GSN Monthly Membership Meeting – Friday, March 19th at 6:30 p.m.

Description

Contact: GSN@GSNV.ORG

GSN Virtual Talk:  FRIDAY, MARCH 19, 2021 – Zoom Opens @ 6:15 PM,  Talk begins @ 6:30 PM (Pacific)

Guest Speaker:  Stan Keith, MagmaChem

Title: “Yellow Gold and Black Gold in the Great Basin: a Magma-Metal Series Approach”

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REVISED ABSTRACT:

Yellow Gold and Black Gold in the Great Basin: a Magma-Metal Series Approach

by

Stanley B. Keith, Jan Rasmussen, Monte Swan, and Troy Tittlemier

Nevada is notorious for its gold cluster centered near Carlin, Nevada. Aficionados of the resource geology in this part of the world know the black gold (oil and hydrocarbons) is also a well known denizen of this region. The economic dominance of the yellow gold makes it tempting to consider northeast Nevada as a gold province. The concept of a northeast Nevada gold province in its regional context however is a major oversimplification. Considered in a more regional geologic and especially temporal context, northeast Nevada is more notable for its highly variable and heterogeneous resource geology in space and time (see the Nevada Test Site region magma-metal series map in Rasmussen and Keith, 2015). Nevertheless, there is no question that northern Nevada for both yellow and black gold has a clustered distribution of gold and petroleum. Comparison to other areas such as SE Arizona reveals contrasts such as a highly economic cluster of economic copper deposits and no notable gold or petroleum clusters. Why is this??

Application of the Magma-metal series approach (Keith and others, 1991; Keith and Swan 1996; Rasmussen and Keith, 2015) reveals a possible answer: crustal oxidation state. In oxidized crusts, oxidation states of average plutonic ferric:ferrous ratios are above 0.9, and early magmatic magnetite-sphene assemblage is present. Hydrothermal porphyry copper deposits (Morenci-Chuquicamata Type) are present in the case of oxidized hydrous calc-alkalic metaluminous intrusions. In areas where plutonic oxidation states are less than 0.9 (and especially less than 0.6), early magmatic ilmenite is present. Correspondingly, hydrothermal porphyry gold deposits (Carlin-Porgera Type) are present in the case of reduced hydrous calc-alkalic metaluminous intrusions (Figure 1). Aqueous chloride complexing is probably the main hydrothermal transport agent in the oxidizing porphyry copper model, whereas bisulfide complex is an important way to transport gold in the case of reducing porphyry gold situations. The influence of crustal oxidation state on gold grades in the associated porphyry metal deposits is shown in Figure 2.

The development of the ultradeep hydrothermal/hydrocarbon model (UDH) applied to Nevada case histories (Rasmussen and Keith, 2015, and Keith and others, 2018) for Kupferschiefer Type Cu-Ag deposits offers a new opportunity to enrich the above model (Figure 3). The UDH model also suggests that oil ultimately has a hydrothermal origin, and that like water, may be stable as kerogenated supercritical water in the supercritical region between its source in a serpentization ‘kitchen’ in the lower-mid crust and a hydrothermal mineral deposit trap in the upper crust. In reduced crust where the hydrocarbon component does not oxidize to carbon dioxide, it is an open unresolved question if petroleum stable fluids like supercritical water can penetrate and mix with magmatic silicate liquids. As such, oil could act incompatibly and leave the magma with metals (e.g. gold) and deposit various hydrocarbon and metal compounds under cooler, ionic conditions.

Experimental data indicates that oil could transport economically interesting amounts of gold. Solubility testing by Williams-Jones and Migdisov, 2007, show that at 250^oC, oil can transport gold at the 50 ppb level. The 250^oC temperature is well above the conventional oil window, and is similar to hydrous pyrolysis experiments by Lewan, 1997, that generated oil above 300^oC. The experimental data is validated by natural occurrence data. At the Rodeo gold deposit in the northern Carlin trend of Nevada, a spectacular pyrobitumen vein has been documented by Williams-Jones and others, 2009, that carries bonanza grade gold at 30 ppm (Figure 4).

It is now becoming increasingly apparent that high temperature hydrothermal oil is not simply an exception but may be the rule and that yellow gold and black gold both owe their origin to deep seated hydrothermal processes. In this context, the concept of magmatic oil is not that outrageous.

Figure 1. Relationship between plutonic oxidation state and gold grades of spatially associated gold deposits.

 

Figure 2: Geographic Patterns of Crustal Oxidation State determined from plutonic ferric:ferrous ratios and mineral assemblages in spatially associated mineral deposits.

 

Figure 3. Ultradeep hydrothermal (UDH) model for crustal scale hydrothermal plumes in incipient continental rift settings like Kupferschiefer in northern Europe and the Hydrothermal Hydrocarbon Petroleum systems of Basin-Range age in the Great Basin.

 

Figure 4. The bonanza grade pyrobitumen vein provided the first evidence we have seen that shows that oil is hydrothermal and can carry a significant amount of metals (in this case, gold) (from Williams-Jones and others, 2009).

References:

Keith, S.B., and others, 1991, Magma series and metallogeny: a case study from Nevada and environs: Nevada Geological Society Guidebook for Field Trips, v. 1 (Field Trip No. 8), pp. 404-493.

Keith. S.B., and Swan M.M., 1996, The great Laramide porphyry copper cluster of Arizona, Sonora, and New Mexico: the tectonic setting, petrology, and genesis of a world class porphyry metal cluster, in Coyner, A.R., and Fahey, P.L., eds., Geology and Ore Deposits of the American Cordillera: Geological Society of Nevada Symposium Proceedings, Reno/Sparks, Nevada, April.

Keith, S.B., Spieth V., and Rasmussen, J.C., 2018, Zechstein-Kupferschiefer Mineralization Reconsidered as a Product of Ultra-Deep Hydrothermal, Mud-Brine Volcanism. In Al-Juboury    AI (ed) Contributions to Mineralization, Chapter 2, pp. 23–66.

Lewan, M. D, 1997, Experiments on the role of water in petroleum formation: Geochimica et Cosmochiica Acta, vol. 61, no. 17, pp. 3691-3723.

Rasmussen, J.C. and Keith, S.B., 2015, Magma-Metal Series classification of mineralization in the vicinity of Yucca Mountain, Nevada, Pennell, W.M., and Garside, L.J. eds.: Geological Society of Nevada Symposium Proceedings, New Concepts and Discoveries, vol. II, pp. 1131-1152.

Williams-Jones, A.E., Migdisov, A.A., 2007, The solubility of gold in crude oil: implications for ore genesis. In: Andrew C.J., et al. (eds) Proceedings of the 9^th Biennial SGA Meeting, Dublin, Millpress, pp 765-768.

Williams-Jones, A.E., and others, 2009, Gold in Solution:  Elements, vol. 5, pp. 281-287.