安徽庐枞盆地龙桥铁矿床赋矿层位碎屑锆石年代学及其对赋矿地层时代的制约研究

龙桥铁矿床是庐枞盆地一个大型层控矽卡岩型铁矿床,顺层产于沉积岩地层中,受地层控制特征明显,具有显著的成矿特色。龙桥铁矿床赋矿层位时代确定对于该类矿床的成因研究和找矿勘探有着重要的意义。龙桥铁矿床赋矿地层为一套厚度20~150m的泥灰岩、角砾状灰岩、白云岩、泥质粉砂岩和石英杂砂岩,由于缺乏标志性化石和岩性标志层,赋矿地层的时代归属长期存在争议。本文在野外研究的基础上,对龙桥铁矿床主矿体顶底板不同位置的4组砂岩样品(ZK109,ZK309,ZK007和ZK1603)和矿区外围侏罗系罗岭组砂岩(LQWW)开展了系统碎屑锆石年代学研究,定年结果表明,ZK109,ZK309,ZK007和ZK1603样品碎屑锆石的最小年龄和最大年龄分别为262.1±3.2 Ma,2589.8±15.0 Ma;261.8±1.9 Ma,2520.1±11.1 Ma;264.6±2.0 Ma,2947.2±51.4Ma和257.1±1.9Ma,2717.0±21.0。LQWW样品的碎屑锆石最小年龄为166.0±1.4Ma,最大年龄为3842.6±6.5 Ma。结合区域地质特征分析,上述碎屑锆石年龄特征表明,龙桥铁矿床赋矿围岩形成时代应为中三叠世。结合近期庐枞地区的勘查成果,本文认为庐枞北部是寻找龙桥式铁矿床的有利地区。

Detrital zircon U-Pb chronology analysis of the ore-bearing strata at Longqiao deposit in the Lujiang-Zongyang basin, Anhui Province, China.

The Longqiao iron deposit is a large stratabound skarn type iron ore deposit in the Luzong ore district. Identification of the ore formation is important for the research and exploration, but distinguishing between two possible host units (Luoling and Dongmaanshan Formations) is controversial. The host formation at Longqiao is marl, brecciated limestone, dolomite, argillaceous siltstone and quartz sandstone, with a thickness of 20-150 m. We collected sandstones from the Luoling Formation outside the deposit and a unit of uncertain stratigraphic affinities about 5 m above the orebody. Using detrital zircon U-Pb chronology we found that there were significant differences in zircon concordant ages of the sandstones from the two locations. Zircon concordant ages of sample LQWW from the Luoling Formation outside the deposit are in the range 166.0 ± 1.4Ma to 3842.6 ± 6.5Ma; zircon concordant ages of samples from the unit hosting the orebody are in the range 262.1 ± 3.2Ma to 2589.8 ± 15.0Ma (ZK109), 261.8±1.9Ma to 2520.1±11.1Ma (ZK309), 264.6 ± 2.0Ma to 2947.2 ± 51.4Ma (ZK007) and 257.1±1.9Ma to 2717.0±21.0 Ma (ZK1603). These data and their age distributions indicate that the unit hosting ore at the Longqiao deposit differs from the Luoling Formation outside the deposit, and we infer that the host unit is not part of the Luoling Formation and probably belongs to the Dongmaanshan Formation. Combining with the recent exploration achievement, it is proposed that the north part of Luzong basin is a good district for looking for Longqiao type deposit.