The Sweet Home Mine, Alma, Park County Colorado - 2

OTHER MINERALS:

The Sweet Home mine rhodochrosite crystals have overshadowed reports of the other species from the mine (Caplan, 1936).  Little information is found in standard mineralogical references.  For example, two noteworthy species known from the mine, tetrahedrite/tennantite and huebnerite, are not listed in Palache and others (1944, 1951), or by Eckel (1961).  Roots (1951a,b) reports collecting rhodochrosite and pyrite at the Sweet Home mine.

The following mineral descriptions focus on the euhedral minerals (‘euhedral’ refers to crystals that are well-formed with sharp, easily recognized faces) that are found on Sweet Home specimens that have been widely distributed since the mine reopened in 1991.  Some historical information on minerals reported from the Alma district is also included.  Information below includes new data on material collected between 1991 and 1995, and, wherever possible, is based on X-ray diffraction or electron microprobe results. Research on Sweet Home mine minerals continues as new material is encountered.  Only a few mineral species new to the Sweet Home mine have been identified since 1990.  Some minerals have not been adequately studied, for example carbonates other than rhodochrosite, and the various dark-colored stains or coatings that are common on many specimens.

Anglesite:  PbSO4

Anglesite was identified by X—ray diffraction on samples collected by J. F. Hurlbut in 1994. Several specimens were collected from a sericite sheer zone in Crosscut No. 2.  The anglesite is in thin, 3-5 mm, compact and massive bands along with dickite, brown fluorite, green fluorite and quartz.  There are no distinct individual crystals on the examined samples.

Barite:  BaSO4

Barite occurs sparingly in many of the mineralized pockets encountered during 1993 to 1995. In one sample, a 6-mm blade of a white, opaque barite grew before or with blue fluorite crystals.  Barite from the Sweet Home needs to be studied further.

Bornite: Cu5FeS4

Bornite appears to be more prevalent in certain veins of the mine than originally thought.  It is a component of a late-stage of mineralization that coats other sulfides.  Bornite was reported from the mine by Corn (1957), and at two other nearby places, namely, the Wyandotte property north of Kite Lake and on an unnamed mine dump southeast of the Sweet Home mine.

Bornite from the Sweet Home mine is probably best known from the Tetrahedrite Drift in an area mined during 1993.  Bornite crystals and encrustations also occur on specimens taken from the Colorado Springs Pocket in July 1992.  The bornite usually forms as isolated and intergrown subhedral, rounded crystals up to 8 mm across that usually range in color from deep blue to black.  However, bornite also occurs as a blue to purple iridescent metallic coating on tetrahedrite/tennantite and sphalerite.

 

Bornite associated with chalcopyrite and pyrite from the Tetrahedrite Pocket, 7.6 cm in length.

Calcite:  CaCO3

Calcite was relatively unknown from the Sweet Home mine until pale yellow, intergrown crystals were encountered in the Tetrahedrite Drift in 1992.  The calcite does not form in typical well-defined crystals; rather, the crystals are modified and intergrown rhombohedra. Calcite occurs in pockets within selected veins with other well-crystallized species, especially rhodochrosite.  The plate containing the large Alma King rhodochrosite contains representative yellow calcite growth.  The finest examples of calcite from the Sweet Home Mine were discovered during 2000 in a rhodochrosite-filled pocket called the Calcite Pocket.  A handful of white to pale yellow, complex, scalenohedral calcite crystals to more than 3 cm in length occurred with and on superb, gemmy red rhodochrosite crystals in the Calcite Pocket.

Chalcopyrite:  CuFeS2

Chalcopyrite is a common mineral in the veins and occurs as small crystals on many specimens.  As far as is known, the finest specimens came from a small vug located above the Rainbow Pocket, the location of the Alma King rhodochrosite crystal.  Sharp, lustrous crystals up to 1 cm were found on a sulfide and quartz matrix.

Chalcopyrite and huebnerite, Rainbow Pocket, 2.5 cm.

