Silver Plume Granite Research Articles

Mineralogic and Geochemical Changes from Alteration of Granitic Rocks, Boulder Creek Catchment, Colorado

In the critical zone of the upland Boulder Creek (Colorado) catchment, a weathered mantle consisting of 3 to 10 m of oxidized bedrock, saprolite, and mobile regolith (grus and soil) overlies fresh and locally hydrothermally altered bedrock that is exposed beneath a low‐relief surface. Saprolite forms from isovolumetric alteration of primary rock materials to secondary minerals by percolating water; mobile regolith includes rock materials in transport downslope, mixed by a variety of biologic and physical processes. Analysis of vadose zone samples by petrographic, inductively coupled plasma mass spectroscopy (ICP–MS) and X‐ray diffraction (XRD) techniques demonstrates that weathering of Precambrian Boulder Creek Granodiorite and Silver Plume Granite has formed small amounts of clay and iron oxides. Bulk geochemical changes during alteration are minor except in the transformation of saprolite to regolith, which is enriched in neoformed minerals. In saprolite, the alteration sequence is: plagioclase > biotite > microcline > quartz. Quantitative XRD analysis shows that smectite and kaolinite form <10% of the rock in weathered samples and that smectite is absent and kaolinite and illite comprise >15% of the rock at several hydrothermally altered sites. Iron oxide and hydroxide minerals, such as goethite and hematite, have formed by weathering and by hydrothermal alteration. Hydrothermal alteration of granodiorite to saprolite at the Hurricane Hill site resulted in substantial losses of CaO, MgO and Na2O, Sr and metals such as Cd and Pb, and enrichment in K2O and the trace elements Cs, Sb, and W. At four critical‐zone profiles studied in detail: (i) increases in kaolinite, smectite, and Fe oxides and decreases in magnetite concentration due to weathering are significant if fresh rock is compared to regolith; (ii) illite is enriched in soil; (iii) minor amounts of clay minerals are inherited from fresh rock; and (iv) clay minerals and Fe oxides are abundant only in hydrothermally altered saprolite. Some of the apparent enrichment of secondary minerals in weathered profiles likely results from dustfall and from material mixed into mobile regolith from heterogeneous bedrock sources. If some portion of the clay minerals in regolith and some fraction of dissolved material leaving Boulder Creek catchments are exotic, rates of clay and Fe oxide formation in the Boulder Creek area must be slow, and the alteration of granitic rocks during the conversion of saprolite to regolith may be driven mainly by volume expansion in this cool, continental climate.

Hydrochemical Characterization of Alpine and Alpine-Subalpine Stream Waters, Colorado Rocky Mountains, U.S.A.

The hydrochemistry of stream waters in an alpine basin and alpine-subalpine basin were characterized after intensive chemical sampling from May 1986 to May 1987. The basins are within the Hourglass drainage basin which is tributary to the Cache la Poudre River in northern Colorado and are underlain by Silver Plume granite. The alpine basin had an annual water yield of 115 cm and was characterized as an ultra-oligotrophic sodium-bicarbonate type water. The alpine stream averaged 130 ,teq L-1 total dissolved solids. Concentrations for sodium, calcium, magnesium, potassium, and hydrogen were 27.3, 22.0, 7.7, 2.6, and 0.7 /ieq L-1, respectively. Concentrations in the alpine stream for bicarbonate, sulfate, nitrate, chloride, and orthophosphate were 31.7, 19.3, 11.0, and 0.7 ,teq L-1, respectively. The alpine-subalpine basin had an annual water yield of 72 cm and was typed as an oligotrophic calcium-bicarbonate water. The alpine-subalpine stream averaged approximately 300 teq L-1 total dissolved solids.

Hydrochemical Characterization of Alpine and Alpine-Subalpine Stream Waters, Colorado Rocky Mountains, U.S.A.

The hydrochemistry of stream waters in an alpine basin and alpine-subalpine basin were characterized after intensive chemical sampling from May 1986 to May 1987. The basins are within the Hourglass drainage basin which is tributary to the Cache la Poudre River in northern Colorado and are underlain by Silver Plume granite. The alpine basin had an annual water yield of 115 cm and was characterized as an ultra-oligotrophic sodium-bicarbonate type water. The alpine stream averaged 130 ,teq L-1 total dissolved solids. Concentrations for sodium, calcium, magnesium, potassium, and hydrogen were 27.3, 22.0, 7.7, 2.6, and 0.7 /ieq L-1, respectively. Concentrations in the alpine stream for bicarbonate, sulfate, nitrate, chloride, and orthophosphate were 31.7, 19.3, 11.0, and 0.7 ,teq L-1, respectively. The alpine-subalpine basin had an annual water yield of 72 cm and was typed as an oligotrophic calcium-bicarbonate water. The alpine-subalpine stream averaged approximately 300 teq L-1 total dissolved solids.

