Stratiform Deposits

mineral deposits concentrated within one or more strata of volcano-sedimentary and sedimentary bedded rock formations. The most characteristic examples are deposits of lead and zinc ores in strata of carbonate rocks (the Missouri Valley deposits in the USA and similar deposits in the USSR, Canada, Poland, Austria, North Africa, and elsewhere) and copper ore deposits in sandstone-shale strata (the Copper Sandstone Deposits in Southern Africa, the German Democratic Republic, and Poland and in Kazakhstan and Central Siberia in the USSR).

In stratiform deposits, there is a predominance of stratified bodies that are conformable with the enclosing rocks. The ores in these bodies have simple mineral compositions determined by the dissemination of copper, zinc, and lead sulfides and the accompanying minerals in one or more strata of the ore-bearing rocks. Stratiform deposits are generally of large size, cover a broad area, and form vast ore regions or provinces (for example, the Mississippi Valley lead and zinc deposits).

Several hypotheses have been advanced on the origin of stratiform deposits. In the view of such geologists as E. Zakharov and K. Satpaev of the USSR, C. Behre of the USA, and C. Davidson of Great Britain, stratiform deposits are of hydrothermal origin. This hypothesis, however, is contradicted by the absence of mag-matic rocks in the areas where the stratiform deposits are found. Another hypothesis, whose adherents include V. Popov and V. Domarev of the USSR and H. Gruszczyk of Poland, regards stratiform deposits as sedimentary formations that arose from marine sediments on the bottoms of ancient seas together with the enclosing rock strata. This view is contradicted by the presence of cross-cutting ore veins along with the bedded ore bodies.

The second half of the 20th century has seen the development of the hypothesis of the protracted formation and complex origin of stratiform deposits. The ore minerals were initially deposited in the ore-bearing strata by sedimentation on the bottoms of ancient seas; extensive deposits of low-grade, noncommercial ores were thereby formed. Subsequently, under the action of hot, chemically active subterranean water that circulated through the strata, the sulfide material was dissolved and redeposited, so that secondary deposits of richer, commercial ores were formed. V. Smirnov of the USSR and P. Zuffardi of Italy are among the geologists supporting this hypothesis.

In the capitalist countries, stratiform deposits account for about 40–60 percent of the lead-ore reserves and 35–40 percent of the zinc-ore reserves.

The term “stratiform deposits” was introduced at a conference on the origin of such deposits held in New York in 1969.