|Outline of the Wyoming Craton (basement rocks of Archean [>2.5 |
billion years old] age) showing locations of known kimberlites,
lamproites, lamprophyres, diamonds, anomalies (Hausel, 1998).
Alnöites, monticellite peridotites, carbonate-rich mica peridotites, monchiquites, lamproites, and kimberlites are all reported in Montana (these are all rare volcanic rocks that could have diamonds - Erlich and Hausel, 2000). Many of these intrusives occur in the central Montana alkalic province in eastern Montana, which overlies the Wyoming Province (Hearn, 1989; Scambos, 1991). A few lamproites have also been found in western Montana off the craton. In eastern Montana, mantle-derived breccias and dikes are described in the Missouri Breaks, Porcupine Dome, and Grassrange areas. More than 30 carbonate-rich dikes and breccia pipes are reported in the Porcupine Dome and Grassrange areas, and a group of kimberlites occur in the Missouri Breaks.
|Map compiled by Hausel (1998) showing known kimberlite, lamproite,|
lamprophyre and diamond occurrences in the US.
A diamond was reported from Grasshopper Creek southwest of Dillon in the southwestern corner of the state (Sinkankas, 1959). No details were given.
Blaine and Hill Counties
Two diamonds were found in gravels of the Etzikom Coulee in the Milk River drainage north of the Sweet Grass Hills. The diamonds weighed 0.14 and 0.17 carats, respectively (Lopez, 1995). The placer diamond occurrence lies near a buried magnetic anomaly aligned with presumed lamproitic rocks in Alberta, Canada, where considerable diamond exploration activity has occurred (D.A. Lopez, personal communication, 1997).
|Location map of the Smoky Butte dike complex |
(from Mitchell and Bergman, 1991).
The lamproites are sanidine-diopside-richterite-phlogopite lamproites. These lie along a dike that swells to 122 feet in width. The rocks include vesicular, massive, glassy, hypabyssal breccias, with minor tuffs and pyroclastics. Smoky Butte is a vent (Mitchell and Bergman, 1991). Fipke and others (1995) reported that a few chromites from the Smoky Butte intrusive yielded geochemistries similar to diamond inclusion chromites. A small (65 kg) sample, however, yielded no diamonds.
In 1883, a 12-grain diamond was discovered in the placer workings near Nelson Hill in the town of Blackfoot, northwestern Montana. The stone was described as a colorless dodecahedron with triangular markings (Kunz, 1885). In 1894, a 0.22-carat flawed stone was reported from the county (Sinkankas, 1959).
|Elk Butte lamproite breccia (photo by the author).|
A phlogopitic lamproite flow, known as the Ruby Slipper (Bearmouth pipe), intrudes Devonian age Jefferson Formation and Tertiary basalts and rhyolites near Bearmouth along Bear Gulch in the Garnet Range. The flow is found in sections 3, 4, and 10, T11N, R14W.
The exposed plug consists entirely of flows which buried the underlying volcaniclastics. The underlying breccias were intersected by drilling (Pete Ellsworth, personal communication, 1996). The flow was initially mapped as a 1800-foot-wide leucite basalt by Carter (1982), but later identified as lamproite. According to Ellsworth (1996), the pipe forms a prominent 1500-foot circular knob, with crater facies volcaniclastics covered by talus and colluvium from the lamproite porphyry. Four other lamproitic prospects have been located in the region (Pete Ellsworth, personal communication, 1996).
Lewis and Clark County
|Ming Bar monchiquite from Montana. Possible source of diamonds? |
Photo by the author.
The only known intrusive with upper mantle material within this area lies along the eastern shoreline of Holter Lake on the Missouri River northeast of Helena. The intrusive, known as the Ming Bar monchiquite, contains abundant cognate metacrysts of olivine, pyroxene, and chromite with granulite, dunite, pyroxenite, and peridotite nodules (Meen and others, 1986). The intrusive originated in the upper mantle, but probably at too shallow a depth for diamonds.
However, some diamonds were apparently found in the vicinity of Ming Bar. Five diamonds were reportedly recovered from placer operations at Spokane Bar, and a 5-carat diamond was reported from Metropolitan Bar. Both placer localities lie upstream from Ming Bar (Tony Irving, personal communication, 1994).
|Kimberlites and lamprophyres in the Grassrange field|
Diamonds have been reported from Greenhorn Gulch in the Greenhorn Range east of Dillon, southwestern Montana (Sinkankas, 1959). No details were given.
Petroleum and Fergus Counties (Grassrange Field)
|Elk Creek Butte & Yellow Water Butte |
lamprophyres (from Mitchell and
Elk Creek and Yellow Water Buttes, located near the center of the field, form large ultramafic lamprophyres. The Yellow Water Butte diatreme, in particular, is carbonate rich (pour some weak 10% hydrochloric acid on the rock and it will fizz as it releases bubbles of carbon dioxide). These two buttes lie within a group of ultramafic intrusives known as the Winnet sills, which are nepheline-haüyne alnöites, a carbonate-free rock (Doden, 1996).