Geology of the Urad and Henderson molybdenite deposits, Clear Creek County, Colorado, with a section on a comparison of these deposits with those at Climax, Colorado

The Urad and Henderson stockwork molybdenite orebodies at Red Mountain, Clear Creek County, Colorado, are related to a rhyolitic subvolcanic center of Oligocene age referred to as the Red Mountain Complex. The two orebodies are well separated in space and are specifically related to different intrusive rocks of the complex. The Urad orebody was deposited first and at shallow depth. The Henderson formed deeper within the complex and is much the larger of the two. The Oligocene rocks intrude a batholith of Precambrian Silver Plume Granite near the western edge of the Colorado mineral belt and are within a mile of the north-north-east-trending Berthoud Pass fault. This fault is the major structural element of the area and has been identified by Tweto and Sims (1963) as a Precambrian fault reactivated in Tertiary time.The part of the Red Mountain Complex crops out over an irregular area about 2,400 feet long and 800 to 1,200 feet wide. Dikes related to the complex surround it and are present as much as 1 mile from the igneous center. Known rocks of the complex represent more than 15 stages and substages of igneous activity. All rocks are chemically and mineralogically similar and consist of essential quartz, K-feldspar, albite, and minor biotite. Clastic and fragmental textures are common features of the rocks exposed at surface, porphyries predominate at intermediate depths, and a granitic rock is present at the greatest depths reached in exploratory drilling. The major rock types of the complex exposed at the surface are columns that either grade into, or are cut off by, an underlying composite stock, the apex of which lies about 2,500 feet below the peak of Red Mountain. The stock, although much larger than the shallower columns, appears to be a steep-walled intrusive to explored depths. The space now occupied by the stock is thought to have been a magma chamber throughout the time the rocks exposed at the surface were emplaced; the magma chamber was their immediate source. The sequence of rocks exposed at surface is as follows: Tungsten Slide and East Knob units (relative ages unknown), Square quartz porphyry, rhyolite porphyry radial dikes, and the Red Mountain porphyry unit. Each unit consists of several members. Rock types which constitute the buried stock are, from oldest to youngest, the Urad porphyry, the Primos porphyry, and the Henderson granite. The Square quartz porphyry is the youngest known igneous rock predating the Urad orebody and is considered to have been the source intrusive for the Urad mineralized zone of which the Urad orebody is a part. The Urad mineralized zone is envisioned as a cylindrical body of mineralized rock. The orebody was localized within part of this zone by a series of moderately dipping arcuate fractures referred to as the main fissure system. The fissures are interpreted as cone sheet fractures caused by the emplacement of the Square quartz porphyry. About half of the orebody was contained in rocks of the Red Mountain Complex; the rest was in the Silver Plume Granite. The Urad mine, now worked out, produced about 14 million tons of ore. Intrusion of the Red Mountain porphyry, the principal rock of the complex at surface, destroyed much of the Square quartz porphyry, a large part of the Urad mineralized zone, and part of the Urad orebody. The magma from which the Red Mountain porphyry crystallized probably vented; a potential orebody may have been lost to the atmosphere during this event.The Henderson orebody is an umbrella-shaped stockwork entirely within the intrusive complex and contains more than 300 million tons of ore. The difference in the shape of the Urad mineralized zone and the Henderson orebody is thought to reflect the difference in the depth of formation. The Henderson deposit is genetically related to one or more phases of the Primos porphyry and to the Henderson granite and is a compound orebody. A master reservoir is postulated at depth beneath Red Mountain to explain the repetition of igneous and hydrothermal events and to provide the quantity of metal necessary for the formation of the ores. We believe that this magma derived its ore metal from crustal rocks anomalously rich in molybdenum during its rise toward the surface. The deposits at Red Mountain are remarkably similar in many respects to those at Climax, Colorado, but there are some notable differences. The abundance of Precambrian metamorphic rocks, possibly available for reaction with, or incorporation by, Oligocene magma at Climax, and the scarcity of these rocks at Red Mountain may account for the large amount of tungsten in the Climax deposit compared to that in the Urad and Henderson orebodies.

