Abstract
Mg-borates were traditionally thought to be diagenetic products of other primary borate minerals. Here we report results from the study of pinnoite deposit from DaQaidam saline lake, indicating that pinnoite minerals are primary in origin. Within the detecting limit of X-ray powder diffraction analysis (XRD) analysis, no other borate minerals than pinnoite are detected from the Mg-borate deposit. The cemented pinnoite orebody shows the sedimentary structure of light-dark lamination couplets, which signal marked seasonal variations in brine chemistry. The scanning electronic microscopy coupled with an energy dispersive X-ray spectrometer (SEM-EDX) examination reveals that all pinnoite minerals displayed euhedral, giving no indication of diagenetic origin. A marked shift in lithology from clastic sediment to evaporitic deposit reflects a critical change in sedimentation regime associated with abrupt changes in hydroclimatic conditions. The deposition of the pinnoite ore-layer containing abundant hydromagnesite marked the beginning of the evaporite formation and the end of the clastic deposition. This suggests that aridification occurred abruptly and the saline lake was much more alkaline than today in the early-stage of the evaporite deposition. The intensified summer evaporation and seasonal variations in water chemistry brought about a shallow to nearly desiccated paleo-lake with pH exceeding 9.3, Mg/Ca ratio >39, and boron concentration >600 mg/L, which favored pinnoite precipitation and the formation of pinnoite deposit in the central DaQaidam saline lake.
Highlights
Pinnoite (MgB2 O4 ·3H2 O) is one of the ~20 naturally occurring Mg-borate minerals identified as of 1996, and it is one of the borates important in commercial deposits under the circumstance that only a comparative few of the 230 borates are that important [1]
In order to better understand the sedimentary process and chemistry of the ore-forming brine associated with the genesis of the Mg-borate deposit, we have performed field investigations to examine the mode of occurrence of the pinnoite ore-layer in the subbottom evaporitic sequence and an integrated lithologic and mineralogical study of a type section, D3, selected from the center of the offshore borate deposit (Figure 1)
In the study of the pinnoite deposit at DaQaidam, we found that a suitable alkalinity of the initial brine is crucial for the formation of the Mg-borate deposit, in addition to the presence of sufficient amounts of B and Mg or, precisely, a very high Mg/Ca ratio, as discussed above in the paper
Summary
Pinnoite (MgB2 O4 ·3H2 O) is one of the ~20 naturally occurring Mg-borate minerals identified as of 1996, and it is one of the borates important in commercial deposits under the circumstance that only a comparative few of the 230 borates are that important [1]. According to Qu et al (1981), hydroboracite occurs as 0.5–2 mm spherical grains in the ore layers within gypsiferous deposits, and the borate ooids comprise ~70% hydroboracite in addition to dolomite or hydromagnesite and clay Both primary and secondary ulexite minerals were identified from the onshore borate deposit. The former was primarily formed in the NaCl supersaturated brine with lower SO4 2− and Mg2+ , but richer in Ca2+. In order to better understand the sedimentary process and chemistry of the ore-forming brine associated with the genesis of the Mg-borate deposit, we have performed field investigations to examine the mode of occurrence of the pinnoite ore-layer in the subbottom evaporitic sequence and an integrated lithologic and mineralogical study of a type section, D3, selected from the center of the offshore borate deposit (Figure 1). Based on the results from our field and laboratory investigations, we discuss hydroclimatic, geochemical and sedimentologic constraints on the pinnoite deposit formation in DaQaidam saline lake
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