Rutile and topaz in Precambrian gneiss, Jefferson and Clear Creek Counties, Colorado

WALTER. J. HICKEL, Secretary

Geological Survey

Abstract - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Introduction - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Geologic environment - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Stratigraphy of the gneissic rocks - - - - - - - - - - - - - - - - - - - - - - Rutile-bearing sillimanitic topaz-quartz gneiss - - - - - - - - - - - - - - - Petrography - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Rutile and topaz content- - - - - - - - - - - - - - - - - - - - - - - - - - - - Chemistry - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Geologic interpretation - - - - - - - - - - - - - - - - - - - - - - - - - - - - Geophysical data- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - Economic implications - - - - - - - - - - - - - - - - - - - - - - - - - - - - References cited- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

2. Mineralogic composition based on mineral separates from
3. Semiquantitative spectrographic analyses of rutile concentrates occurrence, Jefferson and Clear Creek Counties, Colo- - - - -
4. Photomicrograph showing fibrolitic sillimanite replaced in part by topaz - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -
5. Photomicrograph showing rutile grains molded around topaz grains - - - - - - - - - - - .:. - - - - - - - - - - - - - - - - - -
6. Photomicrograph showing prismatic sillimanite intergrown with rutile- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

topaz-quartz gneiss unit and its stratigraphic equivalent - - - -

rutile-bearing topaz-quartz gneiss- - - - - - - - - - - - - - - -

from the sillimanitic topaz-quartz gneiss unit- - - - - - - - - - CONTENTS

Sherman P. Marsh

by folding and cataclastic deformation about 1.45 b.y. z

have been mapped within the report area. The stratigraphic sense-top versus bottom-of the sequence can be best inferred from broad fold patterns outside of this area, and on this basis the tops of the beds are believed to lie to the north.

ently stable pair, and this is interpreted as indicating that the regional metamorphic grade reached the uppermost level of the amphibolite facief'.

To the west, the hornblende gneiss unit interfingers and intergrades with a microcline-quartz-plagioclasebiotite gneiss unit (gnm, fig. 1). This microcline gneiss unit is about 1,000 feet thick in the western part of the area and can be traced westward for more than 4 miles. The unit is more granitic in appearance than the other gneisses, and foliation is defined by oriented biotite flakes in a medium-grained quartz-feldspar rock, rather than by marked compositional banding.

The sillimanitic topaz-quartz gneiss unit that contains the principal rutile concentrations (gnq, fig. 1) is a lenticular layer within the hornblende gneiss unit. The layer is about 100 feet thick from sample locality D to locality G (fig. 1), but it thins to about 11 feet at locality B and grades westward into a feldspathic gneiss with high quartz content. The topaz-quartz gneiss layer is recognized over a strike length of more than 7,000 feet and is 80 feet thick at the point where it is covered by surficial materials. The layer may continue eastward under the alluvium as much as 5,500 feet before it intersects a major fault of northwest trend.

A unit 30 to 500 feet thick (gns, fig. 1), which 1 i e s stratigraphically above the hornblende gneiss unit, is composed of sillimanitic quartzite and sillimanitic biotite-quartz gneiss and has been traced about half a mile east of the report area. Discontinuous lenses of these rocks persist westward at this stratigraphic position for almost a mile west of the report area. Fibrolitic needles of sillimanite form interlensing laminae between granular quartz septa. Sillimanite makes up about 20 to 30 percent of the quartzite and about 10 to 20 percent of the biotite-quartz gneiss. Variable amounts of biotite, feldspar, and muscovite are found. Rutile is a characteristic accessory mineral in amounts of a few tenths of a percent.

All the rock types in this unit are very light colored and are white, with glistening grains, where unweathered. Other than the red to orange rutile grains and weathered pale yellow apatite grains, all the minerals are white or light gray. The rock lacks the blacksand laminae (magnetite, ilmenite, garnet) characteristic of quartzites in the gneissic succession; it lacks the dark brown to black biotite flakes that dot almost all the quartz-feldspar rocks; it lacks even the feldspar grains common to all the quartz-rich rocks.

White, fine- to medium-grained gneisses composed chiefly of quartz and topaz and lesser amounts of sillimanite, rutile, and apatite are common (modes 2, 3, 4, table 1) in the topaz-quartz gneiss unit. Thin sections show that these gneisses are com p o s e d of alternating quartz-rich and topaz-rich layers. Quartz and apatite occur in irregular anhedral grains 1 to 5 mm across; the topaz characteristically occurs in slightly smaller rounded grains flattened parallel to the foliation. The topaz grains commonly exhibit healed cross fractures marked by bubbles concentrated in planar zones that are parallel to the basal cleavage of the topaz grains. These fractures are rudely perpendicular to the foliation of the rock. The rutile grains are commonly anhedral or subhedral, and some are intergrown with topaz or sillimanite. The largest quartz grains contain rutile needles, and a few tiny equant euhedral rutile crystals.

