In the early ERTS (Landsat-1) days, several geologic investigations were aimed at determining whether the MSS (and later, the TM) sensor could produce images (any mode: natural or false color; ratio; PCA; Unsupervised Classification; others) in which tell-tale signs of alteration of minerals in near surface deposits could be detected. Flip back to page 5-1 for the discussion of the principles involved. Most commonly sought were indications of hydrated iron oxides (gossans).
The best known and ballyhooed study was that of Goldfield, an active mining district in south-western Nevada, about 30 km (20 miles) south of Tonapah (a major mining area) on U.S. Highway 95. It was one of many small to medium active or abandoned mines (including the Cuprite district, discussed later) from which gold, silver, and copper have been extracted. The study was directed by Dr. Lawrence C. Rowan of the U.S. Geological Survey, with assistance from Pamela Wetlaufer of the U.S.G.S. and Alexander F.H. Goetz of JPL, and others. The present writer (NMS) was the NASA contract monitor for this investigation and followed its very promising results with excitement. We will review these, plus more recent updates, on this page.
The writer remembers driving through Goldfield enroute to Reno in the mid-80s. Alongside the highway on either side were natural discolorations in browns, yellows, and infrequently orange in the surface soil above the underlying volcanic rocks. This landscape also displayed prospecting pits exposing even fresher gossan alteration. This is a typical view:
Here is the original ERTS-1 scene in color which contains the Goldfield alteration anomalies. It is difficult to find Goldfield in this rendition but as you become familiar with it in subsequent views on this page you can return to this image and probably will find Goldfield. As rendered here, it is hard to spot anything anomalous
Here is a large part of a Landsat-1 image, in quasi-natural color, that includes the Goldfield district, which improves the expression of alteration (in this case mainly hydrated iron oxide (gossan):
The prominent mountain belt near the upper right is the Kawich Range. The near circular white patch is Mud Lake (normally dry - a playa - but may contain a thin water cover after heavy rains). Goldfield lies just below the brown patch that touches Mud Lake at its south end. The Cuprite Mining district is hard to see but occurs west of the peak-like Stonewall Mountain in the lower left of the image.
A perspective view, made by combining ASTER with DEM data shows this area as though seen from an airplane. The alteration zones have been colored in with blues, reds, and greens:
The Investigation Team produced a ratio version of this image, in which MSS Bands 4/5 appear in blue; 5/6 in yellow; and 6/7 in magenta. Vegetation is shown in orange; basalts in gray; silicic extrusives in pinkish-orange. Goldfield is at D; Cuprite is near G; C locates Pihute Mesa in an extension of the Nevada Test Site (this is the area the writer once proposed for a nuclear cratering explosion, when he worked for the AEC; it is also being considered as the main underground facility for eventual storage of nuclear wastes).
The writer tried his hand at bringing out gossan anomalies using ratioing. In this next image, MSS Bands 4/5 = blue; 5/6 = green; and 6/7 = red. Contrary to my expectations, the anomaly at Goldfield showed up as a distinct and separable orange-brown. Its shape is an elongate E-W patch, with a tendency to form a hook at its west end, which in different versions were a "trademark" of the Goldfield anomaly. That color occurs elsewhere, particularly in desert slopes to the east, and also around Cuprite.
Using Landsat TM data, this hook pattern has been more emphatically defined in this ratio composite (5/7, 3/1; 4/5 as RGB); the Cuprite alteration zone also is identified:
Find Goldfield, using Mud Lake as a reference point, in this full TM scene in which Bands 5 and 7 are involved. The color at the anomaly is pinkish.
Larry Rowan's group extracted what they believed to be anomalies relating to several varieties of alteration. Check their map (the form of the Goldfield anomaly is sketched in the lower right):
In the map below are X's that represent individual mining areas and prospects in a region corresponding to the Anomalies map just above. Switch back and forth between the two; you will note that most of the X's coincide with the color patches.
The correlation appears good enough to be reasonably convincing.
