Intrusion and ore deposition in New Mexico

Abstract

In the early part of the twentieth century several investigators proposed that hydrothermal ore deposits are precipitated from solutions or fluids that had been separated from deep-seated magmas. The purpose of this paper is to test this concept, studying the primary petrology, hydrothermal alteration, and distribution of Cu and Zn in silicic igneous plutons genetically related (in the Niggli sense) to nearby hydrothermal ore deposits. Silicic Tertiary plutons were chosen in 3 New Mexico mining districts, the Hanover-Fierro intrusive at Hanover and Fierro, the Granite Mountain pluton at Magdalena, and 4 outliers of the Lordsburg intrusive at Lordsburg. The detailed petrographic and alteration study of each sample taken was accompanied by an analysis for Cu and Zn through a newly developed, sensitive, and accurate dithizone technique. The metal ion distribution of a given metal in an igneous country rock in an area of few veins bearing metals is apt to be related to primary igneous texture, composition, and other features, but is not related to alteration. The average weight percent of alteration in this instance is low, and the amount of metal introduced into the rock from the veins is small. Conversely, the metal ion distribution in an igneous country rock that is cut by numerous veins bearing metals is statistically far more clearly related to alteration than to primary igneous features. The average weight percent of alteration and amount of metal introduced into this area is apt to be large. The same series of relations applies as well on a small scale, for as a single vein is approached from a distance in an igneous country rock, the content of alteration minerals and ore metals increases; both alteration and ore metals are more closely related to each other. The results of the study raised the question whether the ore deposits of the 3 districts studied actually originated from the adjacent intrusives.