Chrysocolla:  (Cu,Al)2H2Si2O5(OH)4.nH2

A pale greenish blue mineral, occurring in veinlets and as coatings on massive quartz and sulfide was originally thought to be turquoise but proved to be chrysocolla (E. E. Foord. Written communication, 1994).  This material was found on the mine dump and has not been seen underground.

Copper:  Cu

Native copper was identified by Jim Hurlbut in sulfide-rich specimens dominated by black coatings on tetrahedrite and bornite. It occurs as very microscopic wires and irregular growths intimately associated with the black surface coatings and as small, skeletal pore fillings.  The material may be pseudomorphous after an unknown mineral.

Covellite:  CuS

Covellite is mainly a fine-grained constituent of the sulfide ore and is primarily recognized in petrographic analysis (Honea, 1992).  Some of the blue to purple coatings on sulfides may be covellite; however, little specific analysis has been done.  Several crystals that petrographically resemble covellite were analyzed by K. J. Wenrich with the electron microprobe and determined to be spionkopite.

Dickite:  Al2Si2O5(OH)4

Microcrystalline pseudohexagonal platelets of dickite about 10 microns in diameter and 1-2 microns thick have been confirmed by XRD and SEM-EDS analysis by Peter J. Modreski (personal communication, 1992) and electron microprobe analyses by K. J. Wenrich (this issue).  Dickite forms the bright white clay material that is found coating rhodochrosite and fluorite in the crystal pockets; it occurs widespread in the quartz-bearing and sulfide-bearing veins.

Digenite:  Cu9S5

Digenite has been reported by petrographic (Honea. 1992) and electron microprobe analyses (Wenrich and Aumente-Modreski).   Digenite, verified by microprobe, was found in association with bornite and spionkopite.  Some of the dark coatings on sulfide matrix with bornite might also prove to be digenite.

Dolomite:  CaMg(CO3)2

Only minor amounts of dolomite have been seen at the Sweet Home mine. So far as is known, no well-formed crystals have been found.  According to Singewald and Butler (1941), iron-bearing dolomite was the most abundant gangue in the silver-lead ores of the Alma district: however, most of the mineral deposits are replacement bodies in limestone where sphalerite and galena are abundant.  Pyrite is less so, and chalcopyrite and argentiferous tetrahedrite occur in small amounts.  Dolomite was reported to contain much iron and some manganese, and was thought to be the most abundant gangue in quartzite-hosted gold deposits.  Wells (1937) gave an analysis of carbonate gangue from the Orphan Boy mine that showed it was iron-bearing dolomite, not ankerite, as previously reported.

Ferrimolybdite:  Fe2(MoO4)3-8H2O

Pale yellow coatings of ferrimolybdite occur with traces of visible molybdenite in quartz in a few places in the mine, especially the Watercourse Raise.  These two minerals are also found in a quartz vein in an old drift that is about 76 meters northwest of the present portal of the Sweet Home mine (Bryan Lees and Dean Misantoni, personal communication, 1994).

Fluorapatite:  Ca5(PO4)3(F,Cl.OH)

Attractive, pale greenish blue fluorapatite crystals are found sparingly in most veins in the Sweet Home mine.  They are one of the more desirable crystallized species from the mine, especially as thumbnails and micromounts.  Fluorapatite is found directly on rhodochrosite and other minerals, and commonly occurs as transparent, doubly terminated crystals.  The largest crystals seen measure about 1 cm.  Typically small clusters of crystals are found with rhodochrosite and needle quartz.  Fluorapatite crystals are intimately associated with white dickite in pockets and may occur as "floaters" in this clay.  The fiuorapatite fluoresces an attractive pink color in shortwave ultraviolet light.

Fluorite:  CaF2

Fluorite occurs at the Sweet Home mine as a gangue mineral throughout the various vein structures.  In different parts of the mine fluorite appears in many colors, from colorless to yellow, blue, green, purple and brown.  Layers of colored fluorite crystals seldom reach 2.5 cm in thickness, and are sometimes interleaved with dickite and sericite.  In some of the pockets it appears that fluorite crystals grew during different periods of mineralization, as evidenced by zonation consisting of multiple layers of fluorite of different colors.

