Plutonic Hydrothermal Deposits           

Porphyry Copper Deposits

Porphyry copper (pCu) deposits are large, low grade copper deposits, which sometimes contain minor Mo, Ag, and Au.  Ore grade is generally around 0.5 % Cu.  Almost all known porphyry copper deposits are Tertiary in age.   They are thought to have formed in island or continental igneous arc settings associated with a subduction zone.  Their composition ranges from tonalite to monzonite.  pCu deposits usually form in relatively fine-grained intrusions of felsic composition.  Where formed in differentiated intrusive sequences, they tend to be formed in the finest-grained and most felsic end members of the suite.

There are two main compositional groupings in the western U.S.:  1) Continental Margin type, and 2) Island Arc type.  Continental margin types are granite-hosted, and typically contain significant Mo values.  Island arc types are quartz diorite or monzonite-hosted, and may contain significant Au values. 

Ore minerals present in pCu deposits include chalcopyrite, bornite, chalcocite, covellite, cuprite and tennorite.  The ores consist of concentrated swarms of quartz-sulfide stockworks and sometimes as sulfide disseminations.  Some deposits have a zone of secondary enrichment at the surface caused by groundwater leaching and redepositing the Cu at a lower elevation.  Characteristic alteration includes potassic (Biot. + K-feldspar), sericitic (Py + Sericite), and propyllitic (Chlorite, Epidote) (more on this in Class 8).

Porphyry Molybdenum Deposits

Porphyry molybdenum pMo deposits are large, low grade molybdenum deposits and the exclusive source of Mo.  As with the pCu deposits, pMo deposits tend to form in the most differentiated members of an intrusive suite.  Unlike pCu deposits, in pMo deposits there is usually only one ore mineral containing Mo, which is the mineral molybdenite.  Many pMo deposits contain some tin and tungsten minerals which are recovered from the ores for additional credits.  There is often a great deal of faulting associated with these types of deposits.  Multiple, overlapping mineralizing events are not uncommon.  Typically ore grades are in the range of 0.1 ­ 0.5 percent MoS₂. 

Tin Greisen Deposits

Tin greisen deposits tend to form in near the top of granite intrusions, an area referred to as a cupola.  The mineralization occurs primarily as big quartz veins containing variable amounts of tin oxide or sulfide minerals.  These deposits tend to form in intrusions which lack accessory magnetite.  This is because tin substitutes for iron in the mineral magnetite, and becomes unavailable to concentrate in the late stage fluids.  Fluid conditions required to form tin oxide (cassiterite) are relatively reducing.  These deposits may have distinct metal zoning associated.  Tin and Tungsten are dominant lower and closer (or “proximal”) to the causitive intrusion.  Base metals (Cu + Pb + Zn) are distinctly elevated in the outer reaches of the intrusion or area (called “distal”).

Skarn Deposits                      

Skarn deposits are replacement deposits.  They form by the replacement of limestone, calcareous rocks (marl or calc-schist), or dolomite.  A wide variety of minerals can form in skarn deposits, but the most common include oxide minerals such as magnetite, sulfide minerals such as chalcopyrite, silicate minerals such as epidote, or the tungstate mineral Scheelite.  Gold is also mined from skarn deposits.  Skarn deposits are a result of the invasion of the country rock by hydrothermal fluids carrying the high metal concentrations outward from the intrusion.  The fluid composition steadily changes as the plutonic source goes through the cooling stages.  Some skarn mineralization is formed by earlier, higher temperature waters (called “prograde” minerals), and some skarn mineralization is formed later at lower fluid temperatures (called “retrograde” minerals).  The waters contain high concentrations of metals, and may even be the same fluids which concurrently formed pCu deposits.  There are many ways to classify skarns, but a simple scheme based on composition is:

Skarns can be either massive and discordant, or stratiform and concordant (with respect to bedding of the host rock).  Sometimes the bedding contacts are favorable places for skarns to form.  Ore minerals in skarns may be associated with  “calc-silicate” minerals such as epidote, tremolite, zoisite, wollastonite,   Silica, iron and magnesium are supplied by hydrothermal fluids that evolve from the magma late in the cooling history.  The minerals form at the expense of calcite or dolomite in limestone or limey sediments.  More abundant impurities in the calcareous sediments appears to enhance the formation of skarn deposits in some examples.

