Geology and mineral deposits of the western portion church hills, Millard County, Utah

Over 1,000 feet of Late Ordovician through Early Devonian dolomite are exposed in a previously unmapped inlier within Late Tertiary (Pliocene?) conglomerate at the western edge of the Church Hills, near Holden, Utah. The Paleozoic rocks are in places capped by shattered Precambrian quartzite, the eroded remnant of a Late Cretaceous (Coloradoan?) thrust sheet. Visible copper mineraliza­tion in the quartzite suggests the possibility of a commercial deposit and has attracted some exploratory effort. Active block faulting following deposition of at least some of the conglomerate triggered rock slides in which slabs of dolomite as much as 1,000 feet long broke away from the upthrown side of the fault and slipped westward and southward out over the conglomerate. Some of the dolomite masses exhibit low-angle thrust-like contacts with the younger but subjacent conglomerate, and at least one block is entirely surrounded by outcrops of the conglomerate. Origin of such relationships by compressive thrusting would be highly unusual, since Late Tertiary tectonics in the Great Basin are dominated by extensional forces. An apparently unique suite of rare copper and lead sulfates belonging to the alunite family occurs in the shattered quartzite near the conglomerate contact. Osarizawaite, beaverite, jarosite, and (in veinlets) alunite are represented. This is the third re­ corded occurrence of osarizawaite, previously noted only in Australia and Japan. Associated with the alunite minerals are iron oxides, chrysocolla, halloysite-alunite-gibbsite veinlets, the arse­ nates conichalcite and mimetite, the carbonates cerrusite, malachite and azurite, and traces of chalcocite and covellite. Together they constitute an oxidized copper-lead supergene mineral occurrence. Zinc minerals are absent. The supergene mineralization is not a product of the present erosion cycle, but rather formed in Miocene(?) time before deposi­tion of the conglomerate, as is suggested both by the spatial re­ triction of the oxide minerals to a zone directly beneath the con­glomerate-quartzite contact and by weathered and iron-stained chips of dolomite and quartzite in the basal few feet of the conglomerate. The present cycle is merely exhuming and dispersing an older deposit. The minerals tend to be rich in aluminum though they occur in alumina-poor rocks. Lack of evidence of significant hydrotherrnal alteration favors the concept of a non-hydrothermal introduction of alumina. The alumina that had been concentrated in a superjacent Miocene.(?) soil could have been solubilized by the acid that was being liberated by the oxidizing mineral deposit and then redistri­buted as veinlets and smears of alunite, halloysite, or gibbsite, either before or after burial by the conglomerate. Osarizawaite is postulated to have formed by a like addition of alumina to pre­ existing oxide phases as follows: 4A1(0H)3 + Cu2(OH)2C03 + 2PbC03 + 5H+ + 4S04 -2 gibbsite malachite cerrusite = 2PbCuAl2 (SO4)2(0H)6 + 3HC0-3 + 2H20. The average concentration of metals in about 180 soil samples taken from the area is 24 ppm Cu, 19 ppm Pb, and 41 ppm Zn, with 90 per cent confidence limits of 1.7, 4.5, and 1.4 ppm, respectively. The zinc trace-element distribution is unrelated to mineralization. Anomalous lead content in soil exceeds 100 ppm, and that of copper 80 ppm, over some parts of the quartzite. Though assessment work completed thus far has not revealed commercial ore, the possibility of minable argentiferous copper and lead accumulations near the base of the quartzite should be further explored.