Patapedia: an Appalachian calc-silicate-hosted copper prospect of porphyry affinity

Abstract

The Patapedia prospect is in several bodies of calc-silicate hornfelses and subordinate marbles in which fracturing and metasomatism are associated with copper-bearing sulphide mineralization. Igneous rocks are restricted to a swarm of thin felsic dykes of calc-alkaline affinity.Prograde metamorphic isograds (anorthite–K-feldspar; tremolite–anorthite; sphene; diopside; and wollastonite) are telescoped into the marginal parts of each metamorphic body, and are estimated from phase equilibrium relationships to have developed at temperatures above 400 °C at XCO2 < 0.5. The prograde metamorphism, which was accompanied by some addition of potassium and a loss of calcium, is interpreted to have been caused by heat transferred from hydrothermal fluids.Garnet- and epidote-bearing assemblages are exclusively metasomatic and formed when conditions of XCO2 < 0.1 prevailed. Epidote-bearing assemblages developed at lower temperatures than those containing garnet, and are associated with the bulk of the copper mineralization. The metasomatic stage, including sulphide deposition, is considered from various lines of evidence (albitization of prograde plagioclase, local marialitic scapolite development, and NaCl-saturated fluid inclusions) to have proceeded from the interaction of the rocks with saline hydrothermal fluids.Dyke rocks show evidence of potassic, sericitic, and propylitic alteration. Those centrally located with respect to the metamorphism are potassically altered. Such areas are also better mineralized.The metasediments display patterns of 13C and 18O depletions that are due in part to the prograde metamorphism and in part to the interaction of the rocks with an isotopically lighter hydrothermal fluid of probable magmatic origin.A model is proposed for the evolution of the deposit and its host, which involves prograde metamorphism of calcareous sediments caused by heat transferred from a largely magmatic hydrothermal system. This fluid altered associated dyke rocks and, after calcite was consumed in the calc-silicate hornfelses, fractured and altered those rocks with concomitant precipitation of copper and iron sulphides. This model is similar to that commonly put forward for the development of porphyry copper deposits.