The general structural trend of the Boxelder Creek and Estes is east-northeast but these rocks are cut by a north-northwest striking, nearly vertical foliation. C l a s t s are reoriented and either elongated or flattened in the plane of the foliation. Many northwest-trending f a u l t s subparallel to the f o l i a t i o n are too small to show on Figure 2.
V i r t u a l l y all the rocks are overturned and have tops to the southeast or south. Folds plunge steeply northwest. Because of the steep dips and plunges, the map—when viewed from the north—resembles a cross section.
Although the details of many of the older events are largely unknown, the structural and depositional history recorded by the dif-ferent rocks indicates the following events:
(1) Folding, faulting, and erosion of the older iron-formation and uplift of the Little Elk Granite prior to the deposition of the a l l u v i a l , f l u v i a l , and marine rocks in the Boxelder Creek Formation and Benchmark Iron-Formation.
(2) Emplacement of the older metagabbro sill in a subhorizontal posi-tion followed by folding of the pre-Estes rocks.
(3) Uplift and block (?) rotation along growth f a u l t s followed by depo-sition of marine fans in the Estes and s h a l l o w marine rocks in the Roberts Draw Formation.
(4) Overturning to the north-northwest of both pre- and post-Estes rocks.
(5) Deformation, folding, and development of the north-northwest, nearly vertical f o l i a t i o n , presumably by northeast-southwest com-pression. This last event was a s s o c i a t e d with north-northwest f o l d i n g and regional metamorphism in the rest of the Black H i l l s which culminated with the emplacement of the 1.7 b.y. Harney Peak Granite.
(6) Post metamorphic faulting.
(7) Laramide uplift of the Black Hills dome and subsequent erosion.
URANIUM DEPOSITS Di stribution
Anomalous r a d i o a c t i v i t y is v i r t u a l l y restricted to the uraniferous conglomerate and granule quartzite tongue in the lower part of the Boxelder Creek Formation. Slightly anomalous r a d i o a c t i v i t y in conglo-merate in the lower chloritic quartzite tongue confirms the gradational contact between the two units. The most uraniferous rocks are the pyri-tic pebble conglomerates, although adjacent granule quartzites are l o c a l l y anomalous, especially where they are coarser grained and pyri-tic.
In general the proportion of pebble conglomerate to q u a r t z i t e decreases along strike to the west. For example, there is about 12 meters of conglomerate where the host unit disappears beneath the Paleo-zoic rocks to the southeast, but the conglomerate f a c i è s is absent in
the northwest part of the district. Most of the conglomerate occurs at two distinct l e v e l s ; one is near the middle of the unit and the other is near the upper contact. However the conglomerate is lensoid and a single 1.3 meter thick bed can thicken to 6 meters in less than 50 meters along strike. A l s o a thick, homogeneous, close packed conglomer-ate bed can also change lconglomer-aterally in a few tens of meters to several thin, loose packed beds separated by quartzite. The shape and rapid lithologie changes indicate the conglomerate was largely confined to channel deposits. Isolated c l a s t s in quartzite along the lateral exten-sions of the conglomerates are typical of gravels in a fluvial system.
The uppermost conglomerate is generally cobble-bearing and forms a relatively continuous subunit adjacent to a 20 meter thick unit of phyllite which marks the top of the uraniferous tongue. Although pyritiferous, this upper cobble conglomerate is generally not as radio-active as some of the underlying conglomerates. Locally there are lenses of cobble conglomerate within the pebble conglomerate subunits near the middle of the tongue.
Petrography and Mineralogy
The conglomerate and granule quartzite are typically poorly foli-ated and gray, tan, or greenish gray in unoxidized drill core but largely tan or iron-stained in surface exposures. The iron staining is due to oxidation of pyrite, which generally makes up a few percent to as much as 10 percent of the conglomerate. Local thin cross beds can contain as much as 30 percent pyrite. Greenish colors are due to a more chloritic matrix or in a few exposures, to higher concentrations of fuchsite.
Clast compositions in the pebble conglomerates consist of--in decreasing order of abundance--quartzite, chert, vein quartz, and phyl-lite. Phyllite clasts are very sparse. The conglomerate a l s o has
single crystal blue quartz clasts,largely of granule size and not exceeding 0.8 centimeter in maximum dimension. The single crystal blue quartz clasts are typically subrounded and undeformed whereas the clasts of chert and quartzite tend to be more rounded and are typi-cally flattened or oriented with their shortest a x i s perpendicular to the foliation. C l a s t s of vein quartz retain their original subrounded shapes. Packing varies from close to loose and some beds grade into quartzite containing only a few widely scattered pebbles.
The matrix of the conglomerate is largely finer grained quartz, muscovite, and chlorite. Accessory heavy minerals--in decreasing order of abundance—include pyrite, chromite, rutile, zircon, apatite, tourmaline, uranothorite(?), pyrrhotite, monazite, xenotime, uraninite and gold. Fuchsite commonly develops from the chromite grains, and in the area of higher metamorphic grade in the northeast part of the d i s t r i c t , virtually all of the chromite is converted to fuchsite. The detn'tal grains of chromite vary from euhedral octahedra to well rounded grains and most are broken as a result of the deformation that accom-panied the metamorphism of the host rocks. Pyrite occurs as either euhedral grains that obviously formed late, or as subrounded grains that are probably detrital. In general the grain size of both types of pyrite increases with increase in pebble size. Zircon, apatite, uranothorite(?), monazite, and gold have typical detrital shapes.
Rutile, however, occurs largely as composite small euhedral crystals that constitute 30 to 70 percent of subrounded to rounded skeletal quartzrich aggregates. Sparse anatase has a similar habit. The t e x -tures indicate that an early detrital titaniferous mineral (ilmenite or magnetite?) has been replaced either during diagenesis or subsequent metamorphism. Similar textures have been described by Ramdohr (1958), Ferris and Rudd (1971), and Saager (1970) in other Precambrian uranifer-ous conglomerates and are the basis for Ramdohr's "Pronto" reaction