Ore Fanglomerates as Exploration Guides for Porphyry Copper Deposits

Three porphyry copper districts in southern Arizona are partly overlain by post-ore fanglomerates which share certain physical and petrologic characteristics and which may comprise a distinct and significant class of rock. These units are the Locomotive fanglomerate of Ajo, the Helmet fanglomerate of Twin Buttes-Mission-Pima, and the Cloudburst formation of San Manuel-Kalamazoo, termed “Locomotive-type fanglomerates” herein. Post-ore units at Poston Butte and Lakeshore, Arizona, and Sar Chesmeh, Iran, also appear to fit this class.

“Locomotive-type fanglomerates” are composed of poorly sorted conglomerates, lenticular sandstone beds, andesitic to rhyolitic flows and tuffs, and unique monothologic landslide breccias. They contain fragments of holyokeite – a rare oligoclase lava of porcellaneous appearance. Their basal contacts are considered to be depositional and overlapping, sometimes complicated by later low-angle faulting.

Locomotive Fanglomerate of AJO

The fanglomerate is presently exposed over about 11 square miles, but had an apparent original areal extent of approximately 25 square miles. The fanglomerate unconformably overlies Precambrian gneiss, Cretaceous volcanics, and Laramide quartz monzonite. The fanglomerate at one time covered the entire New Cornelia orebody, but now only the southern and southeastern parts lie beneath it. Unpublished drill hole information indicates that the base of the fanglomerate has overlapping relations on the pre-fanglomerate rocks, and the contact is depositional and nonconformable. Late Tertiary volcanics and Quaternary alluvium unconformably overlie the Locomotive fanglomerate.

The conglomerates contain fragments of a peculiar lava called holyokeite, which is characterised by a diabasic to fluidal texture and is composed almost entirely of feathery, interlocking laths of oligoclase exhibiting only coarse albite-law twinning. These pebbles are usually dull grey to white, with a porcellaneous appearance, and can be mistaken in the field for quartzite or calc-silicate hornfels. Holyokeite flows have not been found in place in the Locomotive fanglomerate or Ajo volcanics.

Mineralization in the Locomotive fanglomerate is of two types: (1) several small copper occurrences of syngenetic origin derived from erosion of the New Cornelia deposit; and (2) minor epigenetic metallization in narrow quartz- hematite veins with pyrite, chalcocite and argentite.

Helmet Fanglomerate of Twin Buttes-Mission-Pima

Drill hole data have extended its area another five or more square miles under gravel to the east. The fanglomerate is overlain by Quaternary gravel and underlain by Precambrian granite, Paleozoic sedimentary rocks, Cretaceous arkose and volcanics, and Laramide intrusives. The San Xavier “overthrust” fault forms the basal contact of the Helmet fanglomerate locally, that is, in the area immediately north of Twin Buttes. Beds and contacts within the fanglomerate dip south as much as 65°.

The fanglomerate consists of a sucession of lenticular units inferred to total 10,500 feet in thickness. About 18 mappable subunits have been distinguished in three categories: conglomerates, monolithologic sedimentary breccias, and volcanics. The conglomerates comprise about 50 percent of the total; the sedimentary breccias 40 percent; and the volcanics 10 percent.

The conglomerates are poorly sorted with rare bedding in a silty matrix. They are moderately indurated on the surface and in drill holes. Holyokeite fragments are found occasionally in thin sections of conglomerate units of the Helmet fanglomerate.

Any discussion of the Helmet fanglomerate and its origin also becomes embroiled in the San Xavier “overthrust” fault controversy. There are two leading published hypotheses on the timing and direction of movement of the San Xavier, and on its relationship with the Helmet. Cooper’s hypothesis is that the upper plate moved north about 6 miles, relative to the lower, in post-ore and post-Helmet time. Weaver’s hypothesis, based in part on interpretation of drill hole data, holds that the San Xavier “overthrust” fault developed at the base of several sheets transported eastward by gravity, and that the Helmet fanglomerate was shed into the same basin onto the transported sheets late in the process.

Conglomerate at Poston Butte

A small outcrop of pebble conglomerate, which is similar to the “Locomotive-type fanglomerates”, is present on the lower western slope of Poston Butte, about a mile northeast of Conoco’s test shafts. The conglomerate is poorly sorted and essentially non-bedded, but faint layering and orientation of pebbles suggests a dip of 45° to the northeast. Minor amounts of chrysocolla are present in a particular conglomerate layer and were derived from chalcopyrite contained in some Oracle granite clasts.


porphyry copper deposits index map of arizona

locomotive-type post-ore fanglomerates as exploration guides for porphyry copper deposits

By |2019-02-21T18:05:45-05:00February 13th, 2019|Categories: Geology|Tags: |Comments Off on Ore Fanglomerates as Exploration Guides for Porphyry Copper Deposits

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