The predominant fluorite crystal shape is the cube truncated by dodecahedron faces.  A few nearly perfect dodecahedrons have been found in some pockets.  Some cube penetration twins have also been found.  Most of the crystals are smaller than 1 cm.

Many very nice specimens have been produced from different pockets, sometimes as fluorite cubes perched on quartz crystals.  In a few pockets the rate of growth evidently was very rapid, producing crystals with stacked cubic faces.

Galena:  PbS

Galena occurs in well-formed cubes and octahedrons, usually less than 1 cm.  Typically associated with other sulfides, quartz and rhodochrosite.  Historically, the galena was thought to be argentiferous and a significant contributor of the silver production from the mine.  Recent analyses (Wenrich, Modreski) indicate that galena is typically not a major silver-bearing species at the Sweet Home.  Electron Microprobe results on 31 spots from 5 samples yielded silver content values from <0.01 wt % to 0.44 wt %.  The major sources of silver in the Sweet Home Mine are the copper sulfides and silver-copper sulfides: Bornite, Digenite, Spionkopite, Stromeyerite.

Some small, well-formed, galena cubes, usually less than 1 cm, have been found in the Sweet Home mine and make attractive specimens.  These are associated with other sulfides and quartz.

Galena octahedron coated with later chalcopyrite, the Rainbow Pocket, 3.1 cm high.

Goyazite:  SrAl3(PO4)2(OH)5-H2O

Goyazite was first reported from the Sweet Home mine by Kosnar (1979b).  One other identification is known, from a specimen found on the mine dump several years ago by Ray Randstrom (written communication, 1994).  This elusive species has not been found in recent freshly mined material. Kosnar reports attractive, well-formed, brilliant orange goyazite crystals up to 0.6 cm that are associated with pink to purple fluorite crystals, gemmy rhodochrosite, and pale-green fluorapatite crystals in vugs with sulfide and quartz matrix.  Paul Hlava (written communication, 1994) confirmed the identification of the Ransom sample and indicated that in addition to an inner composition of goyzite, there was an outside growth of svanbergite (DMNH #16852).

Greenockite:  CdS

Greenockite was first discovered at the Sweet Home mine by Bryan Lees in 1994.  A small pocket near the Blueberry Pocket in the GPR Drift contained 3-mm to 8-mm pyrite specimens that were coated with a grayish green coating approximately 2-3 microns thick.  SEM analysis by Peter Modreski, in 1994, showed the coating to be greenockite.

Huebnerite:  MnWO4

Huebnerite has long been known from the mine, so finding crystallized specimens was no surprise; in fact, many collectors and curators anticipated that fine material would be located when the mine opened in 1991.  Huebnerite crystals were known to occur in the first drift to the northeast in the main adit by Belser (1956); he reported that 1,000 pounds of ore containing 10 percent W03 was shipped from the talus near the upper workings of the mine. Huebnerite is generally uncommon in the veins exposed between 1992 and 1995.  When it is found, it mainly occurs as crystallized blades with needle quartz, associated with sulfides, rhodochrosite, and fluorite.  Excellent specimens came from the Tetrahedrite Stope, Crosscut No. 3.  Typically, a single blade of huebnerite up to 3-cm long, with a bright luster, is associated with needle quartz and other minerals.  Some large specimens were encountered; one 28 x 20 x 9.5-cm mass of intergrown huebnerite crystals (DMNH #16812) came from the Tetrahedrite Stope.

Huebnerite with rhodochrosite and quartz crystals, Sulfide Pocket, 3.8 cm long.

Malachite:  Cu2(CO3)(OH)

Light green microcrystalline coatings on chalcopyrite crystals are most often malachite (Peter Modreski, personal communication, 1995); however, no detailed study has been done.  Some green to blue-green coatings found on chalcopyrite may be some other mineral, for example, serpierite (E. E. Foord, written communication, 1995).

