科研之友
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摘要
The Beebe vent field (BVF) in the Cayman Trough has built an auriferous massive sulfide deposit on the ultraslow spreading mid-Cayman spreading center. The genesis of auriferous sulfide deposits at mid-ocean ridges is not fully understood, although there is a growing recognition that slow and ultraslow spreading centers are conducive to gold mineralization. Analysis of hydrothermal precipitates from the BVF indicates that the highest gold contents are present within beehive diffusers, which have developed a highly porous pyrrhotite framework. The beehive structure allows vent fluids to effuse slowly while allowing ingress of seawater to cool the fluid. The prevalence of pyrrhotite in the beehive samples, lack of sulfates, association between pyrrhotite and gold grains, and results of thermodynamic modeling suggest gold precipitation occurred under highly reduced conditions even during mixing with seawater. In contrast, high-temperature chimneys, with a single orifice, maintain high temperatures to the primary vent orifice and much of the gold is lost to seawater. Despite this, both chimney types are relatively gold-enriched, which points to a further underlying cause for high gold at the BVF such as interaction of hydrothermal fluids with ultramafic lithologies in the basement. The final gold composition of the deposit is partially controlled by loss of gold during mass-wasting of the material, with gold depletion most prevalent in blocks formed at beehive-type chimneys. The BVF demonstrates that the overall gold content of a massive sulfide deposit is the sum of basement, precipitation, and surface processes. Plain Language Summary Mineral deposits form on the seafloor at hydrothermal vent sites and are rich in metals including copper, zinc, lead, and sometimes precious metals like gold and silver. However, the processes controlling the amount of gold that ends up in these deposits is not clearly understood. In this article we show that as hydrothermal fluid vents on the seafloor, two different types of chimney form. One is relatively gold-poor, while the other is gold-rich. The gold-rich chimneys, so-called beehive chimneys due to their appearance, are a very efficient structure for the formation of gold because the hot hydrothermal fluids are able to be cooled to much lower temperatures in the vicinity of the chimney. In comparison, the other type of chimney vents most of the gold to the ocean. Therefore, at this vent site, the formation of beehive chimneys may control the amount of gold in the deposit.
- 出版日期2017-6
