Gresens' (1967) method of analysis of changes in volume and concentrations during metasomatism have been applied in many studies of hydrothermal alteration. This paper provides a simple method of solution of Gresens' equations, for both volume (or mass) change and concentration changes, one which requires no significant manipulation of analytical data and is readily accomplished both graphically and on a computer spreadsheet. Gresens' equation is rearranged into a linear relationship between the concentration of a component in the altered rock and that in the original. Simultaneous solution of such equations for all components that show no relative gain or loss of mass defines an "isocon." On a graph of the concentrations in the altered rock against those in the original, an isocon is a straight line through the origin. The slope of the isocon defines the mass change in the alteration, and the deviation of a data point from the isocon defines the concentration change for the corresponding component. As is shown, this can be applied to several stages of alteration simultaneously, and to other kinds of mass transfer such as migmatization.

The isocon diagram; a simple solution to Gresens' equation for metasomatic alteration

Lithogeochemical techniques using immobile elements

The Iberian Pyrite Belt (IPB) has, over the past decade, been an area of renewed mining activity and scientific research that has resulted in a wealth of new data and new geological and metallogenic concepts that are succinctly presented in this Thematic Issue. The reason for this interest in the IPB, which forms part of the Hercynian orogenic belt, is that its Late Devonian to Middle Carboniferous rocks host a huge quantity of volcanic-hosted massive sulphide (VMS) mineralization (1700 Mt of sulphides, totalling 14.6 Mt Cu, 13.0 Mt Pb, 34.9 Mt Zn, 46100 t Ag and 880 t Au). The mineralization and its environment display a number of typical signatures that can be related to the mineralogy and zoning of the sulphide orebodies, to the lead isotopes of the mineralization, to the geochemical and mineralogical variations in the hydrothermal alteration halos surrounding the orebodies, to the geochemical characteristics of the bimodal volcanics hosting the VMS, to the complex structural evolution during the Hercynian orogeny, to the presence of palaeofaults and synsedimentary structures that acted as channels and discharge traps for the metalliferous fluids, and to the gossans developed over VMS. Discriminant geological criteria have been deduced for each domain which can be helpful in mineral exploration, complementing the more traditional prospecting techniques. Although the question of the IPB's geodynamic setting is still under debate, any interpretation must now take into account some incontrovertible constraints: for example, the geochemical characteristics of a large part of the basic lavas are comparable to those of mantle-derived basalts emplaced in extensional tectonic settings, and the associated acidic rocks were produced by melting of a basic crustal protolith at low- to medium-pressures and a steep geothermal gradient, thus, the sulphide-bearing volcano-sedimentary sequence differs strongly from recent arc-related series. It is considered here that the tectonic setting was extensional and epicontinental and that it developed during the Hercynian plate convergence, that culminated in thin-skinned deformation and accretion of the South Portuguese terrane to the Iberian Paleozoic continental block.

The volcanic-hosted massive sulphide deposits of the Iberian Pyrite Belt Review and preface to the Thematic Issue

An introduction to remobilization: Information from ore-body geometry and experimental considerations

We have made laboratory measurements of joint closure and permeability of both well- and poorly mated, laboratory-produced joints in Westerly granite under joint normal stresses to 160 MPa, appropriate to midcrustal depths. We have also made detailed topographic measurements of the joint surfaces, both before and after pressure testing. The purpose of the study was to characterize and understand the hydraulic behavior of joints under conditions where fluid migration under natural settings is known to occur but where physical conditions, namely high lithostatic load, suggest that the presence of open, fluid-conducting joints might be improbable. Normal stress was applied by hydrostatic confinement of cylindrical samples, each containing a single axial tensile fracture. Experiments were performed at room temperature under conditions designed to minimize chemical effects of dissolution or precipitation. We found that while a mated joint can be completely closed by the application of high normal stresses, the same joint laterally offset by 0.53 mm remains open and many orders of magnitude more hydraulically conductive than the mated joint even at highest stresses. Detailed topographic observations are consistent with self-propping of the offset joint. Permanent damage to the surfaces of the offset joint is not widespread but occurs at isolated locales covering about 10% of the surface. Damage is confined to regions of negative (convex upward) curvature, such as summits and the edges of plateaus. Approximately 20% of the joint surface is in wall-to-wall contact at 160 MPa, and the flow tortuosity thus induced is probably the cause of the breakdown of the parallel plate approximation for fluid flow in the offset joint.

Self‐propping and fluid flow in slightly offset joints at high effective pressures

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.

G. Riverin

Papers

Wall-rock alteration at the Millenbach Cu-Zn mine, Noranda, Quebec