Photogeology                    

Photogeology is a hugely important area of mineral exploration.  It includes all types of low to high altitude photography as well as satellite photography.  Images are recorded either on films or by recording the image digitally.  Films used include black & white, true color, and false color (Infra-red) (IR) types.   In color IR photos, the red areas indicate live vegetation.  This makes them useful for locating outcrops in highly vegetated areas.  Color IR film is also least affected by haze, and so is more effective in cloudy weather.  It is also good for determining moisture content of soils (darker).  The most common types of aerial photos are those which are taken with the camera lens vertical, ie, with the lens pointed straight downward.  “Obliques”, which are taken with the lens pointed obliquely downward, tend to exaggerate the relief. 

Photos have distortion unless they are specified to be “corrected”.  The map scale towards the center of the photo is different from the map scale toward the edges of the photo.  “Orthophotos” are images which have the distortion rectified, and can be used directly for mapping purposes. 

There are many applications of photogeological methods in mineral exploration work as well as in the studies of environmental geology and geologic hazards.  Most importantly, they are used to make accurate topographic base maps. In mineral exploration work, accurate topographic base maps are essential for recording geological observations.  Rock and soil color changes, or “color anomalies”, can be delineated and possibly investigated with ground traverses. 

 Geological features can also  such as lithologic contacts, alteration zones, and structural information.  With sufficient exposure of layered rocks, it is possible to tell a great deal about the structural features in a area, such as the direction and dip of the beds, fold plunge direction, and fault plane dip (Figure 1).  Any color contrasts in exposed bedrock are probably caused by changes in rock type, or “lithology”, and can be traced on the photograph, or on an acetate overlay, to map out the contact.   The information can be gathered more efficiently and safely than a ground traverse, although there is no substitute for direct observations.   Aerial photos also provide a means for accurate documentation of cultural features, or accurate history of a mine’s development by taking aerial photos at regular intervals (Figure 2). 

Most aerial photographic surveys follow specified flight lines and take the photographs at regular spacings along the path.  The overlap between adjoining photos in a sequence along the line is about 60 %.  The overlapped area is “seen” by the camera from two different “perspectives” in the two different photos.  The two adjoining photos used together make what are called a “stereo pair”.  The two photos can be placed side by side and observed with a stereoscope.  A stereoscope allows each eye to focus on separate photos and merge the two images into one single image.  The new image appears to be in three dimensions.  Try merging the images below with your eyes.  They form the image of a pyramid.

Pending Permission to Use
Figure 1.   Little Dome, Wyoming, double plunging antiform.

Pending Permission to Use
Figure 2.   Bingham, Utah open pit copper mine.

Remote Sensing                                

Remote sensing is the acquisition and use of digital images of the earth’s surface from “Landsat” satellites orbiting the earth at altitudes of up to 438 miles.  A sensor is used with an  electronic scanner to measure specified portions of the electromagnetic spectrum in the radiation from the sun which is reflected off of the surface of the earth.  A variety of spectra are recorded, including color infrared and several different ranges, or “bands” (Figure 3). 

The seventh Landsat satellite was launched in 1999.  It carries an instrument called “Enhanced Thematic Mapper Plus”, which has a resolution (pixel size) as low as 49 feet.  Frequencies which are scanned include visible, near-infrared, and thermal infrared portions of the spectrum. 

Another type of satellite imagery is Side-Looking Airborne Radar.  This method transmits microwave energy from the satellite to the earth’s surface, where it is reflected back to the sensor and recorded digitally or photographically.  Since the method does not rely on the sun’s energy, it can be used to record the reflections in complete darkness and without regard to weather.

Figure 3.  Color infrared Landsat, Talkeetna Mts., Alaska.