Abstract

Pipe-shaped zones of altered rock containing disseminated and stockwork vein mineralization underlie each of the several Archean volcanogenic Cu-Zn ore lenses at the Millenbach mine. The rocks and mineralization have been metamorphosed first to hornblende hornfels assemblages, then retrograded to greenschist facies assemblages, but the metamorphism has not significantly affected the bulk composition of the rocks. The alteration pipes show a distinct mineralogical zoning in which a core zone containing either anthophyllite-bearing assemblages or massive chlorite grades outward into a biotite-rich zone characterized by a spotted texture. Normative calculations were used to show that alteration pipes initially had a chlorite core which graded laterally and vertically into a sericite-rich outer zone and finally into unaltered rocks. Similar zoning is present on the selvedges of individual sulfide veins within the alteration pipes. Volume was conserved, and a distinctive metasomatic trend developed across the alteration pipes and vein selvedges during the alteration process. The most characteristic features of the altered rocks are increased Fe and Mg, and decreased Ca and Na, relative to fresh rocks. K and Al have generally been added at the margins and leached at the cores of pipes. Chemical zoning across vein selvedges is similar. The zoning of alteration types on vein borders, and to a certain extent in the pipes as a whole, is interpreted as due to the progressive reaction of an initially homogeneous hydrothermal solution with the rocks as the fluid moved upward in, and outward from, permeable channelways in the discharge conduit of a submarine, sea water-dominated hydrothermal system. However, an additional cause of vertical, and to a lesser extent lateral, zoning is thought to have been the existence of a strong vertical thermal gradient, as well as lateral variations in the permeability in the pipes. The thermal gradient could have been caused by the discharge fluid column being under hydrostatic pressure and everywhere at its boiling point. The fluid was probably evolved sea water which was in equilibrium with the rocks at depth in the system and then degenerated, causing alteration and sulfide deposition as a result of cooling and chemical changes associated with boiling as it rose in the conduit systems.

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