Prospector's Guide to Diamonds

Diamonds are a Geologist's Best Friend

EXPLORATION & PROSPECTING METHODS

When diamond exploration programs are initiated, priority is given to areas of favorability for finding ?traditional? diamondiferous host rocks. For example, the traditional lodes are associated with kimberlite and lamproite. These are restricted to cratonic regions that have been stable for about 1.5 Ga (Ga= Billion years). Janse (1984, 1994) suggested cratons be separated into areas of favorability known as (1) Archons, (2) Protons and (3) Tectons. This provides an excellent first option priority list.

Archons (Archean basement created >2.5 Ga ago) are considered to have high potential for discovery of commercial diamond deposits hosted by kimberlite and possibly by lamproite and lamprophyre.  The fabulously rich diamond deposits at Ekati and Baffin Bay in Canada occur in kimberlites hosted by a thick Archean craton.

North American craton separated into areas of favorability for finding diamond deposits. 

Protons (Early to Middle Proterozoic 2.5 to 1.6 Ga basement terrains) have moderate to low potential for commercial diamond deposits in kimberlite. Because of the discovery of a very rich diamond deposit intruded in Proterozoic basement rocks in Australia known as Argyle, these terrains are now thought to have high potential for commercial diamond deposits in lamproite and possibly lamprophyre. Most kimberlites found in such terrains have proven to be weakly to poorly mineralized in diamond. Lamprophyres are proving to be more and more interesting with more such rocks yielding diamonds (Erlich and Hausel, 2002).

Tectons (Late Proterozoic 1.6 Ga to 600 Ma [Ma=Million Years] basement terrains) are considered to have low potential for commercial diamondiferous host rock. Unconventional diamond deposits (such as high-pressure metamorphic complexes, astroblemes, subduction-related complexes and volcaniclastics) may occur in tectonically active terrains, but the methods for exploration for these, are not well defined ? but as more of these deposits are discovered, the greater the possibility for finding rich commercial diamond deposits is likely. The discovery of Beni Bousara in Morocco and Rhonda complex in Spain may suggest that some very rich diamond deposits may be found along subduction boundaries some day.

Following selection of a favorable region; topographic and geological maps, aerial and satellite imagery and aerial geophysical data should be examined for traditional anomalies that are characteristic of pipe-like features (Hausel and others, 1979). Unusual circular depressions, circular drainage patterns, noteworthy structural trends and vegetation anomalies are noted.  Geophysics is used to search for distinct (?bull?s eye?) conductors and magnetic anomalies (Hausel, 2006, 2007, 2008). Geochemical data, if available, are examined for Cr, Ni, Mg, and Nb anomalies.

Stream sediment sampling. One of the primary methods used in diamond exploration is stream sediment sampling programs designed to search for ?kimberlitic indicator minerals? (pyrope garnet, chromian diopside, chromian enstatite, picroilmenite, chromian spinel, and diamond). Diamond targets are small and may range from diatremes of several acres to narrow dikes and sills. Diamond-bearing kimberlites and lamproites typically are formed of abundant soft serpentine that encloses resistant mantle-derived xenocrysts, xenoliths and megacrysts. Many of these mantle nodules (peridotites and eclogites) are the source for most kimberlitic indicator minerals used for exploration. The nodules tend to break down during weathering and transport downstream scattering their mineral assemblages into the eluvial material and adjacent drainages. Large megacrysts also provide a good source for indicator minerals.

Peridotite nodule from kimberlite. Note the rounded pyropes in this specimen.

