The Alteration Box Plot: A Simple Approach toUnderstanding the Relationship between Alteration Mineralogy andLithogeochemistry Associated with Volcanic-Hosted Massive Sulfide Deposits

Abstract

Zonal alteration is a common feature in volcanic rocks surrounding sea-floor massive sulfide deposits. Alteration indexes, such as the Ishikawa alteration index (AI) and the chlorite-carbonate-pyrite index (CCPI), have been developed to measure the intensity of sericite, chlorite, carbonate, and pyrite replacement of sodic feldspars and glass associated with hydrothermal alteration proximal to the orebodies. In this paper a simple graphical representation of the Ishikawa AI plotted against the CCPI, termed the "alteration box plot," is used to characterize the different alteration trends related to massive sulfide ores and to assist in the distinction of volcanic-hosted massive sulfide (VHMS)-related hydrothermal alteration from regional diagenetic alteration. Although there are some limitations with the technique, a series of case studies are used to demonstrate that the alteration box plot is a powerful means of understanding the relationship between mineralogy, lithogeochemistry, and intensity of alteration in zoned alteration systems related to VHMS deposits and should assist the exploration geologist in determining vectors to the center of the ore system.

Integrated 3D Geological Modeling to Gain Insight in the Effects of Hydrothermal Alteration on Post-Ore Deformation Style and Strain Localization in the Flin Flon Volcanogenic Massive Sulfide Ore System

“…This trend is typical of weak sericitization at the margins of the hydrothermal system [4]. This trend is not present in the samples of mafic volcanic rocks and is ambiguous for some of the felsic volcanic rock samples.…”

“…Over several decades, lithogeochemical methods were developed for the exploration of altered host rocks providing evidence for hydrothermal fluid circulation associated with the formation of VMS deposits [1][2][3][4]. Although hydrothermal alteration zones of VMS deposits may vary widely in scale, geometry and prevalent alteration mineral assemblages, they typically share characteristic geochemical zoning patterns, particularly in proximal upflow zones in the footwall of the deposit [1].…”

“…Lithogeochemical characterization of hydrothermal alteration was based on the alteration box plot [4] and corundum percentages derived from normative mineral estimation procedures [34]. The alteration box plot is a well-known method to recognize hydrothermal alteration trends in multivariate space, in which the Carbonate-chlorite-pyrite index (CCPI) is plotted against the Ishikawa index (AI) [5].…”

“…The alteration box plot is a well-known method to recognize hydrothermal alteration trends in multivariate space, in which the Carbonate-chlorite-pyrite index (CCPI) is plotted against the Ishikawa index (AI) [5]. It provides a rapid and simple method for identifying common trends, serving as proxies for alkali alteration and feldspar destruction due to hydrothermal alteration of felsic and mafic volcanic rocks [4]. The trends of altered rocks were plotted together with a subset, representing least-altered samples, which can be a priori defined by a number of criteria [35].…”

“…This integrated model was compiled in an Adobe Acrobat ® 3D. A PDF supplementary file ( Figure S1) provides interactive 3D visualization and includes wireframe surfaces of the sulfide ore lenses, lithostratigraphic horizons and the principle D 3 -D 4 and D 7 shear zones, fault structures and a generalized version of the 1:10,000 scale geological map [18]. In addition, the 3D model contains a set of lithostratigraphic reference holes that were used as subsurface constraints for the modeling of the lithostratigraphic horizons and structures.…”