The topaz-quartz gneisses intergrade with interlayered s i 11 i manit i c topaz-quartz gneiss, characterized by prismatic sillimanite (mode 5, table 1 ). The prismatic sillimanite crystals are as much as 3 em Ion~ but are slender and generally are only 1 or 2 mm in cross-section area. These crystals form irregular to radiating aggregates flattened parallel to the foliation, but they lack linear orientation. Certain sillimanitic quartz gneiss laminae (mode 6, table 1) contain fibrolitic sillimanite with linear orientation. These needles are replaced in part by topaz and, less commonly, by randomly oriented prismatic sillimanite.

Pegmatitic stringers or segregations within the quartz gneiss (mode 7, table 1) are composed mainly of quartz and coarsely prismatic sillimanite. Much of the quartz is coarse-grained and rutilated and is 35.8

Figure 2,-Fibrolitic sillimanite replaced in part by topaz. Mag- nification·, X40. Q, quartz; s, sillimanite; t. topaz.

segregated into lenses as much as 5 em thick. Bent sillimanite prisms 5 mm across and as much as 3 em long occur in bundles curving around the quartz lenticles. Rutile is intergrown with the sillimanite and is especially abundant in apatite-rich stringers.

West of the pinchout of the topaz-quartz gneiss unit, a light-gray plagioclase-quartz gneiss is found at the same stratigraphic position and seems to lack both topaz and sillimanite but contains 0.6 percent of rutile (mode 1, table l ). This lithologic type has not been mapped as a part of the topaz-quartz gneiss unit.

Certain mineralogic details observed in thin section seem to be critical in the interpretation of the origin of the rutile-bearing sillimanitic topaz-quartz gneiss.

l. Fibrolitic sillimanite is absent, is partly replaced by topaz (fig. 2), or is partly recrystallized to prismatic sillimanite. As the fibrolitic silliman - ite is linearly oriented parallel to small-s c a 1 e folds related to the early period of folding, it is believed to have formed during the regiona l metamorphism. The topaz and prismatic sillimanite are younger than the fibrolitic sillimanite and therefore are interpreted as being younger than the regional metamor phism.

2, The rutile grains are molded around topaz (fig. 3)

and are intergrown with prismatic sillimanite (fig. 4). The rutile therefore seems to have crystallized after the p r inc i p a 1 regional metamorphism, at the same time as the topaz and prist. matic sillimanite. The rock cannot represent a fossil rutile placer, a lthough the rutile may have formed from titanium alr eady within the rock at the time of r egional metamorphism.

3. Although magnetite, ilmenite, and sphene occur t.'1 rocks bordering the topaz-quartz gneiss , th<::!se minerals do not occur in the same rock WJth rutile. The rutile-bearing rocks are essentially free of ferromagnesian minerals.

RUTILE AND TOPAZ CONTENT A series of samples was taken to evaluate the r u - tile and topaz content of the principal outcrops (table 2). Samples R-1, R-3, and R-6 are composites of a large number of rock chips taken across the entire layer at the only three localities where it crops out fully (fig. 1). Samples R-2A, R-4, and R-7 are grab samples taken at localities where outcrops are sparse, and they may not represent the average composition of the unit.

Each sample was crushed, ground, and screened, and the part between 60 and 150 mesh was washed and sonically cleaned to remove adhering particles of dust. One hundred and fift y grams of this material was split into heavy and light fractions in bromoform

.r .· - ,_ ......:( ... \

(tlens ity 2.8). The weights of the heavy and light fractions were recorded, and percentages were calculated (table 2 ). Further mineral s eparation using the Frantz isudynamic separator and a laboratory micro pan de vi ce yielded an almost pure rutile fraction. Other md.gnetic concentrates also contained small amounts of rutile, and the weight of this rutile was added to that of the pure fraction to calculate the rutile percentages given in table 2. The topaz percentages were obtained by a similar procedure. Because of losses of material to other fractions of the sample during separation, the percentages quoted are conservative estimates of the actual rutile and topaz contents of each sample processed. The major source of error in evaluating the rutile and topaz contents of the topaz-quartz gneiss unit almost certainly lies in the field sampling procedure. Core drilling or other physical exploration would be required to adequately sample this unit.

Sem iquantitative spectrographic analyses of rutile concentrates from the sillimanitic topaz-quartz gneiss unit show that the content of niobium and iron in the rutile is low. The rutile samples contained less than 5 percent impurities, and these impurities were almost entirely made up of sillimanite and topaz.