Stress changes ahead of an advancing tunnel

Instrumentation placed ahead of three model tunnels in the laboratory and ahead of a crosscut driven in a metamorphic rock mass detected stress changes several tunnel diameters ahead of the tunnel face. Stress changes were detected 4 diameters ahead of a model tunnel drilled into nearly elastic acrylic, 2·50 diameters ahead of a model tunnel drilled into concrete, and 2 diameters ahead of a model tunnel drilled into Silver Plume Granite. Stress changes were detected 7·50 diameters ahead of a crosscut driven in jointed, closely foliated gneisses and gneissic granites in an experimental mine at Idaho Springs, Colorado. These results contrast markedly with a theoretical elastic estimate of the onset of detectable stress changes at 1 tunnel diameter ahead of the tunnel face. A small compressive stress concentration was detected 2 diameters ahead of the model tunnel in acrylic, 1.25 diameters ahead of the model tunnel in concrete, and 1 diameter ahead of the model tunnel in granite. A similar stress peak was detected about 6 diameters ahead of the crosscut. No such stress peak is predicted from elastic theory. The 3-dimensional in situ stress determined in the field demonstrate that geologic structure controls stress orientations in the metamorphic rock mass. Two of the computed principal stresses are parallel to the foliation and the other principal stress is normal to it. The principal stress orientations vary approximately as the foliation attitude varies. The average horizontal stress components and the average vertical stress component are three times and twice as large, respectively, as those predicted from the overburden load. An understanding of the measured stress field appears to require the application of either tectonic or residual stress components, or both. Laboratory studies indicate the presence of proportionately large residual stresses. Mining may have triggered the release of strain energy, which is controlled by geologic structure.

Boulder Creek Batholith, Colorado Part III. Fingerprinting Discordant Zircon Ages in a Complex Intrusion

The apparent ages (32 lead/alpha and 6 Pb 206 /U 238 ) of zircon as plotted on an isochron map of the Boulder Creek batholith define the following pattern: (1) very high ages (1600 to 1900 m.y.) within the outermost border zone on the southwest, south, and southeast; (2) transitional high ages (1300 to 1600 m.y.) within an inner border zone on the east and widening to the south and west to include about one-third of the batholith; (3) transitional low ages (1000 to 1300 m.y.) throughout much of the interior of the northern half; and (4) very low ages (1000 m.y. or less) limited to a small area within the northeast corner. The area of minimum age is shown to be part of the reduced-age aureole surrounding the 77 m.y. hornblende granodiorite stock at Jamestown that intrudes the Silver Plume Granite of the Longs Peak-St. Vrain batholith in the region immediately to the north of the minimum-age area of the Boulder Creek batholith. A southeastward elongation of the area of minimum age is attributed to channelway control of the solutions responsible for the recrystallization of the zircon by those northwest-trending breccia reefs that are cut by, or strike toward, the Laramide intrusion. Statistical studies of five zircon separates used for isotopic work showed that the frequency of grains having partial, or complete, rims of colorless zircon on purple to semiopaque zircon cores increased inversely with measured Pb 206 /U 238 age along a smooth curve that, when extrapolated, connected the point representing age of emplacement (0 percent rims) and the point representing the approximate age of re-crystallization (100 percent rims). Consequently, in the Boulder Creek zircon rim frequency gives a useful estimate of the amount of lead lost relative to uranium and thorium from a given sample during its recrystallization. The microstudy indicated: (1) the surface separating core and rim is a major discontinuity; (2) the greater part of the rims appear to be true overgrowths; and (3) the highest frequency of rims is found in the most metamict zircon. However, in any one sample a significant fraction of the most metamict zircon has been sheltered from reaction, presumably by inclusion within relatively impervious minerals, and remains free of rims. These observations coupled with the map evidence of selective channelway control point to warm solutions rather than dry heat as the agent of recrystallization and lead loss.

