H. Ahmed

Low-T formation of high-Cr spinel with apparently primary chemical characteristics within podiform chromitite from Rayat, northeastern Iraq

Abstract Chemical modification of chromian spinel at low-T alteration was examined in detail for a podiform chromitite from a Tethyan ophiolitic mélange belt at Rayat, northeastern Iraq. The chromitite is highly brecciated and the matrix has been completely altered, producing chlorite and carbonate (dolomite and calcite). High-Cr, low-Fe3+ spinel has formed along the margins and cracks of chromian spinel grains throughout the alteration, associated with unaltered primary spinel and magnetite without ferritchromite. In associated harzburgites, only ferritchromite is found instead of the high-Cr, low-Fe3+ spinel. The high-Cr, low-Fe3+ secondary spinel apparently has chemical properties of mantle origin, plotted at the extension of ordinary mantle spinels on compositional spaces. The character is due to subtraction of Al as chlorite with the addition of an amount of magnetite component from the silicate matrix, which is small in volume relative to peridotite and composed of highly magnesian olivine (up to Fo97). We should treat high-Cr chromian spinels with caution in highly altered mantle-derived rocks, especially chromitite and other rocks with highly magnesian olivine, as well as in detrital particles for provenance study.

Environmental impact assessment inside and around Mahd Adh Dhahab gold mine, Saudi Arabia

Environmental impact of the mining activities in the Mahd Adh Dahab gold mine was examined through detailed chemical analyses of a number of samples represent the gold-bearing rocks, soils, dust, and plants from inside and outside the mine area and within the Mahd City as well as the surrounding areas. The obtained data were compared with the standard values of heavy metals concentrations in order to evaluate the pollution levels of these activities on the different systems around the mine area. The data of rock samples shows relatively high concentrations of Cu (up to 11.9%), Pb (up to 4.1%), and Zn (up to 14.2%), which attributed to the outcropping of the sulfide-bearing veins on the surface of the mine area. Soil samples inside the mine area show high concentrations of Zn, Cu, Cd, Pb, and As compared to those from outside the mine indicating pollution impact of the mine activities on soils from inside the mine with these elements. Dust samples collected from inside the mine have considerably high concentrations of Cd, Zn, and Pb compared to those from outside the mine. Values of heavy metals in the collected dust from outside the mine are comparable with those from many big cities including Riyadh City, Saudi Arabia, indicating contaminations with heavy metals to the dust from inside the mine at Mahd Adh Dahab City. Heavy metals concentrations in the stems and leaves samples of the various types of plants inside and outside Mahd Adh Dahab gold mine area show that they have relatively higher concentrations of Cu and Zn compared to other metals. Stem samples show relatively higher concentrations of Zn, Cu, Cr, and V compared to the leaves samples. Although values of most heavy metals in the plants, both stems and leaves, are comparable with those of the typical levels of heavy metals in plants, Co and Cr show relatively higher concentrations suggesting a pollution impact of the mine activities in plants with Co and Cr. As a conclusion, results indicated that the contamination effect of Mahd Adh Dahab gold mine activities are clear in soils and dusts inside the mine area with some metals such Zn, Cu, Cd, and Pb due to their relatively enrichment in mineralization. Therefore, careful treatments of the waste dump materials inside the mine area are recommended to protect soils and air contaminations and to prevent the extension of pollution outside the mine in the future.ملخصتهدف الدراسة الحالية الى قياس مدى التاثير البيئى لعمليات استخلاص الذهب بمنطقة منجم مهد الذهب بالمملكة العربية السعوديه على التربه والهواء والنباتات داخل وحول منطقة المنجم من خلال التحليل الكيميائى لعدد من العينات تمثل الصخور الحاملة للذهب وعينات من التربه والهواء والنباتات التى جمعت من داخل وخارج المنجم ∙ وقد قورنت النتائج التى تم الحصول عليها بالمعدلات القياسية للعناصر الثقيله فى التربه والهواء والنباتات للوقوف على مستوى تلوث منطقه الدراسه تاثرا بعمليات استخلاص الذهب لمنجم مهد الذهباظهرت نتائج تحليل العينات الصخرية الارتفاع النسبى لتركيزات عناصر النحاس (يصل الى 11.9%) والرصاص (يصل الى 4.1%) والزنك (يصل الى 14.2%) نتيجة لوجود العروق الحاملة للكبريتيدات بمنطقة سطح المنجم. عينات التربة من داخل المنجم اظهرت تركيزات مرتفعة من عناصرالزنك والنحاس والكادميوم والرصاص والزرنيخ مقارنة بعينات التربة من خارج المنجم بما يشير الى تاثير عمليات استخراج الذهب على تلوث التربه داخل المنجم. عينات الغبار التى جمعت من داخل المنجم اظهرت تركيزات مرتفعة نسبيا من عناصرالكادميوم والزنك والرصاص مقارنة بعينات الغبار من خارج المنجم والتى تضاهى فى تركيزاتها الغبار فى بعض المدن الكبرى متضمنة مدينة الرياض بالمملكة العربية السعوديه مما يدلل على تاثير عمليات استخراج الذهب على تلوث الهواء داخل المنجم. اظهرت النتائج ايضا ان سيقان واوراق النباتات المختلفة من خارج وداخل منجم مهد الذهب اظهرت تركيزات مرتفعة من عناصر النحاس والزنك مقارنة بباقى العناصر. كما اظهرت عينات سيقان النباتات تركيزات مرتفعة من عناصرالزنك والنحاس والكروميوم والفاناديوم مقارنة بعينات الاوراق. بالرغم من ان معظم تركيزات العناصر الثقيلة فى عينات سيقان واوراق النباتات متطابقة مع القيم النموذجية لهذه العناصر فى النباتات الا ان عنصرى الكوبالت والكروميوم تواجدا بتركيزات مرتفعة نسبيا مما قد يدلل على تاثير عمليات استخراج الذهب على تلوث النباتات بهذين العنصرين.اجمالا يمكن القول بان عمليات استخلاص الذهب بمنطقة منجم مهد الذهب بالمملكة العربية السعوديه كان لها بعض التاثيرات البيئية الضاره على كلا من التربه والهواء داخل المنجم من حيث زيادة تركيزات بعض العناصر الثقيلة مثل عناصرالزنك والنحاس والكادميوم والرصاص نتيجة وجود هذه العناصر بنسب مرتفعة فى الصخور المكونة لمنطقة المجم. وعليه فان هذه الدراسة توصى بضروره التعامل الحذر والتخلص الامن من المخلفات داخل المنجم للحفاظ على البيئه داخل المنجم ومنع امتداد التاثير الضار لهذه المخلفات الى خارج المنجم مستقبلا.

