Spatial Patterns in the Gold Deposits of Nevada, U.S.A.

Transcription

1 Spatial Patterns in the Gold Deposits of Nevada, U.S.A. PowerPoint 6-4 All images and text are Copyright 2005 by Larry Robinson

2 Prepared as part of:

3 The Spatial and Temporal Distribution of the Metal Mineralisation in Eastern Australia and the Relationship of the Observed Patterns to Giant Ore Deposits by Larry Robinson A thesis submitted for the degree of PhD 2006

4 Currently Nevada, which is in the Great Basin, is the third largest gold producer in the World. In 2002 it produced 7.73 million ounces compared to Australia at 9 million ounces and South Africa at 12.7 million ounces. 1 The Great Basin has a long and complex geologic history, spanning at about 800 Ma and involving major episodes of crustal accretion, sedimentation, igneous activity, compressional deformation, and continental rifting and extension. The longevity, variety, and intensity of tectono-magmatic activity in this region has resulted in the formation of a rich geologic, metallogenic province. (p. 224) 2 (Mihalasky and Bonham, 2001)

5 The Geology of Nevada3

6 The Geology and Gold Deposits of Nevada Note that the Large to Giant Sediment hosted gold deposits are concentrated in northern half of Nevada.

7 To analyze the possible patterns in the spatial distribution of gold deposits requires isolation of the data.

8 The current spatial distribution of gold deposits in Nevada 4

9 Ludington et al. (1993) conclude their paper titled Spatial and temporal analysis of precious-metal deposit models for a mineral resource assessment of Nevada with the following question: "Why are they (the giant sediment-hosted deposits) distributed in an area where epithermal deposits are scarce or absent?" (p. 37). 5

10 Note the two arcs of Large to Giant Sediment-hosted deposits.

11 It is proposed that compressional forces, in a NE-SW direction, distorted the original spatial distribution of the deposits.

12 The proposed reconstructed, 200 km circular feature. This was the original spatial distribution of the giant sedimenthosted deposits. Note how the 570 km, semi-circular arc of small, low temperature, epithermal deposits delineate the circle of giant sedimenthosted deposits.

13 The majority of the giant, sedimenthosted deposits occur along the outer rim of the proposed HOT ZONE.

14 Oppliger et al. (1997) propose a genetic relationship between the Ma magmatism, extensional tectonics, and gold mineralisation event centred in the Great Basin of Nevada and the development of the ancestral Yellowstone hotspot. The Yellowstone hotspot (probably a plume) was progressively overridden by the North American plate. Voluminous intracrustal melts were generated and metamorphic devolatization took place as the hotspot broke though the subducted Farallon plate. Convective circulation of hydrothermal fluids in the upper crust occurred in structural and lithologic traps; these characterize the classic Carlin-type (giant sediment-hosted) deposits. 6

15 From Dzurisin, et. al., (1995)7

16 It is proposed that the hot spot (the ancestral Yellowstone Caldera on which the giant sediment-hosted deposits formed) also shaped the current topography in the Great Basin. For this to take place would require the hot spot to be located ~200 km south (relative to the crust) from its position at the time the giant sediment-hosted deposits were being formed. This is shown in the following sequence of slides. This sequence has predetermined timing.

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28 There is supporting evidence that the ancestral Yellowstone hot spot was located at the proposed site in southern, central Nevada.

29 Immediately south of the proposed site of the Yellowstone ancestral plume, reflected in the topography, there is: a significant increase in Bouguer gravity anomalism (~80 mgal), a seismically active region, a major, abrupt, southward decline in elevation (~800 m). The steepness of the gravity gradient requires significant density contrast in the mantle. Gravity, seismic, heat flow, and isotopic observations explain the origin of the topographic step, which is proposed to be coincident with the arrival of the starting head of the Yellowstone thermal plume at the base of the lithosphere. 8

30 The End Thank You

31 References 1. URL for Data on Gold Production in Nevada 2. Mihalasky, M. J., and Bonham, C. G. F., 2001, Lithodiversity and its spatial association with metallic mineral sites, Great Basin of Nevada: Natural Resources Research (International Association for Mathematical Geology), v. 10, p URL for the Geology Map of Nevada 4. United States Geological Survey data base for Nevada, dbf 5. Ludington, S., Cox, D. P., Singer, D. A., Sherlock, M. G., Berger, B. R., and Tingley, J. V., 1993, Spatial and temporal analysis of precious-metal deposits for a mineral resource assessment of Nevada: Geological Association of Canada Special Paper, v. 40, p Oppliger, G. L., Murphy, J. B., and Brimhall, G. H., Jr., 1997, Is the ancestral Yellowstone hotspot responsible for the Tertiary "carlin" mineralization in the Great Basin of Nevada?: Geology (Boulder), v. 25, p Dzurisin, Daniel; Christiansen, Robert L.; Pierce, Kenneth Lee., 1995, Yellowstone; restless volcanic giant, USGS Open-File Report 95-59, 2 p. ill., map 8. Saltus, R. W., and Thompson, G. A., 1995, Why is it downhill from Tonopah to Las Vegas?: a case for mantle plume support of the high northern Basin and Range: Tectonics, v. 14, p