TM data have been processed to extract more information on the alteration present. This is a ratio image, with TM Bands 3/1 = red, indicating iron; 5/4 = green, denoting silica; and 5/7 = blue, picking out clay minerals.
A Principal Components image (subjected to a decorrelation stretch) of the Goldfield anomaly using data obtained from an AVIRIS mission yields this result:
Specific minerals are identified using this hyperspectral data set. For clays, pink = Kaolinite; red = Dickite; dark blue = Illite; for other secondary minerals, green = Alunite;
This group has also tested the ASTER sensor on Terra (page 16-10) for its capability in recognizing specific types of chemical alteration. The next two images show a color composite of the Goldfield area with, in the top image, Silicon oxide (silica) colored blue, and the bottom one, with the clay mineral illite in blue, the clay minerals dickite and kaolinite in green, and alunite and pyrophyllite in red:
Additional information on the above four images, and on other applications of hyperspectral remote sensing can be found by visiting Spectral International, Inc's Web Site.
Spectral International provides services worldwide for mineral exploration using remote sensing. Keeping with the theme of precious materials, here is an ASTER image of the Kimberley District in Australia, where diamonds are mined from an ultrabasic igneous rock type called Kimberlite. Here at the Argyle mine, Kimberlite veins in the open pit and surroundings are shown in two views. The veins are in green in the upper view; the red in the lower view is a kimberlite pipe (a cylindrical-like intrusion).
Spectral International has also surveyed a zinc metal mining district in Namibia, Africa. Three views based on ASTER data (with an image background) of the Skorpion Mine shows left to right 1) a false color image; 2) a computer-enhanced scene using ; and 3) a plot of alteration dominated by variations in illite in reds and greens:
We end this part of Section 5 with a tantalizing look at a powerful remote sensing product - images made with the AVIRIS (Airborne Visible-InfraRed Imaging Spectrometer) now being flown by JPL and the U.S. Geological Survey - which will be covered in detail in Section 13. Shown below (and again, in context, on page 13-10) is an image that delineates iron-bearing minerals and clay minerals around the Cuprite, Nevada mining district. This kind of detailed plot of the distribution of ore-guide mineralogy represents the current state-of-the-art capability of sensors suited for mineral exploration (along with many other uses outside of geology).
Many minerals in an alteration zone can be identified specifically, as indicated from the legend for this AVIRIS image of the Marysvale mining district in Utah.
These last three images above, of the alteration minerals at three well-known mining districts, all support the claim that hyperspectral remote sensing will be the prime tool in exploring for mineral deposits of commercial value from air and space platforms. For geologists, this approach is almost "revolutionary".
Remote sensing has also been applied to the hunt for uranium minerals. Some of these are associated with mineral deposits that oxidize or "rust" giving telltale alteration signatures. But prospecting for uranium minerals is usually done with handheld Geiger counters or scintillometers. Many uranium minerals are sought in certain rock types - often shale, but also sandstones and limestones. This next image is a specially enhanced subscene of a Landsat image in which different colors associate with different lithologies. The area shown is the San Rafael Swell, a broad dome with gentle outward dipping strata. It lies just north of the Waterpocket Fold, Utah, that we studied in Section 2.
As a peripheral observation, but again in Utah, we show here a space image of the famed Bingham Copper Mine, which is the world's largest open pit mine, in the Oquirhh Mountains west of Salt Lake City.
There is an alteration map of the natural alteration and that brought about by mining operations at the Bingham Mine which was made during an AVIRIS overpass. Unfortunately, the colors were not identified as to mineralogy in the online source.
Mining waste is a sure sign that some ore deposit of commercial value is being exploited. After the first California gold rush of 1849, placer deposits within the Sierra Nevada eventually "played out". Some wise prospectors reasoned that gold may also have been carried by the rivers into the Great Valley at the Sierra foothills. Techniques for dredging the river sediment soon proved fruitful, as more gold was discovered. This dredging continued well into the 20th Century and is still active on a limited basis. The visual signatures of this dredging are evident from space, as seen in the patterns formed along the Yuba River in an ASTER image: