PRATIMA DESAI and HELEN REID AN earthquake in Goma, a city in Democratic Republic of Congo near the border with Rwanda, is delaying exports of tin ore from mineral-rich North Kivu province, two sources with direct knowledge have told Reuters. This is likely to exacerbate shortages of the soldering metal, prices of which last week touched 10-year highs at $30,650 a tonne. The sources said tin ore producers have been unable to obtain the permits they need to export the material because government offices in Goma, North Kivu's capital city, are shut due to the earthquake, which destroyed several buildings…