Table 2.--Mineralogic composition, in weight percent, based on mineral separates from rutile-bearing topza-

Table 3.--Semiquantitative spectrographic analyses of 1 rutile concentrates from the sillimanitic topaz-quartz gneiss unit

Semiquantitative spectrographic an a 1y s e s were made by J. C. Hamilton of heavy (density ,.z. 8) and light (density <2.8) fractions separated from the s i 11 i manit i c topaz-quartz gneiss. The rock is abnormally low in iron, calcium, sodium, potassium, and it contains less than 1 part per million of beryllium, an element that is frequently concentrated in topaz greisens. Small amounts of tin (0-lSppm), niobium (0-15 ppm), molybdenum (0-7 ppm), and rareearth elements were detected.

The topaz-quartz gneiss is mineralogically simple, chemically unusual, and rather undistinguished in appearance. Quartz, topaz, and sillimanite are the only abundant minerals and rutile, apatite, and zircon are the only characteristic accessory minerals. These minerals clearly formed at the expense of the previously formed regional metamorphic suite. Chemi-Locality (in fig. 1)

c

[A. L. Sutton, Jr., analyst]

GEOLOGIC INTERPRETATION (ppm)

c

CHEMISTRY [Determinations by S. P. Marsh]

Heavy fraction (density >2.8)

(percent)

0.3 .5 .5 .5 .5 .7 Light fraction (density <2.8)

Grab samples Topaz Rutile

cally, the rock has a very high fluorine content and in comparison with other quartz-rich gneisses in the Front Range is unusually low in the alkali metals, alkaline earths, and iron. The chemically resistant character of the miner a 1 s in the rock, the high fluorine content, and the depletion of several common elements suggest that the topaz-quartz gneiss is the product of fluorine metasomatism. The high fluorine, aluminum, and titanium contents suggest that these elements were added or enriched during the metasomatism. According to this hypothesis, a gneissic progenitor, probably a sillimanitic quartz-rich gneiss, was attacked by hot fluorine-rich solutions that swept the rock clean of many of the common elements and minerals, left a resistant assemblage of zircon, quartz, and sillimanite, and added rutile and the topaz and apatite.

Rutile is apparently stable in this gneissic sequence only in rocks deficient in both iron and c a 1 c i u m. Gneisses adjacent to the topaz-quartz gneiss contain both ilmenite and sphene, but no rutile. Rutile is an accessory mineral (}2 percent) in several other gneiss layers, notably the sillimanitic quartzite unit (gns , fig. 1). This unit probably also underwent limited metasomatic alteration, as it contains no magnetite, . ilmenite, or garnet but does contain rutile and small amounts of topaz.

GEOPHYSICAL DATA Preliminary aeromagnetic data show that the report area lies on a strong linear east-west magnetic gradient. The anomaly is interpreted (Peter Popenoe, oral commun., 1968) as indicating a steeply dipping contact or a major fault that marks the north boundary of a relatively magnetic block. The anomaly extends from the mountain front about 20 miles west Thickness

0.6 1.4 1.0 of unit sampled (feet) into the range. The occurrence of plutonic rocks to the south of the report area suggests that the anomaly marks a steep contact between p 1 u tonic and metasedimentary rocks at depth. The geophysical data are the strongest direct evidence for a major subjacent igneous mass from which fluorine-rich solution could have been derived.

Rutile is an important industrial mineral and source of titanium, and topaz and sillimanite are raw materials for refractories and ceramics. Most rutile is obtained from beach sands, and mostofitis imported. Topaz and sillimanite are members of a group of aluminosilicate minerals-also including kyanite, andalusite, and dumortierite-that has steady demand for production of mullite and other uses. Minerals of this group are produced domestically and also are imported.

Considering the content of rutile in the topaz-quartz gneiss as determined thus far, and considering the purity of the rutile, the gneiss warrants investigation as an ore of rutile, especially since it might also yield a saleable topaz product. Tests to determine the recoverability of the rutile and topaz wo·Jld be required; but first, much more extensive investigations than the preliminary studies reported here would be required to determine the character and c'stribution of the rutile and topaz and their average content in the gneiss unit. Further study of the derosit to the extent permitted by the limited exposures is planned, but the grade of the deposit probably could be determined satisfactorily only by means of core drill holes through the gneiss unit.

The discovery of rutile in the topaz-quartz gneiss suggests that a part of the central Front Range is favorable for prospecting for rutile and that the search should be concentrated on white sillimanitic quartz-rich gneisses.

REFERENCES CITED Lovering, T. S., and Goddard, E. N., 1950, Geology and ore deposits of the Front Range, Co 1or ado: U.S. Geol. Survey Prof. Paper 223, 319 p. Peterman, Z. E., Hedge, C. E., and Braddock, W. A., 1968, Age of Precambrian events in the northeastern Front Range, Colorado: Jour. Geophys. Research, v. 73, no. 6, p. 2277-2296.

Where this page came from

This page was imported from U.S. Geological Survey. Published by the U.S. Geological Survey and, as a work of the United States government, in the public domain.

Nobody has written it yet — it is the source material at a new address, which is why search engines are asked to skip it and why no one earns from it. It is up for grabs: take it on, and it is yours to rewrite and to earn from.