The indicator minerals may be carried downstream for hundreds of feet or a few miles depending on the climatic and geomorphic history of the region. Diamonds however, are thought to be carried considerable distances ? in some cases, hundreds of miles. The indicator minerals may provide a trail leading back to the source.  During stream sediment sampling surveys in the State Line district, Laramie Mountains, and Iron Mountain district, we were able to find a few hundred kimberlitic indicator mineral anomalies that still remain unexplored to this day (2010)!  In the State Line, Iron Mountain and Middle Sybille Creek kimberlite districts, we found that the transportation distances for the classical kimberlite indicator minerals was likely about 1.5 to 2.5 miles for picroilmenite, 1 to 2 miles for pyrope garnet, and 0.25 to 1 mile for chromian diopside. Thus, when these are found by panning (we used the traditional gold pan), all one needs to do is continue panning upstream until the can find no more indicator minerals. At this point, the kimberlite intrusive should lie somewhere between the last positive and negative samples.

As several locations, we found hundreds of indicator minerals ? one of the more impressive sites was the Grant Creek anomaly, where we also recovered diamond stability minerals, and this are too remains unexplored even though I also found a large limestone xenolith next to the kimberlitic indicator mineral anomaly (and limestone does not occur in this region!).

In the planning stages of stream-sediment sampling, proposed sample sites are initially marked in prominent drainages on a topographic map using a sample spacing designed to take advantage of the region. In arid regions, sample spacing should take advantage of relatively short transport distances of the indicator minerals. In subarctic to arctic areas (i.e., Canada, Sweden, Russia, etc) sample density may be considerably lower owing to the greater transport distance and the logistical difficulties of collecting samples. Anomalous areas are then re-sampled at a greater sample density.

The traditional kimberlitic indicator minerals are rare to non-existent in lamproite, thus other minerals (zircon, phlogopite, K-richterite, armalcolite, priderite) may be considered that unfortunately have low specific gravity, poor resistance to abrasion, and are potentially difficult to identify. The better indicators for diamondiferous lamproite have been diamond, magnesiochromite and olivine. 

The classical kimberlitic indicator minerals are described by Hausel (2009), Hausel and others (1979). Chromian diopside is the most readily recognized in the field. This mineral has an average specific gravity of 3.4 thus it can be readily captured in a gold pan with black sand concentrates. Chromian diopside as megacrysts in kimberlite will be rounded, but because of excellent cleavage and twinning, it will easily break apart during stream transport producing rectangular-shaped, emerald-green color. The emerald green color is distinctive and very difficult to capture in photography.

 

Kimberlitic indicator minerals (above left) and large chromian diopside megacryst in kimberlite (right).

Pyrope garnet has a specific gravity of about 3.8 so it too is captured in gold pans. It should be rounded (without crystal faces) due to partial assimilation in the kimberlite magma during transportation to the earth?s surface from depth of 90+ miles. Pyropes are reddish purple, purple and yellow-orange in color. Pyropes found in most diamondiferous kimberlite should exhibit geochemical similarities to tiny mineral inclusions found within diamonds. These pyropes are known as G10s and have relatively high Cr2O3/CaO ratios. It is rare for any prospector to have access to an electron microprobe, which unfortunately is required to test the chemistry of pyropes. Many universities and a few private labs will have these instruments.

Picroilmenite is also commonly found in kimberlite. It occurs as non-magneitic, metallic, rounded mineral grains and megacrysts with a specific gravity of 4.2. In the field, we would look for this mineral until we found one that was nice and rounded and had a white coating of leucoxene ? this provided evidence that it likely came from a kimberlite.

Picroilmenite from kimberlite - note grains with white leucoxene coating used to identify ilmenite from kimberlite.

There are other indicator minerals, but the above three are the principal minerals used in the search for diamondiferous kimberlite.

 To take advantage of the dispersion of kimberlitic indicator minerals, the size of samples are determined based on the environment. For example, where there is a general lack of active streams, larger samples are taken compared to regions with active drainages. In areas with juvenile streams, samples are often panned on site to recover a few pounds of sample concentrate. Recovered indicator minerals are tested for chemistry using an electron microprobe to identify those that have higher probability of originating from the diamond stability field. The data are plotted on maps to facilitate evaluation.