Boulder Creek Batholith, Colorado Part I: Allanite and its Bearing upon Age Patterns

Allanite is abundant and commonly attains unusually large size as a late-replacement mineral in: (1) the comagnatic rocks of the Precambrian Boulder Creek batholith; (2) associated amphibolite xenoliths and related hybrid rocks; and (3) distinctly younger intrusions of Silver Plume Granite that cut the complex. Allanite porphyroblasts develop by replacement of biotite, probably in the presence of emanations from the rare earth-rich and thorium-rich Silver Plume Granite. The largest allanite crystals are made up of nearly isotropic (metamict) cores and birefringent (recrystallized) rims. Smaller crystals are made up exclusively of birefringent material. The maximum birefringence is shown to be that expectable in allanite of Late Cretaceous to early Tertiary age. As plotted on maps, the birefringence increases, and the thorium and uranium contents of the allanite decrease toward a Laramide stock. The variation in birefringence is, therefore, largely relatable to variations in the post-Laramide radiation dosage brought about by differences in the amounts of uranium and thorium lost during recrystallization. The recrystallized allanite is itself partly replaced by epidote which characteristically occurs as a border between allanite and biotite. Total rare-earth oxides for the eight samples of allanite analyzed range from 17.5 to 21.3 percent by weight. In 13 samples, thorium ranged from 0.50 to 1.14 percent by weight, and uranium from 54 to 158 parts pparts per million. Ranges in optical measurements for 20 samples using the spindle stage are: Nα = 1.719–1.759, Nβ = 1.731-1.774, N γ = 1.741–1.784, birefringence = 0.020–0.032, 2V X (calc.) 70 ° –84 ° . Ranges for unit-cell data obtained on 8 samples are: a = 8.948–8.985A, b = 5.721-5.763A, c = 10.184–10.240A, β = 115°7.50′–115°25.89′ and volume = 473.02–478.43A 3 . The average value for the ratio a:b:c = 1.561:1:1.778.

Chronology of Precambrian events in the Front Range, Colorado

The Precambrian rocks of the Front Range have undergone a long and complex history involving multiple periods of metamorphism, deformation, and intrusive activity. Before 1750 m.y. ago sedimentation and volcanism resulted in the accumulation of many thousands of feet of sandstone and shale, and lesser amounts of calcareous and volcanic rocks on a basement terrane that has not yet been recognized.Regional and dynamic metamorphism at 1700 to 1750 m.y. ago converted the sedimentary-volcanic pile to a metamorphic terrane of dominantly medium- to high-grade gneisses and schists. Plutonic rocks typified by Boulder Creek Granite were emplaced during this event. After this orogeny, there is a time span of 250 to 350 m.y. for which there is no obvious geologic record. The interval 1390 to 1450 m.y. was a period of major igneous activity during which the extensive Silver Plume Granite, Sherman Granite, granitic and mafic dikes, and pegmatites were emplaced. Regional heating attendant with this plutonism was effective in resetting most K-Ar and Rb-Sr mineral ages of the older rocks. Tectonic activity along the great shear zone that trends northeast through Idaho Springs probably began during this orogeny and may have continued intermittently until about 1200 m.y. ago.The Pikes Peak batholith and smaller related granites were emplaced at 1040 m.y. ago. This event was not accompanied by regional metamorphism. There is no recognized sedimentary record between the earlier orogeny and this igneous activity. The Pikes Peak igneous activity is the youngest recognized Precambrian event, but it appears to be limited to the Pikes Peak batholith and satellite plutons.

Geology of the Harold D. Roberts Tunnel, Colorado: Station 468+49 to East Portal

The Harold D. Roberts Tunnel is a 23.3-mile water-diversion tunnel that extends through the western part of the Front Range between Grant and Dillon, Colorado. This report concerns the geology of the southeastern segment of the tunnel, which is partly in the Montezuma stock—a Laramide quartz monzonite pluton—but is mainly in Precambrian basement rocks. The Precambrian metamorphic rocks consist of relatively thick, complexly folded units of amphibolite, migmatite, microcline gneiss, and biotite gneiss. These rocks were invaded by plutons of Boulder Creek Granite and Silver Plume Granite, regarded as syntectonic and late tectonic, respectively, in relation to Precambrian deformation and metamorphism. Most of the basement rocks are distinctly foliated. Laramide deformation resulted in elevation of the core of the Front Range along northward-trending marginal upthrusts and in development of the Front Range mineral belt, a broad northeast-trending zone of strike-slip faults, veins, and igneous intrusions that extends diagonally across the range. The Montezuma stock was emplaced approximately at the junction of the mineral belt with the western margin of the range relatively late in the deformation. The stock is massive and shows little indication of flow fabric. Analysis of fault data obtained in the tunnel indicates two complementary sets of strike-slip faults, trending northeast and northwest, apparently related to wrench-fault movement along the mineral belt. Minor faults are randomly oriented but tend to be concentrated in north-trending dip-slip sets approximately parallel to the marginal upthrusts. The fracture pattern in the Montezuma stock conforms to the regional fault pattern. The major joint sets and fault sets are parallel to one another and probably formed concurrently. The fracture pattern in the Precambrian rocks is unlike that in the Montezuma stock. Minor faulting was controlled largely by the rock fabric. Joints bear no obvious relation to the faults or rock fabric and are believed to have formed mainly later than the faults. The anomalous rupture pattern in the foliated basement rocks is attributed to their anisotropy and to a probable transition from pseudoductile to brittle behavior as the core of the range was uplifted.