Yukonite-like alteration products (Ca–Fe arsenate and As-rich Fe-oxyhydroxide) formed by in situ weathering in granodiorite, Bi'r Tawilah gold prospect, Saudi Arabia

Samples from drilling of the Bi9r Tawilah gold prospect in Saudi Arabia reveal the occurrence of a Ca–Fe arsenate phase, which is similar in appearance and chemistry to yukonite. Upon weathering of a granodiorite host, oxidation of arsenopyrite (0–25 m deep) leads to the formation of a very peculiar brown, amorphous to very poorly crystalline aggregate with cellular-like texture. This mixture consists of Ca–Fe arsenate and arsenic-rich ferric oxyhydroxide resulting from the oxidation of arsenopyrite. It is intergrown with colloform ferric oxyhydroxide, the latter resulting from the oxidation of pre-existing pyrite. The EMPA analyses indicate that the Ca-rich domain contains the maximum As 2 O5 content (up to 22.3 wt%) whereas the colloform ferric oxyhydroxide contains the highest amount of Fe 2 O 3 among the sample studied (60.8–63.1 wt%) associated to higher H 2 O content (31.4–33.2 wt%) than in the case of common goethite and lepidocrocite. As far as typical yukonite, scorodite or arsenosiderite are absent in the studied weathered granodiorite, it is believed that oxidation took place at elevated pH (>7) and temperature up to ∼75 °C. The source of Ca 2+ can be derived from alteration of plagioclase in the granodiorite but its possible derivation from strongly corroded marble bands cannot be discarded. It is evident that availability of Ca 2+ and high pH buffered by the dissolution of calcite in the marble, in addition to the prevailing temperature upon weathering, played important roles in the formation of these pseudomorphs at Bi9r Tawilah.

Secular Change of Chromite Concentration Processes From the Archean to the Phanerozoic

Possible secular changes in the way chromitite (accumulations of chromite) have been formed across Earth’s history are reexamined based on our understanding of the origin of podiform chromitites in the mantle. In the Phanerozoic moderately depleted harzburgites have been dominant in the upper mantle and provided optimum conditions for precipitation of podiform chromitite through peridotite-magma reactions and related processes. A relatively large amount of chromium has been consequently left in the mantle as chromitite. In contrast, in the Precambrian, especially in the Archean to Early Proterozoic, the upper mantle peridotite was more depleted and could have produced only a small amount of podiform chromitites. Instead, Mg- and Cr-rich magmas reached the crust and precipitated large amounts of chromite as stratiform chromitites. The ratio of podiform chromitites/stratiform chromitites has been increasing with time, reflecting a decrease in the degree of depletion of peridotite in the upper mantle with time. This difference is due to a secular decrease in Earth’s thermal state.