Geomorphology. Kimberlite and olivine lamproite may be pervasively serpentinized, making outcrops the exception rather than the rule.  In many cases, geomorphic expressions of pipes are subtle to unrecognizable. The Kimberley pipe in South Africa was expressed as a slight mound, but nearby pipes (i.e., Wesselton pipe) were expressed as subtle depressions. Others produced subtle modifications of drainage patterns (Mannard 1968). In the subarctic, where glaciation has scoured the landscape, some kimberlites produce noticeable depressions filled by lakes. In the semi-arid region of Wyoming and Colorado, a few kimberlites are expressed as slight depressions, some are covered by shallow ponds, but most blend into the surrounding topography and may or may not have a subtle vegetation anomaly.

Sloan 5 Kimberlite showing vegetation anomaly - open park with no trees & nice stand of trees growing along a lineament on its right side.

In the Ellendale field, Western Australia, serpentinized diamondiferous olivine lamproites lie hidden under a thin layer of soil in a field of well-exposed leucite lamproite volcanoes. The Argyle lamproite and diamondiferous lamproites in the Murfreesburo area of Arkansas were also hidden by a thin soil cover.

Lineaments. Many kimberlites and lamproites are structurally controlled (Hausel and others 1979; 1981; Macnae 1979, 1995; Nixon 1981; Atkinson 1989; and Erlich and Hausel 2002; Hausel, 2006, 2007, 2008). Controlling lineaments and fractures may be indicated by alignment of a cluster of intrusives or by the elongation of a pipe. In Lesotho, South Africa, Dempster and Richard (1973) reported a close association of kimberlite with lineaments: 96% of kimberlites were found along WNW trends, and many pipes were located where the WNW trends intersected WSW fractures.

Aerial photo showing ponds (probable kimberlites) along distinct northerly lineaments. Note the white bull's eyes around a couple of the ponds (calcium carbonate leached from the rock) and the road running through the Indian Guide area, Wyoming. These remain unexplored.

Lamproites in the Leucite Hills, Wyoming are found on the flank of the Rock Springs uplift where distinct E-W fractures lie perpendicular to the axis of the uplift (Hausel, 2006a; Hausel and others 1995). In the West Kimberley province of Western Australia, some lamproites are spatially associated with the Sandy Creek shear zone, a Proterozoic fault. In the Ellendale field, several lamproites lie near cross faults perpendicular to the Oscar Range trend, even though the intrusions do not appear to be directly related to any known fault. The Argyle lamproite to the east has an elongated morphology suggestive of fault control, and intrudes a splay on the Glenhill fault (Jaques and others 1986).

Remote Sensing. Kingston (1984) reported remote-sensing techniques are widely used to search for kimberlite: these include conventional and false color aerial photography, LANDSAT multispectral scanner satellite data, and airborne multispectral scanning. Multispectral scanning data is used to identify spectral anomalies related to Mg-rich clays (i.e., montmorillonite), carbonate, and other material with low silica content. Image enhancement techniques (contrast enhancements, ratios, principal components and clustering) produce images that are optimum for discrimination of kimberlite and olivine lamproite soils. These and other photo images can be used to search for vegetation and structural anomalies. Airborne multispectral scanning provides higher resolution than LANDSAT, and can also be used to measure reflectance qualities of clay in soil.

Many pipes and dikes possess distinct structural qualities or vegetation anomalies that may allow detection on aerial photographs. Mannard (1968) reported kimberlites in southern and central Africa were identified on aerial photographs on the basis of vegetation anomalies, circular depressions or mounds, and/or tonal differences. With nearly universal access to Google Earth, Virtual Earth and other websites containing aerial photography, the prospector now has a very handy tool to use to search for kimberlites (Hausel, 2009a, b). Low-level aerial photographs (both conventional and false color infrared) have been used to locate kimberlite in the USSR (Barygin 1962) and in the US (Hausel and others 1979, 2000, 2003).

