28. COMPARISON OF THE TAG MOUND AND STOCKWORK COMPLEX WITH CYPRUS-TYPE MASSIVE SULFIDE DEPOSITS1Mark D. Hannington,2 Alan G. Galley,2 Peter M. Herzig,3 and Sven Petersen3 |
ABSTRACTDrilling of the active Trans-Atlantic Geotraverse (TAG) deposit indicates that the size of the mound-stockwork complex is approximately 3.9 million t, including 2.7 million t of massive and semi-massive sulfide (~2% Cu) at the seafloor and 1.2 million t of mineralized breccias (~1% Cu) in a subseafloor stockwork. Quartz-pyrite veining in the stockwork zone extends from about 40 meters below seafloor (mbsf) to a depth of 95 mbsf. Siliceous wallrock breccias in the lower part of the stockwork grade abruptly into chloritized basalt breccias at the margins of the mineralized zone, and massive sulfides at the flanks of the deposit onlap relatively unaltered, partially hematized basalts. The pipe-like dimensions of the stockwork zone do not exceed the diameter of the sulfide mound. Comparisons with samples collected during earlier dive series confirm that the vent complexes at the surface of the mound are not representative of the bulk composition of the deposit. Steep vertical metal zonation within the mound suggests that a long history of hydrothermal reworking has effectively stripped the constituents that are soluble at lower temperatures from the massive sulfides and concentrated them at the top of the deposit through a process of zone refining. The bulk of the mound is composed of massive pyrite and anhydrite-cemented breccias. The massive anhydrite (~165,000 t) occupies a high-temperature zone, immediately beneath the central Black Smoker Complex and above the quartz-rich stockwork. Fracturing in the underlying quartz-pyrite stockwork also has resulted in anhydrite veining at considerable depths in the stockwork zone. Despite the abundance of anhydrite in the mound, the amount of seawater penetrating the region of high-temperature upflow is small in comparison to the total mass flux of hydrothermal fluid. The anhydrite has been deposited by conductive heating of a small amount of entrained seawater at the margins of high-temperature conduits, and little or no mixing has occurred with the end-member fluids. Collapse of the anhydrite-supported portion of the mound following major episodes of hydrothermal upflow has caused extensive in situ brecciation of the mound and is an important mechanism for the formation of "breccia ores" in the deposit. Although anhydrite is not well preserved in the geologic record, given its retrograde solubility, it has likely played an important role in the development of similar ore types in ancient massive sulfides. >The morphology, size, and bulk composition of the TAG mound-stockwork complex is identical to that of some of the largest Cyprus-type massive sulfide deposits in the Troodos ophiolite. Typical Cyprus-type deposits comprise massive brecciated pyrite ores, underlain by a vertically extensive quartz-pyrite-chlorite stockwork. Sandy pyrite or conglomeratic ore, similar to that found in the TAG mound, is characteristic of the upper parts of Cyprus-type deposits. Textures in these ores, previously attributed to seafloor weathering and erosion, are most likely the result of anhydrite dissolution. Massive, granular pyrite (hard, compact ore), with abundant vuggy cavities lined by idiomorphic pyrite and quartz, occur below the conglomeratic ores and closely resemble sections of massive pyrite and pyrite-silica breccias from the TAG mound. At TAG, seafloor oxidation of the sulfides is currently taking place, even as the deposit is forming. Fe-oxide gossans have developed at the surface of the mound as a result of weathering of chimney debris. These deposits are modern analogs of the extensive ochers that typically overlie the massive sulfide deposits in Cyprus. By analogy with TAG, a number of the weathering features of Cyprus-type deposits (e.g., red clays, leached lavas), previously thought to be products of acid alteration by meteoric groundwaters, may have formed while the deposits were still on the seafloor. Low-temperature venting through this material has locally produced distinctive red cherts (silicified Fe oxides). This material is common within the mound and in the underlying basalts and closely resembles the red jaspers found throughout the pillow lava sections in Cyprus. Silicification in the upper part of the TAG mound also has produced a cherty, sulfide carapace at the top of the deposit that inhibits further degradation of the mound by seafloor weathering. This may have important implications for the long-term preservation of the deposit, although dissection of the mound along active fault scarps may eventually expose its interior to seafloor oxidation. An estimated growth rate for the TAG deposit, based on a total accumulation of 2.7 million t of massive sulfides and a cumulative venting history of 5 to 10 k.y., is between 500 and 1,000 t per yr. This is consistent with observed growth rates for the central Black Smoker Complex and with estimates of mass fluxes from heat and fluid flow at black smoker vents on the East Pacific Rise. Although TAG is among the largest of the known mid-ocean ridge deposits, grade-tonnage models for Cyprus-type massive sulfides world-wide suggest that much larger deposits are likely forming elsewhere on the mid-ocean ridges and at similar, slow-spreading centers in extensional back-arc basins. |
|