Kyzylkum desert, Uzbekistan, Ore Geology Reviews, 11, pp. 175-196
3.Hall, G. (2004) SEG Meeting presentation, Perth, Australia, and (2018) personal communication, Denver, Colorado
4.Kempe, U. et al. (2016) The Muruntau gold deposit (Uzbekistan) - a unique ancient hydrothermal system in southern Tien Shan, Geoscience Frontiers V. 7, Issue 3, pp. 495-528
5.Koneev, R.I. et al. (2010) Nanominerology and Nanochemistry of Ores from Gold Deposits of Uzbekistan, Geology of Ore Deposits, Vol. 52, No.8, pp. 755-766
6.Marsh E.E., et al. (2008) New constraints on the timing of gold formation in the Sierra Foothills province, central California, Arizona Geological Society Digest 22, pp. 369-388



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