Muscovite:  KAl2(Si3Al)O10(OH,F)2

Muscovite and sericite are pervasive gangue minerals in the Sweet Home mine.  Unaltered muscovite, in well-formed crystals up to 8 mm, occurs with quartz and sulfides in parts of the mine, especially the Coors Pocket.  Muscovite was analyzed on the microprobe (Wenrich and Aumente-Modreski).  Fine-grained sericite is more common than euhedral muscovite and it is a constituent of the matrix in the veins. It also fills empty spaces in the crystal pockets, along with dickite.  It is intimately associated with massive anglesite in Crosscut No. 2.

Mawsonite:  Cu6Fe2SnS8

Mawsonite was reported in 1994 by Beda Hofmann, Bern Museum, Switzerland.  It was confirmed by electron microprobe on material he collected at the mine in 1992.  It is an intergrown constituent in the sulfide mineralization.  This is the first report of a tin mineral from the Sweet Home mine.

Molybdenite:  M0S2

Coarse, crystalline molybdenite occurs in small amounts with quartz in Sweet Home mine veins, but its occurrence has not been specifically studied.  The mineral was noted by Com (1957).  Molybdenite is reported, with stains of ferrimolybdite, in the Watercourse Raise and from an adit 49 meters northwest of the present portal of the Sweet Home mine (Bryan Lees and Dean Misantoni, personal communication, 1994).

Pyrite:  FeS2

Collectors are usually captivated by the Sweet Home mine pyrites with their distinctive growth habits, specifically the cubic crystals with saw-tooth edges.  Crystallized specimens of pyrites were not well known from the mine until 1993 when Bryan Lees opened up the “Pyrite Pocket” in the “Tetrahedrite Stope” on Crosscut No. 3.  Most of the crystals are of particular interest because of their rounded, rather bulging, striated cubic faces and serrated edges.  The details of these growth patterns are best seen under low magnification, where the step-like striations on the faces can be observed to intersect at the edges of the crystals creating the repetitious saw-tooth patterns.  This texture is not limited to the edges but also selectively occurs on the crystal faces.  Most of the crystals show octahedral modification on the comers.  These pyrites form as attractive intergrown clusters of crystals on white quartz crystals directly associated with dark gray sphalerite.  The cubes range from 5 mm to 2.5 cm across; the largest single cube observed from this pocket is 4.4 cm across (DMNH #16843).  Not all crystals are distorted or have striations.  One (DMNH #16841) is a nearly perfect 2 cm cube with mirror-like faces; the corners are also modified by the octahedron.  Pale purple fluorite cubes and single blades of huebnerite occur on the quartz matrix with these pyrites.

Quartz:  SiO2

Quartz is ubiquitous in the veins and is undoubtedly the major gangue mineral. However, in some veins, sericite is as abundant as or more abundant than quartz.  The quartz may be massive within the veins but it usually forms as attractive white to transparent crystals less than 0.5 cm long in open spaces associated with pyrite, chalcopyrite, huebnerite, rhodochrosite and fluorite.  All plates and hand-sized specimens from the Rainbow and Museum pockets have a myriad of freestanding and intergrown quartz prisms as the background for the other minerals.

Silver:   Ag

Native silver wire was found at the Main stope/Clay seam intersection in 1996.

Sphalerite:  (Zn,Fe)S

Sphalerite is one of the most abundant species found in the Sweet Home mine.  It is an important massive component in the ore, and commonly also occurs in crystal pockets as small, well-formed black, green, yellow or red crystals associated with other sulfides and euhedral fluorites and rhodochrosite.

Sphalerite commonly occurs as opaque, black tetrahedrons that can be confused with tetrahedrite.  Shown is the association of sphalerite with fluorite, 2.4 cm wide.