MINERAL PARAGENESIS OF PRECAMBRIAN ROCKS IN THE TENMILE RANGE, COLORADO

Research Article| September 01, 1960 MINERAL PARAGENESIS OF PRECAMBRIAN ROCKS IN THE TENMILE RANGE, COLORADO A. H Koschmann A. H Koschmann U. S. GEOLOGICAL SURVEY, DENVER, COLO. Search for other works by this author on: GSW Google Scholar Author and Article Information A. H Koschmann U. S. GEOLOGICAL SURVEY, DENVER, COLO. Publisher: Geological Society of America Received: 19 Jan 1960 First Online: 02 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Copyright © 1960, The Geological Society of America, Inc. Copyright is not claimed on any material prepared by U.S. government employees within the scope of their employment. GSA Bulletin (1960) 71 (9): 1357–1370. https://doi.org/10.1130/0016-7606(1960)71[1357:MPOPRI]2.0.CO;2 Article history Received: 19 Jan 1960 First Online: 02 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation A. H Koschmann; MINERAL PARAGENESIS OF PRECAMBRIAN ROCKS IN THE TENMILE RANGE, COLORADO. GSA Bulletin 1960;; 71 (9): 1357–1370. doi: https://doi.org/10.1130/0016-7606(1960)71[1357:MPOPRI]2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract A Precambrian complex of granulite, gneiss, and migmatite, intruded by numerous plutons of granitic rocks correlated with the Silver Plume granite, is exposed in a long narrow belt along the crest and upper slopes of the Tenmile Range, Colorado. The metamorphic rocks are predominantly felsic; bands, lenses, and irregular bodies of mafic rocks rich in biotite, hornblende, and locally in sillimanite and garnet, are interlayered with the felsic rocks. The major lithologic variations in the metamorphic rock complex are believed to be due chiefly to variations in the original sedimentary rocks, which probably were interbedded sandstone, shale, and limestone.The metamorphic rocks and the Silver Plume granite reveal the age relations of quartz and the feldspars, and these relations afford considerable information on the origin and progressive transformation of the rocks. Quartz is the earliest mineral in the metamorphic rocks and is probably a relict mineral of a sandstone. It has been partially replaced by feldspar. It occurs chiefly in irregular clusters, some of which show sutured grains, enclosed in a ramifying network of feldspar. Irregular small apophyses, barbs, and prongs of feldspar penetrate the quartz clusters along grain boundaries and healed fractures in the quartz. In some of the least feldspathized quartzose metamorphic rocks the feldspar is clearly interstitial to the quartz. Quartz also occurs in feldspar as small spherical inclusions. The relations of the quartz to the feldspars show clearly that a quartzose host rock was replaced by feldspar along quartz grain boundaries, pre-existing healed fractures, and margins of shadowy areas in strained quartz grains.The textural relations of the other principal minerals in the metamorphic rocks show that plagioclase formed earlier than the microcline and that the micas were the last of the principal minerals to form.Identical paragenetic relations are found in the Silver Plume granite, and the writer concludes that the Silver Plume granite was derived by partial fusion of quartzose metamorphic rocks. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Turquoise deposits of Colorado

Colorado ranks as the second state in the production of turquois, which comes from near the towns of Villagrove, Manassa, Leadville, and Creede.The deposits occur in weathered acid igneous rocks rich in alkalic feldspar. The turquois is found as veins and nodules in faults, fissures, and cavities. The Hall mine near Villagrove is in felsite porphyry, an Oligocene volcanic flow locally cut by quartz latite. The King mine near Manassa is also in felsite porphyry, probably an extrusive member of the Miocene Santa Fe formation, which is locally cut by felsite, possibly trachyte. The Turquois Chief mine near Leadville is in the Algonkian Silver Plume granite. Turquois is found near Creede in a stream bed, but the source is unknown.The three lode deposits were formed by circulating meteoric waters that leached and concentrated the constituents of turquois from the surficial rocks and precipitated them from cold solution in fracture and shear zones. The phosphorus was probably derived from apatite; the copper, from copper ores; and the aluminum, from the alteration of feldspar; iron and silica are impurities.