The breccias that form Yellow Water Butte consist of massive to brecciated, olivine-phlogopite-diopside-carbonate lamprophyre (?) and massive hypabyssal olivine lamprophyre(?). The Elk Creek vent-dike complex consists of intrusive breccia and lapilli tuffs (Mitchell and Bergman, 1991). Similar, but less extensive outcrops also occur in the area (Hausel, personal field notes, 1994). Doden (1996) suggested that the Yellow Water Butte breccias were part of a highly gas-charged magma that rapidly ascended from the mantle and was virtually unaffected by crustal contamination.
During field investigations of this area, Hausel (personal field notes, 1994) working as a consultant for a major mining company highly recommended the property to company he was working for as a diamond prospect, suggesting this area had as much potential as the Kelsey Lake Kimberlite in Colorado. The company turned down both projects even though diamonds were later recovered from both areas!
|Sample of Yellow Water Butte lamproite breccia (photo by the author).|
In Phillips County north of the Grassrange Field, a group of intrusives, known as the Williams kimberlites, occur near Landusky. The Williams kimberlites form a group of four closely spaced diatremes in the eastern part of an east-northeasterly trending swarm of ultramafic alkalic diatremes, dikes, and plugs (46 to 51 Ma) in the Missouri Breaks area of north-central Montana. These rocks preceded the intrusion of Smoky Butte (27 Ma; Ma=Millions of years ago) located 67 miles to the southeast. The rocks contain a host of xenoliths including garnet-bearing lherzolites, harzburgites, and dunites. The Williams 1 diatreme occupies a surface area of about 750 by 105 feet, and contains the typical kimberlitic indicator minerals pyrope garnet, chromian diopside, and magnesian ilmenite.
|Map of the Williams kimberlites near Zortman, |
central Montana (after Hearn and McGee, 1983).
These intrusives enclose xenoliths from the Precambrian basement, upper crust (schist, gneiss, amphibolite), lower crust (granulite, mafic granulite, amphibolite), and the upper mantle (spinel peridotite, dunite, garnet peridotite, garnet megacrysts), as well as kimberlitic indicator mineral xenocrysts. According to Hearn and McGee (1983) neither diamond nor eclogite has been found in these intrusives.
|Sample of Williams hypabyssal facies kimberlite, Montana (photo by the author).|
Treasure and Rosebud Counties (Porcupine Dome Field)
The Porcupine Dome Field, located in southeastern Montana, includes four intrusives known as Froze-to-Death Butte, Gold Butte, Geyser Spring, and Johnson Ranch Butte. Froze-to-Death Butte is a multiple vent complex located 8.5 miles northwest of Hysham. Gold Butte lies 25 miles northeast of Froze-to-Death Butte; Johnson Ranch and Geyser Spring lie between these two along strike, suggesting structural control. These intrusives are interpreted to be Eocene in age (Doden, 1997).
|Froze-to-Death Butte lamproite breccia, Montana (photo by the author).|
According to Doden (1996), these garnets are similar to some Missouri Breaks garnets derived from upper crustal sources. A second type of garnet is reddish-purple and is similar to peridotitic garnets. These garnets contain 5.7-7.0 wt% Cr2O3, 4.7-7.0 wt% CaO, 18.6-21.6 wt% MgO, and 5.9-7.9 wt% FeO. The peridotitic garnet compositions primarily fall within the G9 compositional field; however, a few have sub-calcic G10 compositions similar to garnets found as diamond inclusions (Doden and Gold, 1993).
Although the lamprophyres apparently sampled the diamond stability field, the picroilmenite compositions are depleted in Cr2O3, indicating that oxidizing conditions prevailed in the magma. This may suggest that diamond preservation was not favorable. However, since the magmas did apparently sample the diamond stability field, the search for undiscovered (buried) pipes of olivine lamprophyre and lamproite in this region may be productive.
Many diamond deposits have been found north of Montana in Canada including several commercial diamond deposits in addition to the great diamond discoveries at Ekati and Diavik diamond mines. The number of discoveries in Canada have been astounding! These deposits do not stop at the Montana border and it is likely other diamond deposits will be found in Montana, Colorado and Wyoming.
For the prospector, one needs to search for cryptovolcanic structures using aerial photography and search for diamonds and kimberlitic indicator minerals while panning for gold. Some articles written by the author and published by the ICMJ Prospecting and Mining Journal describe the use of aerial photography in searching for diamond deposits. The geology of Montana is favorable for discovery of both placer and lode diamonds.
|Diamond-rich kimberlite at Victor, Ontario, Canada (photo by the author).|
|Snap Lake kimberlite from Snap Lake diamond mine, NWT, Canada (photo|
by the author).
|Highwall of Kelsey Lake diamond mine, Colorado (photo by the author)|
Diamond-bearing kimberlite at Sloan Ranch, Colorado (photo by the author).
|If you are prospecting for gold in Montana, get to know the |
characteristics of diamond so you don't miss the mother lode
of diamonds. This photo is taken of the surface of an octahedral
diamond surface showing the common trigons often seen on
rough diamond. These can usually be seen with a 10x
prospectors hand lens (loope).