 Iron Mountain kimberlite sitting under distinct vegetation anomaly on the left side of the photo (higher grass, no trees).

Geophysical Surveys. Geophysical exploration has been successful in the search for hidden kimberlite and lamproite (Litinskii 1963a, b; Gerryts 1967; Burley and Greenwood 1972; Hausel and others 1979, 1981; Patterson and MacFadyen 1984; Woodzick 1980), particularly in districts where kimberlites have previously been discovered. Contrasting geophysical properties are often favorable for distinguishing kimberlite, lamproite and minette from country rock.

INPUT? airborne surveys are effective in identifying both serpentinized and weathered kimberlite owing to the combination of electromagnetics and magnetics used in the survey. Rock exposures of kimberlite may yield magnetic signatures but are poorly conductive, while deeply weathered kimberlites are conductive but poorly magnetic.

Because of the relatively small size of the diamond host rock, close flight-line spacing is necessary. In an airborne INPUT? survey over the State Line district, Wyoming, a flight-line spacing of 200 m effectively detected several kimberlites and identified distinct magnetic anomalies interpreted as blind diatremes (Patterson and MacFayden 1984). An aeromagnetic (200?400m line spacing) survey flown over parts of northeastern Kansas identified several anomalies, some of which were drilled resulting in the discovery of previously unknown kimberlites (i.e., Baldwin Creek, Tuttle, and Antioch kimberlites) (Berendsen and Weis, 2001).  Flight line spacings of 50 to 100 m were used for INPUT?, magnetic and radiometric surveys in the Ellendale field, Australia (Atkinson 1989; Janke 1983; Jaques and others 1986). The olivine lamproites yielded distinct dipolar magnetic anomalies.

In the Yakutia province, Russia, ground magnetic surveys were used where differences between the magnetic susceptibility of kimberlite and the carbonate sedimentary country rock was high.  Anomalies as great as 5,000 gammas were also successfully detected from airborne surveys (Litinskii 1963b).  In Mali, West Africa, the magnetic contrast between kimberlite and schist and sandstone country rock resulted in 2,400-gamma anomalies over kimberlite (Gerryts 1967). In Lesotho, anomalies over kimberlite were comparable with those in the Yakutia province (Burley and Greenwood 1972).

Fipke and others (1995) indicated that barren peridotite phases in Arkansas yielded magnetic highs, but the diamondiferous phases were not detected.  In northeastern Kansas, Brookins (1970) reported large positive (550 to 5,000 gamma) and negative (0 to ?2,800 gamma) anomalies over some kimberlites emplaced in regional sedimentary rocks. The sedimentary rocks had relatively low magnetic susceptibility making magnetic surveys an effective method for exploration.

Most kimberlites in the Colorado?Wyoming State Line district yielded small complex dipolar anomalies in the range of 25 to 150 gammas, with some isolated anomalies of 250 and 1,000 gammas (Hausel and others 1979). Blue ground (weathered) kimberlite tends to mask magnetic anomalies. In the Iron Mountain district, where much of the kimberlite is relatively homogeneous, massive hypabyssal-facies kimberlite, only weak to indistinct magnetic anomalies were detected (Hausel and others 2000).

Magnetite is replaced by hematite during weathering masking near-surface magnetic affinity. Clay produced during weathering promotes water retention, thus weathered blue ground over kimberlite may produce vegetation anomalies that are susceptible to detection by electrical methods.  For example, resistivity surveys in the Colorado?Wyoming State Line district detected apparent resistivity of 25 to 75 ohm-m over weathered kimberlite, compared with 150 to 2,250 ohm-m in the country rock granite (Hausel and others 1979).

Resistivity of weathered lamproite may be lower than that of country rock, owing to the conductive nature of smectitic clay relative to illite, kaolinite and other clay minerals (Gerryts 1967; Janke 1983).  However, the Argyle olivine lamproite yielded moderate to strong resistivity anomalies (40-100 ohm/m) compared to the surrounding country rock (200 ohm/m) (Drew 1986).