Spionkopite:  Cu39S28

Spionkopite is one of the few minerals to be newly identified since the mine was reopened in 1991 (Wenrich and Aumente-Modreski).  It was found during electron microprobe analysis, associated with bornite, digenite, stromeyerite, and possibly jalpaite.

Svanbergite:  (see goyazite)

Tetrahedrite/Tennantite:  (Cu,Fe,Ag,Zn)12(SbAs)4S13

Excellent tetrahedrite/tennantite specimens are recognized from the Sweet Home mine.  Specimens are attractive, with single euhedral crystals or as intergrown clusters of crystals on sulfide matrix, associated with euhedral rhodochrosite, quartz, fluorite, sphalerite and huebnerite.  Typically the crystal surfaces are bright and untarnished, often with a distinctive specular appearance.  Crystals are generally 7 mm to 2.5 cm in size, but individual tetrahedrite/tennantite crystals up to 5 cm are known, and fragments of even larger crystals have been seen.

Tetrahedrite occurs in well-formed tetrahedrons.  Tetrahedrite crystal, the Corner Pocket, 2 cm.

Topaz:  A12(OH.F)(SiO4)

Topaz was reported by Kosnar as part of the pegmatite assemblage in the host rock at the Sweet Home mine.  One very small topaz crystal in a matrix of quartz has been found in the present mining operation.  It is assumed that this mineral is more prevalent because of the abundance of topaz inclusions (<0.0l—40 mm long) in quartz crystals associated with the sulfide minerals, tetrahedrite, rhodochrosite, and fluorite (Reynolds: Wenrich and Aumente-Modreski).

Triplite:  (Mn,Fe.Mg,Ca)2(PO4)

This rare phosphate was discovered while examining inclusions in rhodochrosite, and was subsequently verified by electron micro-probe analysis (Karen Wenrich, personal communication, 1996).  The triplite occurs as solid inclusions in fluorapatite, which itself is an inclusion in rhodochrosite.

Unknown sulfosalt

Tiny, 1-2 mm, black, striated prismatic crystals intimately associated with needle quartz and sphalerite were found on material collected in Crosscut No. 3.  Only two microcrystals are known, not enough for a thorough analysis.  One crystal was studied by Peter J. Modreski and provisionally believed to be eclarite (Pb9(CuFe)Bi12S28), or possibly another orthorhombic lead-copper-iron bismuth sulfosalt, miharaite (PbCu4FeBiS6).

Gemstones:

Gem-clear rhodochrosite crystals are rare but have been found.  Up until the mine was reopened in 1992, gem-quality material was very rare.  Today, transparent cleavage pieces have yielded handsome faceted gems: the largest known, on exhibit at the Denver Museum of Natural History, weighs 65 carats.

Beautiful cherry-red Sweet Home Mine rhodochrosite cut by skilled U.S. gem cutters into faceted stones with incredible color and brilliance

These gemstones look stunning in pendants and earrings!  The rhodochrosite can also be fashioned into cabochons, tongues, rhombs, eggs, and a variety of other polished forms suitable for use in jewelry and perfect for rare gemstone collections.

If you are interested in more information about rhodochrosite gemstones, please contact our gemstone marketing representative, Paul Cory, of ITECO, Inc. at 614-785-0450, or visit ITECO's website at www.itecoinc.com.

CURRENT STATUS:

Operations at the Sweet Home mine ceased and the mine was closed down in 2004.  The portal was removed, the mineshaft plugged, and site reclamation was conducted.  No future work is planned. 

The portal being removed.

The site after reclamation (the portal was located just to the right and behind the building).

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Collector's Edge Minerals, Inc. recognizes and thanks the Mineralogical Record for permission to use portions or all of their original article from the "Mineralogical Record", Volume 29, Number 4, pp. 1-153.   Contributing authors were Bryan K Lees, Paul S. Harter, Steve Voynick, Thomas Moore, Dean Misantoni, Miles K. Silberman, Jack A. Murphy, James F. Hurlbut, Karen J. Wenrich, T. James Reynolds, Regina Aumente-Modreski.

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