Biogeochemical and Geochemical Surveys. Kimberlite and lamproite are potassic alkalic ultrabasic igneous rocks with elevated Ba, Co, Cr, Cs, K, Mg, Nb, Ni, P, Pb, Rb, Sr, Ta, Th, U, V and light rare earth elements (LREE).  The high Cr, Nb, Ni, and Ta may show up in nearby soils (Jaques 1998), but dispersion of these metals in soils is not extensive.  Stream-sediment geochemistry generally is not useful because of efficient dispersion of most metals in streams.  In the Colorado?Wyoming State Line district, Cominco American outlined several known kimberlite intrusives on the basis of Cr, Nb, and Ni soil geochemical anomalies.  However, dispersion patterns were restricted and of little use in exploration in this terrain.

Gregory and Tooms (1969) found that Mg, Ni, and Nb anomalies did not extend farther than 0.6 km from the Prairie Creek lamproite, Arkansas.  Haebid and Jackson (1986) noted that soil geochemical anomalies (Co, Cr, Nb, Ni) were detected in sand and soil immediately above lamproite vents in the West Kimberley province, Australia.  Such anomalies could prove useful in the search for hidden olivine lamproites.  Gregory (1984) used lithochemistry to distinguish olivine lamproite from leucite lamproite on the basis of Mg, Ni, Cr, and Co ratios.

Bergman (1987) suggested that olivine lamproites are generally enriched in compatible elements relative to leucite lamproites as a result of the abundance of xenocrystal olivine in the former. Barren lamproites contain elevated alkali and lithophile contents (K, Na, Th, U, Y, and Zr) relative to diamondiferous (olivine) lamproites.  Diamondiferous lamproites possess twice the Co, Cr, Mg, Nb, and Ni, and half the Al, K, Na and as barren lamproites (Mitchell and Bergman 1991), and lamproites have anomalous Ti, K, Ba, Zr, and Nb compared to most other rocks.  These components may favor the growth of specific flora or may stress local vegetation (Jaques 1998). The Big Spring vent, West Kimberley, Australia, is characterized by anomalous faint pink tones that reflect the growth pattern of grass on the vent (Jaques and others 1986).

Many kimberlites in the Colorado?Wyoming State Line district will not support growth of woody vegetation resulting in open parks over kimberlite in otherwise forested areas.  These same kimberlites may support a lush stand of grass delineating the limit of the intrusive.  Distinct grassy vegetation anomalies over kimberlites in the Iron Mountain district, Wyoming were used successfully to map many intrusives (Hausel and others 2000). The anomalies are especially distinct following a few days of rain in the late spring.

Some Siberian kimberlites support denser stands of larch (Larix dahurica) and abundant undergrowth of shrub willow (Salix) and alder (Alnus) compared to surrounding Cambrian carbonates. In central India, trees over the Hinota pipe are healthier, taller, and denser than those in the surrounding quartz arenite. This may be attributed to greater availability of K, P, micronutrients and water.

Vegetation over the Sturgeon Lake kimberlite in Saskatchewan was tested for 48 elements; the kimberlite showed a consistent spatial relationship with Ni, Sr, Rb, Cr, Mn and Nb, and to a lesser extent with Mg, P and Ba, and relatively high Ni concentrations occurred in dogwood twigs. In hazelnut twigs, Cr levels were greater than 15 ppm near the kimberlite but only 5 to 8 ppm elsewhere, and Nb was higher in hazelnut twigs.  Sr and particularly Rb were relatively enriched in some plant species on kimberlite. The Sr was probably derived from the carbonates associated with the kimberlite, whereas the Rb was derived from phlogopite.  Ni, Rb and Sr distribution and Cr enrichment associated with Mn depletion in the twigs could be used to identify nearby kimberlite.